Most routers feature an “Advanced Settings” section that manages how data packets are handled on the network. The 802.11 standard includes the RTS (Request to Send) threshold function to control station access to the medium. Generally, high-end wireless LANs offer the RTS threshold in radio network interface cards (NICs) and access points — the function is not available in SOHO or inexpensive home products. RTS is used with CTS (Clear to Send).
Understanding RTS/CTS
Through fine-tuning of RTS/CTS, you can optimize the performance of your wireless LAN network. When you activate RTS/CTS on a station, it refrains from sending a data frame until that particular station completes the RTS/CTS connection with another station. Then a station starts the process by transferring the RTS frame. The access point will transfer a CTS frame after it receives the RTS frame. The CTS features a time value that will alert other stations to hold access to the medium while the station that initiates the RTS transmits the data. The RTS/CTS connection provides a framework for the shared medium. The primary reason for implementing this function is to minimize the collision between hidden stations, which occurs when access points and wireless users are spread out in a location and there is a high occurrence of retransmissions on the wireless LAN. The RTS function will then determine if the wireless access point should use CSMA/CD (Carrier Sense Multiple Access with Collision Detection) or CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) mechanism for packet transmission. In the first mechanism, the transmission station will send out the actual packet after waiting for the silence period, while in the latter, the transmission station sends out an RTS packet to the receiving station and waits until the receiving station transmits a CTS packet before sending the actual packet data. If the CTS packet isn’t received within a certain period of time, the transmitting device resends the RTS packet.
Activating RTS/CTS mechanism
If you suspect poor performance of your wireless network because of too many users connecting to the network, interference or collisions, you can halve the RTS threshold and do the same again until there is an improvement in network performance. However, it is important to note that an increase in performance using this mechanism is the net result of adding overhead in the form of RTS/CTS frames and reducing the number of retransmissions. Considering hidden nodes is also important. If there aren’t any hidden nodes, the network may suffer from reduced throughput when RTS/CTS is enabled. A hidden node problem may also have a negative effect on the performance, although the instance of negative impact on performance is quite rare.
Here are some tips to determine whether you should enable RTS/CTS:
Monitor the wireless network for collisions: If there are a large number of collisions and users are far apart, then activate the RTS/CTS in user setup screens. After an RTS frame is received from a user’s radio NIC, the access point will respond with a CTS frame. Check user mobility: A highly mobile user can remain hidden for a short time period during testing, then get closer to other stations most of the time. If collisions occur between users within range of each other, the issue may be the result of RF interference or high network utilization. Check for hidden station problem: In several instances, enabling RTS/CTS in the access point is of no use if the hidden station issue doesn’t exist from the access point perspective. All stations with valid associations are in range and not hidden from the access point; forcing the access point to implement the RTS/CTS connection will reduce throughput. That’s why a network administrator is recommended to focus on using RTS/CTS in the NICs to improve network performance. If there is no hidden node problem, then changing the threshold may not improve performance.
Note: the method for activating RTS/CTS on access points differs from enabling it with NICs. For access points, a specific packet size threshold needs to be set in the user configuration interface. The threshold range is around 0 – 2347 bytes; if the packet transmitted by the access point is larger than the threshold, the RTS/CTS function will initiate. If the packet size is less than the threshold or below average, the function will not initiate.
RTS & fragmentation threshold for improved performance
The recommended standard of the RTS threshold is around 500. A low threshold implies RTS packets are transferred more frequently and the throughput of the packet is on the lower side. However, sending more RTS packets can recover the network from collisions or interference that may occur during electromagnetic interference or overload of traffic on a network. The RTS threshold should always be adjusted in a small amount at a time. After enabling each change, take enough time to consider that the change in network performance is positive before making another adjustment. If the value is lowered too much, you may introduce more latency into the network. This is because the increase in Requests to Send will reserve the shared medium more often than required. If there are many users far from the access point, you can lower the threshold to 2304 bytes and measure the results. After you find success with the RTS threshold, you can also fine-tune the fragmentation threshold to see if there is further improvement in performance. The default size of the fragmentation threshold is 2346 bytes and the standard range is 256 – 2346 bytes. It is used to specify the maximum size for a data packet before being fragmented into multiple packets. Functioning similarly to the RTS threshold, if there are frequent collisions on the network, a user can consider lowering the value of the fragment threshold. A too-low threshold or a misuse of this threshold may negatively affect the performance, so minor changes in the value are recommended. Similarly to the RTS threshold case, this threshold can be set to 2304 bytes to test the results. RTS and fragmentation threshold tuning must be performed one by one and then together, until the user is able to identify the result of each configuration and perform further tuning that improves network performance.
RTS/CTS helping WLAN performance in the real world
In hotels, apartment complexes and offices, there will be often users occupying each channel of a wireless network. In these cases, RTS/CTS will help WLAN performance by eliminating collisions. RTS/CTS does add overhead to the channel as RTS/CTS don’t host data, but they usually help in a crowded place. With a faster RTS threshold, a wireless network can recover from issues faster. If fine-tuning the values of this Advanced Setting causes irregularity in data flow, the values should be set between 256 and 2346 until the data flow is normalized. In the case of the fragmentation threshold, smaller transmissions usually result in better performance in environments with lots of interference around. However, even with significant interference, the results should be tested: in the case where packet transfer error rates are high, values should be set between 1500 and 2346 until transfer rates come back to normal. When both RTS and fragment are set at maximum, they are basically disabled. So adjusting the value is a balancing act between losing overall wireless LAN speed and getting the problem fixed. For testing the RTS threshold in your workplace/home, 802.11 simulation tools such as Opnet can help in determining the optimum RTS threshold sizes, but a simulation network that mimics your network needs to be developed. This is because it can be difficult to portray the collisions and RF interference on the actual network. Therefore, you’re recommended to fine-tune the actual wireless LAN in conjunction with testing results. The RTS/CTS simulation method is superior to the basic access method to see results on a prior basis and then tuning the mechanism exactly as it is required. As with any 802.11 tuning configuration, the goal is to optimize network performance so all users get better connectivity. If you fine-tune a setting that improves throughput and your device driver or WLAN utility displays retries and packets, then you’re likely on the right track.
