DATA LINK PROTOCOLS

Protocols at Layer 2 of the hierarchical structure have evolved from simple, asynchronous, uncontrolled protocols through the character-oriented link con-trols and their many derivatives to the current bit-oriented link control proto-cols. The data link protocol is responsible for the initialization, data inter-change control, and termination of a link, using a circuit established at the lower level. Pemaps its most important task is recovery from abnormal condi-tions that occur on the link.

Character-oriented protocols have been characterized by the use of a com-mon code set for both data and control functions. This led to the mixing oflink, device, and message control and to the requirement for complicated escape mechanisms to achieve transparency. Although these protocols are, and will remain, in widespread use, their deficiencies have led to the development of bit-oriented protocols, which use fields instead of characters for link control.

The entire character set is thus freed for data, and the protocols are naturally code transparent.

Bit-oriented protocols are now coming into widespread use and are the obvious choice for new system development. The major protocol standards of concern are:

• ANSI-Advanced Data Communication Control Procedure (ADCCP), described in X3.66

• ISO-High-Level Data Link Control (HDLC), described in IS 3309, 4335, etal

• CCITT -Link Access Procedure (LAPB), described as Level 2 of Rec-ommendation X.25

• ECMA-High-Level Data Link Control (HDLC), described in ECMA Standard 40

In addition to these, all major vendors as well as the federal government now provide equivalent standards. These bit-oriented standards are all fundamen-tally compatible. The choices that must be made by the data communications manager are described in the following paragraphs.

Classes of Procedure

Bit-oriented protocol standards have identified classes of procedure that are intended to satisfy the requirements of particular applications. In addition, a number of options may be chosen to refine the protocol for a specific applica-tion.

Unbalanced Classes. Control of the data link in this class is unbalanced in favor of the primary control station. This control station carries complete responsibility for link setup, data flow, and error recovery. Secondary stations respond to commands issued by a primary station, and the stations are therefore logically unequal. Unbalanced classes are suitable for point-to-point or multi-point configurations in which centralized control is desirable-for example, a host processor with a family of two-way alternate terminals.

These classes include unbalanced asynchronous (UA) and unbalanced nor-mal (UN). Note that the asynchronous class has nothing to do with the synchronousness of the communications facility, referring instead to a response mode in which the secondary station does not require explicit permission to initiate transmission once the link is active. UA is thus a much less disciplined control than is the normal class, in which a secondary can initiate transmission only after having received explicit permission to do so.

The UN class is ideal for polled multipoint applications requiring orderly interaction between a central control station and remote terminals. The UA class is best suited to situations in which a central control and a single activated remote station wish to exchange data without incurring the overhead penalty of polling. This is usually more efficient in a point-to-point configuration.

Balanced Asynchronous (BA) Class. This class is directed at point-to-point communications between logical equals. Both stations in this arrange-ment are called combined stations, containing the attributes of primary as well as secondary stations. Each combined station is capable of initializing the data link and is responsible for its own data flow and error recovery. The asynchro-nous nature of this class means that no polling overhead is required to start or stop data transfer. Balanced operation is best applied in computer-to-computer situations that require two-way simultaneous, efficient, high-speed data trans-fer. This class also finds application in network-node and host-to-intelligent -terminal configurations.

Each basic class thus identifies a set of elements, including commands, responses, and a sequence number modulus common to that class. Equipment operating in a given class must implement all of these basic elements.

Options

The bit-oriented protocols also derme a set of options that add capability to a basic class or restrict the use of a function. These options are aimed at specific requirements.

Option 1 adds the capability, on a switched network, for exchange identifi-cation (XID) commands and responses; it is used to interchange "I am ... , who are you?" information.

Two-way simultaneous operation can be enhanced with Option 2, which permits more timely error recovery via the introduction of a reject (REJ) capability .

Option 3 permits a single frame to be selectively rejected (SREJ); it is useful for satellite configurations, along with Option 10, which extends the sequence numbering from 8 to 128.

Downline loading or information related to initializing a remote station would call for Option 5, which adds set and request initialization mode (SIM, RIM) capability. Option 4, unnumbered information, can also be useful in these applications since it permits the transfer of information outside the protection of sequence-number checking.

A requirement to solicit responses from one or more remote stations on a one-time basis can be satisfied by selecting the unnumbered poll (UP), Option 6.

An application requiring more than the 128 addresses permitted by the basic address structure may wish to utilize the extended addressing capability pro-vided by Option 7.

Options 8 and 9 permit information frames to flow in one direction only (e.g., going to a printer or corning from a card reader).

Mode reset, Option 11, is useful in balanced operation when it may be desirable to reset the data link parameters in one direction only rather than reset the entire two-way exchange.

A complete class of procedure is identified by the basic class plus selected options. Class BA28, for example, specifies the balanced class with options for adding the reject command and sending information frames in one direction only. (This is the class used at the Level 2 link access procedure of CCITT X.25.)

A high degree of compatibility exists among the bit-oriented protocols.

Since many parameters must be defined, however, the choice involves more than classes of procedure; it encompasses definition of information content, data-field length, addresses, and other parameters. The standards, therefore, should not be used as implementation specifications, although they can be used as a baseline to generate such a specification.