Vertical Xcelerator IP Application Note

Brand: Vertical

Model: Xcelerator IP

Type: Application Note

1 July 2007

Application Note – Calculating Voice Bandwidth Requirements

Xcelerator IP 07010

This Application Note addresses the method for calculating bandwidth requirements for a VoIP

network. This information is not specific to Xcelerator IP and can be used when calculating voice

bandwidth needs on any IP network.

Payload versus Network Bandwidth

Before we begin, we should discuss two key aspects to understanding the impact of voice

connections over the IP network. Payload and bandwidth requirements are used interchangeably

by many people; however, they are different.

Payload defines the bit rate of the connection; i.e., 64Kbps, 32Kbps, etc.

Network Bandwidth is the total bandwidth consumed for the connection and includes the Payload

as well as network overhead. Network overhead consists of

Packetization (RTP, UDP, IP, and Layer 2 headers)

Call Signaling and Control (SIP, H.323, and RTCP)

All connections on the network, whether voice or data, contain overhead in addition to the actual

bandwidth required for the connection.

Packetization Overhead

VoIP requires two classes of protocols: a signaling protocol such as SIP that is used to set up,

disconnect and control the calls and telephony features, as well as a protocol to carry speech

packets. The Real-Time Transport protocol (RTP) carries speech transmission.

An IP phone generates a voice packet every 10, 20, 30 or 40ms, depending on the vendor's

implementation.

RTP/Call Signalling Overhead

VoIP RTP packets are small compared to many other network applications. With Ethernet, the

Maximum Transmission Unit (maximum packet size) is 1500 bytes. With VoIP RTP packets are

generally in the 62-374 byte range.

The overhead, which is a fixed amount of bandwidth, can be large compared to the actual voice

payload.

Real-Time Control Protocol (RTCP) monitors statistics and provides feedback on Quality of

Service (QoS) for the network connections. RTCP requires a small amount of bandwidth in

addition to the RTP requirements. To be conservative, add 5% to the RTP bandwidth requirement

to cover the RTCP component.

2 July 2007

Calculating Bandwidth Utilization

The amount of milliseconds (of voice) in a packet determines the number of packets required per

second. If the RTP is set to 20 milliseconds of voice per packet, you will need 50 packets per

second.

Formula = 1000ms / number of ms of voice per packet = packets / second required.

Example: 1000ms / 20ms = 50 packets / sec)

Each packet acquires various amounts of information depending on the network type. For

example, there is approximately a 40 byte header added for Ethernet packets. Each time a

packet is sent that header must be attached, which will increase the packet size, and by

extension, the amount of bandwidth required for the connection.

To estimate the per-call TCP/IP bandwidth utilization (Layer 3) is simple. Assuming one call with

no silence suppression enabled,

Bandwidth = ((packets / sec * 320 bits / packet) + CODEC bit rate) * 1.05

Example 1: G.711 @ 20ms/packet (50 packets/second)

Bw = ((50packets / sec * 320 bits / packet) + 64Kbps) * 1.05

Bw = ((16Kbps) + 64Kbps) * 1.05

Bw = ((80 Kbps) * 1.05

Bw = 84Kbps

Example 2: G.729a @ 30ms / packet (33.3 packets / second)

Bw = ((33.3packets / sec * 320 bits / packet) + 8Kbps) * 1.05

Bw = ((10.7Kbps) + 8Kbps) * 1.05

Bw = ((18.7 Kbps) * 1.05

Bw = 19.7Kbps

If Silence Suppression is on:

Peak Bandwidth is calculated in the manner above; however, the average bandwidth is

estimated at 50% of peak.

Example 3: G729a with 30ms/packet

Average Bandwidth = 19.7Kbps * 0.5 = 9.9Kbps

Xcelerator IP Per Connection Bandwidth Calculation

The RTP plus UDP plus IP headers will add on 40 bytes. The Ethernet header and trailer account

for another 18 bytes of overhead, for a total of at least 58 bytes of overhead before there are any

voice bytes in the packet. These headers, plus the Ethernet header, produce the overhead for

shipping the packets. This overhead can range from 20% to 80% of the bandwidth consumed

over the LAN and WAN.

3 July 2007

Packet Header Length:

- 14 (Ethernet Header) + 20 (IP) + 8 (UDP) + 12 (RTP) + 4 (Ethernet Trailer) = 58 Bytes

Ethernet IP UDP RTP Payload Trailer

14 bytes 20 bytes 8 bytes 12 bytes Variable 4 bytes

G.711 (64 kbps) with 30 msec Packetization (Xcelerator IP default)

- Payload Length: (160 Bytes / 30 msec) x 30 msec = 160 Bytes

- Packets Per Second: 1000 msec / 20 msec = 50 Packets

- Bandwidth (bps) = (Packet Header + Payload) x Packets Per Second x Bits Per Byte

= (58 + 160) x (1000 / 20) x 8 = 87200

= 87.2 kbps

- For Two-Way Communication (Rx & Tx) Bandwidth (bps) = 87.2kbps x 2 = 174.40 kbps

* For One-Way 10 msec Packetization, Bandwidth (bps) = (58 + 80) x (1000/10) x 8 = 110400 =

110.4 kbps

G.723.1 High Bit Rate compression (6.4 kbps) with 30 msec Packetization (Xcelerator IP default)

- Payload Length: (24 Bytes / 30 msec) x 30 msec = 24 Bytes

- Packets Per Second: 1000 msec / 30 msec = 33.3 Packets

- Bandwidth (bps) = (Packet Header + Payload) x Packets Per Second x Bits Per Byte

= (58 + 24) x (1000 / 30) x 8 = 58133

= 58.13 kbps

- For Two-Way Communication (Rx & Tx) Bandwidth (bps) = 58.13 kbps x 2 = 116.2 kbps

* For One-Way 10 msec Packetization, Bandwidth (bps) = (58 + 12) x 100 x 8 = 56000 =

56 kbps

If you have any questions, please contact Vertical Customer Support.

Vertical Xcelerator IP Application Note

Vertical Xcelerator IP Application Note

Vertical Xcelerator IP Application Note

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