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Resource sizing
Issue 6 January 2008 219
Table 39 and Table 40 express bandwidth usages in Erlangs, because each such usage
actually represents the average number of simultaneous bidirectional media streams through
the IP network in question. To convert those usages into bandwidth requirements in units of
kilobits per second (kbps), one must know how many kbps each call requires. To answer that
question, a closer look at IP packet structure is necessary.
An IP packet consists of a payload and some amount of overhead. The payload consists of
actual sampled voice, and the overhead represents headers and trailers, which serve to
navigate the packet to its proper destination. The overhead due to IP, UDP, and RTP is 40
bytes, while the Ethernet overhead is between 18 and 22 bytes (18 is assumed in this example).
This represents a total overhead of 58 bytes (464 bits), regardless of the nature of the payload.
For this example, Layer 2 (Ethernet) overhead is included in that total. At every router boundary,
because Ethernet overhead is included in this example, our calculations are for bandwidth on a
LAN. Because WAN protocol (for example, PPP) Layer 2 headers are generally smaller than
Ethernet headers, WAN bandwidth is slightly less than LAN bandwidth.
The size of the payload depends on certain parameters that relate to the codec that is used.
The two most common codecs that are used with Avaya products are uncompressed G.711 and
compressed G.729. The transmission rates that are associated with those codecs are 64 kbps
for G.711 (this is the Nyquist sampling rate for human voice) and 8 kbps for G.729.
The packet “size” is sometimes expressed in units of time (specifically, in milliseconds). The
following formula yields the packet size, expressed in bits:
Calls from site
2 to site 3
002.0
Calls from site
3 to site 2
0 0 2.0
Totals 24.0 10.0 4.0
Table 40: IP WAN bandwidth usages (Erlangs) for Example 6: IP bandwidth
considerations (continued)
Endpoints WAN bandwidth
(Erlangs) between
Sites 1 and 2
WAN bandwidth
(Erlangs) between
Sites 1 and 3
WAN bandwidth
(Erlangs) between
Sites 2 and 3
2 of 2
Number of bits of
payload per packet
Transmission Rate
(kbps)
⎝⎠
⎛⎞
ms per Packet()×=