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What is ISDN?

R. Kayne
R. Kayne

ISDN (Integrated Services Digital Network) is a type of digital phone/data and Internet service that preceded ADSL (Asynchronous Digital Subscriber Line) and has for the most part been superseded by it.

Normal telephone lines carry analog signals that must be amplified and converted to digital signals by the phone company. This process introduces not only a slight lag time, but also distortion in the signal. Dial-up modems and telephones are examples of equipment that use analog signals. ISDN makes use of digital signals running along existing copper lines to increase the data throughput, reduce line noise and enhance signal quality.

Man holding computer
Man holding computer

In the mid 1990s, ADSL was very expensive and not widely available. Companies and individuals wanted a faster way to connect to the Internet, but the technology behind dial-up modems had reached its threshold. ISDN became a viable alternative to provide speeds of up to 128 kilobits per second (kbps), versus the standard connection of 30-53 kbps with a dial-up modem.

The most common type of ISDN service for Internet connection is the Basic Rate Interface, or ISDN BRI. This technology creates two B-channels on the existing copper lines of 64 kbps each, along with a single 16 kbps D-channel for the phone line. This separates data channels from the voice channel, allowing telephone or fax use while online.

While ISDN is inexpensive and about twice as fast as dial-up service, it has been largely replaced by affordable DSL service. An inexpensive ADSL service offers speeds up to 384 kbps, while more expensive versions are improving in speed all the time. As of fall 2005, standard ADSL speeds range between 1.5 and 3.0 mbps (megabits per second), or 1536-3072 kbps.

Although ISDN may not be the best choice for packet-switching networks like the Internet, it is still widely used for professional audio and broadcast applications where digital clarity with integrated telephone services is specifically required. Small businesses that often use two voice lines, such as phone and fax, and only require limited Internet connectivity of, say, an hour or less per day, may prefer ISDN. ISDN might also be a better choice for high-speed connections to intranets for video-conferencing, or to remote networks other than the Internet.

Discussion Comments


In the UK, where ADSL over ISDN lines is not supported, what would the operational symptoms of intermittent cross connection from an ISDN copper pair signal wire and an POTS + ADSL copper pair A or B wire but probably A wire? Would there be any audible digital signal noise from the ISDN line on the POTS line, or would it just be the DC voltage level changes of the intermittent connection producing an audible scratching noise?

Would it be expected that leaked ISDN signals would interfere with POTS + ADSL signals and cause dropout as the problem worsens or should G.dmt.bis be able to adapt its BINS usage around the ISDN spectrum usage?


For internet usage, think of ISDN as very fast dial up. Once the 1st "B" channel reaches a pre-set level of usage, say 75 percent, then the ISDN modem dials back into your ISP and starts a second connection on the remaining "B" channel, then the two "B" channels operate in tandem as one dial up connection at 128kp, even though your modem has connected to your ISP on two different digital phone lines. Your ISP has to be provisioned to handle a ISDN call. Your IDSN service is actually comprised of two digital phone numbers.


Think of 30 trains trying to go down one track, isdn30 can send 30 calls down one line at the same time. to send 30 trains down one track at the same time you would have to send each carriage separately. Isdn works in a similar way breaking the calls into segments and putting them back together at the other end, so fast you would never know.

Most large companies who need at least 30 calls at the same time would use or have used isdn at some stage.


What is the disadvantage of a ADSL?


Can some onepleasegive an explanation in layman terms as to what and how ISDN works? And where it is used widely?

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