Computer Networking

Transport Layer

Controlled Access Protocols

In controlled access protocols, there is no chance of collision because stations gets information from one another to find which station has the right to send. Control access protocols allow only one node to send at a time. In this way the collision of messages on shared medium can be avoided.
The 3 basic controlled-access methods which  are:

1.   Reservation

According to reservation, every station needs to make a reservation before to transmit the data.

  • If there are N stations, then reservation slots will also be N, So, each station has its one mini-reservation slot.
  • If station-1 has a frame to send, it transmits bit-1 to its corresponding mini reservation slot.
  • No other station is allowed to transmit data during the reservation slot of any other station.
  • Transmission of frames from a station depends upon the order of reservation of slots.
  • After the first reservation interval (data transmission period), the next reservation interval begins.
  • In this way there will never be any collisions.

Note: As sending one bit in a slot. So it called mini-reservation slot.

Example: The following figure shows a situation with 5 stations and a 5 mini reservation slots.

  • In the first data transmission interval, only stations 1, 3, and 4 are sending data.
  • In the second interval, only station 3 and 4 has made a reservation and sending data.
  • And in the last transmission interval, no station is sending data and so on.

2. Polling

  • In polling, one station acts as a primary station (controller) and the others are considered as secondary stations. All data exchanges are made through the primary station.
  • Polling process is similar to the RollNo-Call performed in class by teacher. As teacher is a controller, all question, answer are made through controller.

In Polling, there are two functions are used, one is POLL and other is SELECT. Let explain

    I. POLL Function

When Primary station wants to receive the data to a secondary station then it uses the POLL function.

  • If the primary station sends a POLL SIGNAL to a secondary station, and corresponding secondary station does not ready to send data then it send NACK (Negative Acknowledgement) instead of data to primary station.
  • Otherwise secondary station send data back to primary station.
  • In the end, if data is received to primary station successfully then primary station sends ACK to corresponding secondary station for confirmation.

    II. SELECT Function

When primary station wants to send the data to a secondary station then it uses Select function.

  • When the primary station sends a SELECT SIGNAL to a secondary station then corresponding secondary station sends ACK back to primary station.
  • As Primary station received ACK, it sends data to corresponding secondary station.
  • In the end, if data is received to corresponding secondary station successfully then it sends ACK to primary station for confirmation.

3. Token Passing

Token passing uses a special signal called a ticket or token. A token is a special bit pattern or a small message that moves from one station to the other in the some predefined order (clock or anti-clock wise).

According to token passing,

  • In token passing, all stations are logically connected with each other in form of ring. If any station wants to send data then first it has to get token. Only one token is used in the entire network.

Any station in the network may transmit the data only if it has a token.  Without token/ticket device cannot communicate with network. So, there is no chance of collision in controlled access protocols.

When a computer in token ring standard network wants to send a message it must follow the following

  • Gets the token
  • Puts the data in token
  • Adds the address of receiving computer

Token passes from computer to computer, the computer whose address is match with the mentioned (destination) address in token will receive the data.

 It also sends the acknowledgement signals to sender to indicate that the data has been received successfully. Token ring follow the ring topology, That’s why it can also use in start topology.

Keep in Mind: the station which contains token is called current station, the station which will gets the token, after the releasing of current station is called successor.  The station, from where the current station gets the token is called predecessor.

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