Topic 7 Local Area Networks (LAN)

1. Topic 7Local Area Networks (LAN) Overview of LANs LAN Bridges LAN Standards Reference A. Leon-Garcia and I. Widjaja, Communication Networks, pp. 421-479. (Reserved in the DC library. Call No. TK5105. L46 2004.)

2. 7.1 Overview of LANs What is a LAN? LAN Topology LAN Protocol Architecture

3. What is a LAN? • Private ownership • freedom from regulatory constraints of WANs • Short distance (~1km) between computers imply • low cost • very high-speed, relatively error-free communication • complex error control unnecessary • Machines are constantly moved • Simply give each machine a unique address • Broadcast all messages to all machines in the LAN • Need a medium access control protocol

4. LAN Topology • A number of nodes (computers and network devices) are interconnected by a shared transmission medium • Nodes are connected to the cabling system through a network interface card (NIC) or LAN adapter card • Nodes can be arranged in bus, ring, or star topology Bus Ring Star

5. OSI IEEE 802 Network layer Network layer 802.2 Logical link control LLC Data link layer 802.11 Wireless LAN Other LANs 802.3 CSMA-CD 802.5 Token Ring MAC Physical layer Fiber, twisted pairs, coax, wireless Physical layer LAN Protocol Architecture

6. Medium Access Control • Coordinate access to medium • Connectionless frame transfer service • Machines identified by MAC/physical address • Broadcast frames with MAC addresses

7. 7.2 LAN Bridges

8. Hub Hub Two Twisted Pairs Two Twisted Pairs Station Station Station Station Station Station Interconnecting Networks • Types of devices • Repeater (physical layer): Signal regeneration - extend the range • Bridge (data link layer): MAC address filtering - reduce LAN saturation • Routers (network layer): Internet routing • Gateway (higher layers): Protocol conversion and security ?

9. Network Network Bridge LLC LLC MAC MAC MAC MAC Physical Physical Physical Physical Bridges • Operation at data link level • Implies capability to work with multiple network layers • However, need deal with • Difference in MAC formats, data rates, maximum frame length • Security • Broadcast storm • Types: transparent, source routing

10. Interconnection of IEEE LANs with complete transparency Use table lookup, and discard frame, if source & destination in same LAN forward frame, if source & destination in different LAN use flooding, if destination unknown Use backward learning to build table observe source address of arriving LANs handle topology changes by removing old entries Prevents loops in the topology Transparent Bridges S1 S2 S3 LAN1 Bridge LAN2 S6 S4 S5

11. Address Port Address Port S5 S1 S2 S3 S4 LAN1 LAN2 LAN3 B1 B2 Port 1 Port 2 Port 1 Port 2

12. S1→S5 S5 S1 S2 S3 S4 S1 to S5 S1 to S5 S1 to S5 S1 to S5 LAN1 LAN2 LAN3 B1 B2 Port 1 Port 2 Port 1 Port 2 Address Port Address Port S1 1 S1 1

13. S3→S2 S5 S1 S2 S3 S4 S3S2 S3S2 S3S2 S3S2 S3S2 LAN1 LAN2 LAN3 B1 B2 Port 1 Port 2 Port 1 Port 2 Address Port Address Port S1 1 S1 1 S3 2 S3 1

14. S4S3 S5 S1 S2 S3 S4 S4 S3 S4S3 S4S3 LAN1 LAN2 LAN3 S4S3 B1 B2 Port 1 Port 2 Port 1 Port 2 Address Port Address Port S1 1 S1 1 S3 2 S3 1 2 2 S4 S4

15. S2S1 Address Port S1 1 S3 1 2 S4 S5 S1 S2 S3 S4 S2S1 S2S1 LAN1 LAN2 LAN3 B1 B2 Port 1 Port 2 Port 1 Port 2 Address Port S1 1 S3 2 2 S4 1 S2

16. Adaptive Learning In a static network, tables eventually store all addresses & learning stops In practice, stations are added & moved all the time Introduce timer (minutes) to age each entry & force it to be relearned periodically If frame arrives on port that differs from frame address & port in table, update immediately

17. Avoiding Loops LAN1 (1) (1) B1 B2 (2) B3 LAN2 B4 LAN3 B5 LAN4

18. Spanning Tree Algorithm Select a root bridge among all the bridges. root bridge = the lowest bridge ID. Determine the root port for each bridge except the root bridge root port = port with the least-cost path to the root bridge Select a designated bridge for each LAN designated bridge = bridge has least-cost path from the LAN to the root bridge. designated port connects the LAN and the designated bridge All root ports and all designated ports are placed into a “forwarding” state. These are the only ports that are allowed to forward frames. The other ports are placed into a “blocking” state.

19. LAN1 (1) (1) B1 B2 (1) (2) (2) (3) B3 LAN2 (2) (1) B4 (2) LAN3 (1) B5 (2) LAN4

20. LAN1 (1) (1) Bridge 1 selected as root bridge B1 B2 (1) (2) (2) (3) B3 LAN2 (2) (1) B4 (2) LAN3 (1) B5 (2) LAN4

21. LAN1 R (1) (1) Root port selected for every bridge except root port B1 B2 R (1) (2) (2) (3) B3 LAN2 R (2) (1) B4 (2) R LAN3 (1) B5 (2) LAN4

22. LAN1 D R (1) (1) Select designated bridge for each LAN B1 B2 R (1) (2) (2) (3) D B3 LAN2 R (2) (1) D D B4 (2) R LAN3 (1) B5 (2) LAN4

23. LAN1 D R (1) (1) All root ports & designated ports put in forwarding state B1 B2 R (1) (2) (2) (3) D B3 LAN2 R (2) (1) D D B4 (2) R LAN3 (1) B5 (2) LAN4

24. 7.3 LAN Standards Ethernet Token Ring FDDI 802.11 Wireless LANs

25. A bit of history… Ethernet: IEEE 802.3 • 1970 ALOHAnet radio network deployed in Hawaiian islands • 1973 Metcalf and Boggs invent Ethernet, random access in wired net • 1979 DIX Ethernet II Standard • 1985 IEEE 802.3 LAN Standard (10 Mbps) • 1995 Fast Ethernet (100 Mbps) • 1998 Gigabit Ethernet • 2002 10 Gigabit Ethernet • Ethernet is the dominant LAN standard Metcalf’s Sketch

26. IEEE 802.3 MAC Protocol • CSMA/CD • Minislot Time is the critical system parameter • upper bound on time to detect collision • upper bound on time to acquire channel • upper bound on length of frame segment generated by collision • quantum for retransmission scheduling • max{round-trip propagation, MAC jam time} • Truncated binary exponential backoff algorthm • For retransmission n, select an integer r equally likely btw 0 and 2k -1 (0 <= r <= 2k -1), where k=min(n,10) • Retransmission time = (minislots time)x(r) • Give up after 16 retransmissions

27. (b) (a) transceivers IEEE 802.3 Physical Layer IEEE 802.3 10 Mbps medium alternatives Hubs & Switches! Thick Coax: Stiff, hard to work with T connectors

28.       Single collision domain High-Speed backplane or interconnection fabric (b) (a)     Ethernet Repeaters & Bridges Twisted Pair Cheap Easy to work with Reliable Star-topology CSMA-CD Twisted Pair Cheap Bridging increases scalability Separate collision domains Full duplex operation

29. Fast Ethernet IEEE 802.3 100 Mbps Ethernet medium alternatives To preserve compatibility with 10 Mbps Ethernet: • Same frame format, same interfaces, same protocols • Hub topology only with twisted pair & fiber • Bus topology & coaxial cable abandoned • Category 3 twisted pair (ordinary telephone grade) requires 4 pairs • Category 5 twisted pair requires 2 pairs (most popular) • Most prevalent LAN today

30. Gigabit Ethernet IEEE 802.3 1 Gbps Fast Ethernet medium alternatives • Slot time increased to 512 bytes • Small frames need to be extended to 512 B • Frame bursting to allow stations to transmit burst of short frames • Frame structure preserved but CSMA-CD essentially abandoned • Extensive deployment in backbone of enterprise data networks and in server farms

31. 10 Gigabit Ethernet IEEE 802.3 10 Gbps Ethernet medium alternatives • Frame structure preserved • CSMA-CD protocol officially abandoned • LAN PHY for local network applications • WAN PHY for wide area interconnection using SONET OC-192c • Extensive deployment in metro networks anticipated

32. Token Ring LAN: IEEE 802.5 • Unidirectional ring network • 4 Mbps and 16 Mbps on twisted pair • Token passing protocol provides access • Fairness • Access priorities • Breaks in ring bring entire network down • Reliability can be improved by using star topology • Relatively low speed

33. A Wiring Center E B D C Star Topology Ring LAN • Stations connected in star fashion to wiring closet • Use existing telephone wiring • Ring implemented inside equipment box • Relays can bypass failed links or stations

34. A E B C D Fiber Distributed Data Interface (FDDI) • Token ring protocol for LAN/MAN • 100 Mbps on optical fiber • Up to 200 km diameter, up to 500 stations • FDDI has option to operate in multitoken mode • Counter-rotating dual ring topology X Dual ring becomes a single ring

35. Wireless LAN (+) • Easy, low-cost deployment • Mobility & roaming: Access information anywhere • Supports personal devices • PDAs, laptops, data-cell-phones • Supports communicating devices • Cameras, location devices, wireless identification (-) • Signal strength varies in space & time • Signal can be captured by snoopers • Spectrum is limited & usually regulated

36. A C B D Wireless LAN Communications • Ad hoc mode

37. B1 A1 Gateway to the Internet Portal Distribution System Server Portal AP1 AP2 A2 B2 Basic Service Set (BSS) A BSS B • Infrastructure mode

38. IEEE 802.11 Wireless LAN • Stimulated by availability of unlicensed spectrum • U.S. Industrial, Scientific, Medical (ISM) bands • 902-928 MHz, 2.400-2.4835 GHz, 5.725-5.850 GHz • Ad Hoc & Infrastructure networks • Based on CSMA with Collision Avoidance (CA) • Why not CSMA/CD? • Cost: requires a full duplex radio • Wireless media: not all stations hear each other

39. IEEE 802.11 MAC • MAC sublayer responsibilities • Channel access • PDU addressing, formatting, error checking • Fragmentation & reassembly of MAC SDUs • MAC security service options • Authentication & privacy • MAC management services • Power management

40. MSDUs MSDUs Contention-free service Contention service Point coordination function MAC Distribution coordination function (CSMA-CA) Physical MAC Services • Contention Service: Best effort • Contention-Free Service: time-bounded transfer

41. IEEE 802.11 Physical Layer Options