Network Allocation Vector (NAV)
The carrier sense mechanism is used by WLAN is CASMA/CA . There are mainly two types of carrier sense:  From Physical Layer  From MAC Layer. NAV is coming under MAC layer. Let’s try to understand the details about it.
♣ What is Network Allocation Vector (NAV)? What is the use of it?
NAV is actually timer in microsecond to indicate how long the medium will be used by the transmitter after the sender got the opportunity post CSMA/CA.
♣ Why it’s called Virtual Carrier Sense?
The actual real carrier sense is done by physical layer. This is required to know that medium is busy or free. In parallel to physical layer by using time the same can be achieved in MAC layer also. But this carrier sense mechanism is like looking into the time value set by other stations and believing that this much time medium will be busy. So this is called virtual and that’s why NAV is also Virtual Carrier Sense.
♣ Where can we find NAV inside 802.11 frame?
Each 802.11 frame contains MAC header. Inside MAC header there is a field called Duration/ID. This field is to represent NAV in time.
♣ What is the use of NAV for other stations?
While any station is in CSMA/CA process, then it checks for Duration/ID of any frame in the air and updates its current NAV value if the received NAV is higher than current NAV value else ignore the received NAV.
♣ Duration/ID field:
In 802.11 MAC header there is one 2 Octets field call Duration/ID. It resides on 2nd place after Frame Control.
♦ When this field contains time in microsecond then it represents NAV.
♦ In PS-POLL frame this field contains association ID of sending station. There is no duration in PS-POLL frame.
In this article we will discuss on NAV.
♣ Duration min and max value meaning:
As duration field is 2 Octets so the duration value range is 0-32767.
Duration value 0 means, there is no further medium reserved for that particular frame exchanges by frame transmitter.
Duration value > 0 and < 32767 means, medium is reserved for that microsecond time by transmitter.
Duration value 32767 means, the transmitter is sending frame in CFP (Contention free period) using PCF (Point coordination function). Just remember, PCF is not used in real time because of complexity, instead DCF/EDCA (mechanism CSMA/CA) is used to enable Contention Period (CP).
♣ Duration/ID field encoding:
Here is the table to understand Duration/ID encoding.
♣ NAV for some 802.11 frames (Examples):
We have to remember that all frames do not contain any duration value (It is set to 0). We will see different types of frames and understand the reason behind it.
Example1 [Broadcast Frame]:
Let’s take Beacon frame as first example.
Duration is 0 for Beacon frame because Beacon frame is broadcast frame; it does not need any ACK. So, CSMA/CA was done by AP just to gain medium for sending Beacon frame and when beacon frame is in air that means no more time is reserved for further frame by sender. See below screenshot.
Same above reason applies for any 802.11 broadcast frames which do not requires any ACK. Example: Probe Request. Check below screenshot for Probe Request and some other broadcast frame like DHCP Discover (receiver is broadcast), ARP Request sent in wireless medium.
Example2 [Unicast Frame]:
There are unicast frames where duration is > 0. Example: Authentication, Association Request, Association Response, QoS Null, Action, DHCP Discover (Receiver is unicast), CTS to Self, DHCP Offer etc.
Example3 [Some special Frames]:
♦ PS-POLL frame does not have any duration value. Instead it’s has AID in same field.
♦ ACK: Duration is always 0 as it’s the last frame for any exchange. See previous screenshot.
♦ RTS/CTS/CTS to Self: These frames have duration value which covers, time required for that frame (RTS/CTS/CTS to Self) itself + further frames (Generally Data and ACK).
So we understood that NAV plays a very important role in wireless network to use the medium effectively. NAV is a important MAC enhancement to do carrier sense in parallel to physical layer carrier sense.
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