IEEE , Token Rings. 10/11/06 CS/ECE UIUC, Fall
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1 IEEE , Token Rings 10/11/06 CS/ECE UIUC, Fall
2 Medium Access Control Wireless channel is a shared medium Need access control mechanism to avoid interference Why not CSMA/CD? 10/11/06 CS/ECE UIUC, Fall
3 Ethernet MAC Algorithm Node A Node B Listen for carrier sense before transmitting Collision: What you hear is not what you sent! 10/11/06 CS/ECE UIUC, Fall
4 Ethernet MAC Algorithm Node A Node B Listen for carrier sense before transmitting Collision: What you hear is not what you sent! 10/11/06 CS/ECE UIUC, Fall
5 CSMA/CD in WLANs? Most (if not all) radios are half-duplex Listening while transmitting is not possible Collision might not occur at sender Collision at receiver might not be detected by sender! 10/11/06 CS/ECE UIUC, Fall
6 Hidden Terminal Problem A B C A s signal strength C s signal strength A B C space 10/11/06 CS/ECE UIUC, Fall
7 Hidden Terminal Problem Node B can communicate with both A and C A and C cannot hear each other When A transmits to B, C cannot detect the transmission using the carrier sense mechanism If C transmits, collision will occur at node B A B C A s signal strength C s signal strength A B C space 10/11/06 CS/ECE UIUC, Fall
8 Hidden Terminal Problem Node B can communicate with both A and C A and C cannot hear each other When A transmits to B, C cannot detect the transmission using the carrier sense mechanism If C transmits, collision will occur at node B A B C DATA A s signal strength C s signal strength A B C space 10/11/06 CS/ECE UIUC, Fall
9 Hidden Terminal Problem Node B can communicate with both A and C A and C cannot hear each other When A transmits to B, C cannot detect the transmission using the carrier sense mechanism If C transmits, collision will occur at node B A B C DATA DATA A s signal strength C s signal strength A B C space 10/11/06 CS/ECE UIUC, Fall
10 Hidden Terminal Problem Node B can communicate with both A and C A and C cannot hear each other When A transmits to B, C cannot detect the transmission using the carrier sense mechanism If C transmits, collision will occur at node B A B C DATA DATA A s signal strength C s signal strength A B C space 10/11/06 CS/ECE UIUC, Fall
11 MACA Solution for Hidden Terminal Problem When node A wants to send a packet to node B Node A first sends a Request-to-Send (RTS) to A On receiving RTS Node A responds by sending Clear-to-Send (CTS) provided node A is able to receive the packet When a node C overhears a CTS, it keeps quiet for the duration of the transfer A B C 10/11/06 CS/ECE UIUC, Fall
12 MACA Solution for Hidden Terminal Problem When node A wants to send a packet to node B Node A first sends a Request-to-Send (RTS) to A On receiving RTS Node A responds by sending Clear-to-Send (CTS) provided node A is able to receive the packet When a node C overhears a CTS, it keeps quiet for the duration of the transfer RTS A B C 10/11/06 CS/ECE UIUC, Fall
13 MACA Solution for Hidden Terminal Problem When node A wants to send a packet to node B Node A first sends a Request-to-Send (RTS) to A On receiving RTS Node A responds by sending Clear-to-Send (CTS) provided node A is able to receive the packet When a node C overhears a CTS, it keeps quiet for the duration of the transfer RTS CTS CTS A B C 10/11/06 CS/ECE UIUC, Fall
14 Exposed Terminal Problem A B C D 10/11/06 CS/ECE UIUC, Fall
15 Exposed Terminal Problem B talks to A C wants to talk to D C senses channel and finds it to be busy C stays quiet (when it could have ideally transmitted) A B C D 10/11/06 CS/ECE UIUC, Fall
16 Exposed Terminal Problem B talks to A C wants to talk to D C senses channel and finds it to be busy C stays quiet (when it could have ideally transmitted) RTS RTS A B C D 10/11/06 CS/ECE UIUC, Fall
17 Exposed Terminal Problem B talks to A C wants to talk to D C senses channel and finds it to be busy C stays quiet (when it could have ideally transmitted) RTS RTS CTS A B C D 10/11/06 CS/ECE UIUC, Fall
18 MACA Solution for Exposed Terminal Problem Sender transmits Request to Send (RTS) Receiver replies with Clear to Send (CTS) Neighbors See CTS - Stay quiet See RTS, but no CTS - OK to transmit A B C D 10/11/06 CS/ECE UIUC, Fall
19 MACA Solution for Exposed Terminal Problem Sender transmits Request to Send (RTS) Receiver replies with Clear to Send (CTS) Neighbors See CTS - Stay quiet See RTS, but no CTS - OK to transmit RTS RTS A B C D 10/11/06 CS/ECE UIUC, Fall
20 MACA Solution for Exposed Terminal Problem Sender transmits Request to Send (RTS) Receiver replies with Clear to Send (CTS) Neighbors See CTS - Stay quiet See RTS, but no CTS - OK to transmit RTS RTS CTS A B C D 10/11/06 CS/ECE UIUC, Fall
21 MACA Solution for Exposed Terminal Problem Sender transmits Request to Send (RTS) Receiver replies with Clear to Send (CTS) Neighbors See CTS - Stay quiet See RTS, but no CTS - OK to transmit RTS RTS RTS CTS A B C D 10/11/06 CS/ECE UIUC, Fall
22 Collisions Still possible RTS packets can collide! Binary exponential backoff Backoff counter doubles after every collision and reset to minimum value after successful transmission Performed by stations that experience RTS collisions RTS collisions not as bad as data collisions in CSMA Since RTS packets are typically much smaller than DATA packets 10/11/06 CS/ECE UIUC, Fall
23 Reliability Wireless links are prone to errors High packet loss rate detrimental to transport-layer performance Mechanisms needed to reduce packet loss rate experienced by upper layers 10/11/06 CS/ECE UIUC, Fall
24 A Simple Solution to Improve Reliability - MACAW When node B receives a data packet from node A, node B sends an Acknowledgement (ACK) If node A fails to receive an ACK Retransmit the packet A B C 10/11/06 CS/ECE UIUC, Fall
25 A Simple Solution to Improve Reliability - MACAW When node B receives a data packet from node A, node B sends an Acknowledgement (ACK) If node A fails to receive an ACK Retransmit the packet RTS A B C 10/11/06 CS/ECE UIUC, Fall
26 A Simple Solution to Improve Reliability - MACAW When node B receives a data packet from node A, node B sends an Acknowledgement (ACK) If node A fails to receive an ACK Retransmit the packet RTS CTS CTS A B C 10/11/06 CS/ECE UIUC, Fall
27 A Simple Solution to Improve Reliability - MACAW When node B receives a data packet from node A, node B sends an Acknowledgement (ACK) If node A fails to receive an ACK Retransmit the packet RTS CTS DATA CTS A B C 10/11/06 CS/ECE UIUC, Fall
28 A Simple Solution to Improve Reliability - MACAW When node B receives a data packet from node A, node B sends an Acknowledgement (ACK) If node A fails to receive an ACK Retransmit the packet RTS CTS DATA CTS A ACK B ACK C 10/11/06 CS/ECE UIUC, Fall
29 Revisiting the Exposed Terminal Problem Problem Exposed terminal solution doesn't consider CTS at node C With RTS-CTS, C doesn t wait since it doesn t hear A s CTS With B transmitting DATA, C can t hear intended receiver s CTS C trying RTS while B is transmitting is useless RTS RTS RTS CTS CTS A B C D 10/11/06 CS/ECE UIUC, Fall
30 Revisiting the Exposed Terminal Problem - MACAW One solution Have C use carrier sense before RTS Alternative B sends DS (data sending) packet before DATA: Short packet lets C know that B received A s CTS Includes length of B s DATA so C knows how long to wait 10/11/06 CS/ECE UIUC, Fall
31 Deafness For the scenario below Node A sends an RTS to B While node C is receiving from D, Node B cannot reply with a CTS B knows that D is sending to C A keeps retransmitting RTS and increasing its own BO timeout A B C D 10/11/06 CS/ECE UIUC, Fall
32 Deafness For the scenario below Node A sends an RTS to B While node C is receiving from D, Node B cannot reply with a CTS B knows that D is sending to C A keeps retransmitting RTS and increasing its own BO timeout RTS A B C D 10/11/06 CS/ECE UIUC, Fall
33 Deafness For the scenario below Node A sends an RTS to B While node C is receiving from D, Node B cannot reply with a CTS B knows that D is sending to C A keeps retransmitting RTS and increasing its own BO timeout RTS CTS CTS A B C D 10/11/06 CS/ECE UIUC, Fall
34 Deafness For the scenario below Node A sends an RTS to B While node C is receiving from D, Node B cannot reply with a CTS B knows that D is sending to C A keeps retransmitting RTS and increasing its own BO timeout RTS RTS CTS CTS A B C D 10/11/06 CS/ECE UIUC, Fall
35 Deafness For the scenario below Node A sends an RTS to B While node C is receiving from D, Node B cannot reply with a CTS B knows that D is sending to C A keeps retransmitting RTS and increasing its own BO timeout RTS RTS CTS CTS A B C D 10/11/06 CS/ECE UIUC, Fall
36 Interframe Spacing Interframe spacing Plays a large role in coordinating access to the transmission medium Varying interframe spacings Creates different priority levels for different types of traffic! uses 4 different interframe spacings DIFS medium busy DIFS PIFS SIFS contention next frame direct access if medium is free DIFS t 10/11/06 CS/ECE UIUC, Fall
37 IEEE CSMA/CA Sensing the medium If free for an Inter-Frame Space (IFS) Station can start sending (IFS depends on service type) If busy Station waits for a free IFS, then waits a random back-off time (collision avoidance, multiple of slot-time) If another station transmits during back-off time The back-off timer stops (fairness) contention window DIFS DIFS (randomized back-off mechanism) medium busy next frame direct access if medium is free DIFS slot time 10/11/06 CS/ECE UIUC, Fall t
38 Types of IFS SIFS DIFS Short interframe space Used for highest priority transmissions RTS/CTS frames and ACKs DCF interframe space Minimum idle time for contention-based services (> SIFS) 10/11/06 CS/ECE UIUC, Fall
39 Types of IFS PIFS EIFS PCF interframe space Minimum idle time for contention-free service (>SIFS, <DIFS) Extended interframe space Used when there is an error in transmission 10/11/06 CS/ECE UIUC, Fall
40 Backoff Interval When transmitting a packet, choose a backoff interval in the range [0,cw] cw is contention window Count down the backoff interval when medium is idle Count-down is suspended if medium becomes busy When backoff interval reaches 0, transmit RTS 10/11/06 CS/ECE UIUC, Fall
41 DCF Example B1 = 25 B2 = 20 cw = 31 B1 and B2 are backoff intervals at nodes 1 and 2 10/11/06 CS/ECE UIUC, Fall
42 DCF Example B1 = 25 wait B2 = 20 data cw = 31 B1 and B2 are backoff intervals at nodes 1 and 2 10/11/06 CS/ECE UIUC, Fall
43 DCF Example B1 = 25 wait B1 = 5 B2 = 20 data B2 = 15 cw = 31 B1 and B2 are backoff intervals at nodes 1 and 2 10/11/06 CS/ECE UIUC, Fall
44 DCF Example B1 = 25 wait B1 = 5 data data wait B2 = 20 B2 = 15 cw = 31 B1 and B2 are backoff intervals at nodes 1 and 2 10/11/06 CS/ECE UIUC, Fall
45 DCF Example B1 = 25 wait B1 = 5 data B2 = 20 data B2 = 15 wait B2 = 10 cw = 31 B1 and B2 are backoff intervals at nodes 1 and 2 10/11/06 CS/ECE UIUC, Fall
46 Backoff Interval The time spent counting down backoff intervals is a part of MAC overhead Large cw Large backoff intervals Can result in larger overhead Small cw larger number of collisions (when two nodes count down to 0 simultaneously) 10/11/06 CS/ECE UIUC, Fall
47 Backoff Interval The number of nodes attempting to transmit simultaneously may change with time Some mechanism to manage contention is needed IEEE DCF Contention window cw is chosen dynamically depending on collision occurrence 10/11/06 CS/ECE UIUC, Fall
48 Binary Exponential Backoff in DCF When a node fails to receive CTS in response to its RTS, it increases the contention window cw is doubled (up to an upper bound) When a node successfully completes a data transfer, it restores cw to Cw min cw follows a sawtooth curve 10/11/06 CS/ECE UIUC, Fall
49 Token Ring Example Token Ring Networks IBM: 4Mbps token ring IEEE 802.5: 16Mbps 10/11/06 CS/ECE UIUC, Fall
50 Token Ring Example Token Ring Networks IBM: 4Mbps token ring IEEE 802.5: 16Mbps 10/11/06 CS/ECE UIUC, Fall
51 Token Ring Focus on Fiber Distributed Data Interface (FDDI) 100 Mbps Was (not is) a candidate to replace Ethernet Used in some MAN backbones (LAN interconnects) Outline Rationale Topologies and components MAC algorithm Priority Feedback Token management 10/11/06 CS/ECE UIUC, Fall
52 Token Ring 10/11/06 CS/ECE UIUC, Fall
53 Token Ring Why emulate a shared medium with pointto-point links? 10/11/06 CS/ECE UIUC, Fall
54 Token Ring Why emulate a shared medium with pointto-point links? Why a shared medium? Convenient broadcast capabilities Switches costly 10/11/06 CS/ECE UIUC, Fall
55 Token Ring Why emulate a shared medium with pointto-point links? Why a shared medium? Convenient broadcast capabilities Switches costly Why emulation? Simpler MAC algorithm Fairer access arbitration Fully digital (802.3 collision detection requires analog) 10/11/06 CS/ECE UIUC, Fall
56 Token Ring: Topology and Components Relay Single Relay Multistation access units 10/11/06 CS/ECE UIUC, Fall
57 Token Ring: Topology and Components Relay Single Relay Multistation access units Host From Previous Host Relay To Next Host 10/11/06 CS/ECE UIUC, Fall
58 Token Ring: Topology and Components Relay Single Relay Multistation access units Host Host From Previous Host Relay To Next Host From Previous Host Relay To Next Host 10/11/06 CS/ECE UIUC, Fall
59 Token Ring: Topology and Components Relay Single Relay Multistation access units Host Host Host From Previous Host Relay To Next Host From Previous Host Relay To Next Host Host From Previous MSAU Host Host To Next MSAU 10/11/06 CS/ECE UIUC, Fall
60 Token Ring: Dual Ring Example Token Ring Networks FDDI: 1000Mbps Fiber Distributed Data Interface 10/11/06 CS/ECE UIUC, Fall
61 Token Ring: Dual Ring Example Token Ring Networks FDDI: 1000Mbps Fiber Distributed Data Interface 10/11/06 CS/ECE UIUC, Fall
62 FDDI Dual ring configuration Self-healing Normal flow in green direction Can detect and recover from one failure 10/11/06 CS/ECE UIUC, Fall
63 FDDI Dual ring configuration Self-healing Normal flow in green direction Can detect and recover from one failure 10/11/06 CS/ECE UIUC, Fall
64 FDDI Dual ring configuration Self-healing Normal flow in green direction Can detect and recover from one failure 10/11/06 CS/ECE UIUC, Fall
65 FDDI Dual ring configuration Self-healing Normal flow in green direction Can detect and recover from one failure 10/11/06 CS/ECE UIUC, Fall
66 FDDI Dual ring configuration Self-healing Normal flow in green direction Can detect and recover from one failure 10/11/06 CS/ECE UIUC, Fall
67 FDDI Dual ring configuration Self-healing Normal flow in green direction Can detect and recover from one failure 10/11/06 CS/ECE UIUC, Fall
68 FDDI Dual ring configuration Self-healing Normal flow in green direction Can detect and recover from one failure 10/11/06 CS/ECE UIUC, Fall
69 FDDI Dual ring configuration Self-healing Normal flow in green direction Can detect and recover from one failure 10/11/06 CS/ECE UIUC, Fall
70 FDDI Dual ring configuration Self-healing Normal flow in green direction Can detect and recover from one failure 10/11/06 CS/ECE UIUC, Fall
71 FDDI Dual ring configuration Self-healing Normal flow in green direction Can detect and recover from one failure 10/11/06 CS/ECE UIUC, Fall
72 FDDI Dual ring configuration Self-healing Normal flow in green direction Can detect and recover from one failure 10/11/06 CS/ECE UIUC, Fall
73 FDDI Dual ring configuration Self-healing Normal flow in green direction Can detect and recover from one failure 10/11/06 CS/ECE UIUC, Fall
74 FDDI Dual ring configuration Self-healing Normal flow in green direction Can detect and recover from one failure 10/11/06 CS/ECE UIUC, Fall
75 FDDI Dual ring configuration Self-healing Normal flow in green direction Can detect and recover from one failure 10/11/06 CS/ECE UIUC, Fall
76 FDDI Dual ring configuration Self-healing Normal flow in green direction Can detect and recover from one failure 10/11/06 CS/ECE UIUC, Fall
77 FDDI Dual ring configuration Self-healing Normal flow in green direction Can detect and recover from one failure 10/11/06 CS/ECE UIUC, Fall
78 FDDI Dual ring configuration Self-healing Normal flow in green direction Can detect and recover from one failure 10/11/06 CS/ECE UIUC, Fall
79 FDDI Dual ring configuration Self-healing Normal flow in green direction Can detect and recover from one failure 10/11/06 CS/ECE UIUC, Fall
80 FDDI Dual ring configuration Self-healing Normal flow in green direction Can detect and recover from one failure 10/11/06 CS/ECE UIUC, Fall
81 FDDI Dual ring configuration Self-healing Normal flow in green direction Can detect and recover from one failure 10/11/06 CS/ECE UIUC, Fall
82 Multistation Access Unit Each station imposes a delay E.g. 50 ms Maximum of 500 Stations Upper limit of 100km Need 200km of fiber Uses 4B/5B encoding Can be implemented over copper 10/11/06 CS/ECE UIUC, Fall
83 Token Ring: Basic Concepts Frames flow in one direction Token Upstream to downstream Special bit pattern rotates around ring Stations Must capture token before transmitting Must remove frame after it has cycled Must release token after transmitting Service Stations get round-robin service 10/11/06 CS/ECE UIUC, Fall
84 Token Ring: Basic Concepts Immediate release Used in FDDI Token follows last frame immediately Delayed release Used in IEEE Token sent after last frame returns to sender 10/11/06 CS/ECE UIUC, Fall
85 Token Release Delayed Release 10/11/06 CS/ECE UIUC, Fall
86 Token Release Delayed Release 10/11/06 CS/ECE UIUC, Fall
87 Token Release Delayed Release 10/11/06 CS/ECE UIUC, Fall
88 Token Release Delayed Release 10/11/06 CS/ECE UIUC, Fall
89 Token Release Delayed Release Early Release 10/11/06 CS/ECE UIUC, Fall
90 Token Release Delayed Release Early Release 10/11/06 CS/ECE UIUC, Fall
91 Token Release Delayed Release Early Release 10/11/06 CS/ECE UIUC, Fall
92 Token Release Delayed Release Early Release 10/11/06 CS/ECE UIUC, Fall
93 Token Ring: Media Access Control Parameters Token Holding Time (THT) Upper limit on how long a station can hold the token Each station is responsible for ensuring that the transmission time for its packet will not exceed THT Token Rotation Time (TRT) How long it takes the token to traverse the ring. TRT ActiveNodes x THT + RingLatency Target Token Rotation Time (TTRT) Agreed-upon upper bound on TRT 10/11/06 CS/ECE UIUC, Fall
94 802.5 Reliability Delivery status Trailer A bit C bit Set by recipient at start of reception Set by recipient on completion on reception 10/11/06 CS/ECE UIUC, Fall
95 802.5 Monitor Responsible for Inserting delay Token presence Should see a token at least once per TRT Check for corrupted frames Check for orphaned frames Header Monitor bit Monitor station sets bit first time it sees packet If monitor sees packet again, it discards packet 10/11/06 CS/ECE UIUC, Fall
96 Token Maintenance: Monitoring for a Valid Token All stations should periodically see valid transmission (frame or token) Maximum gap = ring latency + max frame < = 2.5ms Set timer at 2.5ms send claim frame if timer expires 10/11/06 CS/ECE UIUC, Fall
97 Timing Algorithm: Each node measures TRT between successive tokens If measured-trt > TTRT Token is late Don t send If measured-trt < TTRT Token is early OK to send Worse case: 2xTTRT between seeing token Back-to-back 2xTTRT rotations not possible 10/11/06 CS/ECE UIUC, Fall
98 Traffic Classes: FDDI Two classes of traffic Synchronous Real time traffic Can always send Asynchronous Bulk data Can send only if token is early 10/11/06 CS/ECE UIUC, Fall
99 Timing Algorithm: FDDI Each station is allocated S i time units for synchronous traffic per TRT TTRT is negotiated S 1 + S S N + RingLatency TTRT Algorithm Goal Keep actual rotation time less than TTRT Allow station i to send S i units of synchronous traffic per TRT Fairly allocate remaining capacity to asynchronous traffic Regenerate token if lost 10/11/06 CS/ECE UIUC, Fall
100 Timing Algorithm: FDDI When a node gets the token Set TRT = time since last token Set THT = TTRT TRT If TRT > TTRT Token is late Send synchronous data Don t send asynchronous data If TRT < TTRT Token is early OK to send any data Send synchronous data, adjust THT If THT > 0, send asynchronous data 10/11/06 CS/ECE UIUC, Fall
101 FDDI example Assume RingLatency=12 µs Three active stations Each with Si=20 µs TTRT=100 µs Stations have unlimited supply of asynchronous traffic. 10/11/06 CS/ECE UIUC, Fall
102 FDDI example Arrival time TRT s/a 0 0 0/ / / / / /0 Arrival time TRT s/a 4 0 0/ / / / / /12 Arrival time TRT s/a 8 0 0/ / / / / /8 10/11/06 CS/ECE UIUC, Fall
103 FDDI example Arrival time TRT A Arrival time TRT A Arrival time TRT A /11/06 CS/ECE UIUC, Fall
104 FDDI Performance Synchronous traffic may consume one TTRT worth of time TRT > TTRT Worst case ϒ TRT < 2*TTRT Any asynchronous traffic plus RingLatency TTRT Synchronous traffic < TTRT 10/11/06 CS/ECE UIUC, Fall
105 FDDI Performance Can t have two consecutive TRT = 2*TTRT After a cycle with TRT = 2*TTRT, no asynchronous traffic will be sent 10/11/06 CS/ECE UIUC, Fall
106 Token Maintenance: FDDI Lost Token No token when initializing ring Bit error corrupts token pattern Node holding token crashes Monitoring for a valid token Should see valid transmission (frame or token) periodically within 2*TTRT Maximum gap = RingLatency + MaxFrame 2.5ms 10/11/06 CS/ECE UIUC, Fall
Medium Access Control. IEEE , Token Rings. CSMA/CD in WLANs? Ethernet MAC Algorithm. MACA Solution for Hidden Terminal Problem
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