VoIP Packet Size and Packet Rate Examples
|
Codec and Packetization Period |
G.711 20 ms |
G.711 30 ms |
G.729 20 ms |
G.729 40 ms |
|
Codec bandwidth (kbps) |
64 |
64 |
8 |
8 |
|
Packetization size (bytes) |
160 |
240 |
20 |
40 |
|
IP overhead (bytes) |
40 |
40 |
40 |
40 |
|
VoIP packet size (bytes) |
200 |
280 |
60 |
80 |
|
Packet rate (pps) |
50 |
33.33 |
50 |
25 |
You can see that an increased packetization period increases the size of the IP packet while it reduces the packet rate.
In the table, the IP overhead was assumed to be 40 bytes. This is the normal value for VoIP packets composed of 20 bytes of IP header, 8 bytes of UDP header, and 12 bytes of RTP header. If RTP header compression (cRTP) is used, the IP overhead would be smaller.
Note This table shows the IP packet size only and does not consider the overhead caused by data-link encapsulation.
2-52 Optimizing Converged Cisco Networks (ONT) v1.0 © 2006 Cisco Systems, Inc.
Data-Link Overhead
This topic lists overhead sizes for various Layer 2 protocols.
Data Link Overhead Is Different per Link
|
Data Link Protocol |
Ethernet |
Frame Relay |
MLP |
Ethernet Trunk (802.1Q) |
|
Overhead [bytes] |
18 |
6 |
6 |
22 |
When IP packets are sent over a link within an IP network, they have to be encapsulated using the data-link protocol for that link. Each link can use a different data-link protocol.
The figure illustrates an IP packet being sent from one IP phone to another. The two IP phones are located in different LANs, separated by a Frame Relay network. Before the sending phone transmits the VoIP packet onto the LAN, it has to encapsulate it into an Ethernet frame. The router that receives the frame removes the Ethernet header and encapsulates the VoIP packet into Frame Relay before sending it out to the WAN. The router receiving the VoIP packet from the Frame Relay network removes the Frame Relay header and encapsulates the VoIP packet into an Ethernet frame again before passing it on to the receiving IP phone. As illustrated in the figure, the Ethernet header and the Frame Relay header differ in size.
The overhead of data-link protocols commonly used for VoIP is 18 bytes for Ethernet, 22 bytes for 802.1Q-tagged Ethernet frames, and 6 bytes for Frame Relay or multilink PPP (MLP).
When you are calculating the bandwidth of a VoIP call for a certain link, the appropriate overhead of the data-link protocol has to be considered.
© 2006 Cisco Systems, Inc. Describe Cisco VoIP Implementations 2-53
Continue reading here: The extra bandwidth can be extremely critical for voice packets because of the transmission of small packets at a high rate
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