Back-to-Back Radio (Direct Mode)
Back-to-back radios systems are the simplest of radio systems. They consist only of hand-held radios 'walkie-talkies' (or vehicle mounted radios). These talk to one another without any kind of base station or hub.
Back-to-back is the most reliable radio system, robust against failure because there is no central device which potentially could break down. The radios can be rapidly deployed in any geographical location because there is no base station infrastructure to set up.
Back-to back systems can be the lowest cost solution because there is no infrastructure. A system ideal for small businesses, small sites but also for mission critical communications such as Search and Rescue, Fire Marshalling and other ad-hoc systems operating in no fixed location.
The weakness of Back-to-back radios systems is that range is limited because every radio user is at ground level potentially obstructed from other users. Also there is no central control of radio traffic potentially leading to congestion when many radios are in use.
PMR446 Licence-free radios are a back-to-back system. Licensed radios operating without the use of a Base station or Repeater are also Back-to-back.
To improve range of back to back systems a Base Station or Repeater is normally used.