Calculating the Total Bandwidth for a VoIP Call

Calculating the bandwidth that a VoIP call consumes involves consideration for all the factors discussed thus far. Some fields and protocols are required, each of which might offer implementation alternatives. Other protocols and fields are optional. You use the bandwidth consumed by each VoIP call to calculate the total bandwidth required for the aggregate of simultaneous VoIP calls over LAN and WAN connections. This information is required for the following purposes:

■ Designing and planning link capacities

■ Deployment of CAC

■ Deployment of quality of service (QoS)

QoS can be defined as the ability of a network to provide services to different applications as per their particular requirements. Those services can include guarantees to control end-to-end delay, packet loss, jitter, and guaranteed bandwidth based on the needs of each application. CAC is used to control the number of concurrent calls to prevent oversubscription of the resources guaranteed for VoIP calls.

Computing the bandwidth consumed by a VoIP call involves six major steps:

Step 1 Determine the codec and the packetization period. Different codecs generate different numbers of bits per second (also called codec bandwidth), and they generally range from 5.3 Kbps to 64 Kbps. The number of digital voice samples (each of which is equivalent to 10 ms of analog voice) encapsulated in each IP packet determines the packetization period. A packetization period of 20 ms, which is the default in Cisco voice-enabled devices, means that each VoIP packet will encapsulate two 10-ms digital voice samples.

Step 2 Determine the link-specific information; this includes discovering whether cRTP is used and what the data link layer protocol (encapsulation type) is. You must also find out if any security or tunneling protocols and features are used on the link.

Step 3 Calculate the packetization size or, in other words, calculate the size of voice payload based on the information gathered in Step 1. Multiplying the codec bandwidth by the packetization period and dividing the result by 8 results in the size of voice payload in bytes. Please note that the packet-ization period is usually expressed in milliseconds, so you first must divide this number by 1000 to convert it to seconds. If G.729 with the codec bandwidth of 8 Kbps is used and the packetization period is 20 ms, the voice payload size will equal 20 bytes. 8000 (bps) multiplied by 0.020 (seconds) and divided by 8 (bits per byte) yields 20 bytes.

Step 4 Calculate the total frame size. Add the size of IP, UDP, and RTP headers, or cRTP header if applied, plus the optional tunneling headers and the data link layer header determined in Step 2, to the size of voice payload (packet-ization size) determined in Step 3. The result is the total frame size. If the voice payload size is 20 bytes, adding 40 bytes for RTP, UDP, and IP, and adding 6 bytes for PPP will result in a frame size of 66 bytes (without usage of cRTP and any tunneling or security features).

Step 5 Calculate the packet rate. The packet rate is inversed packetization period (converted to seconds). For example, if the packetization period is 20 ms, which is equivalent to 0.020 seconds, the packet rate is equal to 1 divided by 0.020, resulting in a packet rate of 50 packets per second (pps).

Step 6 Calculate the total bandwidth. The total bandwidth consumed by one VoIP call is computed by multiplying the total frame size (from step 4) converted to bits multiplied by the packet rate (from step 5). For instance, if the total frame size is 66 bytes, which is equivalent to 528 bits, and the packet rate is 50 pps, multiplying 528 by 50 results in a total bandwidth of 26400 bits per second, or 26.4 Kbps.

Figure 1-14 shows VoIP framing and two methods for computing the bandwidth required for a VoIP call. Method 1 displayed in Figure 1-14 is based on the six-step process just discussed.

The second method for calculating voice bandwidth is shown as Method 2 in Figure 1-14. This method is based on the ratio shown on the bottom of Figure 1-14: The ratio of total bandwidth over voice payload is equal to the ratio of total frame size over voice payload size. If G.729 is used and the packetization period is 20 milliseconds, the voice payload size will be 20 bytes. With PPP encapsulation and no cRTP, security, or tunneling, the total frame size adds up to 66 bytes. The ratio of total frame size to voice payload size is 66 over 20, which is equal to the ratio of voice bandwidth over codec bandwidth (8 Kbps for G.729). This 66 multiplied by 8 Kbps and divided by 20 results in voice bandwidth of 26.4 Kbps.

Figure 1-14 Computing the VoIP Bandwidth Requirement

E D C B A

Bytes Bytes Bytes Bytes Bytes

Figure 1-14 Computing the VoIP Bandwidth Requirement

E D C B A

Bytes Bytes Bytes Bytes Bytes

			Either	Digitized Voice
Tunnel Header	Possible Security Header	IP+UDP+RTP	The size of this section depends
Layer 2		Header	on the codec type and the
Header		or	amount (msec) of analog voice
		cRTP Header	that is digitized and encapsulated in each IP packet.

VoIP Bandwidth Calculation Method 1:

A = Amount of digitized voice per packet (Bytes)

= CODEC Bandwidth (bps) x Packetization Period (in Sec) / 8 (bytes) F = Total Frame Size (bits) = 8 x (E + D + C + B + A) R = Packet Rate = 1/(Packetization Period in Seconds) Bandwidth per call (kbps) = F x R divided by 1000

VoIP Bandwidth Calculation Method 1:

A = Amount of digitized voice per packet (Bytes)

= CODEC Bandwidth (bps) x Packetization Period (in Sec) / 8 (bytes) F = Total Frame Size (bits) = 8 x (E + D + C + B + A) R = Packet Rate = 1/(Packetization Period in Seconds) Bandwidth per call (kbps) = F x R divided by 1000

VoIP Bandwidth Calculation Method 2:

A = Amount of digitized voice per packet (bytes)

= CODEC Bandwidth (bps) x Packetization Period (in Sec) / 8 (bytes) F = Total Frame Size (bytes) = E + D + C + B + A

Bandwidth per call = codec bandwidth multiplied by F divided by A Total Frame Size Total Bandwidth Requirement

Voice Payload Size codec Bandwidth (also called Nominal Bandwidth Requirement)

After you compute the bandwidth for one voice call, you can base the total bandwidth for VoIP on the maximum number of concurrent VoIP calls you expect or are willing to allow using CAC. The bandwidth required by VoIP and other applications (non-VoIP) added together generally should not exceed 75 percent of any bandwidth link. VoIP signaling also consumes bandwidth, but it takes much less bandwidth than actual VoIP talk (audio) packets. QoS tools and techniques treat VoIP signaling and VoIP data (audio) packets differently, so VoIP signaling bandwidth and QoS considerations need special attention.

Continue reading here: Voice Gateway Functions on a Cisco Router

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