1. computer networks u1 ver 1.0

Computer networks

Dr. C.V. Suresh Babu


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


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)Dr. C.V. Suresh Babu

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


LAN Architecture SummaryNetwork Type Speed Maximum Number of

StationsTransmission Types Notes

Ethernet 10 Mb/s 1024 Category 3 UTP or better (10BASE-T), Thinnet RG-58 coax (10BASE-2), Thicknet coax (10BASE-5), fiber-optic (10BASE-F)

Replaced by Fast Ethernet; backward compatible with Fast or Gigabit Ethernet when using UTP.

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 Gigabit Ethernet 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 Type 1 STP

UTP, Type 1 STP, and fiber-optic Replaced by Ethernet; obsolete for new installations.

ARCnet 2.5 Mb/s 255 RG-62 coax UTP, Type 1 STP Replaced by Ethernet; obsolete for new installations; uses the same coax cable as IBM 3270 terminals.

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 (12.96Mbps-

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 ChannelsDr. C.V. Suresh Babu

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)Dr. C.V. Suresh Babu

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


FAQ1. 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 frames10.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 protocol15. Discuss sliding window protocol using Go back n.16. How does a Token Ring LAN operate? Discuss.


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