Controlled Access Protocol

Polling

The polling method primarily works with those topologies where one device acts as the primary station and the other as the secondary station.

• All data exchange must be made through the primary device even though the secondary device's final destination.
• Thus, to impose order on a network of independent users and establish one station that will act as a controller and periodically polls over all other stations, this is called polling.
• The Primary device controls the link while the secondary device follows the instructions of the primary device.
• The responsibility is on the primary device to determine which device is allowed to use the channel at a given time.
• Therefore the primary devices are always an initiator of the session.

Efficiency

Assume Tpoll be the time for polling and Tt be the time required for data transmission. Then,

Efficiency = Tt/(Tt + Tpoll)

Poll Function

If the primary devices want to receive the data, it asks the secondary devices if they have anything to send; this is called Poll Function.

• We use a poll function by the primary devices to request transmissions from the secondary devices.
• When a primary device is ready to receive the data, it must ask(poll) each secondary device in order if it has anything to send.
• If the secondary device has some data to transmit, it sends the data frame. Otherwise, it sends a negative acknowledgment (NAK).
• After that, in case of a negative response, the primary then polls the next second in the same fashion until it finds the one with data to send. When the primary device receives a positive response from a data frame, it reads that frame and returns an acknowledgment (ACK ) frame.

Select Function

If primary devices want to send the data, it tells the secondary devices to get ready to receive the data; this is called the Select function.

• Thus, the primary device uses the select function to send something.
• It has been already informed that the primary device always controls the link.
• Before transmitting the data frame, a select (SEL) frame is made and sent by the primary device, and one area of the SEL frame contains the address of the secondary device.
• The primary device alerts the secondary widgets for the upcoming transmission and waits for the secondary devices' acknowledgment (ACK).

Advantages of Polling

The advantages of polling are the following:

• No slot is wasted in polling.
• It has maximum efficiency.
• It has maximum bandwidth utilization.
• The maximum and minimum access times and data rates on the channel are fixed and predictable.
• There is the assignment of priority to ensure faster access from some secondary.

Disadvantages of Polling

The disadvantages of polling are the following:

• Polling results in a waste of time.
• Since each station has an equal chance of winning in each round, link sharing is unfair.
• Only a few stations might run out of data to send.
• An increase in the turnaround time leads to a drop in the data rate of the channel under low loads.

Token Passing

In token-passing methods, we organize all the stations in a logical ring. We can also say that each station has a predecessor and a successor.

• A predecessor is a station that is logically before the station in the ring, while the successor is after the station in the ring. The station that is accessing the channel is the current station.
• A unique bit pattern or a small message circulating from one station to the next in some predefined order is called a token.
• Possessing the token gives the station the right to access the channel and send its data.
• Whenever a station has some data to send, it waits until it acquires a token from its predecessor. After obtaining the token, it holds it and then sends its data. When a station has no more data to send, it frees the token and passes the token to the next logical station in the ring.
• Furthermore, the station cannot transmit the data until it receives the token again in the next round.
• In Token passing, when a station acquires the token and has no data to send, it passes it to the next station.
• The problem due to the Token passing technique is the duplication of tokens or the loss of tokens. The insertion & removal of a station also needs to be tackled for the correct and reliable operation of the token passing technique.

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

Performance:

There are two parameters that conclude the performance of the token ring:

1. Delay: Delay is a measure of time between ‘when a packet is ready’ and ‘when the packet is delivered’. So, the average time (delay) required to send a token to the next station = a/N. Here, N = number of stations & a = Ratio of propagation delay and transmission delay = Tp/Tt.
2. Throughput: It is a measure of successful traffic.

Throughput, S = 1/(1 + a/N), for a < 1 
And S = 1/{a(1 + 1/N)}, for a > 1.
where N = number of stations
a = Tp/Tt 
(Tp = propagation delay and Tt = transmission delay)

Must Read Stop and Wait Protocol, Lock based protocol in DBMS

Advantages of Token Passing

The following are some advantages of token passing:-

• Fairness: Token passing ensures fairness in the network. Each node gets a turn to transmit when it has the token, preventing any single node from dominating the network.
   
• Deterministic: It offers deterministic behavior, as the order in which nodes access the network is predefined by the token passing sequence.
   
• Efficiency: Token passing can be efficient since it eliminates contention and collisions.

Disadvantages of Token Passing

The following are some disadvantages of token passing:-

• Complexity: Token passing protocols can be complex to implement and manage, especially in large networks.
   
• Single Point of Failure: Token loss or failure of the token-passing mechanism can disrupt network communication.
   
• Latency: In networks with a long token rotation time, nodes may experience latency when waiting for their turn to transmit.

Frequently Asked Questions

What is a Controlled Access Protocol?

Controlled access protocols allow only one device to send data at a time, thus avoiding collisions among the shared mediums. No station can send the data unless authorized by the other stations.

What is an example of controlled access?

One example of controlled access in a reservation is: The reservation interval is divided into an equal number of M slots if there are M stations. For each station, a separate slot is available.

How does controlled access work?

All stations seek data from one another under a controlled access protocol to locate the station that is authorised to broadcast data across the medium. It only permits one node for data transfer at a time.

What are all three types of controlled access protocols?

There are three types of controlled access protocols. These are reservation, polling, and token passing. All these three types are used in transferring data. No station can transport the data without permission from other stations.

How many types of controlled access methods are there?

There are several types of controlled access methods, primarily including token passing, where devices pass a token granting permission to transmit; polling, where a central controller invites devices to transmit; and Time Division Multiple Access (TDMA), where devices transmit in assigned time slots. These methods manage network communication to avoid collisions.

Conclusion

This article teaches about Controlled Access Protocol and how we use them. We saw why Controlled Access Protocol could be beneficial for a developer to learn. 

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