About ISDN

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ISDN stands for Integrated Services Digital Network, is an international communications standard, used for carrying voice, data, images, video, everything you could ever need. It involves a digital phone connection. All the traffic is made simultaneously using end-to-end digital connectivity. It transports data over the copper wires, which results in better results regarding quality and speed than the analog systems. Basically, ISDN is the digitization of the PSTN infrastructure.



A couple of years ago, when all the telephone communication was entirely analog there was a problem. Going from switch to switch, farther and farther, the signal lost its intensity and noise appeared. This resulted in poor quality services over long distances.

By turning the analog signal into a digital one the voice does not loose any of its quality. It is encrypted at one end and is transmited using special error correction and syncronization algorithms and then it is decoded at the other end.

This makes digital communication very attractive. Computer users benefit of increased speed, telephone companies remove all the possible noise that could intervene in the audio data. Also, this brings a lot of new services to the public, that were not available with analog systems.

ISDN supports data transfer rates of 64 Kbps (64,000 bits per second). Let’s see what this means. ISDN has two types of channels, used for different things:

  • B channels are used for data, this refers both to voice and data information
  • D channels are intended for signalling and control (but can also be used for data)

Also, these channels provide two types of ISDN service:

  • Basic rate interface (BRI) - there are two B channels, each one benefiting of a bandwidth of 64 kbit/s, and one D channel with a bandwidth of 16 kbit/s. Together these three channels can be called as 2B+D
  • Primary rate interface (PRI) - containing a higher number of B channels and a D channel with a bandwidth of 64 kbit/s, The number of B channels is different depending on the country:
  • North America and Japan: 23B+1D, aggregate bit rate of 1.544 Mbit/s (T1)
  • Europe, Australia: 30B+D, aggregate bit rate of 2.048 Mbit/s (E1)

PRI is meant to be used by companies or people with a higher need for bandwidth. Note that the PRI B channels can be set up for 30 different calls at a time, but there is also the possibility of combining them to get more bandwidth. This can be done using the H channels, like this:

  • H0=384 kb/s (6 B channels)
  • H10=1472 kb/s (23 B channels)
  • H11=1536 kb/s (24 B channels)
  • H12=1920 kb/s (30 B channels) - International (E1) only

What does it take to connect to ISDN?

All you have to do is to subscribe to a ISDN phone line. But there are some limitations. You cannot be at a distance greater than 5.5km from the telephone company central office, because of the need of expensive repeaters. ISDN Terminal Adapters (TA) are required, which are special hardware equipments for communicating with other ISDN devices and with the telephone company.


One of the inventions in computer technology was the modem. But it only permits a maximum of 56kbps (most of them work at 45-50kps, and some phone lines do not support 56kbps at all. ISDN allows, using the same copper wires, to transmit multiple channel digital data. And the latency using a ISDN line is half the latency on a analog one. This means that applications with fast response time requirements (e.g. network games) are possible. Using analog lines, you have to get an extra line and pay for it for every type of communication you need (voice, fax etc.) This is no longer a problem with ISDN, because it can multiplex channels over the same wires.

One disadvantage of using ISDN is the power source. While normal telephones provide their own power (this means that they are powered up from the telephone company and if the power goes down, the telephone will still work), to ISDN devices this rule does not apply. They need more power to run than a normal telephone, so the only place to get it is from inside the house.

ISDN and the future

Even though ISDN is not a very new concept, a lot of standardization is going on. This includes voltage levels, ring volumes, and various things needed for vendor interoperability. This set of interoperatibility standards is called National ISDN. Regardind broadband ISDN, there are still many things to be done. One example is the finalization of ATM. Curently, few people are replacing their whole telephone network with ISDN. For the moment the solution appears to be an entire ISDN network put inside one box which gathers all the ISDN needed devices (NT1, TA and TE1).

About the E1/T1 carrier systems

The E-carrier system is the standardization of digital communications, where a single wire can be used to carry more than one simultaneous voice conversations.

It was first designed in America. There it has the name T-carrier, but the CEPT (European Conference of Postal and Telecommunications Administrations) revised and improved it becoming today the standard for the majority of countries, minus USA, Canada and Japan. The whole idea is to group more circuits to form a high bandwidth network. For now, only the E1 (30 circuits) and E3 (512 circuits) are used. E1 is transmited as 32 time slots and E3 as 512 timeslots. This way, private end-to-end E1 circuits can be provided to the users in different countries that want to share a single high speed link.

Here are some technical data about E1. The 2048Mbit/s speed is achieved using E1 carriers. This is possible by splitting this number into 32 time slots. Each time slot sends and receives a 8-bit sample 8000 times per second. This leads to 8*8000*32=2048Mbit/s. There are two timeslots that are specialized on other things than data transport. These are TS0 (time slot 0), used for framing purposes and TS16, used for singalling and control.

The T-carrier is the American version of the E-carrier. These two types are incompatible. The DS0 (the basic unit) is a channel of 64kbit/s, normally used for voice circuit. The transfer rate is 1544Mbit/s, obtained like this: the highest frequency for human voice is 4000Hz. When converting analog to digital, there is a need to at least double that frequency for the sample rate. Each T1 frame contains 1 byte for each one of the 24 channels and a control bit. S, the result is ( (8*24+1)*2*4000=1544000bit/s.

See also

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