CN Unit 1.PDF. (1)

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Computer networks

Name: K.SUDHA

Designation: Lecturer

Department: Electrical and Electronics Engineering

Subject code: CS2361

Year: III

Unit: I

Title: Introduction to computer networks




Computer Networks

Computer network connectstwo or more autonomouscomputers.

The computers can begeographically locatedanywhere.

Introduction to Computer Networks




LAN, MAN & WAN


Network in small geographical Area (Room, Building or aCampus) is called LAN (Local Area Network)

Network in a City is call MAN (Metropolitan Area Network)

Network spread geographically (Country or across Globe) iscalled 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 topologydefines the way in whichcomputers, printers, andother devices areconnected. A networktopology describes thelayout of the wire anddevices as well as thepaths used by datatransmissions.





Bus Topology

Commonly referred to as alinear bus, all the deviceson a bus topology areconnected by one singlecable.





Star & Tree Topology


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

When installed, the star topologyresembles spokes in a bicyclewheel.

Larger networks use the extendedstar topology also called treetopology. When used with networkdevices that filter frames or packets,like bridges, switches, and routers,this topology significantly reducesthe traffic on the wires by sendingpackets only to the wires of thedestination host.




Ring Topology


A frame travels around the ring,stopping at each node. If a node wantsto transmit data, it adds the data aswell as the destination address to theframe.

The frame then continues around thering until it finds the destination node,which takes the data out of the frame.

Single ring – All the devices on thenetwork share a single cable

Dual ring – The dual ring topologyallows data to be sent in bothdirections.




Mesh Topology

The mesh topologyconnects all devices(nodes) to each other forredundancy and faulttolerance.

It is used in WANs tointerconnect LANs and formission critical networkslike those used by banksand financial institutions.

Implementing the meshtopology is expensive anddifficult.





Network Components


Physical Media

Interconnecting Devices

Computers

Networking Software

Applications




Networking Media

Networking media can bedefined simply as themeans by which signals(data) are sent from onecomputer to another(either by cable or wirelessmeans).





Networking Devices


HUB, Switches, Routers,Wireless Access Points,Modems etc.




Computers: Clients and ServersIn a client/server networkarrangement, networkservices are located in adedicated computer whoseonly function is to respondto the requests of clients.

The server contains thefile, print, application,security, and other servicesin a central computer thatis continuously available torespond 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 developedby 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 networkingcommunications.

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, calledframes, are passed from one or many start-points to one




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




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)

SYH SYH SOH Header STXBody

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 Bits

1.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 each

First 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

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 anacknowledgement (ACK).

• Data frames cannot be sent until the destination station’s replyarrives 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 Receiv er

Frame

ACK

Tim

eou

t

Tim

e

Sender Receiv er

Frame

ACK

Tim

eou

t

Frame

ACK

Tim

eou

t

Sender Receiv er

Frame

ACKTim

eou

t

Frame

ACKTim

eou

t

Sender Receiv er

Frame

Tim

eou

t

Frame

ACK

Tim

eou

t

(a) (c)

(b) (d)




Stop & wait sequence numbers

Sender Receiver

Tim

eout

Tim

eout

Sender Receiver

Tim

eout

Tim

eout

(c) (d)

Sender Receiver

(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




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 betweencomputers,

2. a set of medium access control rules embedded in eachEthernet interface that allow multiple computers to fairlyarbitrate access to the shared Ethernet channel,

3. an Ethernet frame that consists of a standardized set of bitsused 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 airand 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 structureinto 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 andthroughput, 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

0

1 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 E

Host 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; ahost can send data as soon as it is ready.

• Source host has no way of knowing if the network is capable ofdelivering a packet or if the destination host is even up.

• Since packets are treated independently, it is possible to routearound link and node failures.

• Since every packet must carry the full address of thedestination, the overhead per packet is higher than for theconnection-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

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CN Unit 1.PDF. (1)