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:
Used around 1915. The phone company line from the pole connected at the top binding posts marked “L” for “Line”, the house wiring connected to “I” for “Inside” and G for Ground. In later years the Bell System added copper straps that bypassed the fuses… unsure why.The Western Electric Type 58A Protector, circa 1900, protects against lightning and other high voltages.
After that they went to the carbon and porcelain blocks which were much smaller:
The carbon and porcelain block! In the presence of high voltage, the carbon blocks under the pressure of the spring would be moved to earth after the restraining glue melted. The whole thing had to be replaced.In the 1980s, Ma Bell started using a gas-filled protector. In the presence of high voltage, the gas ionizes and provides a path to earth (ground). When the voltage is removed, the protector returns to its normal state. Not a one shot deal like the older carbon ones.
They got fancy with mounts & attached them to the side of your house:
Use before the advent of Network Interfaces.
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.
Internal CSU/DSU WIC
External CSU/DSU Universal Shelf
External CSU/DSU
The functions of the LEDs
LED
Description
TD
Data is being transmitted to the DTE interface.
RD
Data is being received from the DTE interface.
LP
Internal DSU/CSU is in loopback mode.
AL
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.
CD
Internal DSU/CSU in the WIC is communicating with another DSU/CSU. This LED is on during normal operation
Push this button to place the WIC into loopback mode. The service provider can send a signal to test the connection from your site to the central office switch. Push this button again to turn off loopback mode.
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…
Power, PWR
Power
ERR, ER
Error indicator
AL, ALARM
Critical alarm indicator
Loop, LP
Diagnostic loopback indicator
SYNC, RS
DTE sync indicator (Receive signa from telco)
TD, TX, or TXD
Transmit data
RD, RX, or RXD
Receive data
CTS
Clear to send (per flow control)
CLOS
Carrier loss of signal
RLOS
Receiver loss of signal
What you could see
Scenario
Power
Err
Alarm
Loop
Sync
TD
RD
CTS
CLOS
RLOS
Description
1
flash
flash
Normal – up and passing traffic
2
Loopback mode detected from telco or configured in CPE
3
flash
flash
flash
Circuit is experiencing errors, but still passing traffic.
4
CSU detects a total disconnect. Circuit disconnected/no cable.
5
Carrier loss of signal. Possible timing, switch misconfiguration, or circuit degradation
6
Receiver loss of signal. Possible timing, switch misconfiguration
