WIRELESS - Receive Start of Packet (RX-SOP)

Receive Start of Packet (RX-SOP)

The Problem: High Co-channel contention & channel utilization in high-capacity environments. The rules of co-channel interference should always be followed (avoid it!), but in HD environments it is sometimes unavoidable. What results, is typically a situation where an AP is holding off transmitting to it's clients, due to CSMA/CA. More specifically, the CCA-Carrier Sense will kick off at anything above -85 for a STA (AP or Client), and the medium determined 'busy' for the time specified by the Length value of the SIG field in the PLCP Header. Further, CCAEnergy Detect will determine the medium 'busy' at anything 20dBm stronger (-65). I'm skipping over Virtual Carrier Sense (NAV) and sticking to just the PHY for this discussion. For description of both Physical & Virtual mechanisms of CSMA/CA check here, here, here or here.

 u  Consider the following crude drawing:

 

u  This presents a situation where AP-1 "could" successfully transmit to Client-1, assuming sufficient SINR, but it does not, due to CSMA/CA.

u  The Solution:

u   RX-SOP essentially takes any frame received below the set threshold and dumps it in the Noise bucket. It's been described as tuning the AP Receive Sensitivity, or applying "Ear Muffs". Taking our example, if RX-SOP is configured at -80, AP-1 does not "hold-off", because it doesn't determine the medium as "busy" due to AP-2's transmissions. As far as AP-1 is concerned, the Medium is free to use (all be it, a bit more noisy). You can see how this could greatly improve performance in the Downlink direction.

 u  Notes:

Client behavior does not change. If a client determines the medium busy, with or without RX-SOP yields the same result (it will back-off). In other words, this does not improve the Uplink direction. If you configure just RX-SOP, without Optimized Roaming, it is possible that a "sticky client" will fail hard. If the client does nothing (but retransmit), in the absence of any ACK's, it could take a while for it to roam. Probably not a common occurrence, but possible. The success of RX-SOP is dependent on SINR. Your environment will determine whether, or how much, RX-SOP will help and what level it should be set to. I can only speculate on exactly how RX-SOP is implemented. Does the determination happen at CCA-CS; the receipt of the Short Training Field (STF) in the Preamble, or is it after receiving the full PLCP Header? Or is it happening before that? Not sure, but it's been a tool in the bag for quite some time. Check the further reading section for more.