The Role of Digital Signal Processors
Cisco designed its routers with one primary purpose in mind: routing. Moving packets between one location and another is not a very 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 Windows 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 resources you have on the router. This is where Digital Signal Processors (DSP) come into play. DSPs offload the processing responsibility for voice-related tasks from the processor of the router. This is very 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 7.3.
- Figure 7.3 Digital Signal Processor Chip
Some Cisco routers could 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 (converting 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 http://www.cisco.com/pcgibin/Support/DSP/dsp-calc.pl (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!
Key Topic
■ PVDM2-32: Provides 2 DSP chips
■ PVDM2-48: Provides 3 DSP chips
■ PVDM2-64: Provides 4 DSP chips
Alas, not all codecs are created equal. Some codecs consume more DSP resources to pass through the audio conversion process than other codecs consume. Table 7.3 shows the codecs considered medium and high complexity.
|
Medium Complexity |
High Complexity |
|
G.711 (a-law and p-law) |
G.728 |
|
G.726 |
G.723 |
|
G.729a, G.729ab |
iLBC You can add DSP chips either directly to a router's motherboard (if the router supports this) or to the network modules you add to the router to support voice cards. Cisco bundles these DSP chips into packet voice DSP modules (PVDM), which resemble the old memory SIMMs (see image in Figure 7.3). Based on the DSP requirements given by the Cisco DSP calculator, you can then purchase one or more of the following PVDMs: ■ PVDM2-16: Provides 1 DSP chip Generally speaking, the DSP resources are able to handle roughly double the number of medium-complexity calls per DSP as high-complexity calls. Note Newer DSP chips are able to process calls more efficiently and can handle more high-complexity calls per chip than older DSP hardware. To find the exact amount of calls per DSP, use the Cisco DSP calculator tool mentioned in the previous tip. |
Continue reading here: Understanding RTP and RTCP
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