William Stallings Data and Computer Communications

1. William StallingsData and Computer Communications Chapter 13 Local Area Network Technology

2. LAN (Local Area Networks) • A LAN consists of • Shared transmission medium • set of hardware and software for the interfacing devices • regulations for orderly access to the medium

3. LAN Applications (1) • Personal computer LANs • mostly in office environment • to share data and resources (e.g. laser printer) • Back end networks and storage area networks • Interconnecting large systems (mainframes and large storage devices) • High data rate • Limited distance • Limited number of devices

4. LAN Applications (2) • High speed office networks • Desktop image processing • High capacity local storage • Backbone LANs • Interconnect low speed local LANs with a higher capacity LAN

5. LAN Architecture • Layered protocol architecture • Physical (topologies) • Media access control • Logical Link Control

6. Protocol Architecture • Lower layers of OSI model • IEEE 802 reference model • Physical • Logical link control (LLC) • Media access control (MAC)

7. IEEE 802 v OSI

8. 802 Layers - Physical • Signal encoding/decoding • Preamble generation/removal (for synchronization) • Bit transmission/reception • Transmission medium and topology • below the physical layer

9. 802 Layers - Medium Access Control & Logical Link Control • OSI layer 2 (Data Link) is divided into two in 802 • Logical Link Control (LLC) layer • Medium Access Control (MAC) layer • MAC layer • Assembly of data into frame with address and error detection fields (for transmission) • Disassembly of frame (on reception) • Address recognition • Error detection • Govern access to transmission medium • Not found in traditional layer 2 data link control

10. 802 Layers - Medium Access Control & Logical Link Control • LLC layer • Interface to higher levels • flow control

11. LAN Protocols in Context

12. LAN Topologies • Bus • Ring • Star

13. Bus Topology • Multipoint medium • Heard by all stations • Need to identify target station • Each station has unique address • Full duplex connection between station and tap • Allows for transmission and reception • Terminator absorbs frames at end of medium • Need to regulate transmission • To avoid collisions • To avoid continuous transmission from a single station • Solution: Data in small blocks - frames

14. Frame Transmission - Bus LAN

15. Ring Topology • Repeaters joined by point-to-point links in closed loop • Receive data on one link and retransmit on another • Links unidirectional • Stations attach to repeaters • Data transmitted in frames • Circulate past all stations • Destination recognizes address and copies frame • Frame circulates back to source where it is removed • Medium access control determines when station can insert frame

16. Frame TransmissionRing LAN

17. Star Topology • Each station connected directly to central node • Usually via two point-to-point links • Central node can broadcast • Physical star, logically bus • Only one station can transmit at a time • Central node can act as frame switch • retransmits only to destination

18. Medium Access Control • Why? • order • efficient use of medium • Synchronous • everyone knows when to transmit • like TDM • Asynchronous • dynamic • Three categories • Round robin • Reservation • Contention

19. Medium Access Control • Round robin • each station has a turn to transmit • declines or transmits (to a certain limit) • overhead of passing the turn in either case • Good if many stations have data to transmit over extended period • Reservation • Stations reserve slots to transmit • Good for stream traffic • centralized/distributed reservations

20. Medium Access Control • Contention • No control to determine whose turn is it • All stations contend for time/slot to transmit • Good for bursty traffic • Efficient under light or moderate load • Performance is bad under heavy load

21. MAC Frame Format • Actual format differs from protocol to protocol • MAC layer receives data from LLC layer • MAC layer detects errors and discards frames • LLC optionally retransmits unsuccessful frames

22. Bus LANs • Signal balancing • Signal must be strong enough to meet receiver’s minimum signal strength requirements • Give adequate signal to noise ratio • Not so strong that it overloads transmitter and creates distortions • Simultaneous satisfaction for all stations on bus • Solution is to divide network into small segments • Segments are connected with repeaters

23. Transmission Media • Baseband coaxial cable • Used by Ethernet and IEEE 802.3 • not a preferred option nowadays • Broadband coaxial cable • Included in 802.3 specification but no longer made • Optical fiber • Expensive • Not used

24. Baseband Coaxial Cable • Uses digital signaling • Manchester or Differential Manchester encoding • Bi-directional • Few kilometer range • due to attenuation • Ethernet (basis for 802.3) at 10Mbps • 50 ohm cable

25. 10Base5 • Thick coax • Ethernet and 802.3 originally used 0.4 inch diameter cable at 10Mbps • Max cable length 500m • Distance between taps a multiple of 2.5m • Ensures that reflections from taps do not add in phase • Max 100 taps • Vampire taps, twisted pair transceiver cable • 10Base5

26. 10Base2 • Thin coax • 0. 5 cm cable • More flexible • Easier to bring to workstation • Cheaper electronics • T-connectors • easier maintanence • Greater attenuation, lower noise resistance • Fewer taps (30) • Shorter distance (200m)

27. Repeaters • Transmits in both directions • Joins two segments of cable • No buffering • No logical isolation of segments • If two stations on different segments send at the same time, packets will collide • Only one path of segments and repeaters between any two stations

28. Baseband Configuration

29. Star LANs • Use unshielded twisted pair wire in star wiring • Attach to a central active hub • Two links • Transmit and receive • Hub repeats incoming signal on all outgoing lines • Link lengths limited to about 100m • Logical bus - with collisions

30. Two Level Star Topology All stations are logically on the same bus

31. Shared Medium Hub • Central hub • Hub retransmits incoming signal to all outgoing lines • Only one station can transmit at a time • With a 10Mbps LAN, total capacity is 10Mbps

32. Switched Hubs • Hub acts as switch • Incoming frame switches to appropriate outgoing line • Unused lines can also be used to switch other traffic • Each device has dedicated capacity equal to the LAN capacity

33. Switched Hubs • No change to software or hardware of devices • Attachment is not logically different from the attached device point of view • Store and forward switch • Accept input, buffer it briefly, then output • Cut through switch • Take advantage of the destination address being at the start of the frame • Begin repeating incoming frame onto output line as soon as address recognized • May propagate some bad frames

34. Bridges • Need to expand beyond single LAN • Interconnection to other LANs/WANs • Use Bridge or router • Bridge is simpler • Connects similar LANs • Identical protocols for physical and link layers • Minimal processing • Router more general purpose • Interconnect various LANs and WANs

35. Functions of a Bridge • Read all frames transmitted on one LAN and accept those address to any station on the other LAN • Using MAC protocol for second LAN, retransmit each frame • Do the same the other way round

36. Bridge Operation

37. Bridge Design Aspects • No modification to content or format of frame • No additional header • Exact bitwise copy of frame from one LAN to another • that is why two LANs must be identical • Enough buffering to meet peak demand • Routing and addressing intelligence • Must know the addresses on each LAN to be able to tell which frames to pass • May be more than one bridge to reach the destination • May connect more than two LANs

38. Bridge Protocol Architecture • IEEE 802.1D • operates at MAC level • Station address is at this level • Bridge does not need LLC layer

39. Routing in Bridges • Bridge must have routing capability • Bridge must decide whether to forward frame • Bridge must decide which LAN to forward frame on • Alternate routes

40. Fixed Routing • Routing selected for each source-destination pair of LANs • Usually least hop route • Only changed when topology changes • Manual routing data entry needed • Similar to fixed routing in packet switched networks • central routing table shows the first bridge in the route for each source-destination pair • local routing tables can be derived from the central table

41. Example Fixed Routing • LAN E to LAN F • LAN E • Bridge 107 • LAN A • Bridge 102 • LAN C • Bridge 105 • LAN F

42. Spanning Tree Routing • Bridge automatically develops routing tables • Automatically updates in response to changes • Station locations are learned from packets • forwarding databases are constructed for each port of bridges • If the location of a station is known, packet is forwarded accordingly • Otherwise packet is forwarded to all ports • to guarantee delivery • to allow learning

43. Address Learning • When frame arrives at port X, it has come from the LAN attached to port X • Use the source address to update forwarding database for port X to include that address • Example: when Bridge 102 receives a packet from LAN A port with source address 2, it learns that station 2 is on LAN A port • Examples of port databases • Bridge 102 A port • Stations 1, 2, 3, 4 and 5 • Bridge 102 C port • Stations 6 and 7

44. Other Considerations • Address learning works for tree layout • i.e. no closed loops • a spanning tree must be found in case of a loop • Response to topology changes is required • database entries are not permanent • i.e. learning algorithm is repeated periodically

45. Loop of Bridges

46. William StallingsData and Computer Communications Chapter 14 LAN Systems (CSMA/CD - Ethernet, Ring Systems)

47. Ethernet (CSMA/CD) • Carriers Sense Multiple Access with Collision Detection • Xerox - Ethernet • IEEE 802.3 • Random Access (ALOHA) • Stations access medium randomly • Contention • Stations contend for time on medium

48. ALOHA • Packet Radio (applicable to any shared medium) • When station has frame, it sends • collisions may occur • Station listens for max round trip time • If no collision, fine. If collision, retransmit after a random waiting time • Max utilization 18% - very bad

49. Slotted ALOHA • Divide the time into discrete intervals (slots) • equal to frame transmission time • need central clock (or other sync mechanism) • transmission begins at slot boundary • Collided frames will do so totally or will not collide • Max utilization 37%

50. CSMA • First listen for clear medium (carrier sense) • If medium idle, transmit • If busy, continuously check the channel until it is idle and then transmit • If collision occurs • Wait random time and retransmit • Collision probability depend on the propagation time • Longer propagation delay, worse the utilization • Collision occurs even if the propagation time is zero. WHY? • 1-persistent CSMA • Better utilization than ALOHA