Polling Mechanism In Wireless Network and ISMA

Polling is strictly centralized. If one terminal in wireless communication can be heard by all others, this central terminal also known as Base Terminal(BS) can poll all other terminals according to some schemes like the round-robin scheme, random address scheme, and as per reservation scheme. 

Randomly Addressed Polling

1. The first step is the Base Station signals readiness to all mobile terminals if not busy. 

2. The terminals ready to send can now transmit a random number without collision with the help of CDMA or FDMA. The random number can be seen as a dynamic address. 

3. The Base Station now chooses one address for polling from the list of all random numbers. But collision occurs if two terminals choose the same address.

4.  The Base Station acknowledges correct packets and continues polling the next terminal.

5. The cycle from step 1 to step 4 starts again after polling all terminals of the list.

Advantages

• Unlike in TDMA or CSMA, no slot is ever wasted.
• It provides maximum efficiency and bandwidth utilization.

Disadvantages

• Time is wasted during polling as we have to wait for the medium to be free to start transmission.
• Link sharing is not fair since each station has an equal probability of winning in each round.
• Few stations might starve for sending the data.

Inhibit Sense Multiple Access(ISMA)

ISMA is used for packet data transmission in AMPS mobile phone systems. it is also known as Digital sense multiple Access. (DSMA)

Working

1. Here the current state of the medium is signaled via a busy tone.

2. Base Station signals on the downlink from the base station to terminals if the medium is free or not. 

3. Terminals must not send if the medium is busy.

4. Terminals can access the medium as soon as the busy tone stops.

5. If collision occurs, the base station signals collision via the busy tone.

6. If the data has been transmitted successfully, the base station signals the successful transmission via acknowledgments. Media access is not coordinated within this approach. 

Advantages

The inhibit sense multiple access (ISMA) systems is known to obtain high throughput when a number of terminals transmit data packets to a base station over a common radio channel.

Disadvantages

The capture effect and propagation delay causes unfairness of local throughput depending on the distance from the base station. The unfairness of local throughput is because the distance decreases without decreasing the total throughput. 

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