Voice over IP - performance

Wed Feb 12 23:46:12 UTC 2003

At 03:23 PM 2/12/2003 -0500, Charles  Youse wrote:
>I'm assuming in the case of, e.g., a 2650 + dual T-1 PRI interface can 
>actually encode/decode 48 simultaneous g729a voice streams without 
>issues?  Any idea what the CPU utilisation is - or is this handled in 
>separate DSPs in the voice network module itself?

On these particular Cisco boxes, the DSP does the all audio filtering, 
CODEC functions, echo cancellation, jitter buffering & adjustment, silence 
suppression (AKA voice activity detection, if you turned it on), and also 
prepends the RTP and IP headers. The router CPU just has to forward the 
packet that's generated by the DSP.

Router CPU utilization is therefore a function of the number of packets per 
second that the voice card generates and the size of each packet, plus 
signaling overhead. The packet size and rate depend on the CODEC itself 
(higher compression CODECs generate smaller packets), the sample size (20ms 
is the Cisco default, reducing or increasing it makes the packets smaller 
or larger and the packet rate higher or lower, respectively), and whether 
voice activity detection is on (roughly halves the packet rate).

If you leave the default settings in place (no VAD, 20ms sample size), 
you'll be OK with any of the CODECs.

Mathew.

>C.
>
>-----Original Message-----
>From: Bill Woodcock [mailto:woody at pch.net]
>Sent: Wednesday, February 12, 2003 2:43 PM
>To: Charles Youse
>Cc: nanog at nanog.org
>Subject: Re: Voice over IP - performance
>
>
>     > Does anyone have any real-world figures for VoIP performance on
>     > various platforms?  In other words, how many calls can an otherwise
>     > unused e.g., Cisco 2600 be expected to handle if it's the conversion
>     > point from trunked voice calls to IP.  Some rough numbers for
>     > different codecs on different hardware would be very useful.  Most
>     > specifically I'm interested in Cisco router platforms but other
>     > vendor stats would be appreciated as well.
>
>Actually I just ran the dollars-per-simultaneous-call numbers for
>different models for some friends.  I'll append it.  Basically, if you run
>g711, you're limited by the number of PRI channels on the box.  If you run
>g729a, you're limited by the number of DSPs you can fit in the box.  The
>numbers I ran were assuming g729a.
>
>                                 -Bill
>
>
>
>                                                        Cost
>                                                        per
>Package which can handle 23 simultaneous calls:        call
>CISCO1760          10/100 Modular Router      $1,595
>VWIC-1MFT-T1       1-Port RJ-48 Multiflex T1  $1,300
>PVDM-256K-12       3-DSP Module (9 calls)     $1,200
>PVDM-256K-20HD     5-DSP Module (15 calls)    $4,000
>Total                                         $8,095   $352
>
>Different package which can handle 23 simultaneous calls:
>CISCO2650          10/100 Modular Router      $3,295
>NM-HDV-1T1-24E     Single-Port T1 Voice NM    $9,100
>Total                                        $12,395   $539
>
>Package which can handle 45 simultaneous calls:
>CISCO2650          10/100 Modular Router      $3,295
>NM-HDV-2T1-48      Dual-Port T1 Voice NM      $9,800
>Total                                        $13,095   $291
>
>Package which can handle 46 simultaneous calls:
>CISCO2650          10/100 Modular Router      $3,295
>NM-HDV-2T1-48      Dual-Port T1 Voice NM      $9,800
>PVDM-256K-20HD     5-DSP Module (15 calls)    $4,000
>Total                                        $17,095   $372
>
>Upgradeable package which can handle 46 simultaneous calls:
>AS535-2T1-48-AC-V  AS5350-V/2T1              $18,900   $411
>
>Package which can handle 92 simultaneous calls:
>AS535-4T1-96-AC-V  AS5350-V/4T1              $33,600   $366
>
>Package which can handle 184 simultaneous calls:
>AS535-8T1-192-AC-V AS5350-V/8T1              $58,700   $319
>
>Upgradeable package which can handle 184 simultaneous calls:
>AS54HPX-8T1-192AC  AS5400HPX/8T1             $65,500   $356
>
>Package which can handle 644 simultaneous calls:
>AS54HPX-CT3-648AC  AS5400HPX/CT3            $170,300   $265