The hidden node problem in wireless arises from a faulty way of devices talking to each other. This causes a lot of network collisions.
Simply put, collisions are when two network devices try to talk at the same time. This is bad, as it means that the receiving end can’t understand what just happened – and both senders have to try again. It becomes especially troublesome at prime-time as more clients are attempting to use the network simultaneously, and collisions are often the primary cause of prime-time slowdowns.
CSMA: The default won’t work for WISPs
802.11-based equipment is great because of its low cost and wide compatibility, however the protocol was never designed with long-range WISPs in mind. It was designed for a coffee shop or home environment. By default, 802.11 uses a scheme called “CSMA” (Carrier Sense Multiple Access) to avoid collisions, which works as follows:
- Client: “I don’t hear anyone so I’m going to talk now”.
This works great in coffee shops because each device can hear every other device – they’re all using small omnidirectional antennas. With long range WISPs, all the clients have very directional antennas and because of this they can hear the access point (AP) but not each other. This means when they listen using CSMA, they hear nothing, so they can always talk. From the AP’s point of view, everyone is talking at once, so it constantly has to say “try that again” (aka. “retransmit”). This situation where an AP can hear a device but nobody else is able to, is called “hidden node” – because the node is hidden from other clients.
Standards-based solution: RTS/CTS
Author: Matt Beaton