Carrier Sense
Understanding Carrier Sense
Media Access Control and Collision Avoidance
In shared-medium communication networks, multiple transceivers share a common frequency band. To prevent simultaneous transmissions from corrupting packets (collisions), devices implement Carrier Sense Multiple Access (CSMA) protocols. Before transmitting, the RF frontend goes into a listening state. If carrier sense detects a signal with power above a specified threshold, the node defers its transmission, backing off for a random period before checking the channel again.
Wi-Fi networks (IEEE 802.11) utilize CSMA with Collision Avoidance (CSMA/CA). Because wireless transceivers cannot listen for collisions while transmitting (as the local transmit power dwarfs any incoming signal), they rely on active prevention. The system combines physical carrier sensing with virtual carrier sensing. This virtual mechanism uses reservation structures in packets to block other transmitters, minimizing the likelihood of collisions.
Physical and Virtual Sensing Methods
Physical carrier sense involves two techniques: Energy Detection (ED) and Carrier Detection (CD). Energy detection measures the raw RF power in the channel, identifying non-Wi-Fi interferers or distant signals. Carrier detection decodes the preamble of incoming Wi-Fi frames, identifying active transmissions at lower power levels. Virtual carrier sense uses the Network Allocation Vector (NAV), a timer updated by duration fields in overhead frames like Request to Send (RTS) and Clear to Send (CTS).
Key Mathematical Relations
Technical Specifications Comparison
| Carrier Sense Type | Mechanism | Threshold Reference | Target Source |
|---|---|---|---|
| Energy Detection (ED) | Measures total RF energy in the channel | Fixed (e.g., -62 dBm for non-associated signals) | Microwaves, Bluetooth, unlicensed interferers |
| Carrier Detection (CD) | Decodes the frame preamble structure | Sensitive (e.g., -82 dBm for standard Wi-Fi) | Co-channel Wi-Fi transmitters |
| Virtual Sensing (NAV) | Decodes packet duration values in MAC headers | Logical reservation timer | Hidden nodes, controlled network exchanges |
Frequently Asked Questions
How does physical carrier sense differ from virtual carrier sense?
Physical carrier sense directly measures the RF energy level on the antenna or decodes the packet preamble to determine if the channel is busy. Virtual carrier sense uses packet duration info (such as the Network Allocation Vector in Wi-Fi) to predict channel availability.
What is the energy detection threshold in carrier sensing?
The energy detection threshold is the minimum power level of RF noise or signals required for a device to mark the channel as busy. In Wi-Fi systems, this is typically set around -62 dBm to -82 dBm depending on the channel width and packet preamble detection.
How does carrier sense handle the hidden node problem?
Standard carrier sense cannot detect a transmitter that is out of range, leading to collisions at the receiver. Systems solve this by using Request to Send (RTS) and Clear to Send (CTS) frames, which establish a virtual carrier sense period for all nearby nodes.