Network Technology: Point to Point Data Links

Virtually all computers support such data links, using their so-called serial ports. Typically, these ports are able to operate at any standard bit rate between 300 bps and 230400 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

The modem can:

• Dial the telephone number of another modem under program control, or answer the phone for an incoming call
• Negotiate a connection at the highest common bit rate that each modem can support
• perform "on-the-fly" data compression
• perform (under some conditions) error detection and correction

RS232 Interface

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 (or null modem adapter), must normally be used.

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

Asynchronous vs Synchronous Framing

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

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. Was used in Australia for Telecom's old 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

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.

With the emergence of the Internet as a ubiquitous communications medium, these protocols are rarely used nowadays.

High-level Data Link Control

HDLC is an ISO standard, derived from IBM SDLC. In turn, it has begat LAP-B, LAP-D, 802.2 and (most importantly) PPP, the de-facto standard for conveying IP packets over a dial-in link to the Internet, and the most common protocol derived from HDLC. PPP 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

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 (or similar) is needed to access the data link function. This will change...