smaller coverage areas, known as picocells, enabling indoor coverage in places such as subway and train stations, shopping malls, and offices. This is very important in Japan and serves as a key differentiator for PDC networks. This PCS version of PDC is also commonly known as Personal Handyphone System (PHS). PDC offers services that are quite similar to those of GSM, including voice (full and half rate) and the usual supplementary services, such as call forwarding, call waiting, caller ID, three-way conferencing, voice mail, and text mail. It also sup- ports Circuit-Switched Data (CSD) service at up to 9.6Kbps as well as PDC-P, a packet-switched wireless data service that offers data transfer rates of up to 28.8Kbps. Using intelligent network capabilities, PDC also supports prepaid calling, personal numbers, universal access numbers, advanced charging schemes, and wireless vir- tual private networks (VPNs). As with all other 2G technologies, PDC is steadily losing ground to more robust and high-speed 3G networks, as well as various emerging broadband wireless tech- nologies (which are covered in Chapter 15, "WMANs, WLANs, and WPANs"). 2.5G: Enhanced Data Services 2.5G is the realm of enhanced data services, and four primary techniques fall under this category: General Packet Radio Service (GPRS), High-Speed Circuit-Switched Data (HSCSD), Enhanced Data Rates for Global Evolution (EDGE), and the GSM 2.5G: Enhanced Data Services EDGE Radio Access Network (GERAN). These standards, as well as standards for messaging services, continue to evolve within the various working groups of the standards organizations. HSCSD HSCSD is an enhancement to GSM that uses existing circuit-switched equipment with software upgrades. The main difference between HSCSD and the original GSM data transmission mechanism lies in the ability to use different coding meth- ods as well as multiple time slots for a single user in order to increase data through- put. By using time slot aggregation, HSCSD supports data rates from 9.6Kbps to 57.6Kbp