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

2. Prof. Rick Han, University of Colorado at Boulder 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

3. Prof. Rick Han, University of Colorado at Boulder 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

4. Prof. Rick Han, University of Colorado at Boulder 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+5T prop /T trans ), ~ 70% for common values of prop. delay and max pkt size, ->100% for small prop. delays & small pkts

5. Prof. Rick Han, University of Colorado at Boulder 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

6. Prof. Rick Han, University of Colorado at Boulder Physical Layers of 802.11 Variants What does 802.11 use for its physical layer? 2.4 GHz Freq. Hop 1,2 Mbps 2.4 GHz Dir. Seq. 1,2 Mbps Infrared 1,2 Mbps 2.4 GHz Dir. Seq. 5.5,11 Mbps 5 GHz OFDM 6-54 Mbps Original 802.11 Standard 802.11b802.11a 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

7. Prof. Rick Han, University of Colorado at Boulder 802.11b: Direct Sequence Spread Spectrum Multiply data bit stream d(t) by a faster chipping sequence c(t) : BPSK example +1/-1 Chipping Sequence c(t) time +1 100110 110011101001110010 Data d(t) Chipping sequence c(t) also called Pseudo-Noise (PN) spreading sequence depending on usage +1 time

8. Prof. Rick Han, University of Colorado at Boulder Direct Sequence Sender Chipping Sequence c(t) time +1 100110 110011101001110010 Data d(t) +1 d(t)*c(t) +1 time

9. Prof. Rick Han, University of Colorado at Boulder Direct Sequence Receiver Receiver also has c(t) time +1 100110 110011101001110010 d(t)*c(t)*c(t) = Data d(t), since c(t)*c(t) = 1! +1 time Receive d(t)*c(t) +1 time

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

11. Prof. Rick Han, University of Colorado at Boulder 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 c 1 (t) between a base station and user 1, c 2 (t) between base station and user 2, … Base station sends d 1 (t)*c 1 (t) + d 2 (t)*c 2 (t) Ideally, choose c 1 (t) to be orthogonal to c 2 (t), i.e. c 1 (t)*c 2 (t) =0 (reality: only ~orthogonal) At receiver 1, received signal is multiplied by c 1 (t): c 1 (t)*[d 1 (t)*c 1 (t) + d 2 (t)*c 2 (t)] = d 1 (t)! CDMA: multiple data streams simultaneously access the same medium using ~orthogonal DSSS codes

12. Prof. Rick Han, University of Colorado at Boulder 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

13. Prof. Rick Han, University of Colorado at Boulder 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!

14. Prof. Rick Han, University of Colorado at Boulder 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) = d 1 (t)*cos(2  t) + d 2 (t)*cos (2  t) 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

15. Prof. Rick Han, University of Colorado at Boulder 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

16. Prof. Rick Han, University of Colorado at Boulder Token Ring (2) Links are unidirectional Each node has a downstream neighbor and an upstream neighbor Token Ring 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 010010 “Token”

17. Prof. Rick Han, University of Colorado at Boulder 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 Token Ring 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! 010010 “Token”

18. Prof. Rick Han, University of Colorado at Boulder Token Ring (4) When a node has a frame to send, it takes token, and transmits frame downstream Token Ring 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 010010 “Token” 1110011010 Data Frame

19. Prof. Rick Han, University of Colorado at Boulder Token Ring Example Token Ring 010010 “Token” DestinationSource 1110011010 Data Frame1110011010 Data Frame (2) 1110011010 Data Frame (5) 1110011010 Data Frame (4) 1110011010 Data Frame (3) 1110011010 Data Frame (6) (1) (7) Stop Data Frame

20. Prof. Rick Han, University of Colorado at Boulder Token Ring’s Robustness To Failure A given node can fail at any time: Without the token With the token Token Ring 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 010010 “Token” 1110011010 Data Frame

21. Prof. Rick Han, University of Colorado at Boulder 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 Token Ring If a destination node fails without the token: Sender receives unmarked frame, and can retransmit it later 010010 “Token” 1110011010 Data Frame Destination

22. Prof. Rick Han, University of Colorado at Boulder 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 Token Ring 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” 010010 “Token”

23. Prof. Rick Han, University of Colorado at Boulder 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

24. Prof. Rick Han, University of Colorado at Boulder 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

25. Prof. Rick Han, University of Colorado at Boulder FDDI 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 1110011010 Data Frame