Adaptive tree walk: collision resolution

CS3502:Data and Computer NetworksLoca Area Networks - 3Adaptive Tree Walk Token Ring LAN : IEEE 802.5 slotted ringsFDDI

Adaptive tree walk: collision resolution • 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 • assume 8-node network as below; suppose 1, 3, 5 and 7 collide. (“X” is initial collision). x 0 1 3 5 7

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

adaptive tree walk, comments • entire tree must be searched. (why?) • why use depth first search, not breadth first? • ex: suppose 0,1,2,6,7 collide. Show resolution. • 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? • what if another station, not in initial collision, wants to transmit?

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

Token Ring • access controlled by token

Token Ring Network - general • each station receives a bit, checks it, and passes it on; this introduces delay of about 1-1.5 bits per station • note that a station does not receive entire message before passing it on • only node with token transmits; others repeat. • frames removed by station that generated them • THT (token holding timer) limits time a station holds the token • token passed when (a) no more data or (b) THT expires

token ring network - general • transmission medium • 802.5 standard specifies twisted pair; • FDDI standard specifies optical fiber or twisted pair (restrictions). • coaxial cable or wireless media (eg, infrared) could also use the protocol, though none are standardized • transmission rates • 802.5 specifies 4 or 16 Mbps; differential Manchester encoding • FDDI specifies 100 Mbps; 4/5 encoding • 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?

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

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

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

token ring : priorities • priority bits indicate the priority of a frame or token • only frames with priority n >= P(token) may be transmitted; (priorities = 0,1,...,7. ) • frame with priority frame indicates it by raising the RRR bits in passing frames. • 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 802.5 standard • standard has 3 finite state machines : • operational FSM • active monitor FSM • standby monitor FSM • operational FSM has 6 states: T0 Repeat T1 Tx Data_Fr T2 Tx Fill T3 Tx Fill & Strip frames T4 Tx 0s, modify stacks T5 Tx fill, strip SFS

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

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

Adaptive tree walk: collision resolution