What does a channel bank do?

A short guide to using channel banks to build Asterisk solutions.

Introduction

A channel bank is a device that takes up to 24 separate analog telephone lines and combines them into a single digital line. These analog lines can either be telephone lines to the telco's central office or station lines to analog desk phones.

Summary

While channel banks have many uses in the telephone industry, in this guide we will concentrate on their use in building solutions using Asterisk -- the Open Source Telephony system.

Digium, the sponsor of the Asterisk project, manufacturers a four-port PCI interface card that can be used to connect up to four analog telephone lines. Most servers have up to three slots available for such boards but if one is building a system that needs to handle more than a dozen analog lines then using four-port PCI cards can be difficult.

The alternate approach is to use a channel bank to convert 24 separate analog telephone lines into a single digital (T1) line and then connect the T1 line to your Asterisk server using either the T100P (single T1 PCI card) or TE405P (quad-T1 PCI card).

Using this approach one can connect 96 analog lines using four channel banks and a single TE405P board.

Technical Details

When designing a system using a channel bank the first consideration is to determine what types of analog telephone lines are to be connected. There are three types you are most likely to encounter:

Office Trunk (FXO) - a telephone line that connects the channel bank to the telephone company. Your home phone jack is such a type of telephone line.

Station Line (FXS) - a telephone line that connects the channel bank to an analog desk telephone.

E & M Trunk - a telephone line that connects the channel bank to a special port on some types of PBXs.

Each type of line must be plugged into a port of the same type on the channel bank. A channel bank with only FXS ports cannot be used to connect to the telephone company.

Recommended Products

Rhino 24 FXS Port Channel Bank - this channel bank is best when you need to connect lots of analog phones to your Asterisk server. It only supports station lines and cannot be used to connect to telephone trunks.

Rhino 24 FXO/FXS Port Channel Bank - this channel bank accepts four-port FXO and four-port FXS modules to a total of 24 ports. This unit is ideal for connecting your Asterisk server to any combination of telephone trunks and station lines as illustrated above.

Because E&M trunks are less frequently encountered we do not currently handle products that interface to these types of ports but we can provide advice on suitable products that can interface to your PBX.

Please don't hesitate to contact netVOICE communications for any assistance you might need in selecting a channel bank appropriate for your needs.