A Word about Regions and Device Pools
In the system configuration of CCM, you have to create device pools and
regions that deal with how the phone call is treated. These two conditions
do more for quality of service than any other configuration. There
are two basic types of compression and voice handling: g.711, which
uses the full 64 Kbps PRI channel when high bandwidth is available, and
g.729, which compresses the voice packet down to 8 Kbps for transmission
across low speed WAN links such as 56 Kbps frame relay. There are,
however, several other compression types, some that go as low as 5.3
Kbps, but that require more advanced (and more expensive) DSP modules.
When these high-complexity DSPs are employed, you’ll need to use
gateways such as the Cisco 3600 and AS5000 Series devices.
Where and when would you need such high level hardware,
though? The previously mentioned 1750 and 2600 class gateways using
the voice modules can provide adequate voice compression and mixing
for up to four conversations, but fall short when more than four simultaneous
conversations are needed. The 3640 gateway, for instance, can
accept up to 12 DSP modules that each have three individual DSP processors,
with each DSP processor handling one conversation. Also, when
conference calls and bridging are needed, one DSP processor is required
for every three participants in the call. DSP processors can be co joined
for larger conferencing needs, but require the usage of more capable
gateways such as the Catalyst line of switches. These Model 4000 and
Model 6000 Series Catalyst switches utilize the 8-port T-1 DSP module,
with each module supporting three individual DSP processors. However,
these 24 combined processors provide much more VoIP capabilities than
do the lower end gateways.
This diatribe is not meant to say that the 1750 and 2600 class gateways
are not sufficient to do the job—far from it. They each have their
particular place in life as well as an associated cost. Chapter 11 will go
into these more advanced issues in greater detail.