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iperf -b bandwidth option for UDP

I am simulating VoIP traffic with iperf and I am struggling to understand the -b option for bandwidth for UDP traffic (http://crok-linkblog.homelinux.com/links-cisco/how-to-use-iperf-properly-additions-to-the-tcp-throughput-post/).

It says that it is the bandwidth to send at in bits/sec. So I have a 3mbps MPLS link and when I use -b 3000000 -f k, I got a lot of packets being dropped for just one conversation. But when I use -b as 65000, I have no problem with 5 conversations.

Can somebody help me to understand the -b bandwidth option?
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leblanc
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leblanc
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2 Solutions
 
N-WCommented:
The -b option is exactly as described, it sends UDP packets at the specified bandwidth.

If you're sending 5 x 65000 conversations, that's only 0.32Mbits/sec which is well under your 3Mbits/sec connection.

If you're sending 1 x 3000000 conversations, that's pushing your connection to it's limit. If there's other traffic currently on this connection, then it's probably being over utilized and as a result, dropping packets.

Try reducing the UDP stream bandwidth to 2500000 or 2000000 and see there's less dropped packets.
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leblancAccountingAuthor Commented:
I think you just help to understand. So the bandwidth is actually not my actual 3 mbps MPLS connection. It is the bandwidth that I think is required for a conversation. So if I am simulating VoIP g729 codec that requires 65000 kbps per conversation. I will just keep incrementing the number of conversations until I see the packets dropped. That is my limits (roughly) for the number of conversations I can have on my 3mbps MPLS link. Does it make sense?
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N-WCommented:
Yes, that makes sense.

I think you're getting a little confused with your data rates though. G.729 generally runs at either 6.4kbit/s (6400bit/s), 8kbit/s (8000bit/s) or 11.8kbit/s (11800bit/s).

Specifying 65000 when running iperf with run the UDP test at roughly 64kbit/s.

On a 3Mbit/s connection, theoretically you should be able to achieve the following:

467 conversations @ 6.4kbit/s
375 conversations @ 8kbit/s
254 conversations @ 11.8kbit/s

If you're truly running at 64kbit/s, then you'll only achieve 46 conversations.

Note: you're actual results will vary depending on the other traffic running through your connection.
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leblancAccountingAuthor Commented:
You are right. I meant g711 with 64Kbps and g729 is 8Kbps. Thank you
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