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Synchronous Serial Communications

Introduction to Synchronous Serial Communications

Unlike asynchronous serial communications, where each individual individual character contains synchronizing information, synchronous communications exchanges occur in groups of characters, or frames. Protocols are the sets of rules governing how to exchange frames between computers. Over the years, protocols of various kinds have been invented, used, and often discarded. Although data communications using synchronous protocols is more complex than asynchronous schemes, synchronous protocols offer less overhead and higher maximum speeds than asynchronous methods.

All of The Software Group Limited's products contain protocol implementations as well as synchronous serial hardware. Including protocol implementations as part of the product reduces the effort required to deploy synchronous serial information systems to the field.

Our software integrates with the operating system to achieve the following objectives:

  • Carry IP datagrams over a serial communiations link (not a common application in this new millenium)
  • Allow programmers to implement Information Technology systems which communicate with other devices over a synchronous serial communications facility.

To that end, we provide flexible synchronous subsystems which incorporate protocol implementations and the necessary operating system interfaces to make them useful. Our products conform to the OSI (Open Systems Interconnection) model for serial data communications networks.

Layer I: Low-level serial line interface

Considered from the "bottom" of the OSI stack, our serial synchronous controllers handle the lowest layer of a communications stack - electrical signalling and bit manipulation. All of our protocol implementations use the HDLC (High Level Data Link Control) bit oriented protocol as their bottom layer. The HDLC rules ensure:

  • Any data can be transmitted across the link (transparency)
  • The beginning and ends of frames are unequivocally recognized
  • When an error occurs, the receiver will eventually resynchronize
  • The receiver can detect communications errors in each frame it receives

Layer II: Link Layer Protocols

Above HDLC, TSG offers three choices for link protocols (OSI Layer II):

  • LAPB (Link Access Procedure, Balanced)
  • Frame Relay
  • SDLC (Synchronous Data Link Control)

LAPB is a full-duplex sliding window protocol which guarantees an error-free link between two communicating entities. The formal protocol that makes up LAPB is also known as the Asynchronous Response Mode of HDLC (HDLC - ARM). It is the second layer of the CCITT X.25 protocol and corresponds to ISO standard 8077.

Frame Relay is an uncorrected link layer protocol most often used for transmitting IP datagrams between two points on a public packet-switched data network. We provide an API (Application Programmer Interface) to Frame Relay, but the protocol is most often used in the context of an integrated router - providing a method of moving IP datagrams from a LAN into a Frame-based WAN.

SDLC is a component of IBM's SNA/APPC (Systems Network Architecture/ Advanced Peer-to-Peer Communications) networking schemes. Like LAPB, it is a sliding window protocol, but is usually operated in a half-duplex manner. SDLC corresponds to the NRM (Normal Response Mode) operation of HDLC, where the communicating entities have distinct roles - one link station acts as the master (primary) station, while the others operate as slave (secondary) stations.

Layer III: Network Layer Protocols (and above)

  • X.25 & ISO Connection-oriented Network Service
  • SNA/ APPC
The ISO protocol architecture defines a Connection-Oriented Network Service (CONS) which is a subset of the X.25 protocol defined by the CCITT. TSG's X.25 implementations are compatible with the CCITT X.25 and X.32 specifications for interfacing to public packet-switched data networks (PSDN). In addition to an Application Programmer Interface (API) library, TSG provides patches and additional code required for the popular public domain ISODE implementation of the ISO protocols, to support FTAM and other ISODE applications.

Although SNA and APPC don't strictly speaking fit the ISO model, there are aspects of the SNA network architecture which fit the profile of OSI Layer III. TSG has a complete SNA stack available for Intel-base UNIX systems, provided with applications for a variety of IBM terminal emulations (2780/3780, 3270, 3790, 5250) and Logical Unit types, up to and including an APPC End Node.