© 2006 Cisco Systems, Inc. All rights reserved. 2.4: Calculating Bandwidth Requirements for VoIP.
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© 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
Factor Description
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.
Codec Bandwidth
G.711 64 kbps
G.726 r32 32 kbps
G.726 r24 24 kbps
G.726 r16 16 kbps
G.728 16 kbps
G.729 8 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 andPacketization Period
G.711 20 ms
G.711 30 ms
G.729 20 ms
G.729 40 ms
Codec bandwidth (kbps)
64 64 8 8
Packetization size (bytes)
160 240 20 40
IP overhead(bytes)
40 40 40 40
VoIP packet size (bytes)
200 280 60 80
Packet rate(pps)
50 33.33 50 25
© 2006 Cisco Systems, Inc. All rights reserved.
Data-Link Overhead Is Different per Link
Data-Link Protocol
EthernetFrame Relay
MLPEthernet Trunk
(802.1Q)
Overhead [bytes]
18 6 6 22
© 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
Protocol Header Size (bytes)
IPsec transport mode 30–53
IPsec tunnel mode 50–73
L2TP/GRE 24
MPLS 4
PPPoE 8
© 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.
Quick Bandwidth Calculation Total packet size Total bandwidth requirement
————————— = ————————————————
Payload size Nominal bandwidth requirement
Total packet size = All headers + payload
Parameter Value
Layer 2 header 6 to 18 bytes
IP + UDP + RTP headers 40 bytes
Payload size (20-ms sample interval) 20 bytes for G.729, 160 bytes for G.711
Nominal bandwidth 8 kbps for G.729, 64 kbps for G.711
Example: G.729 with Frame Relay:
Total bandwidth requirement = (6 + 40 + 20 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 ExamplesData-Link Overhead
Ethernet
18 bytes
Frame Relay
6 bytes
Frame Relay
6 bytes
MLPP
6 bytes
IP overhead no cRTP
40 bytes
cRTP
4 bytes
no cRTP
40 bytes
cRTP
2 bytes
Codec G.711
64 kbps
G.711
64 kbps
G.729
8 kbps
G.729
8 kbps
Packetization 20 ms
160 bytes
30 ms
240 bytes
20 ms
20 bytes
40 ms
40 bytes
Bandwidth without VAD
87.2 kbps 66.67 kbps 26.4 kbps 9.6 kbps
Bandwidth with VAD (35% reduction)
56.68 kbps 43.33 kbps 17.16 kbps 6.24 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.
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