Advanced Computer Networks: Lecture 12. This lecture will cover the following: fiber distributed data interface; timed token algorithm; FDDI traffic classes; bounded priority traffic; token maintenance procedure; wireless LANs; direct sequence spread spectrum;...
CS716 Advanced Computer Networks By Dr. Amir Qayyum Lecture No. 12 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) (a) (b) 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 Upstream neighbor (DAS) Downstream neighbor (DAS) Concentrator (DAS) SAS SAS SAS SAS 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 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 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 TTRT – Token is late – Can not send data MAC Algorithm • If measuredTRT = (TTRT – measured TRT) 10 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 15 Bounded Priority Traffic • Worse case: 2xTTRT between seeing token – One TTRT is consumed first by asynchronous data – Another TTRT is then consumed by synchronous data • Backtoback 2xTTRT rotations not possible 16 Token Maintenance • Monitoring for the lost token – No token when initializing ring – Bit errors corrupt token pattern – Node holding the token crashes 17 Token Maintenance • Monitoring for a valid token – Should periodically see valid transmission (frame or token) – Max. gap = ring latency + max frame