There are two carrier sense mechanisms used on wireless networks. The first is physical carrier sense. Physical carrier sense functions by checking the signal strength, called the Received Signal Strength Indicator (RSSI), on the RF carrier signal to see if there is a station currently transmitting. The second is virtual carrier sense. Virtual carrier sense works by using a field called the Network Allocation Vector (NAV), which acts as a timer on the station. If a station wishes to broadcast its intention to use the network, the station sends a frame to the destination station, which will set the NAV field on all stations hearing the frame to the time necessary for the station to complete its transmission, plus the returning ACK frame. In this way, any station can reserve use of the network for specified periods of time. Virtual carrier sense is implemented with the RTS/CTS protocol.
The RTS/CTS protocol is an extension of the CSMA/CA protocol. As the wireless LAN administrator, you can take advantage of using this protocol to solve problems like Hidden Node (discussed in Chapter 9, Troubleshooting). Using RTS/CTS allows stations to broadcast their intent to send data across the network.
As you can imagine by the brief description above, RTS/CTS will cause significant network overhead. For this reason RTS/CTS is turned OFF by default on a wireless LAN. If you are experiencing an unusual amount of collisions on your wireless LAN (evidenced by high latency and low throughput) using RTS/CTS can actually increase the traffic flow on the network by decreasing the number of collisions. Use of RTS/CTS should not be done haphazardly. RTS/CTS should be configured after careful study of the network's collisions, throughput, latency, etc.
Figure 8.7 illustrates the 4-way handshake process used for RTS/CTS. In short, the transmitting station broadcasts the RTS, followed by the CTS reply from the receiving station, both of which go through the access point. Next, the transmitting station sends its data payload through the access point to the receiving station, which immediately replies with an acknowledgement frame, or ACK. This process is used for every frame that is sent across the wireless network.
Configuring RTS/CTS
There are three settings on most access points and nodes for RTS/CTS:
- Off
- On
- On with Threshold
When RTS/CTS is turned on, every packet that goes through the wireless network is announced and cleared between the transmitting and receiving nodes prior to transmission, creating a significant amount of overhead and significantly less throughput. Generally, RTS/CTS should only be used in diagnosing network problems and when only very large packets are flowing across a congested wireless network, which is rare.
However, the “on with threshold” setting allows the administrator to control which packets (over a certain size - called the threshold) are announced and cleared to send by the stations. Since collisions affect larger packets more than smaller ones, you can set the RTS/CTS threshold to work only when a node wishes to send packets over a certain size. This setting allows you to customize the RTS/CTS setting to your network data traffic and optimize the throughput of your wireless LAN while preventing problems like Hidden Node.
Figure 8.8 depicts a DCF network using the RTS/CTS protocol to transmit data. Notice that the RTS and CTS transmissions are spaced by SIFS. The NAV is set with RTS on all nodes, and then reset on all nodes by the immediately following CTS.
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