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. {Click, or Press any key to continue}

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 20 + 8 + 12 = 40 bytes 20 bytes {Click, or Press any key to continue}

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. {Click, or Press any key to continue}

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: {Click, or Press any key to continue}

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 64 64 8 8 160 240 20 40 40 40 40 40 200 280 60 80 50 33.33 50 25 We are using the G.729 codec standard, and 20 milliseconds. The corresponding packet rate (measured in packets per second [pps]) is ’50’ {Click, or Press any key to continue}

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 = 16 kbps {Click, or Press any key to continue}

End of Simulation 1