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Principles of Operation
4-113610-A2-GB41-60 March 1999
TDM Backbone Circuit with Extensions
Figure 4-7 shows a 3600 Series DSU with TDM as the
DDS backbone with an analog private line (APL)
extended circuit. In this example, Port 4, which is set to
underspeed, cannot support an extended circuit because
extended circuit equipment cannot accept off-speed
clocks. Port 1 at 2400 bps can be extended via a crossover
cable to the extended circuit (refer to Appendix C).
The extended circuit, if analog, takes timing from the
DDS network; therefore, the extended control modem is
configured for external timing from the local DSU and
received timing at the extended tributary modem. The
tributary DSU is configured for DDS timing and elastic
store on Port 1. Elastic store compensates for phase
differences between the DDS network timing and the
received timing of the analog modem. (All configuration
options, such as elastic store and external/DDS timing, are
discussed in detail in Chapter 5.)
Diagnostic Path Extension
Diagnostic control can be extended to other data
communications equipment (DCE) beyond the tributary
DSU. The diagnostic channel (DC) provides an external
1200 bps asynchronous transparent path for a network
management system’s control channel to extend
diagnostic control to other Paradyne devices at the remote
location.
Digital Sharing
Digital sharing is a TDM or MCMP feature that allows
up to six ports to share the same channel. Typically this is
used for terminal-to-host connections that allow one host
port to service several terminal ports.
Digital sharing can be used on either point-to-point or
multidrop circuits, as shown in Figure 4-8. This
connection is used where multiple terminal devices are to
be attached to a single communications channel. The
terminal devices are typically addressed by the host
computer; the protocol ensures that only one terminal
device transmits at a time.
In single-port multipoint configurations, if
nondisruptive diagnostics are enabled, the configuration
option M-Pt SymPrt must also be enabled. This is because
the TDM circuit card cannot support an asymmetrical port
speed.
In this example, Ports 1 through 6 at each tributary
DSU are in the digital-sharing group. If contention is not
enabled, the DSU allows any DTE to send data whenever
it raises RTS. If contention is enabled, CTS to a
requesting port is not granted until the currently active
port drops its RTS. If multiple RTS signals appear, the
DSU gives CTS to the lower-numbered port.
Data received by a digital-sharing group is broadcast to
all DTEs in the group. Broadcast is the delivery of a
transmission to two or more communicating devices at the
same time.
Figure 4-7. TDM with Extended Circuit