TDMA users share a common frequency channel but use the channel for only a very short time. They are given a time slot and only allowed to transmit during that time slot. When all available time slots in a given frequency are used, the next user must be assigned a time slot on another frequency. These time slices are so small that the human ear does not perceive the time slicing. Examples of TDMA-based systems are the following:
- Global System for Mobile communications (GSM) in Europe
- TDMA IS-136 in US
In TDMA one user can transmit and receive data only during the predetermined time interval.
Some networks provide Visitor Location Registers (VLR) at their MSCs (Mobile Switching Center), such that, one an MS (Mobile Station) roams from a different traffic area, the subscriber information from its HLR (Home Location Register) can be transmitted to and temporarily stored in the VLR. VLRs prevent unnecessary, potentially long-distance, repeated communication with the HLR.
Each MSC (Mobile Switching Center) has access to a database called Home Location Register (HLR). This contains information on all subscribers who are initially registered within its traffic area. This information includes the subscriber identity numbers, MS (Mobile Station) identity numbers, service subscription details, usual and present traffic area locations plus authentication and encryption keys.