Page 104 - Asterisk™: The Future of Telephony

In the first line, in addition to indicating whether we are using FXO or FXS signaling,
we specify one of the following protocols for channel 2:

• Loop start (ls)
• Ground start (gs)
• Kewlstart (ks)

The difference between loop start and ground start has to do with how the equipment
requests a dial tone: a ground-start circuit signals the far end that it wants a dial tone
by momentarily grounding one of the leads; a loop-start circuit uses a short to request
a dial tone. Though not common for new installations, analog ground start lines still
‖
exist in many areas of the country. Ground start is really a rather strange thing, because
it doesn’t exist in its analog form in Asterisk, so technically, there is no ground signal
happening, but is rather a signaling bit that is intended for analog circuitry that his-
torically would have been at the end of the T1. If this does not make much sense, don’t
sweat it; chances are you won’t have to worry about ground-start signaling. All home
lines (and analog telephones/modems/faxes) in North America use loop-start signaling.
Kewlstart is in fact the same as loop start, except that it has greater intelligence and is
#
thus better able to detect far-end disconnects. Kewlstart is the preferred signaling
protocol for analog circuits in Asterisk.
To configure a signaling method other than kewlstart, replace the ks in fxsks with either
ls or gs (for loop start or ground start, respectively).
loadzone configures the set of indications (as configured in zonedata.c) to use for the
channel. The zonedata.c file contains information about all of the various sounds that
a phone system makes in a particular country: dial tone, ringing cycles, busy tone, and
so on. When you apply a loaded tone zone to a Zap channel, that channel will mimic
the indications for the specified country. Different indication sets can be configured for
different channels. The defaultzone is used if no zone is specified for a channel.

After configuring zaptel.conf, you can load the drivers for the card. modprobe is used to
load modules for use by the Linux kernel. For example, to load the wctdm driver, you
would run:
# modprobe wctdm
*
If the drivers load without any output, they have loaded successfully. You can verify
that the hardware and ports were loaded and configured correctly with the use of the
ztcfg program:

‖ Yes, there is such a thing as ground-start signaling on channelized T1s, but that has nothing to do with an
actual ground condition on the circuit (which is entirely digital).
# A far-end disconnect happens when the far end hangs up. In an unsupervised circuit, there is no method of
telling the near end that the call has ended. If you are on the phone this is no problem, since you will know
the call has ended and will manually hang up your end. If, however, your voicemail system is recording a
message, it will have no way of knowing that the far end has terminated and will, thus, keep recording silence,
or even the dial tone or reorder tone. Kewlstart can detect these conditions and disconnect the circuit.

76 | Chapter 4: Initial Configuration of Asterisk