Line Discipline

Enquiry / Acknowledgement (ENQ/ACK)

Used primarily in systems where there is no question of wrong receiver getting the transmission
In other words when there is a Dedicated Link b/w the two devices so that the only device that can receive data is the intended one
Enquiry / Acknowledgement (ENQ/ACK)
ENQ/ACK coordinates who may start a transmission and whether or not the intended recipi nt is ready and enabled

Using ENQ/ACK, a session can be initiated by either station on a link as long as both are of equal rank- a printer for example cannot initiate communication with a CPU
Enquiry / Acknowledgement (ENQ/ACK)
In both half duplex and full duplex TX, the initiating device establishes the session
In half-duplex, the initiator then sends its data while the responder waits
The respondent may take over the link when the initiator is finished or has requested a response
In full duplex, both the devices can TX simultaneously once the link has be established

The receiver must answer either with an acknowledgement (ACK) frame if it is ready to receive or with a negative acknowledgement ( NAK), if it is not
By requiring a response, even if the answer is negative, the initiator knows that his enquiry was in fact received even if the receiver is currently unable to receive
If neither an ACK or a NAK is received within a specified time limit, the initiator assumes that an ENQ frame was lost in transit, it disconnects and sends a replacement
An initiating system ordinarily makes 3 such attempts before giving up
If the response to the ENQ is negative for 3 attempts, the initiator disconnects and begins the process again at another time

If the response is positive the initiator is free to send its data
How It Works

The initiator first transmits a frame called an enquiry (ENQ) asking if the receiver is available to receive data
Once all of its data have been transmitted, the sending system finishes with an End of Transmission (EOT) frame

Primary-Secondary communication

This method works with topologies where one device is designed as a Primary station and the other devices are Secondary stations
Multipoint systems must coordinate several nodes, not just two
The questions are not only Are you Ready? But also Which of the device has the right to use the channel
Whenever a multipoint link consists of a primary device and multiple secondarydevices using a single TX line , all exchanges must be made through the primary device even when the ultimate destination is a secondary device
The primary device controls the link and the secondary device follow sits instruction
It is up to the primary to determine which device is allowed to use the channel ata given time
The primary therefore is always the initiator of the a session

If the primary wants to receive data, it asks the second-arise if they have anything to send, This is called POLLING
If the primary wants to send data, it tells the target secondary to get ready to receive, This function is called SELECTING

For point-to-point configuration, there is no need for addressing
Any TX put onto the link by one device can be intended only for the other
For the primary device in a a multipoint topology to be able to identify and communicate with a specific secondary device, there must be some addressing convention
For this reason, every device on the link has an address that can be used for identification
Inny transmission, this address will appear in a specified portion of each frame, called the Address Field or Header depending upon the protocol
If the TX comes from a secondary device, the address indicates the originator of the data

The select mode is used whenever the primarydevice has something to send
Primary control the link and if primary is not sending or receiving data, it knows that the link is available
If it has something to send, it sends it
As a frame makes its way to the intended device, each of the other devices check the address field
Only when the device recognizes its own address, does it open the frame and read the data
In case of a SEL frame, the enclosed data consists of an alert that data is forthcoming
What it does not know, however is if the target device it ready to receive (ON)
So the primary must alert the secondary to the upcoming TX and wait for an acknowledgement of the secondary ready status
Primary send a SEL frame, one field of which includes the address of the intended RX
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

Used by the primary device to receive transmissions from the secondary devices
The secondaries are not allowed to TX data until asked
By keeping control with the primary, the multipoint system guarantees that only one TX can occur at a time
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 if it does
If the response is negative, primary then polls the next secondary
When the primary has received data, it acknowledges by sending an ACK

Two possibilities for termination:

2nd aspect of data link control is Flow Control
In most protocols, flow control is a set of procedures that tells the sender how much data it can transmit before it must wait for an ACK from the receiver
The flow of data must not be allowed to overwhelm the receiver

Section 10.1,10.2 “Data Communication and Networking” 4th Edition by Behrouz A. orouzan