> PBX Trunks

PBX Trunks
A trunk is a special kind of line that connects two telephone switches. If one of the
two switches is the PBX, the other could be a local or long-distance switch for
PSTN access, in which case we would call these local trunks or long-distance
trunks, respectively (though it’s worth pointing out that even if you don’t have dedicated
long-distance trunks you likely are able to get long distance services through
local trunks). On the other hand, if the other end of the trunk is another privately
owned PBX, we would call these private trunks or tie lines, even if they happen to
be routed through the PSTN (since the telephone numbers they can reach can only
be dialed from within the private network).There are also trunks that can act like
both types through the use of Centrex or something called a Virtual Private
Network (VPN—but it’s not the remote access VPN you may be familiar with from
the data world—this VPN is created by a carrier to let you keep a private dial plan
across many sites on the same trunks that you use for regular PSTN access).
Some say trunks are so named because in the old days, Ma Bell saw fit to use
thick, lead-covered cables to connect the switches.These cables resembled an elewww.
phant’s trunk. Others claim the word’s origin is derived from the way the local loop
network resembles the branches of a tree, with the trunks having similarity to…
well, a tree trunk. Regardless, trunks are the main lines of the communications
system, and the only case where a trunk is not connecting to a switch is when an
adjunct server is involved (like a voice messaging server, an Automatic Call
Distribution (ACD) server, an Interactive Voice Response (IVR) system, or similar
system). In some cases, these servers may use station emulation instead of trunking,
so you’ll need to verify what actually is being used.
Trunks can be analog, digital, or VoIP-based, just like station lines. Analog trunks
can be as simple as a regular 2-wire POTS line to the local CO switch, or a 4-wire
analog E&M trunk that provides improved signaling response (less glare).
Channelized digital T1 trunks come in two main flavors.The first and oldest type of
T1 can have 24 channels of 64 kilobit per second voice with robbed-bit signaling
(signaling bits are stolen from the voice stream in a way that’s not noticeable to the
ear).This type of T1 sends much less signaling data but cannot be used with 64 kbps
switched data because of the robbed bits used for signaling, but can pass 56 kbps
switched data. ISDN T1 trunks have 23 channels of voice (bearer, or B channels) and
a separate 64 kbps channel for signaling (the data, or D channel) that can support
ISDN User Part (ISUP) messages, including Automatic Number Identification,
which allows calling and called number information to be sent (although it can be
spoofed; this is discussed in Chapter 4). In Europe and internationally, the E1 is the
typical digital interface, with an ISDN BRI carrying 30 bearer channels (30B+D) as
opposed to the 23 channels supported by ISDN over T1 (23B+D).
VoIP trunks also come in various flavors, including H.323, SIP, and proprietary
protocols like Inter-Asterisk eXchange (IAX). In some cases, IP-enabled PBX systems
also use gateway control protocols with VoIP trunks, such as Simple Gateway
Control Protocol (SGCP), H.248/Megaco/Media Gateway Control Protocol
(MGCP), Skinny Gateway Control Protocol. One of the difficult problems with
VoIP trunks, however, is feature transparency between vendors. ISUP/Q.931 or its
private line equivalent (QSIG) has the most complete feature interworking capability,
and standards for mapping these onto H.323 and SIP exist, but these are not
evenly supported by PBX vendors at this point. Robust, reliable interworking
between different PBX vendors over VoIP is not easy to find today (and is still a
challenge over private tie lines).