TOKEN RING FIBER EXTENDERS
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3.
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Overview Of Operating Modes
To successfully install the LCL112A-TR extenders an understanding of the basic
interoperability issues is important. This section describes those aspects of Token
Ring important to LCL112A-TR interoperability.
3.1
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Variants of Token Ring
Token Ring products have come a long way since the early 4Mbps systems based on
IBM cabling of the mid-eighties. Today there is a mix of devices supporting 4 and
16Mbps data transfer on passive and active MAUs, bridges, switches and routers
with cable choices of STP, UTP and fiber optics.
Many manufacturers have added to the complexity by introducing proprietary smart
mechanisms that increase ring integrity at the expense of interoperability.
The major issues when using a fiber extender in a network are:
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Phantom current mechanisms
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Frequency lockout on active MAUs
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Switch to switch auto-negotiation
3.1.1
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Phantom Current Mechanisms
Phantom Current mechanisms are defined by IEEE 802.5 for station to concentrator
links. Phantom Current is a DC (Direct Current) signal sent with the data down the lobe
cable to the MAU to open the port at the MAU. This was the principle that allowed early
MAUs to be unpowered devices as the controlling power came from the station. The
station measures the amount of current flowing to detect wire faults in both open and
short-circuit conditions.
All lobe links, including those between the station and concentrator ports of switches,
make use of phantom current in a standard way.
Whenever fiber is used in a link it is not possible to send the phantom current from one
end of the link directly to the far end device. The transceiver local to the station has to
mimic a MAU port and conversely the transceiver next to the MAU has to mimic a
station. Special 802.5J keying sequences are used as messages between the
transceivers on the fiber link to indicate when phantom should be generated.
