ch the number of tones, or subcarriers, used to transport the data and signaling traffic scales with the channel bandwidth. Mobile WiMax uses SOFDMA. softswitch A device that implements service logic to control external trunking gateways, access gateways, and remote access servers. Softswitches run on commercial computers and operating systems, and they provide open applications programming interfaces. Also called a call agent or a media gateway controller. software Computer instructions that perform a common function for all users as well as spe- cific applications for particular user needs. solid-state device Electronic pathways made of solid materials (e.g., chips, bubble memories). SONET (Synchronous Optical Network) An ANSI standard, incorporated into the SDH standard, that defines a line rate hierarchy and frame formats for use with high-speed optical fiber transmission systems (50.84Mbps to 2.488Gbps). source route bridging A bridging protocol supported by the 802.5 Token Ring standard. With source routing, the sending station is responsible for providing routing information for messages that cross multiple network segments. The sending station acquires routing informa- tion by first issuing a command to find the location of a particular destination. When the mes- sage is sent, the source station specifies the route to use by using the routing information field of the frame. source routing transparent The combination of source routing and transparent bridging in the same device. SP (service provider) A generic term for providers of different types of services. space division The carving up of a cellular service area into smaller coverage areas. Space Division Multiplexing Space Division Multiplexing The use of multiple antennas to simultaneously transmit data to the receivers in small pieces. The receivers then process the data flows and put them back together. This process proportionally boosts the data transmission speed by a factor equal to number of transmitting antennas (i.e., using four antennas incr