THD204 - TMA05 Part 2 - Digital Communications Services
ISDN Basic Rate
Integrated Services Digital Network (ISDN) is an international standard for global digital communications networks developed by the Commité Consultatif International de Télégraphique et Téléphonique (CCITT) between 1984 and 1989.
ISDN is based upon an extended form of the Public Switched Telephone Network (PSTN). ISDN services can be delivered over a standard twisted copper pair local loop networks, but require line-encoding equipment at the installation site. ISDN is capable of providing access both to the digital public switched telephone network and to packet-switched services, such as X.25 and frame relay.
An ISDN line is normally described as having two types of channel, a bearer channel B-channel (IDSN Basic Rate has two B channels) and a Data channel D-Channel The differences between the two are as follows:
B-channels are circuit-switched (the whole connection is switched to one destination, the same as a standard telephone call). They are the prime carrier for digital voice and data services and have a transmission capacity of 64 Kbps. They can transmit and receive voice or data at the same time.
D-channels (data) are packet-switched data channels and operate at 16 Kbps. The D channel is used for signalling i.e. messages to do with starting and ending a call, dialling numbers and finding a number is busy.
ISDN Basic Rate known to domestic consumers as BT Home Highway. The analogue devices (phone, fax etc.) can be connected to the D channels via the installed Home Highway distribution box enabling simultaneous phone and data transfers to take place, two phone calls or to data transfers. Equipment is required to be installed at the exchange and customer premises the Home Highway box includes a terminal adapter, which converts analogue input to digital for transmission.
ADSL
Asymmetrical Digital Subscriber Line (ADSL) is a method of sending data at high-speed over existing copper twisted-pair telephone wires typical of the 'local loop' connecting business and domestic users to a telephone exchange.
ADSL is a line-encoding scheme. Line encoding schemes describe how data or information is encoded as a signal and how it is transmitted across a network. A 'line code' is the term used to describe the electrical representation of a digital signal.
The user must be within 5.5Km of the exchange (extended reach ADSL available from 1-Aug-2001 - BT). It has an ADSL "modem" at each end of the line, creating three information channels - a high speed downstream channel, a medium speed duplex channel, and a POTS channel. The course notes also show a 9.6Kbps upstream channel used for selection and administrative of VOD services The POTS channel is split off from the digital modem by splitters thus guaranteeing uninterrupted POTS, even if ADSL fails. These have the job of combining a splitting the three different signals, which are running over this pair of copper wires.
An Example ADSL Home Installation (courtesy www.yellowhead.com)
Speed depends on how far from the exchange you are, for distances over 3.7Km the max data rate is 1.5 - 2.0Mbps. If you live under 3.7Km from the exchange you may get up to 6-8Mbps but in reality the high speeds will be at a premium and only available to business. See appendix to this question. Any bandwidth mentioned in my answer is only indicative as to what is available in reality it will depend on how far form the exchange you are, what services are offered and how much you are willing to pay.
Justification for Upgrading to a Digital Service.
Present modem speed has reached its maximum rate using POTS (Plain Old Telephone System) and PSTN (Public Switched Telephone Network) which cannot accommodate connection speeds higher than the present 56.6Kbps. The Local Loop is the part between the exchange and the subscriber.
To able to be online and to use the telephone at the same time is a major benefit especially when you only have one line. The increase in speed of access to the Internet would be extremely beneficial. At present downloading large files takes a great deal of time and multiple connections are required to complete a large download as my ISP cuts connection after 60mins.
I also do quite a lot of shopping (books, CD's etc.) on the internet and the increased speed would be very useful here by reducing the time taken for online catalogues to download. In fact the increased speed would be beneficial for surfing the Net in general as sites with a large graphic content especially those constructed with Macromedia's Flash can be extremely slow.
BT's Home Highway gives speeds of up to 64Kbps or 128Kbps if both lines are used but using both lines doubles the cost of the call (two calls). However the Home Highway doesn't really give any great increase in speed considering the cost. BT's Broadband (ADSL) provides download speeds of up to 40 times that of a standard 56K modem. Broadband also allows for VOD services (when available), which may be of interest to consumers. Broadband should also allow access to online catalogues, databases, CMC, fully presented online courses, etc. ADSL also provides an always-on connection, which doesn't risk call blocking within the telephone network.
The following table gives some idea of the relative download speeds - comparing a 56K modem with ISDN and ADSL.
Bandwidth Range - Comparison to 56K home modem |
||
Dial up Modem |
28-56Kbps |
0.5-1.0x |
ISDN |
64-128Kbps |
1.1-3.3x |
BT ADSL |
0.5-2Mbps |
9-143x (see note 1) |
Cable Modem |
0.5-15Mbps |
29-270x (see note 2) |
(Source: http://www.broadband.bt.com/common_questions/content12.html - visited 01-Aug-2001)
Note 1: 0.5 Mbps is the maximum bandwidth available to home users from BT at present. As there are no VOD services available no information was found for bandwidth for these services.
Note 2: Modern Cable TV is capable of delivering up to 30Mbps but most companies share this with fifty or so subscribers who share the local repeater capping each user to about 512Kbps.
Appendix:
BTIgnite announced that as of the 1st August they would be rolling out Extended ADSL effectively increasing the distance from the exchange to subscriber to 5.5Km.
Excerpts from Introduction To ADSL downloaded from bt.com/adsl
ADSL Data Rates
ADSL can deliver asymmetric transmission rates typically up to 8Mbps and up to 1Mbps upstream in some cases. The normal average is 6Mbps.