MAC_ Overview & allocation methods.md (1930B)
1 +++ 2 title = "MAC: Overview & allocation methods" 3 +++ 4 5 # MAC: Overview & allocation methods 6 having multiple users on a channel in a way that can lead to conflicts is a contention system 7 8 broadcast channels are multiaccess or random access channels 9 10 contention is especially important in LANs, most of all wireless, since they broadcast naturally 11 12 MAC sublayer determines how to how to allocate a channel between multiple users 13 14 **Static channel allocation** 15 16 - chop up its capacity using a muxing scheme (e.g. FDM) 17 - if N users, bandwidth is split up into N equal-sized portions, one for each user 18 - no interference between users because each user has a private freq band 19 - best for small and constant number of users 20 - if number of senders is large and varying, or bursty traffic, FDM has more problems 21 - if spectrum is cut up into N regions, the number of users might be <=> N and lead to waste or lack of enough spectrum 22 - not good for bursty traffic 23 24 **Dynamic channel allocation** 25 Assumptions: 26 27 1. Independent traffic — N independent stations, each with a program/user generating frames 28 29 2. Single channel — one channel for all communication, all stations transmit and receive on it 30 31 3. Observable collisions — if two frames are transmitted at same time, they overlap and the result is garbled: “collision”. All stations can detect that a collision occurred, and the frame must be transmitted again later 32 33 4. Continuous/slotted time 34 35 - time may be assumed continuous, so frame transmission can begin at any time 36 - time may also be divided into slots, with frame transmissions starting at the beginning of a slot. 37 38 5. Carrier sense/no carrier sense 39 40 - carrier sense — stations can tell if channel is in use before using it, no station will try to use the channel while it’s busy 41 - no carrier sense — stations just go ahead and transmit and later determine whether they transmitted successfully