configuration). Figure represents the bandwidth
calculation as a formula. Following the steps of the bandwidth
calculation procedure, use this formula: Bandwidth [kbps] =
(Total packet size [bytes per packet] * 8 / 1000) * packet rate
[pps] (Multiply the total packet size by 8 and divide by 1000
to convert bytes to kilobits per second.) Calculate the
Total Packet Size
Use this formula to calculate the
total packet size: 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
that is caused by the data-link protocol during Layer 2
encapsulation. "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 and 4 bytes when the UDP checksum is sent. The
packetization size is the size of the voice payload that is
encapsulated per packet. If the size is not known, use this
formula: Packetization size [bytes per packet] = (Packetization
period [ms per packet] / 1000) * codec bandwidth [kbps] * 1000
/ 8 Because the packetization size is in bytes and the codec
bandwidth is in kilobits per second, convert the codec
bandwidth by dividing by 8 and multiplying by 1000. In
addition, because the units in the codec bandwidth (kilobits
per second) and packetization period (milliseconds per packet)
are not the same, the packetization period has to be converted
by dividing by 1000. Balancing the conversions, the formula
simplifies to this: Packetization size [bytes per packet] =
Packetization period [ms per packet] * codec bandwidth [kbps] /
8 Calculate 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 instead of the
packetization period is configured. When the packetization size
is configured, 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 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 is 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 for calculating the total bandwidth are 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
to use this formula: 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])
Content 2.4 Calculating Bandwidth
Requirements for VoIP 2.4.8 Quick Bandwidth
Calculation Figure shows a quick way to calculate the total
bandwidth when the packetization size is given. Remember that
the ratio of the total bandwidth requirement to the nominal
bandwidth for the payload is the same as the ratio of total
packet size to payload size.The payload size depends on the
sample interval and the codec that is used and is usually 20
bytes for G.729 and 160 bytes for G.711, assuming a 20-ms
sample interval.The headers are always 40 bytes for the IP,
UDP, and RTP headers, plus the Layer 2 header size. The Layer 2
size is 6 bytes for Frame Relay or PPP or 18 bytes for
Ethernet.To calculate the total bandwidth, find the total
packet size, including all the headers plus payload and divide
by the payload size. Multiply the result by the nominal
bandwidth for the codec. The result is the total bandwidth
requirement. Note
You can calculate the bandwidth
using the Voice Codec Bandwidth Calculator at
http://tools.cisco.com/Support/VBC/do/
CodecCalc1.do.
Access to this URL requires a valid Cisco.com account.
Content 2.4 Calculating Bandwidth
Requirements for VoIP 2.4.9 Effects of VAD on
Bandwidth In a circuit-switched telephony network, because
of the nature of the network, the bandwidth of a call is
permanently available and dedicated to that call. There is no
way to take advantage of speech pauses, one-way audio
transmission, or similar instances when a link is not being
used. In a packet network, however, voice activity detection
(VAD) can take advantage of the fact that one-third of the
average voice call consists of silence. VAD detects silence
caused, for instance, by speech pauses or by one-way audio
transmission while a caller is listening to music on hold (MoH)
when being transferred. VAD suppresses the transmission of
silence and, therefore, saves bandwidth. The amount of
bandwidth saved by VAD depends on several factors: -
Type of audio: During a human conversation, the two
parties do not generally talk at the same time. When MoH is
played, the call usually turns into a one-way call. Because of
the constantly playing music, no bandwidth can be saved in this
direction of the call. However, the caller listening to the
music does not send any audio and no packets have to be
transmitted while the call is on hold.
- Level of
background noise: VAD needs to detect silence to be able to
perform silence suppression. If the background noise is too
high, VAD cannot detect silence and continues the
transmission.
- Other factors: Differences in
the language and character of speakers have an impact on the
amount of silence that is detected in a call. Some calls, such
as conferences or broadcasts where only one or a few
participants are speaking and most of the participants are
listening allow higher bandwidth savings than other
calls.
On average, the use of VAD can save about 35
percent of bandwidth. Because of the factors mentioned, there
is considerable deviation between each individual call.
Therefore, the average of 35 percent assumes a certain
statistical distribution of call types, which is usually
achieved only if a link carries at least 24 calls. If you are
calculating bandwidth for fewer calls, you should not take VAD
into account. Figure summarizes the characteristics of VAD.
VAD Bandwidth Reduction Examples
The table in Figure
shows the reduced bandwidth needs of various calls, assuming
that VAD can save 35 percent of bandwidth.
Content
2.5 Implementing VoIP in an Enterprise Network
2.5.1 Enterprise Voice Implementations
Enterprise voice implementations use components such as
gateways, gatekeepers, Cisco Unified CallManager, and IP
phones. Cisco Unified CallManager offers PBX-like features to
IP phones. Gateways interconnect traditional telephony systems,
such as analog or digital telephones, PBXs, or the PSTN to the