1. computer networks u1 ver 1.0

Computer networks

Dr. C.V. Suresh Babu


Topics
•
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•
•
•
•
•
•
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•
•
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Introduction to networks –
network architecture –
network performance –
Direct link networks –
encoding –
framing –
error detection –
transmission –
Ethernet –
Rings –
FDDI Wireless networks –
Switched networks –
bridges


Introduction to Computer Networks

Computer Networks
Computer network connects
two or more autonomous
computers.

The computers
geographically
anywhere.

can be
located


LAN, MAN & WAN
Network in small geographical Area (Room, Building or a
Campus) is called LAN (Local Area Network)
Network in a City is call MAN (Metropolitan Area Network)
Network spread geographically (Country or across Globe) is
called WAN (Wide Area Network)



Applications of Networks
Resource Sharing
Hardware (computing resources, disks, printers)
Software (application software)
Information Sharing
Easy accessibility from anywhere (files, databases)
Search Capability (WWW)
Communication
Email
Message broadcast
Remote computing
Distributed processing (GRID Computing)


Network Topology
The network topology
defines the way in which
computers, printers, and
other
devices
are
connected. A network
topology describes the
layout of the wire and
devices as well as the
paths used by data
transmissions.



Bus Topology
Commonly referred to as a
linear bus, all the devices
on a bus topology are
connected by one single
cable.



Star & Tree Topology

The star topology is the most
commonly used architecture in
Ethernet LANs.

When installed, the star topology
resembles spokes in a bicycle
wheel.
Larger networks use the extended
star topology also called tree
topology. When used with network
devices that filter frames or
packets, like bridges, switches, and
routers, this topology significantly
reduces the traffic on the wires by
sending packets only to the wires of
the destination host.


Ring Topology
A frame travels around the ring,
stopping at each node. If a node wants
to transmit data, it adds the data as
well as the destination address to the
frame.
The frame then continues around the
ring until it finds the destination node,
which takes the data out of the frame.
Single ring – All the devices on the
network share a single cable
Dual ring – The dual ring topology
allows data to be sent in both
directions.


Mesh Topology
The
mesh
topology
connects
all
devices
(nodes) to each other for
redundancy and fault
tolerance.
It is used in WANs to
interconnect LANs and for
mission critical networks
like those used by banks
and financial institutions.
Implementing the mesh
topology is expensive and
difficult.


Network Components
Physical Media
Interconnecting Devices
Computers
Networking Software
Applications



Networking Media
Networking media can be
defined simply as the
means by which signals
(data) are sent from one
computer
to
another
(either by cable or wireless
means).



Networking Devices
HUB,
Switches,
Wireless
Access
Modems etc.

Routers,
Points,



Computers: Clients and Servers
In a client/server network
arrangement,
network
services are located in a
dedicated computer whose
only function is to respond
to the requests of clients.
The server contains the
file, print, application,
security,
and
other
services in a central
computer
that
is
continuously available to
respond to client requests.


Networking Protocol: TCP/IP



Applications
E-mail
Searchable Data (Web Sites)
E-Commerce
News Groups
Internet Telephony (VoIP)
Video Conferencing
Chat Groups
Instant Messengers
Internet Radio


Network Architecture
• Provides a general, effective, fair, and robust connectivity of
computers
• Provides a blueprint
– Types
• OSI Architecture
• Internet Architecture


OSI ARCHITECTURE
•

Open Systems Interconnection (OSI) model is a reference model developed
by ISO (International Organization for Standardization) in 1984
OSI model defines the communications process into Layers
Provides a standards for communication in the
network
Primary architectural model for inter-computing and Inter networking
communications.
network communication protocols have a structure based on OSI Model


OSI Architecture


Ethernet

10 Mb/s

LAN Architecture Summary
Maximum Number of
Transmission Types
Notes
Stations
1024
Category 3 UTP or better (10BASE- Replaced by Fast Ethernet; backward
T), Thinnet RG-58 coax (10BASE- compatible with Fast or Gigabit Ethernet
2), Thicknet coax (10BASE-5),
when using UTP.
fiber-optic (10BASE-F)

Fast Ethernet

100 Mb/s

1024

Category 5 UTP or better

The most popular wired networking
standard, rapidly being replaced by gigabit
Ethernet.

Gigabit Ethernet

1000 Mb/s

1024

Category 5 UTP or better

Recommended for new installations; uses all
four signal pairs in the cable.

10 000 Mb/s

1024

Category 6a UTP or better

Uses all four signal pairs in the cable.

802.11a Wireless Ethernet

Up to 54 Mb/s

1024

RF 5 GHz band with dual-band
802.11n

Short range; interoperable with dual-band
802.11n.

802.11b Wireless Ethernet

Up to 11 Mb/s

1024

RF 2.4 GHz band

Interoperable with 802.11g/n.

802.11g Wireless Ethernet

Up to 54 Mb/s

1024

RF 2.4 GHz band

Interoperable with 802.11b/n.

802.11n Wireless Ethernet

Up to 600 Mb/s

1,024

RF 2.4/5 GHz bands

Longest range; interoperable with
802.11a/b/g; dual-band hardware needed to
interoperate with 802.11a; recommended for
new installations.

Token-Ring

4/16/100 Mb/s

72 on UTP; 250–260 on UTP, Type 1 STP, and fiber-optic
Type 1 STP

Replaced by Ethernet; obsolete for new
installations.

2.5 Mb/s

255

Replaced by Ethernet; obsolete for new
installations; uses the same coax cable as
IBM 3270 terminals.

Network Type

10 Gigabit Ethernet

ARCnet

Speed

RG-62 coax UTP, Type 1 STP

UTP = unshielded twisted pair, STP = shielded twisted pair, RF = Radio Frequency

Direct Links: Outline
• Physical Layer

– Link technologies
– Encoding

• Link Layer
–
–
–
–

Framing
Error Detection
Reliable Transmission (ARQ protocols)
Medium Access Control:

• Existing protocols: Ethernet, Token Rings, Wireless


Link Technologies
• Cables:
–
–
–
–

Cat 5 twisted pair, 10-100Mbps, 100m
Thin-net coax, 10-100Mbps, 200m
Thick-net coax, 10-100Mbps, 500m
Fiber, 100Mbps-2.4Gbps, 2-40km

• Leased Lines:
– Copper based: T1 (1.544Mbps), T3 (44.736Mbps)
– Optical fiber: STS-1 (51.84Mbps), STS-N (N*51.84Mbps)


Link Technologies
• Last-Mile Links:
– POTS (56Kbps), ISDN (2*64Kbps)
– xDSL: ADSL (16-640Kbps, 1.554-8.448Mbps), VDSL (12.96Mbps55.2Mbps)
– CATV: 40Mbps downstream, 20Mbps upstream
• Wireless Links: Cellular, Satellite, Wireless Local Loop


FRAMING
• An efficient data transmission technique
• It is a message forwarding system in which data packets, called
frames, are passed from one or many start-points to one


Approaches
• Byte oriented Protocol(PPP)
BISYNC
Binary Synchronous Communication
DDCMP
Digital Data Communication Message Protocol

• Bit oriented Protocol(HDLC)
• Clock based Framing(SONET)


Byte oriented Protocol(PPP)
BISYNC FRAME FORMAT
SYH

SYH

SOH

Header

STX

Body

ETX

PPP Frame Format
Flag

Address

Control

Protocol


Payload

Flag

CRC

DDCMP Frame Format

SYN

SYN

Class

Count


Header

Body

CRC

Bit Oriented Protocol(HDLC)

• Collection of Bits
1.HDLC
High-Level Data Link Control
2.Closed Based Framing(SONET)
Synchronous Optical Network


HDLC Frame Format
Beginning
sequence

Header

Body

CRC

Bit Stufffing
After 5 consecutive 1s insert 0
Next bit is 0 – stuffed removed
Next bit is 1 –end of frame or erorr


Ending
sequence

Closed Based Framing(SONET)
• STS-1 Frame
9 rows of 90 byte each
First 3 byte for overhead rest contains data
Payload bytes scrambled- exclusive OR
Supports Multiplexing
Payloads

9 rows

90 columuns

ERROR DETECTION
• Detecting Errors In Transmission
Electrical Interference, thermal noise
Approaches
Two Dimensional Parity
Internet Checksum Algorithm
Cyclic Redundancy Check


Two Dimensional Parity
7 bits of data

8 bits including parity

Number of 1s

even

odd

0000000 (0)

00000000

100000000

1010001 (3)

11010001

01010001

1101001 (4)

01101001

11101001

1111111 (7)

11111111

01111111


Transmission sent using even parity:
• A wants to transmit: 1001
• A computes parity bit value: 1^0^0^1 = 0
•

A adds parity bit and sends: 10010

• B receives: 10010 B computes parity: 1^0^0^1^0 = 0
• B reports correct transmission after observing expected even result.


Transmission sent using odd parity:
•
•
•
•
•
•

A wants to transmit: 1001
A computes parity bit value: ~(1^0^0^1) = 1
A adds parity bit and sends: 10011
B receives: 10011
B computes overall parity: 1^0^0^1^1 = 1
B reports correct transmission after observing expected odd result.


Reliable Transmission
Deliver Frames Reliably
Accomplished by Acknowledgements and Timeouts
ARQ-Automatic Repeat Request

Mechanism:
Stop and Wait
Sliding Window
Concurrent Logical Channels

Stop And Wait ARQ
• The source station transmits a single frame and then waits for an
acknowledgement (ACK).
• Data frames cannot be sent until the destination station’s reply
arrives at the source station.
• It discards the frame and sends a negative acknowledgement (NAK)
back to the sender
• causes the source to retransmit the damaged frame in case of error


Acknowledgements & Timeouts
Sender

Receiver

Sender

Timeout

Timeout

Fram
e

ACK

ACK

Timeout

Fram
e

(a)

Timeout

Fram
e

Sender

Timeout

Receiver
Fram
e

ACK

(c)

Timeout

Sender

Timeout

Time

Fram
e

Receiver

Receiver
Fram
e
ACK

Fram
e
ACK

ACK

(b)


(d)

Stop & wait sequence numbers

Timeout

Timeout

Fram
e

0

ACK

Fram
e

0

0

0
ACK

(c)

Sender
Timeout

Receiver

Timeout

Sender

Receiver
Fram
e

Sender

0

0
ACK

Fram
e

0

0
ACK

Receiver
Fram

e0

0
ACK

Fram
e

1
ACK

Fram
e

(d)

0

0
ACK

(e)

• Simple sequence numbers enable the client to discard
duplicate copies of the same frame
• Stop & wait allows one outstanding frame, requires two
distinct sequence numbers

1

Stop And Wait


Sliding Window
•

bi-directional data transmission protocol used in the data link layer
(OSI model) as well as in TCP

• It is used to keep a record of the frame sequences sent
•

respective acknowledgements received by both the users.


Sliding Window: Sender
•
•

•
•
•

Assign sequence number to each frame (SeqNum)
Maintain three state variables:
– send window size (SWS)
– last acknowledgment received (LAR)
– last frame sent (LFS)

Maintain invariant: LFS - LAR <= SWS
Advance LAR when ACK arrives
≤ SWS
Buffer up to SWS frames

…

LAR

…
LFS


Sequence Number Space
•
•
•

SeqNum field is finite; sequence numbers wrap around
Sequence number space must be larger then number of outstanding frames
SWS <= MaxSeqNum-1 is not sufficient
–
–
–
–
–
–

•
•

suppose 3-bit SeqNum field (0..7)
SWS=RWS=7
sender transmit frames 0..6
arrive successfully, but ACKs lost
sender retransmits 0..6
receiver expecting 7, 0..5, but receives the original incarnation of 0..5

SWS < (MaxSeqNum+1)/2 is correct rule
Intuitively, SeqNum “slides” between two halves of sequence number space


Sliding Window: Receiver
• Maintain three state variables

•

– receive window size (RWS)
– largest frame acceptable (LFA)
– last frame received (LFR)
Maintain invariant: LFA - LFRRWSRWS
≤ <=

…

…
LFR

LFA

• Frame SeqNum arrives:
•

– if LFR < SeqNum < = LFA
accept
– if SeqNum < = LFR or SeqNum > LFA
discarded
Send cumulative ACKs – send ACK for largest frame such that all
frames less than this have been received


Ehernet
• local-area network (LAN) covered by the IEEE
802.3.
• two modes of operation:
– half-duplex
– full-duplex modes.
.


Three basic elements :
1. the physical medium used to carry Ethernet signals between
computers,
2. a set of medium access control rules embedded in each
Ethernet interface that allow multiple computers to fairly
arbitrate access to the shared Ethernet channel,
3. an Ethernet frame that consists of a standardized set of bits
used to carry data over the system


IEEE 802.5 Format


Frame Format IEEE 802.5


IEEE 802.3 MAC Data Frame Format


Wireless
• The process by which the radio waves are propagated through air
and transmits data
• Wireless technologies are differentiated by :
• Protocol
• Connection type—Point-to-Point (P2P)
• Spectrum—Licensed or unlicensed


Types
• Infrared Wireless Transmission

– Tranmission of data signals using infrared-light
waves
• Microwave Radio

– sends data over long distances (regions, states,
countries) at up to 2 megabits per second
(AM/FM Radio)
• Communications Satellites
– microwave relay stations in orbit around the earth.


UNIT III Packet Switching
•
•
•
•
•
•

Is a network communications method
Groups all transmitted data, irrespective of content, type, or structure
into suitably-sized blocks, called packets.
Optimize utilization of available link capacity
Increase the robustness of communication.
When traversing network adapters, switches and other network nodes
packets are buffered and queued, resulting in variable delay and
throughput, depending on the traffic


Types
• Connectionless
• each packet is labeled with a connection ID rather than
an address.
• Example:Datagram packet switching

• connection-oriented
– each packet is labeled with a destination address
– Example:X.25 vs. Frame Relay


Star Topology


Source Routing
0 Switch 1
3

0

1
2 Switch 2
2

3 0 1

3

3

1

2

1 3 0
0

Host A
0 1 3
1

0 Switch 3
3
2


1

Host B

Virtual Circuit Switching
• Explicit connection setup (and tear-down) phase
• Subsequence packets follow same circuit
• Sometimes called connection-oriented model
0 Switch 1
1

3

2
5





Analogy: phone
call

3
11

2 Switch 2
1
0

Host A
7

Each switch
maintains a VC
table

1

0 Switch 3
3
2


4

Host B

Datagram Switching
• No connection setup phase
• Each packet forwarded independently
• Sometimes called connectionless model
Host D




Analogy: postal
system
Each switch
maintains a
forwarding
(routing) table

3
Host C

Host E

0 Switch 1
1
2

Host F
3

2 Switch 2
1
0

Host A

Host G
1

0 Switch 3 Host B
3
2
Host H


Virtual Circuit Model
• Typically wait full RTT for connection setup before sending first
data packet.
• While the connection request contains the full address for
destination
• each data packet contains only a small identifier, making the
per-packet header overhead small.
• If a switch or a link in a connection fails, the connection is
broken and a new one needs to be established.
• Connection setup provides an opportunity to reserve resources.

Datagram Model
• There is no round trip delay waiting for connection setup; a
host can send data as soon as it is ready.
• Source host has no way of knowing if the network is capable of
delivering a packet or if the destination host is even up.
• Since packets are treated independently, it is possible to route
around link and node failures.
• Since every packet must carry the full address of the
destination, the overhead per packet is higher than for the
connection-oriented model.

Bridges and Extended LANs
• LANs have physical limitations (e.g., 2500m)
• Connect two or more LANs with a bridge
– accept and forward strategy
– level 2 connection (does not add packet header)
A

B

C
Port 1
Bridge
Port 2

• Ethernet Switch = Bridge on Steroids

X


Y

Z

Spanning Tree Algorithm
• Problem: loops

A
B
B3
C

B5
D

B2

B7

E

K
F

B1
G

H
B6

B4

I
J

• Bridges run a distributed spanning tree algorithm
– select which bridges actively forward
– developed by Radia Perlman
– now IEEE 802.1 specification

Algorithm Details
• Bridges exchange configuration messages
– id for bridge sending the message
– id for what the sending bridge believes to be root bridge
– distance (hops) from sending bridge to root bridge
• Each bridge records current best configuration message for
each port
• Initially, each bridge believes it is the root


FAQ
1. Explain the ISO-OSI model of computer network with a neat diagram.
2. Discuss the major functions performed by the Presentation layer and Application layer of the ISO OSI model.
3. Explain Transport Layer and Physical Layer.
4. What are the major components of an optical communication system? Discuss.
5. Distinguish between point to point links and multi point links. Give relevant diagrams.
6. Explain Data Link Layer and Network Layer.
7. Compare Connection oriented and connectionless service.
8. a) What is the need for data encoding and explain the various data encoding schemes and compare their features. (8)
b) Explain how hamming code can be used to correct burst errors. (8)
9. Explain the operation of the bit-oriented protocol HDLC with the required frames
10.Explain the various error detection and correction Mechanisms used in computer network.
11. Write short notes on:
a) Go back NARQ (8)
b) Selective repeat ARQ (8)
12. a) Discuss the major functions performed by the Presentation layer and Application layer of the ISO - OSI model. (8)
b) Compare Connection oriented and connectionless service. (4)
c) What are the major components of an optical communication system? Discuss. (4)
13. a) A block of 32 bits has to be transmitted. Discuss how the thirty two bit block is transmitted to the receiver using Longitudinal Redundancy Check.
(4)
b) Consider a 32 bit block of data 11100111 11011101 00111001 10101001 that has to be transmitted. If Longitudinal Redundancy Check is used
what is the transmitted bit stream?(4)
c) In the Hamming code, for a data unit of 'm' bits how do you compute the number of redundant bits 'r' needed? (4)
d) What kinds of errors can Vertical Redundancy check determine? What kinds of errors it cannot determine? (4)
14. Discuss stop and wait protocol
15. Discuss sliding window protocol using Go back n.
16. How does a Token Ring LAN operate? Discuss.


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