ZETRON 748, Technical Manual

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Operation
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The local topography does not lend itself to good overall coverage from a single site. Multiple sites
have been set up to establish overlapping coverage areas. As mobile units move from one coverage
area to another, they are only able establish communications with other units who are currently in
the same coverage area. Interconnect gateway solves this problem. It allows mobile units to directly
contact other coverage areas by linking the two sites together through the PSTN.
A mobile in area 52 wants to establish contact with other mobiles in his user group, who are
currently in area 51. Initiating a call to area 51 in can be accomplished one of two ways, using short
sign-on with his normal dispatch tone or temporarily switching to a second tone to initiate the call. A
Mobile-to-Phone call, using forced autodial, to the Model 748 in area 51 is the result. When the
Model 748 in area 51 (destination site) answers the incoming call, the Model 748 in area 52
(originating site) responds by over dialing a user number. This then setups up a Phone-to-Mobile call
in area 51 from the Model 748 in area 52. Mobiles in the called user group will hear ringing and can
just key up to answer (PTT to answer). We have now established a link between two units of the
same dispatch group on two different sites through the PSTN. Call clear down can be either DTMF
or time-out. If either Model 748 shuts the call down, it will send a disconnect code to the Model 748
at the other site.
Call Setup
There are two ways to initiate an Interconnect Gateway call; 1) sub-audible alone 2) DTMF + sub-
audible. Both have advantages and disadvantages. Using sub-audible alone requires a multi-channel
radio but not DTMF encode capability, using channel (sub-audible) selection to initiate calls. If more
than two sites are involved, using sub-audible alone can get quite complicated in a hurry. Using
DTMF + sub-audible requires a DTMF capable multi-channel radio. When more than two sites are
involved, this method is the most straight forward in terms of operation from a user stand point and
in flexibility of programming.
Once an Interconnect Gateway call has been setup (phone line off hook and digits being dialed), the
operation is the same regardless of which method was used to setup the call.