## Sunday, August 30, 2009

### Calculating number of voice calls How many voice calls can a WAN link support given the amount of bandwidth, codec, packet header, payload and cRTP? This is a sample CCDA question on voice. Well, I have come up with a simple calculation. See below sample questions:

Q1) Given a VoIP network with these attributes:
Codec: G.728, Bit rate: 16 Kbps, WAN Bandwidth: 256 Kbps, Packet Header: 6 bytes, Payload: 40 bytes, CRTP: Yes

How many calls can be made? Ans: 13
Amount of b/w per call = [voice overhead + packet header + payload] * 50 pps * 8
= [ 2(cRTP)+ 6 + 40 ] * 50 * 8 = 19,200 bps
Number of calls supported = Total bandwidth / bandwidth per call
= 256,000 / 19,200 = 13 (round-down)

Q2) Given a VoIP network with these attributes:
Codec: G.711, WAN bandwidth: 768Kbps, Packet Header: 6 bytes, Payload: 160 bytes, CRTP: No

How many calls can be made? Ans: 9
Using the same formula,
Amount of b/w per call = [voice overhead + packet header + payload] * 50 pps * 8
= [ 40 (no cRTP) + 6 + 160 ] * 50 * 8 = 82,400 bps
Number of calls supported = Total bandwidth / bandwidth per call
= 768,000 / 82,400 = 9 (round-down)