lation that produces one bit per symbol time. GSM uses GMSK. GPRS (General Packet Radio Service) An always-on nonvoice value-added service that enables information to be sent and received across a mobile telephone network via GSM phones. GPS (global positioning system) A navigation system in which satellites broadcast pre- cise timing signals by radio to GPS receivers, allowing them to accurately determine their loca- tion in any weather, at any time of day, anywhere on earth. GRE (Generic Routing Encapsulation) A simple stateless protocol that allows for the tunneling of IP in IP. GRE tunnels can be used to form VPNs, connecting remote sites by using private IP addresses via a public network. grooming grooming The process of selectively removing channels from a digital facility for routing to a designated remote location via another digital facility. Grooming basically enables the flexible dropping and adding of payload. ground circuit (1) A circuit in which energy is carried one way over a metallic path and returned through the earth. (2) A circuit connected to the earth at one or more points. ground start A signaling method whereby one station detects that a circuit is grounded at the other end. Group 3 fax An ITU-T standard for encoding an image and transmitting it over dialup lines. Group 4 fax An ITU-T standard for encoding an image and transmitting it over ISDN or other wideband digitized services. G.SHDSL (Symmetric High-Speed DSL) A member of the DSL family that offers sym- metrical service with options to operate over one pair or two pairs of copper wires, and also has rate-adaptive capability. G.SHDSL supports data rates of up to 5.6Mbps in each direction over a distance of up to 2 miles (3 km). G.SHDSL was the first international standard for DSL. Also referred to as SHDSL. GSM (Global System for Mobile Communications) A European standard for 2G wire- less digital communications that is globally implemented and supports both voice and data com- munications. GSM operates in several frequency bands: GSM