RANDOM ACCESSRANDOM ACCESS• InIn random accessrandom access oror contentioncontention methods, no stationmethods, no stationis superior to another station and none is assignedis superior to another station and none is assignedthe control over another.the control over another.• No station permits, or does not permit, anotherNo station permits, or does not permit, anotherstation to send.station to send.• At each instance, a station that has data to sendAt each instance, a station that has data to senduses a procedure defined by the protocol to make auses a procedure defined by the protocol to make adecision on whether or not to send.decision on whether or not to send.12.4
Frames in a pure ALOHA network12.61. Each station sends a frame when it has a frame to send. Thus likelihood of collisions.2. If Ack timed-out, each station waits a random amount of time, TB, before resending.3. After Kmax (normally = 15) No. of resending attempts, must give up and try later.
Behavior of three persistence methods- What to do if channel busy or idle12.10After finding line idlesends immediately, ie.with probability 1. Maxchance of collisionsIf line idle sendsimmediately, else waitsrandom amount oftime. ReducedcollisionsIf line idle sends withprob p, else waits fornext slot.
Flow diagram for three persistence methods12.11
12.12Note, quickly terminating damaged frames saves time and bandwidth.
A network using CSMA/CD has a bandwidth of 10 Mbps.If the maximum propagation time (including the delays inthe devices and ignoring the time needed to send ajamming signal, as we see later) is 25.6 μs, what is theminimum size of the frame?ExampleSolutionThe frame transmission time is Tfr = 2 × Tp = 51.2 μs.This means, in the worst case, a station needs to transmitfor a period of 51.2 μs to detect the collision. Theminimum size of the frame is 10 Mbps × 51.2 μs = 512bits or 64 bytes. This is actually the minimum size of theframe for Standard Ethernet.12.16
CONTROLLED ACCESSCONTROLLED ACCESSInIn controlled accesscontrolled access, the stations consult one another, the stations consult one anotherto find which station has the right to send. A stationto find which station has the right to send. A stationcannot send unless it has been authorized by othercannot send unless it has been authorized by otherstations. We discuss three popular controlled-accessstations. We discuss three popular controlled-accessmethods.methods.12.25