Previous Table of Contents Next


A single BRI line can support up to eight devices, which might be in the form of telephones, facsimile machines, or computers. Additionally, up to 64 individual telephone numbers can be supported [8-31]. While BRI supports as many as three simultaneous calls, only one can be a voice conversation.

BRI uses an 8-pin connector which is defined by the International Standards Organization (ISO) in ISO 8877 and which uses a RJ-45 jack. Full duplex (FDX) connectivity is accomplished over a digital twisted-pair local loop through the application of special carrier electronics. A NT1 (Network Termination 1) device provides for compatibility with network protocols.

Primary Rate Interface (PRI)

Primary Rate Interface (PRI) also is known as 23B+D in the US and Japan. The European or ITU version is known as Primary Rate Access (PRA) or 30B+D. PRI offers 23 B (Bearer) Channels, plus 1 D (Data) channel. Both the B and D channels operate at 64 Kbps. The individual B channels can be used as discussed in the case of BRI; the D channel is reserved exclusively for signaling. As the standards provide for a D channel to support up to five PRI connections, numerous carriers have recently embraced this concept, thereby yielding additional usable bandwidth for user application. For example, the first PRI in a PBX trunking application would be provided at 23B+D; the next four PRIs would be deliver 24B+0D.

PRI provides a full duplex (FDX) point-to-point connection through a NT2-type intelligent CPE switching device (e.g., a PBX or router) device for protocol interface with the carrier CO exchange switch.

While designed for transmission over a standard T1 trunk, PRI is a significant improvement over T1, because the channels can be allocated dynamically. Each channel can act as an incoming, outgoing, combination, or DID trunk as the need arises. The nature of the channel can be determined as required or as specified, varying based on user-definable parameters. Additionally, multiple B channels can be aggregated to serve bandwidth-intensive applications, such as videoconferencing.

H-Channels

H-channels (High-speed channels), are functionally equivalent to B-channels, but provided greater aggregate bandwidth in PRI applications. H0 channel signals have a aggregate bit rate of 384 Kbps, while H1 channels operate at an aggregate of 1.536 Mbps for the North American version (H11) and 1.920 Mbps for the European version (H12). This capability of channel aggregation allows multirate communications on a dynamic basis through inverse multiplexing over multiple B-channels. It does have a drawback, however, when compared to traditional Inverse MUXs as the connection must be torn down and reinitiated when channels are added or dropped. The feature is known variously as Multirate ISDN, Nx64, Channel Aggregation and bonding. H channels find application in fast faxing (Group IV), videoconferencing, high-speed data transfer and high-quality audio transmission.

ISDN Equipment

ISDN hardware, at the end user side of the connection, includes Terminal Equipment (TE), Terminal Adapters (TAs), and Network Terminations (NTs) as shown in Figure 8.10. The carrier requires digital COs that are equipped with ISDN and SS7 software.


Figure 8.10  ISDN terminations in TE1 and TE2 CPE.

Terminal Equipment (TE)

Terminal Equipment (TE) is the term for a functional device that connects a customer site to ISDN services. Examples include computers, telephones, facsimile machines, and videoconferencing units. TE1 has a built-in ISDN interface, while TE2 devices do not have native ISDN compatibility.

Terminal Adapters (TAs)

Terminal Adapters (TAs) are interface adapters for connecting one or more TE2 (non-ISDN) devices to an ISDN network. TAs act as ISDN DCE, serving a function equivalent to protocol or interface converters. Applied to equipment which does not have ISDN capability built within it; the TAs must be exactly tuned to the specific CPE/DTE. TAs can be in the form of either standalone units or printed circuit boards fitting into an expansion slot of a PC.

A key function of the TA is that of rate adaption, which effectively throttles down the transmission rate from 64 Kbps to the rate at which the non-ISDN device is capable [8-32]. As an example, a non-ISDN PC might be capable of only 19.2 Kbps through the serial port. Another example might involve a connection supported at only 56 Kbps (non-ISDN or Pacific Bell “ISDN”) on the receiving end; the device would throttle down to that rate, rather than 64 Kbps. Rate adaptation is accomplished in North America through the ITU-T V.120 protocol; the European standard is V.110.

Network Terminations (NTs)

Network Terminations 0(NTs) are Network Termination devices, NT1s and NT2s.

NT2s

NT2s (Network Termination type 2) are intelligent devices responsible for the user’s side of the connection to the network, performing such functions as multiplexing, switching or ISDN concentration. A NT2 device would likely be in the form of a PABX, LAN router or switching hub.

NT1s

NT1s (Network Termination type 1) physically connect the customer site to the carrier side of the connection, performing such functions as signal conversion and maintenance of the local loop’s electrical characteristics. In a PRI environment, these functions are similar to those provided by Data Service Units (DSUs) and Channel Service Units (CSUs). In a BRI environment, these devices are TE1 devices.

Inverse MUXs,

Inverse MUXs, offered by some manufacturers, allow multiple BRIs to be bonded, or linked, for greater aggregate transmission over a BRI circuit(s). For example, four BRIs can be linked to support a 512 Kbps data transmission. Such an approach competes effectively with Fractional T1 (FT1).

D-Channel Contention Devices

D-Channel Contention Devices, offered by some manufacturers, allow as many as eight devices to share a BRI circuit, contending for access to the B-channel. The individual devices identify themselves to the network through contention for the D channel.


Previous Table of Contents Next