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2-4
System Overview
FIGURE 2-2. TANDEM VOICE CHANNEL INSTALLATION
Tandem Voice Channel Configuration
A Voice II/CVSD channel may be installed in a tandem arrangement. With this arrangement, a
Voice channel at one end is connected to a Data II channel at the opposite end. This enables the use
of tandem voice channels, where two MEGAMUX PLUS links operate back-to-back, and a syn-
chronous data channel link completes a voice channel link through two MEGAMUX PLUS sys-
tems. Tandem voice channel systems provide much higher voice quality in this arrangement
because unnecessary voice-to-data and data-to-voice conversions are eliminated. Figure 2-2 illus-
trates a tandem voice channel installation.
Aggregate Frame
When MEGAMUX PLUS multiplexes data, the process is controlled by an aggregate "frame." The
frame is a sequence of select codes; each select code instructs the TDM to select a bit of data from
a channel and transmit it across the aggregate link. The TDM uses an identical frame to sort receive
data and pass each data bit to the correct channel. For proper multiplexing, the transmit frame at
one node must operate in synchronization with the receive frame at the opposite-end node. Each
time the frame select code at one node instructs the TDM to select a data bit from a particular chan-
nel, the frame select code at the opposite-end node must instruct that TDM to pass the bit to the
proper channel. When the frames at both TDM nodes are synchronized, the TDM is said to be "In
Sync."
The AUTO FRAME Plus module or Universal AUTO FRAME module at both nodes stores a very
simple frame called a "sync frame." The sync frame has no channel data select codes in it; it is com-
posed only of "sync bit" selects and "overhead bit" selects. These bits establish synchronization and
exchange system information between TDM units. When an aggregate link is first established be-
tween TDMs, each end uses the sync frames to transmit and receive data until synchronization is
achieved. Once the TDMs are In Sync, they switch over to a full "data frame" (one with channel
data select codes) in unison. If some disruption causes the TDMs to lose synchronization (an Out
of Sync condition) they revert to the sync frames until synchronization occurs again. In this manner,
the TDMs are able to maintain synchronization automatically, even if there are no channel modules
in the system.
036R660-000, Issue 10
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