Information Technology Reference
In-Depth Information
Default voice payload= 20 bytes * (8 bits/bytes) = 160 bits.
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WA N h e a d e r = 6 b y t e s .
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Vo i c e p a c k e t s i z e = 6 b y t e s + 2 b y t e s + 2 0 b y t e s = 2 8 b y t e s * ( 8 b i t s / b y t e ) = 2 2 4 b i t s .
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PPS = 8 kbps / 160 bits = 8000/160 = 50 pps.
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BW per c all = 224 (bits/packet) * 50 (pps) = 11200 bps = 11.2 kbps.
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BW for 10 c alls = 11.2kbps * 10 = 112 kbps.
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Here is a second example: Calculate the WAN bandwidth used by a G.711 calls with no
cRTP and a default voice payload.
From this description, we obtain the following:
G.711 codec is used: 64 kbps codec bit rate
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IP/UDP/RTP header = 40 bytes
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Default voice payload= 160 bytes * (8 bits/bytes) = 1280 bits
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WA N h e a d e r = 6 b y t e s
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Vo i c e packet size = 6 bytes + 40 bytes + 160 bytes = 206 bytes * (8 bits/byte) =
1648 bits
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PPS = 64 kbps / 1280 bits = 64,000/1280 = 50 pps
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BW per c all = 1648 (bits/packet) * 50 (pps) = 82400 bps = 82.4 kbps
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Cisco has developed a tool, available on its website, that you can use to obtain accurate
estimates for IPT design. The tool is the Voice Codec Bandwidth Calculator, and it is avail-
Delay Components in VoIP Networks
The ITU's G.114 recommendation specifies that the one-way delay between endpoints
should not exceed 150 ms to be acceptable, commercial voice quality. In private networks,
somewhat longer delays might be acceptable for economic reasons. The ITU G.114 recom-
mendation specifies that 151-ms to 400-ms one-way delay might be acceptable provided
that organizations are aware that the transmission time will affect the quality of user ap-
plications. One-way delays of above 400 ms are unacceptable for general network plan-
ning purposes.
Key
To p i c
Delay components are one of two major types: fixed delay and variable delay.
As shown in Figure 14-23, fixed delay includes the following types.
Propagation delay
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Processing delay (and packetization)
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Serialization delay
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Propagation delay is how long it takes a packet to travel between two points. It is based on
the distance between the two endpoints. You cannot overcome this delay component. The
