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Erlangs and VoIP Bandwidth Calculator
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Here is a
brief introduction to the calculator. For more detailed information, press the Help
Button. For a running record of the results calculated, press the Results button.
Both these functions open new browser windows on your desktop.
- This calculator can be used to estimate the bandwidth required to transport a known busy
hour traffic figure through an IP based network. Reverse calculations are also
possible. These estimate traffic that can be offered to an IP network if the
available bandwidth is known. The calculator is based on the Erlang B traffic model.
- Before a calculation can be performed, details of the voice compression scheme must be
entered into the first two areas of the calculator.
- Use the first drop down box to select the CODEC being used. CODECs convert
analogue voice signals into data streams through sampling and quantisation. CODECs
vary in their quality and delay characteristics and, although there is not yet an agreed
standard, G.723.1 and G729A are the most common CODECs used for Internet voice
transmission.
- The frequency at which the voice packets are transmitted have a significant bearing on
the bandwidth required. The selection of the packet duration (and therefore the
packet frequency) is a compromise between bandwidth and quality. Lower durations
require more bandwidth. However, if the duration is increased, the delay of the
system increases, and it becomes more susceptible to packet loss; 20ms is a typical
figure.
- The three variables involved in these calculations are Busy Hour Traffic (BHT), Blocking
and Bandwidth:
- Busy Hour Traffic (in Erlangs) is the number of hours of call traffic there are
during the busiest hour of operation of a telephone system.
- Blocking is the failure of calls due to an insufficient number of lines being
available. E.g. 0.03 mean 3 calls blocked per 100 calls attempted.
- Bandwidth is the amount of bandwidth in kbps required through an IP based network
to carry the traffic.
- If you know two of the figures, you can work out the third as follows:
- Click on the Unknown radio button representing the unknown variable.
- Enter the 2 known figures into their edit boxes.
- Press Calc. and the third figure will be calculated and displayed in the
remaining edit box.
- The number of discrete voice paths which the network needs to be able to simultaneously
transport is shown in the Voice paths box; this box is only used to display the
intermediate result, and is not used for entry.
- For example, to work out how much bandwidth is required to transmit 45 Erlangs of busy
hour traffic through an IP network using the G.723.1 (6.4kbps) coding scheme with 30ms
packet duration, follow these steps:
- Use the Coding algorithm drop down list to select G.723.1 (MP-MLQ) 6.4 kbps
compression.
- 30ms should automatically be selected as the Packet duration. With
this coding scheme, lower packet durations are not possible.
- Ensure that the Unknown radio button in the B/W (bandwidth) area is
selected.
- Enter 45 into the edit box within the B.H.T. area.
- Press the Calc. button. After a short time, 990 should appear in
the Bandwidth edit box, indicating that a bandwidth if 990kbps would be required
to carry the 58 voice paths required.
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| A Windows version of this calculator is available for only 99 US Dollars. VoIP Lite is a low cost calculator based on the calculator on this
page. Additionally, it supports cRTP header compression to 2 and 4 octets. 
VoIP Lite, showing the Erlangs and IP Bandwidth
Calculator
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| Westplan is a revolutionary software package which simplifies the
complicated design process involved in creating effective voice networks that are based on
either traditional circuit switching or Voice over IP. All calculations are based on an
inter-nodal traffic model which you can specify yourself, or allow Westplan to estimate. 
Westplan main window, with a network drawing in view
By allowing you to try different network topologies, Westplan quickly helps you to make
informed judgements on the suitability of particular voice networking strategies to your
company. You will be in a stronger position to understand your voice network costs and
therefore to control your network resources.
Westplan supports these features:
- Point and click network diagrams
- Voice over IP bandwidth calculations
- Layer 2 support for PPP, Frame Relay, Ethernet, ATM and HDLC.
- Analogue, T1 and E1 transmission facilities
- User-definable routing rules
- RTP compression for VoIP
- Full justification reports for each analysis
- Clear printed reports
- Context sensitive help
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