Integrated Services Digital Network (ISDN)


The current public phone system is based on analog signals used primarily for voice transmission. The Integrated Services Digital Network (ISDN) specification is a digital replacement for the analog phone system. An ISDN phone system is similar to a computer network. Currently, to send data you must use a modem to convert from digital signals generated by computers to analog signals that can be transferred on analog phone lines. Since ISDN is digital, no modem is needed.

Starting in the 1960's, phone carriers began to install digital backbone networks in their phone systems. ISDN extends this digital network to the consumer site. Some phone systems still do not provide support for ISDN, but many do; contact your local phone carrier for more information. Windows NT Remote Access Service (RAS) supports ISDN functionality. This article discusses the following topics:

  • ISDN at Hardware Level
  • ISDN at Service Level
  • Optimizing RAS on ISDN
  • ISDN Setup Options


An ISDN network requires ISDN lines and adapters. Each ISDN line is divided into one of the following categories:

  • Two B-channels for voice and data that can transfer data at 64K BPS (bits per second) per channel.
  • One D-channel for signaling and line control. This channel is used for out-of-band signaling for call setup. It is possible to transfer data on the D-channel also. The X.25 protocol is sometimes available over the D-channel. Currently, neither Digiboard nor Windows NT support this capability.
Digiboard makes three ISDN adapters that are supported by Windows NT:

  • Digiboard PCIMAC ISA
  • Digiboard PCIMAC/4 ISA. Expandable from 1 to 4 ports.
  • Digiboard PCIMAC MCA


There are two ISDN protocols: primary rate interface (PRI) and basic rate interface (BRI). Digiboard and Windows NT currently support only BRI. BRI uses two B-channels and one D-channel. The European implementation of PRI uses 32 B-channels, while the United States version uses 23. PRI uses a different style of signaling and type of interface that is not compatible with BRI. PRI is more prevalent in Europe than in the United States at this time.

Currently Digiboard is the only company providing ISDN drivers for Windows NT and Windows NT Advanced Server. You can install the Digiboard drivers from the Networks option of Control Panel. Choose Add Adapter and select one of the three Digiboard adapters.

The driver is actually a network media access control (MAC) driver. It talks with the ISDN hardware on the bottom and exposes an NDIS 3.0 interface on the top. The ISDN MAC driver binds with the RASHUB driver on top. RASHUB is then bound with the transport (that is, NBF).

The ISDN MAC driver does not use the Windows NT serial I/O driver and, therefore, does not need to work with the ASYNCMAC driver that normally converts between serial I/O and NDIS 3.0.


RAS can use one or more B-channels per connection. You can optimize RAS for the maximum number of connections or the best throughput. To maximize the number of connections, use one B-channel per incoming call. This transfers data at a rate of 64K per second. The 16K D- channel control is shared between the two B-channels, and control information is passed over the D-channel so that the adapter knows what B-channel is being referenced.

You can maximize throughput by using multiple B-channels per connection. (This will reduce the number of possible connections.) On a one ISDN port Windows NT RAS client, you can transfer data using two B-channels, giving you an effective rate of 128K BPS. If you have more ISDN ports available, as in the case of the Digiboard adapter that has four ports, you can increase the throughput even further:

4 <ISDN ports> * 2 <B-channels> * 64K BPS = 512K BPS
To do this, you need hardware support on both the client and server side. The incoming call determines how many channels are used. If there are not enough channels available, the call fails. (Currently you cannot instruct the server to limit the number of B-channels to use per connection.)

Because of Digiboard's proprietary method of aggregating B-channels, Digiboard drivers cannot connect to other vendors' ISDN drivers. Currently there is no standard protocol for B-channel aggregation, but a group is putting one together. It is called Bandwidth On Demand (BONDING). If this becomes an industry standard, Digiboard may support it in the future.


ISDN Setup options for a RAS client can be found in the Remote Access application and in the Networks portion of Control Panel.

From Remote Access, choose Add, Advanced, and ISDN for configuration options. The following options are available:

Line Type

Determines the highest line type to try when initially dialing an ISDN connection.

Negotiate Protocol

This enables RAS to attempt to use multiple line types. The following notes apply to this setting:

  • Intermediate devices may not allow a connection to go through at a certain rate or line type.
  • RAS first attempts to make the connection by using the line type specified (via the D-channel).
  • If you enable Negotiate Protocol and an intermediate device rejects the line type, the call is terminated and a lower line type is tried.
  • Attempt order: 64K digital, 56K digital, 56K voice.
  • If 56K Voice is accepted, RAS still sends data, not voice. The connection only seems to be voice to the intermediate device.
  • The server is reached after the intermediate device has accepted the line type. The RAS ISDN server then accepts that rate and does not need to be manually configured.

Enable Hardware Compression

This enables hardware compression on the ISDN card. This could allow throughput speeds to be effectively doubled, depending on data type. For example: A two B-channel connection (2 * 64 BPS = 128 BPS) could yield an effective BPS rate of over 200K.

Channels in Use

This indicates the number of B-channels that the client ISDN connection should use.

From Control Panel, choose Networks, select the Digiboard driver in use, and choose Configure. The following options are available:


This option is set to Disable by default. The driver can poll the card using a system timer instead of an interrupt request. Testing has shown that this provides very good performance. Disabling the IRQ also frees an IRQ for another device to use. If you want to use an IRQ, it is software configurable.

I\O Address

This must match the switch settings on the card. Each ISDN card must have a unique I\O address.

Memory Address

This address is software configurable. Shared memory is used to transfer data and can be shared between multiple Digiboard ISDN cards.

Switch Type

You must contact your local phone carrier to determine what switch type to specify. There is no way the Digiboard driver or adapter can get this information from the switch service.

The following are the currently supported ISDN switch types:

- ATT (AT&T)
- NI-1 (National ISDN-1)
- NTI (Northern Telecom)
- Generic (CCITT Standard)
- Auto (Attempts auto-detection)

Line Options

These settings vary depending on the ISDN line configuration. You should ask your local phone carrier for more information on these settings. The defaults are correct for an AT&T line.