Intel mcs-48 manual 8273, 8273·4, 8273·8, Brief Description of HDLC/SDLC Protocols

8273, 8273·4, 8273·8

A BRIEF DESCRIPTION OF HDLC/SDLC PROTOCOLS

types of frames; an Information Frame is used to transfer data, a Supervisory Frame is used for control purposes, and a Non-sequenced Frame is used for initialization and control of the secondary stations.

General

The High Level Oata Link Control (HOLC) is a standard communication link protocol established by International Standards Organization (ISOL HOLC is the discipline used to implement ISO X.25 packet switching systems.

The Synchronous Oata Link Control (SOLC) is an·IBM communication link protocol used to implement the System Network Architecture (SNAL Both the protocols are bit oriented, code independent, and ideal for full duplex communication. Some common applications include terminal to terminal, terminal to CPU, CPU to CPU, satellite communication, packet switching and other high speed data links. In systems which require expensive cabling and interconnect hardware, any of the two protocols could be used to simplify interfacing (by going serial), ttlereoy reducing interconnect hardware cosis. Since both the protocols are speed independent, reducing interconnect hardware could become an important application.

Network

In both the HOLC and SOLC line protocols, according to a pre-ass.igned hierarchy, a PRIMARY (Control) STATION controls the overall network (data link) and issues commands to the SECONOARY (Slave) STATIONS. The latter comply with instructions and respond by sending appropriate RESPONSES. Whenever a transmitting station must end transmission prematurely it sends an ABORT character. Upon detecting an abort character, a receiving station ignores the transmission block called a FRAME. Time fill between frames can be accomplished by transmitting either continuous frame preambles called FLAGS or an abort character. A time fill within a frame is not permitted. Whenever a station receives a string of more that fifteen consecutive ones, the station goes into an IOLE state.

Frames

A single communication element is called a FRAME which can be used for both Link Control and data transfer purposes. The elements of a frame are the beginning eight bit FLAG (F) consisting of one zero, six ones, and a zero, an eight bit AOORESS FIELO (A), an eight bit CONTROL FIELO (C), a variable (N-bit) INFORMATION FIELO (I), a sixteen bit FRAME CHECK SEQUENCE (FCS), and an eight bit end FLAG (F), having the same bit pattern as the beginning flag. In HOLC the Address (A) and Control (C) bytes are extendable. The HOLC and the SOLC use three

Frame Characteristics

An important characteristic of a frame is that its con- tents are made code transparent by use of a zero bit insertion and deletion technique. Thus, the user can adopt any format or code suitable for his system - it may even be a computer word length or a "memory dump". The frame is bit oriented that is, bits, not characters in each field, have specific meanings. The Frame Check Sequence (FCS) is an error detection scheme similar to the Cyclic Redundancy Checkword (CRC) widely used in magnetic disk storage devices. The Command and Response information frames contain sequence numbers in the control fields identifying the sent and received frames. The sequence numbers are used in Error Recovery Procedures (ERP) and as implicit acknowledge- ment of frame cUlnrnunication, enhancing the tiue fu:l- duplex nature of the HOLC/SOLC protocols.

In contrast, BISYNC is basically half-duplex (two way alternate) because of necessity to transmit immediate acknowledgement frames. HOLC/SOLC therefore saves propagation delay times and have a potential of twice the throughput rate of BISYNC.

It is possible to use HOLC or SOLC over half duplex lines but there is a corresponding loss in throughput because both are primarily designed for full-duplex communi- cation. As in any synchronous system, the bit rate is determined by the clock bits supplied by the modem, protocols themselves are speed independent.

Abyproduct of the use of zero-bit insertion-deletion technique is the non-return-to-zero invert (NRZI) data transmission/reception compatibility. The latter allows HOLC/SOLC protocols to be used with asynchronous data communication hardware in which the clocks are derived from the NRZI encoded data.·

References

IBM Synchronous Data Link Control Genera/Information, IBM, GA27-

3093-1.

Standard Network Access Protocol Specification, DATAPAC, Trans-

Canada Telephone System CCGlll

Recommendation X.25. ISO/CCITT March 2. 1976.

IBM 3650 Retail Store SysternLoop Interface OEM Information, IBM, GA 27 -3098-0

Guidebook to Data Communications, Training Manual, Hewlett-Packard

5955-1715

IBM Introduction to Teleprocessing,. IBM, GC 20-8095-02

System Network Architecture, Technical Overview, IBM, GA 27-3102System Network Architecture Format and Protocol, IBM GA 27-3112

OPENING	ADDRESS	CONTROL	INFORMATION	FRAME CHECK	CLOSING
FLAG (F)	FIELD (A)	FIELD (C)	FIELD III	SEQUENCE (FCS)	F1.AG (F)
01111110	8 BITS	8 BITS	VARIABLE LENGTH	16BITS	01111110
			(ONLY IN I FRAMESI

Figure 1, Frame Format

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