Calculate the total bandwidth
To calculate the total bandwidth of a VoIP call, perform these steps:
Step 1 Gather required packetization information: First, you must determine the bandwidth of the codec that is used to digitize the analog signals. The codec bandwidth is specified in kilobits per second and is usually in the range of approximately 8-64 kbps. In addition, the packetization period (specified in milliseconds) or the packetization size (specified in bytes) is required. Given the codec bandwidth and one of these two values, the other value can be calculated.
Step 2 Gather required information about the link: The amount of overhead that will be added per packet on each link is the next piece of information needed. Will cRTP be used? Which data-link protocol is in use, and what is its overhead per packet? IP, UDP, and RTP overhead is 40 bytes unless cRTP is used, then it is 2 (the default) or 4 bytes. Make sure to include the overhead (in bytes) of the data-link protocol that is used. Finally, you must observe whether any other features that cause additional overhead are being used. These could be security features, such as VLANs, IPsec, or any special tunneling applications.
Step 3 Calculate the packetization size or period: Depending on the voice device, you might know either the packetization period or the packetization size (determined in Step 1). You have to calculate the missing information based on the known value plus the codec bandwidth, also noted in Step 1. The packetization size is expressed in bytes, the packetization period in milliseconds.
Step 4 Sum up the packetization size and all headers and trailers: Add the overhead of IP, UDP, and RTP (or cRTP), the data-link protocol, and any other protocols that you noted in Step 2 to the voice payload (packetization size), which you determined either in Step 1 or Step 3. All values must be in bytes.
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Step 5 Calculate the packet rate: Calculate how many packets will be sent per second by using the multiplicative inverse of the packetization period. Because the packet rate is specified in packets per second, make sure to convert the milliseconds value of the packetization period to seconds.
Step 6 Calculate the total bandwidth: Multiply the total size of the packet or frame by the packet rate to get the total bandwidth. Because the packet size is specified in bytes and the bandwidth is specified in kilobits per second, you need to convert bytes to kilobits.
Based on this procedure, you can calculate the bandwidth used by a VoIP call on a specific link. For planning the capacity of physical links, you will have to consider the maximum number of calls that should be supported and the bandwidth that is needed for applications other than VoIP. In addition, you have to ensure that enough bandwidth is available for call setup and call teardown signaling. Although signaling messages need relatively little bandwidth, you should not forget to provision the bandwidth for signaling protocols (especially in your QoS configuration).
© 2006 Cisco Systems, Inc. Describe Cisco VoIP Implementations 2-59
Illustration of the Bandwidth Calculation
This section represents the bandwidth calculation in a form of a formula.
Illustration of the Bandwidth Calculation
Following the steps of the bandwidth calculation procedure, the following formula can be generated:
Bandwidth [kbps] = (Total packet size [bytes per packet] * 8 / 1000) * packet rate [pps]
Because the total packet size is specified in bytes while the bandwidth is specified in kilobits per second, the total packet size has to be multiplied by 8 and divided by 1000 to convert bytes to kilobits per second.
Calculation of the Total Packet Size
The total packet size can be calculated by using this formula:
Total packet size [bytes per packet] = Data-link overhead [bytes per packet] + other overhead [bytes per packet] + IP overhead [bytes per packet] + packetization size [bytes per packet]
The data link overhead is the overhead caused by the data-link protocol during Layer 2 encapsulation.
The other overhead is any additional overhead, for example, the overhead caused by using IPsec or any of the tunneling protocols.
If cRTP is not used, the IP overhead is 40 bytes for the IP, UDP, and RTP headers. If cRTP is used, the IP overhead is 2 bytes, when the UDP checksum is not transmitted, or 4 bytes, when the UDP checksum is sent.
The packetization size is the size of the voice payload that is encapsulated per packet. It is either known (instead of the packetization period) or calculated using this formula:
Packetization size [bytes per packet] = (Packetization period [ms per packet] / 1000) * codec bandwidth [kbps] * 1000 / 8
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Because the packetization size is specified in bytes and the codec bandwidth is specified in kilobits per second, the codec bandwidth has to be converted by dividing it by 8 and multiplying it by 1000. In addition, because of the mismatch of the units in the codec bandwidth (kilobits per second) and packetization period (milliseconds per packet), the packetization period has to be converted by dividing it by 1000. Balancing the conversions, the formula can be simplified to this:
Packetization size [bytes per packet] = Packetization period [ms per packet] * codec bandwidth [kbps] / 8
Calculation of the Packet Rate
The packet rate, specified in packets per second, is the multiplicative inverse of the packetization period, which is specified in milliseconds per packet. Therefore, you have to convert the packetization period from milliseconds to seconds when building the reciprocal value:
Packet rate [pps] = 1 / (packetization period [ms per packet] / 1000)
Sometimes, you will not know the packetization period (milliseconds per packet), but you will know the packetization size (bytes per packet). This is because on some devices, the packetization size is configured instead of the packetization period. In that case, you have to calculate the packetization period first, using this formula:
Packetization period [ms per packet] = (Packetization size [bytes per packet] * 8 / 1000) / (codec bandwidth [kbps] / 1000)
Because of the different units used for the packetization size (bytes per packet) and the codec bandwidth (kilobits per second), you have to multiply the packetization size by 8 and then divide it by 1000. In addition, you must convert the codec bandwidth value (because the packetization period uses milliseconds instead of seconds) by dividing the codec bandwidth by 1000. Balancing the two conversions, the formula can be simplified to this:
Packetization period [ms per packet] = Packetization size [bytes per packet] * 8 / codec bandwidth [kpbs]
Summary
Assuming that you know the packetization period (in milliseconds per packet), the formulas can be aggregated and then simplified to this:
Bandwidth [kbps] = (8 * (data-link overhead [bytes per packet] + other overhead [bytes per packet] + IP overhead [bytes per packet]) + packetization period [ms per packet] * codec bandwidth [kpbs]) / packetization period [ms per packet]
If the packetization size (in bytes per packet) is known instead of the packetization period (milliseconds per packet), the simplest way to calculate the total bandwidth is the following:
Bandwidth [kbps] = (Codec bandwidth [kbps] / packetization size [bytes per packet]) * (packetization size [bytes per packet] + data-link overhead [bytes per packet] + other overhead [bytes per packet] + IP overhead [bytes per packet])
© 2006 Cisco Systems, Inc. Describe Cisco VoIP Implementations 2-61
Continue reading here: Quick Bandwidth Calculation
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