Allworx 24X Manuel d'utilisateur télécharger pdf (Page 19)

VoIP White Paper

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Revised: February 8, 2007

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10 QoS across a WAN

This section relates to configurations where the Allworx server and one or more IP phones are located at

different physical sites. It also applies to cases where multiple Allworx sites are connected together in a site-to-

site manner or when the Allworx server is configured to take advantage of an Internet Telephony Service

Provider (ITSP) for calls to/from the Allworx server.

QoS topics across a WAN are of particular concern for both physical and historical reasons. The historical part

of the problem is that IP protocols and the Internet in general were originally engineered to move only data – all

treated pretty much equally on best effort basis. It didn’t matter if the data was e-mail or coded voice traffic. As

it sits today, there is no standardized way for the public Internet to support prioritized traffic between arbitrary

end-points. Those protocols are still evolving, and the installed base of Internet infrastructure is not fully

equipped to support the protocol standards, even where they do exist currently.

The basic physical problem here is what to do when bursts of data exceed the bandwidth of a limited size pipe.

This is a complex topic that has several aspects. For example, there are priority and traffic shaping trade-offs

that affect both the effective latency and available throughput of different traffic classification types. The traffic

patterns and needs of various sites are different and have to be managed with site-specific knowledge of

policies and priorities.

In many circumstances, ordinary Internet connections carry voice traffic pretty well most of the time. For most

users, the potential reliability disadvantages are greatly outweighed by the cost advantages of a simple ad-hoc

WAN setup. While guaranteed operation is only possible through a carefully engineered and managed QoS

plan, adhering to the following guidelines will pave the way for a cost-effective solution using only ordinary

Internet connections that may already be in place:

• Do not attempt to deploy VoIP service using a dial-up connection – these are too easily overloaded by

even modest data traffic.

• For remote telephony applications to work through Allworx, the Allworx server may require to have its

WAN interface directly connected to the public Internet. This is discussed in more detail in previous

sections. In particular, calls to/from an ITSP service will typically not work if the Allworx server is behind a

firewall.

• Prior to deploying VoIP between two sites, it is highly recommended that you first test your Internet

connection to determine the speed of your link. Test the speed several times per day over the course of a

week, and base your planning on the slowest rate measured.

• A VoIP call consumes symmetrical data on the network. Be sure your speed test results account for

uplink and downlink performance. The lesser of the two values should be used for bandwidth planning.

• Determine the percentage of available bandwidth to be used for voice. Generally, it is better to not use

more than about 50 percent of the available bandwidth for voice, leaving the remaining 50 percent for

data applications.

• Compute the maximum number of calls your voice-allocated bandwidth will support and configure Allworx

VoIP server settings to limit the maximum number of calls accordingly (so the desired limits are not

exceeded).

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Allworx 24X Manuel d'utilisateur Page 19