1. www.hope.ac.uk Faculty of Sciences and Social Sciences HOPE Local Area Network Overview Part 1 of 2 Dr. Jia Hu huj@hope.ac.uk FML 413 0151 291 ???? Stewart Blakeway blakews@hope.ac.uk FML 213 0151 291 3113

2. www.hope.ac.uk Faculty of Sciences and Social Sciences HOPE Required Reading Data 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

3. www.hope.ac.uk Faculty of Sciences and Social Sciences HOPE Aims of the Presentation Background Topologies Transmission Media Media Access Control

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

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

6. www.hope.ac.uk Faculty of Sciences and Social Sciences HOPE Technologies of Local Area Network Peer to Peer Client Server Ad-Hoc

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

8. www.hope.ac.uk Faculty of Sciences and Social Sciences HOPE Disadvantages Stealing of resources Accountability Single Point of Failure Infection Additional Cost – Structural – Administration

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

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

11. www.hope.ac.uk Faculty of Sciences and Social Sciences HOPE Storage Area Networks

12. www.hope.ac.uk Faculty of Sciences and Social Sciences HOPE LAN Applications (3) high speed office networks – desktop image processing – high capacity local storage backbone LANs – interconnect low speed local LANs – reliability – capacity – cost

13. www.hope.ac.uk Faculty of Sciences and Social Sciences HOPE LAN Architecture topologies transmission medium layout medium access control

14. www.hope.ac.uk Faculty of Sciences and Social Sciences HOPE Local Area Network Topologies BUS Tree Ring Star Complex Star (snowflake)

15. www.hope.ac.uk Faculty of Sciences and Social Sciences HOPE Bus T

16. www.hope.ac.uk Faculty of Sciences and Social Sciences HOPE Tree R T T T T T T

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

18. www.hope.ac.uk Faculty of Sciences and Social Sciences HOPE Ring S D R

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

20. www.hope.ac.uk Faculty of Sciences and Social Sciences HOPE Star S or H S D

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

22. www.hope.ac.uk Faculty of Sciences and Social Sciences HOPE Complex Star S or H S D

23. www.hope.ac.uk Faculty of Sciences and Social Sciences HOPE Choice of Topology reliability expandability performance needs considering in context of: – medium – wiring layout – access control

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

25. www.hope.ac.uk Faculty of Sciences and Social Sciences HOPE Access Control Methods Polling Token Passing CSMA (Carrier Sense Multiple Access) CSMA/CD (Collision Detection) CSMA/CA (Collision Avoidance)

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

27. www.hope.ac.uk Faculty of Sciences and Social Sciences HOPE Polling Server Poll Client 1 Client 1 send NAK Server Poll Client 2 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) Server Poll Client 3 … S or H

28. www.hope.ac.uk Faculty of Sciences and Social Sciences HOPE Advantages Easy to Implement No collisions – No need to implement CA or CD techniques Every client gets the opportunity to transmit

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

30. www.hope.ac.uk Faculty of Sciences and Social Sciences HOPE Token Originally developed for small ring networks – Adapted for use with bus Does not requires a central server A form of distributed polling

31. www.hope.ac.uk Faculty of Sciences and Social Sciences HOPE Token Passing Token Send Data Token Data ACK Data Token S D R

32. www.hope.ac.uk Faculty of Sciences and Social Sciences HOPE 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)

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

34. www.hope.ac.uk Faculty of Sciences and Social Sciences HOPE Frame Transmission on Bus LAN

35. www.hope.ac.uk Faculty of Sciences and Social Sciences HOPE 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!

36. www.hope.ac.uk Faculty of Sciences and Social Sciences HOPE CSMA - Collision Cable Quiet 2 Machines Listening 2 Machines Transmit Collision Occurs Listen All Clear - TRANSMIT

37. www.hope.ac.uk Faculty of Sciences and Social Sciences HOPE CSMA/CA 1.Transmitter Listen 2.When cable is free, transmit initial carrier burst 3.Transmitter Listen (any other carrier bursts?) 4.If cable is still free, send 2 nd carrier burst 5.Short time delay 6.Send Data 7.Receiver replies with ACK This technique is explained at length by Colvine (1983)

38. www.hope.ac.uk Faculty of Sciences and Social Sciences HOPE 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)

39. www.hope.ac.uk Faculty of Sciences and Social Sciences HOPE 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)

40. www.hope.ac.uk Faculty of Sciences and Social Sciences HOPE Worst Case S D ListenAll Clear - TRANSMIT

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

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

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

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

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

46. www.hope.ac.uk Faculty of Sciences and Social Sciences HOPE 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?

47. www.hope.ac.uk Faculty of Sciences and Social Sciences HOPE Choice of Medium constrained by LAN topology capacity reliability types of data supported environmental scope

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

49. www.hope.ac.uk Faculty of Sciences and Social Sciences HOPE What have we covered? (cont.) The Medium (Cable) Access Control Methods – Polling – Token – Token Passing – CSMA CSMA/CA CSMA/CD

50. www.hope.ac.uk Faculty of Sciences and Social Sciences HOPE Any Questions ?