Information Technology Reference
In-Depth Information
Role of Digital Signal Processors
Cisco designed its routers with one primary purpose in mind: routing. Moving packets be-
tween one location and another is not a processor-intensive task, thus Cisco routers are
not equipped with the kind of memory and processing resources typical PCs are
equipped with. For example, from a router's perspective, having 256 MB of RAM is quite
a bit. From a PC's perspective, 256 MB will barely help you survive the Microsoft Win-
dows boot process.
Moving into the realm of VoIP, the network now requires the router to convert loads of
voice into digitized, packetized transmissions. This task would easily overwhelm the re-
sources you have on the router. This is where DSPs come into play. DSPs offload the pro-
cessing responsibility for voice-related tasks from the processor of the router. This is
similar to the idea of purchasing an expensive video card for a PC to offload the video
processing responsibility from the PC's processor.
Specifically, a DSP is a chip that performs all the sampling, encoding, and compression
functions on audio coming into your router. If you were to equip your router with voice
interface cards (VIC), allowing it to connect to the PSTN or analog devices, but did not
equip your router with DSPs, the interfaces would be worthless. The interfaces would be
able to actively connect to the legacy voice networks, but would not have the power to
convert any voice into packetized form.
DSPs typically come in chips to install in your Cisco router that look like old memory
SIMMs, as shown in Figure 1-14.
Figure 1-14
DSP Chip
Some Cisco routers can also have DSPs embedded on the motherboard or added in riser
cards. Above all, it is important for you to add the necessary number of DSPs to your
router to support the number of active voice call, conferencing, and transcoding (convert-
ing one codec to another) sessions you plan to support.
Tip:
Cisco provides a DSP calculator that provides the number of DSP chips you need to
purchase based on the voice network you are supporting. This tool can be found at
www.
cisco.com/web/applicat/dsprecal/index.html
(Cisco.com login required). Keep in mind that
a growing network will always require more DSP resources. It is usually best to pack the
router full with as many DSP resources as you can fit in it; you're going to need them!
