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Question number: 256

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Describe in Detail

What are the Data link protocols?


  • Data link protocol specifications implement the data link layer.

  • Protocol provides a set of rules to implement layers of the OSI model.

Categories of Data Link Protocols

Asynchronous Protocols

  • ACL provides packet-switched connections between the master and all active slaves.

  • Packet retransmissions are usually applied to assure data integrity.

  • Simple, character oriented protocol’s generally used on two-point networks using asynchronous data and asynchronous modems.

  • Examples: X-MODEM, Y-MODEM, IBM 83B

Synchronous Protocols

  • The SCO link is point-to-point between master and slave.

  • Master maintains the link by using reserved timeslots at regular intervals.

  • Packets retransmissions are not allowed.

  • Protocols on point to multi-point networks use synchronous data and synchronous modems.

  • Examples: IBM BSC, SDLC, HDLC

  • There are 3 types are character oriented protocol, byte-count-oriented protocol, and bit-oriented protocols.

Character Oriented Protocols

  • Character-oriented protocols use special characters to indicate the beginnings and ends of messages.

  • Best-known character-oriented protocol is the binary synchronous communication protocol also known as BISYNC.

  • BISYNC is defined to only support the 6-bit trans code SBT, EBCDIC and ASCII.

Character-oriented protocol

Character-Oriented Protocol

Given the image is defining the character oriented protocol frame structure.

Byte-Count-Oriented Protocols

  • Byte-count-oriented protocols use special characters to mark the beginning of the header, followed by a count field that indicates number of characters in the data portion of the message.

  • The best known byte-count-oriented protocol is the digital data communication message protocol.

Byte-count-oriented protocol

Byte-Count-Oriented Protocol

Given the image is defining the Byte-count-oriented protocol frame structure.

Bit-Oriented Protocol

  • These do serial bit-by-bit information transfer over the data communications channel

  • Information is transferred as a series of successive bits interpreted individually on a bit-by-bit basis or in groups of several bits as opposed to a fixed length group of bits

  • Example: SDLC, HDLC

Bit-oriented protocol

Bit-Oriented Protocol

Given the image is defining the Bit-oriented protocol frame structure.