When studying for CCNP, it was kind of interesting to dive deeper into how ethernet frames are constructed and how they function in a switched network. With wireless, though, you have to, in the words of Yoda, “…unlearn what you have learned!” Wireless frames can be deceptive to those of use familiar with standard ethernet frames because they do use MAC addresses and there are ACK frames sent back to a client to acknowledge transmission. However, it turns out that the differences seem to be more common than the similarities.
Frame Length – An ethernet frame has a Maximum Transmission Unit (MTU) of 1500 bytes that you can adjust up to 1515 or a little bit larger, depending on your trunking protocol and what you need. A wireless frame, just by default is 2346 bytes long! In order to help accomodate the transition that the data will need to make from a wireless frame to an ethernet frame in the AP, wireless frames have a built in field to deal with fragmentation, a Fragment Number. This field is essentially a sequence number that will tell a station if this is the last fragment of a series of fragments.
MAC Addresses - A standard ethernet frame has 2 MAC addresses, the MAC address of the sending device and the MAC address of the receiving device, so it’s pretty easy to see where it came from and where it is going. A wireless frame, however, has to take into account that the MAC address of the device that sent the frame may not be the last MAC address the frame “saw” and might not be the first MAC address the frame has to return to. For example, if a wireless client is behind a wireless repeater, then frames coming from that client will need the MAC address of that repeater in addition to the actual source and destination address. A wireless frame can have up to 4 MAC addresses.
Types of Frames - While all wireless frames have pretty much the same kind of header, what happens from then on depends a lot on what kind of frame it is. Wireless networks use 3 types of frames. Management frames are used for beacons, probes, disassociations, and pretty much anything else that pertains to the AP and client associations. Control frames pretty much have to do with preparing to send data and then acknowledging that data has been received. These include the frames that are used to request to send and are mostly used when the AP has taken control of the medium, specifying when it is a station’s turn to send data. Data frames are exactly what they sound like…they contain data.
It’s easy to see how wireless frames evolved from the existing ethernet frames and interesting to see how they were changed to better fit the type of medium used here.
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