Erlang bandwidth calculator instructions
This Erlang bandwidth calculator estimates the bandwidth you need through your IP-based network to carry your voice traffic, specified as busy hour traffic in Erlangs. It will also calculate the busy hour traffic that a specific bandwidth can cater for. Before performing a calculation, you need to provide details of your voice compression scheme in the first two areas of the calculator:
- Use the first drop down box to select the CODEC your equipment uses to convert between analogue voice signals and data streams. CODECs vary in their quality and delay characteristics, with G.723.1 and G729A being most common for low-bitrate compressed applications and G.711 being used for higher quality lossless coding.
- The rate at which voice packets are transmitted has a significant bearing on bandwidth. The selection of the packet duration (and therefore the packet frequency) is a compromise between bandwidth and quality. A lower duration requires more bandwidth but, as the duration is lengthened, a system’s delay increases and it becomes more susceptible to packet loss. 20 ms is a typical value.
There are three further variables involved:
- Erlangs: the hours of call traffic during the busiest hour of a telephone system’s operation (its busy hour traffic)
- Blocking: the ratio of calls that fail through insufficient transmission facilities being provided (e.g. 0.03 mean 3 calls blocked per 100 calls attempted)
- B/W: the bandwidth, in kbps
If you know two of the figures, you can work out the third:
- Click on the button representing the variable you want to calculate (bandwidth is selected by default)
- Enter the two known figures into their edit boxes
- Press Calculate and the unknown value will be calculated and displayed in the remaining box
For example, to work out how much bandwidth is required to transmit 45 Erlangs busy hour traffic through an IP network using the G.723.1 (6.4 kbps) coding scheme with 30 ms packet duration, enter those values into the calculator. When you press Calculate, the result will be displayed: 990 kbps. (This assumes a 1% blocking target).
Note that this calculator does not take RTP compression mechanisms or datalink protocol overheads such as Ethernet into account.
VoIP Calculator is our simple-to-use Windows products that includes an Erlang bandwidth calculator and a lines to IP bandwidth conversion tool. Although based on our online calculators, it also models RTP header compression (cRTP) and supports calculations up to 25,000 Erlangs and 20 Gbps.
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