CANOGA PERKINS 2240 Fiber Optic Modem - Manual (Page 19)

Chapter 1 Introduction
Loss Budget
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The receiver first extracts the clock and data information from the Manchester-coded optical
signal. After frame-bit lock is established, the de-multiplexer separates out the clock, data and
control lead signals, as well as the supervisory information. The supervisory states are mainly
routed to control status indicators, while the remaining signals are routed to the interface circuits.
The operation of the receiver is somewhat dependent on the 2240 operating mode, but much
less dependent than the transmitter.
1.5.5 Expanded Interface Control Channels
The 400 series of 2200 Series Fiber Optic Modem Interfaces can support additional Control
Leads up to a maximum of four. There are three channels dedicated to use for Control. For
descriptions of these interfaces, see Chapter 4, "Data Interfaces". The fourth is the Aux Channel
1 input and output which is available on the expanded interface connector.
1.5.6 Expanded Interface Auxiliary Channels
The 2240 has five Auxiliary Channels. One of these channels is available on the expanded
interface connector and the other four on the Auxiliary Interface Connector (see Figure 3-4). The
MC1 and MC2 interfaces make use of all eight control and auxiliary channels (see Chapter 4,
"Data Interfaces").
1.5.7 Fiber Optics
Each interface signal input to the modem is converted to logic level for use by the modem circuit.
The logic level signal is then multiplexed and encoded into a biphase data stream, which in turn
is converted to optical signal level for transmission over the fiber optic cable.
1.6 Loss Budget
The maximum possible transmission distance is dependent on the overall power loss over the
fiber optic link. This is called the link loss. The modem’s loss budget is determined by comparing
the launch power at the modem with receiver sensitivity at the other end of the link. The
difference is the loss budget.
For reliable operation over a long term, such as several years, the link loss should be at least 3
dB less than the modem's loss budget. This allows for minor increases in link loss through
terminations and any slight deterioration in optical power output.
The connectors are clearly marked as to their function, either Transmit (Tx) or Receive (Rx), on
the back panel of the 2240 standalone units, and on the rear of the 2201 Rack Chassis.
The 2240 modem can be used with most popular sizes of multimode and single mode optic
cable, including 50/125, 62.5/125 and 8-10/125.
NOTE: When using 85/125 or 100/140 micron fiber optic cable, an inline
attenuator may need to be installed between the 2240 and the Receive
(Rx) fiber optic cable for proper modem operation.
1.7 Initial Unit Testing
The Remote and Local Sync indicators on the front panel constantly indicate link integrity. The
Local Sync indicator blinks off momentarily if an error has been detected. The Loopback Test
feature can be used to verify that the fiber optic modem link and electrical interface are installed
correctly.