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ISDN - Integrated Services Digital Network

ISDN is replacing our old analog phones and offers a lot of new services. It is the easy way how to transmit voice and data simultaneously at the same time using an andvantages of a digital communication. Now I want to explain you, how does it really work.

B, D channels, BRI, PRI

ISDN uses for communication two different channels. B channel (bearer) has a bandwith of 64kb/s and carry voice and data. D channel has a bandwith of 16kb/s or 64 kb/s (dependent on the service type) and is used for signaling only.

BRI (Basic Rate Interface) and PRI (Primary Rate Interface) are basic types of service comprising of B and D channels.


  • 2 B channels to carry user data
  • 1 D channel (16kb/s) to carry control and signaling information
  • total of 144 kb/s


  • offers a greater capacity
  • 30 B channels
  • 1 D channel (64kb/s)
  • total of 1984 kb/s

There are many available combinations of B channels mostly depending on company. Only limitation for BRI service is distnace and it's about 5.5km between user and company switch. The longer distance is available, but needs more expensive equipment.

ISDN Network

The basic structure of ISDN network is on the demonstrative picture below.

Fig.1: Basic ISDN Network topology


The most important thing here is the Network terminator. It's the end of line for company and beginning of your network. And as we can see, it's up to you if you connect one computer or your own network, but it's necessary to have the network terminator always.

There are many of ISDN devices, some could be connected directly to network, some not. Typical example of incompatible device is your old analog modem. You can't connect it directly although the connector seems to be the same. ISDN is a digital network and you need a special equipment - terminal adapter. This is like convertor and all analog devices need this one to work in ISDN network. Better to use are fully compatible ISDN devices, which are designed as digital and can exploit all available sevices.

It seems, there is no limitation in connecting devices but there is. Directly to Network terminator you can connect only one device. If you want more, you need connect them to S-Bus and there are three possibilities how to make it.

With Point to Point there is only one device connected to network. Naturally you can connect more devices, but only one of them can be „online“. This type of connecting is limited by cable characteristics. Lenght should be less then 1000 metres – it mostly depends on cable quality.

Short S-Bus has maximal length of S-Bus is here about 200 metres and 10 metres for connecting devices to S-Bus. This type is most widely used, because we can connect up to 8 devices which can operate simultaneously.

Fig.2: Short S-Bus

Extended S-Bus allows connect device up to 500 metres from NT, but we can't connect it closer then 450 metres. Thanks to this, it's not as common as Short S-Bus.

Fig.3: Extended S-Bus

Power supply

There are different ways how to supply devices with electrical power. An analog phone is supplied by your provider. Analog transmission doesn't need a high power, so it is not a big problem to supply all devices directly from the provider. Phone needs a high voltage only when is ringing and it is not so often. ISDN uses the same line, it means there should be a way for ISDN devices how to work without additional power supply. And there is, but is used only for emergency situations. For example, when the power in your house is off. In this case, ISDN device, mostly your ISDN phone, can be supplied from provider.

There is a voltage of 40V in the ISDN line. It could be enough for your devices, but the power cunsumption would be unacceptable for your provider. The solution is very simply. The Network terminator is powered by 120/220V (depending on your country) and can be used as a power supply up to 4,5mW. It is the limitation for S-Bus in contrast to unlimited analog phone - theoretically, you can connect as many phones in parallel combination as you want. 4,5mW is not much and it allows you to connect up to four devices. If you want more, you have to have an additional power supply. Of course, this method is usable for devices like a phone or modem. You can not supply bigger devices like a fax by this way.


As I said, connector seems to be the same as in the analog telephony. It's 8 pin connector RJ45, known of ethernet network. The cable for connecting ISDN devices is twisted pair (TP), the same as for ethernet too. ISDN and ethernet are both digital services so different cables are not necessary. For better understandind of connector description look at the next picture. S-Bus uses two pairs for streaming - pair 2 for sending data and pair 3 for receiving data, the other two pairs are not used here.

Fig.4: RJ-45 Connector


For communicating with phone company, ISDN uses the DSS1 protocol (Digital Subscriber Signaling System) which defines data format, addressing and service messages too. Between companies is used another protocol - SS7, containing communication definitions similar to DSS1. The advantage of SS7 is Common Channel Signaling (CCS). You can separate signaling and B-channel. Without this you are able to use only 56kb/s for data transferring instead of 64kb/s.

If you are using analog devices and terminal adapter, there is a problem with the bandwith of 64kb/s. So some standards have been developed to rate adoption.

V.110 is the earlier standard of the two, and is mainly concerned with synchronous transmissions. It was designed for putting low-rate synchronous data onto 56Kbps channels. V.110 does also support asynchronous data up to 19.2Kbps and doesn't have any error correction.

V.120 is a frame-oriented protocol based on LAPD protocol. It's possibility to support a numbeer of multiplexed low-speed devices over one channel and also supports both synchronous and asynchrounous data streams. Because of its use of LAPD, it provides error correction.

Frequency, coding and modulation

ISDN bandwidth is 80kHz, the baud rate is 80kbaud, and the raw data rate is 160kbits/s. On the next picture is ISDN in comparison to other services. The lowest frequencies are used for analog phone, the highest for services like ADSL.

Fig.5: ISDN spectrum

ISDN uses a 2B1Q (2 binary, 1 quarternary) code and PCM (pulse coded modulation) for sending data over ordinary telephone lines. Two binary bits are encoded into one quarternary signal. This method doubles effectiveness of transmission. 2B1Q is a four level pulse amplitude modulation system, where each level represents a dibit. Thanks to this coding system, we are able to reach up to 160kb/s. The figure 6 illustrates demonstration of data stream.

Fig.6: 2B1Q line-coding rule

Each dibit has it's specific voltage amplitude. The combinations of dibit is illustrated in the next table.

Dibit Voltage Symbol
00 -2.5V -3
01 -0.833 -1
10 +2.5 +3
11 +0.833 +1

ISDN offers much more than the old analog phone line. Big advantage of this service is compatibility with phone lines. It is more stable for data transmitting, faster in establishing a connection and give us faster transferring speed in due to analog devices, e.g. computer modem with speed only up to 56kb/s. So if you need faster internet connection, this is very friendly solution how to make it. You only have to contact your phone provider. Disadvantage of ISDN could be initial costs for you (ISDN devices) and still not so low monthly flat fee, but massive increasing of digital services give us a chance to make it cheaper.

Jan Popelka


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