 SHA-1 (Secure Hash Algorithm-1) A message authentication mechanism that hashes a file of arbitrary length into a 160-bit value. It is more processor intensive than other mechanisms (such as MD5) but renders greater security. Glossary SHDSL (Symmetric High-Speed DSL) SHDSL (Symmetric High-Speed DSL) See G.SHDSL. SHF (superhigh frequency) Frequencies from 3GHz to 30GHz. shielded cable A cable in which the signal-carrying wire is enclosed by an outer sheath to reduce the effects of EMI on the signals. The shielding also reduces the effect of these signals on nearby electrical components and helps to prevent electronic eavesdropping. ships-in-the-night routing An approach to routing multiple protocols by which each pro- tocol is routed based on a separate routing mechanism. For example, IP routed via OSPF and Connectionless Network Protocol (CLNP) routed via IS-IS might be used to handle the routing for both IP and CLNP. sideband The frequency band on either the upper or lower side of the carrier frequency band within which the frequencies produced by the process of modulation fall. Various modulation techniques make use of one or both of the sidebands, and some of them also suppress the carrier frequency. signal A physical, time-dependent energy value used for conveying information through a transmission line. signaling The process by which a caller or equipment on the transmitting end of a line informs a particular party or equipment at the receiving end that a message is to be communicated. SIM (subscriber identity module) card A smart card that defines the accounting and personal details of a service and can be used in any GSM handset to activate service. SIMPLE (SIP for Instant Messaging and Presence Leveraging Extensions) A set of SIP extensions for IM and presence capabilities. simplex Pertaining to the capability to transmit in one direction only. simplex circuit A circuit that permits the transmission of signals in one specified direction only. single-attached station A station connected to an FDDI network 