Frequently Asked Question
Codec informatie en bandbreedte
Last Updated 10 years ago
VoIP – Per Gesprek bandbreedte
Bron: http://www.cisco.com/en/US/tech/tk652/tk698/technologies_tech_note09186a0080094ae2.shtmlThese protocol header assumptions are used for the calculations:
- 40 bytes for IP (20 bytes) / User Datagram Protocol (UDP) (8 bytes) / Real-Time Transport Protocol (RTP) (12 bytes) headers.
- Compressed Real-Time Protocol (cRTP) reduces the IP/UDP/RTP headers to 2or 4bytes (cRTP is not available over Ethernet).
- 6 bytes for Multilink Point-to-Point Protocol (MP) or Frame Relay Forum (FRF).12 Layer 2 (L2) header.
- 1 byte for the end-of-frame flag on MP and Frame Relay frames.
- 18 bytes for Ethernet L2 headers, including 4 bytes of Frame Check Sequence (FCS) or Cyclic Redundancy Check (CRC).
Note:
| Codec Information | Bandwidth Calculations | ||||||||
|---|---|---|---|---|---|---|---|---|---|
| Codec & Bit Rate (Kbps) | Codec Sample Size (Bytes) | Codec Sample Interval (ms) | Mean Opinion Score (MOS) | Voice Payload Size (Bytes) | Voice Payload Size (ms) | Packets Per Second (PPS) | Bandwidth MP or FRF.12 (Kbps) | Bandwidth w/cRTP MP or FRF.12 (Kbps) | Bandwidth Ethernet (Kbps) |
| G.711 (64 Kbps) | 80 Bytes | 10 ms | 4.1 | 160 Bytes | 20 ms | 50 | 82.8 Kbps | 67.6 Kbps | 87.2 Kbps |
| G.729 (8 Kbps) | 10 Bytes | 10 ms | 3.92 | 20 Bytes | 20 ms | 50 | 26.8 Kbps | 11.6 Kbps | 31.2 Kbps |
| G.723.1 (6.3 Kbps) | 24 Bytes | 30 ms | 3.9 | 24 Bytes | 30 ms | 33.3 | 18.9 Kbps | 8.8 Kbps | 21.9 Kbps |
| G.723.1 (5.3 Kbps) | 20 Bytes | 30 ms | 3.8 | 20 Bytes | 30 ms | 33.3 | 17.9 Kbps | 7.7 Kbps | 20.8 Kbps |
| G.726 (32 Kbps) | 20 Bytes | 5 ms | 3.85 | 80 Bytes | 20 ms | 50 | 50.8 Kbps | 35.6 Kbps | 55.2 Kbps |
| G.726 (24 Kbps) | 15 Bytes | 5 ms | 60 Bytes | 20 ms | 50 | 42.8 Kbps | 27.6 Kbps | 47.2 Kbps | |
| G.728 (16 Kbps) | 10 Bytes | 5 ms | 3.61 | 60 Bytes | 30 ms | 33.3 | 28.5 Kbps | 18.4 Kbps | 31.5 Kbps |
| G722_64k(64 Kbps) | 80 Bytes | 10 ms | 4.13 | 160 Bytes | 20 ms | 50 | 82.8 Kbps | 67.6Kbps | 87.2 Kbps |
| ilbc_mode_20(15.2Kbps) | 38 Bytes | 20 ms | NA | 38 Bytes | 20 ms | 50 | 34.0Kbps | 18.8 Kbps | 38.4Kbps |
| ilbc_mode_30(13.33Kbps) | 50 Bytes | 30 ms | NA | 50 Bytes | 30 ms | 33.3 | 25.867 Kbps | 15.73Kbps | 28.8 Kbps |
Per Call Bandwidth requirements for various codecs
| Codec & Bit Rate (Kbps) | Codec Sample Size (Bytes) | Codec Sample Interval (ms) | Voice Payload Size (Bytes) | Voice Payload Size (ms) | Bandwidth Ethernet (Kbps) |
| G.711 (64 Kbps) | 80 Bytes | 10ms | 160 bytes | 20ms | 87.2Kbps |
| G.729 (8 Kbps) | 10 Bytes | 10ms | 20 bytes | 20ms | 31.2Kbps |
| G.723.1 (6.3 Kbps) | 24 Bytes | 30ms | 24 bytes | 30ms | 21.9Kbps |
| G.723.1 (5.3 Kbps) | 20 Bytes | 30ms | 20 bytes | 30ms | 20.8Kbps |
| G.726 (32 Kbps) | 20 Bytes | 5ms | 80 bytes | 20ms | 55.2Kbps |
| G.726 (24 Kbps) | 15 Bytes | 5ms | 60 bytes | 20ms | 47.2Kbps |
| G.728 (16 Kbps) | 10 Bytes | 5ms | 60 bytes | 30ms | 31.5Kbps |
| G722_64k(64 Kbps) | 80 Bytes | 10ms | 160 bytes | 20ms | 87.2Kbps |
| ilbc_mode_20(15.2Kbps) | 38 Bytes | 20ms | 38 bytes | 20ms | 38.4Kbps |
| ilbc_mode_30(13.33Kbps) | 50 Bytes | 30ms | 50 bytes | 30ms | 28.8Kbps |
| speex(24.6 Kbps) | 66 Bytes | 20ms | 56 bytes | 25ms | 42.4 Kbps |
| speex(18.2 Kbps) | 46 Bytes | 20ms | 56 bytes | 20ms | 42.4 Kbps |
Explanation of Terms
| Codec Bit Rate (Kbps) | Based on the codec, this is the number of bits per second that need to be transmitted to deliver a voice call. (codec bit rate = codec sample size / codec sample interval). |
|---|---|
| Codec Sample Size (Bytes) | Based on the codec, this is the number of bytes captured by the Digital Signal Processor (DSP) at each codec sample interval. For example, the G.729 coder operates on sample intervals of 10 ms, corresponding to 10 bytes (80 bits) per sample at a bit rate of 8 Kbps. (codec bit rate = codec sample size / codec sample interval). |
| Codec Sample Interval (ms) | This is the sample interval at which the codec operates. For example, the G.729 coder operates on sample intervals of 10 ms, corresponding to 10 bytes (80 bits) per sample at a bit rate of 8 Kbps. (codec bit rate = codec sample size / codec sample interval). |
| MOS | MOS is a system of grading the voice quality of telephone connections. With MOS, a wide range of listeners judge the quality of a voice sample on a scale of one (bad) to five (excellent). The scores are averaged to provide the MOS for the codec. |
| Voice Payload Size (Bytes) | The voice payload size represents the number of bytes (or bits) that are filled into a packet. The voice payload size must be a multiple of the codec sample size. For example, G.729 packets can use 10, 20, 30, 40, 50, or 60 bytes of voice payload size. |
| Voice Payload Size (ms) | The voice payload size can also be represented in terms of the codec samples. For example, a G.729 voice payload size of 20 ms (two 10 ms codec samples) represents a voice payload of 20 bytes [ (20 bytes * 8) / (20 ms) = 8 Kbps ] |
| PPS | PPS represents the number of packets that need to be transmitted every second in order to deliver the codec bit rate. For example, for a G.729 call with voice payload size per packet of 20 bytes (160 bits), 50 packets need to be transmitted every second [50 pps = (8 Kbps) / (160 bits per packet) ] |
Bandwidth Calculation Formulas
These calculations are used:- Total packet size = (L2 header: MP or FRF.12 or Ethernet) + (IP/UDP/RTP header) + (voice payload size)
- PPS = (codec bit rate) / (voice payload size)
- Bandwidth = total packet size * PPS
Sample Calculation
For example, the required bandwidth for a G.729 call (8 Kbps codec bit rate) with cRTP, MP and the default 20 bytes of voice payload is:- Total packet size (bytes) = (MP header of 6 bytes) + ( compressed IP/UDP/RTP header of 2 bytes) + (voice payload of 20 bytes) = 28 bytes
- Total packet size (bits) = (28 bytes) * 8 bits per byte = 224 bits
- PPS = (8 Kbps codec bit rate) / (160 bits) = 50 pps
Note: 160 bits = 20 bytes (default voice payload) * 8 bits per byte - Bandwidth per call = voice packet size (224 bits) * 50 pps = 11.2 Kbps