Controlled Access Protocols in Computer Network

In this tutorial, you will learn the basics of controlled access. You will also learn about controlled access protocols that are used to control channel access between devices. After reading this tutorial, you will be able to understand the working of the Controlled Access Protocols.

Contents:

1. What is Controlled Access?
2. Reservation Access Method
3. Working of Reservation Access Method
4. Polling Method
5. Working of Polling Method
6. Select and Poll Function
7. Token Passing
8. Logical Ring and Physical Topology in Token Passing

What is Controlled Access?

Controlling access to a channel is known as controlled access. Multiple devices are connected on a channel, and they all are trying to access the channel for communication. But in controlled access, a device cannot send a frame unless another device authorizes it.

• In controlled access, devices communicate with each other, and devices consult each other about which device has the right to send.
• By controlling channel access, network performance is increased, and frame collisions are reduced.

The figure below shows three popular controlled-access methods.

As shown in the above figure, mainly, Reservation, Polling, and Token Passing are controlled access methods that control access to the channel.

Reservation Access Method?

In the reservation method, the device makes a reservation before sending data over a channel. Time is divided into intervals, and at a particular interval, the device sends frames over a channel that has made the reservation.

• If there are N devices on a channel that wants to communicate, the reservation frame will have a total of N reservation mini-slots.
• Each device is assigned a mini-slot, so when a device needs to send a data frame, it makes a reservation in its mini-slot and then transmits the data.
• Reservation is made in such a way that the path to the destination is decided. Hence the chances of congestion on one channel will be less.
• The bit-map protocol is the reservation protocol used to reserve a channel in advance to prevent collisions.

Working of Reservation Access Method

Now we know that the reservation access method is used to control the access on the channel to prevent collision of frames. So, let us now understand how it works.

The diagram below explains the working of the reservation access method.

• As shown in the figure, 5 devices are communicating on a channel. There are a total of 5 stations, so five mini-slot reservation frame will be created.
• In the first interval, station-1, station-2, station-3, and station-4 have made reservations. So, when their interval comes, they will send the data.
• In the second interval, only stations 2 and 5 have made reservations. Similarly, in the third interval, only station 1 has made a reservation.
• When a station sends data on a channel within its interval, other stations can’t send data frames until the interval end.

Polling Method

In the polling system, one primary station and several secondary stations are connected on a single channel. So that, all the data exchange is done through the primary station by controlling the channel.

• The primary station controls the channel, while the secondary stations follow its instructions for communication. The primary station decides which station is allowed to access the channel to communicate.
• The primary station allowed a secondary station to use the channel at a given time to communicate.
• In simple words, we can say that the primary station is the originator of the communication.
• When the primary station wants to communicate and receive data, it asks secondary stations if they have data to send, this method is known as polling.
• If a primary station wants to send data, it tells the secondary station that it sends data and secondary have to be ready, this is known as the select function.

Working of Polling Method

The diagram below explains the working of polling method and its functions.

• As shown in the figure, there is one primary station and two secondary stations connected on a channel.
• The primary station wants to send data to secondary station B. It sends the SEL (SELECT) function to the secondary station.
• When Secondary Station B receives the SEL, it will send an acknowledgment to the Primary Station to inform that it is ready to receive data.
• Therefore, the primary station sends the data to the secondary station B, and the secondary station sends a reply acknowledgment for the received data to the primary station.
• Figure on the right side, the primary station wants to receive data from secondary station-A. Therefore, it will send the poll function to station-A.
• But, station-A sends a negative acknowledgment (NAK), so the primary station will understand that it does not want to communicate. Therefore, the primary station sends the poll function to secondary station B.
• As soon as station B receives the poll function, it accepts the primary station’s request for data. So, in response, station B sends data to the primary station, and the primary station sends an acknowledgment for the successful arrival of the data.

Select and Poll Function

As we learned, the select and poll functions of poling method play a major role in communication. Now, let us understand the select and poll functions in depth.

Select: As we discussed, whenever the primary station wants to send something, the Select function is used.

• The primary station controls the link, so if it has something to send, the primary device sends it.
• But the primary station has no information about whether the receiver is ready to receive. Therefore, the primary station has to alert the receiver before sending the data to the receiver.
• When the primary station wants to send data, it alerts the secondary station for incoming transmission and waits for the secondary station’s ready status acknowledgment.
• To know whether the receiver is ready for communication, the primary station will create a select (SEL) frame, one of which will contain the desired secondary station’s address in the field and sends it to the secondary.
• If the primary station receives an acknowledgment, only then it starts transmitting the data.

Poll: The poll function is used when the primary station wants to receive something from the secondary station.

• Using the poll function, the primary station demands a transmission from the secondary station.
• When the primary station is ready to receive data, it sends a pole frame to each secondary station to receive the data.
• When the pole frame is received by the secondary station, and if the secondary station does not have data to send, it sends a negative acknowledgment (NAK).
• If the primary station receives a negative acknowledgment from the secondary station, it sends the pole frame to the other secondary devices until it finds a device with the data to send it.
• When the secondary station accepts the poll, it reads the information in the poll frame and gives an acknowledgment confirming its receipt.

Token Passing

In token passing, each station has a predecessor and a successor station, they are arranged in a logical ring. The current station is using the channel for communication, as it gets access to the channel from the predecessor. This right is passed to the successor station when the current station does not have data to send on the channel.

• In token passing, a packet called a token passes through the ring, which gives the station access to the channel.
• The token is transmitted over a channel, and whichever station has the token has the right to access the channel for communication.
• When a station has data to send, it has to wait until it receives a token from its predecessor station. Until the data is sent, one station retains the token, and after completing the task of sending the data, it sends the token to its successor station.
• The station is not able to send data until the token is received again in the next round.
• On a channel, it may be possible that the station receives the token but has no data to send, so in this case, it only sends the token to the next station.
• However, in the token passing method, the tokens are monitored to ensure that it is not lost or destroyed. For example, if a station is holding a token and it fails due to unavoidable circumstances, the token will disappear from the network.
• Also, the token function assigns priority based on the type of data the station has. To release the tokens from low-priority stations to high-priority stations, token management is required.

The below diagram explains the structure of Token Ring.

As shown in the above figure, all the stations are connected in a ring topology. When the current device completes its transmission, it sends the token to the successor station.

Logical Ring and Physical Topology in Token Passing

In a token passing method, on the network, stations do not need to be physically connected in a ring because the ring they form is a logical ring. There are four physical topologies of a logical ring.

1. Physical ring (IFS)
2. Dual ring
3. Bus ring
4. Star ring

The figure below shows four logical ring topologies.

• As shown in the figure, a physical ring, a dual ring, a bus ring, and a star ring are the topologies of the logical ring.
• Physical ring: When a station sends a token to its successor station, other stations cannot see the token because the token does not contain the address of the next successor.
• The problem with this topology is that if one link between two adjacent devices fails, the entire topology fails.
• Dual ring: The dual ring consists of two rings, one is the main ring, and the other is the auxiliary (second) ring. The second ring is only used for emergencies such as the main ring failing.
• If one of the links in the main ring fails, the auxiliary ring forms a temporary ring. When the failed link is restored, the auxiliary ring will be deactivated again.
• Each station should have two transmitter and receiver ports.
• Fiber Distributed Data Interface (FDDI) and Copper Distributed Data Interface (CDDI) ring networks use a dual ring topology.
• Bus ring: All the stations are connected by a single cable called a bus. Each station is aware of the address of its successor station.
• When the data transmission is finished, the station adds the address of the successor station to the token. A station whose address matches the token’s address is only capable of accessing shared media.
• Star ring: All stations are connected to a central device, called a hub, and form a star topology. Wire connections connect stations to the hub, and the wiring inside the hub forms the ring.
• If a channel goes down due to congestion, it will be controlled by the hub, and the rest of the stations can function without being affected.
• Easy to add or remove stations to or from the hub. Token Ring LAN uses the Star Ring topology, designed by IBM.