CMPE 80N Spring 2003 Week 4

1. CMPE 80NSpring 2003Week 4 Introduction to Networks and the Internet

2. Announcements • HTML tutorial on 04.29 (in class). • Links to on-line HTML tutorials/tools.

3. Layer 2: Data Link Layer

4. Data Link Layer • So far, sending signals over transmission medium. • Data link layer: responsible for error-free (reliable) communication between adjacent nodes. • Functions: framing, error control, flow control, addressing, and medium access (in shared networks).

5. APPLICATION logical link control Sharing of link and transport of data over the link PRESENTATION SESSION medium access control TRANSPORT NETWORK LINK PHYSICAL Medium Access Control Protocols Coordinate competing requests for medium.

6. Medium Access Control • Problem: • Computers in a shared network environment. • Only one computer can transmit at a time. • If two computers try to use the same line at the same time, their messages get garbled. • Collision! • How can we organize the transmission so that all computers are given an opportunity to exchange messages?

7. Medium Access Control • Control access to shared medium. • How?

8. The Multiplexing Problem frequency Shared channel (how to divide resource among multiple recipients?) time Analogy: a highway shared by many users

9. Frequency-Division Multiplexing frequency user 1 user 2 user 3 user 4 guard-band time Analogy: a highway has multiple lanes

10. Time-Division Multiplexing frequency user 1 user 2 user 3 user 4 user 1 user 2 guard-band time Requirement: precise time coordination

11. Frequency-Time-Division frequency time-slot (usually of the same size) time

12. Centralized versus Distributed MAC • Centralized approaches: • Controller grants access to medium. • Simple, greater control: priorities, qos. • But, single point of failure and performance bottleneck. • Decentralized schemes: • All stations collectively run MAC to decide when to transmit.

13. Round-Robin MAC • Each station is allowed to transmit; station may decline or transmit (bounded by some maximum transmit time). • Centralized (e.g., polling) or distributed (e.g., token ring) control of who is next to transmit. • When done, station relinquishes and right to transmit goes to next station. • Efficient when many stations have data to transmit over extended period (stream).

14. Scheduled Access MAC • Time divided into slots. • Station reserves slots in the future. • Multiple slots for extended transmissions. • Suited to stream traffic.

15. Contention-Based MAC • No control. • Stations try to acquire the medium. • Distributed in nature. • Perform well for bursty traffic. • Can get very inefficient under heavy load. • NOTE: round-robin and contention are the most common.

16. MAC Protocols • Contention-based • ALOHA and Slotted ALOHA. • CSMA. • CSMA/CD. • Round-robin : token-based protocols. • Token bus. • Token ring.

17. Standardized MACs Topologies Bus Ring Techniques Token bus (802.4) Polling (802.11) Token ring (802.5; FDDI) Round robin Scheduled DQDB (802.6) Contention CSMA/CD (802.3) CSMA/CA (802.11)

18. Contention-Based MACs

19. The ALOHA Protocol • Developed @ U of Hawaii in early 70’s. • Packet radio networks. • “Free for all”: whenever station has a frame to send, it does so. • Station listens for maximum RTT for an ACK. • If no ACK, re-sends frame for a number of times and then gives up. • Receivers check FCS and destination address to ACK.

20. Collisions • Invalid frames may be caused by channel noise or • Because other station(s) transmitted at the same time: collision. • Collision happens even when the last bit of a frame overlaps with the first bit of the next frame.

21. ALOHA’s Performance 1 t0+t t0+3t t0 t0+2t Time vulnerable

22. ALOHA’s Performance 2 • S = G e-2G, where S is the throughput (rate of successful transmissions) and G is the offered load. • S = Smax = 1/2e = 0.184 for G=0.5.

23. Slotted Aloha • Doubles performance of ALOHA. • Frames can only be transmitted at beginning of slot: “discrete” ALOHA. • Vulnerable period is halved. • S = G e-G. • S = Smax = 1/e = 0.368 for G = 1.

24. ALOHA Protocols • Poor utilization. • Key property of LANs: propagation delay between stations is small compared to frame transmission time. • Consequence: stations can sense the medium before transmitting.

25. Carrier-Sense Multiple Access (CSMA) • Station that wants to transmit first listens to check if another transmission is in progress (carrier sense). • If medium is in use, station waits; else, it transmits. • Collisions can still occur. • Transmitter waits for acknowledgement (ACK); if no ACKs, retransmits.

26. CSMA/CD 1 • CSMA with collision detection. • Problem: when frames collide, medium is unusable for duration of both (damaged) frames. • For long frames (when compared to propagation time), considerable waste. • What if station listens while transmitting?

27. CSMA/CD Protocol 1. If medium idle, transmit; otherwise 2. 2. If medium busy, wait until idle, then transmit. 3. If collision detected, abort transmission. 4. After aborting, wait random time, try again.

28. Ethernet

29. Ethernet • Most popular CSMA/CD protocol. • What if a computer transmits a very long message? • It keeps the line busy for very long time, while all other computers in the LAN must wait for the long message to end • Rule #1 of resource sharing: All “messages” must be “small”, to allow other computers to access the line • For Ethernet, the maximum size of the payload is 1,500 bytes

30. Ethernet (cont’d) • What is expected performance? • When only one computer needs to transmit: it can immediately access the line. • When many computers want access (high traffic): • Average waiting time is high. • There is high probability of “collision”. • For every collision, Xmission must start agan • Fairness?. • Conclusion: expected delay depends on the traffic on the LAN!

31. Ethernet Frame Format 8 6 6 2 4 1 CRC Data Preamble Type DA Postamble SA Type: identifies upper layer protocol (for demux’ing) Data: 0-1500 bytes (min. is 46 bytes). DA and SA: destination and source addresses.

32. Round-Robin MACs • Polling. • Token passing.

33. Token Passing • Consider the following game: • A group of friends sitting in a circle • A ball is passed from friend to friend • When somebody receives the ball, it passes to the friend to his/her left • A person is allowed to talk only when s/he has the ball in his/her hands • This guarantees that only one person talks at a time!

34. Token Passing (cont’d) • Let’s make the game more difficult: • A person that receives the ball and has something to say, rather than saying it, s/he writes it on a letter • Including the name of the addressee • Before passing along the ball, s/he passes along the letter • Everyone who receives the paper passes it to the person to his/her left • If s/he is the recipient of the letter, s/he signs it after reading it • Once the letter arrives back to the sender, s/he throws it away • The ball is still circulating in the circle

35. Token Ring • A Token Ring MAC works similarly: • A special pattern (3-bytes word) of bits called token moves from one computer to the next. • If a computer does not have a message to send, it just passes the token along. • Otherwise, it “seizes the token” and transmits its message (including the address) instead. • The message is passed from one computer to the next, until it arrives back to the sender, which “destroys” it (does not pass it along anymore). • The addressee may “write” something on the message so that the sender knows it has been received correctly. • Once the computer is done transmitting the message, it “releases” (transmits) the token.

36. Token Ring (cont’d) • When station wants to transmit: • Waits for token. • Seizes it. • Transmits frame. • When station seizes token and begins transmission, there’s no token on the ring; so nobody else can transmit.

37. Token Ring (cont’d) • What is expected performance of Token Passing? • It is fair. • Each computer is given in turn an opportunity to transmit, even when the traffic is high. • However, even if only one computer needs to transmit a message, it has to wait that it receives the token. • Again, long messages should not be allowed, because otherwise one computer may “hold the token” for too long.

38. Token Ring Frame Format 1 1 2 or 6 4 1 1 2 or 6 1 SD DA FCS AC FC SA Data ED FS SD Token frame AC FC SD: starting delimiter; indicates starting of frame. AC: access control; PPPTMRRR; PPP and RRR priority and reservation; M monitor bit; T token or data frame. FC: frame control; if LLC data or control. DA and SA: destination and source addresses. FCS: frame check sequence. ED: ending delimiter; contains the error detection bit E; contains frame continuation bit I (multiple frame transmissions). FS: frame status.

39. Ethernet versus Token Ring • Token ring: • Efficient at heavy traffic. • Guaranteed delay. • Fair. • But, ring/token maintenance overhead. • But, under light traffic? • Ethernet is simple!

40. Wireless LANs • IEEE 802.11. • Distributed access control mechanism (DCF) based on CSMA with optional centralized control (PCF). Contention-free Service (polling) MAC layer PCF Contention Service (CSMA) DCF Physical Layer

41. MAC in Wireless LANs • Distributed coordination function (DCF) uses CSMA-based protocol (e.g., ad hoc networks). • CD does not make sense in wireless. • Hard for transmitter to distinguish its own transmission from incoming weak signals and noise. • Point coordination function (PCF) uses polling to grant stations their turn to transmit (e.g., cellular networks).