Vertical Xcelerator IP Application Note

Application Note – Calculating Voice Bandwidth Requirements
This Application Note addresses the method for calculating bandwidth requirements for a VoIP
network. This information is not specific to Xcelerator IP and can be used when calculating voice
bandwidth needs on any IP network.

Payload versus Network Bandwidth

Before we begin, we should discuss two key aspects to understanding the impact of voice
connections over the IP network. Payload and bandwidth requirements are used interchangeably
by many people; however, they are different.
Payload defines the bit rate of the connection; i.e., 64Kbps, 32Kbps, etc.
Network Bandwidth is the total bandwidth consumed for the connection and includes the Payload
as well as network overhead. Network overhead consists of
Packetization (RTP, UDP, IP, and Layer 2 headers)
Call Signaling and Control (SIP, H.323, and RTCP)
All connections on the network, whether voice or data, contain overhead in addition to the actual
bandwidth required for the connection.

Packetization Overhead

VoIP requires two classes of protocols: a signaling protocol such as SIP that is used to set up,
disconnect and control the calls and telephony features, as well as a protocol to carry speech
packets. The Real-Time Transport protocol (RTP) carries speech transmission.
An IP phone generates a voice packet every 10, 20, 30 or 40ms, depending on the vendor's
implementation.

RTP/Call Signalling Overhead

VoIP RTP packets are small compared to many other network applications. With Ethernet, the
Maximum Transmission Unit (maximum packet size) is 1500 bytes. With VoIP RTP packets are
generally in the 62-374 byte range.
The overhead, which is a fixed amount of bandwidth, can be large compared to the actual voice
payload.
Real-Time Control Protocol (RTCP) monitors statistics and provides feedback on Quality of
Service (QoS) for the network connections. RTCP requires a small amount of bandwidth in
addition to the RTP requirements. To be conservative, add 5% to the RTP bandwidth requirement
to cover the RTCP component.
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Xcelerator IP 07010
July 2007