Simplex, Duplex and Infrared
Simplex, Duplex and Infrared are terms you will hear. They are related to various types of untethered or walk-around throttles.
First, the basics.
Simplex is the simplest method of communication technology. Your car radio is a good example, as it only receives a signal, it cannot transmit.
Some throttles are simplex in nature, in that they can send signals to the layout, but cannot get feedback from the layout. Often, they have to be physically connected to the throttle bus in order to do certain things, like select or dispatch a locomotive. Many throttles will have a cable which will allow the connection to be made. They can function fully tethered, or in wireless mode.
A duplex (Full Duplex) throttle allows both functions to occur at the same time. Such as a telephone. It has both receive and transmit sections that do not share components. Duplex throttles often have all the features of a tethered throttle, without the need to be tethered (physically connected) at times.
This is a slightly more complex method than simplex, where the handheld or portable device can transmit or receive, but not at the same time. Such as the press-to-talk button on a portable radio. The button simply reconfigures the circuitry from listen to talk mode. Much like a tape recorder that has a single record/play head, and a number of components are shared between both functions, so they cannot do both at once.
The advantage of radio based wireless systems is you often only need one receiver or receiver/transmitter pair to get decent results. Some also incorporate IR capabilities too.
Infra-red or IR is a method of communication using light instead of radio waves. It is part of the spectrum just below visible light (the low end is red), ranging from 300 Gigahertz to 430 Terahertz. For comparison, microwaves are 300MHz to 300GHz. Infrared is felt as heat in sunlight. Ultraviolet, the radiation that gives you a suntan, is from 790THz and up. It is above the visible spectrum. Radio waves are found below 300GHz.
IR is used for remote controls for televisions, Blu-ray players and other electronic devices. It is a simple technology to implement, but it has disadvantages. It can be very directional, with limited range. It also does not pass through walls or other barriers. Fluorescent lighting can throw off enough IR to interfere with remote controls, and the sun radiates so much IR it will swamp a receiver.
Some throttles have an IR feature, which may be simplex or duplex. IR Receivers are needed, and must be placed so they can see the IR signals from the throttle. Unlike radio systems you will probably need multiple receivers for satisfactory operation. Many operators place a few of their IR receivers high up in the layout room to minimize signal disruptions caused by something blocking the transmitter's path to the receiver. Same concept as your TV, where you might have to hold the remote in a certain way to get a clear path to the TV.
Interference will be a problem. Many Radio and Digital devices will have a label that says ICES-XXXX. ICES means Interference Causing Emission Standard. Basically, you may not interfere with other devices, and you must accept any interference.
With IR, interference will come from any radiation source that emits light, such as the sun or fluorescent lighting. If flourescent lights cause erratic operation, replacing them with a different colour temperature may help.
With radio based throttles, many of them share the same spectrum as other devices in what is known as the ISM Band (Industrial, Scientific, Medical). Devices such as cordless phones, baby monitors, microwave ovens and WiFi networks also occupy this band. Low power devices, which meet certain technical requirements, are permitted to use this band, and may interfere with other devices nearby.
Another source of interference is wireless throttles from another manufacturer. This is often seen during train shows with layouts using wireless DCC systems from a variety of manufacturers, which will sometimes cause problems by being on the same or a nearby frequency. The same way pushing PLAY on the VCR remote caused the TV to mute or the cable box to change channel. The device received a command not meant for it, but which it understood (incorrectly). Or it will just cause enough interference, and be powerful enough to render a nearby layout useless while feeling no effects from other wireless systems around it. It is also possible that the offending transmitter has a wider channel that spills over onto other channels in use. There is not much that can be done other than aligning antennas and physically separating the offenders to reduce the chances of interference occuring.
Some WiFi base stations allow you to define the channels being used to reduce or eliminate conflicts with other devices. This can also help with things like smartphones or other WiFi devices being used as throttles.
There are also issues related to reception, both by the base stations and the throttles. Walls, especially concrete, or those with metal studs can attenuate the RF energy passing through them. Positioning and alignment of receivers/transmitters can also be an issue, but with experimentation can often be mitigated. Sometimes adding additional base stations can improve operations. Even the position of the operator(s) can have some influence.
Infrared (IR) uses light, so anything that blocks light is an issue. Mounting recievers in the ceiling helps, as does positioning them with the least amount of obstruction between them and the transmitter. Again, additional recievers can solve some issues while increasing reliablity.