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Week3 The Medium Access Sublayer

Explore multiple access protocols at the Medium Access Control (MAC) sublayer, including Aloha, CSMA, and channel allocation methods in computer networks. Learn about frame generation, collision assumptions, and throughput calculations for efficient data traffic management.

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Week3 The Medium Access Sublayer

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  1. Week3The Medium Access Sublayer Multiple Access Protocols

  2. Overal Internet Architecture

  3. MAC Sublayer

  4. MAC Architecture

  5. Computer Networks 1 Broadcast through a Single Channel • Determining who will use the channel next is a problem • Medium Access Control (MAC) sublayer solves this problem • MAC is a sublayer (bottom part) of data link layer • Broadcast Channels, also called • Multiaccess Channels • Random Access Channels

  6. Static Channel Allocation • Usually done by FDM or TDM • Not an efficient method for data traffic: • Let • The capacity of a channel be C bps • The mean time delay of the channel be T (seconds) • Frame arrival rate is a random variable from Poisson distribution with mean  frames/second • Frame length is a random variable from exponential probability density function with mean 1/ bits/frame • Then • T = 1 / (C -  ) (result from queuing theory) • Now, let the channel be divided into N subchannels with capacity C/N and mean input rate /N • TFDM= 1 / ((C/N) – (/N) = N / (C - ) = NT • Tis means that he average delay is N times worse

  7. Dynamic Channel Allocation Assumptions 1 • Station Model: block and wait • Generates frames at a rate of  frames/unit time (Frame generation is Poisson Distribution) • Once a frame is generated, the station is blocked until the frame is successfully transmitted • Single Channel Assumption: equal rights • All stations transmit and receive with equal priority over a unique channel

  8. Dynamic Channel Allocation Assumptions 2 • Collision Assumption • Overlapping transmission by two or more stations at the same time garbles the frames (collision) • All stations detect collisions • There are no errors other than those generated by collisions • Continuous Time • Frame transmission can begin at any instant of time • Slotted Time • Time is divided into very narrow time slots • Frame transmission always begins with a slot

  9. Dynamic Channel Allocation Assumptions 3 • Carrier Sense • Stations can detect if the channel is in use • LANs generally have carrier sense • No Carrier Sense • Stations can not sense the channel before trying to use it • Satellite networks do not have carrier sense

  10. Pure ALOHA • Users transmit any time • If there is a collision • sender knows about it after a certain time, • waits random amount of time, • sends the frame again • Contention systems • Systems in which multiple users share a common channel in a way that can lead to conflicts • To maximize throughput, frames must have uniform sizes

  11. Frames in Pure ALOHA

  12. ALOHA Assumptions • Frame time=time to transmit one frame • Number of frames generated in a frame time is a Poisson Distribution with mean N. • If N>1, every frame will suffer a collision • 0<N<1 is reasonable • Probability of k transmission attempts in a frame time is Poisson with parameter G. Pr[k]=Gk e-G/k! • For small N, G  N • For large N, G>N

  13. ALOHA cont’d • P0 = probability that a frame does not suffer a collision • S = Probability of a transmission succeeding • S = G P0

  14. ALOHA Frame Collision Period

  15. Efficiency of ALOHA • Referring to the figure on prev. page, the vulnerable period is two frame times • The probability that no frame is transmitted during this period is e-2G • Pr[0]=e-G in one frame period so P0=e-2G in two frame periods • Therefore troughput S = G e-2G • The maximum of S occurs at G=0.5, S=1/2e

  16. ALOHA Throughput

  17. Slotted ALOHA 1 • Can only transmit at the beginning of a slot • Vulnerable period is halved • Hence S = G e-G • S peaks at G = 1 • Probability that a frame avoids a collision is e-G • The probability of a collision is 1-e-G • Probability of a transmission requiring exactly k attempts is Pk=e-G(1-e-G)k-1

  18. Slotted ALOHA 2 • Expected number of transmissions, E, per each created frame is  E =  k Pk =  ke-G(1-e-G)k-1=  d/dG(1-e-G)k = k=1 k=1 k=1  d/dG  (1-e-G)k = d/dG eG = eG k=1 • Conclusion: Performance exponentially degrades by the load

  19. Carrier Sense Multiple Access (CSMA) Protocols • ALOHA does not listen to the channel before it transmits, ending up with poor performance • Carrier Sense Protocols • Stations listen the channel if there is any transmission going on before they transmit

  20. Persistent and Nonpersistent CSMA • 1-persistent CSMA • Stations transmit with probability 1 whenever they find the channel idle • Nonpersistent CSMA • If the channel is idle before the first attempt, transmit • If the channel is already in use, wait for a random amount of time, and then listen to the channel for transmission • P-persistent CSMA • Applies to slotted channels • If the channel is idle, • transmit with probability p • Defer transmission until the next slot with probability q = 1 – p • If, in the mean time, someone else transmits, wait a random time • If channel busy • Wait for the next slot

  21. Channel Utilization for Random Access Protocols

  22. CSMA with Collision Detection (CSMA/CD) • collision Detection • Abort transmission as soon as detect collision • If  is the time the signal propagates between two farthest stations, the station has to wait 2  to make sure that no collision has occurred • CSMA/CD model has contention, transmission and idle periods • Contention period is modeled as a slotted ALOHA with slot size 2

  23. CSMA/CD States

  24. Collision-Free Protocols • Assumptions • There are N stations • Each station has a unique address (0 to N-1) hardwired to it • Question • Which station gets the channel after a successful transmission?

  25. A Bit-Map Protocol (Reservation Protocol) • Two rounds of transmission cycle • First Round (Contention Period) • Consists of N slots each reserved for a particular station • In this period, each station transmits • 1 if it has a frame to transmit • 0 if it has no frame to transmit • At the completion of the first round everybody knows who wants to transmit • Second Round (Transmission Period) • Stations transmit according to the order formed in the first round • There will not be any collisions

  26. The basic bit-map protocol

  27. Reservation Protocol Performance :Binary Countdown • Each station has a binary station address • A station wanting to transmit broadcasts its address starting with the high-order bit • The bits from each station are boolean Or’ed • Arbitration Rule • As soon as a station sees that a high-order bit position that is 0 in its address is overwritten by 1, it gives up • Channel Efficiency is d/(d+log2N) • If station address is the first field in the frame then efficiency is 100%.

  28. Binary Countdown Example

  29. Wavelength Division Multiple Access (WDMA) Protocols • All optical LANs divide the spectrum into wavelength bands • Each station is assigned two channels • Narrow channel: Control channel to signal the station • Wide channel: Station outputs data frames • Narrow channel is divided into m time slots • Wide channel is divided into n+1 slots • n for data output • 1 for status (to indicate which slots on both channels are free)

  30. Computer Networks 1 WDMA 2

  31. WDMA 3 • Both connection-oriented and connectionless traffic is supported • Each station has: • A fixed-wavelength receiver for listening to its own control channel • A tunable transmitter for sending on other station’s control channel • A fixed-wavelength transmitter for outputting data frames • A tunable receiver for selecting a data transmitter to listen to

  32. WDMA Connection Setup Procedure • A tunes its data receiver to B’s data channel and waits for the status slot to learn about a free B control slot (on 4 of A) • A chooses a free control slot and sends a CONNECTION REQUEST (on 2 of A) • B assigns a data slot to A by announcing it in the status slot (on 3 of B, B also tunes 4 to A’s 3) • A reads this announcement and a unidirectional connection from A to B is established (A transmits on 4 in the slot assigned by B) • If the request was for two way communications, B would repeat the same procedure

  33. The Medium Access Sublayer IEEE Standard 802.3 and Ethernet Ethernet Frame Structure (Ethernet Encapsulation) 7 1 6 6 2 4 preamble SFD DA SA type Data CRC 60 to 1514 bytes synchronize the receiver type Cyclic Redundancy Check 0800: IPv4 datagram 0806: ARP request/reply 8035: RARP request/reply 86DD: IPv6 start frame delimiter

  34. Ethernet Frame (cont’d) • 2 byte type that indicates what kind of data follows, e.g., 0800 for an IP packet • Then the data, maximum 1500 bytes, minimum 46 bytes • Data field must be padded with extra bytes if fewer than 46 bytes are supplied

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