E3Switch DS3 Operating information manual - page 19
Chapter 10: Troubleshooting
The Ethernet networks to which the converter connects are complex and may contain thousands of devices,
each of which requires proper configuration and performance. As such, network configuration and
topology issues dominate when problems arise. When troubleshooting, solutions can be reached more
rapidly by remembering that the most frequent cause of problems arises from improper network
configurations.
The next most frequent source of problems can arise from the E3, T3/DS3 link configurations or faults – in
other words, the microwave radio or fiber optic link or the interface and associated configuration settings
between such equipment and the converter. It is important that the telecom link is operating in
unchannelized mode and that the circuit provider has not mixed C-Bit with M13 or auto-detect framing
among the various intermediate pieces of equipment.
The next most frequent source of problems generally arises from faulty cabling or connectors or incorrect
cable type. Cabling must be UTP5 or better for LAN and 75-Ohm rather than 50-Ohm for telecom. If long
telecom cable runs or an electrically noisy environment exists, high-quality coaxial cable will be required.
Loopback of DS3
The converters have the ability to loopback DS3 data in several ways, including responding to SNMP
requests and responding to FEAC loopback requests from intermediate carrier equipment. Remote loopback
is supported. Local loopback is not implemented – being of limited utility.
Terminology
·
Remote Loopback: A DS3 signal received at the “In” port of the converter is duplicated onto
the converter's “Out” port. In this case the DS3 signal has traversed the entire link in both
directions.
·
Local Loopback: The DS3 signal being sent at the converter's “Out” port is duplicated in
internal circuitry as if it is also being received at the “In” port.
·
FEAC Loopback Request: FEAC command bits in a C-Bit-framed DS3 can instruct equipment
receiving the DS3 signal to enter or leave remote loopback.
Limitations
Loopback is typically not the best way to diagnose or confirm a DS3 connection and can be misleading.
Typically, the web management screen or SNMP interface of the converter provides a less misleading and
more informative, sophisticated, detailed and user-friendly mechanism than a loopback test. Loopback
testing is most useful for a carrier who has few other tools at their disposal and simply wants to see if
sending a FEAC loopback request to something at the end of the line results in a loopback signal initiation.
The carrier, however, doesn't even know if they are actually connected to an E3Switch unit in such a case –
whether the loopback succeeds or fails.
·
Remote Loopback: A carrier or test equipment requesting loopback of a remote device does
not know what device they are connected to. The loopback may succeed or fail, but the
carrier may not even be on the correct DS3 line. Little information is gained. If the carrier is
receiving the E3Switch's PMDL ID signal, then the carrier has a better idea that they are on
the correct receive DS3 line, but is the converter connected to the correct DS3 cable in the
other direction? This can be better determined by examining the receive signal of the web
status screen, including the PMDL section, of the remote converter.
·
Local Loopback: This would mainly diagnose a broken receive DS3 path on the local
converter board. Such failures are extremely rare. A much better test would be a physical
local loopback using a short DS3 cable, since connectors and cables are a much more likely
source of problems.
·
FEAC Loopback Requests: All intermediate equipment on the DS3 path and the converter
must be in C-Bit mode for a FEAC request to be forwarded and acted upon.
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