Chapter 2 More on Wireless Ethernet, Token Ring, FDDI

1. Chapter 2More on Wireless Ethernet, Token Ring, FDDI Professor Rick Han University of Colorado at Boulder rhan@cs.colorado.edu

2. Announcements • Previous lecture now online • Homework #1 is on the Web site, due Feb. 5 • Programming assignment #1 is now available on Web site, due Feb. 19 (3 weeks) • Next, Chapter 2, more on Wireless Ethernet, Token Ring, FDDI Prof. Rick Han, University of Colorado at Boulder

3. Recap of Previous Lecture • Multiple Access Protocols • Designed for shared-media links • Channel reservation protocols: TDMA, FDMA, CDMA • Random access protocols: CSMA/CD (Ethernet), CSMA/CA (802.11 wireless Ethernet) • Random Access Protocols • ALOHA, slotted ALOHA – packet collisions • CSMA – “listen before you talk” • CSMA/CD – “listen while you talk” • CSMA/CA – see next slide Prof. Rick Han, University of Colorado at Boulder

4. 802.11 MAC Layer • Uses CSMA/CA = CSMA + Collision Avoidance • Collision Avoidance equated with exponential backoff • Hidden terminal RTS/CTS is required feature but may be disabled • 802.11’s CSMA/CA is called the Distributed Coordination Function (DCF) • Useful to send non-delay-sensitive data such as Web, ftp, email <- asynchronous traffic • 802.11b’s MAC is ~70% efficient • slotted ALOHA ~37% • Ethernet’s efficiency: ~ 1/(1+5Tprop/Ttrans), • ~ 70% for common values of prop. delay and max pkt size, • ->100% for small prop. delays & small pkts Prof. Rick Han, University of Colorado at Boulder

5. 802.11 MAC Layer (2) • Contention in CSMA causes delay • Point Coordination Function (PCF) Mode gives delay-sensitive traffic priority over asynchronous traffic • Useful for interactive audio/video • Define a “superframe”. Delay-sensitive traffic gets access to first part of superframe via shorter random wait times. • Inside the first part of superframe, a central PCF master polls each user with delay-sensitive data • In second part of superframe, asynchronous data is carried • Built on top of DCF Prof. Rick Han, University of Colorado at Boulder

6. 2.4 GHz Dir. Seq. 5.5,11 Mbps 5 GHz OFDM 6-54 Mbps 2.4 GHz Freq. Hop 1,2 Mbps 2.4 GHz Dir. Seq. 1,2 Mbps Infrared 1,2 Mbps Physical Layers of 802.11 Variants • What does 802.11 use for its physical layer? Original 802.11 Standard 802.11b 802.11a Also, 802.11g at 2.4 GHz, OFDM or PBCC, up to 54 Mbps. 802.11a @ 5 GHz ok in U.S., but conflicts abroad Prof. Rick Han, University of Colorado at Boulder

7. 1 0 0 1 1 0 802.11b: Direct Sequence Spread Spectrum • Multiply data bit stream d(t) by a faster chipping sequence c(t) : BPSK example +1/-1 +1 time Data d(t) -1 110011101001110010 +1 Chipping Sequence c(t) time -1 • Chipping sequence c(t) also called Pseudo-Noise (PN) spreading sequence depending on usage Prof. Rick Han, University of Colorado at Boulder

8. 1 0 0 1 1 0 Direct Sequence Sender +1 time Data d(t) -1 110011101001110010 +1 Chipping Sequence c(t) time -1 +1 d(t)*c(t) time -1 Prof. Rick Han, University of Colorado at Boulder

9. 1 0 0 1 1 0 Direct Sequence Receiver +1 Receive d(t)*c(t) time -1 110011101001110010 +1 Receiver also has c(t) time -1 +1 d(t)*c(t)*c(t) = Data d(t), since c(t)*c(t) = 1! time -1 Prof. Rick Han, University of Colorado at Boulder

10. Direct Sequence Spreads the Spectrum • Benefit of modulating data d(t) by chipping sequence: spreading the spectrum to improve immunity to noise and fading Spectrum of data d(t) frequency Spectrum of chipping sequence c(t) frequency Spectrum of d(t)*c(t) frequency Prof. Rick Han, University of Colorado at Boulder

11. CDMA Employs Direct Sequence • Each c(t) can be looked upon as a code that only the sender and receiver pair both know • Assign code c1(t) between a base station and user 1, c2(t) between base station and user 2, … • Base station sends d1(t)*c1(t) + d2(t)*c2(t) • Ideally, choose c1(t) to be orthogonal to c2(t), i.e. c1(t)*c2(t) =0 (reality: only ~orthogonal) • At receiver 1, received signal is multiplied by c1(t): c1(t)*[d1(t)*c1(t) + d2(t)*c2(t)] = d1(t)! • CDMA: multiple data streams simultaneously access the same medium using ~orthogonal DSSS codes Prof. Rick Han, University of Colorado at Boulder

12. CDMA Employs Direct Sequence (2) • Original 802.11 at 1 Mbps • used 11 chips/bit (Barker sequence), and BPSK (+1/-1 signalling) for 11 Mcps, or 11 MHz • 802.11b is more sophisticated: • 8 chips per symbol, and 8 bits/symbol, chipping rate is 11 MHz = 1.375 Msps = 11 Mbps • 2.4 GHz ISM band has 14 channels (11 in U.S.) • Each channel occupies 22 Mhz. Within each channel, uses Direct Sequence CDMA Prof. Rick Han, University of Colorado at Boulder

13. 802.11 Specifics (2) • 2.4 GHz ISM band has 14 channels (11 in U.S.) • Interference from adjacent Access Points (AP) or base stations: Only 3 channels (1,6,11) are non-overlapping • reuse frequencies in beehive pattern to avoid degraded throughput • Interference from Bluetooth, microwaves, garage door openers – unlicensed spectrum! Prof. Rick Han, University of Colorado at Boulder

14. 802.11a: OFDM • OFDM = Orthogonal Frequency Division Multiplexing • Special case of Multi-Carrier Modulation (MCM), or Discrete Multi-Tone (DMT) • Divide data bit stream d(t) over different frequencies. For example: • Transmit(t) = d1(t)*cos(2p3000t) + d2(t)*cos (2p6000t) • 48 subcarriers in 802.11a over a 20 MHz channel • Delivers better performance than DSSS, especially indoors • High spectral efficiency, resistance to multipath, … • Various flavors of DSL also employ this technique Prof. Rick Han, University of Colorado at Boulder

15. Token Ring • Not very popular, even being phased out at IBM – primarily of historical interest • Why did Ethernet win? “Cheaper and good enough” • Conceptual Topology of Token Ring: Token Ring Ethernet Prof. Rick Han, University of Colorado at Boulder

16. Token Ring (2) • Links are unidirectional • Each node has a downstream neighbor and an upstream neighbor • Topology resembles N point-to-point links forming a ring rather than continuous wire loop • but access to ring is shared via tokens • A “token” is a special flag that circulates around the ring Token Ring 010010 “Token” Prof. Rick Han, University of Colorado at Boulder

17. 010010 “Token” Token Ring (3) • Each node receives token, then transmits it to its downstream neighbor • Round-robin ensures fairness, as every node eventually can transmit when it receives token • Suppose token was passed from source to destination rather than around the ring as in Token Ring • some hosts could be passed over indefinitely – unfair! Token Ring Prof. Rick Han, University of Colorado at Boulder

18. 010010 1110011010 “Token” Data Frame Token Ring (4) • When a node has a frame to send, it takes token, and transmits frame downstream • Each node receives a frame and forwards it downstream • Destination host saves copy of frame, but keeps forwarding frame. • Inefficient • Forwarding stops when frame reaches original source Token Ring Prof. Rick Han, University of Colorado at Boulder

19. (2) (3) (4) 1110011010 1110011010 1110011010 1110011010 1110011010 010010 1110011010 (5) (6) Data Frame “Token” Data Frame Data Frame Data Frame Data Frame Data Frame Token Ring Example Destination Source (1) Token Ring (7) Stop Data Frame Prof. Rick Han, University of Colorado at Boulder

20. 010010 1110011010 “Token” Data Frame Token Ring’s Robustness To Failure • A given node can fail at any time: • Without the token • With the token • If a node fails without the token: • An electromechnical relay closes at failing node, keeping the ring intact • Data frame continues to be forwarded as before Token Ring Prof. Rick Han, University of Colorado at Boulder

21. 010010 1110011010 “Token” Data Frame Token Ring’s Robustness To Failure (2) • In Token Ring, when frame reaches a destination node, it is marked as read • When marked-as-read frame reaches sender, it acts as “ACK” to sender Destination • If a destination node fails without the token: • Sender receives unmarked frame, and can retransmit it later Token Ring Prof. Rick Han, University of Colorado at Boulder

22. 010010 “Token” Token Ring’s Robustness To Failure (3) • If a node fails with the token, then the ring must somehow introduce a new token • After a timeout, in which no token is detected, a “designated monitor” introduces a new token • If designated monitor fails • Its periodic keep-alive not detected • A node sends “claim” token around ring • If claim token returns to sender, then sender becomes “designated monitor” Token Ring Prof. Rick Han, University of Colorado at Boulder

23. Token Ring : Other Points • Token Holding Time (THT) by default is 20 ms • Token Ring data rates are 4 and 16 Mbps • If a token is held until data frame returns, then called “delay-release” • Inefficient, original version of 802.5 • Solution: release token as soon as send has transmitted data frame • More efficient, called “early release”, now supported in later version of 802.5 • Token Rotation Time <= (# Nodes)*THT + Ring Latency Prof. Rick Han, University of Colorado at Boulder

24. FDDI • Fiber Distributed Data Interface • Dual ring topology originally using optical fibers instead of copper wire • 100 Mbps • Second ring helps with robustness/ fault recovery • Some nodes may be part of only one ring: single attachment station (SAS) FDDI Prof. Rick Han, University of Colorado at Boulder

25. 1110011010 Data Frame FDDI (2) • Recall the inefficiency of Token Ring: frames are forwarded even after they’ve reached destination • Solution: in FDDI, destination node removes frame from ring Destination FDDI Prof. Rick Han, University of Colorado at Boulder