Wi-Fi performance Issues

1
Wi-Fi performance Issues

• Throughput subject to packet size
• High latency 11 32 ms
• Not flexible
• Choose high throughput or low latency

2
Atheros Throughput Example
packet size packet size 64 Bytes 128 Bytes 256 Bytes 512 Bytes 1024 Bytes 1280 Bytes 1518 Bytes 1522 Bytes (Vlan tagged)
UL DL Wi100 T..... Wi100 T..... Wi100 T..... Wi100 T..... Wi100 T..... Wi100 T..... Wi100 T..... Wi100 T.....
0 100 41.7 0.09 42.1 1.1 42.6 5.0 42.1 21.5 40.6 30.4 42.6 33.2 40.5 33.2 40.5 34.5
20 80 43.9 0.04 44.5 0.3 43.9 1.4 44.6 7.7 43.5 37.0 45.1 39.4 42.0 38.9 46.4 38.8
40 60 45.8 0.03 46.5 0.3 43.9 1.4 46.8 10.1 46.0 41.5 47.2 43.2 42.0 39.9 46.4 40.4
50 50 46.5 0.03 47.1 0.3 43.9 1.4 47.3 9.5 46.5 42.8 47.2 43.1 42.0 44.8 46.4 44.6
60 40 45.8 0.03 46.5 0.3 43.9 1.4 46.8 10.1 46.0 41.5 47.2 43.2 42.0 39.9 46.4 40.4
80 20 43.9 0.04 44.5 0.3 43.9 1.4 44.6 7.7 43.5 37.0 45.1 39.4 42.0 38.9 46.4 38.8
100 0 41.7 0.09 42.1 1.1 42.6 5.0 42.1 21.5 40.6 30.4 42.6 33.2 40.5 33.2 40.5 34.5
Wi-Fi is extremely sensitive to packet size Max
throughput can only be achieved when big packet
sizes are transmitted.
3
Concatenation

• The small packet problem can be solved in Wi-Fi
  through concatenation.
• Small packets are received in the radio and
  grouped into larger packets.
• The end result is similar data throughput as with
  large packets.

PROBLEM Concatenation adds 10 14 ms Delay!
4
Fragmentation

• The latency problem in Wi-Fi can be resolved by
  fragmenting packets so that smaller packets are
  sent on a fixed time interval.
• This means that all packet sizes are fixed and at
  a smaller size.
• As seen on the previous slides, Wi-Fi performance
  suffers when packet-sizes are small.

PROBLEM Fixed frames lower data throughput!
5
Wi-Fi performance Issues

• Throughput subject to packet size
• Concatenation solves problem
• Adds significant delay (10 14ms)
• High latency
• 11 18 ms (21 32ms w/concatenation)
• Fixed frame sizes (fragmentation) solves latency
• Turn-off concatenation
• Choose latency or throughput
• Wi-Fi based product can offer
• High Throughput with high latency
• Low latency with low throughput at fixed frame
  rate