Compatibility with a/b/g
802.11n devices will have to operate with 802.11a, b and g devices for a long time. Therefore, it is absolutely necessary to guaranty the backward compatibility with these previous generations. Here are two examples of backward compatibilities protection techniques that can be used:
Multiple PPDU frame format
PPDU stands for Physical layer convergence procedure Protocol Data Unit. A PPDU is a frame containing a PSDU + a Phy header + a preamble. The Physical layer creates it. Multiple preamble and Phy header assures the backward compatibity. A preamble can have different formats. It can be a legacy format (also called non-HT format) or it can be an HT format. A legacy format (or non-HT format) is the format used by 802.11a, b and g. An HT format is the format used by 802.11n. Therefore, an HT format cannot be understood by 802.11a, b or g. The same thing applies with Phy header.
If a PPDU only contains HT format elements, (such a PPDU is called HD-Greenfield) it can only be understood by 802.11n compatible devices. There is therefore no backward compatibility. The solution is to use mixed-formats. As shown in the next figure, such a PPDU contains a HT format Phy header and preamble but also a legacy format Phy header and preamble. This way a non-802.11n device will be able to read the information.
CTS-to-self
CTS stands for Clear-to-Send. It deals with medium access control which objective is to prevent collision over the medium. It is normally used with RTS, Request To Send. When a device aims at sending a packet to another device, it first sends an RTS packet to everyone. Every receivers of this RTS send back a CTS to the sender to indicate that they don’t intent to use the medium. Finally, the data is sent to the targeted device. This limit the bandwidth since before every sending, several RTS and CTS has to circulate. 802.11g introduced the CTS-to-self packet. When a station wishes to use the medium, it first send a CTS-to-self packet to everyone. All the devices that receive the CTS-to-self delay their emissions (called backoff) and the transmitter is able to transmit his data.
Imagine now two devices, A and B, connected to an AP. Let's suppose that the AP and A are 802.11n compatible devices and B is an 802.11a device. A wants to send a data to the AP. It sends a CTS-to-self to reserve the medium. If this packet does not use a legacy data rate, B will not decode the CTS-to-self. Therefore, it can decides to use the medium at the same time and there will be a collision. That is the reason why, every time a 802.11n device sends a CTS-to-self to reserve the medium for a certain time, it uses a legacy data rate that can be decode by every non-HT devices, so that every others stations will perform a backoff.