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

© 2006 Cisco Systems, Inc. All rights reserved.

2.4: Calculating Bandwidth Requirements for VoIP


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.


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)


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


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)


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


Data-Link Overhead Is Different per Link

Data-Link Protocol

EthernetFrame Relay

MLPEthernet Trunk

(802.1Q)

Overhead [bytes]

18 6 6 22


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.


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


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


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


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.


Bandwidth Calculation Example


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


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


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


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?


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.

Documents

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