By N.Gopinath AP/CSE Unit – I Introduction to Computer Networks

ByN.Gopinath

AP/CSE

Unit – IIntroduction to Computer Networks

Computer Networks

Computer network connects two or more autonomous computers.

The computers can be geographically located anywhere.

Introduction to Computer Networks

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 SharingHardware (computing resources, disks, printers)Software (application software)

Information SharingEasy accessibility from anywhere (files, databases)Search Capability (WWW)

CommunicationEmailMessage 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, Routers, Wireless Access Points, Modems etc.

Computers: Clients and ServersIn 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-mailSearchable Data (Web Sites)E-CommerceNews GroupsInternet Telephony (VoIP)Video ConferencingChat GroupsInstant 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 thenetwork

Primary architectural model for inter-computing and Inter networking communications.

network communication protocols have a structure based on OSI Model

OSI Architecture

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 (12.96Mbps-

55.2Mbps)– 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

http://en.wikipedia.org/wiki/File:Frame_relay.jpg

Approaches

• Byte oriented Protocol(PPP)BISYNCBinary Synchronous CommunicationDDCMPDigital Data Communication Message Protocol

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

Byte oriented Protocol(PPP)

SYH SYH SOH Header STX Body ETX CRC

BISYNC FRAME FORMAT

Flag Address Control Protocol Payload Flag

PPP Frame Format

SYN SYN Class Count Header Body CRC

DDCMP Frame Format

Bit Oriented Protocol(HDLC)

• Collection of Bits1.HDLC

High-Level Data Link Control

2.Closed Based Framing(SONET)Synchronous Optical Network

HDLC Frame Format

Beginning sequence

Header Body CRC Ending sequence

Bit Stufffing

After 5 consecutive 1s insert 0

Next bit is 0 – stuffed removed Next bit is 1 –end of frame or erorr

Closed Based Framing(SONET)

• STS-1 Frame9 rows of 90 byte eachFirst 3 byte for overhead rest contains data

Payload bytes scrambled- exclusive OR Supports Multiplexing

90 columuns

Payloads

9 rows

ERROR DETECTION• Detecting Errors In Transmission

Electrical Interference, thermal noise

ApproachesTwo Dimensional ParityInternet Checksum AlgorithmCyclic 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 & TimeoutsSender Receiver

Frame

ACK

Tim

eout

Tim

e

Sender Receiver

Frame

ACK

Tim

eout

Frame

ACKTim

eout

Sender Receiver

Frame

ACKTim

eout

Frame

ACKTim

eout

Sender Receiver

Frame

Tim

eout

Frame

ACKTim

eout

(a) (c)

(b) (d)

Stop & wait sequence numbersSender Receiver

Frame 0

ACK 0

Tim

eo

ut

Frame 0

ACK 0

Tim

eo

ut

Sender Receiver

Frame 0

ACK 0Tim

eo

ut

Frame 0

ACK 0Tim

eo

ut

(c) (d)

Sender Receiver

Frame 0

ACK 0

Frame 1

ACK 1

(e)

Frame 0

ACK 0

• 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

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 • Buffer up to SWS frames SWS

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 - LFR <= RWS

• 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

RWS

LFR LFA

… …

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

13

2

0

1 3

2

0

13

2

0

13

2

3 0 1 3 01

30 1

Switch 3

Host B

Switch 2

Host A

Switch 1

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

0

13

2

01 3

2

0

13

25 11

4

7

Switch 3

Host B

Switch 2

Host A

Switch 1

Analogy: phone call

Each switch maintains a VC table

Datagram Switching

• No connection setup phase• Each packet forwarded independently • Sometimes called connectionless model

0

13

2

0

1 3

2

0

13

2

Switch 3Host B

Switch 2

Host A

Switch 1

Host C

Host D

Host EHost F

Host G

Host H

Analogy: postal system

Each switch maintains a forwarding (routing) table

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)

• Ethernet Switch = Bridge on Steroids

A

Bridge

B C

X Y Z

Port 1

Port 2

Spanning Tree Algorithm • Problem: loops

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

B3

A

C

E

DB2

B5

B

B7 K

F

H

B4

J

B1

B6

G

I

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

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

Thank u

Documents

By N.Gopinath AP/CSE Unit – I Introduction to Computer Networks