CANOGA PERKINS 2240 Fiber Optic Modem - Manual (Page 16)

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Canoga Perkins
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 to be found in Section 3. 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
µ sec, respectively.
1. 4.2 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. Refer to Sections 5 and 6 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. 4.3 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.