Data Communication questions and answers

What are the differences between datagram approach virtual circuit approaches in packet switching network? Which one is advantageous & why?

The datagram service, coupled with internal datagram operation, allows for efficient use of the network; no call setup and no need to hold up packets while a packet in error is retransmitted. This latter feature is desired in some real-time applications.

The virtual-circuit service can provide end-to-end sequencing and error control; this service is attractive for supporting connection-oriented applications, such as file transfer and remote-terminal access.

In practice, the virtual-circuit service is much more common than the datagram service. The reliability and convenience of a connection-oriented service is seen as more attractive than the benefits of the datagram services

What is HDLC?

HDLC (High-Level Data Link Control) is a bit-oriented synchronous data link layer protocol developed by the International Organization for Standardization (ISO). It provides both connection oriented & connectionless service. HDLC can be used for point to multipoint connections, but is now used almost exclusively to connect one device to another, using what is known as Asynchronous Balanced Mode (ABM).


Describe supervisory frame in HDLC


Supervisory Frames (S-frames) in HDLC are used for flow and error control whenever piggybacking is impossible or inappropriate, such as when a station does not have data to send. S-frames do not have information fields.

The S-frame control field includes a leading “10” indicating that it is an S-frame. This is followed by a 2-bit type, a poll/final bit, and a sequence number. If 7-bit sequence numbers are used, there is also a 4-bit padding field. The first 2 bits mean it is an S-frame. Poll/Final bit is called Poll when used by the primary station to obtain a response from a secondary station, and Final when used by the secondary station to indicate a response or the end of transmission.


Write the sequences for HDLC initialization


Ans: Initialization may be requested by either side by issuing one of the six set-mode commands. This command serves three purposes:

It signals the other side that initialization is requested

It specifies which of the three modes ( NRM, ABM, ARM ) is requested.

It specifies whether 3- or 7-bit sequences numbers are to be used.


If the other side accepts this request, then the HDLC module on that end transmits an unnumbered acknowledged (UA) frame back to the initiating side. If the request is rejected, then a disconnected mode (DM) frame is sent.