Basic Rate Access (BRA) is used when two channels are required, and consists of two products ISDN2 (used for digital equipment) and ISDN2a (used for digital and analogue equipment). BRA offers two 64 kbps channels (or 128 kbps when bonded) and one 16 kbps channel (known as the D Channel). The B Channel may be used for voice; data, video or image communications by the user and the D channel is used for signalling. The two B Channels that are available can be used simultaneously and independently, for example; you may use the Internet, receive a phone call, send or receive a fax - any two at the same time.
The ISDN BRA service (ISDN2 & ISDN2a) allows different information such as telephone calls, Internet access, video conferencing and faxes to be transmitted with digital technology that is more advanced than traditional analogue lines.
Two B Channels are available and can be used simultaneously and independently, for example; you may use the Internet, receive a phone call, send or receive a fax - any two at the same time. The speed of each B Channel is 64 kbps and may be bonded to provide the speed of 128 kbps.
Supplementary services provide additional functions to your existing ISDN2 BRA service. One such service is Multiple Subscriber Numbers, (MSN), and allows up to 4 additional telephone numbers.
A network terminator (NT) unit will be installed at your premises. Multiple devices may be simultaneously connected to the network terminator, including telephones, personal computers or fax machines
The only difference between ISDN2 and an ISDN2a service is the NT unit. A NT1 is installed for ISDN2 service and consist of two digital ports; therefore only digital equipment may be used (unless a Terminal Adaptor (TA) is purchased to allow connectivity to analogue equipment). For and ISDN2a service, a NT2 is installed and consist of two digital and two analogue ports (digital as well as analogue equipment will function on the ISDN 2a service).
Graphic illustration of a Typical Telkom ISDN2 Basic Rate Access
Graphic illustration of a Typical Telkom ISDN2a Basic Rate Access
ISDN2/2a as classified under Basic Rate Access
Basic Rate Access enables the user to transmit up to 128 kbps on a standard twisted copper pair. This bandwidth is divided into three logical channels, namely two independent B Channels of 64 kbps each and one signalling D Channel of 16 kbps. BRA can also be referred to as 2B + D. The B Channels are simultaneously available for voice, data, text or image calls, or any combination of these. These B Channel calls are seen as normal, independent telephone calls to separate destinations. The D Channel, on the other hand, will set up and control these B Channel calls. Therefore, a Basic Rate Access will allow up to two terminals to communicate simultaneously, using the two B Channels.
With only a digital line Terminating Card in the exchange and a Network Terminator (NT) on the customer's premises, Basic Rate Access of 128 kbps can be provided over the standard twisted copper pair used for standard telephone connections. This interface (twisted copper pair) is specified by ITU-T and is commonly referred to as the U bus.
On the customer's side of the NT, ITU-T has specified another interface, called the So bus. Customer Premises Equipment (CPE) may be connected to the S bus of an ISDN BRA. CPE can be categorised into two main functional groups namely ISDN S bus compatible CPE and non-compatible CPE. Compatible CPE is equipment which can be connected directly to the S bus, and which interfaces with the ISDN signalling protocol (e.g. ISDN PC-cards, ISDN digital telephone). Non-compatible CPE is equipment that cannot be connected to the S bus and which needs a mediation device to convert its protocol to an ISDN signalling protocol (e.g. V24, X25, analogue telephones and facsimile). This mediation device is called a Terminal Adapter (TA) and is applicable to ISDN2.
Basic Rate Access Network Terminator (NT)
A network terminator may consist of two functional components (NT1 and NT2). The NT1 has two digital ports for digital equipment. Should customers require analogue equipment to function a terminal adapter needs to be purchased.
The NT2a has two analogue and two digital ports, and both analogue and digital equipment can function. The network terminator form part of the ISDN basic rate access, and is supplied by Telkom.