Network Allocation Vector (NAV)

The Network Allocation Vector (NAV) is a virtual carrier sensing mechanism that forms an important part of the Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) protocol. It is implemented in wireless network protocols like IEEE 802.11 and IEEE 802.16, operating at the Medium Access Control (MAC) layer to prevent collisions in wireless networks.

NAV addresses the hidden terminal problem in wireless networks by providing a virtual mechanism to reserve the wireless medium, ensuring that stations defer transmission even when they cannot physically detect ongoing transmissions.

How NAV Works

The Network Allocation Vector operates as a countdown timer that indicates how long the wireless channel will remain busy. When a station wants to transmit, it announces the expected transmission duration, and all other stations set their NAV timers accordingly.

• NAV Timer − A counter that decrements to zero, indicating channel availability

• Maximum Duration − Set to the total time required for the complete frame exchange sequence

• Channel Reservation − Non-zero NAV value prevents other stations from contending for the channel

• Channel Release − When NAV reaches zero, the channel becomes available for contention

RTS/CTS Frame Exchange with NAV

The NAV mechanism is most commonly implemented with the Request to Send/Clear to Send (RTS/CTS) handshake:

1. RTS Transmission − Sender waits for DIFS (Distributed Inter-Frame Space) and transmits RTS frame containing the duration of the entire exchange

2. NAV Setting − All stations hearing the RTS set their NAV timers to the specified duration

3. CTS Response − Receiver waits for SIFS (Short Inter-Frame Space) and sends CTS with updated duration

4. Data Transmission − Sender transmits data frame after SIFS

5. ACK Frame − Receiver sends acknowledgment after SIFS

6. Channel Release − NAV timers expire, making the channel available for contention

Advantages of NAV

• Collision Avoidance − Prevents hidden terminal collisions by virtual carrier sensing

• Energy Conservation − Eliminates need for continuous physical carrier sensing during reserved periods

• Improved Efficiency − Reduces overhead by preventing unnecessary transmission attempts

• Fair Channel Access − Ensures all stations respect ongoing transmissions

Energy Conservation through NAV

Wireless stations are typically battery-powered, making energy conservation critical. Without NAV, mobile stations must continuously monitor the channel to detect transmissions, consuming significant power. The NAV mechanism allows stations to enter power-saving mode during reserved periods, as no carrier sensing is required when NAV is non-zero.

Conclusion

The Network Allocation Vector is a crucial component of wireless MAC protocols that provides virtual carrier sensing to avoid collisions and conserve energy. By implementing a countdown timer mechanism, NAV enables efficient channel reservation and helps solve the hidden terminal problem in wireless networks.