VoIP Bandwidth Requirments
This documentation addresses the method for calculating bandwidth requirements when
implementing VoIP into a network. This information is not specific, and can be used when
calculating voice bandwidth needs on any IP network.
Payload versus Network Bandwidth
When trying to understand the impact of voice connections over the IP network, there are two key
aspects that need to be addressed. Payload and bandwidth may seem one in the same, but they are
not:
• 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)
Voice and data connections contain overhead in addition to the actual bandwidth required for the
respective connection.
Packetization Overhead
VoIP requires two different 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 Signaling Overhead
VoIP RTP packets are generally in the 62374 byte range which is minimal when compared to other
network applications. For example, when it comes to Ethernet, the maximum packet size is 1500
bytes.
Real Time Control Protocol (RTCP) controls Quality of Service (QoS) for network connections. RTCP
requires an additional amount of bandwidth above and beyond RTP requirements. Add 5% for
RTCP when calculating bandwidth requirement. Remember, because of this requirement, you must
account for RTCP when calculating bandwidth requirements.
Calculating Bandwidth Usage
Milliseconds per packet determine the number of packets required per second. Example, if the RTP
is set to 20 milliseconds of voice per packet, you will need 50 packets per second.
The formula for calculating bandwidth is as follows:
1000ms / number of ms of voice per packet = packets / second required.
(Example: 1000ms / 20ms = 50 packets / sec)
Each packet contains varying amounts of information dependent upon the network type. For
example, there is approximately a 40 byte header added for Ethernet packets. Each time a packet is
sent, that header is attached, which increases the packet size, and in turn, the amount of bandwidth
required for the connection.
To estimate the per call TCP/IP bandwidth usage, follow the below equations:
Bandwidth = ((packets / sec * 320 bits / packet) + CODEC bit rate) * 1.05
Example 1: G.711 @ 20ms/packet (50 packets/second)
Bw = ((50packets / sec * 320 bits / packet) + 64Kbps) * 1.05Bw = ((16Kbps) + 64Kbps) *
1.05Bw = ((80 Kbps) * 1.05Bw = 84Kbps
Example 2: G.729a @ 30ms / packet (33.3 packets / second)
Bw = ((33.3packets / sec * 320 bits / packet) + 8Kbps) * 1.05Bw = ((10.7Kbps) + 8Kbps) *
1.05Bw = ((18.7 Kbps) * 1.05Bw = 19.7Kbps
If Silence Suppression is on:
Peak Bandwidth is calculated in the manner above; however, the average bandwidth is
estimated at 50% of peak.
IP Per Connection Bandwidth Calculation
The RTP plus UDP plus IP headers add 40 bytes. The Ethernet header and trailer add another 18
bytes of overhead, for a total of at least 58 bytes of overhead before there are any voice bytes in the
packet. Do not overlook this when calculating bandwidth requirements. These headers, plus the
Ethernet header, produce an overhead for shipping the packets that can range from 20% to 80% of
the bandwidth consumed over the LAN and WAN.
{Example: Packet Header Length:14 (Ethernet Header) + 20 (IP) + 8 (UDP) + 12 (RTP) +
4 (Ethernet Trailer) = 58 Bytes}