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1 Lecture Topics Direct Link Networks: Multiaccess Protocols (.7) Multiaccess control IEEE 80.5 Token Ring and FDDI CS/ECpE 556: Computer Networks Originally by Scott F. Midkiff (ECpE) Modified by Marc Abrams (CS) Virginia Tech courses.cs.vt.edu/~cs556 ECPE/CS 556 (//00) Direct Link Networks: Multiaccess Protocols - Multiaccess Communication () Previous discussion considered point-to-point links Received signal is transmitted signal (plus noise) Many networks are such that received signal at one node depends on transmitted signal at two or more other nodes Satellite systems Radio networks Multi-tap bus systems Multiaccess Communication () Multiaccess media are communication media where received signal is sum of attenuated transmitted signals plus effects of delay, distortion, and noise Examples: Multitap bus (Ethernet) Radio (wireless) network ECPE/CS 556 (//00) Direct Link Networks: Multiaccess Protocols - ECPE/CS 556 (//00) Direct Link Networks: Multiaccess Protocols - Medium Access Control -- MAC () With multiaccess media, protocol is needed to coordinate sharing of media Medium access control (MAC) protocol performs this function MAC is sublayer between data link control (DLC) layer and physical layer (usually grouped with DLC) Medium Access Control -- MAC () LLC provides link to adjacent node MAC coordinates access to shared media Physical provides hardware interface Data Link LLC MAC physical ECPE/CS 556 (//00) Direct Link Networks: Multiaccess Protocols - 5 ECPE/CS 556 (//00) Direct Link Networks: Multiaccess Protocols , Scott F. Midkiff
2 Medium Access Control -- MAC () Separation of layer functions in multiaccess networks is not as well-defined as in networks with point-to-point links Feedback about errors is part of ARQ strategy of DLC, but may depend on how media is shared Flow and congestion control needed to provide fair, efficient access to shared media Broadcast nature of shared media implements some routing functions Token Ring Networks Token ring networks are common form of LAN & MAN IEEE 80.5 (Token Ring): Mbps or 6 Mbps Fiber Distributed Data Interface (FDDI): 00 Mbps ECPE/CS 556 (//00) Direct Link Networks: Multiaccess Protocols - 7 ECPE/CS 556 (//00) Direct Link Networks: Multiaccess Protocols - 8 Nodes Arranged in Ring Topology Nodes Arranged in Ring Topology 7 8 Node receives bit stream from last node relays bit stream to next node Node can repeat or replace each bit interface logic Point-to-point links between stations At least bit delay at each node: Propagation Processing Regeneration & transmission ECPE/CS 556 (//00) Direct Link Networks: Multiaccess Protocols - 9 ECPE/CS 556 (//00) Direct Link Networks: Multiaccess Protocols - 0 Token () To transmit its own data, node must discard input & output its data interface logic But we can't discard data until it has reached its destination Token is used to coordinate use of ring Ring is shared medium, so network is multiaccess system Token () Conceptually, token is passed from node to node Only send your data when you've got token Pass token when data reaches destination or you've got no data to send So what is a token anyway? Special pattern -- distinguished from data Similar to framing flags ECPE/CS 556 (//00) Direct Link Networks: Multiaccess Protocols - ECPE/CS 556 (//00) Direct Link Networks: Multiaccess Protocols - 999, Scott F. Midkiff
3 Token () Token can be in states Free token (or idle token): ring available Discard bits following free token Busy token: ring in use Follow busy token with data Token indicates upcoming data (if busy), as well as permission to transmit (if free) Basic Token Ring Operation () When node with data to transmit receives free token, it marks token as busy and appends its own data Subsequent nodes forward data since token is marked busy Destination node both forwards and stores data Destination node may mark data as received, but token is still busy Data returns to originating node where it is discarded ECPE/CS 556 (//00) Direct Link Networks: Multiaccess Protocols - ECPE/CS 556 (//00) Direct Link Networks: Multiaccess Protocols - Basic Token Ring Operation () Ring Example () After node finishes transmission, it marks token as free forwards token next node follows token with idle fill (until it sees busy or idle token again) data busy Node receives free token Node transmits busy token followed by data for node ECPE/CS 556 (//00) Direct Link Networks: Multiaccess Protocols - 5 ECPE/CS 556 (//00) Direct Link Networks: Multiaccess Protocols - 6 Ring Example () Ring Example () idle free token Busy token followed by data continues around ring, node stores data Busy token completes round trip and is stripped at node Node strips old data from ring and transmits new data until finished When finished, node puts free token on ring, followed by idle fill ECPE/CS 556 (//00) Direct Link Networks: Multiaccess Protocols - 7 ECPE/CS 556 (//00) Direct Link Networks: Multiaccess Protocols , Scott F. Midkiff
4 Ring Example () Ring Example (5) Node forwards free token (no data to send), node still storing data Node receives all of data from, forwards free token (no data to send) Node receives free token, transmits busy token followed by data Node forwards bits (busy token) after its last data bit arrives ECPE/CS 556 (//00) Direct Link Networks: Multiaccess Protocols - 9 ECPE/CS 556 (//00) Direct Link Networks: Multiaccess Protocols - 0 Additional Details of Ring Operation Propagation delay around ring must be long enough to store complete token Otherwise first part of free token would be discarded to transmit last part Error detection Receiving node can check CRC and put an ACK or NAK in packet trailer on its way back to sender Sending node can also check CRC since it sees all transmitted data Numerous variations are possible in ring operation Token Holding Time How long can node hold free token? Option : Transmit only packet Lets token rotates at maximum rate Minimizes latency Option : Transmit all waiting packets Reduces token transmission overhead Maximizes throughput : Transmit waiting packets up to time limit Best of both worlds ECPE/CS 556 (//00) Direct Link Networks: Multiaccess Protocols - ECPE/CS 556 (//00) Direct Link Networks: Multiaccess Protocols - Retransmission Schemes () options in handling retransmissions Selfless operation (FDDI): Give up token when done transmitting; if error detected, reacquire token & retransmit Selfish (IEEE 80.5): Hold token for round trip time, to be sure receiver got data correctly Pros/Cons Retransmission Schemes () Selfless (FDDI): Penalty: higher latency on error Selfish (80.5): Ring transmits idle fill until sender gets ack Penalty: lower throughput for low error rates Advantage: Lower latency for retransmissions ECPE/CS 556 (//00) Direct Link Networks: Multiaccess Protocols - ECPE/CS 556 (//00) Direct Link Networks: Multiaccess Protocols - 999, Scott F. Midkiff
5 Type of Token Failures () Lost token -- no node can transmit! Corrupted by noise (bit errors alter token code) Node holding token fails Token is permanently marked busy -- no node can transmit Idle token corrupted by noise (is marked busy) Multiple tokens created -- conflicts for access Non-token corrupted by noise to become token Node failure Ring protocol recognizes token failures & recovers (e.g. generating new token) ECPE/CS 556 (//00) Direct Link Networks: Multiaccess Protocols - 5 Fiber Distributed Data Interface -- FDDI 00 Mbps timed token ring network based on fiber optics Developed under auspices of ANSI committee XT9 formed in 98 Limited popularity Lack of high BW apps in 980's High cost of NIC's ($5000) and concentrators ($$) Was popular for backbones & switching fabric ECPE/CS 556 (//00) Direct Link Networks: Multiaccess Protocols - 6 FDDI Standard LLC Data Link Physical MAC PHY PMD SMT LLC Logical Link Control MAC Media Access Control PHY Physical PMD Physical Media Dependent SMT Station Management FDDI Versus Token Ring Token Ring: sender waits until all of transmitted data goes round ring before releasing token FDDI: sending node releases token after sending last bit of data Busy token not sent Data frame header recognized as busy token Improves FDDI s throughput FDDI supports low-priority (asynchronous) and high-priority (synchronous) packets Guarantees throughput and latency Suitable for digitized voice, real-time control, etc. ECPE/CS 556 (//00) Direct Link Networks: Multiaccess Protocols - 7 ECPE/CS 556 (//00) Direct Link Networks: Multiaccess Protocols - 8 Capacity Allocation in FDDI () Capacity allocation for high-priority data is provided by timed token scheme Each node measures times between token arrivals Low-priority traffic can be sent only if intertoken time is sufficiently small High-priority traffic can be sent anytime token arrives Limited amount of high priority traffic can be sent for each token arrival (token holding time is limited) Guaranteed transmit time a i is allocated to node i ECPE/CS 556 (//00) Direct Link Networks: Multiaccess Protocols - 9 Capacity Allocation in FDDI () Target token rotation time (TTRT), t, is established when FDDI ring is initialized Used to determine when to send low-priority traffic It can be shown that: TTRT, t, is upper bound on time-average intertoken arrival time t is worst-case intertoken arrival time Transmission time for node i, a i, i = 0,,..., m- (m-node network), allocated such that α0 + α α m τ ECPE/CS 556 (//00) Direct Link Networks: Multiaccess Protocols , Scott F. Midkiff
6 Capacity Allocation in FDDI () To be more exact, other factors must be considered in setting TTRT, t Maximum propagation time around ring, T P,max Time to transmit maximum length frame (500 bytes), T F,max Token transmission time, T T So, allocations a i must be set such that m T + T + T + α τ P,max F,max T i Can group other factors into values i = 0 for a i Capacity Allocation in FDDI () Let t 0, t,..., t m-, be times at which token arrives at nodes 0,,..., m-, for some given cycle Assume that node k = (i mod m) receives token at time t i, i 0; node measures intertoken arrival time, t i - t i-m If t i - t i-m < t, node can send low-priority traffic for t - (t i - t i-m ) seconds and can send high-priority traffic for its allocated time of a k seconds ECPE/CS 556 (//00) Direct Link Networks: Multiaccess Protocols - ECPE/CS 556 (//00) Direct Link Networks: Multiaccess Protocols - Capacity Allocation in FDDI (5) If t i - t i-m t, node cannot send low-priority traffic, but it can still send high-priority traffic for a k seconds Capacity Allocation Algorithm () All stations know same value for TTRT (t) and each has its own value a i Each node maintains two timers and counter Token rotation timer (TRT): Times intertoken arrival time (using LC) Token holding timer (THT): Times token holding time at node Late counter (LC): Counter for number of times that TRT expires ECPE/CS 556 (//00) Direct Link Networks: Multiaccess Protocols - ECPE/CS 556 (//00) Direct Link Networks: Multiaccess Protocols - Capacity Allocation Algorithm () TRT is initialized to TTRT (t) and counts down, LC is 0 If token is received before TRT expires, TRT is reset to TTRT If TRT expires before token is received, LC is incremented to and TRT is reinitialized to TTRT If TRT expires second time, LC is incremented to and token is considered lost Capacity Allocation Algorithm () If token is received before TRT expires once (early token) THT is set to TRT (t - [t i - t i-m ]) TRT is reset to TTRT and started Station transmits high-priority frames until all are transmitted, but for at most a i seconds Station starts THT and transmits low-priority frames until done or THT expires ECPE/CS 556 (//00) Direct Link Networks: Multiaccess Protocols - 5 ECPE/CS 556 (//00) Direct Link Networks: Multiaccess Protocols , Scott F. Midkiff
7 Capacity Allocation Algorithm () If TRT expires before token is received (late token) LC reset to 0, TRT resets ( rolls over ) Station can transmit high-priority frames for at most a i seconds (cannot transmit low-priority frames) Latency in FDDI For high-priority stream-type traffic, delay is bounded by t + T, where T is allocated traffic T m = i = 0 αi Delay is loosely bounded by t Short TTRT decreases delay, but at expense of efficiency (throughput) ECPE/CS 556 (//00) Direct Link Networks: Multiaccess Protocols - 7 ECPE/CS 556 (//00) Direct Link Networks: Multiaccess Protocols - 8 You should now be able to () Describe IEEE 80.5 LAN protocol Describe operation of Token Rings and FDDI Compare operation of FDDI to IEEE 80.5 Token Ring Analyze allocated capacity and latency in an FDDI network ECPE/CS 556 (//00) Direct Link Networks: Multiaccess Protocols , Scott F. Midkiff
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