www.hope.ac.uk faculty of sciences and social sciences hope local area network overview part 1 of 2...

www.hope.ac.uk Faculty of Sciences and Social Sciences

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Local Area Network OverviewPart 1 of 2

Dr. Jia Hu [email protected] 4130151 291 ????

Stewart [email protected] 2130151 291 3113


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Required ReadingData and Computer Communications by William Stallings. Eight Edition.

Pearson Education International.

ISBN 13: 978-0-13-507139-9. ISBN 10: 0-13-507139-9.

Today is based on the first part of chapter 15


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Aims of the Presentation

• Background• Topologies• Transmission Media• Media Access Control


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Local Area Network Overview

The whole of this operation is described in minute detail in the official British Naval History, and should be studied with its excellent charts by those who are interested in its technical aspect. So complicated is the full story that the lay reader cannot see the wood for the trees. I have endeavored to render intelligible the broad effects.

—The World Crisis, Winston Churchill


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Definition and Background

• What is a LAN?

– Two or more computers connected together over some form of medium for the communication of data

Currie (1988) “LANs [sic] have rapidly become the

major growth area within the computing industry, and are

widely expected to remain so until at least the early 1990’s


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Technologies of Local Area Network

• Peer to Peer• Client Server• Ad-Hoc


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Advantages• High Speed Communication

• Sharing of resources– Memory

– Hard Disk Space

– Processing Power

– Data

– Software

– Devices (printers, scanners, modems ect)

• Accountability

In 1986 only 3% of PCs were connected to a LAN. Today

90% of PCs are connected to a LAN and a huge

percentage connected to a WAN


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Disadvantages

• Stealing of resources• Accountability• Single Point of Failure• Infection• Additional Cost

– Structural – Administration


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LAN Applications (1)• personal computer LANs

– low cost– limited data rate

• back end networks– interconnecting large systems (mainframes and large

storage devices)• high data rate• high speed interface• distributed access• limited distance• limited number of devices


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LAN Applications (2)• storage area networks (SANs)

– separate network handling storage needs

– detaches storage tasks from specific servers

– shared storage facility• eg. hard disks, tape libraries, CD arrays

– accessed using a high-speed network• eg. Fibre Channel

– improved client-server storage access

– direct storage to storage communication for backup


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Storage Area Networks


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LAN Applications (3)• high speed office networks

– desktop image processing– high capacity local storage

• backbone LANs– interconnect low speed local LANs– reliability– capacity– cost


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LAN Architecture

• topologies• transmission medium• layout• medium access control


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Local Area Network Topologies

• BUS• Tree• Ring• Star• Complex Star (snowflake)


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Bus

T


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Tree

R

TT T

T

TT


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Bus and Tree• used with multipoint medium

• transmission propagates throughout medium

• heard by all stations

• full duplex connection between station and tap– allows for transmission and reception

• need to regulate transmission– to avoid collisions and hogging

• terminator absorbs frames at end of medium

• tree a generalization of bus

• headend connected to branching cables


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RingS

D

R


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Ring Topology• a closed loop of repeaters joined by point to point links• receive data on one link & retransmit on another

– links unidirectional– stations attach to repeaters

• data in frames– circulate past all stations– destination recognizes address and copies frame– frame circulates back to source where it is removed

• media access control determines when a station can insert frame

• very high speed links over long distances

• single link or repeater failure disables network


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Star

S or H

S

D


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Star Topology

• each station connects to central node– usually via two point to point links

• either central node can broadcast– physical star, logical bus– only one station can transmit at a time

• or central node can act as frame switch• uses natural layout of wiring in building• best for short distances• high data rates for small number of devices


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Complex Star

S or H

S

D

S or H

S or H


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Choice of Topology

• reliability• expandability• performance• needs considering in context of:

– medium– wiring layout– access control


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Media Access Control• where

– central• greater control, single point of failure

– distributed• more complex, but more redundant

• how– synchronous

• capacity dedicated to connection, not optimal

– asynchronous• in response to demand


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Access Control Methods

• Polling• Token Passing• CSMA (Carrier Sense Multiple Access)• CSMA/CD (Collision Detection)• CSMA/CA (Collision Avoidance)


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Polling

• Originally developed for small star networks– Adapted for use with bus or ring

• Requires a central server (not peer to peer)– Server knows of its clients– Asks each client in turn if it wishes to transmit


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Polling

Server Poll Client 1

Client 1 send NAK


Client 2 ACK

Server SEND Client 2 (open connection)

Client 2 send DATA

Server ACK (received OK)

Client 2 send DATA

Server ACK (received OK)

Client 2 send NAK

Server send DATA (close connection?)

Client 2 ACK (close connection)

Server ACK (close connection)


…

S or H


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Advantages

• Easy to Implement• No collisions

– No need to implement CA or CD techniques

• Every client gets the opportunity to transmit


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Disadvantages

• Additional Network Overhead (load) with constant polling messages

• A greedy client makes this network unfair (although advanced polling techniques do include time slicing)

• Clients waiting to transmit have to wait their turn (even if no data is being sent by any other clients)

• Client to Client transfers have to be passed via the server (for both directions)

• Single point of failure– Server crashes– Polling algorithm fails


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Token

• Originally developed for small ring networks– Adapted for use with bus

• Does not requires a central server• A form of distributed polling


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Token Passing

Token

Send Data Token

Data

ACK Data Token

S

D

R


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Advantages

• Easy to Implement• No collisions

– No need to implement CA or CD techniques

• Every client gets the opportunity to transmit• No bottle neck (centralised server)


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Disadvantages

• Additional Network Overhead (load) with constant token passing (but less than polling)

• A greedy client makes this network unfair (although advanced token techniques do include slotted ring)

• Clients waiting to transmit have to wait their turn (for the token to get to them) even if no data is being sent by any other clients

• Client to Client transfers have to be passed via all other intermediate clients in the ring

• Single point of failure– Medium Fails– Token algorithm fails


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Frame Transmissionon Bus LAN


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CSMA

• Carrier Sense Multiple Access• Most common technique used in LANs• Restricted to BUS and TREE• Client listens (Carrier Sense) and only

transmits when quiet• Collision Problems!


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CSMA - Collision

• Cable Quiet• 2 Machines

Listening• 2 Machines

Transmit• Collision Occurs

Listen

Listen

All Clear - TRANSMIT

All Clear - TRANSMIT


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CSMA/CA

1. Transmitter Listen2. When cable is free, transmit initial carrier burst3. Transmitter Listen (any other carrier bursts?)4. If cable is still free, send 2nd carrier burst5. Short time delay6. Send Data7. Receiver replies with ACK

This technique is explained at length by Colvine (1983)


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CSMA/CD

• Originally developed by Xerox (called Ethernet)

• If two or more stations transmit at the same time the voltage level of the cable decreases

• Fully described by Metcalf & Boggs (1976)


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CSMA/CD

1. Transmitter Listen

2. When cable is free, transmit data

3. Transmitter Listen (collision?)

4. If collision then transmit jamming signal and backoff

5. Steps 1- 4

This technique is explained at length by Metcalf & Boggs (1976)


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Worst Case

S

D

ListenAll Clear - TRANSMIT


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Worst Case

• Machine S had finished transmitting• Machine D listened – All clear so transmitted• Machine D detected the collision so sent a

jamming signal and backed off• Machine S – at this stage had stopped

listening as it had finished transmitting! Packet Lost.


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Overcoming Worst Case

• Packet Sizes were increased– The beginning of the packet would reach the destination

before the source had finished sending

• Cable Length was decreased– The length of the cable was decreased to restrict the

destination being too far from the source

• The greater the cable length the greater the packet size had to be increase (thus adding overhead for each and every packet sent)

Many people associate restricted cable length to attenuation. Although this is a very important factor, the CSMA/CD determined the maximum length of cable in an Ethernet network.


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Media Available

• Voice grade unshielded twisted pair (UTP)

• Cat 3 phone, cheap, low data rates

• Shielded twisted pair / baseband coaxial– more expensive, higher data rates

• Broadband cable– even more expensive, higher data rate

• High performance UTP– Cat 5+, very high data rates, witched star topology

• Optical fibre– security, high capacity, small size, high cost


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Bus LAN Transmission Media (1)

• twisted pair– early LANs used voice grade cable– didn’t scale for fast LANs– not used in bus LANs now

• baseband coaxial cable– uses digital signalling– original Ethernet


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Bus LAN Transmission Media (2)

• broadband coaxial cable– as in cable TV systems– analog signals at radio frequencies– expensive, hard to install and maintain– no longer used in LANs

• optical fiber– expensive taps– better alternatives available– not used in bus LANs

• less convenient compared to star topology twisted pair

• coaxial baseband still used but not often in new installations


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A closer look at coaxial

Central Core

Insulator

Metal Shield

Plastic Jacket • Cables do not carry data!• Cables carry a voltage• Signals transmit across

the cable using this electrical current

• What about access?– Multiple access or not?

• What about collision?


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Choice of Medium

• constrained by LAN topology• capacity• reliability• types of data supported• environmental scope


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What have we covered?

• Definition and Background• Advantages of Networking• Disadvantages of Networking• Technologies of Local Area Network• Topologies of Local Area Network

– Ring– Bus– Star– Complex Star– Tree


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What have we covered? (cont.)

• The Medium (Cable)• Access Control Methods

– Polling– Token– Token Passing– CSMA

• CSMA/CA• CSMA/CD


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Any Questions ?

www.hope.ac.uk faculty of sciences and social sciences hope local area network overview part 1 of 2...

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