© 2006 Cisco Systems, Inc. All rights reserved. Optimizing Converged Cisco Networks (ONT) Module 2: Cisco VoIP Implementations

© 2006 Cisco Systems, Inc. All rights reserved. Lesson 2.4: Calculating Bandwidth Requirements for VoIP

© 2006 Cisco Systems, Inc. All rights reserved. Objectives  Describe factors influencing encapsulation overhead and bandwidth requirements for VoIP.  Explain how the packetization period impacts VoIP packet size and rate.  Explain how link encapsulation effects data-link overhead on a per link basis.  Explain the bandwidth impact of adding a tunneling protocol header to voice packets.  Use the bandwidth calculation process to calculate bandwidth needs for various VoIP call types.  Describe how VAD is used in VoIP implementations.

© 2006 Cisco Systems, Inc. All rights reserved. Factors Influencing Encapsulation Overhead and Bandwidth FactorDescription Packet rate–Derived from packetization period (the period over which encoded voice bits are collected for encapsulation) Packetization size (payload size) –Depends on packetization period –Depends on codec bandwidth (bits per sample) IP overhead (including UDP and RTP) –Depends on the use of cRTP Data-link overhead–Depends on protocol (different per link) Tunneling overhead (if used) –Depends on protocol (IPsec, GRE, or MPLS)

© 2006 Cisco Systems, Inc. All rights reserved. Bandwidth Implications of Codecs  Codec bandwidth is for voice information only.  No packetization overhead is included. CodecBandwidth G kbps G.726 r3232 kbps G.726 r2424 kbps G.726 r1616 kbps G kbps G.7298 kbps

© 2006 Cisco Systems, Inc. All rights reserved. How the Packetization Period Impacts VoIP Packet Size and Rate  High packetization period results in: Larger IP packet size (adding to the payload) Lower packet rate (reducing the IP overhead)

© 2006 Cisco Systems, Inc. All rights reserved. VoIP Packet Size and Packet Rate Examples Codec and Packetization Period G ms G ms G ms G ms Codec bandwidth (kbps) Packetization size (bytes) IP overhead (bytes) 40 VoIP packet size (bytes) Packet rate (pps)

© 2006 Cisco Systems, Inc. All rights reserved. Data-Link Overhead Is Different per Link Data-Link Protocol Ethernet Frame Relay MLP Ethernet Trunk (802.1Q) Overhead [bytes]

© 2006 Cisco Systems, Inc. All rights reserved. Security and Tunneling Overhead  IP packets can be secured by IPsec.  Additionally, IP packets or data-link frames can be tunneled over a variety of protocols.  Characteristics of IPsec and tunneling protocols are: The original frame or packet is encapsulated into another protocol. The added headers result in larger packets and higher bandwidth requirements. The extra bandwidth can be extremely critical for voice packets because of the transmission of small packets at a high rate.

© 2006 Cisco Systems, Inc. All rights reserved. Extra Headers in Security and Tunneling Protocols ProtocolHeader Size (bytes) IPsec transport mode30–53 IPsec tunnel mode50–73 L2TP/GRE24 MPLS4 PPPoE8

© 2006 Cisco Systems, Inc. All rights reserved. Example: VoIP over IPsec VPN  G.729 codec (8 kbps)  20-ms packetization period  No cRTP  IPsec ESP with 3DES and SHA-1, tunnel mode

© 2006 Cisco Systems, Inc. All rights reserved. Total Bandwidth Required for a VoIP Call  Total bandwidth of a VoIP call, as seen on the link, is important for: Designing the capacity of the physical link Deploying Call Admission Control (CAC) Deploying QoS

© 2006 Cisco Systems, Inc. All rights reserved. Total Bandwidth Calculation Procedure  Gather required packetization information: Packetization period (default is 20 ms) or size Codec bandwidth  Gather required information about the link: cRTP enabled Type of data-link protocol IPsec or any tunneling protocols used  Calculate the packetization size or period.  Sum up packetization size and all headers and trailers.  Calculate the packet rate.  Calculate the total bandwidth.

© 2006 Cisco Systems, Inc. All rights reserved. Bandwidth Calculation Example

© 2006 Cisco Systems, Inc. All rights reserved. Quick Bandwidth Calculation Total packet size Total bandwidth requirement ————————— = ———————————————— Payload size Nominal bandwidth requirement Total packet size = All headers + payload ParameterValue Layer 2 header6 to 18 bytes IP + UDP + RTP headers40 bytes Payload size (20-ms sample interval)20 bytes for G.729, 160 bytes for G.711 Nominal bandwidth8 kbps for G.729, 64 kbps for G.711 Example: G.729 with Frame Relay: Total bandwidth requirement = ( bytes) * 8 kbps ————————————— = 26.4 kbps 20 bytes

© 2006 Cisco Systems, Inc. All rights reserved. VAD Characteristics  Detects silence (speech pauses)  Suppresses transmission of “silence patterns”  Depends on multiple factors: Type of audio (for example, speech or MoH) Level of background noise Other factors (for example, language, character of speaker, or type of call)  Can save up to 35 percent of bandwidth

© 2006 Cisco Systems, Inc. All rights reserved. VAD Bandwidth-Reduction Examples Data-Link Overhead Ethernet 18 bytes Frame Relay 6 bytes Frame Relay 6 bytes MLPP 6 bytes IP overheadno cRTP 40 bytes cRTP 4 bytes no cRTP 40 bytes cRTP 2 bytes CodecG kbps G kbps G kbps G kbps Packetization20 ms 160 bytes 30 ms 240 bytes 20 ms 20 bytes 40 ms 40 bytes Bandwidth without VAD 87.2 kbps66.67 kbps26.4 kbps9.6 kbps Bandwidth with VAD (35% reduction) kbps43.33 kbps17.16 kbps6.24 kbps

© 2006 Cisco Systems, Inc. All rights reserved. Self Check 1.Describe the relationship between packetization period and packet size and packet rate. 2.How does the data-link protocol used effect bandwidth considerations? 3.What is the default packetization period on Cisco devices? 4.What is VAD? 5.How much bandwidth can be saved, on average, using VAD?

© 2006 Cisco Systems, Inc. All rights reserved. Summary  VoIP packet size and rate are determined by the packetization period.  Data-link overhead must be considered with calculating bandwidth requirements. Different links have different overhead requirements.  Adding a tunneling protocol header effects the bandwidth requirements for voice packets. This additional overhead must be considered when calculating bandwidth requirements.  Voice Activity Detection (VAD) is a process used to detect silence in order to save bandwidth. VAD can save 34% on average.

© 2006 Cisco Systems, Inc. All rights reserved. Q and A

© 2006 Cisco Systems, Inc. All rights reserved. Resources  Voice Over IP - Per Call Bandwidth Consumption h_note09186a ae2.shtml#topic1  Voice Codec Bandwidth Calculator

© 2006 Cisco Systems, Inc. All rights reserved.