Have phone, will travel: Inmarsat services 257 control of a Satellite Access Station (SAS), which controls and monitors the traffic based on the terminal-supplied GPS information. 6.4 Packet and circuit switching In Chapter 5, we discussed the differences between packet and circuit switching, and it is important to reiter- ate the differences, as with the BGAN system, both methods of transport are available according to the service. A telephone circuit is a single dedicated line established for the duration of the call ­ this is based on a concept called `circuit switching'. This is like a railway train that travels along one track from A to B, with a string of carriages all trailing along together. Packet switching is a system where the best (quickest) path for each railway carriage or packet is decided dynamically, re-ordered on-the-fly to use any track or path available to get from A to B. Other packets from other sources run on these circuits as well, making the most use of each path, unlike circuit switched calls that occupy a single path to the exclusion of all others. So the train starts from the departure point as a complete string of carriages, gets split up along the way, but comes back together as a single train with all the carriages in the right order at the destination. However, while packet switching can also be used for voice calls sent over digital networks, delay becomes critical. Packet switching as used for data is acceptable when accessing a web page or down- loading a file, since small delays are hardly noticed. But the human brain is particularly sensitive to