over a single fiber pair using a concentrator as the dual-attached station. Single-attached stations can be attached to only one ring. These stations are less expensive than dual-attached stations but are less reliable because the optics, electronics, and physical link must all be operational for the single-attached station to connect to the network. single-carrier modulation A modulation scheme in which a single channel occupies the entire bandwidth. SIP (Session Initiation Protocol) An application-layer control or signaling protocol for creating, modifying, and terminating sessions with one or more participants. SIP can establish sessions for features such as audio- or videoconferencing, interactive gaming, and call forward- ing to be deployed over IP networks, thus enabling service providers to integrate basic IP tele- phony services with Web, e-mail, and chat services. SIP telephony Voice telecommunications that occur over a data network, using the SIP pro- tocol. SIP telephony enables capabilities such as presence management across both voice and instant messaging, instant messaging archiving, click-to-talk voice functionality, speech-to-text functionality, personal mobility, and time-of-day routing. skin effect skin effect An occurrence in which as electricity migrates to the medium's skin, resistance increases because less of the wire is used. SLA (service-level agreement) A contract between an ASP and an end user that stipulates and commits the ASP to a required level of service. An SLA should contain a specified level of service, support options, enforcement or penalty provisions for services not provided, a guaran- teed level of system performance as related to downtime or uptime, a specified level of customer support, and what software or hardware will be provided and for what fee. sleeve The third contacting part of a telephone plug, preceded in the location by the tip and ring. (Serial Line Internet Protocol) A minimal character-oriented protocol that provides basic IP encapsulation over dedicated or