eases the data rate by a factor of four). In addition, because all data is transmitted both in the same frequency band and with sep- arate spatial signatures, Space Division Multiplexing uses spectrum very efficiently. Also referred to as spatial multiplexing. space division switching A method for switching circuits in which each connection through the switch takes a physically separate path. spanning tree A loop-free subset of a network's topology. spatial capacity A measure of the number of bits per second per square meter that can be supported over a given technology. UWB has a much greater spatial capacity than the 802.11a and 802.11b standards and than Bluetooth. S-PCS (Satellite Personal Communications System) A system that uses satellites to provide ubiquitous mobile communications. SPE (synchronous payload envelope) The payload portion of an STS or STM frame. spectral compatibility The capability of two DSL systems to operate inside a single cable bundle. The spectral compatibility of the DSL systems depends on the effect of crosstalk that one DSL system has on the other inside the cable bundle. spectral efficiency A measure of the number of digital bits that can be encoded in a single cycle of a waveform. spectrum utilization The efficient use of the available bandwidth by many competing devices. Spectrum utilization techniques include space division, multiple-access techniques, spread spectrum, duplexing, and compression techniques. splitter A filter used at each end of a copper pair to split the frequency bands. spooling Temporarily storing input and output data streams on disk or tape files until the processor is ready. spread spectrum technique A technique by which a signal is transmitted in a bandwidth considerably greater than the frequency content of the original information. FHSS, DSSS, CDMA, and OFDM are examples of spread spectrum techniques. SPX (Sequenced Packet Exchange) The NetWare communications protocol used for interprocess communications. SPX guarantees that an entire message arrives in