Page 55 - Asterisk™: The Future of Telephony
P. 55

Digital                  Analog


                                 T1
                                             FXO
                                             FXS
                                             FXS
                                      Channel bank
                                                                        Central office
                          PBX


               Figure 2-2. One way you might connect a channel bank
               competition there is in your area, the better chance you have of finding a good deal. Be
               sure to shop around.
               The Zapata Telephony Project originally produced a T1 card, the Tormenta, that is the
               ancestor of most Asterisk-compatible T1 cards. The original Tormenta cards are now
               considered obsolete, but they do still work with Asterisk.
               Digium makes several different digital circuit interface cards. The features on the cards
               are the same; the primary differences are whether they provide T1 or E1 interfaces, and
               how many spans each card provides. Digium has been producing Zaptel cards for Linux
               longer than anyone else, as they were deeply involved with the development of Zaptel
               on Linux, and have been the driving force behind Zaptel development over the years.

               Sangoma, which has been producing open source WAN cards for many years, added
                                                            ‖
               Asterisk support for its T1/E1 cards a few years ago.  Rhino has had T1 hardware for
               Asterisk for a while now, and there are many other companies that offer digital interface
               cards for Asterisk as well.

               Channel banks
               A channel bank is loosely defined as a device that allows a digital circuit to be de-
               multiplexed into several analog circuits (and vice versa). More specifically, a channel
               bank lets you connect analog telephones and lines into a system across a T1 line.
               Figure 2-2 shows how a channel bank fits into a typical office phone system.
               Although they can be expensive to purchase, many people feel very strongly that the
               only proper way to integrate analog circuits and devices into Asterisk is through a
               channel bank. Whether that is true or not depends on a lot of factors, but if you have
                                              #
               the budget, they can be very useful.  You can often pick up used channel banks on
               eBay. Look for units from Adtran and Carrier Access Corp. (Rhino makes great channel


               ‖ It should be noted that a Sangoma Frame Relay card played a role in the original development of Asterisk
                 (see http://linuxdevices.com/articles/AT8678310302.html); Sangoma has a long history of supporting open
                 source WAN interfaces with Linux.

                                                                       Telephony Hardware | 27
   50   51   52   53   54   55   56   57   58   59   60