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Simulation 1: Calculate the total bandwidth required for a VoIP call

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. Simulation 1: Calculate the total bandwidth required for a VoIP call.

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Simulation 1: Calculate the total bandwidth required for a VoIP call

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  1. 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 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 Let’s use the G.729 codec standard and 20ms as our packetization period. {Click, or Press any key to continue}

  2. Ethernet, PPP, etc. Internet Protocol (20) The packetization size is the voice payload User Datagram Protocol (8) Real Time Transport Protocol (12) IP UDP RTP Voice Sample 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 18 bytes 20 + 8 + 12 = 40 bytes 20 bytes {Click, or Press any key to continue}

  3. Before cRTP: After cRTP: cRTP reduces the IP/UDP/RTP headers to 2 or 4 bytes 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) 40 bytes 2 bytes After cRTP compression, this IP/UDP/RTP header is reduced to 2 bytes. {Click, or Press any key to continue}

  4. The number of packets to be sent in a certain time interval (measured in packets per seconds [pps]) is the packet rate Simulation 1: Calculate the total bandwidth required for a VoIP call Step 4: Determine 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}

  5. 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 Codec bandwidth (kbps) Packetization size (bytes) IP Overhead (bytes) VoIP packet size (bytes) Packet rate (pps) Simulation 1: Calculate the total bandwidth required for a VoIP call 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}

  6. Bandwidth per call = voice packet size (320 bits) * 50 pps = 16 kbps 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 {Click, or Press any key to continue}

  7. End of Simulation 1

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