Making Changes to the LAN
to Handle Large Call Volumes
Before attempting this volume of traffic, we’ve found that the head office backbone
must be up to speed as far as its rate of transmission and routing topology.
The most modern LAN installations in the head office are using the Catalyst
6509 chassis with the Multilayer Switch Feature Card (MSFC) and Policy Feature
Card (PFC) to enable Layer 2/3/4 traffic controls.The 6509 also employs the 8-
port T-1 card with 24 DSP units on board this T-1 card. Lastly, the 6509 runs the
48-port 10/100 switch card that has in-line power ports.
With the LAN switches in place and operational, the baseline has been set to
support CallManager clusters, multiple Exchange servers, Unity servers, and the
VLANs between locations.The switched backbone network should be, at minimum,
Fast Ethernet, but should also be Gigabit Ethernet between the servers
and the 6509 chassis when possible and economically feasible.
The purpose for the T-1 DSP card is to provide conference calls, group
bridges, and media mixing for AVVID applications that require such services.The
T-1 card has 8 ports for PRI circuits, but you’ll not be using the PRI for an
actual circuit. Each of the DSPs can handle three mixed communications sessions
at one time, so up to 24 conferences can be held at any given time. However,
you’ll need to reduce the 24 number by however many conference bridges you
may dedicate to other compression protocols and dedicated functions, such as
retaining one DSP for strictly internal office uses. Among these internal uses is
the capability for mobile or home-based employees to call into the office and
have dedicated processing capabilities.
DSPs are also used for transcoding purposes.Transcoding occurs when a
device speaking one call type (such as an IP phone using g.711) contacts another
device that uses a different call type (like an IP phone using g.729a) of compression.
Since this is like two humans speaking different languages, the DSP acts like
a translator to complete the call in an acceptable manner. Of these call types,
conferencing only uses g.711 compression.