Information Technology Reference
In-Depth Information
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 bun-
dles these DSP chips into packet voice DSP modules (PVDM), which resemble the old
memory SIMMs (refer to Figure 1-14). Based on the DSP requirements given by the Cisco
DSP calculator, you can then purchase one or more of the following PVDMs:
PVDM2-8: Provides .5 DSP chip
PVDM2-16: Provides 1 DSP chip
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 1-3 shows the
codecs considered medium and high complexity.
Table 1-3
Medium- and High-Complexity Codecs
Medium Complexity
High Complexity
G.711 (a-law and μ-law)
G.728
G.726
G.723
G.729a, G.729ab
G.729, G.729b
iLBC
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 capable of handling 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.
Understanding RTP and RTCP
When you walk into the VoIP world, you encounter a whole new host of protocol stan-
dards. Think of the Real-time Transport Protocol (RTP) and Real-time Transport Control
Protocol (RTCP) as the protocols of voice. RTP operates at the transport layer of the OSI
model on top of UDP. Having two transport layer protocols is odd, but that's exactly what
is happening here. UDP provides the services it always does: port numbers (that is, session
multiplexing) and header checksums (which ensure that the header information does not
become corrupted). RTP adds time stamps and sequence numbers to the header informa-
tion. This allows the remote device to put the packets back in order when it receives them
at the remote end (function of the sequence number) and use a buffer to remove jitter
(slight delays) between the packets to give a smooth audio playout (function of the time
stamp). Figure 1-15 represents the RTP header information contained in a packet.
 
 
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