Page 120 - DCAP406_DCAP_207_Computer Networks

Unit 8: Data Link Protocols

Petri nets are used to detect protocol failures similar to the use of finite state machine. For Notes
example, if some firing sequence included transition Tn twice without transition Tn+1
intervening, the protocol would be incorrect.

Self Assessment

Fill in the blanks:
1. ...................... describes the techniques to access a shared communication channel and reliable
transmission of data frame in computer communication environment.

2. ...................... does not include any connection setup or release and does not deal with
frame recovery due to channel noise.
3. ...................... refers to a reliable transfer of bit streams to the network layer for which the
data link layer breaks the bit stream into frames.
4. ...................... controls mismatch between the source and destination hosts data sending
and receiving speed and therefore dropping of packets at the receiver end.
5. In stop and wait protocol, the acknowledgement frame has ...................... bits that the
destination node sends back to the source machine.
6. Positive Acknowledgement with Retransmission Protocol (PAR) uses ...................... to
determine if any frames is lost or damaged.

7. The Go Back N protocol overcomes the problem of PAR by enabling the source machine
to have more than ...................... at a time by using buffers.

8.4 Example Data Link Protocols

Data link protocols regulate communication flow between various computers. A wide variety
of computers and communication technologies are used to carry out useful tasks. These include
mainframe computers, local area networks, workstations, personal computers, and proprietary
and standards based networking platforms. All of these products are not interoperable and it is
not easy to exchange data across different systems and applications. Therefore, standards were
developed to ensure the interrelationship of many different standards adopted by various
vendors. To establish a meaningful session, a certain sets of rules need to be adopted by vendors
of networking device.
Some of the examples of data link protocol are High Level Data Link Control (HDLC) protocol
and Point-to-Point Protocol (PPP). The discussion here of the simpler HDLC and PPP protocols
will explore many of the most important features of data link protocols.

8.4.1 High-level Data Link Control (HDLC)

HDLC procedure is standardized by ISO. This is suitable for high-speed transmission of large
amounts of data, something the basic control procedure cannot provide. HDLC procedure has
been standardized based on the SDLC. In addition to characters, bit strings of a desired length
can also be transmitted through this procedure. The unit of data transmission is called a frame.
With the basic control procedure, receipt of data is checked after multiple frames are sent for
improved transmission efficiency. It also offers a form of advanced error control called CRC
(Cyclic Redundancy Check).
The advantage of this procedure is that the sending equipment can send multiple blocks of data
at one time for improved transmission efficiency. As the receiving equipment must inform the

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