925VST-TestingTVoIPphones.wpd
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jls August 17, 2005
SAGE INSTRUMENTS, 240 AIRPORT BLVD., FREEDOM, CA 95019-2614 Ph (831) 761-1000 Fax (831) 761-2452
www.sageinst.com
Sage 925VST Application Note
Testing Through WiFi and VoIP Phones
Introduction
This document describes how to interface a Sage 925VST Handheld VoIP Service Tester to a WiFi phone
or VoIP phone. It also includes a complete example of how to conduct an SMOS test.
Background
On classic analog telephone circuits, test equipment can be connected directly to the pair of telephone
wires that extend from the telephone central office to the customer's premises. The test equipment directly
substitutes the telephone set. This 2-wire interface is expected to have a known fixed test impedance of
about 600 ohms, no matter what phone line you connect to, and no matter what phone was being used.
There are also industry-wide expectations about the required transmit level (0 dBm). And with the fixed
interface impedance, it is very straightforward to accurately measure the receive level (nominally
-10 dBm)
Telephone circuits employing WiFi or VoIP phones, however, don't have this "standard" 2-wire interface.
Instead, the connection from phone to the "telephone central office" can employ 2 or more wires and any
of a number of transport protocols and modulation schemes. So, the most practical method of testing
through such phones is to connect to the "coil-cord" handset interface of a VoIP phone or the headset
interface of a WiFi phone.
These interfaces are inherently 4-wire (two wires for the microphone, and two wires for the earphone). In
WiFi phones, though,
you may find the
physical connections
consist of only three
wires. On such
connections, one lead
of the microphone and
one lead of the
earphone have been
joined as a "common"
lead.
Unfortunately, none
of these interfaces are
"standardized" with
respect to impedance,
transmit level, or
receive sensitivity. So,
this document will
also describe how to
adjust the 925VST
transmit level and
receive sensitivity as a
method to cope with
these variances.
IP NETWORK
925VST
IP PHONE
Y-MODULAR TEST ACCESS CABLE
SAGE PART NO. 2100-0237-01
Figure 1
— Connecting the 925VST to an IP Phone
