Medium Access Control MAC Sublayer Outline Bus Ethernet
Medium Access Control (MAC) Sub-layer Outline Bus (Ethernet, IEEE 802. 3) Token ring (IEEE 802. 5, FDDI) Wireless (IEEE 802. 11) Spring 2001 CS 585 1
Token Ring Overview • Examples – 16 Mbps IEEE 802. 5 (based on earlier IBM ring) – 100 Mbps Fiber Distributed Data Interface (FDDI) Shared Media: Access control All nodes see all frames Spring 2001 CS 585 2
Token Ring (cont) • Idea – – Frames flow in one direction: upstream to downstream special bit pattern (token) rotates around ring must capture token before transmitting release token after done transmitting • immediate release • delayed release – – remove your frame when it comes back around Other nodes simply forward the frame Destination node makes a copy and forward the frame stations get round-robin service Spring 2001 CS 585 3
Token Ring (cont) • Frame Format 8 8 48 48 Start of frame Control Dest addr Src addr 32 Body 8 CRC End of frame 24 Status • Reliability – A bit (initial = 0, set to 1 by receiver) – C bit (initial = 0, set to 1 by receiver if successfully copied) Spring 2001 CS 585 4
Timed Token Algorithm • Token Holding Time (THT) – upper limit on how long a station can hold the token – The impact of THT on performance (10 ms in 802. 5) • Token Rotation Time (TRT) – how long it takes the token to traverse the ring. – TRT <= Active. Nodes x THT + Ring. Latency • Target Token Rotation Time (TTRT) in FDDI – agreed-upon upper bound on TRT Spring 2001 CS 585 5
Algorithm (cont) • Each node measures TRT between successive tokens – if measured-TRT > TTRT: token is late so don’t send – if measured-TRT < TTRT: token is early so OK to send • Two classes of traffic – synchronous: can always send – asynchronous: can send only if token is early • Worst case: 2 x. TTRT between seeing token Spring 2001 CS 585 6
Token Maintenance • Lost Token – no token when initializing ring – bit error corrupts token pattern – node holding token crashes • Generating a Token (and agreeing on TTRT) – – execute when join ring or suspect a failure send a claim frame that includes the node’s TTRT bid when receive claim frame, update the bid and forward if your claim frame makes it all the way around the ring: • your bid was the lowest • everyone knows TTRT • you insert new token Spring 2001 CS 585 7
Maintenance (cont) • Monitoring for a Valid Token – should periodically see valid transmission (frame or token) – maximum gap = ring latency + max frame < = 2. 5 ms – set timer at 2. 5 ms and send claim frame if it fires • Monitoring orphaned frames – monitor bit is set to 0 initially – set to 1 when the monitor sees it – discard frames with monitor bit = 1 Spring 2001 CS 585 8
FDDI (Fiber Distributed Data Interface) • Physical Properties – Fiber – Dual ring in opposite directions (fault-tolerance) • Target Token Rotation Time (TTRT) – Synchronous and asynchronous traffic Spring 2001 CS 585 9
Wireless LANs • IEEE 802. 11 • Bandwidth: 1 or 2 Mbps • Physical Media – spread spectrum radio (2. 4 GHz) – diffused infrared (10 m) • The sender and receiver do not have to be aimed at each other and do not need a clear line of sight Spring 2001 CS 585 10
Spread Spectrum • Idea – spread signal over wider frequency band than required – originally deigned to thwart jamming • Frequency Hopping – transmit over random sequence of frequencies – sender and receiver share… • pseudorandom number generator • seed – 802. 11 uses 79 x 1 MHz-wide frequency bands Spring 2001 CS 585 11
Spread Spectrum (cont) • Direct Sequence – – for each bit, send XOR of that bit and n random bits random sequence known to both sender and receiver called n-bit chipping code 802. 11 defines an 11 -bit chipping code 1 0 Data stream: 1010 1 0 Random sequence: 01001011001 1 0 XOR of the two: 101110101001 Spring 2001 CS 585 12
Collisions Avoidance • Similar to Ethernet (not quite) • Problem: hidden and exposed nodes A and C both send to B A to B, D to C ? B sends to A, Can C Send to D ? Spring 2001 CS 585 13
MACAW (Medium Access Control Avoidance) • Sender transmits Request. To. Send (RTS) frame • Receiver replies with Clear. To. Send (CTS) frame • Neighbors… – see CTS: keep quiet – see RTS but not CTS: ok to transmit • Receiver sends ACK when has frame – neighbors silent until see ACK • Collisions – no collisions detection – known when don’t receive CTS – exponential backoff Spring 2001 CS 585 14
Supporting Mobility • Case 1: ad hoc networking • Case 2: access points (AP) – connected by wire – each mobile node associates with an AP Spring 2001 CS 585 15
Mobility (cont) • Scanning (selecting an AP) – node sends Probe frame – all AP’s w/in reach reply with Probe. Response frame – node selects one AP; sends it Association. Request frame – AP replies with Association. Response frame – new AP informs old AP via tethered network • When – active: when join or move – passive: AP periodically sends Beacon frame Spring 2001 CS 585 16
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