For effective digital data communications, much more is needed to control and manage the exchange. In this chapter, we shift our emphasis to that of sending data over a data communications link. To achieve the necessary control, a layer of logic is added above the physical transmission layer; this logic is referred to as data link control or a data link control protocol. When a data link control protocol is used,
the transmission medium between systems is referred to as a data link. To see the need for data link control, we list some of the requirements and objectives for effective data communication between two directly connected transmitting-receiving stations
• Frame synchronization. Data are sent in blocks called frames. The beginning and end of each frame must be recognizable.
• Flow control. The sending station must not send frames at a rate faster than the receiving station can absorb them.
•Error control. Any bit errors introduced by the transmission system must be corrected.
• Addressing. On a multipoint line, such as a local area network (LAN), the identity of the two stations involved in a transmission must be specified.
• Control and data on same link. It is usually not desirable to have a physically separate communications path for control information. Accordingly, the receiver must be able to distinguish control information from the data being transmitted.
• Link management. The initiation, maintenance, and termination of a sustained data exchange requires a fair amount of coordination and cooperation among stations. Procedures for the management of this exchange are required.
A data link protocol that satisfies these requirements is a rather complex affair. First we look into the various techniques used to implement the error control and flow control mechanism. Then we look at the most important example of a data link control protocol: HDLC (High-level Data Link Control). This protocol is important for two reasons: First, it is a widely used standardized data link control protocol. And secondly, HDLC serves as a baseline from which virtually all other important data link control protocols are derived.
the transmission medium between systems is referred to as a data link. To see the need for data link control, we list some of the requirements and objectives for effective data communication between two directly connected transmitting-receiving stations
• Frame synchronization. Data are sent in blocks called frames. The beginning and end of each frame must be recognizable.
• Flow control. The sending station must not send frames at a rate faster than the receiving station can absorb them.
•Error control. Any bit errors introduced by the transmission system must be corrected.
• Addressing. On a multipoint line, such as a local area network (LAN), the identity of the two stations involved in a transmission must be specified.
• Control and data on same link. It is usually not desirable to have a physically separate communications path for control information. Accordingly, the receiver must be able to distinguish control information from the data being transmitted.
• Link management. The initiation, maintenance, and termination of a sustained data exchange requires a fair amount of coordination and cooperation among stations. Procedures for the management of this exchange are required.
A data link protocol that satisfies these requirements is a rather complex affair. First we look into the various techniques used to implement the error control and flow control mechanism. Then we look at the most important example of a data link control protocol: HDLC (High-level Data Link Control). This protocol is important for two reasons: First, it is a widely used standardized data link control protocol. And secondly, HDLC serves as a baseline from which virtually all other important data link control protocols are derived.
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