Computer_Networks

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Oct 26, 2013 (3 years and 7 months ago)

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CS2302
-
Computer networks



Department of CSE & IT



2012
-
2013





1









QUESTION BANK

CS2302
-
Computer networks

UNIT I


PART A

1.

C
ompare LAN

and

WAN.


LAN

1.Scope of L
ocal
A
rea
N
etwork

is
restricted to a small/ single

building



2. LAN is owned by same organization


3. Data rate of LAN 10
-
100mbps
.


WAN

1.

scope of W
ide
A
rea
N
etwork

spans
over large geographical area country/
Continent
.

2.

a part of n/w asserts are owned or not
owned


3 .Data rate of WAN is Gigabyte.

2.

What is circuit switching?

In a circuit
-
switched network, a dedicated communication path is estab
lished between two stations through the nodes
of the network. That path is a connected sequence of physical links between nodes.

3.

What is packet switching?

In a packet
-
switched network, it’s not necessary to dedicate transmission capacity along a path throu
gh the network.
Rather, data are sent out in a sequence of small chunks, called packets. Packet switching is mainly used in terminal
-
to
-
computer and computer
-
to
-
computer communications.

4.

Define Full Duplex and simplex transmission system.

With Full duplex t
ransmission, two stations can simultaneously send and receive data from each other. This mode is
known as two
-
way simultaneous. The signals are transmitted in only one direction. One is the sender and another is
the receiver.

5.

Why sliding window flow cont
rol is considered to be more efficient
than stop and wait flow control?

In sliding window flow control, the transmission link is treated as a pipeline that may be filled with frames in transit.
But with stop
-
and
-
wait flow control only one frame may be in
the pipe at a time.

6.

Differentiate between lost frame and damaged frame? What is the difference between stop and wait and
sliding window protocol?

Lost frame is the frame that fails to arrive at the other side. The damaged frame is a recognizable frame doe
s arrive,
but some of the bits are in error. In stop and wait protocol, we can send one frame at a time where as in sliding
window protocol we can send multiple frames at a time.

7.

Define piggybacking.

The technique of temporarily delaying outgoing acknowled
gment so that they can be hooked onto the next outgoing
data frame is widely known as piggybacking.

8
. What is OSI? Define HDLC.

OSI is Open Systems Interconnection and is developed by the International Organization for Standardization (ISO).
HDLC stands f
or High Level Data Link Control. It has three stations, two links, and three types of data transfer.

9
. What is a protocol? What are the key elements of a protocol?


Protocol is used for communications between entities in a system and must speak the same
language. Protocol is the set of
rules governing the exchange of data between 2 entities. It defines what is communicated, how it is communicated, when it
is communicated

Key elements of Protocol:

Syntax



It refers to the structure or format of data meani
ng the order in which they are presented.

Semantics


It refers to the meaning of each section of bit. How to do interpretation.

Timing


When data should be sent and how fast they can be sent.

1
0
. What are the uses of transport layer?



Reliable data exchan
ge



Independent of network being used



Independent of application

1
1
. What is protocol data unit (PDU)?

CS2302
-
Computer networks



Department of CSE & IT



2012
-
2013





2









At each layer, protocols are used to communicate and Control information is added to user data at each layer.
Transport layer may fragment user data. Each

fragment has a transport header added and header consists of
Destination SAP, Sequence number and Error detection code.

12.
What are the uses of internet layer in TCP/IP?



Systems may be attached to different networks



Routing functions across multiple net
works



Implemented in end systems and routers

13
. What is a layered Network Architecture?



A layer is created when a different level of abstraction occurs at protocol. Each layer should perform a well
defined function.



Function of each layer should be cho
sen using internationality standardized protocols. Boundaries between
should be chosen to minimize information flow across the interfaces.



A set of layers and protocol is called network architecture. A list of protocols used by a system is called
protocol

stack.

1
4
.Compare OSI and
TCP
.


O
pen
S
ource
I
nterconnection



T
ransmission
C
ontrl
P
rotocol



It distinguishes between



It does not distinguish between

service,Interface,protocol


service, interface, protocol



Protocols are well hidden



P
rotocols are not just hidden



Dejure. Standard Fit Model



Defacto standard Fit protocol then

then protocol





model



In transport layer only connection


In Transport layer choice is for

Oriented services are available

Connection oriented/conne
ction less.



It contains 7 layers




It contains 5 layers

15
. How do layers of the internet model correlate to the layers of the OSI model?

OSI

TCP/IP

Physical Layer

Physical Layer

Data Link Layer

Network Access Layer

Network Layer

IP Layer

Transport L
ayer

TCP Layer

Session Layer

Application Layer

Presentation Layer

Application Layer

16
. What is the use of data link layer in OSI?



Frame synchronization
: Data is divided by data link layer as frames ,a manageable unit.



Flow Control
: Sending st
ation does not overwhelm receiving station.



Error Control
: Any error in bits must be detected and corrected using some mechanism.



Addressing
: Two stations in a multi point that involved in transmission must be specified using physical
address



Access Contr
ol
: When two or more devices are connected to the same link, Access control mechanism is
needed to determine which device has control over the link at any given time.


17
. Why is flow control and error control duplicated in different layers?

Like the data
link layer, the transport layer is responsible for flow and error control . Flow control and error control
at data link layer is node
-
to
-
node level. But at transport layer, flow control and error control is performed end
-
end
rather than across a single lin
k.


18
. List the key ingredients of technology that determines nature of a LAN. List the common topologies available
for LAN.


Topology, Transmission medium and Medium access control technique are the technology that determines nature of a
LAN. Star Topol
ogy, Ring Topology Bus Topology and Tree Topology are the topologies available for LAN.


CS2302
-
Computer networks



Department of CSE & IT



2012
-
2013





3










19
. What are the functions of physical layer and
presentation layer
?

Encoding/ decoding of signals, preamble generation/removal (for synchronization) and Bit transm
ission/ reception
are the functions of physical layer. Translation, Encryption / Decryption, Authentication and Compression are the
functions of
presentation layer.

20.

What d
o you mean by Flow Control? (Nov.

2011)

Fl
ow control is a technique for assuring that

a transmitting entity does not overwhelm a receiving entity with data.


F
low
control

a feedback mechanism by which the receiver is able to throttle the sender. Such a mechanism is used to keep the
sender from overrunning the receiver, i.e., from transmitt
ing more data than the receiver is able to process

21.

Define

error detection and
correction.

(
Nov.

2011)

Error detection: Sender transmits every data unit twice. Receiver performs bit
-
by
-
bit comparison between that two
versions of data. Any mismatch would ind
icate an error, which needs error correction.


22
. What are the functions of
Application Layer
? (May 2011)



It enables the user

(human/software) to access the network. It provides user interfaces and support for services such
as electronic mail, rem
ote file access and transfer, shared database management and other types of distributed information
services. Services provided by the application layer are Network Virtual terminal
,
File transfer, access and management.
Mail services
,
Directory services.


23.
Define bit stuffing.

(May 2011)

HDLC denotes both the beginning and the end of a frame with

the distinguished bit sequence 01111110.


This
sequence might appear anywhere in the body of the frame,

it can be avoided by bit stuffing.

On the sending side,

any
time five consecutive 1’s have been

transmitted from the body of the message (i.e., excluding when

the sender is
trying to transmit the distinguished 01111110

sequence), the sender inserts a 0 before transmitting the next

bit.

24
. What are the two typ
es of line configuration?

(Nov. 2010)


Point to point line configuration and multipoint line configuration.


Point to point
:



It provides a dedicated link between 2 devices.



Entire capacity of the link is reserved for transmission between 3 devices only



Eg
: connection between remote control and TV’s control system

Multipoint
:



Also called as multi drop connection



Here the channel capacity is shared



If many devices share the link simultaneously it is called spatially shared connection

25.

What do you mean by erro
r control?

(Nov. 2010)

Error control refers to mechanism to detect and correct errors that occur in the transmission of frames.


PART
-
B

1.

Explain in detail the

error detection.

(Nov. 2010)

Error Detection
-

Sender transmits every data unit twice. Receiver pe
rforms bit
-
by
-
bit comparison between that two versions
of data. Any mismatch would indicate an error, which needs error correction. Advantage is it is very accurate. Disadvantage
is time consuming.

Instead of repeating the entire data stream, a shorter gro
up of bits may be appended to the end of each unit called as
“redundancy” because the extra bits are redundant to the information. Redundant information will be discarded as soon as the
accuracy of the information has been determined

Types of Redundancy Ch
ecks



Parity Check



Simple Parity Check



Two Dimensional Parity Check / Longitudinal Redundancy Check (LRC)



Cyclic Redundancy Check (CRC)



Check Sum

Simple Parity Check



A redundant bit called “Parity Bit” is added to every data unit



Even Parity : total number
of 1’s in the data unit becomes
even

CS2302
-
Computer networks



Department of CSE & IT



2012
-
2013





4











Odd Parity : total number of 1’s in the data unit becomes
odd

Error Detection
-

2D/LRC



Adds an additional character (instead of a bit)



A block of bits is organized in a table



The Parity Bit for each data unit is calculat
ed



Then Parity Bit for each column is calculated



Parity Bits are attached to the data unit


Error Detection
-

CRC



Powerful error detection scheme



Rather than addition, binary division is used



A sequence of redundant bits, called “CRC” or “CRC remainder” is

appended to the data unit, so that the resulting
data unit becomes divisible by a predetermined binary number



At the receiver side, the incoming data unit is divided by the same predetermined number.



If there is no remainder, the data unit is accepted



If
there is a remainder, the receiver indicates that the data unit has been damaged during transmission


CS2302
-
Computer networks



Department of CSE & IT



2012
-
2013





5











Error Detection
-

Check Sum



The Check Sum generator subdivides the data unit into equal segments of “n” bits (usually 16)



These segments are added usi
ng one’s complement arithmetic in such a way that the total is also “n” bits long




Total is complemented and appended to the end of the original data unit as redundancy bits, called the check sum
field



The sender follows these steps:



The data unit is divid
ed into “k” sections, each of “n” bits



All sections are added using one’s complement to get the sum



The sum is complemented and becomes the checksum.



The checksum is appended and sent with the data.



The receiver follows these steps:



The unit is divided int
o “k” sections, each of “n” bits



All sections are added using one’s complement to get the sum.



The sum is complemented.



If the result is zero, the data are accepted; otherwise, rejected

CS2302
-
Computer networks



Department of CSE & IT



2012
-
2013





6











Data:


10101001

00111001

Computing Checksum:





10101001





00111
001




---------------


Sum


11100010

CheckSum 00011101

Data Sent :



10101001


00111001 00011101

Receiver Side:





10101001





00111001





00011101





---------------


Sum


11111111

Complement 00000000


2.

Explain about inter
net architecture.


The Internet architecture, also called the TCP/IP architecture after its two main protocols, is

depicted in Fig.1
.
An alternative repre
sentation is given in Fig.2
.


Fig.1

Internet protocol graph.

CS2302
-
Computer networks



Department of CSE & IT



2012
-
2013





7










Fig
2.

Alternative view of the Inter
net architecture.

At the lowest level are a wide variety of network protocols, denoted NET1, NET2, and so on.

The
Internet Protocol
(IP) supports the interconnection of multiple networking technologies into a single, logical
internetwork

The third layer co
ntains two main protocols: the
Transmission Control Protocol
(TCP) and the
User Datagram Protocol
(UDP).

TCP and UDP provide alternative logical channels to application programs: TCP provides a reliable byte
-
stream channel,
and UDP provides an unreliable
datagram delivery channel.

The Internet architecture has three features:



1.

The application is free to bypass the defined transport layers and to directly use IP or one of the

underlying
networks.


2. IP serves as the focal point for the architec
ture

it defines a common method for exchanging


packets
among a wide collection of networks. Above IP can be arbitrarily many transport

protocols, each offering a different
channel abstraction to application programs.


Below IP, the architecture allows

for arbitrarily many different network technologies, ranging

from Ethernet to
FDDI to ATM to single point
-
to
-
point links.

3.

The existence of working implementations is required for standards to be adopted by the IETF.




3.
Discuss in detail about the

layers
O
S
I

m
ode
l
.
(Nov. 2010, Nov. 2011)

Purpose of the reference model was to provide a framework for the development of protocols


Fig.1 OSI model layers

Physical Layer



It coordinates the functions required to transmit a bit stream over a physical mediu
m.



It deals with the mechanical and electrical specifications of the interface and transmission media.

CS2302
-
Computer networks



Department of CSE & IT



2012
-
2013





8









Mechanical: cable, plugs, pins...


Electrical/optical: modulation, signal strength, voltage levels,
bit times.



It also defines the

procedures and functions that physical devices and interfaces have to perform for
transmission to occur
.


Major responsibilities of Physical layer are

Physical characteristics of interfaces and media:
It defines the characteristics of the interface b
etween the devices
and the transmission media. Also defines the type of transmission medium.

Representation of bits
: To transmit the bits, it must be encoded into electrical or optical signals. It defines the type
of representation how 0s and 1s are change
d to signals.

Data rate:
The number of bits sent each second is also defined by the physical layer.

Synchronization of bits:
Sender and the receiver must be synchronized at the bi
t level .i.e the

sender and the
receiver clocks must be synchronized.


Fig.2

Information flows from top to bottom at the sender and bottom to top at the receiver.

Data link layer

The data link layer is responsible for hop
-
to
-
hop (node
-
to
-
node) delivery. It


transforms the physical layer a raw transmission facility to a

reliable link. It makes physical layer appear error free to
the network layer. The duties of the data link layer are



Framing: The data link layer divides the stream of bits received from the network layer into manageable data
units called frames.



Physical

Addressing: If the frames are to be distributed to different systems on the network the data link layer
adds a header to the frame to define the receiver or sender of the frame. If the frame is intended for a system
located outside the
sender’s

network th
en the receiver address is the address of the connecting device that
connects the network to the next one.



Flow Control: If the rate at which the data absorbed by the receiver is less than the rate produced in the
sender, the data link layer imposes a f
low control mechanism to overwhelming the receiver.



Error control
:

Reliability is added to the physical layer by data link layer to detect and ret
ransmit loss or
damaged frames
and also to prevent duplication of frames. This is achieved through a trailer a
dded to the end
of the frame



Access control
:

When two or more devices are connected to the same link it determines which device has
control over the link at any given time.



Network Layer

The network layer is responsible fo
r source
-
to
-
destination delivery of a packet across multiple networks. It ensures
that each packet gets from its point of origin to its final destination .It does not recognize any relationship between
those packets. It treats each one independently as tho
ugh each belong to separate message.

CS2302
-
Computer networks



Department of CSE & IT



2012
-
2013





9









The functions of the network layer are



Logical Addressing
:

If a packet has to cross the network boundary then the header contains information of the
logical addresses of the sender and the receiver.



Networking
:

When ind
ependent networks or links are connected to create an internetwork or a large network the
connective devices route the packet to the final destination.

Transport Layer

The network layer is responsible for process
-
to
-
process
delivery that

is source to desti
nation delivery of the entire
message.

The responsibilities of Transport layer are



Service
-
point (port) addressing:

Computers run several programs at the same time. source
-
to
-
destination delivery
means delivery from a specific process on one computer to a

specific process on the other. The transport layer
header therefore includes

a type of address called a service


point address.



Segmentation and reassembly A message is divided into segments and each segment contains a sequence number.
These numbers ena
ble the Transport layer to reassemble the message correctly upon arriving at the destination. The
packets lost in the transmission is identified and replaced.



Connection control: The transport layer can be either connectionless or connection
-
oriented. A co
nnectionless
transport layer treats segment as an independent packet and delivers it to the transport layer. A connection
-
oriented
transport layer makes a connection with the transport layer at the destination machine and delivers the packets.
After all th
e data are transferred
,
the connection is terminated.



Flow control:

Flow control at this layer is performed end to end.



Error Control: Error control is performed
end to end. At the sending side
,

the transport layer makes sure that the
entire message arrive
s at the receiving transport layer with out error. Error correction is achieved through
retransmission.

Session Layer
:


Session layer is the network di
alog controller. It establishes
,

maintains, and synchronizes the interaction between
communicating system
s. Specific responsibilities of the layer are



Dialog Control: Session layer allows two systems to enter in to a dialog.
Communication between two processes
takes

place either in half
-
duplex or full
-
duplex. Example
:

the dialog between a terminal connected t
o a mainframe.
Can be half
-
duplex.



Synchronization. The session layer allows a process to add checkpoints into a stream of data. Example If a system
is sending a file of 2000 pages, check points may be inserted after every 100 pages to ensure that each 100

page
unit is advised and acknowledged independently. So if a crash happens during the transmission of page 523,

r
etransmission begins at page 501
,

pages 1 to 500 need not be retransmitted.

Presentation layer
:


It is concerned with the syntax and semantics

of the information exchanged between two systems.

Responsibilities of the presentation layer are



Translation .The processes in two systems are usually exchanging information in the form of character strings,
numbers, and so on. Since different computers

use different encoding systems,

the presentation layer is responsible
for interoperability between these different
encoding methods. At the sender
,

the presentation layer changes the
information from its sender
-
dependent format into a common format. The p
resentation layer at the receiving
machine changes the common format into its receiver dependent format.



Encryption. The sender transforms the original information from to another form and sends the resulting message
over the entire network. Decryption rev
erses the original process to transform the message back to its original form.



Compression. It reduces the number of bits to be transmitted. It is important in the transmission of text, audio and
video.

Application Layer
:


It enables the user

(human/softwa
re) to access the network. It provides user interfaces and support for services
such as electronic mail, remote file access and transfer, shared database management and other types of distributed
information services. Services provided by the application l
ayer are



Network Virtual terminal
:

A network virtual terminal is a software version of a physical terminal and allows a user
to log on to a remote host.

CS2302
-
Computer networks



Department of CSE & IT



2012
-
2013





10











File
transfer, access and management:

This application allows a user to access files in a remote compu
ter, to
retrieve files from a remote computer and to manage or control files in a remote computer.



Mail services:

This application provides the basis for e
-
mail forwarding and storage.



Directory services:

It provides distributed database sources and access

for global information about various objects
and services.





4
.
Explain briefly about various physical links available.



Physical links:

Network connectivity occurs at many different levels. At the lowest level, a network can consist of
two or more com
puters directly connected by some physical medium, such as a coaxial cable or an optical fiber. We call such
a physical medium a
link
.


Figure A Direct links: (a) point
-
to
-
point; (b) multiple
-
access.

In Figure A, physical links are sometimes limited to a
pair of nodes (such a link is said to be
point
-
to
-
point
), while more than
two nodes may share a single physical link (such a link is said to be
multiple access
). Whether a given link supports point
-
to
-
point or multiple access connectivity depends on how th
e node is attached to the link.

Explain about Leased Lines, Last
-
Mile Links and Wireless Links.


Channel access on links

The network as providing logical
channels
over which application
-
level processes can communicate with each other; each
channel provides

the set of services required by that application. A channel Is connecting one process to another. Figure B
shows a pair of application
-
level processes communicating over a logical channel that is, in turn, implemented on top of a
cloud that connects a set

of hosts.

The channel is like a pipe connecting two applications, so that a sending application can put data in one end and expect that

data to be delivered by the network to the application at the other end of the pipe.

Applications supported on any net
work is a file access program like FTP (File Transfer Protocol) or NFS (Network File
System).The process that requests access to the file is called the
client
, and the process that supports access to the file is
called the
server
.

The client sends a large

message containing the data to be written to the server, and the server responds with a small message
confirming that the write to disk has taken place.

A client process makes a request, and a server process responds by returning the requested data.

The
following two types of channels:
request/reply
channels and
message stream
channels.

The request/reply channel would be used by the file transfer and digital library applications.

Every message sent by one side is received by the other side and that only
one copy of each message is delivered.

The request/reply channel might also protect the privacy and integrity of the data that flows over it, so that unauthorized
parties cannot read or modify the data being exchanged between the client and server process
es.


CS2302
-
Computer networks



Department of CSE & IT



2012
-
2013





11










Figure B Processes communicating over an abstract channel.

The message stream channel could be used by both the video
-
on
-
demand and video conferencing applications, provided it is
parameterized to support both one
-
way and two
-
way traffic and to supp
ort different delay properties.


The message stream channel might not need to guarantee that all messages are delivered, since a video application can
operate adequately even if some frames are not received.

Those messages that are delivered arrive in the
same order in which they were sent, to avoid displaying frames out of
sequence. The message stream channel might want to ensure the privacy and integrity of the video data.

The message stream channel might need to support multicast, so that multiple part
ies can participate in the teleconference or
view the video.


5.

Explain Sliding window flow control and stop and wait flow control in detail.



CS2302
-
Computer networks



Department of CSE & IT



2012
-
2013





12
















CS2302
-
Computer networks



Department of CSE & IT



2012
-
2013





13









6
.

Discuss in detail about the Byte
-

oriented Protocols

(PPP), Bit

oriented Protocols(HDLC)

and
SONET.

(apr. 2011).


Point
-
to
-
Point Protocol (PPP)


The more recent Point
-
to
-
Point Protocol (PPP). The format of

PPP frame is


Fig: PPP Frame Format


The Flag field has 01111110 as starting sequence.


The Address and Control fields usually contain default values


The Protocol field is used for demultiplexing.


The frame payload size can he negotiated, but it is 1500 bytes

by default.


The PPP fram
e format is unusual in that several of the field

sizes are negotiated rather than fixed.


Negotiation is conducted by a protocol called LCP (Link

Control Protocol).


LCP sends control messages encapsulated in PPP frames


such messages are denoted by an L
CP identifier in the PPP

Protocol.

Bit
-
Oriented Protocols (HDLC)

In this, frames are viewed as collection of bits. High level data

link protocol is used. The format is


Fig: HDLC Frame Format


HDLC denotes both the beginning and the end of a frame with

t
he distinguished bit sequence 01111110.


This sequence
might appear anywhere in the body of the frame,

it can be avoided by bit stuffing.


On the sending side, any time five consecutive 1’s have been

transmitted from the body of the message (i.e., exclud
ing
when

the sender is trying to transmit the distinguished 01111110

sequence), the sender inserts a 0 before transmitting the next

bit.


On the receiving side, five consecutive 1’s arrived, the receiver

makes its decision based on the next bit it sees (i
.e., the bit

following the five is).



If the next bit is a 0, it must have been stuffed, and so the

receiver removes it. If the next bit is a 1, then one of two things

is
true, either this is the end
-
of
-
frame marker or an error has

been introduced into t
he bit stream.


By looking at the next bit, the receiver can distinguish between

these two cases:



If it sees a 0 (i.e., the last eight bits it has looked at are

01111110), then it is the end
-
of
-

frame marker.



If it sees a 1 (i.e., the last eight b
its it has looked at are

01111111), then there must have been an error and the whole

frame
is discarded
.


Clock Based Framing(SONET)

Synchronous optical network is used for digital transmission over
o
ptical n/w.

It is mainly applied for long distance
trans
missions.


* Sonet addresses both framing & encoding issues. Also several


low speed links can be multiplexed on to 1 high speed link.


* A Sonet frame has spl info (overhead info) that tells receiver where frame starts and ends. No bit

stuffi
ng is used here
and frame’s length does not depend on data.

STS
-
1 Frame


9 rows of 90 byte each

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Computer networks



Department of CSE & IT



2012
-
2013





14












First 3 byte for overhead rest contains data of which 2 bytes tells start and end of each frame


If start and end patterns are repeated in data part, rcvr
identifies it as, it looks for spl bit pattern

consistently,
hoping to see it appearing once every 810 bytes, since each frame is 9 x 90 = 810 bytes.



Overhead bytes of Sonet are encoded using NRZ encoding where highs are 1 and lows are 0. If

continuous

1s or 0s
are present in data,
r
e
c
ei
v
e
r

may misinterpret the sequence.




Hence a bit sequence of length 127 bits are


xored with data to produce enough transitions for the r
e
c
ei
v
e
r to
correctly synchronize with sender. Such a method is called
scrambling
.


Multiplexing Support in Sonet



A single Sonet frame can contain sub frames for multiple lower channels. i.e., an STS
-
3 frame contains 3 STS
-
1
frames.



Such multiplexed sonet frame’s bytes are interleaved, ensuring each STS
-
1 frame are evenly placed.



Durin
g multiplexing, payloads of all STS
-
1 frames can be linked together forming a larger payload, denoted as STS
-
NC ( C
-

Concatenated) . An STS
-
3c frame differs form a STS
-
3 frame. Former is regarded as a single link and latter
as 3 STS
-
1 frames.



Another featu
re of sonet frames is that frame boundaries are not strictly followed. i.e., a frame may extend and float
in another frames’ region. In such cases the start and end of frame could be identified by bits present in overhead.


7.
With a neat diagram explain in

detail about the network architecture
.

(Apr. 2011)

When the system gets complex, the system designer introduces another level of abstraction.

The challenge is to identify abstractions that simultaneously provide a service that proves useful in a large num
ber of
situations and that can be efficiently implemented in the underlying system.

Abstractions naturally lead to layering, especially in network systems.

we might imagine a network as having two layers of abstraction sandwiched between the application pr
ogram and the
underlying hardware, as illustrated in Figure 1.


Figure 1 Example of a layered network system.


The layer immediately above the hardware in this case might provide host
-
to
-
host connectivity, abstracting away the fact that
there may be an ar
bitrarily complex network topology between any two hosts.

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The next layer up builds on the available host
-
to
-
host communication service and provides support for process
-
to
-
process
channels.

Layering provides two features
.

1.

It decomposes the problem of buil
ding a network into more manageable components.

2.

It provides a more modular design. If you decide that you want to add some new service, you may only need to
modify the functionality at one layer, reusing the functions provided at all the other layers.


Fi
gure 2 Layered systems with alternative abstractions available at a given layer.

the abstract objects that make up the layers of a network system are called
protocols
. That is, a protocol provides a
communication service that higher
-
level objects use to ex
change messages.

Each protocol defines two different
interfaces.

1.

it defines a
service interface
to the other objects on the same computer that want to use its communication services.

This service interface defines the operations that local objects can pe
rform on the protocol. For example, a request/reply
protocol would support operations by which an application can send and receive messages.

2.

a protocol defines a
peer interface
to its counterpart (peer) on another machine. This second interface defines th
e
form and meaning of messages exchanged between protocol peers to implement the communication service.


Fig: Service and peer interfaces.

Except at the hardware level, peer to peer communication is indirect.

Peer
-
to
-
peer communication is indirect

each
pr
otocol communicates with its peer by passing messages to some lower
-
level protocol, which in turn delivers the message to
its
peer.

We represent the suite of protocols that make up a network system with a
protocol graph
. The nodes of the graph
correspond t
o protocols, and the edges represent a
depends
-
on
relation.

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Fig:

Example of a protocol graph.

The protocols RRP (Request/Reply Protocol) and MSP (Message Stream Protocol) implement two different types of
process
-
to
-
process channels, and both depend on H
HP (Host
-
to
-
Host Protocol), which provides a host
-
to
-
host connectivity
service.

Suppose that the file access program on host 1 wants to send a message to its peer on host 2 using the communication
service offered by protocol RRP. The file application asks
RRP to send the message on its behalf.

To communicate with its peer, RRP then invokes the services of HHP, which in turn transmits the message to its peer on the
other machine.

Once the message has arrived at protocol HHP on host 2, HHP passes the message

up to RRP, which in turn delivers
the message to the file application. The application is said to employ the services of the
protocol stack
RRP/HHP.

protocol specification

are expressed using a combination of prose, pseudocode, state transition diagrams,
pictures of packet
formats, and other abstract notations.

Two or more protocol modules that do accurately implement a protocol specification are said to
interoperate
with
each other
.
We can imagine many different protocols and protocol graphs that satisfy
the communication requirements of a
collection of applications.

For example, the file access program on host 1 wants to send a

message to its peer on host 2 using the communication
service offered

by protocol RRP. In this case, the file application asks RR
P to send the

message on its behalf. To
communicate with its peer, RRP then invokes

the services of HHP, which in turn transmits the message to its peer on

the
other machine. Once the message has arrived at protocol HHP on

host 2, HHP passes the message up

to RRP, which in
turn delivers the

message to the file application. In this particular case, the application

is said to employ the services of
the protocol stack RRP/HHP.

Encapsulation

Control information must be added with the data to instruct the

peer h
ow to handle with the received message. It will be
added into

the header or trailer.


Header
-

Small data structure from few bytes to few kilobytes

attached to the front of message.

Trailer


Information will
be added at the end of the message

Payload or m
essage body


Data send by the program

In this case data is encapsulated with new message created by

protocol at each level.


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Fig: High
-
level messages are encapsulated inside of low
-
level

messages.

In this example HHP encapsulates RRP’s message by attac
hing a

header of its own. Then HHP sends the message to its
peer over some

network, and then when the message arrives at the destination host, it

is processed in the opposite order.

Multiplexing and De
-
Multiplexing

The fundamental idea of packet switching
is to multiplex multiple flows of data over a single physical link. This can be
achieved by adding identifier to the header message. It is known as
demultiplexing or demux key.
It gives the address to
which it has to communicate. The messages are demultipl
exed at the destination side. In some cases same demux key is used
on both sides and in some cases different keys are used.

RRP Is implementing a logical communication channel, with
messages from two different applications multiplexed over this channel at
the source host and then demultiplexed back to the
appropriate application at the destination host.

The header that RRP attaches to its messages contains an identifier that records the application to which the message
belongs. We call this identifier RRP’s

demultiplexing key
, or
demux key.


At the source host, RRP includes the appropriate
demux key in its header. When the

message is delivered to RRP on the destination host, it strips its header, examines the

demux key, and demultiplexes the message to the c
orrect application.


8.

Explain various channel access techniques.



In
telecommunications

and
computer networks
, a
channel access me
thod

or
multiple access method

allows several
terminals

connected to the same multi
-
point
transmission medium

to tran
smit over it and to share its
capacity. Examples of shared physical media are
wireless networks
,
bus networks
,
ring networks
,
hub networks

and
half
-
duplex

point
-
to
-
point links. A channel
-
access scheme is based on a
multiplexing

method, that allows
several data streams or signals to share the same
communication channel

or
physical medium
. Multiplexing is in
this context provided by the
physical layer
. Multiplexing may be used in
full
-
duplex

point
-
to
-
point

communication
between nodes in a switched network, which should not be considered as multiple access.

A channel
-
access scheme is also based
on a multiple access protocol and control mechanism, also known as
media access control

(MAC). This protocol deals with issues such as addressing, assigning multiplex channels to
different u
sers, and avoiding collisions. The MAC
-
layer is a sub
-
layer in Layer 2 (
Data Link Layer
) of the
OSI
model

and a component of the
Link Layer

of the
TCP/IP model
.

These are the four fundamental types of channel access schemes:

1.

Frequency division multiple access (FDMA)

The
frequency division multiple access

(FDMA) channel
-
access scheme is based on the
frequency
-
division
multiplex

(FDM
) scheme, which provides different frequency bands to different data
-
streams. In the FDMA case,
the data streams are allocated to different users or nodes. An example of FDMA systems were the first
-
generation
(1G) cell
-
phone systems. A related technique is

wave
-
length division multiple access (WDMA), based on
wavelength division multiplex

(WDM), where different users get different colors in fiber
-
optical communication.

2.

Time division
multiple access (TDMA)

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The
time division multiple access

(TDMA) channel access scheme is based on the
time divisi
on multiplex

(TDM) scheme, which provides different time
-
slots to different data
-
streams (in the TDMA case to different
transmitters) in a cyclically repetitive frame structure. For example, user 1 may use time slot 1, user 2 time slot 2,
etc. until the l
ast user. Then it starts all over again.

Packet mode

Packet mode

multiple
-
access is typically also based on time
-
domain multiplexing, but not in a cyclically
repetitive frame structure, and therefore

it is not considered as TDM or TDMA. Due to its random character it can
be categorised as
statistical multiplexing

methods, making it possible to provide
dynamic bandwidth allocation
.

The following are examples of
packet mode

channel access methods:

Contention

based
random multiple access

methods


Aloha


Slotted Aloha


Multiple Access with Collision Avoidance

(MACA)

Multiple Access with Collision Avoidance for Wireless

(MACAW)

Carrier sense multiple access

(CSMA)

Carrier sense multiple access with collision detection

(CSMA/CD)
-

suitable for wired networks

Carrier s
ense multiple access with collision avoidance

(CSMA/CA)
-

suitable for wireless networks

Distributed Coordination Function

(DCF)

Point Coordination Function

(PCF)

Carrier sense multiple access with collision avoidance and Res
olution using Priorities

(
CSMA/CARP
)

Carrier Sense Multiple Access/Bit
wise Arbitration

(
CSMA/BA
) Based on constructive interference (
CAN
-
bus
)

3.

Code division multiple access (CDMA)

The
code division multiple access

(CDMA) scheme is based on
spread spectrum
. An example is the
3G

cell phone
system.

4.

Space division multiple access (SDMA)

Duplexing methods

Where these methods are used for dividing forward and reverse communication channels, they are known as
d
uplexing

methods, such as:



Time division duplex

(TDD)



Frequency division duplex

(FDD)

Code Division Multiple
Access: CDMA

In CDMA, one channel carries all transmissions simultaneously
.
CDMA uses unique spreading codes to spread the
baseband data before transmission. The signal is transmitted in a channel, which is below noise level. The recei
ver then uses a
corre
lator to de
spread the wanted signal, which is passed through a narrow band

pass filter. Unwanted signals will not be
despread and will not pass through the filter. Codes take the form of a carefully designed one/zero sequence produced at a
much higher rate

than that of the baseband data. The rate of a spreading code is referred to as chip rate rather than bit rate.


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Simple idea of communication with CDMA

One channel carries all transmissions simultaneously

Two properties that the assigned cods have:



If we

multiply each code by another, we get 0.



If we multiply each code by itself, we get number of

channel

Each station wants to receive data from one of other stations multiplies the common data on the channel by the code of the
sender
.



9.Explain the dif
ferent types of multiplexing. (nov. 2011)

Multiplexing is the set of techniques that allows the simultaneous transmission of multiple signals across a single data link
.
The need for multiplexing in data transmission:

Whenever the transmission capacity of
a medium linking two devices is greater than the transmission needs of the
devices, the link can be shared. MULTIPLEXING is the set of techniques that allows simultaneous transmission of multiple
signals across a single data link. As data and telecommunica
tions usage increases traffic also increases. We can accommodate
this increase by continuing to add individual lines each time a new channel is needed, or we can install a higher capacity li
nks
and use each to carry multiple signals.

Signals are multiplexe
d using 3 basic techniques

FDM


Frequency Division Multiplexing


an analog technique, applied when the BW of a link is greater than the
combined bandwidths of the signals to be transmitted.

WDM


Wave Division Multiplexing


involves light signals trans
mitted through fiber
-
optic channels.


TDM


Time Division Multiplexing


a digital process, applied when the data rate capacity of the transmission medium
is greater than the data rate required by the sending and receiving devices. TDM is of 2 types


Sync
hronous and
Asynchronous/statistical TDM/concentrator.

The types of mu
ltiplexing:

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Upward multiplexing



Downward multiplexing

1.

Freq
uency division

When bandwidth (Hz) of link is greater then combined bandwidth of signals.

Each sending device modulate Signals a
t different carrier frequency.

Modulated signals are combined into a single signal.

Channels are formed through which various signals travel.


Time division multiplexing



Instead of sharing portion of bandwidth as in FDM, time is shared.



Each connection oc
cupies a portion of time in link.



10.Discuss about

Error Correction
.

A receiver can use an error
-
correcting code, which automatically corrects certain errors



Single
-
bit errors:



Can be detected by the addition of parity bit which helps to find “error” or

“no error” which is
sufficient to detect errors



To correct errors the receiver can simply invert 0 to 1 or 1 to 0, but the problem is “locating” the
position of error



To do so requires enough redundancy bits



Condition:
2
r

>= m + r + 1



Hamming Code can be

applied to data units of any length and uses the relationship between data and
redundancy bits




For example: a 7
-
bit ASCII code requires 4 redundancy bits that can be added to the end of the data unit or
mixed with the original data bits, which are placed

in positions 1, 2, 4 and 8 i.e x
0
,x
1
,x
2
,x
3

and so on.




In the
Hamming Code, each “r” bit for one combination of data bits as below:


r1:

bits 1, 3, 5, 7, 9, 11


r2:

bits 2, 3, 6, 7, 10, 11


r3:

bits 4, 5, 6, 7


r4:

bits 8, 9, 10, 11

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UNIT
-

II

PA
RT A

1.

List the advantages of a centralized scheme.



It may afford greater control over access for priorities, overrides, and guaranteed capacity.



It enables the use of relatively simple access logic at each station.



It avoids problems of distributed coordina
tion among peer entities.

2.

Mention some of the physical properties of

Ethernet?

(may 2011)

T
he Ethernet is a multiple
-
access network, meaning that a set of nodes send and receive frames over a shared link.
An Ethernet is like a bus that has multiple station
s plugged into it.

What is CSMA/CD?

(Nov. 2011)

It is a protocol used to sense whether a medium is busy before transmission but it has the ability to check whether a
transmission has collided with another.

3.

List the rules for CSMA/CD.

1. If the medium is i
dle, transmit; otherwise go to step 2.

2. If the medium is busy, continue to listen until the channel is idle, and then transmit


i
mmediately.

3. If a collision detected during transmission, transmit a brief jamming signal to all


station to i
ndicate collision has occurred and then cease transmission.

4. After transmitting a jamming signal, wait for some time, then transmit again.

4.

What is preamble

and MAC
?

A 7
-
octet pattern of alternating 0s and 1s is used by the receiver to establish bit synch
ronization is called as
preamble.

M
edium
A
ccess
Control

field contains any protocol control information needed for the functioning of
the MAC protocol. For example, a priority level could be indicated here.

5.

When a transmitting station will insert a new tok
en on the ring?

It will insert a new token when the station has completed transmission of its frame.

The leading edge of the transmitted frame has returned to the station.

6.

What is Early Token Release (ETR)?

ETR allows a transmitting station to release a to
ken as soon as it completes frame transmission, whether or not the
frame header has returned to the station.

7.

What is Frame Status (FS)?

It contains the error detected (E), address recognized (A), and frame coped (F) indicators. Each indicator is
represente
d by a symbol, which is R for “reset” or “false” and S for “set” or “true”.

8.

Give the applications of wireless LANs.

LAN extension, cross building interconnect, nomadic access, and advantages hoc networks.

9.

What is a bridge?
(Nov. 2011)

Bridge is a hardware
networking device used to connect two LANs. A bridge operates at data link layer of the OSI
layer.

A bridge observes and forwards all frames that it receives.
It does forwarding & filtering frames using LAN
destination address. Bridges are used to connect
LAN or WAN and works at data link layer level. Collision
Probability is more.

10.

What is No
-
transition?

A station of this type is either stationary or moves only within the direct communication range of the communicating
stations of a single BSS(
B
asic
S
ervice

S
et)
.

11.

What is spanning tree routing?

The spanning tree approach is a mechanism in which bridges automatically develop a routing table and update that
table in response to changing topology.


12.

Ethernet stipulates a minimum size of a frame. Why is it necessa
ry?


Ethernet is a networking topology developed in 1970 which is governed by the IEEE 802.3 specification.

To
detect collision and t
o identify valid frame from garbage, valid full format should contain 64 bytes from destination address
to checksum. So if
the data portion is less than 46 bytes, pad field is used to fill out the frame to minimize size.



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13.

What is the advantage of
FDDI
over a basic token ring?

(NOV. 2010)

FDDI

802.5

No priority and reservation bits
.

It has priority scheme by using reservation

bits.

No need of converting a token to start of data frame
by inverting token bits because of high data rate
.

It converts a token to data frame changing token frame.

A station that transmits data frames releases a new
token as soon as it completes data.

A station that data transmissions after releasing back its
own transmission, release the token.

14.

Give the format of Ethernet address.


Preamble
64

Dest addr

48

Src addr

48

Type

16

Body

CRC

32

15.

What is meant by the contention period of Ethernet?
How
many lines are required to connect n


systems in
Direct Mesh topology?

When several stations on an Ethernet have data to send, there are contention periods during which collisions happen and n
o
data is successfully transmitted. n(n
-
1)/2 lines are req
uired.


16.

What does IEEE 10 Base 5 standard signify?

10 represents data rate 10 Mbps.

5 refers to segment length 5* 100 m that can run without repeaters

Base represents Base band communication
.


17.


Define Repeater and

Hub.


Repeaters and hubs are interco
nnecting devices.
Repeater:

Repeaters extends the Ethernet segment and it repeats
the signal. It does not
amplify

the signal.
Hub:

A Hub has several point to point segments coming out. It is a multi
way repeater. It broadcasts any signal through all outgoi
ng lines.


18.

What is meant by Exponential back of algorithm?

After first collision, each station waits either 0 or 1 slot time before trying again. If 2 stations collide and each one pic
ks
same random number 0/1.

After second collision, each one picks either

0,1,2 or 3 slot at random and waits. If collision
occurs again, then next time the number of slots to wait is chosen at random from 0 to [2
3



1]. This algorithm is called
binary exponential “back off algorithm”.


1
9
. Define a switch
.

Switch
es are hardwar
e or software device capable of creating temporary connections between more devices which
are not directly connected. It is a multi input/output port device. It transfers data coming from one input port to one
or more output ports. This function is called
as forwarding.
Reliability, performance, security, and geography are the
reason for using bridges in LAN
.


2
0
.Define Spanning Tree Algorithm
.

Bridge connects n/w and removes loop in the path using spanning tree algorithm It constructs a spanning tree of ed
ges
between hosts that maintain connectivity of the graph with no loops. It is a dynamic algorithm. The algorithm works as Frame
Forwarding, Address Learning and Loop Resolution.


21
.
Mention the

different types of brid
ge.

What are the limitations of bridg
es?



Simple Bridge connect 2 LAN



Multi port Bridge connect more than 2 LANs



Transparent Bridge it learns on its own about connected LANs
.

The
limitations of bridges:
Scalability and Hetrogenity
.


22
.
What are

the
functions of

Bridges?

(Nov. 2010)


1.

A bridge should have enough buffer space to store the frames until it is transmitted.


2. It should be able to distinguish addresses of host on different LAN.

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3. It can contain information about other bridges.


4. It should follow congesti
on control mechanisms to overcome congestion.


5.
It works at layer 1 and layer 2 level.


23
.Name any two network connecting devices? Can a bridge replace repeater for interconnecting 2 segments of a n/w?

Repeater
repeats the signal to the actual st
rength so that they can travel and works at physical layer. Repeater
operates on the physical layer level. Here collision probability is more.

A bridge cannot replace repeater for interconnecting 2 segments of a network because functions of them are entir
ely
different.

2
4
.What are the advantages of switches? Write the frame format for FDDI.

In switches, the sending information are directly transmitted to the concern receiver.

8

8

48

48



32

8

24















Start of

Control

Dest

Src

Body





CRC

End of

Status







frame

addr

A
ddr





frame












25
. What is Token ring? What is the use of bit stuffing?

Token ring is a set of nodes are connected together in a ring. Data flow always in a particular direction around the ring.
Bit
s
tuffing

is bit oriented protocol. It is used to detect the error during the transmission of the stream of bits.


PART
-
B

1.

Explain
in detail
about token ring

and its frame format
.

(nov. 2011)

Token ring network
(IEEE 802.5)

c
onsists of a set of nodes connecte
d in a ring.



Data always flows in a particular direction around the ring



Each node receiving frames from its up stream neighbor and then forwarding them to its downstream
neighbor



The ring is viewed as a single shared medium



It does not behave as a collec
tion of independent point
-
to
-
point links that just happen to be configured in a
loop




Requires that stations take turns sending data



Token is a specially formatted three
-
byte frame that circulates; station wishing to transmit must first have
possession



To
ken passes from NIC to NIC in sequence; if station has data to send, station takes token and sends data
frame; if not, passes to neighbor



Each station receives the frame one by one and examines the destination address



If it matches, frame is copied; statio
n checks the frame for errors; changes bits to indicate the frame was
received and copied



Packet continues around the ring and is passed back to originating station

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Once the sender receives the frame and recognizes its address in the sender field, it exami
nes the address
-
recognized bits



If they are set, it knows the frame was received and copied



Sender then discards the frame and releases the token back to the ring

Token Ring Maintenance



Lost tokens
-

timer is issued each time a frame or token is generated



If no frame is received within time period, new token is generated by a
monitor station




Orphan frames

result if a sending station neglects to remove a used data frame from the ring



Monitor sets a bit in the AC field in each frame; as frame passes, bit is

set; if the frame passes again, the
monitor discards, will remove it, and generate a new token



the detection of dead stations

Priority and Reservation



Higher priority stations may access the token sooner,



Every station has a priority code



As token passes

by, station waiting to transmit can place its priority code in the access control (AC) field of
the token or data frame



Higher priority stations may remove a lower priority reservation; if stations have equal priority, it’s first
-
come, first
-
served



Once a

station has the token, it is allowed to send one or more packets

exactly how many more depends
on some factors.



how much data a given node is allowed to transmit each time it possesses the token, or said another way,
how long a given node is allowed to ho
ld the token. this the
token holding time
(THT).



no lower
-
priority packets get sent when higher
-
priority packets are waiting
-
lower
-
priority packets to be
locked out of the ring for extended periods if there is a sufficient supply of high
-
priority packets.





Token Ring Frame




2.

Discuss
t
he v
a
ri
ou
s aspects of

FDDI

in detail
.

(nov. 2011)


FDDI Basics

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Fiber Distributed Data Interface (FDDI) high
-
speed LAN which uses fiber optics, dual ring topology and
the token passing access method.


FDDI is frequently used

as a backbone technology and to connect high
-
speed computers in a LAN.






















FDDI has four specifications:

1.

Media Access Control
-

defines how the medium

is accessed

2.

Physical Layer Protocol

defines data encoding/decoding procedures

3.

Physica
l Layer Medium

defines the characteristics of the transmission medium

4.

Station Management

defines the FDDI station configuration


Physical properties

This runs on fiber, not copper. The copper variant is called CDDI.


It has dual ring. The 2 independent rings

send the data in opposite directions. The second ring is not used
during normal operation but instead comes into play only if the primary ring fails.


FDDI network is able to tolerate a single break in the cable or the failure of one station.




Instead o
f designating one node as a monitor, all the nodes participate equally in maintaining the FDDI
ring.



FDDI uses 4B/5B encoding instead of Manchester.




FDDI allows nodes to attach to the network by means of a single cable. Such nodes are called
single
attach
ment stations
(SAS); their dual
-
connected counterparts are called, not surprisingly,
dual attachment
stations
(DAS). A concentrator is used to attach several SASs to the dual ring.

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When a SAS fails, the concentrator detects this and uses an optical byp
ass to isolate failed SAS, thereby
keeping the ring connected.



Like 802.5, each n/w adaptor holds some no. of bits between its i/p and o/p interfaces. But here the size of
buffer varies from station to station, with a minimum of 9 bits to a maximum of 80 b
its.



A station may start to transmit before its buffer becomes full and buffer size of each station determines the
total time taken by token to pass around the ring.



A single FDDI n/w can contain a maximum of 500 hosts with a distance of 2Km between 2 stat
ions.
Overall the n/w is limited to 200Km, resulting to a total distance of 100Km, because of the dual nature of
the ring.

Timed


Token Algorithm




THT is the token holding time for each station. TTRT is the average time taken by all stations to complete
o
ne rotation. TTRT value is accepted and agreed by all stations.



Each node measures the successive arrivals of token which is named as that node’s measured TRT.



If measured TRT > agreed TTRT, it implies the token is late and node does not transmit any data
. If TRT <
TTRT, it implies token is early and token stays at node for the calculated time difference. During this time,
node transmits its data.



There are 2 types of data held by a node


synchronous and asynchronous. Synchronous data are real time
info a
nd should not be delayed. Asynchronous data are of less priority and could be delayed.



During each node’s hold of token, it transmits the synchronous data first and if THT still remains positive,
then asynchronous data could be sent. If THT becomes 0 afte
r sending synchronous data, asynchronous
data is not sent.



Generally whenever a node receives token it sends its synchronous data regardless of token being early /
late.


During worst case, i.e., when only synchronous / asynchronous data is present in any

station, then

measured TRT could be as much as 2 x TTRT.


If a station has already used up one TTRT’s worth of time for its single rotation in sending its

synchronous data, it cannot send continuously asynchronous data.


Therefore it is possible to have

a single rotation to consume 2 x TTRT but it cannot have back to back

rotations each consuming 2 x TTRT time.

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If the difference in time between TRT and TTRT is very less such that an entire frame cannot be sent, it still
goes ahead in sending its full fr
ame. This is because the measured TRT is actually bounded by TTRT plus
the time it takes to send a full FDDI frame.


Token Maintenance



All nodes monitor the token. Timer is set to 2.5ms between which either a token or data frame should

be
seen by each s
tation.


If not, timer expires and any station can send a claim token containing its TRT. If any station finds that

TRT higher than its own, then it can overwrite a lower TRT value.


In case of tie, station with a higher address wins.


In this way an agr
eed upon TTRT value is fixed.1



3.

Explain the following: Switches and Bridges
.

Switches

A

switch is a mechanism that allows us to interconnect links to form a larger network. A switch is a multi
-
input,
multi
-
output device, which transfers packets from an
input to one or more outputs. Thus, a switch adds the star topology (see
Figure.1) to the point
-
to
-
point link, bus (Ethernet), and ring (802.5 and FDDI) topologies
.

A star topology has several
attractive properties:

Even though a switch has a fixed number
of inputs and outputs, which limits the number of hosts that can be
connected to a single switch, large networks can be built by interconnecting a number of switches.
Fig.
A switch provides a
star topology.








We can connect switches to each other
and to hosts using point
-
to
-
point links, which typically means that we can
build networks of large geographic scope.

Adding a new host to the network by connecting it to a switch does not necessarily mean that the hosts already connected will

get worse pe
rformance from the network.

A packet switch is a device with several inputs and outputs leading to and from the hosts that the switch
interconnects. The core job of a switch is to take packets that arrive on an input and
forward

(or
switch
) them to the rig
ht
output so that they will reach their appropriate destination. There are a variety of ways that the switch can determine the
“right” output for a packet, which can be broadly categorized as connectionless and connection
-
oriented approaches.

Bridges




LAN
may need to cover more distance than the media can handle effectively

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When the bridge receives a frame on
port1 that is addressed by host A

the bridge would not forward the frame out on
port2.



Bridges uses connectionless model.



Bridge decides on which outp
ut to send a frame by looking up address in the table.


Bridges are used to logically separate network segments within the same network.


They operate at the OSI data link layer (Layer 2) and are independent of higher
-
layer protocols.


The function of the b
ridge is to make intelligent decisions about whether or not to pass signals on to the next segment
of a network.


When a bridge receives a frame on the network, the destination MAC address is looked up in the bridge table to
determine whether to filter, fl
ood, or copy the frame onto another segment


Broadcast Packets are forwarded




Multi

input device and multi

output device.



Operate in both physical and data link layers.



Ethernet segment can carry 10Mbps of total traffic.



Ethernet bridge can carry 10n Mbps

where n is the input

and output ports of the bridge.



Used to divide a network into smaller segments.



May

relay frames between separate LANs.



Keeps traffic from each segment separate; useful for controlling congestion and provides isolation, as well as
sec
urity.



Checks address of frame and only forwards to segment to which address belongs
.


Transparent Bridges & Learning Bridges
:




Builds table by examining destination and source address of each packet it receives



Learning bridges



If address is not recognize
d, packet is relayed to all stations



Stations respond and bridge updates routing table with segment and station ID information.



Changes on the network are updated as they occur
.



A bridge has a table used in filtering decisions.

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A bridge does not change
the physical (MAC) addresses in a frame


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4.

Explain how bridges run a distributed spanning tree algorithm. (8) (apr. 2011)

Spanning tree
algorithm
:



developed by Radia Perlman at Digital Equipment Corporation



is a protocol used by a set of brid
ges to agree upon a spanning tree for a particular extended LAN.



is dynamic algorithm.



bridges are always prepared to reconfigure themselves into a new spanning tree should some bridge fail.



Fig.
Extended LAN with loops.

Problem:


1. The network is
managed by more than one administrator


-

It is possible that no single person knows the entire configuration of the network, meaning that a bridge that closes
a loop might be added without anyone knowing.


2.Loops are built into the network

on purpose
-

to provide redundancy in case of failure.


Bridges must be able to correctly handle loops. This problem is addressed by having the bridges run a distributed
spanning tree
algorithm.


Extended LAN as being represented by a graph t
hat possibly has loops (cycles), then a spanning tree is sub graph of
this graph that covers (spans) all the vertices, but contains no cycles.


That is, a spanning tree keeps all of the vertices of the original graph, but throws out some of the edges
.

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Ex
ample of (a) a cyclic graph; (b) a spanning tree.




Each bridge has a unique identifier; ie B1, B2, B3, and so on.



First elects the bridge with the smallest id as the root of the spanning tree;

Procedure:



The root bridge always forwards frames out over al
l of its ports.



Each bridge computes the shortest path to the root and notes which of its ports is on this path. This port is also
selected as the bridge’s preferred path to the root.



Finally, all the bridges connected to a given LAN elect a single
desig
nated
bridge that will be responsible for
forwarding frames toward the root bridge.



If two or more bridges are equally close to the root, then the bridges’ identifiers are used to break ties; the smallest id
wins.

Information of new configuration messages



The bridges have to exchange configuration messages with each other and then decide whether or not they are the
root or a designated bridge based on these messages.



It identifies a root with a smaller id or



It identifies a root with an equal id but with a

shorter distance or



The root id and distance are equal, but the sending bridge has a smaller id.



If the new message is better than the currently recorded information, the bridge discards the old information and
saves the new information



It first adds 1 to

the distance
-
to
-
root field since the bridge is one hop farther away from the root than the bridge that
sent the message.



5
.

Discuss in detail about the Ethernet?

The Ethernet is a multiple
-
access network, meaning that a set of nodes
send and receiv
e frames over a
shared link.
A
n Ethernet
i
s like a bus that has multiple stations plugged into it. The “carrier sense” in CSMA/CD
means that all the nodes can distinguish between an

idle and a busy link, and “collision detect” means that a node
listens as
it transmits and can therefore detect when a frame it is transmitting has interfered (collided) with a frame
transmitted by another node
.


Ethernet and a 1000
-
Mbps version called Gigabit Ethernet. The rest of this section focuses on10
-
Mbps Ethernet,
since
it is typically used in multiple
-
access mode and we

are interested in how multiple hosts share a single link. Both 100
-
Mbps and 1000
-
Mbps Ethernets are designed to be used in full
-
duplex and point
-
to
-
point configurations.

Ethernet Frame Format



Consists of
seven fields



No mechanism for acknowledging received frames; considered an unreliable medium

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Preamble


seven bytes of alternating 0s and 1s to notify receiver of incoming frame and to provide
synchronization



Start frame delimiter (SFD)


one byte sig
naling the beginning of the frame



Destination address (DA)


six bytes containing the physical address of the next destination; if packet must
reach another LAN, this field contains the physical address of the router; upon reaching the target network,
fiel
d then contains the physical address of the destination device



Source address (SA)


six byte field containing physical address of last station to forward packet, sending
station or most recent router



Length/type


two bytes indicating number of bytes in c
oming PDU; if fixed length, can indicate type



Data


46 to 1500 bytes



CRC


CRC
-
32 error detection information


Ethernet Addressing



Each station on the network must have a unique physical address



Provided by a six
-
byte physical address encoded on the netw
ork interface card (NIC)



Normally written in hexadecimal notation



Categories of traditional Ethernet



Baseband


digital signals using Manchester encoding



10Base5, 10Base2, 10
-
Base
-
T, 10Base
-
FL



First number indicates data rate in Mbps
.



Last number indica
tes maximum cable length or type



Broadband


analog signals using digital/analog conversion (differential PSK)



Only specification: 10Broad36

6.
Explain
the functioning of wireless LAN
in detail.

(Nov. 2010)

IEEE 802.11:
Collision avoidance

-

Hidden termin
al problem

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Exposed Terminal:

B transmits to A, C wants to transmit to D.

C needlessly assumes a full channel


Multiple Access with Collision Avoidance (MACA)


Before every data transmission


Sender sends a Request to Send (RTS) frame containing the len
gth of the transmission


Receiver responds with a Clear to Send (CTS) frame, echoing back the length of frame to sender.


Any node nearer to receiver also receives CTS, becomes aware that it cannot transmit for a period
specified inside CTS.


Also any node s
eeing RTS and not CTS is not close to receiver, hence free to transmit.


Sender sends data


Receiver sends an ACK; After seeing this ACK only other nodes can send data


If more than 1 node transmits RTS simultaneously, collision will occur. Nodes realize abou
t collision if
they don’t get a CTS back.


Nodes follow random back off time procedure before retransmitting

Distribution system

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All the nodes are not alike, some roam (Laptops), some are connected to a wired n/w (base stations).



Such base stations are ca
lled Access points, which are connected to each other by a Distribution System,
which in turn could be any wired n/w (Ethernet, Token Ring etc
.

).



Even though each node can communicate directly with other node, if they are within reach of each other,
they
associate themselves with an Access point.



That is for eg., if node A wishes to communicate with node E,A first sends a frame to its AP (AP
-
1), which
forwards the frame across the Distribution system to AP
-
3, which finally forwards the frame to node E.





Scanning is done by a node if it joins the n/w and alos if it is unhappy with its current AP.



This might happen when the signal reaching from its current AP is weak, since the node has moved away
from it.



Whenever a node acquires a new AP, the new AP infor
ms such migration to its old AP thru the Distribution
system.



Node migration is depicted in the fig below.

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Scanning could be of 2 types.



Active scanning
, the one initiated by node itself, if it sends probes continuously , actively searching for an
AP.



AP
s also periodically send a
Beacon frame
, advertising their own capabilities like transmission rates
supported by them. Such type of scanning is referred as
passive scanning
. Any node on receiving such
Beacon frame can send a
Associate

Request

and join with

any specific AP.



There are 4 addresses present in 802.11 frame whose interpretations depend on ToDs and FromDs bits
present in Frame Control field.



If frame is forwarded across Distribution system, the original sender might change to the recent transmitti
ng
node i.e., the node which forwards finally to the ultimate destination.



W
hen a node sends frame directly to another, both DS bits are 0, ADDR1 identifies target node, ADDR2
identifies source node.ADDR3 & 4 are not applicable.


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When a frame has to cross

Distribution system to reach its destination, both DS bits will be 1, ADDR1
identifies ultimate destination, ADDR2 will be immediate sender i.e., the node which forwarded the frame
from Distribution system to ultimate destination, ADDR3 will be intermedia
te destination i.e., the node that
accepted the frame from a wireless node and forwarded it across the Distribution system and ADDR4
identifies original source.



Consider for eg., frame transmission from A to E from previous figure:


In this case, ADDR1 wil
l be E, ADDR2 will be AP
-
3, ADDR3 will be AP
-
1 and ADDR4 will

be A.









7
.
a)

Explain CSMA in detail
.

(Apr. 2011)




CSMA
:


listen before transmit.



A network station wishing to transmit will first check the cable plant to ensure that no other station
is currently
transmitting (
CARRIER SENSE
).



The communications medium is one cable, therefore, it does allow multiple stations access to it with all being able to
transmit and receive on the same cable (
MULTIPLE ACCESS
).



Error detection is implemented throu
ghout the use of a station "listening" while it is transmitting its data.



A jam signal is transmitted to network by the transmitting stations that detected the collision, to ensure that all
stations know of the collision. All stations will "back

off" fo
r a random time.



Detection and retransmission is accomplished in microseconds.



Two or more stations transmitting causes a collision (
COLLISION DETECTION
)





If channel sensed idle: transmit entire frame





If channel sensed busy, defer transmission

colli
sions
can

still occur.




Propagation

delay means two nodes may not hear

each other’s transmission
.




Before transmit sense the medium whether the medium is busy or idle. Whether the medium is idle, the
sender ready to transmit the data , medium is busy t
he sender waits for certain time then sense the medium always.
Suppose multiple user sense the medium is idle, all are trying to send the data, in this case collision is happened. To av

oid this we are using relay and also using some the control frames lik
e RTS and CTS.

7 b.Discuss about
RPR
.

Resilient Packet Ring RPR (IEEE 802.17)
:


The ability to recover quickly from the link or node failure.


Dual ring (like FDDI) and use the both the ring for transmission


Buffer Insertion


instead of token


when they ar
e no packet to forward then start sending its own packet.


Efficiency



Spatial reuse
-

Receiver removes the frame


instead of sender



Temporal reuse
-

Unused BW reclaimed and distributed



Weighted fairness


Not necessarily equality



Lossless



No single point of
failure



Steering: optimized for minimizing packet re
-
ordering for TDM and video services and to preserve
bandwidth utilization


Default protection method that is always supported


node failures will be notified by the adjacent nodes



Wrapping: optimized to mi
nimize immediate packet
-
loss for data services


Optional protection method that may be supported


RPR can work over the existing physical layers like


SONET , Ethernet


QoS




class A low latency and low jitter




class B Predictable latency and jitter





class C best effort transport.

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8
.

Describe about token bus.







Token Bus (IEEE 802.4)








Uses broadcast channel, but the stations form a logical ring (13576824)
.



There is a special packet called the “token”
.




a station that has the token is allo
wed to transmit for a time




when the time is up it passes the token to next station in the ring




a station may only transmit what it has when the token arrived. If it has no frames to send then it simply
passes the token on

Properties of the Token Bus


The
token bus allows priorities. For example, high priority can be given to voice packets


The token bus can allow for quick turnaround on acknowledgements. The station that has the token allows the
recipient to ack before sending the next frame


IEEE 802.4 is a

standard for token buses running on broadcast channel


Useful in the real
-
time application when a guaranteed level of service is required


In heavy loads there is a very good utilization since token passing is only a small percentage of the traffic and ther
e
are no collisions


In very light loads there are delays caused by the token passing


If a station goes down there is a potential of a token being lost. A lost token can be detected and can be regenerated
by the remaining active stations
.

Token Bus Frame Fo
rmat
:








Token Bus Control Frame
:

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9.


Explain about token passing.






Station can send only when it receives a special frame called a token



Token circulates around the ring



If station wishes to send, it captures the token and sends one

or more frames



Token is then released so next station can transmit.


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Token Passing inToken Ring
:



Requires that stations take turns sending data



Token passing coordinates process



Token is a specially formatted three
-
byte frame that circulates; station

wishing to transmit must first have possession



Token passes from NIC to NIC in sequence; if station has data to send, station takes token and sends data frame; if
not, passes to neighbor



Each station receives the frame one by one and examines the destina
tion address



If it matches, frame is copied; station checks the frame for errors; changes bits to indicate the frame was received and
copied



Packet continues around the ring and is passed back to originating station



Once the sender receives the frame and r
ecognizes its address in the sender field, it examines the address
-
recognized
bits



If they are set, it knows the frame was received and copied



Sender then discards the frame and releases the token back to the ring


10a)
Explain the basic network topologies

and give all the relevant features
.

(8)

Star Topology:

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Here all computers are attached to a central point called as hub.

In star topology, each station is directly connected to a
common central node. Central node
is referred as star coupler which uses two point
-
to
-
point links, one for transmission in
each direction. Its advantages are scalable and easy to identify the fault.






Here computers are connected in a closed loop forming a loop.

In the r
ing topology, the network consists of a set of
repeaters joined by point
-
to
-
point links in a closed loop. The repeater is a device which receives data in one link and
transmits them in other link.




Here all computers are attached to a si
ngle long cable.

Bus topology uses a multipoint medium and all stations are attached
through appropriate hardware interfacing known as a tap. A full duplex operation is used for transmission and reception of
data in a bus.



Tree topology is generalizatio
n of bus topology. Transmission medium is a branching cable with no closed loops. It
begins at a point known as head

end, where one or more cables start, and each of these may have branches.

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10 b)
Explain the following: Segmentation and reassembly
.

(April
2011)

Segmentation and reassembly A message is divided into segments and each segment contains a sequence number. These
numbers enable the Transport layer to reassemble the message correctly upon arriving at the destination. The packets lost in
the transmi
ssion is identified and replaced.

When the size of the data unit received from the upper layer is too long for the

network layer datagram or data link
layer frame to han
dle, the transport protocol divides it into smaller usable blocks. The dividing process is called
segmentation.