Specifications

MBG Engineering Guidelines, Release 8.0
Step Four: Erlang-B Calculator
An Erlang-B calculator can now be used with the values above to find the number of lines required to handle the
load. (Free Erlang-B calculators are widely available online.) Following the teleworker example above, the
Erlang-B calculation is:
λ = 40, μ = 12, P(b) = 0.01
c = 9
The site will need 9 lines to handle the load. In MBG terms, this is 9 simultaneous calls. The number of
simultaneous calls is the key value in determining MBG resource requirements.
For the call center example:
λ = 3333.3, μ = 6, P(b) = 0.01
c = 583
The call center would require 583 lines (and agents) to handle the call volume. Again, this is 583 simultaneous
calls going through the MBG.
12.2Determine Call Equivalents
The next step is to determine the impact of transcoding, taps, and codec choices on CPU load and network
bandwidth. The baseline is a single non-transcoded call with one RTP stream. The procedure in this step applies
a load factor to convert more complex calls into an equivalent number of simple calls.
Usage Call Equivalent (CPU)
Basic (non-transcoded) call 1
Transcoded call 2.5
SIP trunk call 1
Tapped call 1.5
Example:
4 MiNet sets all in calls with other parties, plus 2 SIP trunk calls, and two of the calls being tapped, all using
G.711, would constitute:
CPU use = (4 MiNet calls – 2 tapped) + 2 SIP trunk + 2 tapped
= 2 untapped + 2 trunk + 2 tapped
= 2 + 2 + 2*1.5
= 7 calls
12.3 Determine Bandwidth Requirements
VoIP devices, including phones and SIP trunks, use RTP/SRTP for voice communication. The bandwidth
required for the RTP stream depends on the codec selected by the device. Mitel Border Gateway supports the
use of G.711, G.729, and G.722.1. Typically, there will be other requirements for Internet access, and these
requirements (such as e-mail, web browsing, e-commerce) must be considered as well.
Failure to provide sufficient bandwidth for all Internet activities may compromise the quality of
service of the Mitel Border Gateway.
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