1. Simulation 1: Calculate the total bandwidth required for a VoIP call
This simulation calculates the total bandwidth required for a VoIP call in five (5) steps:
Step 1:
Determine the packetization period and codec standard
The time over which encoded voice bits are collected for encapsulation is the packetization period.
Codec is a program (or device) that encodes / decodes digital data signals
Let’s use the G.729 codec standard and 20ms as our packetization period.
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2. Simulation 1: Calculate the total bandwidth required for a VoIP call
The voice payload is determined by codec samples. We are using G.729 codec, and 20 ms (or, two 10 ms codec samples). Therefore, voice payload =
20 bytes [ (20 bytes * 8 bits) ÷ (20 ms) = 8 kbps ]
Step 2:
Determine the packetization size
The packetization size is the voice payload
Ethernet, PPP, etc.
Internet Protocol (20)
User Datagram Protocol (8)
Real Time Transport Protocol (12) IP
UDP
RTP
Voice Sample
18 bytes
= 40 bytes
20 bytes
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3. Simulation 1: Calculate the total bandwidth required for a VoIP call
Step 3:
Determine the link-specific information, such as cRTP (compressed Real-Time protocol)
cRTP reduces the IP/UDP/RTP headers to 2 or 4 bytes
Before cRTP:
After cRTP:
40 bytes
2 bytes
After cRTP compression, this IP/UDP/RTP header is reduced to 2 bytes.
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4. Simulation 1: Calculate the total bandwidth required for a VoIP call
Step 4:
Determine the packet rate
The number of packets to be sent in a certain time interval (measured in packets per seconds [pps]) is the packet rate
 20 ms is the default time interval for Cisco equipment
The following chart shows codec standards with corresponding packet rates:
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5. Simulation 1: Calculate the total bandwidth required for a VoIP call
Codec bandwidth (kbps)
Packetization size (bytes)
IP Overhead (bytes)
VoIP packet size (bytes)
Packet rate (pps)
Codec and Packetization period
G.711 G.711 G.729 G.729
20 ms 30 ms 20 ms 30 ms
We are using the G.729 codec standard, and 20 milliseconds. The corresponding packet rate (measured in packets per second [pps]) is ’50’
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6. Simulation 1: Calculate the total bandwidth required for a VoIP call
Step 5:
Calculate Total Packet Size, and determine Total Bandwidth required per VoIP call
Total packet size (in bytes) =
(18 bytes for the Ethernet L2 headers) + (2 bytes for compressed IP/UDP/RTP header) + (20 bytes for the voice payload) = 40 bytes
Total packet size (in bits) = (40 bytes) * 8 bits per byte = 320 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 (320 bits) * 50 pps = kbps
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7. End of Simulation 1