Poll/Select
- If primary device is asking to the secondary device that if you have anything to send known as Polling.
- If secondary device have something and send it to primary device known as Selecting.
- The poll/select method of line discipline works with typologies where one device in designated as primary station and the other devices are secondary stations.
Working of Poll/Select method
- Whenever a multi point link consists of a primary device and must be secondary devices using a single transmission line, all exchanges must be such through the primary device even when the ultimate destination is a secondary device.
- The primary device controls the link; the secondary devices follow its instructions. It is up to the primary to determine which devices allowed to use the channel at a given time. The primary, therefore always the initiator of a session.
- If the primary wants to receive data, it asks the secondary’s if they have anything to send; this function is called polling. If the primary device to send data, it tells the target secondary to get ready to receive.
Addresses
- For point-to-point configuration, there is no need for addressing; any transmission put onto the link by one device can be intended only for the other. For the primary device in a multi point topology to be able to identify and communicate with a specific secondary device, however, the must be an addressing convention.
- For this reason, every device on a link has address that can be used for identification.
- Poll/select protocol identify each frame as being either to or from a specific device on the link.
- Each secondary device has an address that differentiates it from the others. In any transmission, that address will appear in a specified portion of each frame called an address field or header depending on the protocol.
- If the Data Link Control In Data Communication transmission comes form the primary device, the address indicates the recipient of the data. If the transmission comes from a secondary device, the address indicates the originator of the data.
Select
- The select mode is used whenever the primary device has something to send. Remember that the primary controls the link. If the primary is not either sending or receiving data, it knows the link is available. If it has something to send, it sends it.
- What it does not know, however, is whether the target device is prepared to receive (usually, prepared to receive means on).
- So the primary must alert the secondary to the upcoming transmission and wait for an acknowledgment of the secondary’s ready status. Before sending data, the primary creates and transmits select.
- If the secondary is awake and running, it returns an ACK frame to the primary. The primary then sends one or more data frames, each addressed to the intended secondary
Poll
- The polling function is used by the primary device to solicit transmissions from the secondary devices. the secondary’s are not allowed to transmit data unless asked.
- By keeping all control with the primary, the multipoint system guarantees that only one transmission can occur at a time thereby ensuring against signal collisions without requiring elaborate precedence protocols. When the primary is ready to receive data,
- it must ask (poll) each device in turn if it has anything to send. When the first secondary is approached, it responds either with a NAK frame if it has nothing to send or with data (in the form of a data frame) it does.
- If the response is negative (a NAK frame), the primary then polls the next secondary in the same way until it finds one with data to send. When the response is positive a data frame, the primary reads the frame and returns an acknowledgment (ACK frame) verifying its. receipt.
- The secondary may send several data frames one after the other, or it may be required to wait for an ACK before sending each one, depending on the protocol being used.
- There are two possibilities for terminating the exchange: either the secondary sends all its data, finishing with an end of transmission (EOT) frame, or the primary says, “Time’s up.” Which of these occurs depends on the protocol and the length of the message. Once a secondary has finished transmitting, the primary can poll the remaining devices.
