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VoIP Troubleshooting 101 – Bandwidth needs for VoIP March 31, 2010

Posted by TelUS Consulting Services in VoIP catagory.
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An organization can determine how much bandwidth to set aside for voice traffic using simple math. However, in a converged voice and data network, administrators have to make decisions on how much bandwidth to give each service. These decisions are based on careful consideration of the organization’s priorities and the available bandwidth that can be afforded. If an administrator allocates too little bandwidth for voice service, there might be unacceptable quality issues. Another consideration is that voice services are less tolerant to bandwidth depletion than that of Internet traffic. Therefore,
bandwidth for voice services and associated signaling must take a priority over that of best-effort Internet traffic. If a network were to use the same prevailing encoding (CODEC) scheme as the current PSTN system, bandwidth requirements for VoIP networks would tend to be larger than that of a circuit-switched voice network of similar capacity. The reason is the overhead in the protocols used to deliver the voice service. Typically, an organization would need speeds of OC-12c/STM-4 and higher to
support thousands of call sessions. However, VoIP networks that employ compression and silence suppression could actually use less bandwidth than a similar circuit switched network. The reason is because of the greater granularity in bandwidth usage that a packet-based network has in comparison to a fixed, channel size TDM network.

Allocations of network bandwidth are based on projected numbers of calls at peak hours. Any over-subscription of voice bandwidth can cause a reduction in voice quality. Also, you must set aside adequate bandwidth for signaling to ensure that calls are complete and to reduce service interruptions.

The formula for calculating total bandwidth needed for voice traffic is relatively straightforward. The formula to calculate RTP bearer voice bandwidth usage for a given number of phone calls is as follows:

bits per sec = packet creation rates per sec x packet size x number of calls x 8 bits per sec
where samples per sec = 1,000 ms / packet creation rate

Example

2,000 full-duplex G.711 encoded voice channels that have a packet creation rate of 20 ms,
with a packet size of 200 bytes (40 byte IP header + 160 byte payload)

50 samples per second = 1,000 ms / 20 ms

160 Mbps = 50 x 200 x 2,000 x 8

Note that this number is a raw measure of IP traffic and does not take in account the
overhead used by the transporting media (links between the routers) and data-link layer
protocols. Add this raw IP value to that of the overhead to determine the link speeds
needed to support this number of calls. Note this value represents only the bearer (voice)
content.

Signaling bandwidth requirements vary depending on the rate at which the calls are generated and the signaling protocol used. If a large number of calls are initiated in a relatively short period, the peak bandwidth needs for the signaling could be quite high. A general guideline for the maximum bandwidth requirement that an IP signaling protocol needs is roughly three percent of all bearer traffic. Using the previous example, signaling bandwidth requirements, if all 2,000 calls were initiated in one second, would be approximately 4.8 Mbps (3 percent of 160-megabits).

With the calculation of bearer and signaling, the total bandwidth needed to support two thousand G.711 encoded calls would be an approximate maximum of 164.8 MB. This bandwidth requirement is a theoretical maximum for this specific case. If the parameters change, such as call initiation rate, voice encoding method, packet creation rate, employment of compression and silence suppression, the bandwidth requirements would change as well.

With large VoIP implementations requiring sizable bandwidth, it becomes imperative
that the IP network delivers the needed service at predictably high performance.

our final installment of VoIP Troubleshooting will be on Packet Loss and Reliability

Joe Buck, N.C.E.

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