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Collision-Free Protocols in Computer Network

Almost all collisions can be avoided in CSMA/CD but they can still occur during the contention period. The collision during the contention period adversely affects the system performance, this happens when the cable is long and length of packet are short. This problem becomes serious as fiber optics network came into use. Here we shall discuss some protocols that resolve the collision during the contention period.

Pure and slotted Aloha, CSMA and CSMA/CD are Contention based Protocols:

Collision Free Protocols:

1. Bit-map Protocol:
Bit map protocol is collision free Protocol. In bitmap protocol method, each contention period consists of exactly N slots. If any station has to send frame, then it transmits a 1 bit in the corresponding slot. For example, if station 2 has a frame to send, it transmits a 1 bit to the 2nd slot.

In general, Station 1 Announce the fact that it has a frame questions by inserting a 1 bit into slot 1. In this way, each station has complete knowledge of which station wishes to transmit. There will never be any collisions because everyone agrees on who goes next. Protocols like this in which the desire to transmit is broadcasting for the actual transmission are called Reservation Protocols.

Bit Map Protocol fig (1.1)

For analyzing the performance of this protocol, We will measure time in units of the contention bits slot, with a data frame consisting of d time units. Under low load conditions, the bitmap will simply be repeated over and over, for lack of data frames. All the stations have something to send all the time at high load, the N bit contention period is prorated over N frames, yielding an overhead of only 1 bit per frame.

Generally, high numbered stations have to wait for half a scan before starting to transmit low numbered stations have to wait for half a scan(N/2 bit slots) before starting to transmit, low numbered stations have to wait on an average 1.5 N slots.

2. Binary Countdown:
Binary countdown protocol is used to overcome the overhead 1 bit per binary station. In binary countdown, binary station addresses are used. A station wanting to use the channel broadcast its address as binary bit string starting with the high order bit. All addresses are assumed of the same length. Here, we will see the example to illustrate the working of the binary countdown.

In this method, different station addresses are read together who decide the priority of transmitting. If these stations 0001, 1001, 1100, 1011 all are trying to seize the channel for transmission. All the station at first broadcast their most significant address bit that is 0, 1, 1, 1 respectively. The most significant bits are read together. Station 0001 see the 1 MSB in another station address and knows that a higher numbered station is competing for the channel, so it gives up for the current round.

Other three stations 1001, 1100, 1011 continue. The next station at which next bit is 1 is at station 1100, so station 1011 and 1001 give up because there 2nd bit is 0. Then station 1100 starts transmitting a frame, after which another bidding cycle starts.

Binary Countdown fig (1.2)

3. Limited Contention Protocols:

  1. Behave like the ALOHA scheme under light load 
  2. Behave like the bitmap scheme under heavy load.

4. Adaptive Tree Walk Protocol:

  1.  treat every stations as the leaf of a binary tree 
  2. first slot (after successful transmission), all stations
    can try to get the slot(under the root node). 
  3. If no conflict, fine. 
  4. Else, in case of conflict, only nodes under a subtree get to try for the next one. (depth first search)

Adaptive Tree Walk Protocol fig (1.3)

Slot-0 : C*, E*, F*, H* (all nodes under node 0 can try which are going to send), conflict
Slot-1 : C* (all nodes under node 1 can try}, C sends
Slot-2 : E*, F*, H*(all nodes under node 2 can try}, conflict
Slot-3 : E*, F* (all nodes under node 5 can try to send), conflict
Slot-4 : E* (all nodes under E can try), E sends
Slot-5 : F* (all nodes under F can try), F sends
Slot-6 : H* (all nodes under node 6 can try to send), H sends.

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