CAS Signaling Types
CAS supports the following signaling types:
Loop Start Signaling
Loopstart signaling is one of the simplest forms of CAS signaling. When a handset is picked up, this action closes the circuit that draws current from the telephone company Central Office (CO) and indicates a change in status, which signals the CO to provide dial tone. An incoming call is signaled from the CO to the handset by sending a signal in a standard on/off pattern, which causes the telephone to ring.
Ground Start Signaling
Groundstart signaling is similar to loopstart signaling in many regards. It works by using ground and current detectors that allow the network to indicate off-hook or seizure of an incoming call independent of the ringing signal and allow for positive recognition of connects and disconnects. For this reason, ground start signaling is typically used on trunk lines between PBXs and in businesses where call volume on loop start lines can result in glare.
The advantage of groundstart signaling over loopstart signaling is that it provides far-end disconnect supervision. Another advantage of groundstart signaling is the ability for incoming calls (network ––> CPE) to seize the outgoing channel, thereby preventing a glare situation from occurring.
E and M Signaling
E&M Signaling is typically used for trunk lines. The signaling paths are known as the E-lead and the M-lead. Descriptions such as Ear and Mouth were adopted to help field personnel determine the direction of a signal in a wire. E&M connections from routers to telephone switches or to PBXs are preferable to FXS/FXO connections because E&M provides better answer and disconnect supervision.
E&M signaling has many advantages over the previous CAS signaling methods that are discussed in this document. It provides both disconnect and answers supervision and glare avoidance. E&M signaling is simple to understand and is the preferred choice when you use CAS.
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