e attached to port 2 is out for the day, nobody else can make use of time slot 2. Hence, there is a tendency to waste bandwidth when vacant slots occur because of idle stations. However, this type of TDM is more efficient than standard FDM because more subchannels can be derived. Time division muxes are used a great deal in the PSTN, with two gener- ations currently in place: those used in the Plesiochronous Digital Hierarchy (PDH) infrastructure, better known as T-/E-/J-carrier muxes, and those used in the SDH/SONET optical networks (described in Chapter 4). FDM and TDM can be combined. For example, you could use FDM to carve out individual channels and then within each of those channels apply TDM to carry multiple conversations on each channel. This is how some digital cellular systems work (e.g., Global System for Mobile Communications [GSM]). Digital cellular systems are discussed in Chapter 14, "Wireless WANs." STDM was introduced to overcome the limitation of standard TDM, in which sta- tions cannot use each other's time slots. Statistical time division multiplexers, sometimes called statistical muxes or stat muxes, dynamically allocate the time slots among the active terminals, which means you can actually have more terminals than you have time slots (see Figure 1.16). A stat mux is a smarter mux, and it has more memory than other muxes, SO if all the time slots are busy, excess data goes into a buffer. If the buffer fills up, the additional access data gets lost, so it is important to think about how much traffic to put through a stat mux in order to maintain performance variables. Dynamically Chapter 1 Telecommunications Technology Fundamentals Dynamically Allocated Time Slots Personal Central Computers Computer Port 1 Port 2 Port 3 Port N Statistical 4-wire Statistical Leased Data Communication Equipment (modems, data service units) Figure 1.16 STDM allocating the time slots enables you to make the most efficient use of bandwidth. Furthermore, because these are smarter muxes, they have the additional in