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
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
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 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
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 (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
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. 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
Simple serial data links (usually asynchronous) were once 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.
With the emergence of the Internet as a ubiquitous communications
medium, these protocols are rarely used nowadays.
High-level Data Link Control
HDLC used to be the standard control protocol
used in point-to-point links
forming part of a larger network.
It specifies synchronous signalling, with frames having the following
format:

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
ISDN is the Next Big New Thing in point-to-point
links. A minimal
("Basic Rate") 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 (or similar) 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