1. 1 CS716 Advanced Computer Networks By Dr. Amir Qayyum

2. Lecture No. 12

3. 3 Fiber Distributed Data Interface Similar to 802.5/IBM token rings but runs on fiber Consists of a dual ring: two independent rings that transmit data in opposite directions at 100Mbps Tolerates a single link break or node failure (self- healing ring)

4. 4 FDDI - Concentrator Allows nodes to attach using a single cable - SAS Dual connected nodes still exist - DAS Concentrator attaches several SASs to dual ring –Uses optical bypass to isolate failed SAS Downstream neighbor (DAS) Upstream neighbor (DAS) Concentrator (DAS) SAS

5. 5 FDDI – Physical Properties Variable size buffer (9 - 80 bits) between input and output interfaces (10 ns bit time) –Not required to fill buffer before starting transmission Maximum 500 stations, maximum 2 km distance between any pair of stations

6. 6 FDDI – Physical Properties Total 200 km fiber: dual nature implies 100 km cable connecting all stations Physical media can be coax or twisted pair cable Uses 4B/5B encoding

7. 7 Timed Token Algorithm Token Holding Time (THT) –Upper limit on how long a station can hold the token –configured to some suitable value Token Rotation Time (TRT) –How long it takes the token to traverse the ring (time since a host released the token) –TRT <= ActiveNodes x THT + RingLatency

8. 8 Timed Token Algorithm Target Token Rotation Time (TTRT) –“agreed-upon” or negotiated upper bound on TRT

9. 9 MAC Algorithm Each node measures TRT between successive token arrivals If measured-TRT > TTRT –Token is late –Can not send data

10. 10 MAC Algorithm If measured-TRT < TTRT –Token is early so OK to send –Send data for remaining time until either No more data to send THT >= (TTRT – measured TRT)

11. 11 FDDI MAC Issue If a node has lots of data, it holds the token for the maximum allowed time When a downstream neighbor gets the token, its measured TRT >= TTRT –It cannot transmit its frame!

12. 12 FDDI MAC Issue What if the downstream neighbor has some urgent data to send ?

13. 13 FDDI Traffic Classes Synchronous traffic –Latency sensitive –Gets higher priority –Can always send data

14. 14 FDDI Traffic Classes Asynchronous traffic –Sensitive to throughput rather than delay –Lower priority –Can send only if token is early May cause the time to exceed by one FDDI frame

15. 15 Bounded Priority Traffic If a node has large amount of synchronous data –It will send regardless of measured TRT –TTRT will become meaningless !!! Therefore, total synchronous data during one token rotation is bounded by TTRT

16. 16 Bounded Priority Traffic Worse case: 2 x TTRT between seeing token –One TTRT is consumed first by asynchronous data –Another TTRT is then consumed by synchronous data Back-to-back 2 x TTRT rotations not possible

17. 17 Token Maintenance Monitoring for the lost token –No token when initializing ring –Bit errors corrupt token pattern –Node holding the token crashes

18. 18 Token Maintenance Monitoring for a valid token –Should periodically see valid transmission (frame or token) –Max. gap = ring latency + max frame <= 2.5ms Set 2.5ms timer; start negotiations if it fires

19. 19 Token Maintenance The procedure when a node –Joins the ring (startup) –Suspects a failure Claim frame is used in order to –Generate a new Token –Agree on TTRT (so that an application can meet its timing constraints) A node can send a claim frame without holding the token

20. 20 Token Maintenance Procedure A node sends a claim frame including its TTRT bid When a node receives a claim frame, it compares the bid with its own bid –If its bid is higher, it updates TTRT & forward the frame –If its bid is lower, it replaces with its own claim frame –If bids are equal, higher address node wins

21. 21 Token Maintenance Procedure If a node’s claim frame returns back to it, it knows: –Its bid was the lowest –Everyone knows TTRT –It can now insert new token

22. 22 Frame Format 4B/5B control symbols for start and end of frame Control Field –1st bit: asynchronous (0) versus synchronous (1) data –2nd bit: 16-bit (0) versus 48-bit (1) addresses –Last 6 bits: demux key (includes reserved patterns for token and claim frame) Status Field –From receiver back to sender; error in frame –Recognized address; accepted frame (flow control) BodyCRC Src addr Variable48 Dest addr 4832 End of frame 8 Status 24 Control 8 Start of frame 8

23. 23 Feedback Error detection –Host attaches “error” marker to frame –Sender detects error marker, resends later Flow control –Host attaches “my address but did not copy” –Sender detects problem, resends later (backs off)

24. 24 Wireless LANs

25. 25 Wireless LANs IEEE 802.11 standard –Designed for use in a small area (offices, campuses) Bandwidth: 1, 2 or 11 Mbps –Up to 56Mbps in newer 802.11a standard Targets three physical media –Two spread spectrum radio (2.4GHz freq) –One diffused infrared (10m range, 850 nm band)

26. 26 Spread Spectrum Spread signal over wider freq band –Uses more frequency spectrum than strictly necessary Originally designed to thwart jamming/ interference –Pseudo-random sequence, signal looks like a noise Introduce pseudo-random component into signal

27. 27 Spread Spectrum Sender and receiver share –Pseudorandom number generator and the seed Frequency Hopping –Transmit over pseudo-random sequence of frequencies –802.11 uses 79 x 1MHz-wide frequency bands

28. 28 Direct Sequence Spread Spectrum For each bit, send XOR of the bit and n random bits Random sequence is known to sender and receiver n random bits are called n-bit chipping code

29. 29 Direct Sequence Spread Spectrum 802.11 defines an 11-bit chipping code 83MHz band Random sequence: 0100101101011001 Data stream: 1010 XOR of the two: 1011101110101001 0 0 0 1 1 1