Networks - Computer Engineering Department

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23 Οκτ 2013 (πριν από 3 χρόνια και 5 μήνες)

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ETM 555

1



Networks


ETM 555

2

Computer Networks


“An interconnected collection of autonomous end
-
systems”


End Systems: hosts, PCs, dedicated computers, network


components


Connection can be via various means: copper wire, fiber optic


cables, microwaves, satellites, ham radio.


Distributed Systems:


Existence of multiple autonomous computers is transparent
(not visible), machines managed automatically


Network:


Users explicitly log onto a machine, machines managed
personally

ETM 555

3

Network Hardware


Broadly speaking : two types of transmission technology

1)
Broadcast Networks

2)
Point
-
to
-
Point Networks

Broadcast (multiaccess)

Point
-
to
-
point

ETM 555

4

Broadcast Networks


Single communication channel that is shared by all the
machines on the network


Share wire, frequency etc.


Packets (small messages) sent by one mechanism is received
by the others.


Each machine checks the address field in the provided
message. If the address is the same as receiving machine’s
address, packet is processed, otherwise it is ignored.


Some arbitration mechanism is needed when more than one
machine want to send message at the same time.


Multicasting: transmission to a subset of machines.

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Point
-
to
-
Point Networks


Consists of many connections between individual pair of
machines.


To go from source machine to destination machine, packet may
have to visit intermediate machines.


Various routes (multiple paths) possible


Routing algorithms
are required.


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Connecting Networks

-

Repeaters
-


Copies bits from one network to another


Does not look at (interpret) bits


Allows the extension of a network beyond physical length
limitations

REPEATER

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Connecting Networks

-

Bridges
-



Copies frames from one network to another


Can operate selectively


does not copy all forms


Extends the network beyond physical length limitations

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Connecting Networks


-

Routers
-



Switching Elements, packet switching nodes, intermediate
systems, data switching exchanges


Copies packets from one network to another


Makes decisions about what route (path) a packet should take


If two routers not connected by direct link want to
communicate, they do so by using intermediate routers


Subnets in which intermediate routers can store a packet and
forward it is called point
-
to
-
point or store
-
and
-
forward, packet
switched.

ETM 555

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Subnet

LAN

host

LAN

LAN

LAN

Router

SUBNET

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Connecting Networks

-

Gateways
-


Operates as a router


A gateway is a host that is connected to two or more physical
networks simultaneously and is configured to switch packets
between them


Can perform


Data conversions


Translation: connect different application protocols


encryption

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Hardware vs. Software


Repeaters are typically hardware devices


Bridges can be implemented in hardware or software


Routers and gateways are typically implemented in software so
that they can be extended to handle new protocols


Many workstations can operate as routers or gateways

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Local Area Networks (LANs)


LANs connect computers that are physically close together


LANs are restricted in size


Worst case transmisson time is bounded and known in advance


High speed


Transmission technology: multi
-
access (broadcast)


Speeds


Ethernet: 10 Mbps


Token Ring: 16 Mbps


FDDI: 100 Mbps


Fast Ethernet: 100 Mbps


Gigabit Ethernet: 1 Gbps

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Classification of Networks


Smaller, geographically localized networks tend to use broadcasting. Larger
networks are usually point
-
to
-
point.

Interprocessor Distance

Processors located in the same:

Example

1m

system

multicomputer

10m

100m

1km

Room

Building

Campus



LAN

10km

City

Metropolitan Area

Network (MAN)

100km

1000km

Country

Continent

Wide Area

Network (WAN)

10000km

Planet

The Internet

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Transmission Media


Can be classified as:


Guided : waves are guided along a solid medium such as
copper wire, optical fiber etc.


Unguided : wireless transmission, atmosphere, outer space,
transmit electromagnetic signals, radio, laser etc.


Design issues in transmission systems:


Data rate, distance, bandwidth, transmission impairments,
interference, number of receivers etc.

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Twisted Pair (guided trans. media)


Oldest, least expensive, most widely used


Telephones connected to telcos by twisted pair (subscriber
loops)


Can transmit both analog and digital signals:


For analog: amplifiers needed every 5 to 6 km


For digital: repeaters required every 2 to 3 km


Limited distance, bandwidth compared to others



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Twisted Pair (guided trans media)


2 kinds:

1) Unshielded (UTP, 10 Base T) is ordinary telephone wire,
least expensive transmission media for LANs

2) Shielded: shield twisted pair with a metalic braid/sheating
to reduce interference, more expensive


UTP kinds:


Category 3: 3 or 4 twists per foot, (data rates: 16 Mbps)


Category 5: 3 to 4 twists per inch (data rates: 100 Mbps)

ETM 555

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Coaxial Cable (guided trans. media)


Can be used over longer distances and supports more stations
on a shared line than twisted pair.


Bandwidth possible depends on cable length. For 1 km cables
1
-
2 Gbps feasible.


Widespread use: television distribution, long distance telephone
transmission, short
-
run computer system, local area networks


Varieties: thin, thick coaxial

ETM 555

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Fiber Optics (guided trans. media)


Physiscs: light ray is refracted (bent) when passing from one
medium to another)


Above a critical angle of incidence, light is refracted back into
silica (hence trapped inside fiber) and can travel for many
kilometers

ETM 555

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Fiber Optics (guided trans. media)


Lower attenuation


Fibers thinner and lighter weight (considerably)


Data Rates: 1 Gbs, 100 Gbps, 1 Tbps


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Fiber Optics (guided trans. media)


Electromagnetic isolation (i.e. not affected by external
electromagnetic fields) no interference


Difficult to tap, therefore better security


Enables greater spacing between repeaters


Multimode fiber: allows many different rays incident at
different angles to travel


Singlemode fiber: employs fiber core which has small distance
(of the order of a wavelength) and allows only a single ray to
pass. Singlemode fiber is more expensive but can be used for
longer distances.

ETM 555

21

Global Wireless Network Standards



IEEE 802.15 (Bluetooth) for the Personal Area Network (PAN)


IEEE 802.11 (Wi
-
Fi) for the Local Area Network (LAN)


IEEE 802.16 (WiMax) for the Metropolitan Area Network
(MAN)


IEEE 802.20 standard for the Mobile Broadband Wireless
Access (MBWA)


ETM 555

22

Bluetooth


Can link up to 8 devices in 10
-
m area


Can transmit up to 722 Kbps in the 2.4 GhZ band


Low power requirements


Radio based communication

ETM 555

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Bluetooth

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Wi
-
Fi


Three standards:


802.11a
: up to 54 Mbps (10


30 meters)



802.11b

: up to 11 Mbps (30


100 meters)


802.11g
: up to 54 Mbps (30


100 meters)


Infrastructure mode: Devices use access point to communicate
with wired network


Ad
-
hoc mode (peer
-
to
-
peer): Wireless devices communicate
directly with each other


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Wireless LAN



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The Telephone System


Telephone was patented by Alexander Graham Bell, 1876.


Evolution of the structure of telephone system:


(a) Fully
-
interconnected network.

(b) Centralized switch.

(c) Two
-
level hierarchy.

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The Telephone System


Basic Bell system model remained essentially intact up to now.


Presently, we have highly redundant multilevel hierarchy.


Hierarchy: Home, End Office, Toll Office, Primary Office,
Sectional Office, Regional Office


Transmission media used:


Local loops consist of twisted pair


Between switching offices: coaxial cables, microwaves,
fiber optics


Transmission:


Local loops: analog signaling


Trunks: mostly digital

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Structure of Telephone System

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Local Loop


Between home and the end office


Modems can be used to connect computer at home


Baud Rate: Signalling speed


the number of times per second that signal
changes its value (i.e. its voltage)


b
-
baud line does not necessarily transmit b bits/sec


each signal may convey several bits


Example: if voltages 0,1,2,3,4,5,6,7 are used, each signal can convey 3
bits. So bit rate: 3 X baud rate


DSL (Digital subscriber lines) : Use existing telephone lines to transmit
signals. 500Kbps
-
1.5Mbps possible.


ADSL is asymmetric DSL : download and upload speeds are different.
Upload speed is slower than download speed.

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Trunks and Multiplexing


Cost of installing and maintaining both high
-
bandwidth and
low bandwidth trunk the same (essentially)


Therefore, choose high
-
bandwidth trunk installation


To efficiently use high bandwidth, multiplex many
conversations over a single trunk


Multiplexing schemes:


Frequency Division Multiplexing


Wavelength Division Multiplexing


Time Division Multiplexing

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Example of Time Division Multiplexing


T1 Carrier (1.544 Mbps):


24 voice channels multiplexed together


Analog signals are sampled on a round robin basis

ETM 555

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Multiplexing T1 streams into higher carriers

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Layered Network Software Architecture Model

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Protocol Hierarchies


Design issues for the layers: addressing, error control,
flow control, multiplexing, routing

ETM 555

35

OSI Reference Model


Network architecture model based on a proposal developed by
International Standards Organization (ISO)


OSI: Open Systems Interconnection


Standardized protocols for network layers


OSI has 7 layers

ETM 555

36

OSI Layers

Layer

Responsibility

1) Physical Layer

Transmission of raw bits over a communication
channel

2) Data Link Layer


Data Link Control


MAC Sublayer


-

Provide an error free communication link. Issues:
framing, addressing

-

Medium Access Control needed by multiaccess
communications. MAC provides DLC with virtual
wires on multiaccess networks

3) Network Layer

-
Path selection between end
-
systems (routing)

-
Subnet flow control

-

Translation between different network types

4) Transport layer

-
Provides virtual end
-
to
-
end links between peer
processes. Fragmentation and reassembly. End
-
to
-
end
flow control

5) Session Layer

-

Establishes, manages and terminates sessions
between applications. Service location lookup.

6) Presentation Layer

-

Data encryption, data compression, data conversion

7) Application Layer

-

Anything not provided by other layers

ETM 555

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TCP/IP Reference Model


First used in ARPANET (Advanced Research Projects Agency)
sponsored by US Department of Defense


Now used in worldwide Internet


Goal: ability to connect multiple networks in a transparent way


Requirements: Connections should remain intact as long as
source and destination machines are functioning even if some
of the intermediate machines, links are out of operation


Internetwork Layer (Internet) : Packet switching network based
on a connectionless internetwork layer.

ETM 555

38

TCP/IP Reference Model

ETM 555

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Protocols and networks in the TCP/IP model

ETM 555

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Why OSI Model/Protocols were not implemented


Bad timing


Bad technology


Bad implementations


Bad politics