CANOGA PERKINS 2240 Fiber Optic Modem - Manual (Page 18)

Chapter 1 Introduction 2240 Fiber Optic Modem User Manual
Modem Operation
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The modem functions as a 10-channel multiplexer. The following discussion assumes an 8.19
MHz composite. Lower composite speeds result in proportionally lower submultiples. Clock and
data are carried on a 4.096 Mbps and 2.048 Mbps channel, respectively. Each of the three
control leads and five Auxiliary lines are carried on a 64 Kbps channel. The remaining 1.536 kbps
bandwidth splits into 1.024 Mbps for multiplexer synchronization, 256 kbps for low-speed channel
synchronization and
256 kbps for supervisory channels. Each 64 kbps channel can be used to carry an async data
signal if the user's equipment can tolerate the 16 microseconds of pulse distortion due to
sampling.
The composite speed of the 2240 Modem varies between 4.1 and 8.2 Mbps, depending on the
selected mode of operation. A detailed description of mode selection is in Chapter 3, "Mode and
Rate Selection". A brief description follows.
The modem has two basic external clock operating modes: Sampled and External Locked. In the
Sampled mode, the composite speed is fixed at 8.192 MHz and clock, data and control/auxiliary
channels are sampled at 4.096, 2.048 and .064 MHz, respectively. This mode is recommended
for low data speed applications (less than 128 kbps).
For the External Locked modes, the composite speed is a multiple of an external clock. For T1
and E1, the multiple is four and the resulting composite rates are 6.176 and 8.192 MHz,
respectively. Also, for the External Locked modes, the sampling frequency for the control and
auxiliary channels is 1/128th of the composite rate. Therefore, this sampling rate can vary from
32 to 64 kHz, resulting in sampling jitter of 32 to 16 msec, respectively.
1.5.2 System Test and Diagnostics
Both Local and Remote test modes can be invoked via a front panel switch. These are useful for
diagnosing system problems. See Chapter 5, "Troubleshooting" and Chapter 6, "Diagnostic
Procedures" for more details on these test modes. Two front panel LEDs, Loc and Rem Sync,
also help to isolate system problems by indicating whether the local and remote composites are
synchronized.
1.5.3 Transmit Section
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 bi-phase data stream, which in turn
is converted to an optical signal for transmission over the fiber optic cable.
The heart of the 2240 transmitter is a ten-channel multiplexer. This multiplexer takes the clock,
data and control lead inputs from the interface, multiplexes them, then adds framing and
supervisory information. This composite data is then converted into a Manchester-coded signal
which drives the modulator of the optical transmitter.
The function of the multiplexer is highly dependent on the operating mode of the modem (see
Chapter 3, "Mode and Rate Selection"). Supervisory information is related to frame
synchronization and loopback status.
1.5.4 Receive Section
An optical receiver circuit converts the incoming signal to a biphase logic signal. It is then de-
multiplexed into all necessary interface signals.