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Codec Complexity and Compression


Hi Everyone

I am bit confused with the two terminologies - Codec Complexity (Medium, high, flex) and compression algos (G711, G729 etc).

I have read in one of the cisco documents that under flex complexity, we can place 16 G711 calls per DSP and under medium complexity, 8 G711 calls can be placed. Now, we know that G711 uses 15 MIPs per call (Since one DSP has 240 MIPS so 240 /15 = 16 calls). Is this equation valid only for flex complexity? Coz for medium complexity, G711 should use 30 MIPs per call then only the statement that "only 8 G711 calls can be placed under medium complexity" becomes true ( 240 / 30 = 8 calls). Then for high complexity, G711 should use 40 MIPs per call as under high complexity, we can only place 6 G711 calls?
What about other compression algos?

So far i know

g729a/g729ab = 30 MIPS per call

g729/g729b = 40 MIPS per call

Surely, the above two equations are only true for flex complexity.

If i follow the same pattern then

for medium complexity

g729a/g729ab = 60 MIPS per call

g729/g729b = 80 MIPS per call

This implies that under medium complexity, we can only place 4 G729a calls per DSP (240 /60 = 4) ?

for High complexity

g729a/g729ab = 120 MIPS per call

g729/g729b = 160 MIPS per call

Which means under high complexity, only 2 G729 calls can be placed?

I have done the above calculation on the basis of following statement which i took from one of the cisco documents

"Note: The difference between medium and high complexity codecs is the amount of CPU utilization necessary to process the codec algorithm, and therefore, the number of voice channels that can be supported by a single DSP. For this reason, all the medium complexity codecs can also be run in high complexity mode, but fewer (usually half) of the channels are available per DSP."

But still I doubt my calculation. I must be missing something here? Plz help me out.

Thanx in advance


4 Replies 4

Chris Deren
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This is little dated as it pertains to older PVDM models, but the concept is the same:

Codecs fall into different compression complexity (the amount of PVDM channels needed to compress/decompress TDM signal into/from voip), some codec i.e. G729 are complex codecs as they require a lot DSP power to do that as they are highly compressed, others such as G711 are medium complexity as they don't need as much power. Flex simply means you leave it up to the gateway to determine what is needed with an assumed risk that you may oversubscribe your DSPs.



Thanx Chris for ur prompt reply

I got quite fair picture now. I was under the impression that complexity codec and compression algos are two different things but now its clear that codec can either be high in complexity or medium and if we use flex mode then we can use either of the comlexity codec. But i still have some doubts. Would really appreciate if u can clarify them too.

I have read in one of the documents that

The codecs under the High Complexity category use 40 MIPS per call. And

The codecs under the medium complexity category use 30 MIPS per call.

High Complexity Codecs are

G.723.1, G.728, G.729, G.729b, GSM-EFR and Modem Relay

Medium Complexity Codecs are

G.711, g.729a, G.729ab, G.726, GSM-FR and Fax Relay

So its mean if i select high complexity mode then i will not have the option to select G711 ? (as it is medium complex codec). Moreover, it should not matter which high complex codec i use, all will use 40 MIPs per call...right?? This is bit strange coz G.723.1 compresses voice at 5300 bps and G728 compresses at 16000 bps. In my point of view, G723.1 should have used more MIPs per call than G728 as it compresses more but according to above statement, since both are high complexity codec therefore both should use 40 MIPs per call?

Similarly, G711 should use 30 MIPs per call as it is a medium complexity codec but when i use it under flex mode, it only consumes 15 MIPs per call?? That is the reason it states that u can place 16 G711 calls per DSP under flex mode (as it only uses 15 MIPs per call) and only 8 under medium mode (30 MIPs per call) so then which type of codec exactly G711 is??

Last but not least, the link u provided above states that "all the medium complexity codecs can also be run in high complexity mode, but fewer (usually half) of the channels are available per DSP." According to this statement, i should be able to use G711, G729a and other medium complex codec under high complexity mode but only with 60 MIPs per call??

Thanx in advance



Any help??


Anyone can help me please??

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