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6-4 DISTRIBUTED COMMUNICATIONS SYSTEM (DCS)
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administrable amount of time elapses, the packet is retransmitted. The link can be reset and restarted if
packets are not delivered or if the packets arrive out of sequence.
Transmission Media
Both analog and digital facilities can carry DCS signals between network nodes (see the DCS Signaling
Link Connections section later in this chapter).
When multiplexed digital facilities are used, two scenarios are possible. One scenario is to multiplex DCS
signaling onto a DS1 facility externally through an channel division multiplexer (CDM) (see chapter 1 for
more details) that interfaces with the DCIU or PI link. The other scenario is to internally multiplex the
signaling onto the DS1 facility through a DS1 port. DCS signaling channels can be redirected through DS1
ports through a dedicated connection. On the System 75 and Generic 1 communications system dedicated
connections are set up through switch administration that connects the PI channel and the DS1 channel over
an internal communications path. On a System 85 or Generic 2 communications system, the DS1 channel
is permanently connected to a general purpose port (GPP). In turn, the GPP communicates with the DCIU
over a MPDM. (See figure 6-2.)
MPDM
AVD
G
P
P
D
C
I
U
D
S
1
SYSTEM 85/G2
SOFTWARE
CONNECTION
SYSTEM 75/G1
PI
D
S
1
SOFTWARE
CONNECTION
Figure 6-2. Internally Connected DCS Signaling Channel Over a DS1
On the System 75 and Generic 1 communications systems these dedicated connections are called
permanent switched connections (PSC); on the System 85 and Generic 2 communications systems, they are
called dedicated switched connections (DSC).
Note:
• When a dedicated connection must be implemented between a System 75 or Generic 1 endpoint, and a
System 85 or Generic 2 endpoint, the connection should be implemented as a PSC on the System 75 or
Generic 1 communications system. Because PSCs can be established across communications systems,
no DSC is necessary on the System 85 or Generic 2 communications system when the PSC is defined
on the System 75 or Generic 1.
• On the System 75 and Generic 1 communications system, PSCs can be defined at either of two
administration screens: the Permanent Switched Connection screen or the Interface Link screen. PSCs
established through the Interface Link screen are more properly called software connections, rather than
PSCs.