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130 SIGNALING IN ACCESS NETWORKSTime-Slot Management. This function assigns and/or activates time slots in adigital link for connections to analog or ISDN lines, then releases and/or deactivatesthem when they are no longer needed. With statistical multiplexing (Section 1.8),signaling handles the dynamic assignment of time slots to lines at the start of acall and their release at the end of it. With fixed (semipermanent) assignment,signaling handles the activation and deactivation of preset connections of timeslots to lines.Call Control for Analog Lines. This function starts after a line has beenassigned a time slot and the connection between line and TS has been activated. Subscriberloop signals from the subscriber are converted to digital signals that can becarried by the digital link; digital signals from the LE are converted to analog signalsthat can be applied toward the subscriber. DTMF signals from subscribers andaudible signals from the LE (such as dial-tone and busy-tone) go through the AStransparently, like voice.Call-control signaling for analog lines is based on stimulus protocols. Mostsignaling protocols, both common-channel and channel-associated, are functionalprotocols, requiring equal processing complexity in sender and receiver. Stimulusprotocols, on the other hand, use simple commands that require only one end tokeep track of call states, thus keeping complexity at the other end to a minimum.In access networks, stimulus signaling keeps call processing simple in the AS andconcentrates complexity in the LE. Signaling from the AS to the LE consists of indicationsof changes detected in the electrical configuration of the subscriber line (e.g.,open or closed loop). Signaling from the LE to the AS consists of instructions todeploy electrical configurations (e.g., ringing current or reverse polarity) towardthe subscriber line.Protection Switching. This function is the ability to maintain communicationbetween the AS and the LE when a digital link fails. Protection is realized byconfiguring active (protected) and standby (protecting) channels. Obviously, thisfunction applies only to multilink configurations, and protecting and protectedchannels must be in different links.Call Control for ISDN Lines. In addition to analog lines, ASs support digitalsubscriber lines that use ISDN (DSS1) signaling (Chapter 10). ISDN call-controlsignaling is carried in D-channel messages, which are transported in time slotsassigned semipermanently by provisioning and transit through the AS with littleor no involvement by AS signaling.Signaling Protocols. The two most commonly used AS protocols are:. GR-303, issued by Telcordia. V5.1 and V5.2, issued by ITU-T and ETSI