APPENDIX B
AUTOMATION OF CALL SETUP IN
IP TELEPHONY FOR TESTS AND
MEASUREMENTS1
In IP telephony, a call is usually established in multiple stages. In the first stage,
an ingress or call-originating IP-PSTN GW is accessed. This is followed by a
PIN-based caller authentication. Finally, the destination telephone number is
entered. If the GWs have enough digital signal processing (DSP) channels and
processing capacity, and the backbone (transport) network can support one
T1 CAS port’s worth of calls, we should be able to start 24 voice connection
attempts simultaneously. The call-originating GW should be able to process all
24 connection requests. However, it appears that most of the currently avail-
able IP-PSTN GWs cannot handle all 24 connection requests simultaneously.
Therefore, it is necessary to develop a method to determine the number of calls
that can be started simultaneously. It is also necessary to determine the inter-
call-burst time gap (in milliseconds or seconds) so that all 24 calls will be pro-
cessed using the existing hardware and software configuration and capacity
of the GW. This appendix describes techniques used to perform both of the
above functions. They are implemented using Hammer’ HVB language [1]
(www.hammer.com) for testing some commercially available IP telephony
GWs.
INTRODUCTION
The emerging IP telephony GWs for enterprise networking applications usually
support (a) one to four T1 ports per line card for interfacing to the PBX or
PSTN network and (b) one or two auto-sensing 10/100 BT Ethernet ports for
1 The ideas and viewpoints presented here belong solely to Bhumip Khasnabish, Massachusetts,
USA.
152
interfacing to the IP network via a LAN. One possible configuration for sup-
porting and testing black phone to black phone voice calls over an IP network
is shown in Figure B-1. Generic information on computer telephony testing

of intercall-burst time gap (in milliseconds) needed so that all 24 calls will be
processed with the existing hardware and software configuration and capacity
of the GW.
A set of HVB [1] language-based scripts that can perform both of the above
functions is presented in this appendix. The proposed technique and the results
obtained are discussed. Some concluding remarks are presented next. The
scripts for emulating the call-originating and call-terminating parties are also
presented.
THE PROPOSED TECHNIQUE
In this section, we first present a simple example, followed by a generic
description of the proposed technique in the form of flowcharts. Next, another
example is discussed. Implementations using HVB are then presented.
A Simple Example
As mentioned earlier, with one set of T1 connection from the PSTN network,
the ingress IP-PSTN GW should be able to support gracefully a maximum
of 24 incoming call requests. However, because of limited DSP resources, very
often most of the call attempts fail. To solve this problem, a precall wait can
be added. The precall wait determines the amount of time the call-originating
script will wait before actually making a connection attempt over a selected
channel.
In this example, only one call setup request is in progress at any point in time,
and a precall wait time of 1 sec is used.
Let us define a parameter called ‘‘waitTick’’ which controls the intercall time
gap or precall waiting time, called ‘‘waitTime.’’ In this simple example, only
one call is started at a time, and a 1-sec intercall gap is added. In case of one
T1, we have 24 channels to start the calls or connection requests. The call in the
first channel starts at the same time that the script starts running on the chan-
nel, the call in channel 2 starts 1 sec after the script starts running, the call in
channel 3 starts 2 sec after the script starts running, the call in channel 4 starts 3
sec after the script starts running, the call in channel 24 starts 23 sec after the

pause waitTime, HT_SECONDS
logmsg ") Pre-Call Wait is " & waitTime & " sec", HT_LOG_DEBUG
’***---- Enter the called party’s number ***
logMsg ") Placing call LU-GW4 B-to-A", HT_LOG_DEBUG
set event ¼ placeCall (dialnumber,)
Figure B-3 A simple example of adding a precall waiting time of 1 sec between each
call.
THE PROPOSED TECHNIQUE
155

Determining the intercall-burst time gap, which may vary from hundreds
of milliseconds to a few seconds, depending on the GW and GK soft-
ware, hardware, and processing capacity of the DSP modules of the GWs;
and

Developing a script or suite for determining the number of stages for set-
ting up a prespecified number of calls in multiple stages. For example, if a
GW can handle only four simultaneous incoming call requests, one would
need six (¼ 24/4) stages to set up 24 incoming call requests with a reason-
able amount of intercall-burst time gap.
Step 1: In this step, the objective is to determine the maximum number of
calls that can be started simultaneously so that the call attempts are
successful. For one T1 connection between the PSTN network and
the ingress/egress GW, it is possible to start the calls on all 24 channels
of a T1 line of the Hammer tester [1,2] at the same time. Then one
can visually monitor the status of the call requests in the Hammer
tester’s channel monitor. Usually, successful calls or connections are
indicated by a green color and failed connections are indicated by red.
If the connection requests are successful on all 24 channels, the ingress
GW is capable of handling 24 simultaneous connection establishment

In this example, attempts are made to start multiple calls simultaneously with
an empirically determined size of the call bursts. Although it is possible to
implement a nonuniform distribution of bursts, for the sake of simplicity we
use uniform call bursts here.
Let us set the size of the call burst that represents the number of calls to start
at the same time—that is, the parameter ‘‘no_of_simult_calls’’—to 4. The
intercall-burst interval, waitTick, is set to 3 sec. With this combination of
no_of_simult_calls and waitTick, one would need six [¼ 24/4] stages and 15
[¼ 3 Â ((24/4) À 1)] sec to start all 24 calls. If all of the calls are made and the
call setup time is 5 sec, then successful establishment of all the connections
Figure B-4 Determination of the number of calls that can be started simultaneously so
that successful connections are established to the called telephones.
THE PROPOSED TECHNIQUE
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