Network Technology: Point to Point Data Links

These are the most basic data communications links, eg:

Virtually all computers support such data links, using their so-called serial ports. Typically, these ports are able to operate at bit rates between 300 bps and 57600 bps.

The most common hardware interface standard used in such data links is called RS232: in fact, the term RS232 port is widely used as a synonym for serial port. A connection may be "direct" or may use a modem.

Modems

A modem allows a point-to-point data link to be carried over the telephone system (PSTN), thus:

The modem can:

Dial the telephone number of another modem under program control, or answer the phone
Negotiate a connection at the highest common bit rate that each modem can support
(often) perform error detection and correction and/or data compression

RS232 Interface

This interface was originally based on a 25 pin connector[1]. It defines:

The RS232 interface was designed to connect a modem (DCE) to a computer (DTE). To make a direct (DTE-DTE) link, a null modem cable, or interface, must normally be used.

[1] 9 pin connectors are also used, but there is no standard pin assignment for this.

Asynchronous vs Synchronous Framing

All data links have to provide byte-level synchronisation of the sender and receiver.

In the (more common) asynchronous systems, each character (or byte) sent on a data link is prefixed with an extra start bit and has a stop bit appended, thus:

In synchronous systems, a whole block or frame of data is sent as a unit, with a synchronising preamble of a few bytes. This has much lower overhead, and is by far the most common in real networks.

Other Serial Interfaces

There are many, much more modern, alternatives to RS232:

RS449/422/423: designed by EIA to replace RS232 and technically far superior, these standards have never enjoyed wide success in the market.
X.21: A CCITT specification which was designed for connecting DTE equipment directly to digital public carrier links. Used in Australia by Telecom's Digital Data Service (DDS) and X.25 Austpac services. The X.21bis variation is essentially the same as RS242/V.24 for connecting older equipment
V.35: is a CCITT variation on RS449 et al.

Basic File Transfer Protocols

Simple serial data links (usually asynchronous) are commonly used to transfer files of data between computers, instead of forming part of a larger network. Typical protocols used for this include Kermit and Zmodem.

File transfer programs (or packages) allow the user to specify the name of a file to transfer, and perform any necessary format translations between computer systems at each end of the link.

They also perform error detection and recovery, using checksummed data packets and resend-on-error, to guarantee correct delivery of the file.

High-level Data Link Control

HDLC is the standard control protocol used in point-to-point links forming part of a larger network. It uses synchronous signalling, with frames having the following format:

HDLC is an ISO standard, derived from IBM SDLC. In turn, it has begat PPP, LAP-B, LAP-D, 802.2 and many other control protocols. It has many advanced features, including:

Full-duplex data transfer - data can flow in both directions on a link simultaneously
Two different methods of recovering from transmission errors
Able to use long-delay links efficiently, etc

Integrated Services Digital Network

ISDN is the Next Big New Thing in point-to-point links. A minimal (Microlink) ISDN service provides:

An ISDN service is integrated in the sense that a B channel can be used to carry either a digitised voice call, or a 64 kbps data call. The D channel is normally used for signalling, but may also be used as a 16 kbps data link.

ISDN links are (still!) pretty expensive in Australia. In addition, an expensive terminal adaptor is needed to access the data link function. This will change...

This lecture is also available in PostScript format. The tutorial for this lecture is Tutorial #9.
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Phil Scott