Controlled Access Protocols

In this tutorial, we will be covering the concept of Controlled access protocols in Computer networks.

In the Controlled access technique, all stations need to consult with one another in order to find out which station has the right to send the data.

• The controlled access protocols mainly grant permission to send only one node at a time; thus in order to avoid the collisions among the shared mediums.

• No station can send the data unless it has been authorized by the other stations.

The protocols lies under the category of Controlled access are as follows:

1. Reservation

2. Polling

3. Token Passing

Let us discuss each protocol one by one:

1. Reservation

In this method, a station needs to make a reservation before sending the data.

• Time is mainly divided into intervals.

• Also, in each interval, a reservation frame precedes the data frame that is sent in that interval.

• Suppose if there are 'N' stations in the system in that case there are exactly 'N' reservation minislots in the reservation frame; where each minislot belongs to a station.

• Whenever a station needs to send the data frame, then the station makes a reservation in its own minislot.

• Then the stations that have made reservations can send their data after the reservation frame.

Example

Let us take an example of 5 stations and a 5-minislot reservation frame. In the first interval, the station 2,3 and 5 have made the reservations. While in the second interval only station 2 has made the reservations.

2. Polling

The polling method mainly works with those topologies where one device is designated as the primary station and the other device is designated as the secondary station.

• All the exchange of data must be made through the primary device even though the final destination is the secondary device.

• Thus to impose order on a network that is of independent users, and in order to establish one station in the network that will act as a controller and periodically polls all other stations is simply referred to as polling.

• The Primary device mainly controls the link while the secondary device follows the instructions of the primary device.

• The responsibility is on the primary device in order to determine which device is allowed to use the channel at a given time.

• Therefore the primary device is always an initiator of the session.

Poll Function

In case if primary devices want to receive the data, then it usually asks the secondary devices if they have anything to send. This is commonly known as Poll Function.

• There is a poll function that is mainly used by the primary devices in order to solicit transmissions from the secondary devices.

• When the primary device is ready to receive the data then it must ask(poll) each secondary device in turn if it has anything to send.

• If the secondary device has data to transmit then it sends the data frame, otherwise, it sends a negative acknowledgment (NAK).

• After that in case of the negative response, the primary then polls the next secondary, in the same manner until it finds the one with the data to send. When the primary device received a positive response that means (a data frame), then the primary devices reads the frame and then returns an acknowledgment (ACK )frame,

Select Function

In case, if the primary device wants to send the data then it tells the secondary devices in order to get ready to receive the data. This is commonly known as the Select function.

• Thus the select function is used by the primary device when it has something to send.

• We had already told you that the primary device always controls the link.

• Before sending the data frame, a select (SEL ) frame is created and transmitted by the primary device, and one field of the SEL frame includes the address of the intended secondary.

• The primary device alerts the secondary devices for the upcoming transmission and after that wait for an acknowledgment (ACK) of the secondary devices.

Advantages of Polling

Given below are some benefits of the Polling technique:

1. The minimum and maximum access times and data rates on the channel are predictable and fixed.

2. There is the assignment of priority in order to ensure faster access from some secondary.

Drawbacks

There are some cons of the polling method and these are as follows:

• There is a high dependency on the reliability of the controller

• The increase in the turnaround time leads to the reduction of the data rate of the channel under low loads.

3. Token Passing

In the token passing methods, all the stations are organized in the form of a logical ring. We can also say that for each station there is a predecessor and a successor.

• The predecessor is the station that is logically before the station in the ring; while the successor is the station that is after the station in the ring. The station that is accessing the channel now is the current station.

• Basically, a special bit pattern or a small message that circulates from one station to the next station in some predefined order is commonly known as a token.

• Possessing the token mainly gives the station the right to access the channel and to send its data.

• When any station has some data to send, then it waits until it receives a token from its predecessor. After receiving the token, it holds it and then sends its data. When any station has no more data in order to send then it releases the token and then passes the token to the next logical station in the ring.

• Also, the station cannot send the data until it receives the token again in the next round.

• In Token passing, when a station receives the token and has no data to send then it just passes the token to the next station.

• The problem that occurs due to the Token passing technique is the duplication of tokens or loss of tokens. The insertion of the new station, removal of a station, also needs to be tackled for correct and reliable operation of the token passing technique.

The performance of a token ring is governed by 2 parameters, which are delay and throughput.

Delay is a measure of the time; it is the time difference between a packet ready for transmission and when it is transmitted. Hence, the average time required to send a token to the next station is a/N.

Throughput is a measure of the successful traffic in the communication channel.

Throughput, S = 1/ (1 + a/N) for a<1

S = 1/[a(1+1/N)] for a>1, here N = number of stations & a = Tp/Tt

Tp = propagation delay &Tt = transmission delay

In the diagram below when station-1 posses the token, it starts transmitting all the data-frames which are in its queue. now after transmission, station-1 passes the token to station-2 and so on. Station-1 can now transmit data again, only when all the stations in the network have transmitted their data and passed the token.

Note: It is important to note that A token can only work in that channel, for which it is generated, and not for any other.