Specifications

Bandwidth, Codecs and Compression
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Bandwidth, Bandwidth Management, Codecs and
Compression
An IP packet carrying voice information has a number of additional “wrappers” (see graphic
below) so that different network devices know how to route the information (IP address), how
to forward information between physical devices (MAC address), and how to identify when a
packet starts and finishes (Ethernet).
These additional wrappers add overhead to the overall packet. This overhead increases the
bandwidth required to transport a voice packet. To understand the true bandwidth requirements,
this overhead must be taken into account.
Codecs are devices or programs that encode or decode a signal into a digital format, in this
case, the voice payload. Different codecs can provide different sized voice payloads given the
same input information. A reduction in payload is often referred to as compression.
The following sections discuss bandwidth, codecs and compression in further detail.
Calculating and Measuring Bandwidth
Bandwidth can be described in a number of ways:
• Payload bandwidth, voice: sufficient bandwidth to transfer the usable information.
• IP bandwidth: bandwidth to transfer the data between the two end devices. Note that this
doesn’t include the transport protocol, which may change between devices and network.
• Wire bandwidth: This includes all of the bits and timing gaps that are transmitted onto the
media. This includes the payload, the IP address information and the transportation and
synchronization information.
It is important to note which bandwidth is being described when comparing information. For
instance, a G.711 Ethernet connection with 20 ms frames will have the following values:
• Payload bandwidth: 64 kbps
• IP bandwidth: 80 kbps
• Wire bandwidth: 96.8 kbps
Ethernet MAC IP UDP RTP Voice R U I M E
Notes:
1. To calculate and measure bandwidth, use the Mitel calculator rather than a
third-party tool. The Mitel calculator uses 802.1p/Q (8100) frames, which ensure the
highest degree of accuracy. Many third-party tools use standard Ethernet (0800)
frames, which are less accurate and do not account for VLANs.
2. PC-based applications for monitoring IP network traffic often do not indicate the
actual bandwidth being used. These applications usually do not include IP packet
overhead information, and as a result using these applications to try and measure
bandwidth will provide misleading results