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2-16
System Overview
Transmit External/Receive Internal
This selection is made for modem "tail" circuits operating from a channel interface. The timing sig-
nal from the MEGAMUX PLUS channel is provided to the modem connected to the channel, along
with the receive data output from the channel. The modem encodes the timing signal with the data,
and transmits the combined modulated signal to the far-end modem (remember that the receive data
from the channel is the transmit data into the modem). The far-end modem decodes the timing sig-
nal, and uses the timing for both transmit and receive data. That timing is encoded by the far-end
modem with its transmit data; the combined signal is transmitted to the modem at the TDM site,
which decodes the timing signal and passes the data and timing signals to the channel interface. The
data channel module receives the timing as an external signal; however, the timing is the channel's
receive data timing, looped through the entire modem link and back to the channel. Figure 2-5 il-
lustrates this application.
This timing arrangement is known as "wrap" timing, because the timing is sent to the far-end mo-
dem site and wrapped back to the channel interface. Although there is some delay between the in-
ternal receive timing and the external transmit timing, the delay is fixed; there is no drift between
the timing signals. The phase-locking of MEGAMUX PLUS system timing is therefore preserved
by this arrangement.
Transmit External/Receive External
This arrangement is only used when an external device connected to MEGAMUX PLUS is the
source of all timing for the MEGAMUX PLUS system. Channel 22 may be the source of the master
timing signal for the system; the aggregate link connecting MEGAMUX PLUS nodes may also be
the source of timing.
The data channel module receives transmit timing from the channel or aggregate link device; this
timing signal is passed to the Clock Generator Plus module, which phase-locks all system clocks to
the external timing signal.
If the channel 22 device is the master timing source, the channel 22 timing must be set to external
transmit and receive timing. (Since all system clock signals are phase-locked to the channel 22
transmit timing, receive data for channel 22 may also be timed by the external timing signal.)
FIGURE 2-5. EXTERNAL TRANSMIT/INTERNAL RECEIVE TIMING
FOR SYNCHRONOUS CHANNELS
036R660-000, Issue 10
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