RTP: Voice payloads are encapsulated by RTP, then by UDP, then by IP. A Layer 2 header of the correct format is applied; the type obviously depends on the link technology in use by each router interface. A single voice call generates two one-way RTP/UDP/IP packet streams. UDP provides multiplexing and checksum capability; RTP provides payload identification, timestamps, and sequence numbering.
Each RTP stream is accompanied by a Real-Time Transport Control Protocol (RTCP) stream. RTCP monitors the quality of the RTP stream, allowing devices to record events such as packet count, delay, loss, and jitter (delay variation). A single voice packet by default contains a payload of 20 msec of voice (either uncompressed or compressed). Because sampling is occurring at 8000 times per second, 20 msec gives us 160 samples. If we divide 8000 by 160, we see that we are generating 50 packets with 160 bytes of payload, per second, for a one-way voice stream.
| G711 | G729 | ||||
| Payload | 160 b | 20 b | |||
| Codec BW | 64kbps | 8kbps | |||
| L3/L4 header: | |||||
| RTP | 12 | 12 | |||
| UDP | 8 | 8 | |||
| IP | 20 | 20 | |||
| Protocol Payload | 40 | 40 | |||
| Total before L2 | 200 b | 60 b | |||
| Bamdwidth/call = Payload + Protocol Payload / Payload x CodecBW | |||||
| BW/call (without L2) | (160+40)/160 x 64 = 80kbps | (20+40)/20 x 8 = 24kbps | |||
| Frame Relay Overhead | |||||
| Frame Relay | 6 b | 6 b | |||
| Total L2 | 206 b | 66 b | |||
| BW/call | (160+(40+6))/160 x 64 =82.4 kbps | (20+(40+6))/20 x 8 = 26.4kbps | |||
| Multilink PPP Overhead | |||||
| Multilink | 6 b | 6 b | |||
| Total L2 | 206 b | 66 b | |||
| BW/call | (160+(40+6))/160 x 64 =82.4 kbps | (20+(40+6))/20 x 8 = 26.4kbps | |||
| Ethernet Overhead | |||||
| Ethernet | 18 b | 18 b | |||
| Total L2 | 218 b | 78 b | |||
| BW/call | (160+(40+18))/160 x 64 =87.2 kbps | (20+(40+18))/20 x 8 = 31.2kbps | |||
| cRTP Applied | |||||
| CRTP | 40 => 4 | 40 => 4 | |||
| Before L2 | 160+4 / 160 x 64 =65.6kbps | 20+4 / 20 x 8= 9.6kbps | |||
| After L2 | 160+(4+6) / 160 x 64 =68kbps | 20+(4+6) / 20 x 8= 12kbps | |||
| % GAIN with cRTP | 18% | 60% | |||
voiceonbits.com 
sir i have a small issue on a simple practical on 1 voice vlan site.
here is description :-
R1 –
ephone 1
number 2001
{ codec g729 }
ephone 2
number 2002
{ codec g711}
ephone 3
number 2003
{codec g711}
In a lan network in 1 ip phone i m running codec g729 instead of g711.
when communication between ephone2 & ephone3 happens they use g711 codec but
when communication between ephone1 & ephone2 happens they use g729 codec. WHY.?
why it is happening. plz give explanation..?
–
Regards –
Suraj Singh Rana
Nice one! Helpful, easy understanding!
I have been desperately looking for the equation to find the BW utilization by codecs.This one helped me to understand it while preparing for my preperations …Kudos
for g729 does packet size in ms need be a multiple of 10?
for piss poor ISP jankiness would decreasing to 10ms result in fewer lost sound chunks?