Uninformed: Informative Information for the Uninformed

Vol 5» 2006.Sep

Physical and Virtual Carrier Sense

The 802.11 standard specifies two ways to determine if the medium is busy. The first is a physical carrier sense. 802.11 specifies that any PHY must provide a technique to sense if the medium is busy. The function in the PHY layer responsible for this is called the clear channel assessment (CCA).

Two clients that belong to the same BSS may not be within radio range of each other. Therefore, neither will be able to detect energy on the medium necessary to do a CCA. Further, it is more efficient in some cases for a client to reserve the medium in advance, for instance, for an acknowledgement which can be sent immediately upon receipt of a frame. Both cases are handled using a virtual carrier sense mechanism. It consists of a Network Allocation Vector (NAV) maintained by each client. The NAV can be thought of as a client's best guess as to how long the medium will be busy. The client's NAV is updated in response to receiving a frame whose duration field contains a value that exceeds the current NAV value.

The duration field is found in nearly every packet. It is not included in Power-Save Poll frames, as the bits are used for the association ID field. Conceptually the duration field of a frame is the amount of time the transmitting client wishes to reserve the medium for itself to send subsequent frames, including any replies expected of the recipient such as acknowledgements. How this value is computed depends on the exact type of frame it is in. The duration field is 16 bits. Therefore the largest value it could reserve the media for is 65,535 microseconds. However the standard explicitly says to ignore any values greater than 32,767.

In a typical scenario where a client is not sending an unfragmented data frame, the duration field will be the amount of time it takes for the inter-frame spacing, combined with the time required for the receiving station to send an ACK packet; in other words, a constant. In management types (such as beacons) or some control types (such as ACKs) no more traffic is needed, and the duration field is set to zero.

In more complicated scenarios involving fragmentation, the duration field will include the time required not only for the inter-frame spacing and ACK, but for the rest of the fragments. When using frames to explicitly reserve the media (RTS/CTS) it is the duration field that specifies how long the media is reserved. Finally an important aspect of the PCF is implemented by using the duration field to interoperate with stations on the same channel using the DCF.