AN2832 Freescale Semiconductor / Motorola, AN2832 Datasheet - Page 2

• Manufacturer Part Number: AN2832
• Description: Packet Telephony Remote Diagnostics on the StarCore SC140 Core
• Manufacturer: Freescale Semiconductor / Motorola

Basics of Remote Diagnostics
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Basics of Remote Diagnostics
Remote diagnostic tests are becoming important maintenance applications in packet telephony systems for
monitoring and sometimes correcting anomalies that affect performance of voice or data communication services
[1], [2]. Maintenance tests are performed either in-service while other users are accessing the same channel of the
communication network or out-of-service (same channel not in use). The tests employ either intrusive methods that
change transmit and/or receive signals or non-intrusive methods that do not change the signals.
The International Telecommunication Union (ITU) developed the P.561 Recommendation [3], which defines
preferred interfaces, measurement ranges, and accuracy requirements for measuring voice-grade transmission
parameters in the communication network. This recommendation is intended primarily for in-service non-intrusive
measurement devices (INMDs). Typical P.561 measurements include speech level, noise level, echo loss, and
speech echo path delay; optional measurements, such as double talk, signal classification, and speech activity
factors, are sometimes needed.
Among the transmission parameters, echo is one of the most relevant impairments affecting quality of service.
Echo is caused by a hybrid network element, which is a circuit to convert a four-wire physical interface to a two-
wire connection. The main role of this hybrid is to provide an electrical interface for signals traveling in both
transmit and receive directions at the same time. The hybrid is designed to minimize echo (reflection) of the
receive signal, but in most practical cases the echo becomes quite noticeable, especially when communication
delay is large. Therefore, an echo canceller is required to mitigate this kind of distortion [4].
Hybrid circuits are typically modeled as linear time-invariant systems so that their signal processing characteristics
can be determined by estimating impulse responses. Estimates usually employ finite impulse response (FIR) filters,
as detailed in the ITU recommendation G.168 [5], which defines eight basic FIR hybrid models for testing network
echo canceller devices. When the impulse response is known (that is, coefficients of the FIR filter), the echo signal
of a hybrid circuit can be mimicked by a linear convolution of the receive signal and the hybrid impulse response
estimate, which then cancels the echo component of the signal. This is the basic foundation of echo cancellers, but
additional nonlinear processing techniques are used to handle residual echo.
Echo cancellers are deployed in media gateways, which are platforms for handling communications between IP
packet-based networks and circuit-switched networks. However, some network configurations may violate certain
design assumptions, resulting in degraded echo canceller performance and customer complaints. To resolve
configuration problems, expensive measurement devices are brought to the field to estimate telephone channel
characteristics. A far more cost effective approach is to design the media gateway with a software module that is
efficiently programmed to perform reliable measurements. A protocol is defined to transmit this measurement data
to a remote media gateway controller (see Figure 1) so that specialized engineers can retrieve and analyze it.
Media Gateway
Media
Hybrid
Controller
Gateway
Circuit
Figure 1. Portion of a Packet Telephony System
Packet Telephony Remote Diagnostics on the StarCore SC140 Core, Rev. 1
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Freescale Semiconductor