Advanced Computer Networks: Lecture 10. This lecture will cover the following: multiple access with nodes send and receive frames over a shared link; carrier sense nodes can distinguish between an idle and busy link; collision detection with a node listens as it transmits to detect collision;...
CS716 Advanced Computer Networks By Dr. Amir Qayyum Lecture No. 10 Ethernet MAC – CSMA/CD • Multiple access – Nodes send and receive frames over a shared link • Carrier sense – Nodes can distinguish between an idle and busy link • Collision detection – A node listens as it transmits to detect collision CSMA/CD MAC Algorithm • If line is idle (no carrier sensed) – Send immediately – Upper bound message size of ~1500 bytes Mustwait9.6àsbetweenbackưtoư backframes CSMA/CDMACAlgorithm ã Iflineisbusy(carriersensed) Waituntilthelinebecomesidleand thentransmitimmediately – Called 1persistent (special case of p persistent) • If collision detected – Stop sending data and jam signal – Try again later Collision Detection mymachine yourmachine start transmission at time 0 start transmission at time T almost there at time T collision !!! How to ensure that mymachine knows about the collision? Constraints on Collision Detection • In our example, consider – mymachine’s message reaches your machine at T – yourmachine’s message reaches my machine at 2T • Thus, mymachine must still be transmitting at 2T Constraints on Collision Detection • Specifics of IEEE 802.3 – bounds 2T to 51.2 microseconds – packet must be at least 64B long • Jam after the collision, for 32 bits, then stop transmitting frame (runt frame of 96 bits) – ensures that all hosts notice collision Ethernet Min. Frame Size • RTT on a maximally configured Ethernet of 2500m, with 4 repeaters is about 51.2 μs – 2500m / 2 x 108 m/s = 12.5 us – 2 x 12.5 = 25 us + repeater delays • 51.2 μs on 10 Mbps corresponds to 512 bits (64 bytes) • Therefore, the minimum frame length for Ethernet is 64 bytes (header +46 bytes data)9 Collision Detection mymachine yourmachine start transmission at time 0 start transmission at time T almost there at time T collision mymachine notices collision before transmission ends at 2T 10 Ethernet Collision Detection • Bus topology Ethernets – Transceiver handles • Carrier detection • Collision detection • Jamming after a collision – Transceiver sees voltage sum • Outgoing signal + Incoming signal – Looks for voltages impossible for local alone • Attenuation can prevent detection • Limits segment length 18 Ethernet Collision Detection • Hub topology Ethernets – Controller/card handles carrier detection – Hub handles • Collision detection • Jamming after a collision – Detect transmission activity on each line – If more than 1 line (host) active • Assert collision to all lines • Continue until no lines active – Attenuation is less critical 19 Experience With Ethernet • Number of hosts limited to 200 in practice, standard allows 1024 • Range much shorter than 2.5 km limit in standard • Roundtrip time is typically 5 or 10 μs, not 50μs 20 Experience With Ethernet • Higherlevel flow control (i.e., TCP) limits load – 30% maximum load recommended – Large packets recommended • Star topologies easier to administer than Bus – Failure isolation, node addition w/o complex topology • (Backwards) Compatibility more important than elegance or raw performance (1 > 0) 21 Token Ring 22 Token Ring Overview • Token Ring network “was” a candidate to replace Ethernet; used in some MAN backbones – 16Mbps IEEE 802.5 (based on earlier 4Mbps IBM ring) – 100Mbps Fiber Distributed Data Interface (FDDI) 23 Token Ring Rationale • Why emulate a shared medium with point topoint links? • Why a shared medium? – convenient broadcast capabilities – switches are costly • Why emulation? – simpler MAC algorithm: only have 2 wires – fairer access arbitration – fully digital (802.3 collision detection requires 24 analog) IBM Token Ring – IEEE 802.5 • Data flows in a particular direction – a node receives frames from its upstream neighbor – a node forwards frames to its downstream neighbor 25 IBM Token Ring – IEEE 802.5 • Ring is viewed as a single shared medium – each node is allowed to transmit according to some distributed algorithm for medium access – all nodes see all frames; destination saves a copy of frame as it flows past • The term “token” indicates the way the access to shared channel is managed 26 Token Ring State Diagram No token (pass frames along) get token release token Token (insert new frames) 27 Token in a Token Ring • Token is a special bit pattern that rotates around the ring – A node must capture token before transmitting – A node releases token after done transmitting • Immediate release token follows last frame (FDDI) • Delayed release – after last frame returns 28 to sender Token Release e m a Fr Token (a) immediate release Token Fra me (b) delayed release 29 Token in a Token Ring • Remove your frame when it comes back around – Transmit another frame or reinsert the token • Stations get roundrobin service as the token circulates around the ring 30 Review Lecture 10 • • • • • • • • CSMA/CD MAC algorithm Constraints on collision detection Min frame size – RTT 51.2us Retries after collision Binary exp backoff: Min value unique Frame reception: acceptable frames Bus and Hub topology: CD handle Exp: nodes, dist, RTT, 30% load, largeframe31 Review Lecture 10 • • • • • • • • Token Ring MAN backbone IBM, 802.5 token ring, FDDI Rationale: emulate shared med p2p links Directional data flow Shared med: see all frames, distr MAC algo Token – special bit pattern Capture and release – round robin service 32 .. .Lecture? ?No.? ?10 Ethernet MAC – CSMA/CD • Multiple access – Nodes send and receive frames over a shared link... RTT on a maximally configured Ethernet of 2500m, with 4 repeaters is about 51.2 μs – 2500m / 2 x? ?108 m/s = 12.5 us – 2 x 12.5 = 25 us + repeater delays • 51.2 μs on? ?10? ?Mbps corresponds to 512 bits (64 bytes) • Therefore, the minimum frame length for ... Retry After the Collision • Delay and try again algorithm – 1st time: 0 or 51.2us – 2nd time: 0, 51.2, or? ?102 .4us – 3rd time51.2,? ?102 .4, or 153.6us – nth time: k x 51.2us, for randomly selected k=0 2n 1 – Give up after several tries