DATA
COMMUNICATIONS
© Copyright Brian Brown, 1995-2000. All rights reserved.
Part 4: The Public Telephone Switched Network
Public Switched Network | ISDN | Nominal Voice Channel | Summary
Objectives
At the end of this section you should be able to
The
Public Telephone Switched Network (PTSN)
The Public Telephone Switched Network (PTSN) refers to the
publicly available dial-up telephone network (offered in New
Zealand by Telecom
New Zealand). It is an interconnection of switching centers
and connections to subscribers, that offers voice dial-up between
any two subscribers connected to the PTSN. In addition, overseas
connections to other countries is possible.
As telephone communications evolved, switching centers were used to manually connect users together. A pair of overhead wires was run from the switching center to each subscriber, and the wires were terminated on a switch-board. A manual operator was used to interconnect subscribers. With more and more subscribers, the number of operators required became a problem. In addition, the sheer amount of cabling and interconnection became unmanageable. The development of automatic exchanges (using Electro-mechanical devices) simplified the interconnection of subscribers by placing it under the control of the subscriber (provide a telephone with a numbered dial).
The problem of subscriber wires was overcome by the use of multi-strand underground telephone cable (500/1000 pair). Each switching center (called a CENTRAL EXCHANGE or METROPOLITAN EXCHANGE) was interconnected to the next, and located geographically center of the existing population to minimize the amount of cable required to reach all subscribers.
As data communications evolved, the only connection mechanisms available was via DIAL-UP lines provided over the switched telephone network. These dial-up lines were suitable for voice use, but severely affected digital data. They were prone to noise from the Electro-mechanical switches used to interconnect subscribers, and they placed limits on the speed of which data could be transmitted.
Special circuits that are suitable for data are available. These circuits are an END-TO-END connection, and no switching is involved (a direct hard-wired connection that bypasses the switching mechanism in the exchange). These are called LEASED LINES, and are suitable for data communications at a number of different speeds (customers pay a monthly rental based on the speed of the line they lease). A typical use of leased lines is to provide a permanent connection to the Internet.
The need for dial-up data connections has necessitated other services being made available. In addition, the computerization of switching centers has reduced the interference and noise associated with voice channels, and high grade (low error rate), high speed (9600bps or higher) communication is now possible over most dial-up voice circuits.
The
Integrated Services Digital Network (ISDN)
The Integrated Services Digital
Network (ISDN) is a new integrated network design that allows a
network to support voice, data, video and other data over twisted
pair cable. It is fully digital, and requires voice users to
convert speech into digital signals before connection. Rather
than a company having three separate lines to handle three
machines like phone, fax and computer, it can now have a single
line and connect all three simultaneously. The single line is a
digital line that connects into the telephone companies digital
network. ISDN is not the same as the PTSN. The PTSN is an analgue
network designed for speech, whereas ISDN is a digital network
designed for transferring digital data.
The basic level of services available for ISDN (the single line) are,
2B + D
where B = a 64Kbps data channel
D = a 16Kbps secondary channel used for signally or low speed data use.
Higher capacity circuits are also offered (called H channels).
The advantages of ISDN are,
Typical uses of ISDN are,
The ISDN connection is digital end-to-end. Calls are setup to a destination on demand, just as in the PSTN. Once the connection is made, the user has access to the full digital capacity of the link. Users are often charged on combination of time and the amount of data transferred.
The Nominal Voice Channel
The voice channel supplied by telecommunication companies for
speech circuits meets internationally defined standards. These
define such characteristics as,
Voice circuits are suited to conveying speech. When used to carry digital data, they limit the speed at which signals can be transmitted, and also introduce errors as the voice circuits generally suffer from noise. Each speech circuit provided by the telecommunication companies is measured against the nominal speech channel standard. This guarantees it suitable for voice transmission only.
Summary
A dial up line is a non-permanent switched
connection. It is suited to telephone conversations (analogue
speech). In data communications, it is often used by users to
connect to the Internet using a modem.
A leased line is a permanent non-switched connection. It is suited to higher speed communication and attracts a monthly fee. However, users have exclusive use of the leased line and can send as much data over it as they want. It is used to establish a permanent connection between two sites, such as connecting a company to the Internet 24 hours a day.
ISDN provides much higher speeds than dial-up, less errors in transmission of data, but currently has slower speeds than that available for leased line. It is a digital temporary dial up service, and the basic ISDN connection is two lots of 64Kbps and one 16Kbps. If a company is using a connection more than 5 hours per day, a leased line will be cheaper, as ISDN charges include call connection charges and time charges.
© Copyright B Brown. 1995-2000.
All rights reserved.