CCIE Collaboration Exam: Bandwidth Allocation for VoIP Calls

# Calculating Bandwidth Allocation for VoIP Calls

### Question

Assume a 30-millisecond voice payload, 6 bytes for the Layer 2 header, 1 byte for the end-of-frame flag, and the IP, UDP, and RTP headers are compressed to 2 bytes, how much bandwidth should be allocated to the strict priority queue for eight VoIP calls that use a G.729 codec over a multilink PPP link with cRTP enabled?

### Explanations

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A. B. C. D. E.

D.

References: http://www.cisco.com/en/US/tech/tk652/tk698/technologies_tech_note09186a0080094ae2.shtml.

To calculate the required bandwidth for eight VoIP calls that use a G.729 codec over a multilink PPP link with cRTP enabled, we need to follow these steps:

Step 1: Calculate the Layer 2 overhead: The Layer 2 overhead includes the Layer 2 header and the end-of-frame flag. The total overhead is 6+1=7 bytes.

Step 2: Calculate the IP/UDP/RTP header compression overhead: The IP/UDP/RTP headers are compressed to 2 bytes using cRTP.

Step 3: Calculate the total packet size: The total packet size is the sum of the voice payload, Layer 2 overhead, and IP/UDP/RTP header compression overhead. Therefore, the total packet size is 30 + 7 + 2 = 39 bytes.

Step 4: Calculate the bandwidth for one VoIP call: To calculate the bandwidth required for one VoIP call, we need to multiply the total packet size by the codec bit rate and the packetization interval. For G.729 codec, the bit rate is 8 kbps and the packetization interval is 20 ms (50 packets per second). Therefore, the bandwidth required for one VoIP call is:

8 kbps x 39 bytes x 8 bits/byte x 50 packets/second = 39.6 kbps

Step 5: Calculate the bandwidth for eight VoIP calls: To calculate the bandwidth required for eight VoIP calls, we need to multiply the bandwidth required for one VoIP call by 8. Therefore, the bandwidth required for eight VoIP calls is:

39.6 kbps x 8 = 316.8 kbps

Step 6: Allocate bandwidth to the strict priority queue: In a Cisco router, the strict priority queue is typically reserved for voice traffic. Therefore, we need to allocate 316.8 kbps of bandwidth to the strict priority queue.

However, in the real world, it's recommended to allocate additional bandwidth to the strict priority queue to accommodate for any network congestion or traffic bursts. In this case, a bandwidth allocation of 20% to 30% above the calculated value (316.8 kbps) would be appropriate.