Information and Communication

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Information and Communication

1. INTRODUCTION.............................................................................................................................................................................................3
1.1

U
SES OF COMPUTER NETWORKS
[3]..............................................................................................................................................................3
1.1.1. Networks for companies [3]................................................................................................................................................................3
1.1.2. Networks for people [4].......................................................................................................................................................................3
1.2

N
ETWORK
H
ARDWARE
[7]............................................................................................................................................................................3
1.2.1. Local Area Networks (LAN) [9]..........................................................................................................................................................4
1.2.2. Metropolitan Area Networks (MAN) [10]...........................................................................................................................................4
1.2.3. Wide Area Networks (WAN) [11]........................................................................................................................................................4
1.2.4. Wireless Networks [13].......................................................................................................................................................................5
1.2.5. Internetworks [16]...............................................................................................................................................................................5
1.3.

N
ETWORK
S
OFTWARE
[16]..........................................................................................................................................................................6
1.3.1. Protocol Hierarchies [17]...................................................................................................................................................................6
1.3.2. Design Issues for the Layers [21].......................................................................................................................................................6
1.3.3. Interfaces and Services [22]................................................................................................................................................................6
1.3.4. Connection-Oriented and Connectionless Services [23]....................................................................................................................6
1.3.5. Service Primitives [25]........................................................................................................................................................................7
1.3.6. The Relationship of Services to Protocols [27]..................................................................................................................................7
1.4.

R
EFERENCE
M
ODELS
[28]............................................................................................................................................................................7
1.4.1. The OSI Reference Model [28]...........................................................................................................................................................7
1.4.2. The TCP/IP Reference Model [35].....................................................................................................................................................8
1.4.3. A Comparison of the OSI and TCP Reference Models [38]...............................................................................................................9
1.4.4. A Critique of the OSI Model and Protocols [40]................................................................................................................................9
1.4.5. A Critique of the TCP/IP Reference Model [43]................................................................................................................................9
1.5.

E
XAMPLE
N
ETWORKS
[44]...........................................................................................................................................................................9
1.5.4. The Internet [52].................................................................................................................................................................................9
1.6.

E
XAMPLE
D
ATA
C
OMMUNICATION
S
ERVICES
[56]...................................................................................................................................10
1.6.2. X.25 Networks [59]............................................................................................................................................................................10
1.6.4. Broadband ISDN and ATM [61].......................................................................................................................................................10
1.6.5. Comparison of Services [66].............................................................................................................................................................11
2. THE PHYSICAL LAYER [77]......................................................................................................................................................................12
2.1.

T
HE
T
HEORETICAL
B
ASIS FOR
D
ATA
C
OMMUNICATION
[77]...................................................................................................................12
2.1.1. Fourier Analysis [78]........................................................................................................................................................................12
2.1.2. Bandwidth-Limited Signals [78].......................................................................................................................................................12
2.1.3. The Maximum Data rate of a Channel [81]......................................................................................................................................12
2.2.

T
RANSMISSION
M
EDIA
[82].......................................................................................................................................................................13
2.2.1. Magnetic Media [82].........................................................................................................................................................................13
2.2.2. Twisted Pair [83]...............................................................................................................................................................................13
2.2.3. Baseband Coaxial Cable [84]...........................................................................................................................................................13
2.2.4. Broadband Coaxial Cable [85].........................................................................................................................................................13
2.2.5. Fiber Optics [87]...............................................................................................................................................................................13
2.6.

B
ROADBAND
ISDN
AND
ATM

[144].........................................................................................................................................................14
2.6.1. Virtual Circuits versus Circuit Switching [145]...............................................................................................................................14
2.6.2. Transmission in ATM Networks [146]..............................................................................................................................................14
2.6.3. ATM Switches [147]..........................................................................................................................................................................14
3. THE DATA LINK LAYER [175]..................................................................................................................................................................15
3.1.

T
HE
D
ATA
L
INK LAYER
D
ESIGN
I
SSUES
[176]...........................................................................................................................................15
3.1.1. Services Provided to the Network Layer [176].................................................................................................................................15
3.1.2. Framing [179]...................................................................................................................................................................................15
3.1.3. Error Control [182]..........................................................................................................................................................................16
3.1.4. Flow Control [183]...........................................................................................................................................................................16
3.2.

E
RROR
D
ETECTION AND
C
ORRECTION
[183].............................................................................................................................................16
3.2.1. Error-Correcting Codes [184]..........................................................................................................................................................16
3.2.2. Error-Detecting Codes [186]............................................................................................................................................................16
3.3.

E
LEMENTARY
D
ATA
L
INK
P
ROTOCOLS
[190]...........................................................................................................................................16
3.3.1. An Unrestricted Simplex Protocol [195]..........................................................................................................................................16
3.3.3. A Simplex Protocol for Noisy Channel [197]...................................................................................................................................17
3.4.

S
LIDING
W
INDOW
P
ROTOCOLS
[202]........................................................................................................................................................17
3.4.1. A One Bit Sliding Window Protocol [206].......................................................................................................................................17
3.3.2. A Protocol Using Go Back n [207]...................................................................................................................................................17
3.5.

P
ROTOCOL
S
PECIFICATION
A
ND
V
ERIFICATION
[219]..............................................................................................................................18
3.5.1. Finite State Machine Models [219]..................................................................................................................................................18
3.6.

E
XAMPLE
D
ATA
L
INK
P
ROTOCOLS
[225]..................................................................................................................................................18
3.6.1. HDLC – High-Level Data Link Control [225].................................................................................................................................18
3.6.2. The Data Link Layer in the Internet [229]........................................................................................................................................19
3.6.3. The Data Link Layer in ATM [235]..................................................................................................................................................19
4. THE MEDIUM ACCESS SUBLAYER [243]...............................................................................................................................................21
4.1.

T
HE
C
HANNEL
A
LLOCATION
P
ROBLEM
[244]...........................................................................................................................................21
4.1.1. Static Channel Allocation in LANs and MANs [244].......................................................................................................................21
Information and communication Seite 2
4.1.2. Dynamic Channel Allocation in LANs and MANs [245]..................................................................................................................21
4.2.

M
ULTIPLE
A
CCESS
P
ROTOCOLS
[246].......................................................................................................................................................21
4.2.1. ALOHA [246]....................................................................................................................................................................................21
4.2.2. Carrier Sense Multiple Protocols [250]...........................................................................................................................................22
4.3.

IEEE

S
TANDARD
802
FOR
LAN
S AND
MAN
S
[275].................................................................................................................................22
4.3.1. IEEE Standard 802.3 and Ethernet [276].........................................................................................................................................22
4.3.3. IEEE Standard 802.5: Token Ring [292]..........................................................................................................................................23
4.3.4. Comparison of 802.3, 802.4 and 802.5 [299]...................................................................................................................................24
4.3.6. IEEE Standard 802.2: Logical Link Control [302]..........................................................................................................................24
5. THE NETWORK LAYER [339]....................................................................................................................................................................25
5.1.

N
ETWORK
L
AYER
D
ESIGN
I
SSUES
[339]....................................................................................................................................................25
5.1.1. Services Provided to the Transport Layer [340]..............................................................................................................................25
5.1.2. Internal Organization of the Network Layer [342]..........................................................................................................................25
5.1.3. Comparison of Virtual Circuit and Datagram Subnets [344]..........................................................................................................25
5.2.

R
OUTING
A
LGORITHMS
[345]....................................................................................................................................................................26
5.2.1. The Optimality Principle [347].........................................................................................................................................................26
5.2.2. Shortest Path Routing [349]..............................................................................................................................................................26
5.2.3. Flooding [351]..................................................................................................................................................................................26
5.2.5. Distance Vector Routing [355].........................................................................................................................................................26
5.2.6. Link State Routing [359]...................................................................................................................................................................26
5.2.9. Broadcast Routing [370]...................................................................................................................................................................27
5.2.10. Multicast Routing [372]..................................................................................................................................................................27
5.5.

T
HE
N
ETWORK
L
AYER IN THE
I
NTERNET
[412].........................................................................................................................................27
5.5.1. The IP Protocol [413].......................................................................................................................................................................28
5.5.2 IP Address [416]................................................................................................................................................................................28
5.5.3. Subnets [417].....................................................................................................................................................................................28
5.5.4. Internet Control Protocols [419]......................................................................................................................................................28
5.5.7. Internet Multicasting [431]...............................................................................................................................................................29
5.5.10. Ipv6 [437]........................................................................................................................................................................................29
5.6.

T
HE
N
ETWORK
L
AYER IN
ATM

N
ETWORKS
[449]....................................................................................................................................30
5.6.1. Cell Formats [450]............................................................................................................................................................................30
5.6.2. Connection Setup [452].....................................................................................................................................................................30
5.6.3. Routing and Switching [455]............................................................................................................................................................30
5.6.4. Service Categories [458]...................................................................................................................................................................30
5.6.5. Quality of Service [460]....................................................................................................................................................................31
5.6.8. ATM LANs [471]...............................................................................................................................................................................31
5.7.

S
UMMARY
[473].........................................................................................................................................................................................31
6. THE TRANSPORT LAYER [479]................................................................................................................................................................32
6.1.

T
HE
T
RANSPORT
S
ERVICE
[479]................................................................................................................................................................32
6.1.1. Services Provided to the Upper Layer [479]....................................................................................................................................32
6.1.2. Quality of Service [481]....................................................................................................................................................................32
6.1.3. Transport Service Primitives [483]...................................................................................................................................................32
6.5.

T
HE
ATM

AAL

L
AYER
P
ROTOCOLS
[545]................................................................................................................................................33
6.5.1. Structure of the ATM Adaptation Layer [546]..................................................................................................................................33
6.5.2. AAL 1 [547].......................................................................................................................................................................................33
6.5.3. AAL 2 [549].......................................................................................................................................................................................33
6.5.4. AAL 3/ 4 [550]...................................................................................................................................................................................33
6.5.5. AAL 5 [552].......................................................................................................................................................................................33
6.5.6. Comparison of AAL Protocols [554]................................................................................................................................................33

Information and communication Seite 3
1. Introduction
20
th
century: The key technology has been information gathering, processing and distribution
computer networks: interconnected collection of autonomous computers
distributed system: existence of multiple autonomous computers is transparent (not visible). The user is not
aware that there are multiple processors.
The difference between a distributed system and a computer network lies in who invokes the movement of files,
etc..., the system or the user.

1.1 Uses of computer networks [3]
1.1.1. Networks for companies [3]
Here, ressource sharing is very important: The goal is to make all programs, equipment and especially data
available to anyone on the network without regard to the physical location of the ressource and the user.
High reliability: having alternate sources of supply
saving money: Small computers have much better price/performance ratio than larger ones
Scalability: The ability to increase system performance gradually as the workload grows just by adding more
processors.
Communication medium: communication in the company
1.1.2. Networks for people [4]
Acces to remote information
person-to-person-interaction
Interactive entertainment

1.2 Network Hardware [7]
Broadcast networks: They have a single communication channel that is shared by all the machines on the
network. Short messages, called packets in certain context, sent by any machine are received by all others.
Broadcast system allow to address all machines by using a special code in the address field. Some broadcast
systems also allow transmissions to a subnet. This is called multicasting.
Point-to-point networks: Many connections between individual
pairs of machines. Routing algorithms play an important role.

Smaller, geographically localized networks tend to use
boroadcasting, whereas larger networks usually are point-to-point.

Information and communication Seite 4
1.2.1. Local Area Networks (LAN) [9]
LAN’s have 3 characteristics: size, transmission
technology, topology
LAN’s are restricted in size.
LAN’s use a single cable to which all machines are
attached. They run at speed of 10 to 100Mbps.
Ethernet (IEEE 802.3): Bus based broadcast network
with decentralized control operating at 10 or 100Mbps.
Computers on an Ethernet can transmit whenever they
want to: if two or more packets collide, each computer
just waits a random time and tries again laiter.
IBM token ring (IEEE 802.5): ring –based LAN operating at 4 and 16 Mpbs
Broadcast networks can be further divided into static and dynamic.
Static broadcast: Divide up time into discrete intervals and run a round robin algorithm, allowing each machine
to broadcast only when its time slot comes up.
Dynamic broadcast: DB can be centralized or decentralized. In the centralized DB there is a single entity which
determines who goes next. In the decentralized DB there is no such entity. Each machine must decide for itself
wether or nor to transmit.


1.2.2. Metropolitan Area Networks (MAN) [10]
A metropolitan area network (MAN) is basically a bigger version of a LAN and normally also uses the same
technology.
A standart has established DQDB (Distributed Queue
Dual Bus). DQDB consists of two unidirectional
buses to which all the computers are connected.
The key aspect of a MAN is that there is a broadcast
medium to which all the computers are attached.


1.2.3. Wide Area Networks (WAN) [11]
A WAN contain a collection of machines intended for running user programs (hosts or end system). The hosts
are connected by a communication subnet (or just subnet).
By separating the pure communication aspect of the network (the subnet) from the application aspects (the hosts)
the complete network design is greatly simplified.
In most WAN’s the subnet consists of two distinct components:
transmission lines (circuits, channels or trunks) and switching elements.
The switching elements are specialized computers used to connect two or
more transmission lines. These computers are called routers.

There exists a lot of different router interconnection topology:

Information and communication Seite 5
1.2.4. Wireless Networks [13]
Typically they have a capacity of 1-2Mbps,
which is much slower than wired LAN’s. The
error rates are often much higher too, and the
transmissions from different computers can
interface which each other.


1.2.5. Internetworks [16]
A collection of interconnected networks is called an internetwork or just internet. The machines which connect
the networks is called a gateway.
A common form of internet is a collection of LAN’s connected by a WAN.
If the system within the closed curve contains only routers, it is a subnet. If it contains both routers and hosts
with their own users, it’s a WAN.
Differences between a subnet, networks and internetworks:
Subnet: WAN, it refers to a collection of routers and network lines owned by the network operator (AOL,
CompuServe).
The combination of a subnet and ist hosts forms a network. In the case of a LAN, the cable and the hosts from
the network. There really is no subnet.

Information and communication Seite 6
1.3. Network Software [16]
1.3.1. Protocol Hierarchies [17]
To reduce their design complexity, most networks are organized as a series of layers or levels.
Layer n on one machien carries on a conversation with layer n on another machine.
Below layer 1 is the physical medium trough which actual communication occurs. Between each pait of
adjacent layers there is an interface.
A set of layers and protocols is called a network architecture.


1.3.2. Design Issues for the Layers [21]
Every layer needs a mechanism for identifying senders and receivers.
There exists some rules of data transfer: Travel in one direction (simplex communication), either direction, but
not simultaneously (half-douplex comm.) and both directions at once (full-duplex comm.).
Not all communication channels preserve the order of the messages sont on them. To deal with a possible loss of
sequencing, the protocol must makje explicit provision for the receiver to allow pieces to be put back together
properly.
Another problem that must be solved at several levels is the inability of all processes to accept arbitrarily long
messages.
Multiplexing: To have several connections established trough one physical cable.


1.3.3. Interfaces and Services [22]
Active elements in each layer are often called entities, entities in the same layer on different machines are called
peer entities.


1.3.4. Connection-Oriented and Connectionless Services [23]
Connection-Oriented service is modeled after the telephone
system.
Connectionless service is modeled after the postal system.
Each message carries the full destination address and each one
is routed through the system independent of all others.
Each service can be characterized by a quality of service. Some
services are reliable in the sense that they never lose data.
Reliable connection-oriented services has two variations:
message sequence and byte stream.
Message sequence: When two 1KB messages are sent, they
arrive as two distinct 1-KB messages.
Information and communication Seite 7
Byte stream: Here, the message is simply a stream of bytes without message boundaries.
Datagram service: Unreliable conectionless service
Acknowledged datagram service: conectionless, but reliable
Request-reply service: The sedner transmits a single datagram containing a request, the reply contains the
answer.


1.3.5. Service Primitives [25]
Services can be either confirmed or unconfimed. In a
confirmed service, there is a request, an indication, a
response and a confirm. In an unconfirmed service, there is
just a request and an indication.


1.3.6. The Relationship of Services to Protocols [27]
A service is a set of primitives that a layer provides to the layer above it. A protocol in contrast is a set of rules
governing the format and meaning of the frames.


1.4. Reference Models [28]
1.4.1. The OSI Reference Model [28]
The model is called the ISO OSI (Open Systems Interconnection) Reference Model. The OSI model has seven
layers. The principles that were applied to arrive at the seven layers are as follows:
1. A layer should be created where a different level of abstraction is needed.
2. Each layer should perform a well defined function.
3. The function of each layer should be chosen with an eye toward defining internationally standardized
protocols.
4. The layer boundaries should be chosen to minimize the information flow across the interfaces.
5. The number of layers should be large enough that distinct functions need not be thrown together in the same
layer out of necessity, and small enough that the architecture does not become unwieldy.
Information and communication Seite 8
The Physical Layer
Transmitting raw bits over a communication channel.

The Data Link Layer
Take raw transmission facility and transform it into a line that appears free of undetected transmission errors to
the network layer. Break the input data up into frames, transmit the frames sequentially and process the
acknowledgement frames. It is up to the data link layer to create and recognize frame boundaries.
It is up to this layer to solve the problem caused by damaged, list, and duplicated frames. Another issue is to
keep a fast transmitter from drowning a slow receiver in data.
Broadcast networks have a different additional issue in data link layer: how to control access to the shared
channel. A special sublayer of the data link layer, the medium access sublayer, deals with this problem.

The Network Layer
Controlling the operation of the subnet. Determining how packets are routed from source to destination. If too
many packets are in the subnet at the same time, the will get in each other’s way – The control of such
congestion also belongs to the network layer. It must also count how many packets (or chars or bits) sent by each
customer to produce billing information.
It is up to the network layer to overcome all these problems to allow heterogeneous networks to be
interconnected. In broadcast, the routing problem is simple, so the network layer is often thin or even
nonexistent.

The Transport Layer
The basic function is to accept data from the session layer, split it up into smaller units if need be, pass these to
the network layer and ensure that the pieces all arrive correctly at the other end. Furthermore all this must be
done efficiently and in a way that isolates the upper layers from the inevitable changes in the hardware
technology. Another task is to make the multiplexing transparent to the session layer. It must also take care of
establishing and deleting connections across the network. There must also be a mechanism to control the flow so
that a fast host cannot overrun a slow one. This is called flow control.

The Session Layer
A session allows ordinary data transport. The session layer has to manage dialogue control. Another session
service is token management and synchronization.

The Presentation Layer
The presentation layer performs certain functions that are requested sufficiently often to warrant finding a
general solution for them rather than letting each user solve the problems. The presentation layer is concerned
with the syntax and semantics of the information transmitted.

The Application Layer
Contains a variety of protocols that are commonly needed. Another function is file transfer.



1.4.2. The TCP/IP Reference Model [35]
Connections may get established as long as the source and the destination machines are functioning.

The Internet Layer
A packet switching network based on a connectionless internetwork
layer. This alyer, clled the internet layer, is the linchpin that holds the
whole architecture together. The job of the internet layer is to deliver
IP packets where they supposed to go.

The Application Layer
It contains all the higher level protocols (TEéNET, FTP, SMTP)
Domain Name Service (DNS) for mapping host names onto their
nertwork addresses, NNTP (news), HTTP (WWW).
Information and communication Seite 9
1.4.3. A Comparison of the OSI and TCP Reference Models [38]
Both are based on the concept of a stack of independent protocols. Also, the functionality of the layers is roughly
similar.
The biggest contribution of the OSI model is to make the distinction between the three concepts explicit (Service
– Interfaces – Protocols). The TCP/IP model did not originally clearly distinguish between service, interface and
protocol.
As a consequence the protocols in the OSI model are better hidden than in the TCP/IP model and can be replaced
relatively easily as the technology changes.
The OSI reference model was devised before the protocols were invented. With the TCP/IP the reverse was true:
the protocols came first and the model was really just a description of the existing protocols.
They differ also in the number of layers.
The OSI model supports both connectionless and connection-oriented communication in the network layer, but
only connection-oriented communication in the transport layer, where it counts. The TCP/IP model has only one
mode in the network layer (conectionless) but supports both modes in the transport layer, giving the user a
choice.


1.4.4. A Critique of the OSI Model and Protocols [40]
Bad Timing: The OSI model came in the false moment. It now appears that the standard OSI protocols got
crushed. The competing TCP/IP protocols were already in widespread use by research universities by the time
the OSI protocols appeared.
Bad Technology: The session layer has little use in most applications and the presentation layer is nearly empty.
The data link and network layer are so full that subsequent work has split them into multiple sublayers, each with
different functions.
The idea behind OSI was to produce an IBM-like reference model.
Another problem in OSI is that addressing, flow control and error control reappear again and again in each layer.
Data security and encryption were controversial that no one could agree which layer to put them in, so they were
left out altogether. Network management was also omitted from the model for similar reasons.
Perhaps the most serious criticism is that the model is dominated by a communication mentality.
Bad Implementation: The initial implementation was huge, unwieldy and slow.
Bad Politics


1.4.5. A Critique of the TCP/IP Reference Model [43]
First the model does not clearly distinguish the concepts of service, interface and protocol.
Second the TCP/IP model is not at all general and is poorly suited to describing any protocol stack other than
TCP/IP.
Third, the host-to-network layer is not really a layer at all in the normal sense that the therm is used in the
context of layered protocols. It is an interface (between the network and the data link layer).
Fourth, the TCP/IP model does not distinguish (for even mention) the physical and data link layers.
Finally, although the IP and TCP protocols were carefully thought out and well implemented, many of the other
protocols were ad hoc, generally produced by a couple of graduate students hacking away until they got tired.


1.5. Example Networks [44]
Networks differ in their history, administration, facilities offered, technical design and user communities.

1.5.4. The Internet [52]
The number of networks, machines and users connected to the ARPANET grew rapidly after TCP/IP became the
only official protocol on Jan 1, 1983. When NSFNET and the ARPANET were interconnected, the growth
became exponential. Many regional networks joined up and connections were made to networks in Canada,
Europe and the Pacific.
Our definition is that a machine is on the internet if it runs the TCP/IP protocol stack, has an IP address and the
ability to send IP packets to all other machines on the internet.
Information and communication Seite 10
Traditionally the internet has four main applications:
Email, News, Remote login, FTP. Recently (1990s) the WWW was added.



1.6. Example Data Communication Services [56]
The subnet is owned by the network operator, providing communication service for the customers’ host and
terminal. Such a system is called a public network.

1.6.2. X.25 Networks [59]
X.25 was developped during the 1970s.
The physical layer protocol, called X.21, specifies the physical, electrical and procedural interface between the
host and the network. It requires digital signaling.
The data link layer standard has a numbver of (slightly incompatible) variations. They all are designed to deal
with transmission errors on the telephone line between the user’s equipement (host or terminal) and teh public
network (router).
The network layer protocol deals with addressing, flow control, delivery confirmation, interrupts and related
issus.
X.25 is connection-oriented and supports both switched virtual curcuits and permanent ones. A switched virtual
circuit is created when one computer send a packet to the networl asking to make a call to a remote computer. A
permanent virtual circuit is used the same way as a switched one, but it is set up in advance by agreement
between the customer and the carrier.


1.6.4. Broadband ISDN and ATM [61]
The perceived solution is to invent a single new network for the future that will replace the entire telephone
system an all the specialized networks with a single integrated network for all kinds of information transfer.
B-ISDN (Broadband Integrated Services Digital Network). It will offer video on demand, live television from
many sources, full motion multimedia electronic mail, CD-quality music, LAN interconnection, high-speed data
transport for science and industry and many others all over the telephone line. Th underlying technology that
makes B-ISDN possible is called ATM (Asynchronous Transfer Mode). The basic idea behind ATM is to
transmit all information in small, fixed-size packets called cells. The cells are 53 bytes long, of which 5 bytes are
header and 48 bytes are payload.
There are a variety of reasons why cell switching was chosen:
First, cell switching is highly flexible and can handle both constant traffic (audio, video) and variable rate traffic
(data) easily. Second, it is very fast (gigabits/sec). Third for television, broadcasting is essential.
ATM networks are connection-oriented. ATM networks are organized like traditional WANs with lines and
switchers (routers) The intended speed for ATM networks are 155Mbps and 622Mbps.

The B-ISDN Reference Model
Three layers: The physical, ATM and ATM
adaptation layer plus whatever the users want to put
on top of that.
The physical layer deals with the physical medium:
voltage, bit timing, ...
The ATM layer deals with cells and cell transport.
The ATM interface segments these packets, transmits
the cells individually and reassembles them at the
other end. This layer is the AAL (ATM Adaptation
Layer).

The PMD (Physical Medium Dependent) sublayer
interfaces to the actual cable. The other sublayer of
the physical layer is the TC (Transmission
Convergence) sublayer. When cells are transmitted, the TC layer sned them as a string of bits to the PMD layer.
The AAL Layer is split into a SAR (Segmentation And Reassembly) sublayer and a CS (Convergence Sublayer).
The lower sublayer breaks packets up into cells on the transmission side and puts them back together again at the
Information and communication Seite 11
destination. The upper sublayer makes it possible to have ATM systems offer different kinds of services to
different applications.


1.6.5. Comparison of Services [66]

Information and communication Seite 12
2. The Physical Layer [77]
2.1. The Theoretical Basis for Data Communication [77]
Information can be transmitted on wires by varying some physical property such als voltage or current.

2.1.1. Fourier Analysis [78]

2.1.2. Bandwidth-Limited Signals [78]
Unfortunately, all transmission facilities dimish different Fourier components by different amounts, thus
introducing distortion. The number of changes per second is measured in baud.


2.1.3. The Maximum Data rate of a Channel [81]
maximum data rate (low-pass filter of bandwidth H, V discrete levels)
signal-to-noise ratio: signal power to the noise power (S/N)
decibels (dB): 10*log
10
S/N
maximum number of bits per second (bandwidth H, signal-to-noise ratio S/N)


sec/log2
2
bitsVH
)/1(log
2
NSH +
Information and communication Seite 13
2.2. Transmission Media [82]
Guided media, such as copper wire and fiber optics and unguided media such as radio and lasers through air.

2.2.1. Magnetic Media [82]
Never underestimate the bandwidth of a station wagon full of tapes hurtling down the highway.


2.2.2. Twisted Pair [83]
A twisted pair consists of two insulated copper wires, typically 1mm thick. The wires are twisted together in a
helical form. The purpose of twisting is to reduce electrical interference from similar pairs close by.
Several megabits/sec can be achieved for a few kilometers.
Category 3 twisted pair consist of two insulated wires gently twisted together.
Category 5 twisted pair are similar to category 3 pairs, but with more twists per cm and Teflon insulation, which
results in less crosstalk and better quality signal over longer distances.


2.2.3. Baseband Coaxial Cable [84]
It has better shielding than twisted pairs, so it can span longer distances at higher speeds. They have high
bandwidth and excellent noise immunity. For a 1-km cables, a data rate of 1 to 2 Gbps is feasible.


2.2.4. Broadband Coaxial Cable [85]
The cables can be used up to 300 MHz.


2.2.5. Fiber Optics [87]
The current practical signaling limit of about 1 Gbps is due our inability to convert between electrical and optical
signals any faster. In the laboratory, 100 Gbps is feasible on short runs. A speed of 1 terabit/sec is only a few
years down the road.
An optical transmission system has three
components: the light, the transmission
medium and the detector.
A light ray incident at or above the critical
angle is tapped inside the fiber.
Different rays will be bouncing around at
different angles => Multimode fiber.
Fibers can be connected in three different
ways: They can terminate in connectors and
be plugged into fiber sockets. They can be
spliced mechanically. Two fibers can also be
fused (melted) together to form a solid
connection.
As light there are lasers or LED’s.

Fiber Optic Networks
Two types of interfaces are used. A passive
interface consisting of two taps fused the main
fiber. The other interface type is the active
repeater.


Comparison of Fiber Optics and Copper Wire
Fiber has a much higher bandwidth, repeaters
are only needed about every 30km on long
Information and communication Seite 14
lines, versus about every 5 km for copper. Fiber is not being affected by power surges, electromagnetic,
interference, or power failures. Nor ist it affected by corrosive chemicals in the air. It is thin and lightweight.
Finally fiber do not leak light and is quite diffucult to tap.
On the downside, fibers is an unfamiliar technology requiring skills most engineers do not have. And fiber is
unidirectional, so we need two fibers or two bands.


2.6. Broadband ISDN and ATM [144]
ATM is fundamentally a packet-switching technology not a circuit-switching technology (although it can
emulate circuit switching fairly well). Broadband ISDN can not be sent over existing twisted pair wiring. BISDN
needs category 5 twisted pair or fiber.

2.6.1. Virtual Circuits versus Circuit Switching [145]
Permanent virtual circuits are requested by the customer manually and typically remain in place for months.
Switched virtual circuits are like telephone calls: they are set up dynamically as needed and potentially torn
down immediately afterward.


2.6.2. Transmission in ATM Networks [146]
Asynchronous transfer mode. ATM does not standardize the format for transmitting cells. ATM is normally fiber
optics.


2.6.3. ATM Switches [147]
Cells arrive at ATM speed, normally about 150Mbps. This works out to slightly over 360000 cells/sec, which
means that the cycle time of the switch has to be about 2.7 usec. A commercial switch might have anywhere
from 16 to 1024 input lines, which means that it must be prepared to accept and start switching a batch of 16 to
1024 cells every 2.7 usec. At 622 Mbps, a new batch of cells are injected into the switching fabric every 700
nsec. The fact that the cells are fixed length and short (53 bytes) makes it possible to build such switches. An
ATM switch has two common goals: 1. Switch all cells with as low a discard rate as possible, 2. Never reorder
the cells on a virtual circuit.
Goal 1 says that it is permitted to drop cells in emergencies.
Goal 2 says that cells arriving on a virtual circuit in a certain order must also depart in that order.
A problem that occurs in all ATM switches is what to do if the cells arriving at two or more input lines want to
go to the same output port in the same cycle. Solving this problem is one of the key issues in the design of all
ATM switches. One solution is a queue for each input line.

The problem with input queuing is that when a cell has to be held up, it blocks the progress of any cells behind
it, even if they could otherwise be switched. This effect is called head-of-line blocking. An alternative design is
queuing on the output side.

The Knockout Switch
It uses output queuing. Problem here is that this switch is basically a crossbar switch, so the number of
crosspoints is quadratic in the number of linbes!

The Batcher-Banyan Switch
Like knockout switches, Batcher-Banyan switches are synchronous, accepting a set of cells on each cycle. Each
of the 12 switching elements in the banyan switch has two inputs and two outputs. When a cell arrives at a
switching element, 1 bit of the output line number is inspected and based on that, the cell is routed either to port
o or 1 (the lower). In the event of a collision, one cell is routed and one is discarded.
Depending on the input, the banyan switch can do a good job or a bad job of routing.
The idea behind the Batcher-Banyan switch is to put a switch in front of the banyan switch to permute the cells
into a configuration that the banyan switch can handle without loss. In principle the Batcher-Banyan switch
makes a fine ATM switch, but there are two complications that we have ignored: output line collision and
multicasting.
Information and communication Seite 15
3. The Data Link Layer [175]
This study deals with the algorithm for achieving reliable, efficient communication between two adjacent
machines at the data link layer.

3.1. The Data Link layer Design Issues [176]
Providing a well-defined service interface to the network layer, determing how the bits of the physical layer are
grouped into frames, dealing with transmission errors, and regualting the flow of frames so that slow receiver are
not swaped by fast senders.

3.1.1. Services Provided to the Network Layer [176]
The job of the data link layer is to transmit the bits to the destination machine, so they can be handed over to the
network layer there.
Three services are provided:
Unacknowledged connectionless service, Acknowledged connectionless service and acknowledged connection-
oriented service
Unacknowledged connectionless service consists of having the source machine send independent frames to the
destination machine without having the destination machine acknowledge them.
With the acknowledged connectionless service is like the unacknowledged one but now, each frame sent is
individually acknowledged.
With the connection-oriented service, the source and destination machine establish a connection before any data
are transferred. Each frame sent over the connection is numbered, and the data link layer guarantees that each
frame sent is indeed received. Furthermore, it guarantees that each frame is received exactly once and that all
frames are received in the right order.


3.1.2. Framing [179]
It is up to the data link layer to detect and if necessary correct errors. The usual approach is for the data link layer
to break the bit stream up into discrete frames and compute the checksum for each frame.
There are four methods of breaking the stream up into packets:
Charcter count, Starting and ending character (with character stuffing), starting and ending flags (with bit
stuffing) and Physical layer coding violations.

Character count: Character stuffing:


Bit stuffing: Each frame begins and end with a special bit pattern, 01111110, called a flag byte. Whenever the
sender’s data link layer encounters five consecutive ones in the data, it automatically stuffs a 0 bit into the
outgoing bit stream. This bit stuffing is analogous to character stuffing, in which a DLE is stuffed into the
outgoing character stream before DLE in the data.


Information and communication Seite 16
3.1.3. Error Control [182]
When the sender starts to transmit a frame, it also starts a timer.
It is generally necessary to assign sequence numbers to outgoing frames, so that the receiver can distinguish
retransmissions from originals.


3.1.4. Flow Control [183]
The usual solution is to introduce flow control to throttle the sender into sending no faster than the receiver can
handle the traffic. This throttling generally requires some kind of a feedback mechanism, so the sender can be
made aware of whether or not the receiver is able to keep up.


3.2. Error Detection and Correction [183]
The disadvantage of burst errors is that they are much harder to detect and correct than isolated errors.

3.2.1. Error-Correcting Codes [184]
There are error-detecting codes (which only detects that there is an error) and error correcting codes (which are
able to correct the error).
The number of bit positions in which two codewords differ is called the Hamming distance.
The error-detecting and error-correcting properties of a code depend on ist Hamming distance. To detect d
errors, you need a distance d+1 code because with such a code there is no way that d single-bit errors can change
a valid codeword into another valid codeword. When the receiver sees an individual codeword, it can tell that
transmission error has occurred. Similarly, to correct d errors, you need a distance 2d+1 code because that way
the legal codewords are so far apart that even with d changes, the original codeword is still closer than any other.


3.2.2. Error-Detecting Codes [186]


3.3. Elementary Data Link Protocols [190]
A frame consists of an embedded packet and some control (header) information. The transmitting hardware
computes and appends the checksum, so that the data link software need not worry about it.
As long as the network layer knows nothing at all about the data link protocol or the frame format, these things
can be changed without requiring changes to the network layer’s software.
A frame is composed of four field: kind, seq, ack and info. The first three of which contain control information,
and the last of which may contain actual data to be transferred. These control fields are collectively called the
frame header. The kind field tells wether or not there are any data in the frame, because some of the protocols
distinguish frames containing exclusively control information from those containing data as well. The seq and
ack fields are used for sequence numbers and acknowledgements, respectively. The info field of a data frame
contains a single packet; the info field of a control frame is not used. A more realistic implementation would be
to use a variable-length info field.

3.3.1. An Unrestricted Simplex Protocol [195]
A protocol that is as simple as it can be.
Protocols in which the sender send one frame and then waits for an acknowledgement before proceeding are
called stop-and-wait.

Information and communication Seite 17
3.3.3. A Simplex Protocol for Noisy Channel [197]
The network layer has no way of knowing that a packet has been lost or duplicated, so the data link layer must
guarantee that no combination of transmission errors, no matter how unlikely, can cause a duplicate packet to be
delivered to a network layer.


3.4. Sliding Window Protocols [202]
In most practical situations, there is a need for transmitting data in both directions.
The technique of temporarily delaying outgoing acknowledgements so that they can be hooked onto the next
outgoing data frame is known as piggybacking.
The essence of all sliding window protocols is that at any instant of time, the sender maintains a set of sequence
numbers corresponding to frames it is permitted to send. These frames are said to fall within the sending
window. Similarly, the receiver also maintains a receiving window corresponding to the set of frames it is
permitted to accept.


3.4.1. A One Bit Sliding Window Protocol [206]
Maximum window size of 1. Such a protocol uses stop-and-wait, since the sender transmits a frame and waits for
its acknowledgement before sending the next one.


3.3.2. A Protocol Using Go Back n [207]
Until now we have made the tacit assumption that the
transmission time required for a frame to arrive at the
receiver plus the transmission time for the acknowledgement
to come back is negligible. Sometimes this assumption is
clearly false.


At all times, 25 or 26 unacknowledged frames are
outstanding. Put in other terms, the sender’s maximum
window size is 26- This technique is known as pipelining.

There are two basic approaches to dealing with errors in the
presence of pipelining. One way, called go back n, is for the
Information and communication Seite 18
receiver simply to discard all subsequent frames, sending no acknowledgements for the discarded frames.
The other general strategy for handling errors when frames are pipelined, called selective repeat, is to have the
receiver data link layer store all the correct frames, following the bad ones.

These two alternative approaches are trade-offs between bandwidth and data link layer buffer space. Depending
on which resource is more valuable, one or the other can be used.


3.5. Protocol Specification And Verification [219]
3.5.1. Finite State Machine Models
[219]
The key concept here is the finite state machine.
With this technique, each protocol machine is
always in a specific state at every instant of time.
Then there are possible transitions. One particular
state is designated as the initial state.
It is possible to determine which states are
reachable and which are not. This technique is
called reachability analysis.

An important property of a protocol is the absence
of deadlock: 1, There is no transition out ob the
subset and 2. There are no transitions in the subset that cause forward progress.



3.6. Example Data Link Protocols [225]
3.6.1. HDLC – High-Level Data Link Control [225]
In IBM’s SNA a protocol is used called SDLC (Synchrnonous Data Link Control). ANSI modified it to become
ADCCP (Advanced Data Communication Control Procedure) and ISO modified it to become HDLC.
All of these protocols are based on the same principles. All are bit-oriented, and all use bit stuffing for data
transparency.
All the bit-oriented protocols use the frame
structure shown below. The Address field is
primarily of importance on lines with multiple
terminals.


The control field is used for sequence numbers, acknowledgements and other purposes.
The Data field may contain arbitrary information.
The Checksum field is the checksum using CRC-CCITT.

There are three kind of frames: Information, Supervisory and
Unnumbered.
The P/F bit stands for Poll/Final. It is used when a computer is
polling a group of terminals.
The various kinds of Supervisory frames are distinguished by the
Type field. Type 0 is acknowledgement frame, type 1 is negative
acknowledge frame, type 2 is receive not ready and type 3 is
elective reject.
Control frames may be lost or damaged, just like other data frames, so they must be acknowledged too.


Information and communication Seite 19
3.6.2. The Data Link Layer in the Internet [229]
It is the routers and the leased lines that make up the communication subnets on which the Interner is built.
For both the router-router leased line connection and the dial-up host-router connection, some point-to-point data
link protocol is required on the line from framing, error control and the other data link layer functions. Two such
protocols are widely used in the Internet, SLIP and PPP.

SLIP – Serial Line IP
Just sends raw IP packets over the line, with a special flag byte (0xC0) at the end for framing. If the flag byte
occurs inside the IP packet, a form of character stuffing is used, and the two byte sequence (0xDB, 0xDC) is sent
in its place. If 0xDB occurs inside the IP packet, it, too is stuffed.
SLIP has some serious problems. First it does not do any error detection or correction, so it is up to higher layers
to detect and recover from lost, damaged or merged frames. Second, SLIP supports only IP. Third each side mist
know the other’s IP address in advance. Fourth, SLIP does not provide any form of authentication. Fifth, SLIP is
not an approved Internet Standard.

PPP – Point-to-Point Protocol
PPP handles error detection, supports multiple protocols, allows IP address to be negotiated at connection time,
permits authentication and has many other improvements over SLIP.

PPP provides three things:
1. A framing method that unambiguously delineates the end of one frame and the start of the next one. The
frame format also handles error detection.
2. A link control protocol for bringing lines up, testing them, negotiating options, and bringing them down
again gracefully when they are no longer needed. This protocol is called LCP (Link Control Protocol).
3. A way to negotiate network-layer options in a way that is independent of the network layer protocol to be
used. The method chosen is to have a different NCP (Network Control Protocol) for each network layer
supported.

All PPP frames begin with the standard HDLC
flag byte (01111110), which is charcter stuffed if
it occurs within the payload field.




3.6.3. The Data Link Layer in ATM [235]

Transmission
The first step in header checksumming. Each cell contains a 5-byte header consisting of 4 bytes of virtual circuit
and control information followed by a 1-byte checksum. HEC (Header Error Control)

If no data cell is available when needed, the TC sublayer must invent one. These are called idle cells. Another
kind of nondata cell is the OAM (Operation And Maintenance) cell.


Information and communication Seite 20
Cell Reception
On output, the job of the TC sublayer is to take a sequence of cells, add a HEC to each one, convert the result to
a bit stream and match the bit stream to the speed of the underlying physical transmission system by inserting
OAM cells as filter. The hardest part is locating the cell boundaries in the incoming bit stream.
Information and communication Seite 21
4. The Medium Access Sublayer [243]
In any broadcast network, the key issue is how to determine who gets to use the channel when is competition for
it.
Broadcast channels are sometimes referred to as multiaccess channels or random access channels. The
sublayer of the data link layer is called the MAC (Medium Access Control) sublayer. He takes care of this
problem.

4.1. The Channel Allocation Problem [244]
4.1.1. Static Channel Allocation in LANs and MANs [244]
Frequency Division Multiplexing (FDM). If there are N users, the bandwidth is divided into N equal sized
portions.
If the spectrum is cut up into N regions, and fewer than N users are currently interested in communicating, a
large piece of valuable spectrum will be wasted. If more than N users want to communicate, some of them will
be denied permission. So, dividing a single available channel into subchannels is inherently inefficient.


4.1.2. Dynamic Channel Allocation in LANs and MANs [245]
There are five key assumptions:
1. Station model: The model consists of N independent stations, each with a program or a user to generate
frames. Once a frame has been generated, the station is blocked and does nothing until the frame has been
successfully transmitted.
2. Single Channel Assumption: A single channel is available for all communication. All station can transmit
on it and all can receive from it.
3. Collision Assumption: All stations can detect collisions. A collided frame must be transmitted again later.
There are no errors other than those generated by collisions.
4a. Continuous Time: Frame transmission can begin at any instant. There is no master clock dividing time into
discrete intervals.
4b. Slotted Time: Time is divided into discrete intervals (slots). Frame transmission always begins at the start
of a slot.
5a. Carrier Sense: Stations can tell if the channel is in use before trying to use it. If the channel is sensed busy,
no station will attempt to use it until it goes idle.
5b. No Carrier Sense: Stations cannot sense the channel before trying to use it. They just go ahead and
transmit. They can only later determine if the transmission was successful or not.


4.2. Multiple Access Protocols [246]
4.2.1. ALOHA [246]
Pure ALOHA
Let users transmit whenever they have data to be sent. There will be collisions, of course, and the colliding
frames will be destroyed. A sender can always find out whether or not its frame was destroyed by listening to the
channel.
Throughput of ALOHA systems is maximized by having a uniform frame size rather than allowing variable
length frames. The frames also have a checksum. A checksum cannot distinguish between a total loss and a near
miss. Bad is bad.
At low load, there will be few collisions, at high load there will be many.

Slotted ALOHA
Divide time up into discrete intervals, each interval corresponding to one frame. Since the vulnerable period is
now halved, the probability of no other traffic during the same slot is decreasing.


Information and communication Seite 22
4.2.2. Carrier Sense Multiple Protocols [250]
Protocols in which stations listen for a carrier are called carrier sense protocols.

1-persistent CSMA (Carrier Sense Multiple Access).
When a station has data to send, it first listens to the channel to see if anyone else is transmitting at the moment.
If the channel is busy, the station waits until it becomes idle. When the station detects an idle channel, it
transmits a frame. If a collision occurs, the station waits a random amount of time and starts all over again. The
protocol is called 1-persistent because the station transmits with a probability of 1 whenever it finds the channel
idle.

Nonpersistent CSMA
Before sending, a station senses the channel. If no one else is sending, the station begins doing so itself.
However, if the channel is already in use, the station does not continually sense it for the purpose of seizing it
immediately upon detecting the end of the previous transmission. Instead it waits a random period of time and
then repeats the algorithm.

P-persistent CSMA
When a station becomes ready to
send, it senses the channel. If it is
idle, it transmits with probability p.
With a probability q=1-p it defers
until the next slot. If that slot is
also idle, it either transmits or
defers again, with probability p and
q.

CSMA with Collision Detection
If two stations sense the channel to
be idle and begin transmitting
simultaneously, the y will both
detect the collision almost
immediately. Rather than finish transmitting their frames, which are irretrievably garbled anyway, the should
abruptly stop transmitting as soon as the collision is detected. Quickly terminating damaged frame saves time
and bandwidth. This protocol, known as CSMA/CD (Carrier Sense Multiple Access with Collision Detection) is
widely used on LANs in the MAC sublayer.


4.3. IEEE Standard 802 for LANs and MANs [275]
IEEE 802 include CSMA/CD, token bus and token ring. The various standards differ at the physical layer and
MAC sublayer but are compatible at the data link layer.
802.1 standard gives an introduction to the set of standards
802.2 standard describes the upper part of the data link layer which uses the LLC (Logical Link Control)
protocol.
802.3 – 802.5 describes three LAN standards, the CSMA/CD, token bus and token ring.

4.3.1. IEEE Standard 802.3 and Ethernet [276]
The IEEE 802.3 standard is for a 1-persistent CSMA/CD LAN. It is also called Ethernet.

Manchester Encoding
None of the versions of 802.3 uses 0
volts for a 0 bit and 5 volts for 1 bit.
They use Manchester Encoding and
differential Manchester encoding. With
Manchester encoding, each bit period
is divided into tow equal intervals. A
binary 1 bit is sent by having the
voltage set high during the first interval and low in the second one. A binary 0 is just reverse. The disadvantage
of teh Manchester encoding is that it requires twice as much bandwidth as straight binary encoding, because
Information and communication Seite 23
pulses are half the width.
Differential Manchester encoding is a variation of the basic Manchester encoding. In it, a 1 bit is indicated by the
absence of a transition at the start of the interval. A 0 bit is indicated by the presence of a transition.


The 802.3 MAC Sublayer Protocol
The frame contain two addresses, one for the destination and one for the source. It is possible to send to a group
(multicast) and also to all (broadcast).

There are 46bit od addressing to distinguish local from global
addresses.
The Length field tells how many bytes are present in the data
field, from a minimum of 0 to a maximum of 1500.
The final field is the Checksum field. It is effectively a 32-bit hash code of the data.

The Binary Exponential Backoff Algorithm
After the first collision, each station waits either 0 or 1 slot times before trying again. After the second collision,
each one picks either 0,1,2 or 3 at random and waits that number of slot times. And so on...
After 16 collisions, the controller throws in the towel and reports failure back to the computer.
By having the randomization interval grow exponentially as more and more consecutive collisions occur, the
algorithm ensures a low delay when only a few stations collide but also ensures that the collision is resolved in a
reasonable interval when many stations collide.

802.3 Performance
It is instructive to formulate the equation in terms of frame length, F, the network bandwidth B, the cable Length
L and the speed of signal propagation, c, for the optimal case of e contention slots per frame.


4.3.3. IEEE Standard 802.5: Token Ring [292]
A ring is not really a broadcast medium, but a collection of individual point-to-point links that happen to form a
circle.
Each bit arriving at an interface is copied into a 1-bit buffer and then copied out onto the ring again. While in the
buffer, the bit can be inspected and modified. This copying step introduces a 1-bit delay at each interface.
In a token ring a special bit pattern, called the token, circulates around the ring whenever all stations are idle.
When a station wants to transmit a frame, it is required to seize the token and remove it from the ring before
transmitting.
Ring interfaces have two operation modes: listen and transmit.
Signals are encoded using differential Manchester encoding.
One problem with a ring network is that if the cable breaks somewhere, the ring dies. This problem can be
solved very elegantly by the use of a wire center. So, the
network looks more like a star and is called star-shaped
ring. In this topology, a cable can get broken and the
network will still be working.

The Token Ring MAC Sublayer Protocol
When there is no traffic on the ring, a 3-byte token
circulates endlessly, waiting for a station to seize it by
setting a specific 0 bit to a 1 bit, thus converting the token
into the start-of-frame sequence.
Consequently, the transmitting station must drain the ring
cFBLe
efficiencyChannel
/
21
1
+
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Information and communication Seite 24
while it continues to transmit. A station may hold the token for the token-holding time, which is 10 msec unless
an installation sets a different value. If there is enough time left after the first frame has been transmitted to send
more frames, these may be sent as well.

The Starting delimiter and Ending delimiter fields mark the beginning and ending of a frame. The access control
byte contains the token bit, and also the Monitor bit, Priority bits and Reservation bits. The Frame control byte
distinguishes data frames from various possible control frames.
Frame status contains the A and C bits:
A=0 & C=0: destination not present or not powered up
A=1 & C=0: dest. Present but frame not accepted
A=1 & C=1: dest present and frame copied.


4.3.4. Comparison of 802.3, 802.4 and 802.5 [299]
They have roughly similar technology and they get roughly similar performance.
Advanteges of 802.3: widly used, protocol is simple, stations can be istalled on the fly, the delay at low load is
practically zero.
Disadvantages of 802.3: substantial analog component, minimum valid frame is 64bytes (which represents
substantial overhead when the data consist of just a single character). Furthermore 802.3 is nondeterministic, has
no priorities, cable length is limited to 2.5 km, speed increases then the efficiency drops (because frame
trans.time drops), with high loads the collisions becomes a major problem.
Advantages Token Ring: engineering is easy and can be fully digital, can use any transmission medium, the use
of wire centers make the token ring the only LAN that can detect and eliminate cable failures automatically,
priorities are possible, throughput and efficency at high load is excellent.
Disadvantages Token Ring: Presence of a centralized monitor function, delay at low load



4.3.6. IEEE Standard 802.2: Logical Link Control [302]
LLC (Logical Link Control) provides three service options: unreliable datagram service, acknowledged
datagram service and reliable connection-oriented service.
Information and communication Seite 25
5. The Network Layer [339]
The network layer is concerned with getting packets from the source all the way to the destination. Getting to the
destination may require making many hops at intermediate routers along the way. This function clearly contrasts
with that of the data link layer, which has the more modest goal of just moving frames from one end of a wire to
the other. Network layer is the lowest layer that deals with end-to-end transmission.

5.1. Network Layer Design Issues [339]
5.1.1. Services Provided to the Transport Layer [340]
The layer has been designed with the following goals:
1. The services should be independent of the subnet technology
2. The transport layer should be shielded from the number, type and topology of the subnets present.
3. The network addresses made available to the transport layer should use a uniform numbering plan.

One camp argues that the subnets job is moving bits around and nothing else, the other argues that the subnet
should provide a reliable, connection-oriented service.
So there exist all sort of combinations from connection-oriented or connectionless and reliable or unreliable. The
dominant combinations are reliable connection-oriented and unreliable connectionless.


5.1.2. Internal Organization of the Network Layer [342]
A connection is usually called a virtual curcuit and independent packets of the connectionless . The idea behind
virtual curcuits is to avoid having to choose a new route for every packet or cell.


5.1.3. Comparison of Virtual Circuit and Datagram Subnets [344]


Information and communication Seite 26
5.2. Routing Algorithms [345]
The main function of the network layer is routing packets from the source machine to the destination machine.
The routing algorithm is part of the network layer software. Desirable in a routing algorithm: correctness,
simplicity, robustness, stability, fairness and optimality.
Nonadaptive algorithms do not base their routing decisions on measurements or estimates of the current traffic
and topology. Instead, the choice of the route to use to get from I to J is computed in advance, off-line, and
downloaded to the routers when the network is booted. This procedure is sometimes called static routing.
Adaptive routing change their routing decision to reflect changes in the topology, and usually the traffic as well.


5.2.1. The Optimality Principle [347]
The optimality principle states that if router J is on the optimal path from router I to router K, then the optimal
path from J to K also falls along the same route.
The set of optimal routes from all sources to a given destination is called a sink tree.


5.2.2. Shortest Path Routing [349]
The algorithm just finds the shortest path between them on the graph.
In most general cases, the labels on the arcs could be computed as a function of the distance, bandwidth, average
traffic, communication cost, mean queue length, measured delay and other factors.
Labeling algorithm (see 350).


5.2.3. Flooding [351]
Another static algorithm is flooding in which every incoming packet is sent out on every outgoing line except the
one it arrived on. Flooding generates a vast number of duplicate packets. For this reason, the packets have a
lifetime, which decrements at every hop. When it reaches 0, it dies.
An alternate technique for damming the flood is to keep track of which packets have been flooded to avoid
sending them out a second time.
There exists also selective flooding where each incoming packet is only sent on those lines that are going
approximately in the right direction.


5.2.5. Distance Vector Routing [355]
Vector routing and link state routing are the most popular dynamic routing algorithms,
Distance vector routing algorithms operate by having each router maintain a table giving the best known distance
to each destination and which line to use to get there.

Count-to-infinity Problem
Distance vector routing works in theory but has a serious
drawback in practice: although it converges to the
correct answer, it may do so slowly.

The Split Horizon Hack
The split horizon algorithm works the same way as
distance vector routing, except that the distance to X is not reported on the line that packets for X are sent on.
But sometimes also this algorithm fails an we have again a count-to-infinity problem.


5.2.6. Link State Routing [359]
The idea behind link state routing:
1. Discover its neighbors and learn their network addresses
2. Measure the delay or cost to each of its neighbors
3. Construct a packet telling all it has just learned
Information and communication Seite 27
4. Send this packet to all other routers
5. Compute the shortest path to every other router

Learning about the Neighbors
It will send special HELLO packets on each point-to-point line.

Measuring Line Cost
It will send an ECHO packet.
To factor the load in, the round-trip timer must be started when the ECHO packet is queued. To ignore the load,
the timer should be started when the ECHO packet reaches the front of the queue.

Building Link State Packets
The packet starts with the identity of the sender, followed by a sequence number and age and a list of neighbors.
These can be built periodically or when a significant event occurs.

Distributing the Link State Packets
The trickiest part of the algorithm is distributing the link state packets reliably. The fundamental idea is to use
flooding to distribute the link state packets.

Computing the New Routes
For a subnet with n routers, each of which has k neighbors, the memory required to store the input data is
proportional to kn. For large subnets, this can be a problem. Also, the computation time can be an issue.
Nevertheless, in many practical situations, link state routing works well.


5.2.9. Broadcast Routing [370]
One broadcasting method that requires no special features from the subnet is for the source to simply send a
distinct packet to each destination.
Flooding is another obvious candidate.
A third algorithm is multidestination routing. If this method is used, each packet contains either a list of
destinations or a bit map indicating the desired destination.
A fourth broadcasting algorithm makes explicit use of the sink tree for the router initiating the broadcast, or any
other convenient spanning tree for that matter.

Another broadcast algorithm works like that:
When a broadcast packet arrives at a router, the router check to see if the packet arrived on the line that is
normally used for sending packets to the source of the broadcast. If so, there is an excellent chance that the
broadcast packet itself followed the best route from the router and is therefore the first copy ton arrive at the
router. This being the case, the router forwards copies of it onto all lines except the one it arrived on. If,
however, the broadcast packet arrived on a line other than the preferred one for reaching the source, the packet is
discarded as a likely duplicate.


5.2.10. Multicast Routing [372]
Sending packets to a subgroup is called multicasting and the routing algorithm multicast routing.
When a process joins a group, it informs its host of this fact. It is important that routers know which of their
hosts belong to which group.
To do a multicast routing, each router computer a spanning tree covering all other routers in the subnet.
One potential disadvantage of this algorithm is that it scales poorly to large networks.
An alternative design uses core-base trees. Here a single spanning tree per group is computed, with the root near
the middle of the group. To send a multicast message, a host sends it to the core, which then does the multicast
along the spanning tree.


5.5. The Network Layer in the Internet [412]
The Internet can be viewed as a collection of subnetworks or Autonomous Systems (ASes) that are connected
together. The network layers job is to provide a best-efforts way to transport datagrams from source to
Information and communication Seite 28
destination, without regard to whether or not these machines are on the same network, or whether or not there are
other networks in between them.

5.5.1. The IP Protocol [413]
It transmits in big endian order.
IHL stand for how long the header is, Type of service
what kind of service it wants. The precedence field is a
priority, from 0 (normal) to 7 (network control packet).
The three flag bits allow the host to specify what it cares
most about from the set {Delay, Throughput,
Reliability}.
Total length has a maximum of 65535 bytes.
DF stand for don’t fragment, MF for more fragments.
Fragment offset tells where in the current datagram this
fragment belongs.
The time to live field is a counter used to limit packet lifetimes. The
protocol field tells if ist TCP or UDP.
The header checksum verifies the header only.



5.5.2 IP Address [416]
Network number are assigned by the NIC (Network Information Center)


5.5.3. Subnets [417]
Allow a network to be split into several parts for internal use but still act like a single network to the outside
world.

In fact, all that needs to be changed is to have each router do a Boolean AND with the network’s subnet mask to
get rid of the host number and look up the resulting address in its tables.


5.5.4. Internet Control Protocols [419]
The Internet Control Message Protocol
An event (something unexpected) is reported by the ICMP
(Internet Control Message Protocol).

The Address Resolution Protocol
How do IP addresses get mapped onto data link layer
addresses such as Ethernet. One solution is to have a
configuration file somewhere in the system that maps IP
addresses onto Ethernet addresses. A better solution is for
host 1 to output a broadcast packet onto the Ethernet asking:
“Who owns IP address 192.31.65.5?”.
Information and communication Seite 29
The protocol for asking this question and getting the reply is called ARP (Address Resolution Protocol).

The Reverse Address Resolution Protocol (RARP)
This protocol is there for finding the IP address to a given Ethernet address.

An Ethernet address is on every Ethernet card and it is a 48bit address.


5.5.7. Internet Multicasting [431]
IP supports multicasting, using class D addresses. Each class D address identifies a group of hosts.
Multicasting is implemented by special multicasting routers, which may or may not be colocated with standard
routers. About once a minute, each multicast router send a hardware multicast to the hosts on its LAN asking
them to report back on the groups their processes currently belong to. These query and response packets use a
protocol called IGMP (Internet Group Management Protocol). Multicast routing is done using spanning trees.


5.5.10. Ipv6 [437]
The number of days of IP in its current form (Ipv4) are numbered. The goals
for the new IP (Ipv6) are:
1. Support billions of hosts, even with inefficient address space allocation
2. Reduce the seize of the routing tables
3. Simplify the protocol, to allow routers to process packets faster
4. Provide better security (authentication and privacy) than current IP
5. Pay more attention to type of service, particularly for real-time data
6. Aid multicasting by allowing scopes to be specified
7. Make it possible for a host to roam without changing its address
8. Allow the protocol to evolve in the future
9. Permit the old and new protocols to coexist for years.

The SIPP (simple Internet Protocol Plus) was selected and
given the designation Ipv6 (Ipv5 was already in use for an
experimental real-time stream protocol).
In general Ipv6 is not compatible with Ipv4, but compatible
with all other Internet protocols.
Ipv6 has longer addresses than Ipv4. They are 16 bytes long.
The header has been simplified and there is better support for
options and security.


Controversis
Hop limit field: One camp felt that limiting the maximum number of hops to 255 was a gross mistake.
Another hot potato was the maximum packet size, so a compromise was reached: normal packets are limited to
64kb, but the hop-by-hop extension header can be used to permit jumbograms.
Another hot topic was removing the Ipv4 checksum.
But the biggest battle was about security.

Information and communication Seite 30
5.6. The Network Layer in ATM Networks [449]
The lowest layer that goes from source to destination, and thus involves routing and switching is the network
layer. The ATM layer performs the work expected of the network layer. The ATM layer is connection oriented.
The basic element of the ATM layer is the virtual circuit. Cells sent along virtual circuit will never arrive out of
order.

5.6.1. Cell Formats [450]

5.6.2. Connection Setup [452]
ATM supports both permanent virtual circuits and
switched virtual circuits. The former are always
present and can be used at will, like leased lines. The
latter have to be established each time they are used,
like making phone calls.
The normal way is to first acquire a virtual circuit for
signaling and use it.
Virtual circuit establishment uses the six messages
from the grfx.

ATM networks allow multicast channels to be set up.
ATM addresses comes in 3 forms: The first is 20 bytes long and is based on OSI addresses. The first byte
indicates which of three formats the address is in. In the first format, byte 2 and 3 specify a country, byte 4 gives
the format of the rest of the address, which contains 3 byte authority, a 2-byte domain, a 2-byte area and a 6 byte
address plus some other items. In the second format bytes 2 and 3 designate an international organization instead
of a country. The rest of the address is the same as in format 1. Alternatively, a older form of addressing using
15-digit decimal ISDN telephone numbers is also permitted.


5.6.3. Routing and Switching [455]
The ATM standard does not specify any particular routing algorithm, so the carrier is free to choose.
Virtual paths makes it easier to offer closed user groups.


5.6.4. Service Categories [458]
Information and communication Seite 31

5.6.5. Quality of Service [460]

5.6.8. ATM LANs [471]
The major problem that must be solved is how to provide connectionless LAN service over a connection-
oriented ATM network. One possible solution is to introduce a connectionless server into the network. Every
host initially sets up a connection to this server and sends all packets to it for forwarding.
An alternative approach: Every host has a ATM virtual circuit to every other host. These virtual circuits can be
established and released dynamically as needed, or they can be permanent virtual circuits.
ATM LANs do not support broadcasting. This problem is solved by adding an LES (LAN emulation Server) and
a BUS (Broadcast/ Unknown Server)


5.7. Summary [473]
In virtual circuit subnets, a routing algorithm decision is made when the virtual circuit is set up. In datagram
subnets, it is made on every packet.
Static algorithms include shortest path routing, flooding, and flow-based routing. Dynamic algorithms include
distance vector routing and link state routing.
Unlike the datagram-based Internet, ATM networks use virtual circuits inside.
Information and communication Seite 32
6. The Transport Layer [479]
6.1. The Transport Service [479]
6.1.1. Services Provided to the Upper Layer [479]
The ultimate goal of the transport layer is to provide efficient, reliable and cost-effective service to its users.
The connection-oriented transport service is similar to the connection-oriented network service in many ways. In
both cases, connections have three phases: establishment, data transfer and release.
The transport layer is there to improve the quality of service (QoS). Thanks to the transport layer, it is possible
for application programs to be written using standard set of primitives, and to have these programs work on a
wide variety of networks.


6.1.2. Quality of Service [481]
There are some possible QoS parameters, see fig.




6.1.3. Transport Service Primitives [483]




Information and communication Seite 33
6.5. The ATM AAL Layer Protocols [545]
The AAL (ATM Adaptation Layer) is the end-to-end layer on the top of the ATM layer.
The service is organized in three axes:
1. Real-time service versus nonreal-time service
2. Constant bit rate service versus variable bit rate service
3. Connection-oriented service versus connectionless service


6.5.1. Structure of the ATM Adaptation Layer [546]
The upper part of the ATM adaptation layer is called the convergence sublayer. Ist job is to provide the interface
to the application.
The lower part of the AAL is called SAR (Segmentation And Reassembly) sublayer. It can add headers and
trailers to the data units given to it by the convergence sublayer to form cell payloads.


6.5.2. AAL 1 [547]
AAL 1 is the protocol used for transmitting class A traffic, that is real-time, constant bit rate, connection-
oriented traffic, such as uncompressed audio and video.

6.5.3. AAL 2 [549]
AAL 1 is designed for simple, connection-oriented, real-time data streams without error detection, except for
missing and missinserted cells. AAL 2 is for a more variable data stream.

6.5.4. AAL 3/ 4 [550]
AAL 3/ 4 can operate in tow modes: stream or message. In message mode, each call from the application to
AAL 3/ 4 injects one message into the network. The message is delivered as such, that is, message boundaries
are preserved. In stream mode the boundaries are not preserved.
A feature of AAL 3/ 4 not present in any of the other protocols is multiplexing.

6.5.5. AAL 5 [552]
AAL 5 is also called SEAL (Simple Efficient Adaptation Layer). AAL 5 offers several kinds of services to ist
application. One choice is reliable service, another one is unreliable service (with options to have cells with
checksum errors either discarded or passed to the application layer). Both unicast and multicast are supported,
but multicast does not provide garanteed delivery. It also supports both message mode and stream mode.

6.5.6. Comparison of AAL Protocols [554]