Bandwidth estimates

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Revision as of 22:52, 30 January 2007 by Gijs (talk | contribs) (→‎Trunking)
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Packet overhead

Communication over an IP-network has some overhead for each data-packet. This overhead can be seen as a constant value in calculations, although it is dependant on a few parameters like the networkstructure and wether it is an TCP or UDP packet.

For TCP, an overhead of 52 Bytes is average and for UDP it is 28 Bytes.

See for more detailed information ethernet payload datarates.


Along with the data used by the codec(s), some control data also has to be transmitted. This means the overhead per packet is even greater than only the IP-headers. This overhead is different for several protocols (e.g. SIP and IAX) and may also be dependant on the number of streams combined in the packet. (e.g. audio and video or even multiple conversations)

Trunking

Because of this packet-overhead, several protocols (like IAX2) support some kind of multiplexing. This means in each datapacket which is sent over the network, several packets of multiple conversations are put together in order to minimize the amount of packets on the network, thus maximizing the payload and minimizing the overhead.

This means the bandwith used between 2 PBX stations can be seen as:

  • BRcodec * #channels + IP-overhead + PBX-protocol-overhead.

This saves (n-1) * (IP-overhead + PBX-overhead) in traffic and even more important, all traffic can be sent over one connection.

Audio codecs

Bandwidth Requirements for Several Common VoIP Compression Algorithms

Codec Bit rate NEB
G.711 64 Kbps 87.2 Kbps
G.729 8 Kbps 31.2 Kbps
G.723.1 6.4 Kbps 21.9 Kbps
G.723.1 5.3 Kbps 20.8 Kbps
G.726 32 Kbps 55.2 Kbps
G.726 24 Kbps 47.2 Kbps
G.728 16 Kbps 31.5 Kbps
iLBC 15 Kbps 27.7 Kbps

NEB = Nominal Ethernet Bandwidth (one direction)

  • G711u/a - Requires 64+Kbps (inbound and outbound) continuous data stream for a quality call, plus the SIP signal. A 128Kbps connection has shown to be the minimal connection needed in our testing.
  • G723 - Requires 6.4+Kbps (I and O) continuous data stream and also needs room for the SIP signal, 28.8Kbps connection has shown to be the minimal connection.

(Source)