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CS601 - Data Communication - Lecture Handout 37

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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

Enquiry   Acknowledgement

  • 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

How It Works?

  • 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

Poll / Select

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

Poll  Select


  • 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:

  • A secondary sends all its data and finishes with EOT frame
  • Primary says “ Time is Up


Flow Control -Definition

  • 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


  • Line Discipline
    • ENQ/ACK
  • Flow Control
    • Stop-and-Wait
    • Sliding Window

Reading Sections

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