The demarcation point (DMARC) is the point at which the public switched telephone network (PSTN)ends and connects with the customer’s on-premises wiring (called Inside Wire or IW.) Don’t confuse this with a NIU, it’s the same but different.
For residential locations you might have started off with an old Western Electric Company (WECO) lightning protector:
After that they went to the carbon and porcelain blocks which were much smaller:
They got fancy with mounts & attached them to the side of your house:
Then we come to the modern Network Interface Device, Telephone Network Interface, NID, NI, dmarc:
A Data Service Unit/Channel Service Unit (DSU/CSU) WAN Interface Card ( WIC) these days is usually a blade on a router. In the past, these were separate. The CSU originated at AT&T as an interface to their non-switched digital data system. The DSU provides an interface to the data terminal equipment (DTE) using a standard (EIA/CCITT) interface. It also provides testing capabilities. They evolved from standalone hardware, to shelf type systems and are now just a blade or Wan Interfacde Card (WIC) in a router.
The functions of the LEDs
Data is being transmitted to the DTE interface.
Data is being received from the DTE interface.
Internal DSU/CSU is in loopback mode.
One of these alarm conditions is present: no receive signal, loss of frame signal from the remote station, or out of service signal from the remote station. This LED is off during normal operation.
Internal DSU/CSU in the WIC is communicating with another DSU/CSU. This LED is on during normal operation
There are many manufactures, each with their own ideas of abbreviations so TD, TX, or TXD all mean you’re transmitting data. You may not have every LED but in general…
Critical alarm indicator
Diagnostic loopback indicator
DTE sync indicator (Receive signa from telco)
TD, TX, or TXD
RD, RX, or RXD
Clear to send (per flow control)
Carrier loss of signal
Receiver loss of signal
What you could see
Normal – up and passing traffic
Loopback mode detected from telco or configured in CPE
Circuit is experiencing errors, but still passing traffic.
CSU detects a total disconnect. Circuit disconnected/no cable.
Carrier loss of signal. Possible timing, switch misconfiguration, or circuit degradation
Receiver loss of signal. Possible timing, switch misconfiguration