Basic Rate ISDN
The Basic Rate ISDN interface provides a bandwidth of 144Kbit/s which is divided into two bearer channels of 64Kbit/s each and one data (D) channel for signalling with an available bandwidth of 16Kbit/s.

Bearer (B) Channel
A 64Kbit/s time slot available to carry voice or data across the ISDN network. Two bearer channels are available on a basic rate interface (2B+D) and thirty bearer channels are available on a primary rate interface (30B+D).

Clearing Cause Values
A range of more than 50 reasons which can be given by the network or the terminal in Disconnect or Release messages to indicate why a call has ended or failed. A successful call will always end with a clearing cause of 016 (normal call clearing) but a failed call will clear with another cause such as 088 (incompatible destination). Cause values are very useful for fault diagnosis.

CLI
If CLI (calling line identification) is enabled it will display the telephone number of the calling party in a display on the called terminal before the call is accepted. This supplementary service may be de-selected by the calling party for reasons of privacy.

DDI (Direct Dialling Inward)
Allows a large range of telephone numbers to share one or a small number of lines. Each member of staff can enjoy their own direct dial number without the need for an operator. DDI is called DID in North America.

Dual Number Mode
Dual number mode is where separate telephone numbers are provided for each of the two B (bearer) channels available on an ISDN port. Dialling one number will always access B1 and the other number will always access B2 assuming that both are available.

ETSI (Euro-ISDN)
This is the latest Pan-European protocol specification which supersedes, and is generally compatible with, the earlier NET 3 specification for basic rate ISDN.

High/Lower Layer Compatibility
This is used to identify the type of service required or expected by the calling party which may include 'Speech, Unrestricted Digital, Restricted Digital, 3.1KHz audio, 7KHz audio or Video' (Low Layer) or 'Telephony, Fax group 2/3, Fax group 4, Teletex….etc' (High Layer). Some called terminals need to see this information in order to ensure compatibility.

E1 Interface (ISDN)
2Mbit/s HDLC framed digital interface used for Primary Rate ISDN where it is divided into 32 64kbit/s channels, comprising of one for signalling (TS16) and one for synchronisation (TS0) and 30 bearer channels.

S/T Interface (ISDN)
144kbit/s HDLC framed digital interface used for Basic Rate ISDN where it is divided into 3 channels, comprising of one for signalling (16Kbit/s D channel) and two 64Kbit/s bearer channels. The Physical interface used is RJ45.

MSN
The use of a Multiple Subscriber Number allows ten fully independent numbers to be specified to access one ISDN basic rate port. Each connected terminal may be programmed with one of the allocated numbers so that it will only ring or connect when that specific number is dialled by the calling party. This allows particular calls to be directed to a particular terminal for example dial 'x' for sales or 'y' for service.

Network Termination Unit (NT)
The Network Termination unit is the box provided at the customer site which converts between the U interface required to connect to the local exchange and the S/T interface required by the customer.

Passive Bus
This is an ISDN feature which allows up to 6 terminal devices and two voice devices to share the same twisted pair each with unique identification.

PH
Physical Layer (Layer 1) for synchronisation and activation.

Primary Rate ISDN (E1)
An E1 Primary Rate line is a 2.048 Mbit/s framed interface divided in to 32 channels one of which is reserved for framing and synchronisation, another is reserved for signalling and the 30 remaining channels are available to the user. Each user channel provides the same facilities as one ISDN basic rate bearer channel (64Kbit/s bandwidth, supplementary service options, etc). Some network providers will allow channels to be delivered in groups of 6 or 15 channels to allow for future expansion.

PS1
Power Source 1 or Phantom Power provided by the network to the ISDN S bus.

PS1 Normal
Most ISDN lines will provide 'Phantom Power' to supply one connected terminal. In 'Normal' mode the level of this power feeding is 40V at up to 1W of power and it is usually derived at the NT1 from a local mains source.

PS1 Restricted
In the event of a local mains failure the Phantom Power level is limited to 420mW and will be supplied from the ISDN network to ensure a basic telephone service in these emergency conditions. To indicate this state to the terminal(s) the polarity will be reversed. Please note that some network providers always supply power in the restricted mode.

S/T Interface
The standard interface between ISDN terminals or terminal adapters and the network channel termination.

Single Number Mode
Single number mode is the most common configuration for ISDN basic rate lines. In this mode just one telephone number is provided to gain access to the ISDN port and the B (bearer) channels will be allocated in sequence so the first caller receives B1 and the second caller receives B2 assuming that both are available.

Subaddressing
Subaddressing is another supplementary service which allows a string of letters or digits to be included when a call is made so that these are received by the called party. The called party may then use this information to permit acceptance of the call. It is a form of call security.

Supplementary Service
These are optional services above the basic ability to make a call. Public Telephone Operators offer a range of Supplementary Services as cost options which can include: Calling Line Identification, Call Waiting, Call forwarding, Multiple Subscriber Number and Subaddressing.

TEI Mode
When an ISDN terminal is connected to a PBX or NT1 for the first time it needs to establish an identity on the network. Some terminals send an identity request as soon as they are connected while others wait until they need to send or receive a call. When the terminal is temporarily disconnected from the NT1 some will automatically lose their identity while others will retain it and expect to use it again when re-connected. This process of terminal endpoint identification (TEI) can vary when a different TEI mode is adopted.

U Interface
The physical, electrical and informational format of the two wire local loop connection that provides Basic Rate Access for North American ISDN networks.