Chapter 4 Medium Access Control Sublayer.docx

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Information Technology Management
ITM 301
Franklyn Prescod

Chapter 4 The Medium Access Control SublayerNetwork Links are divided into two categories PointtoPoint connections and Broadcast channelsBroadcast channels referred to as multiaccess channels or random access channelsProblem how to determine who gets to use the channel when there is competition for it When only a single channel is available it is much harder to determine who will go nextMedium Access Control MAC the protocol used to determine who goes next on a multiaccess channel belongs to a Sublayer of the data link layer It is very important in LANS especially wireless ones because wireless is a natural broadcast channelWANs use point to point links except for satellite networks41 The Channel Allocation Problem A single broadcast channel might be a portion of the wireless spectrum or a single wire or optical fiber to which multiple nodes are connected In both cases the channel connects each user to all other users and whichever user makes full use of the channel interferes with other users who wish to use it411 Static Channel Allocation Traditional Way of allocating a single channel is to chop up its capacity by using one of the multiplexing schemes If there are N users the bandwidth is divided into N equal sized portions Useful when there is small and constant number of users Example is FM radio stationsProblems when the spectrum is cut up into N regions and fewer than N users are currently using it a large piece of valuable spectrum is wasted Some N users may be denied permission for lack of bandwidth when the user is not using it no one else is allowed to either412 Assumptions for Dynamic Channel Allocation Five key Assumptions 1 Independent Traffic the model consists of N independent stations computers telephones each with a program or user that generates frames for transmission Once a frame has been generated the station is blocked and does nothing until the frame has been successfully transmitted not good model of network traffic because the packets come in bursts over a range of time scales 2 Single Channel a single channel is available for all communication All stations can transmit on it and all can receive on it The stations are assumed to be equally capable though protocols may assign them different roles heart of the model because no external way to communicate 3 Observable Collisions if two frames are transmitted simultaneously they overlap in time and the resulting signal is garbledcollision All stations can detect that a collision has occurred A collided frame must be transmitted again later No errors other than those generated by collision occur 4 Continuous or Slotted Time time may be assumed continuous in which case frame transmission can begin at any instant Time may be slotted or divided into discrete intervals slots Frame transmission must begin at the start of a slot A slot may contain 01 or more frames corresponding to an idle slot a successful transmission or a collision 5 Carrier Sense or No Carrier Sense With the carrier sense stations can tell if the channel is in use before trying to use it No station will attempt to use the channel while it is sense busy If there is no carrier sense stations cannot sense the channel before using it they just go ahead and transmit Only later can they determine whether the transmission was successful
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