Specifications

Engineering Guidelines
180
Inter-zone bandwidth settings
As well as defining the zones and links between locations, the available bandwidth also needs
to be defined. Generally the available bandwidth on these links is also determined by the WAN
link protocol. This could be a dedicated link running cPPP, or may be a more general purpose
connection such as MPLS, or xDSL. Although the payload (IP) is common to these WAN
protocols, the bandwidth on the physical wire link may not be. The MiVoice Business system
considers the throughput, or payload bandwidth, with some minor overhead and is defined in
Table 55.
Therefore, define the link bandwidth based on the IP throughput.
An alternative method is to determine the physical wire bandwidth and define the number of
voice streams, or “channels”, that are required or achievable across the link, using the physical
wire bandwidth per connection. Once the number of “channels” is defined, multiply this by the
numbers defined in the table above to define the Inter-zone bandwidth limit.
For example, suppose a link has a physical bandwidth of 200kbits/s and running DSL. The
protocol is PPPoEoATM and on such a link, a G.729 call uses ~64kbits/s. With this link it should
be possible to achieve 3 voice streams, albeit with high utilization (200/(3x64)). Therefore, a
bandwidth value of 96 should be defined for the link or maybe 64 in order to maintain usage
below 80%. See Table 56 and Table 57 for more details of wire bandwidth, codec type, frame
rate and WAN protocols.
Codec – Introduction
The word CODEC is a concatenation of two words: Coder and Decoder. The CODEC is actually
two functions, coding and decoding, for the conversion of media, in this case, voice, into some
data format that can be returned at the far end into something akin to the original. For voice,
this usually involves converting the analog signals into digital signals and levels and returning
them back to analog.
The most popular CODEC, G.711, has become standardized across large parts of the telephony
network. As such, it has become the baseline for IP devices to perform to. But to make life
interesting, the G.711 CODEC comes in two varieties: A-Law and µ-Law. Typically these coding
laws were kept separated by geographic boundaries, but with increasingly global IP traffic, both
types are regularly encountered. Therefore a G.711 CODEC has to negotiate which coding law
to use as well.
Table 55: CODEC Throughput
CODEC type IP Payload + %overhead
G.711 32
G.722.1 (32k) 56
G.729 88