Chapter 10: Troubleshooting
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.
The least frequent cause of problems will be the hardware of the converter itself. This statement is not due
to a narcissistic point of view, but rather to the simplicity of configuration features on the converter and the
low-component count. A microwave radio link, for example, has waveguides that can fill with water,
antennas that can become misaligned, foreign objects that can block the path. From a failure point of view,
the converter is a simpler device.
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.
Warnings
•
The LAN port may become inoperable/inaccessible once loopback testing is initiated.
“Sophisticated” switches and routers monitor incoming LAN MAC addresses, and when an
incoming address matches the outgoing address being sent (as in the case of loopback), the
switch or router will often disable the LAN port as a flood-prevention measure. This can
result in an inability to further access the converter through the switch or router.
•
The local converter's configuration setting will affect whether LAN packets sent to the DS3
are returned to the LAN during DS3 loopback. At the web screen settings of the converter,
refer to the “Packet Flow” section and the “Loopback: Disable to LAN Port...traffic when
loopback detected” setting.
•
A remote converter that has been put into loopback mode will no longer be accessible for
management across the DS3 until a FEAC Reset code is sent on a C-Bit-framed DS3 link.
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
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