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Three WAN technologies are commonly used with IP Telephony:
ATM
Frame Relay
Point-to-point (PPP) circuits
These technologies all have good throughput, low latency, and low jitter. ATM has the added
benefit of enhanced QoS. Frame Relay and PPP links are more economical, but lack some of
the traffic-shaping features of ATM.
Of the three technologies, Frame Relay is the most difficult WAN circuit to use with IP
Telephony. Congestion in Frame Relay networks can cause frame loss, which can significantly
degrade the quality of IP Telephony conversations. With Frame Relay, proper sizing of the
committed information rate (CIR) is critical. In a Frame Relay network, any traffic that exceeds
the CIR is marked as discard eligible, and is discarded at the option of the carrier if it
experiences congestion in its switches. Because voice packets must not be dropped, CIR must
be sized to maximum traffic usage. Also, Service Level Agreements (SLAs) must be established
with the carrier to define maximum levels of delay and frame loss, and remediation if the
agreed-to levels are not met.
Network management is another important area to consider when implementing IP Telephony.
Because of the stringent requirements imposed by IP Telephony, it is critical to have an
end-to-end view of the network, and ways to implement QoS policies globally. Products such as
HP OpenView Network Node Manager, Avaya Integrated Management, Concord NetHealth,
and MRTG help administrators maintain acceptable service. Outsource companies are also
available to assist other companies that do not have the resources to implement and maintain
network management.
Voice quality
Voice quality is always a subjective topic. Defining "good" voice quality varies with business
needs, cultural differences, customer expectations, and hardware and software. The
requirements set forth are based on the ITU-T and EIA/TIA guidelines and extensive testing at
Avaya Labs. Avaya requirements meet or exceed most customer expectations. However, the
final determination of acceptable voice quality lies with the customer's definition of quality, and
the design, implementation, and monitoring of the end-to-end data network.
Quality is not one discrete value where the low side is good and the high side is bad. A trade-off
exists between real-world limits and acceptable voice quality. Lower delay, jitter, and packet loss
values can produce the best voice quality, but also can come with a cost to upgrade the network
infrastructure to get to the low values. Another real-world limit is the inherent WAN delay over
an IP trunk that links the west coast of the United States to India. This link could add a fixed
delay of 150 milliseconds (ms) into the overall delay budget.
Perfectly acceptable voice quality is attainable, but will not be "toll" quality. Therefore, Avaya
presents a tiered choice of elements that make up the requirements.
Voice quality
Issue 3.4.1 June 2005
279
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