Bandwidth Calculation Formulas

Bandwidth Calculation Formulas

total packet size = (Layer 2 header) + (IP/UDP/RTP header) + (voice payload size)

packets per second (PPS) = (codec bit rate) ÷ (voice payload size)

bandwidth = total packet size * PPS

Layer 2 header can be Multilink Point-to-Point Protocol (MP), Frame Relay Forum (FRF), or Ethernet.

The approximate protocol header sizes used in these calculations are as follows:

• 40 bytes for IP (20 bytes) / User Datagram Protocol (UDP) (8 bytes) / Real-Time Transport Protocol (RTP) (12 bytes)
• Compressed Real-Time Transport Protocol (cRTP) reduces the IP/UDP/RTP headers to 2 or 4 bytes (cRTP is not available over Ethernet).
• 6 bytes Layer 2 Header for MP or FRF
• 1 byte for the end-of-frame flag on MP and Frame Relay frames
• 18 bytes for Ethernet Layer 2 headers, including 4 bytes of Frame Check Sequence (FCS) or Cyclic Redundancy Check (CRC)
• Sample Bandwidth Calculation for G.711 and for G.729 

The required bandwidth for a G.711 call (64 Kb/s codec bit rate) with Ethernet L2 header and the default 160 bytes of voice payload is:

●	total packet size (bytes) = (18 bytes Ethernet Layer 2 header) + (40 bytes IP) + (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 Kb/s codec bit rate) ÷ (1280 bits) = 50 pps

●	bandwidth per call = (1744 bits voice packet size) * 50 pps = 87.2 Kb/s

The required bandwidth for a G.729 call (8 Kb/s codec bit rate) with cRTP, MP and the default 20 bytes of voice payload is:

●	total packet size (bytes) = (18 bytes Ethernet Layer 2 header) + (40 bytes IP) + (voice payload of 20 bytes) = 78 bytes

●	total packet size (bits) = (78 bytes) * 8 bits per byte = 624 bits

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

●	PPS = (8 Kb/s Codec bit rate) ÷ (160 bits) = 50 pps

●	bandwidth per call = (624 bits voice packet size) * 50 pps = 31.2 Kb/s