Motorola OM 2000 Installation & Operation Manual page 21

2 OVERVIEW
The multiplexer uses a queuing scheme to transfer packets from input ports to
output buffers. Each destination has a dedicated queue composed of multiple data
buffers set up to handle packets from particular input packet streams. As an input
packet enters a given input port, the packet multiplexer reads the stream PID map
and transfers the packet to the correct output destination queue. To generate the
output multiplex for a given destination, the multiplexer extracts packets from the
destination's queue by taking one packet from each buffer in a round-robin fashion.
Figure 2-3 illustrates how the packet multiplexer uses a destination queue to create
an output multiplex. In this example, the multiplexer routes three different input
packet streams labeled A, B, and C to the RF modulator for output. As a packet from
stream A enters the queue, the multiplexer transfers it to its associated buffer in the
RF modulator destination queue. The multiplexer handles packets from streams B
and C are in the same way:
Figure 2-3 — Creating an output multiplex
The multiplexer then builds the output multiplex for the RF modulator by extracting
packets from each buffer in the RF modulator queue in sequence (that is, one packet
from the A buffer, one from the B buffer, one from the C buffer, and so on).
The input devices supplying the data control the PID numbers assigned to incoming
packet streams. As a result, it is possible that packet streams received from two
different devices may use the same PID number. To avoid collision of these packets
when multiplexed for output, the OM 2000 re-assigns the PID numbers of streams
routed to the same destination. PID numbers can be re-assigned to any value or left
unchanged, as necessary, to guarantee PID uniqueness in the output. PID number
re-assignment for a packet stream is defined in the PID map for that stream.
Inserting PID 0 and PID 1 Control Streams
Depending on your system configuration, the cable-terminal control-stream input to
the OM 2000 can include the PID 0 and PID 1 control streams required to define the
OOB transport multiplex. If not, these streams are added to the multiplex before it is
input to the RF modulator.
The internal MPEG message stores the PID 0 and PID 1 control streams received
from the addressable controller and inserts packets containing messages from these
streams into the multiplexer at defined intervals. The messages are input to the
internal MPEG message generator through the Ethernet and they include data to
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