Netw250 week 5 ilab: voip traffic engineering



In this lab, students will examine the following objectives.


·       Calculate the VoIP traffic load in access trunks to the packet network.

·       Calculate the number of IP PBX access channels to the packet network.

·       Calculate the number of backup access channels to PSTN.




ABC Inc. is planning to retire an analog PBX serving its location, and to deploy an IP PBX to integrate telephone services into the TCP/IP network using VoIP technologies. It plans to use primarily the packet network (Internet) to make and receive phone calls. To meet its phone service objectives, ABC Inc. must size access links to the Internet and determine the number of SIP trunk connections that are needed. In addition, the company wants to have a backup connection (i.e., a subscription to a few analog lines) to the PSTN through a trunk gateway in case of failure in the packet network.

Your task is to calculate the following traffic engineering parameters to correctly size access connections to WAN.

1)              Calculate VoIP traffic load in access trunks to the Internet.

2)              Calculate the access bandwidth required and the number of SIP trunks needed for VoIP calls to the Internet.

3)              Calculate the number of backup access channels to PSTN.



The figure below shows the IP PBX network at the company’s location.


Note: Text in red indicates required iLab questions.


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 company’s 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 company’s 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 company’s location?

Roughly 307.2


Q2. What is the traffic load in Erlangs during the busy hour? Show calculation and units.



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


Task 2—Calculate 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 company’s 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

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



Q5. What is the number of SIP trunks (lines from the online calulcator) required to meet company’s needs?



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



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.


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


Task 3—Calculate 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


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

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.

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