Fragmentation was an enhancement to 802.11a/b/g WLAN networks, which helped in a highly interfered environments with usage of SISO antennas. As a final effect there were a lot of collisions in these conditions, therefore there were a lot of retransmissions. It was easier to fragment a frame into smaller chunks and transmit it. As a drawback, the throughput was decreased.
In Cisco WLC the fragmentation can be find in global 802.11a/n/ac settings for 5GHz radios. The default value is 2346 bytes.
For a test I use simple ICMP packets with 600 bytes threshold generated every 0.2 second with application Net Analyzer Pro.
We focus on the packet capture in Wireshark, we see frame 214 from the mobile client to the default gateway (DG) and frames 218 and 220 from the DG to the mobile client:
The Frame 214 shows the length of 738 bytes, including data payload. As there is no requirement advertised in a Beacon regulating usage of fragmentation of MSDU, the mobile client doesn’t use it and send MSDU with no restrictions related to length.
The AP fragments the MSDU as instructed by the configuration. The MSDU is too big for the initial value of 500 bytes plus overhead and that’s why it is fragmented into 2 smaller chunks and transmitted separately:
The Frame number 218:
There are 3 important elements for fragmentation for frame 218:
- More fragments – normally 0, when the MSDU is fragmented every frame (but not last one) will have value of 1, which means there is more fragments coming
- Fragment number – as we work with binary system, the first fragment is with number 0
- Sequence number – the number in sequence indicating which QoS data frame it is since association to the radio. In this example it is number 129.
The frame number 220:
Again 3 important elements in frame 220:
- More fragments – value of 0, which indicates it is the last fragment of the MSDU
- Fragment number – value of 1 – the second fragment of the MSDU
- Sequence number – value of 129. As we take a look at the previous frame, there was the same value.
The Sequence number is the same for all fragments of the MSDU
This is how the fragmentation works. It is legacy as with 802.11n and 802.11ac the technology focuses on frames aggregation into one bigger frame to boost the throughput with 802.11n and 802.11ac MCS data rates, rather than fragment MSDU into smaller chunks to decrease the throughput.