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How Do I? Part 2 Voice Over IP - Page 101
Bandwidth Requirements Calculations
When considering the deployment of VoIP, establish the goals for data and voice
integration and determine the main traffic types the integrated network is
expected to support. Determine if baseline data networking requirements can be
met when the INDeX solution is in place on the network. Within this section the
calculation to determine to load of VoIP call is given.
In a correctly designed network, VOIP traffic is prioritised over non-voice traffic.
The bandwidth for VOIP must be available in the networking, when congestion
occurs non-voice traffic will be dropped in favour of voice traffic. The TCP/IP
protocol will handle the re transmission of non-voice packets. In the absence of
VoIP traffic non-voice traffic will have full occupancy of the link. Congestion
mainly occurs on slow speed WAN links. Ethernet switches can be used to
ensure congestion does not occur on the LAN.
The Bandwidth used by a given compression type can be calculated using the
formula shown below.
(Layer2 Header + IP_ header + UDP_header + RTP_Header + Payload) X Num_Sample_per_sec X 8
The variables in this formula are detailed in the proceeding table.
Variable Description Value
Type Bytes
LAN 14
Layer2_Header
VoIP may be encapsulated using
Ethernet or PPP.
PPP 4
IP_Header 20 bytes
UDP_header 8 bytes
RTP_Header 12 bytes
Type Size
G711 160
G729 20
Payload
G723 24
num_Sample_per_sec The IPNC generates one sample
(payload) either every 20ms (50 per
sec.) or 30ms (33 per sec.).
G711/729
50 per sec.
G723
33 per sec.
Notes: 1. Multiplication factor of 8 (i.e. X 8) is used to convert the calculation to bits per seconds.
Transmission speed is expressed in bits per second or Kbps).
2. The effect of running IPHC is to reduce the total byte size of the IP, UDP and RTP headers.
For the purposes of general calculations when running IPHC a nominal value of 7 bytes
should be used to represent the sum of IP, UDP and RTP headers.
3. The LAN calculation does not include the 4-byte CRC of the Ethernet frame.
INDeX IPNC Cassette Administration Manual How Do I? - Page 101
38DHB0002UKDD – Issue 7 (22/11/02) Part 2 Voice Over IP