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Medium Access Control. Channel Allocation. Static channel allocation in LANs and MANs FDMA, TDMA, CDMA Dynamic channel allocation in LANs and MANs MAC protocols: with collisions, polling, token. Static Channel Allocation. Delay for one fast channel: Delay for multiple FDM slower channels:.

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## Medium Access Control

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**Channel Allocation**• Static channel allocation in LANs and MANs • FDMA, TDMA, CDMA • Dynamic channel allocation in LANs and MANs • MAC protocols: with collisions, polling, token**Static Channel Allocation**• Delay for one fast channel: • Delay for multiple FDM slower channels:**Poisson Process**• Probability of k arrivals in time t: • Probability that packet duration exceeds t: • Note that λ is the average packet arrival rate, and 1/μ is the average packet duration.**M/M/1 Queue**• Queue equations, pj(t) is the probability that the number of packets at time t in a queue is j • The solution of the previous recursion for stationary probabilities to which pj(t) converge**Delay**• Little’s formula for average delay E[D] E[Q] is the average number of packets in a queue • Delay is:**Static Channel Allocation**• Delay for one fast channel: where C is the channel bit-rate and E[L] is the average packet length. • Delay for multiple FDM N times slower channels:**Multiple Access Protocols**• ALOHA • Carrier Sense Multiple Access (CSMA) protocols • CSMA/CD • CSMA/CA • Collision-Free protocols • Reservation based • Token based**Pure ALOHA**Vulnerable period for the shaded frame.**ALOHA Throughput**• Throughput is S=GPs, where Ps is the probability of successful transmission. • The k frames per f frame slots is Ps=e-fG • For pure ALOHA f=2, for slotted ALOHA f=1, so:**Pure and Slotted ALOHA**Throughput versus offered traffic for ALOHA systems.**Carrier Sense Multiple Access**• 1-Persistant CSMA • Nonpersistant CSMA • P-Persistant CSMA**CSMA with Collision Detection**CSMA/CD can be in one of three states: contention, transmission, or idle.**Wireless LAN: CSMA-CA**The MACA protocol. (a) A sending an RTS to B. (b) B responding with a CTS to A.**Collision-Free Protocols:Reservations**The basic bit-map protocol.**Collision-Free Protocols: Bidding**The binary countdown protocol. A dash indicates silence.**Collision Free Protocols: Fiber Distributed Data Interface**(FDDI) • Station transmits only when it has a token • Timers count the time while the token is away • Two timers determine how much data a station may transmit, so that the token delay is limited**Ethernet**• Ethernet, IEEE 802.3 • 10Base (10Mbps) • Fast Ethernet (100Mbps) • Gigabit Ethernet**Ethernet MAC Sublayer Protocol**Frame formats. (a) DIX Ethernet, (b) IEEE 802.3. Preamble-synchronization, Type-upper layer protocol, Pad-to make the minimum packet size 64B**CSMA with Collision Detection**CSMA/CD can be in one of three states: contention, transmission, or idle.**Back-Off Mechanism**• After a collision, user accesses medium with probability 1/W where W is the window size. • With each collision W doubles.**Throughput of CSMA/CD (Ethernet)**• Assume that requests form a Poisson process with rate g, T is time slot duration, and Tpis a packet duration. The throughput equals S=Tp/(Tp+I), where I is the average time between packet transmissions. • The probability of a packet transmission is equal to the probability that there is only one request in some previous time slot which is Ps=gTe-gT. • The average time between transmissions is**S**2 1 l g Throughput of CSMA/CD • The throughput is • It tends to 0 when g increases . • Protocol is unstable like ALOHA.**Throughput of CSMA/CD**• The throughput is • If p is the packet generation probability and k is the number of active users Ps=kp(1-p)k-1 • The maximum throughput is achieved for p=1/k and it is tends to e when k tends to infinity**Ethernet Performance**Efficiency of Ethernet at 10 Mbps with 512-bit slot times.**10Mbps Ethernet Cabling**The most common kinds of Ethernet cabling.**10Mbps Ethernet Cabling**Three kinds of Ethernet cabling. (a) 10Base5, (b) 10Base2, (c) 10Base-T.**Ethernet Cabling**Cable topologies. (a) Linear, (b) Spine, (c) Tree, (d) Segmented.**10Mb Ethernet Coding**(a) Binary encoding, (b) Manchester encoding, (c) Differential Manchester encoding.**10 Mb Ethernet Collision Detection**10Base5 cabling, Kadambi, Crayford and Kalkunte, Gigabit Ethernet, Prentice Hall, 1998**10 Mb Ethernet Collision Detection**10Base2 and 10BaseT cabling, Kadambi, Crayford and Kalkunte, Gigabit Ethernet, Prentice Hall, 1998**Fast Ethernet**The original fast Ethernet cabling.**Fast Ethernet**• Auto negotiation enables communication with 10Mb Ethernet • Manchester code → 4B/5B code • Full duplex mode is optional with using PAUSE command**Switched Ethernet**A simple example of switched Ethernet.**Gigabit Ethernet**(a) A two-station Ethernet. (b) A multistation Ethernet.**Gigabit Ethernet**Gigabit Ethernet cabling.**Gigabit Ethernet**• Prioritization of fiber over copper • 4B/5B coding → 8B/10B coding • Full duplex mode is preferred with PAUSE message • Carrier extension, and frame bursting introduced in half-duplex mode**IEEE 802.2: Logical Link Control**(a) Position of LLC. (b) Protocol formats.**A Sample HFC System**narrowcast Downstream: 500 MHz shared by ~50,000 (broadcast) 200 MHz by 1200 (narrowcast) Upstream: ~37 MHz shared by 300 HOME o o o o o o o o lup lb ln(4ln/fiber) Secondary Hub Fiber Node o o o o o o o o o o RF Spectrum on coax: return 80 broadcast channels30 QAM channels (~150 video channels) broadcast 5-42 MHz 550 MHz 750 MHz Sheryl Woodward, AT&T Labs-Research**Justification for Using Shared Medium**• Equivalent circuit rate (ECR) on a cable with many users is the rate of a dedicated link that would provide the same e.g. average delay (similar results is obtained for 90th percentile page delay). By Shankar, Jiang and Mishra: where tON is the transmission tim, and tOFF is the think time, r is the channel rate, tON/(tON+tOFF)<<1, on periods have an exponential distribution.**Justification for Using Shared Medium**• Let’s calculate how many users can be allocated one DOCSIS channel of 32Mbps to get the same experience as DSL user with dedicated rate of 2Mbps. According to traffic statistics page size is 68KB on average, and tOFF is 14.5s on average, which is much more than 32/2=16 users.Price: high user speed.**DOCSIS MAC Protocol**• Traffic that is transmitted downstream to the users is controlled by CMTS (cable modem termination system) in headend. It polices and shapes the traffic, and perform algorithms such are WFQ and RED. • Users requests are resolved at headend, and they are informed about the resolution through the downstream channel. If there is a collision of requests, users repeat their requests according to exponential back-off mechanism, otherwise they send data in specified time slot(s).**Performance for BE service in DOCSIS**• Assume that requests form a Poisson process with rate g, T is time slot duration, and Tpis a packet duration. The throughput equals S=Tp/(Tp+I), where I is the average time between packet transmissions. • The probability of a packet transmission is equal to the probability that there is only one request in some previous time slot which is gTe-gT. • The average time between transmissions is**S**2 1 l g Performance for BE service in DOCSIS • The throughput is • It tends to 0 when g increases . • Protocol is unstable like ALOHA.**Wireless LANs**• Distributed coordination function (DCF) • Point coordination function (PCF)**The 802.11 Protocol Stack**Part of the 802.11 protocol stack.

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