PRI for Asterisk

PRI is an abbreviation for Primary Rate Interface and is a telecommunication standard for carrying multiple data or voice DS0 transmissions between 2 physical locations.

All data and voice channels are (ISDN) and operate at 64 kbit/s.

North America and Japan use a T1 of 23 B channels and one D channel. Europe, Australia and most of the rest of the world use a slightly higher capacity E1 of 30 B channels and one D channel.

A B-channel is used to transmit the voice or data and is sometimes also called a user channel, a D channel is used for control messages and signalling.

In some T1 configurations, no D-channels might be used, instead the signalling will be sent inband over the B channels, this is called in-band signalling or bit robbing, resulting in lower transmission rates than the E-carrier system. This resulted in many US ISDN installations only having an effective data rate of 56 kbit/s over a nominal 64 kbit/s channel. See also A&B. This depends on the framing format used.

In the states its common to order a fractional T1, with less than 23 B channels.


1.3. signalling

On both E1 and T1, one timeslot is usually reserved for a D-channel for call setup and call teardown. (called signalling).

- CAS: Channel associated Signalling: with this kind of signalling, a set of bits is used to replicate opening and closing the circuit (as if picking up the telephone receiver and pulsing digits on a rotary phone), or using tone signalling which is passed through on the voice circuits themselves.

- CCS: Common Channel signalling: A more recent kind of signalling, (ISDN signalling and ss7 signalling are a subgroup of CCS signalling.) In this kind of signalling, short messages are sent over the signalling channel, with more information about the call, including caller ID, type of transmission required, etc. etc.


1.4. Framing

- HDB3
- AMI: Aternate Mark Inversion
- B8ZS:


1.5. Timing or clock sources

A PRI connection needs a timing device on one of both ends.
A PRI line can be clocked internally or can be clocked by the telco.

USB 2.0 port support heavy telephony traffic

The USB 2.0 interface provides a theoretical speed of 480,000,000 bits per second. A typical uncompressed phone conversation uses about 64,000 bits per second per direction, plus some overhead; in total less than 200,000 bits per second per phone call is used. Thus, the theoretical concurrent number of conversations that the USB 2.0 interface can handle is 480,000,000 divided by 200,000: roughly 2,400 calls for a single USB 2.0 port.

This is a theoretical number, of course, and the typical host processor will not be able to handle this amount of concurrent calls, but it gives a clear answer to the question: "Can the USB port support heavy telephony traffic with multiple Astribanks?"

Connecting multiple Astribank units to a single USB 2.0 port is easy: simply use a USB 2.0 Hub to connect many Astribank units.

Xorcom Astribank XPP Technology

Xorcom has developed a revolutionary concept in telephony interfaces for Asterisk systems which we call Astribank XPP Technology. Using USB 2.0 ports to connect to any Asterisk server, Astribank eliminates the requirement for a PCI (E1/T1) card, and even for PCI slots.

The USB 2.0 interface provides a theoretical speed of 480 Mbits per second, which for typical uncompressed phone conversations translates to roughly 2,400 calls per port. The USB architecture employed by Xorcom products affords many advantages, including the ability to build large systems that support hundreds of analog extensions, rapid set-up and easy maintenance.