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CMPE 252A: SET 5: Medium edium Acces ccess Cont ontrol ol Prot otocols ocols 1 Collision Avoidance Collision avoidance emulates collision detection in networks where stations are half duplex. First protocol was proposed by


  1. CMPE 252A: SET 5: Medium edium Acces ccess Cont ontrol ol Prot otocols ocols 1 Collision Avoidance  Collision avoidance emulates collision detection in networks where stations are half duplex.  First protocol was proposed by Kleinrock and Tobagi (Split Reservation Multiple Access).  Many protocols have been proposed since then: MACA, MACAW, FAMA, RIMA.  The objective of collision avoidance protocols is to eliminate the hidden-terminal problem of CSMA: S, R, and N hear one another, and R R, N, and H hear one another H N hears S ’ s transmission S However, S and H cannot hear each other ’ s transmissions to R, and cause N interference at the receiver R. 2 Collision Avoidance  Because of hidden terminals, the vulnerability of a data packet is just as in pure ALOHA, twice its length.  With collision avoidance, stations exchange small control packets to determine which sender can transmit to a receiver.  The collision avoidance dialogue can be controlled by the sender or the receiver.  In sender-initiated collision avoidance we have: RTS (S to R) -> CTS (R to S) -> DATA (S to R) -> ACK (R to S)  In receiver-initiated collision avoidance we can have: RTR (R to S) -> DATA (S to R) -> ACK (R to S) 3 1

  2. Sender-Initiated Collision Avoidance  Examples are MACA, MACAW, FAMA, and IEEE 802.11.  MACA and MACAW do not use carrier sensing, FAMA and 802.11 do.  MACA, MACAW, and IEEE 802.11 do not prevent collisions in the absence of base stations. C. Fullmer and JJ Garcia-Luna-Aceves, “ Solutions to Hidden-Terminal Problems in Wireless Networks, ” Proc. ACM SIGCOMM 97 (in the ccrg web page)  Objective is to force hidden sources to hear the feedback from a receiver when they are causing interference during the collision-avoidance handshake. 4 Example of CSMA/CA: Floor Acquisition Multiple Access  Stations use carrier sensing to send any packet.  The CTS lasts much longer than an RTS to force the interfering sources to detect carrier (from the receiver) and back off. RTS RTS S S S S to R R to S CTS CTS RTS CTS time RTS H to R noise is heard 2 τ RTS from S arrives at R with no collisions. RTS from H must start within one prop. delay from CTS from R to S. H must hear noise from CTS and backs off! 5 Basic FAMA Protocol Packet Non-persistent ready strategy. Same basic algorithm for all Floor CSMA/CA schemes Taken? yes no send RTS delay packet transmission k times wait for a round-trip time CTS compute random back? send packet backoff integer k yes no 6 2

  3. Throughput of FAMA Now we consider the effect of collision avoidance overhead.  Remember: Fully connected net, arrivals of RTSs to the channel are  Poisson with parameter lambda. Performance is always below that of CSMA/CD, because feedback  incurs more overhead. first packet starts ( A ) last interfering packet starts ( B ) A B NEW RTS NEW CTS NEW DATA time ' γ τ idle τ ' τ P τ γ γ period Y ≤ τ collision interval: average successful packet interval: C Y 2 idle period: = + γ + τ ≤ τ + γ P ' + 3 + γ + γ τ I 1 / = λ 7 Throughput of FAMA Typical (over) simplification: Think of two mutually exclusive events: packet is successful or a collision occurs. B P ( P ) ( 1 P ) C Therefore, …. but that is not correct = + τ + − S S Note that: • A successful packet occurs when the first and the last packet of a busy period are the same packet. • The average length of a collision interval includes the case when Y = 0 i.e., the first and the last pkt starts in busy period are the same! Therefore, we know two things: • The length of an average busy period must include the length of the average collision interval . • The busy period includes a CTS and a packet only when it is successful with probability P S 8 Throughput of FAMA first RTS starts ( A ) Length is Y + γ + τ last interfering RTS starts ( B ) Y ≤ τ A B NEW time ' τ γ Length is Y P 2 Y ! + γ + τ + γ + + τ ≤ τ A single RTS Y 0 = RTS CTS DATA NEW NEW time γ τ ' τ P τ γ Y 0 = 9 3

  4. Throughput of FAMA Therefore: B Y P ( P 2 ) with Y $ = + γ + τ + γ + + τ ≤ τ S A packet is successful with probability P P { 0 packets in } e − λτ = τ = S For P we can approximate: τ << B 2 e ( ' P 2 ) − λτ ≈ γ + τ + γ + + τ The utilization period is only that portion of a packet transmission that has no overhead, that is: U = Pe − λτ Notice the impact of Pe − λτ the RTS-CTS S Substituting: ≈ 1 overhead! 2 e − λτ ( ' P 2 ) + γ + τ + γ + + τ λ 10 Throughput of FAMA  FAMA (and all collision-avoidance protocols) is always below CSMA/CD. Collision interval in CA is much longer than in CD, because detecting collisions is done using small packets, rather than listening to self transmission. 11 Collision Resolution and Backoff Strategies  Used to stabilize the system by preventing traffic loads that exceed its capacity.  Collision resolution: Let packet that collide resolve when each is transmitted and block new traffic from entering the system.  Backoff strategies: Increase the time between retransmissions when traffic load (that creates collisions) increases. 12 4

  5. Collision Resolution and Backoff Strategies  Backoff strategy in Ethernet:  After experiencing the nth collision of a frame, pick a value K randomly from the set {0, 1, 2,…, 2 m -1} with m = min{10, n }.  Wait K .512 bit times before attempting a retransmission.  Goal is to reduce offered load to the channel; however, it provides no assurance that a retransmission will be sent ahead of another new transmission from other node. 13 Conflict-Free MAC Protocols  Conflict-free:  Fixed assignment (TDMA, FDMA)  Reservations  Polling  Dynamic scheduling  Token passing 14 TDMA TDMA: time division multiple access  access to channel in "rounds"  each station gets fixed length slot (length = pkt trans time) in each round  unused slots go idle  example: 6-station LAN, 1,3,4 have pkt, slots 2,5,6 idle 15 5

  6. FDMA FDMA: frequency division multiple access  channel spectrum divided into frequency bands  each station assigned fixed frequency band  unused transmission time in frequency bands go idle  example: 6-station LAN, 1,3,4 have pkt, frequency bands 2,5,6 idle. time frequency bands 16 Channel Partitioning (CDMA) CDMA (Code Division Multiple Access)  unique “ code ” assigned to each user; i.e., code set partitioning  used mostly in wireless broadcast channels (cellular, satellite, etc.)  all users share same frequency, but each user has own “ chipping ” sequence (i.e., code) to encode data  encoded signal = (original data) X (chipping sequence)  decoding: inner-product of encoded signal and chipping sequence  allows multiple users to “ coexist ” and transmit simultaneously with minimal interference (if codes are “ orthogonal ” ) 17 Token Passing  Basic Scheme:  A token granting the right to transmit is circulated among stations.  Station with something to send receiving token changes the token into a start of packet and sends its packet.  The token is sent back to the system when the sender is done.  Two transmission strategies:  Release after transmission (RAT): Sender releases the token immediately after transmitting its packet.  Release after reception (RAR): Sender waits until it hears the last bit of its own transmission before releasing the token.  Token Passing protocols can be used in any network topology; however, token management is simpler in rings. 18 6

  7. RAT Strategy D D D TK S S S SFD SFD D Token circulates until it reaches S Source changes token to start-of-frame delimiter S D copies the data SFD S takes frame out (packet length is actually longer than token circulation time) 19 Efficiency of RAT  Let p be the probability that a station has something to send when the token arrives to it, P is the packet length, T is the token length, there are N stations in the ring, and the propagation delay from one station to the next is τ τ τ τ τ 1 2 3 N ... PKT PKT PKT TK TK TK TK time p ( N P ) p ( N P ) × × back to 1 η = = RAT TOTAL N ( T pP ) + τ + 1 T + τ ; with a η = = RAT a P 1 + p 20 Average Delays in Token Ring  Delays are bounded in token ring nets!  Each station can hold the token for a maximum amount of time, and there is a finite number of stations in the net.  The maximum medium access time (MMAT) is defined to be the time elapsed from the start of a current packet transmission by a node to the time when it can have the “ floor ” of the network again.  Assume that each station is allowed a token holding time of THT sec. 21 7

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