Dynamic Protection MechanismsBy CWNP On 11/14/2008 - 8 Comments
Protection mechanisms are essential to the interoperability of multiple PHY's, like 802.11b on an 802.11g network. The problem with protection mechanisms is that they are very static, either on or off. Either you don't allow a "mixed mode" environment or your do allow it and consequently run the risk of losing 40+% of your throughput. What is the answer you ask?
Right now, you have two choices when it comes to protection. Either allow protection mode to automatically kick on or, turn it off completely, most of the time losing the ability to support legacy devices. I am proposing that vendors implement a new feature I call dynamic protection.
First, let’s explore the problem.
You may still want to support 11b devices for guest access or those few 11b devices still straggling on your network; however, you are penalized either way. 11g AP’s will detect those slow-going 11b devices and implement protection as necessary. Protection mechanisms change the operation of the entire 11g BSS. Without going into crazy whitepaper detail, it will reduce the throughput of a given cell by approximately 35 – 45%, depending on the speed at which the CTS frame is sent. (Ok, that was whitepaper-level detail.) This throughput loss exists even if the 11b device doesn’t transmit much at all. Even a mouse-quiet device will invoke protection - and boom!...you are out 40% of your throughput. What's worse, it doesn’t stop there. Other 11g APs in the area will also use protection, even if they don’t have an 11b device in the area. For more (insane) detail on this topic, see Devin’s whitepaper on the protection ripple effect. In today’s environment of very few 11b devices, protection mechanisms can be more of a problem than a solution.
The argument is, 'without protection, 11b devices will cause collisions on the network' and they can. However, it's often a matter of 'acceptable loss'. If a few 11b devices on your network cause 3% extra collisions, do you want to protect your network by using a system that will guarantee that you lose 40% throughput? No way, not for me.
So, to the answer... The more frames an 11b device transmits, the more collisions it will cause. So the answer is simple: Have a configuration setting on the Controller/AP that has a threshold. If the number frames from an 11b device exceeds X value per second (configurable by the administrator) then invoke protection. Otherwise, let the 11b devices associate and operate without sending the entire system into protection.