Per packet overhead on VDSL2 – part 2

A few days ago I wrote about some interesting latency results I observed on my home Internet connection with small packets. This post adds a bit more data.

In this experiment I disabled all upstream traffic shaping and then used iperf to blast UDP packets of various sizes to a destination host I control. The transmitted rate was 10Mbps and the upstream link rate is ~6.5Mbps. On the destination I captured the packets with tcpdump and generated the charts below with Wireshark.

The charts show ten sub-experiments – 10 seconds of traffic for each data size (iperf -l): 25, 50, 75, 100, 200, 300, 400, 500, 1000, 1400 bytes. T0 is when the first packet is received.

Packets per second observed at the destination

The first chart shows the packets per second received at the destination. Not surprisingly, the packet rate is much higher with small packets.

Bitrate observed at the destination

The second chart shows the bitrate observed at the destination. Notice that for small packets the effective bitrate is much lower. This seems to support the theory that this link has a lot of per-packet overhead.

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