Task 1 Calculate the VoIP traffic load in access trunks to the Internet

In this exercise, you will calculate the phone service traffic load in Erlangs on the access links connecting the companys location with the Internet through the ISP. The following assumptions are given.

1. The PSTN connection is only used as a backup. No phone calls will go to the PSTN trunk, which is only used in case of packet network WAN failure.

2. There are 510 phones at the companys location, including IP phones, soft phones, and analog phones.

3. Sixty percent of the phones generate simultaneous call attempts in the busy hour, with an average duration of 7 minutes per call.

Q1 What is the number of call attempts during the busy hour at the companys location?

Number of phones that generate continuous call attempts during the busy hour;

60% of the phones represents 0.6 x 510 = 306 calls

Q3. What is the dimension of traffic measurement unit Erlang?

Erlang which is denoted by letter E is a dimensionless unit of measurement that is used to measure offered load, sometimes called load on service in telephony and therefore is useful in the provision of features that include telephone switching equipment. The unit was named in honor of Agner Krarup Erlang who discovered the unit of measurement.

Task 2Calculate the number of SIP trunks and access link bandwidth required for VoIP calls to the Internet

In this exercise, you will calculate the access bandwidth and the number of SIP trunks required to meet the companys objective. Use the busy-hour traffic load to the Internet that was calculated in Task 1. The following assumptions are given.

1. The call-blocking probability during busy hour should not exceed 1% (i.e., 0.01).

2. Use the G.711 codec with a 20-millisecond payload duration per packet.

3. Use the Erlang-to-VoIP bandwidth calculator at http://www.erlang.com/calculator/eipb/. 

Q4. What is the difference between a completed call and a call attempt?

A completed call is taken to be complete when it is active up to a point that the measurement equipment measuring the call actively terminates it while a call attempt implies a call setup procedure, which, if the call becomes successful it leads to a connected call while if the call procedure is unsuccessful it leads to a failed call attempt.

Q5. What is the number of SIP trunks (lines from the online calculator) required to meet companys needs?

0.010 implying 1 call is blocked per 100 calls that are attempted.

Q6. What access link bandwidth (Kbps from the online calculator) is required to connect the companys location to the Internet?

3840 kbps are required to carry the 48 voice paths that are required.

Q7. If T1 bandwidth is 1.544 Mbps, T3 bandwidth is 44.736 Mbps, and there are 28 T1 in a T3, what access connection(s) does the company need to lease in terms of T1 and/or T3? Show the calculation.

A G.711 (PCM) call uses the following bandwidth:

28kbps per call full rate with headers

48.2 kbps per call with VAD

2 kbps per call with cRTP

6.3 kbps per call with VAD and cRTP

Therefore, the bandwidth needed on the link between the T1 and the T3 is as follows:

Full Rate: 1.544Mbps/48.2kbps = 32.03DSoS

VAD: 1.544Mbps/2kbps = 772DSoS

cRTP: 44.736Mbps/28kbps = 1597.71DSoS

VAD/cRTP: 1597.71DSoS/6.3kbps = 253.61DSoS

Q8. Include a screenshot below showing the results from the Erlang-to-VoIP bandwidth calculator.

Task 3Calculate the number of backup access channels to the PSTN

ABC Inc. is concerned with performance degradation and service interruptions due to the Internet availability. It decides to have a backup connection to the PSTN in case of any disruptions of phone services through the packet network.

You will calculate the number of analog lines the company needs to lease from a telephone service provider (TSP) as a backup in case of packet network failures. The following assumptions are given.

1. Use the same busy-hour traffic load as in Task 1.

2. The call blocking should not exceed 10% (i.e., 0.01).

3. Use the Erlang B calculator at http://www.erlang.com/calculator/erlb/. 

Q9. How many analog lines does the company need to subscribe from a TSP? Include a screenshot below that shows the calculator results.

48

Q10. Assume that each analog line can carry 1 Erlang of voice traffic. What is the utilization of the PSTN trunks if an Internet failure occurs during the busy hour? Show the calculation.