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Q. Sometimes we read "PVDM", and other times "DSP". What are differences
between these two terminologies in the context
of this document and the PVDM2 data sheet?
A. PVDM stands for packet voice DSP module; it is the Cisco product name for
the module that provides digital signal
processing resources to a system. DSP stands for Digital Signal Processor; it
is a generic Industry terminology. A PVDM
module could be staffed with one or multiple DSPs.
PVDM2-16 is one DSP chip (32=2, 48=3, 64=4)
PVDM2-8 is one DSP chip but less processing capacity than the DSP on the 16.
PVDM2-8 contains one TNETV2505GGW DSP;
other PVDM2 modules contain 1 to 4 TNETV2510GGW DSPs (referred to C5510 in
command outputs)
High Complexity Codecs
G.723.1, G.728, G.729, G.729b, GSM-EFR and Modem Relay
Medium Complexity Codecs
G.711 (see G711 note), g.729a, G.729ab, G.726, GSM-FR and Fax Relay
You can configure each DSP separately as either medium complexity, high
complexity, or flex mode (C5510 only).
Configure with voice-card x. Then codec complexity (flex | high | medium).
The DSP treats all calls according to its
configured complexity, regardless of the actual complexity of the codec of the
call. A resource with configured
complexity equal or higher than the actual complexity of the incoming call must
be available, or the call will fail.
For example, if a call requires a high-complexity codec but the DSP resource is
configured for medium complexity mode,
the call will fail. However, if a medium-complexity call is attempted on a DSP
configured for high complexity mode,
then the call will succeed and Cisco IOS will allocate a high-complexity mode
resource.
Flex mode has an advantage when calls of multiple codecs must be supported on
the same hardware because flex mode can
support more calls than when the DSPs are configured as medium or high
complexity. ***However, flex mode does allow
oversubscription of the resources, which introduces the risk of call failure if
all resources are used. With flex mode
it is possible to have fewer DSP resources than with physical TDM interfaces.
***
G711 Note:
----------
Compared to medium or high complexity mode, flex mode has the advantage of
supporting the most G.711 calls per DSP. In
medium complexity mode a DSP can support max 8 G.711 calls, while flex mode
supports max 16 G.711 calls.
Note IP Phones and Unity support G711 and G729a (no other g729 variant.) Cisco
Unity has support for accepting calls
with G.729a, but the codec is implemented in software and is CPU-intensive.
PVDM2-8 Signaling
- 8 calls per DSP for G711 (flex mode only)
- 4 calls for other medium and all high complexity codecs.
- 120 MIPS in FLEX Mode
PVDM2-16 - Signaling
- 16 calls for G711 (flex mode only)
- 8 calls for the other medium complexity codecs and g711 in medium complexity
mode
- 6 calls per DSP for high complexity codecs
- 240 MIPS in flex mode
- G711 uses 15 MIPS per call (240 MIPS / 15 MIPS per call = 16 calls per DSP)
- The codecs under the High Complexity category use 40 MIPS per call.
- The codecs under the medium complexity category use 30 MIPS per call.
- The D channel doesn't count for resources
- Each FXO or FXS port takes 1 channel.
MTP
- CCM SW MTP is G711 only (all versions including CCM7.x)
- IOS SW MTP
- Supports G711 and any G729 variant. But can choose only one
codec on the dspfarm profile at a time.
- Need the capacities. (500 for a 2811?)
- IOS HW MTP
- 16 G711 sessions per DSP
- 6 G729 sessions per DSP
PVDM2-16 - Conferencing
- IOS 12.4(15)T has new capability for 32 participants per conference. G711
must be only codec in dspfarm profile.
Use the maximum conference-participants 8, 16 or 32 command.
- If use 32 participants then only get 2 G711 max sessions (conferences.)
- If use 16 participants, get 4 G711 max sessions
- If use 8 participants, get 8 G711 conferences
- If configure any of the g729s in dspfarm profile (medium or high complexity),
get 8 or 16 participants per
conference. (if 16, get one session. If 8, get two sessions)
- Each DSP supports 2 conferences (of 8 participants each) if G.729 is
CONFIGURED on the dspfarm.
(even if all participants on all conferences are using G.711.) When
configured to accept both G.711 and G.729 calls,
a single DSP provides 2 conferences because it is also reserving its resources
for performing transcoding of streams.
- Can't share conference on DSP with xcode or voice signaling. DSPs that are
configured through Cisco IOS as
conference resources will load firmware into the DSPs that is specific to
conferencing functionality only, and these
DSPs cannot be used for any other media feature.
- Config max-sessions for 2 or 8 (depending on configured codec). Doesn't
make sense to configure less--wasting resources. Again, the figures are for
a 1 DSP PVDM2-16. Scale it up for
others.
PVDM2-16 - Transcoding
- Can share Transcoding with voice signaling.
- 8 Sessions - G711a/u to G729a/ab (must turn off high complexity g729 and
g729b to get 8 sessions) in dspfarm profile.
- 6 sessions - G711a/u to G729 / G729b
Universal Transcoding with capacities is given in:
http://www.cisco.com/en/US/docs/ios/12_4t/12_4t15/it_unitr.html
If source phone is g711 to a POTS (or T1 or PRI) it will use 15 credtis.
If source phone is g729 to a POTS it will use 30 or 40 credits depending on
codec complexity. This includes the source
phone locally (could be a SIP Phone doing g729) or a source phone over the WAN
at g729r8 doing TEHO. In the later
case, no transcoding at HQ or the TEHO location will be used. It just takes
more processing power to decode g729 back
to analog.
Need to verify formula for mixed transcoding and signaling sessions on same
DSP. Don't think we can
trust max-sessions.
I have this written up and tested. The basic conclusion is 12.4(5b) (the IOS
used in the v2 lab) allows you to
obersubscribe the resources. IOS 12.4(22)T doesn't allow you to. I have a
long writeup and testing procedure on this.
Please feel free to correct or amend this document.
