DATA COMMUNICATION PRINCIPLES Data Link Control Layer Functions

DATA COMMUNICATION PRINCIPLES : Data Link Control Layer Functions and Procedures

Data Link Control Layer Functions and Procedures Right above the physical layer, the data link control layer adds logical functions to the data exchanged between two PHYs. DLC protocols provide functions and procedures to implement a point-to-point logical connection for reliable exchange of information. In a network path consisting of many links between two communicating nodes, problem on any link could slow down or even halt all communications. A link problem could occur in many ways. Each PDU may require proper addressing mechanism understood by the next receiving station on the link. Addressing could take many forms, such as, a station-specific address, a group-specific address, or even an address for links. Then, there is a need to have a way of identifying the beginning and end of a PDU so that the received information is interpreted in the same way as it was meant to be. Each link has a specific amount of bandwidth restraining the maximum signal bandwidth. In digital communications, bandwidth translates into bit rate. Thus, each link has a maximum allowable bit rate, the link capacity. If the bit rate of information coming to a node temporarily increases beyond the outgoing link capacity then data can be stored in buffers.

However, if this happens a little too often, then the buffers can overflow and data may be lost. Signals may get lost or be deteriorated due to the channel impairments discussed in earlier chapter. This may result in receiver interpreting a signal with wrong logical equivalent (e. g. , a ‘ 1’ as a ‘ 0’) or miss the signal altogether. In summary, when information is exchanged between two nodes across a link, packetization and transmission are not sufficient. The information has to be reliable and exchanged with the maximum exchange rate for a given link capacity and buffer sizes. This makes the job of DLC layer quite complex. However, the functions required by a typical DLC can be easily spotted out. This makes DLC layer one of the easiest to understand for a beginner in data communications. This is a layer that provides functions and procedures for an efficient and reliable exchange of PDUs between two nodes connected directly without an intervening node.

REFERENCES • Ahmad A. - Data Communication Principles. For Fixed and Wireless Networks • Cornelius T. Leondes - Database and Data Communication Network Systems, Three-Volume Set_. . -Academic Press