When operating as a pure ADM, the 454 could add and drop circuits from OC-192 down to Digital Signal 1 (DS1) -- later it would support Wavelength Division Multiplexing (WDM).
The Cisco 2500 series routers are a series of 19" rack mount access routers typically used to connect Ethernet or token ring networks via ISDN or leased serial connections (i.e. Frame Relay, T1 etc.).
Digital Signal 1, a T-carrier signaling scheme devised by Bell Labs
The protocol accommodated for a variety of expansion boards, including trunk interfaces (usually T1 or ISDN), voice processing boards equipment speech recognition or fax processing.
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The improved AIS originated with the T-carrier system, and became a standard feature of subsequent plesiochronous and synchronous circuit-based communication systems, and is also part of the ATM standards.
This type of multiplexing is needed when the customer wants to terminate the DS1 or DS3 in the central office and wants to ‘pick up’ lower level services.
The probe passively monitors the E1/T1 or SDH/SONET bearers, and extracts the signalling information for onward presentation to a network application.