Bandwidth Calculation

Note: The IP/UDP/RTP headers combined are equal to 40 bytes. If header compression is used, the total for the IP/UDP/RTP headers will be only 2 bytes. For Ethernet, no compression is allowed and the frame flag is equal to zero.

Required Bandwidth Formulas

The following is the method used to calculate the estimated bandwidth:

a. Total Packet Size

For Media Access types MPPP, Frame-Relay, HDLC, PPP, and Ethernet use the following formula for determining total packet size in bytes:

1. • Total packet size = (Media Access header) + (IP/UDP/RTP header) + (Voice payload size) + Frame Flag
   • For all other Media Access types or a Codec of “Other”, use the following formula for determining total packet size in bytes:

Total packet size = (Media Access header) + (IP/UDP/RTP header) + (Voice payload size) + Cell bytes wasted per voice frame

1. • Total packet size (in bits) = Total packet size in bytes * 8 bits per byte

b. Voice Payload Size

1. • Voice payload size (in bits) = Voice payload size in bytes * 8 bits per byte

c. Calculate Bandwidth

1. • Packets per second (PPS) = (Codec bit rate) / (Voice payload size)
   • Bandwidth (in bits) = Total packet size * PPS * Peak Busy Hour Concurrent Calls

d. Convert the bandwidth in bits to megabits, 1 bit = 0.000000954 megabits.

Sample Bandwidth Calculations for G.711 and for G.729:

The following examples show how to use the formulas to calculate required bandwidths for codecs G.711 and G.279.

1. The required bandwidth for G.711 calls (64 kbps Codec bit rate) with Ethernet media access header, default 160 bytes of voice payload, PPS of 50, and peak busy hour concurrent calls of 23 is:

Total packet size (bytes) = (18 bytes Ethernet Media Access header) + (40 bytes IP/UDP/RTP) + (Voice payload of 160 bytes) = 218 bytes

Total packet size (bits) = (218 bytes) * 8 bits per byte = 1744 bits

Voice payload size = (160 bytes default voice payload) * 8 bits per byte = 1280 bits

Packets per second (PPS) = (64 kbps codec bit rate) / (1280 bits) = 50 PPS

Bandwidth per call = (1744 bits Total packet size) * 50 PPS * 23 calls = 2005600 bits = 1.913 mbps

Additional signaling overhead (5% for this example) = 0.096 mbps

Total Bandwidth with additional signaling overhead = 2.009 mbps

2. The required bandwidth for a G.729 call (8 kbps Codec bit rate) with a compressed IP/UDP/RTP header, MPPP media access header, the default 20 bytes of voice payload, and peak busy hour concurrent calls of 34 is:

Total packet size (bytes) = (6 bytes MPPP Media Access header) + (2 bytes compressed IP/UDP/RTP) + (voice payload of 20 bytes) + (Frame Flag of 1 byte) = 29 bytes

Total packet size (bits) = (29 bytes) * 8 bits per byte = 232 bits

Voice payload size = (20 bytes default voice payload) * 8 bits per byte = 160 bits

PPS = (8 kbps Codec bit rate) / (160 bits) = 50 PPS

Bandwidth per call = (232 bits voice packet size) * 50 PPS * 34 calls = 394400 bits = 0.3761 mbps

Additional signaling overhead (5% for this example) = 0.0188 mbps

Total Bandwidth with additional signaling overhead = 0.395 mbps