CCIE Collaboration Exam: Bandwidth Allocation for VoIP Calls

Calculate Bandwidth Allocation for VoIP Calls

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Question

Assume 6 bytes for the Layer 2 header, 1 byte for the end-of-frame flag, and a 40-millisecond voice payload, how much bandwidth should be allocated to the strict priority queue for five VoIP calls that use a G.729 codec over a multilink PPP link?

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Explanations

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A.

Voice payloads are encapsulated by RTP, then by UDP, then by IP.

A Layer 2 header of the correct format is applied; the type obviously depends on the link technology in use by each router interface: A single voice call generates two one-way RTP/UDP/IP packet streams.

UDP provides multiplexing and checksum capability; RTP provides payload identification, timestamps, and sequence numbering.

To calculate the bandwidth required for five VoIP calls using the G.729 codec over a multilink PPP link, we need to consider the size of the Layer 2 header, the end-of-frame flag, and the voice payload.

Let's break down the calculation step by step:

  1. Voice payload size: The G.729 codec produces a voice payload of 20 bytes per packet, and the voice payload is transmitted every 20 milliseconds. Therefore, the bandwidth required for one VoIP call is:

(20 bytes/packet) / (20 milliseconds/packet) = 1,000 bytes/second = 8,000 bits/second

  1. Layer 2 header and end-of-frame flag: The Layer 2 header is 6 bytes, and there is an additional 1-byte end-of-frame flag. Therefore, the total size of the Layer 2 header and end-of-frame flag is 7 bytes.

  2. Total bandwidth required for five VoIP calls: To calculate the total bandwidth required for five VoIP calls, we need to multiply the bandwidth required for one call (8,000 bits/second) by five (the number of calls) and add the bandwidth required for the Layer 2 header and end-of-frame flag (7 bytes).

Total bandwidth = (8,000 bits/second/call) x 5 + (7 bytes x 8 bits/byte) = 40,000 bits/second + 56 bits/second = 40,056 bits/second

  1. Bandwidth allocation for the strict priority queue: The strict priority queue should be allocated enough bandwidth to handle the highest-priority traffic, which in this case is the VoIP traffic. Therefore, the bandwidth allocation for the strict priority queue should be equal to the total bandwidth required for the five VoIP calls.

Converting bits per second to kilobits per second (kb/s):

40,056 bits/second = 40,056/1000 kb/s ≈ 40 kb/s

Therefore, the answer is A. 87 kb/s.

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