Bandwidth Calculation for VoIP Calling

This is required when we need to configure LLQ (Low Latency Queuing) based on the number of calls according to the VoIP codec used. For the calculations, following parameters are required..

1. Codec Payload size
2. Layer 4 header size (RTP & UDP headers)
3. Layer 3 header size (IP headers)
4. Layer 2 header + tail size (Ethernet, Frame-relay, HDLC etc headers)

Accounting Layer 2 header is important because LLQ policer on Cisco IOS takes this overhead into account when enforcing the maximum rate.

Ex:- G.711

By default G.711 generates 50 VoIP packets per second.
G.711 has a codec framing rate of 10 ms (100 samples per second) which means 1 VoIP packet carries 2 samples. 1 sample has 80 bytes. So the Payload is 160 bytes.

So the Code Payload Rate = 160 x 50 x 8  = 64 kbps

This is why we call the G.711 is 64 kbps..

But actual bit stream is bit more larger because of the Layer 4,3,2 headers.

The RTP header is 12 bytes & UDP header is 8 bytes, therefor Layer 4 header is 20 bytes.

A typical Layer 3 IP header (with no options) is 20 bytes.

Layer 2 Ethernet header + tail is typically 18 bytes (14 bytes for the Ethernet header and 4 bytes for FCS (32 bits). If the frame carries a VLAN tag, add another 4 bytes, for 22 bytes of total overhead.

So the total bandwidth for a G.711 call = (160 + 20 + 20 + 18) x 50 x 8  = 87.2 kbps

Note:-

Basically if the Layer 3 packet size is given, you need only to add the Layer 2 overhead and multiply with the packet rate and 8 to get the bit rate..

Ex:- Calculate the bandwidth required in Ethernet for a G.729 call which generates 50 packets per second with a Layer 3 size of 60 bytes..

= (18+60) x 50 x 8  = 31.2 kbps

Very useful indeed

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