8 | Section 2
The “gateway” could also be a full-up IP PBX, such as from Cisco, Digium (Asterisk), and many
others. In this case, compatible VoIP phones can be used for general office locations. SIP lets
you move calls between the office and studio systems with no audio degradation. A PBX such
as Asterisk could add additional capability, such as automated attendant functionality. With
appropriate cards, Asterisk makes an excellent gateway to T1 and ISDN PRI Telco lines.
Using VoIP to Connect to the Telco Network
Recognizing the growth in market share of VoIP PBXs, Telcos are beginning to offer “SIP Trunk-
ing” service, which delivers phone network connectivity directly over a controlled IP link. With
this service, you wouldn’t need a gateway.
While it remains a niche in early 2011, SIP trunking is growing in support from both PBX
vendors and carriers. Over time, this will almost certainly reduce the use of the older POTS and
T1 trunking. Eventually, it may replace it completely. See the Resources section for a list of SIP
VoIP providers that we’ve tested with.
What’s a ‘Line’?
While we are talking about Telco lines and VoIP services, we should take a moment to consider
– just what is a ‘line’ these days? Use of the word ‘line’ is becoming bothersome at this point in
telecom history. Back when an analog line was associated with a single telephone number, the
word had a clear meaning. When you ordered ten lines, you received ten physical wire pairs
and had ten telephone numbers. (Yes, “roll-over” rotary service exposed only one number to
the public, but the others were still there.) ISDN was the first step on the path to trouble. We
engineers began to speak of a BRI “line” with two “voice channels”. But producers and hosts
still communicated with each other as if channels were lines, saying things like, “The tree-
hugger is on line 3. Do you want to take him now?” We knew that meant BRI line 2, channel 1,
or T1 channel 8, but users had no clue.
ISDN PRI made matters yet more knotty, since the telephone number and physical channel
were divorced. Now we are faced with SIP trunking and other IP-based services, where a single
link (which might be connected via one, two, or more physical copper pairs, or an optical cable,
or via a wireless system) might carry any number of voice channels. (And what is going to
happen when phone numbers become obsolete, as they already are for VoIP applications such
as Skype?)
Users, blissfully unaware of all this, will undoubtedly continue with their conditioned habit,
referring to a certain caller as being on a particular line. Thus is the word ‘line’ destined to join
‘dial’ in a peculiar departure from original meaning.
In this manual, we refer to “lines’ as your users would. Usually that means a particular tele-
phone number that is probably associated with a button on the VSet phone and/or console
controller. To keep the ambiguity down, we’ll try to remember to call the physical links “con-
nections” or “interfaces”. And we’ll sometimes refer to ‘calls’ when the sense points more to
the conversation than the connection.
Grandstream GXW4004 4-port FXO gateway
To Next Page
Loading...