Definitely Erlang C should be used. Here is an example of how to use it:
You are designing your backbone connection between two routers. You know that you will generally see about 600 packets per second and 200 bytes per packet or 1600 bits per packet. The result of multiplying 600 pps by 1600 bits per packet gives us the amount of bandwidth we will need to support, 960,000 bps. We know that we can buy circuits in increments of 64,000 bps. This will give us the amount of data necessary to keep the circuit busy for 1 second. How many circuits will we need to keep delay under 10 milliseconds?
To get the traffic load we will need to:
960,000 bps/64,000 bps = 15 Erlangs of traffic load
To get the average transmission time we will need to multipy bytes per packet by 8 to get bits per packet. Then divide by 64000 bps (circuit speed) to get the average transmission time per packet
200 bytes/packet * 8 bits = 1600 bits/packet /64000 bps = 0.025 seconds to transmit or 25 milliseconds.
Delay factor 10 ms/25 ms = 0.4 delay factor
25ms = AHT
10ms = avg delay
When I look in my erlang tables, I see that 0.4 blocking probability or delay factor with 15.25 Erlangs of traffic load yields, 18 Agents (traffic sensitive facilities)
In this context an erlang is used as the time it takes to keep a traffic sensitive facility busy for 1 second.