5
A TESTBED FOR EVALUATING VoIP
SERVICE1
A new service must be prototyped and tested in a laboratory environment
before massive deployment. This allows objective and subjective evaluation
of the service in question. In addition, the findings can be used for tuning
the network operations and performance control parameters, as required for
maintaining acceptable QoS (as discussed in Chapter 4).
The testbed presented in this chapter consists of a variety of PSTN and IP
domain network elements [1]. These elements are required to emulate PSTN
and IP networks, IP network impairments, and elements of SS7 networks like
SCP and STP. Other network elements include (a) the network timing server,
(b) software- and hardware-based IP and SIP phones, (c) analog and digital
(including ISDN BRI) circuit or PSTN phones, and (d) test equipment to
emulate and analyze single and bulk phone calls. This testbed is used for a
variety of VoIP tests and measurements, as described in Appendixes A, B, and C.
Appendix A discusses how this testbed can be used to measure call prog-
ress time in IP telephony. A multistage call setup method is proposed, and its
implementation using a set of scripts written in Hammer visual basic (HVB)
language (www.hammer.com, www.empirix.com, 2001) is described.
Appendix B presents techniques to determine the bulk-call-setup request-
handling performance of IP-PSTN GWs. To achieve this, both call burst size
and intercall burst time gap must be determined so that the call setup requests
are properly processed. These are implemented using HVB language for testing
some commercially available IP telephony GWs.
59
1 The ideas and viewpoints presented here belong solely to Bhumip Khasnabish, Massachusetts,
USA.
Implementing Voice over IP. Bhumip Khasnabish
Copyright


(www.madge.com, 2001) called Madge Access Switch 60 and a GTD-5 switch
from AG Communication Systems are used in the testbed.
An IP network can be emulated by using multiple EtherSwitches connected
via a router that can be programmed to introduce various types of network
impairments. For example, NIST-Net ( />2001) and Shunra’s (www.shunra.com, 2001) cloud or storm product can be
used to introduce IP-layer impairments in a controlled fashion. We use NIST-
Net in the testbed described in this chapter.
To emulate the elements of SS7 network elements like SCP and STP, various
types of equipment can be used. These include Tekelec’s (www.tekelec.com,
2001) MGTS, Eagle’s signal transfer point (STP), Inet’s (www.inetinc.com,
2001) test equipment, and so on. We use a small STP from Tekelec in our test-
bed to emulate SS7 network elements, and both MGTS and Inet’s spectrum as
SS7 test equipment.
60
A TESTBED FOR EVALUATING VoIP SERVICE
Any general-purpose server running the network time protocols (NTPs)
(IETF’s RFC 1305/1119, RFC 2030, RFC 867/8, etc.) can be used as the IP
domain network time server (NTS). For example, TrueTime’s (www.truetime.
net, 2001, www.truetime.com, 2001) NTS can be used in the testbed. Without
proper synchronization of the asynchronously operating IP-PSTN GWs and
other IP domain network elements like routers and SIP or IP phones, voice
quality and service reliability cannot be assured.
The traditional PSTN switch suppliers such as Lucent (www.lucent.com,
2001), Nortel (www.nortelnetworks.com, 2001), and Siemens (www.siemens.
com, 2001) manufacture ISDN BRI and analog and digital phones. We use the
BRI phones from Lucent and Siemens, and digital and analog phones from
Nortel. IP and SIP phones from a number of suppliers including Pingtel, Sie-
mens, Cisco, Ploycom, and 3Com (e.g., www.pingtel.com, www.siemens.com,
www.cisco.com, www.ploycom.com, www.3com.com, 2001) can be used in the
testbed described in this chapter.

DESCRIPTION OF THE TESTBED/NETWORK CONFIGURATION
61
phones (optiSet NI-1200S) from Siemens. Any of Lucent’s BRI phones can
support up to 10 calls or connections.
The two 24-port EtherSwitches and the IP network impairment emulator,
a PC-based simple router, comprise the Intranet of the testbed. The Ether-
Switches provide connectivity to the IP side interfaces of the IP-PSTN GWs (or
VoIP GWs) under test.
The VoIP gateway A (GW-A) and gateway B (GW-B) are the near-end
(or call-originating) and far-end (call-terminating) GWs. Usually GW-A and
GW-B are connected to two di¤erent subnets, which are interconnected via the
simple PC-based router mentioned above. However, if necessary, it is also
possible to connect the two GWs using the same subnet as well, that is, to
connect both GWs to the same EtherSwitch. Depending on the type of link
interface supported on the PSTN side, an IP-PSTN GW (GW-A, GW-B, etc.)
could be either a line-side, trunk-side, or residential GW.
Line-side GWs usually support multiple T1 (CAS or PRI) lines for con-
nectivity to the PSTN network. Trunk-side GWs usually support multiple
T1- and/or T3-type intermachine trunks (IMTs) for connectivity to the PSTN
network. Residential GWs usually support one (rarely more than one) T1 or
Figure 5-1 Block diagram of a VoIP testbed. The softswitch contains various VoIP
CC functions, such as, H.323 GK, MGCP/H.248 MGC, and SIP servers for registra-
tion, redirect and proxy functions, and may contain the SG and others. The SG can be
implemented in a physically separate network element (NE) as well. Clustering or hier-
archical interconnections can be used to interconnect the layers of the softswitch. The
global positioning system (GPS) antenna attached to the NTS extracts the clock infor-
mation from a globally synchronized time source and delivers the timing information to

messages (M3UA and M2UA) for transmission over IP.
The Inet SS7 tester (www.inetinc.com, 2001) supports a variety of interfaces
including V.35, BRI, RS-449, DS0, and DS1 for connections to an SS7 net-
work. It can emulate the SS7 signal transfer point (STP) and service switching
and control points (SSP and SCP). Inet can be used to monitor the flow of SS7
messages for a preset group of originating point codes (OPCs) and destination
point codes (DPCs). It can be also used to generate SS7 ISUP messages for
setting up and terminating PSTN calls, either repetitively or in bulk.
PSTN EMULATION
For emulating a CO switch of the PSTN network, we use the Madge Access
Switch 60 (www.madge.com, 2001) and a CLASS-5 switch such as a GTD-5
switch (see www.agcs.com for details). The Madge switch can accommodate a
maximum of six 4- or 8-port cards, with 4 ports in one card reserved for local/
remote configuration, network, and timing management. The remaining ports
PSTN EMULATION
63