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,
Protocols are the sets of rules governing how to exchange frames
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
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
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
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)
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.
- X.25 & ISO Connection-oriented Network Service
- SNA/ APPC
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
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.