In controlled access methods, the stations consult one another to find which station has the right to send. A station cannot send unless it has been authorized by other stations.

There are three main approaches for controlled-access methods:

i. Reservation System

• The principle of reservation system can be understood from Fig. 3.1
• In this system each station transmits a single packet at the full rate R bps.The transmissions from the stations can be organized into frames of variable length.
• Before each frame a reserved slot (interval) is being transmitted.

• Figure 3.2 shows the details of the reservation interval “r”. The reservation interval consists of M minislots, one per station.
• Stations use their minislots to indicate that they have a packet to transmit in the corresponding frame.
• The station that wants to transmit packet by broadcasting their reservation bit during the appropriate minislot.
• By listening to the reservation interval, the stations can determine the order of packet transmissions in the corresponding frame.
• The frame length corresponding to the number of stations which have a packet to tansmit.
• If the length of the packet is variable, then it can be handled if the reservation message includes packet length information.

ii. Polling

• In this system the stations take turns accessing the medium.
• At any given time only one of the stations will transmit into the medium.
• One device act as a primary device and the other rest of the devices act as a secondary device.
• Secondary device follows the instruction of the primary devices and primary device will decide which medium to access the data.
• Consider the example as shown in Figure 3.3, Sender sends a poll message to Machine A, to check whether they have anything to send.
• If Machine A has nothing to send then it will send a NACK (No Acknowledgement) to the sender.
• If Machine B wants to send data, then it will pass the data to the destination machine and destination machine will send ACK (Acknowledgement).

iii. Token passing

• Token is a special frame which is used to authorize a particular station for transmission.
• In the token passing method, a station is authorized to send its data receives the token.
• Each station has a predecessor and successor.
• The frames comes from the predecessor and go to the successor
• When no data is being transmitted and the line is idle, the token circulates around the ring.
• If a station is ready to send data, then it will wait for the token. The station will capture the circulating token and transmit one or more frames.
• This station will keep sending the frames as long as it has frames to send or the allocated time is not complete.
• It then passes this token to the next station in the ring.

• Consider the example above, token revolve inside the ring. If machine 1 wants to transmit the data then it will occupy the token.
• After processing Machine 1 releases the token and the token is received by the other machine that wants to transmit the data.