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6.2 THE GR-303 STANDARD 133For path protection, the TMC/CSC and the EOC each have an active and astandby channel. When path protection is triggered, traffic is switched from theactive to the standby channel. Switching is managed via operations messages onboth active and standby channels.Facility protection switching uses a n þ 1 scheme and is managed via messageson the ESF link.Protection switching can be manual or automatic. Manual switching can beinitiated by an operations system (OS) or by the human–machine interface at theLE or AS. Automatic switching is initiated by the LE or AS, depending on whichside detects failure or deterioration in the protected resource.Signaling. GR-303 supports two signaling methods:. Hybrid signaling. Common Signaling Channel (CSC) signalingSupport of hybrid signaling is mandatory; support of CSC signaling is optional. Asignaling method applies to the whole GR-303 interface and cannot be configuredon a per-line basis.For analog lines each signaling method manages both TS assignment and callcontrol. Hybrid signaling uses the TMC for TS assignment and the channelassociatedsignaling subchannels for call control. CSC signaling (also calledout-of-band signaling) uses the CSC for both TS assignment and call control.ISDN lines use only the TS assignment function of each signaling method: theTSC protocol with hybrid signaling and the TS assignment part of the CSC protocolwith CSC signaling. The call-control part of the signaling method is not used, sincecall control is carried in D-channel messages (Section 6.2.4).Signaling on the TMC and the CSC is out-of-band and message-oriented; that is,it is a form of common-channel signaling (Chapter 5) with messages carried in adedicated TS. The protocol stack is shown in Fig. 6.2-1.6.2.1 Data Link Layer ProtocolThe data link layer protocol for the TMC and the CSC is a subset of DSS1 LAPD(Section 10.2 and [2]), which uses the multiframe transfer mode. The subset ofLAPD Command/Response Frames is listed in Table 6.2-1.Figure 6.2-1. Protocol stack for TMC and CSC signaling.