CS3502: Data and Computer Networks Loca Area Networks - 3 Adaptive Tree Walk Token Ring LAN : IEEE slotted rings FDDI

Adaptive tree walk: collision resolution u alternative to binary exponential back-off Basic idea: 1. network nodes grouped as leaves of a binary tree T 2. time after collision measured in slots. 3. tree T searched depth first to resolve collisions; each time slot corresponds to a node of the tree --> 1st collision is “root” of tree --> during each time slot, the stations which have that node as an ancestor transmit. Another collisions go down otherwise go up.

adaptive tree walk example u assume 8-node network as below; suppose 1, 3, 5 and 7 collide. (“X” is initial collision) x

adaptive tree walk example u in 1st time slot following collision, 1 and 3 collide again --> go down 1 level u in next slot 1 transmits without collision --> branch up and over u next slot, 3 transmits without collision --> branch up and over u next slot, 5 and 7 collide --> go down 1 level u next slot, 5 transmits without collision u next, 7 transmits without collision

adaptive tree walk, comments u entire tree must be searched. (why?) u why use depth first search, not breadth first? u ex: suppose 0,1,2,6,7 collide. Show resolution. u is there an upper bound on time to resolve collisions? If so what? If used with CSMA/CD, does this alter U.B. on time to transmit? u what if another station, not in initial collision, wants to transmit?

LANs : Token Ring Network u IEEE standard 802.5; originated at IBM Zurich, early 1970s. u topology: a ring, that is a series of unidirectional point-to-point links forming a cycle. u each station receives bits from its “upstream” neighbor, and repeats them to its “downstream” neighbor. u only one station, which holds the “token” transmits at a time. All others receive and repeat.

Token Ring u access controlled by token

Token Ring Network - general u each station receives a bit, checks it, and passes it on; this introduces delay of about bits per station u note that a station does not receive entire message before passing it on u only node with token transmits; others repeat. u frames removed by station that generated them u THT (token holding timer) limits time a station holds the token u token passed when (a) no more data or (b) THT expires

token ring network - general u transmission medium u standard specifies twisted pair; u FDDI standard specifies optical fiber or twisted pair (restrictions). u coaxial cable or wireless media (eg, infrared) could also use the protocol, though none are standardized u transmission rates u specifies 4 or 16 Mbps; differential Manchester encoding u FDDI specifies 100 Mbps; 4/5 encoding u could be done at higher rates on fiber, though not standardized (yet)

Token Ring Network Q : is T.R. a broadcast network? physically? in any other sense?

Token Ring : frame formats token SD: JK0JK000 ED: JK1JK1IE frame AC: access control (8) FS: frame status (8) SD AC ED FC DA SAINFOFCS ED FSSDAC P P PR R R T M A C r r A C r r

token ring : notes on frame formats u “I” bit : intermediate frame (more follow if set) u J,K bits : violations of diff. manchester encoding; no transition in middle of bit; mark start, end of frame. u P, R bits : priority, reservation bits u FS : frame control. For control purposes. (details not included here!) u FS field: A : address recognized; C : frame copied

toke n ring : monitor u 1 station is “elected” as the “active monitor” (AM), or just “monitor” u the other stations are “standby monitors” u monitor performs routine operations such as watching for u lost token u broken connection/ring u garbled frames u latency buffer u timers (TAM,TSM) used for this purpose

token ring : priorities u priority bits indicate the priority of a frame or token u only frames with priority n >= P(token) may be transmitted; (priorities = 0,1,...,7. ) u frame with priority frame indicates it by raising the RRR bits in passing frames. u when a station with the token sees that the priority bits are set, it raises the priority of the token and releases it. Same station that raises P must eventually lower it.

IEEE standard u standard has 3 finite state machines : u operational FSM u active monitor FSM u standby monitor FSM u operational FSM has 6 states: T0 Repeat T1 Tx Data_Fr T2 Tx Fill T3Tx Fill & Strip frames T4Tx 0s, modify stacks T5 Tx fill, strip SFS

token ring : performance u light load u heavy load u overhead in token passing - small u no time lost due to collisions u can approach 100% when all stations have data u even when load more than 100%, no time wasted on collisions u what are best, worst case time waiting to transmit? Compare to CSMA/CD.

slotted rings u same topology as token ring, different protocol u rather than a single “token”, slots continuously circulate around the ring. u a bit in the front marks the slot as “empty” or “full” u stations with data to transmit can write into the empty slots u slot removed by the receiving station u must insure that slots don’t overrun each other u potentially higher throughput than token ring