>If I have a CSU/DSU which connects to the
>demarc (smartjack), then it is the smartjack's job to regenerate that
>signal and send it to the CO, correct?
A smartjack isn't a repeater. It's just an RJ48 jack with enough circuitry
("smart") to answer a remote loopback command. The CSU is responsible for
putting enough db of signal on the wire so that it arrives at the CO (or
perhaps the first repeater) neither "too hot nor too cold," as Goldilocks
might have said.
>So, is there a special inband signal which the telco can
>send which is picked up by just the smartjack and not the customer's
>CSU/DSU which raises/drops loopback?
I've never been clear whether the SJ and CSU remote loopbacks (and OCU, for
that matter) involve different command codes or different frequencies. At
any rate, they differ somehow so that the correct device (usually) loops
back. If the signal on the circuit is degraded enough, of course, the
loopback can fail.
>Is there a normal smartjack -like device on the
>telco side as well, or does the leased line connect directly to a >CSU/DSU?
Out past the smartjack, you may or may not have a repeater or two. The T1
may or may not go to a SLCC hut (SLCC = Subscriber Line Carrier Circuit),
where your cable pair might join a large bundle of other pairs. The voice
line at my home gets muxed onto dark fiber at the SLCC; I don't know if they
do that with T1's at SLCC's or not. At the CO, the T1 is usually terminated
on an OCU (Office Channel Unit), which recovers the signal from your cable
pair (it's a card in a big box in the CO, generally). The OCU<>Smartjack
portion of the circuit is the local loop, AKA the UNI (User-to-Network
Interace). From the OCU it will generally go through a DACS (Digital Access
Crossconnect System), or perhaps a manually wired crossconnect panel, into a
MUX (Multiplexer), which will put your T1 into a timeslot on a T3 or some
larger pipe. From there, it will make similar hops through as many CO's as
it takes to take the circuit to its destination, getting demuxed and remuxed
as needed to ride whatever pipes it needs to reach its destination. Each hop
along the way is an NNI (Network-to-Network Interface), although often this
term is used to refer more specifically to where one telco hands off the
circuit to another.
The OCU is something like a CSU without a DSU. The individual DS0's
("channels," "timeslots,") on the T1 can be broken out there for individual
treatment if need be (such as on a T1 local loop carrying both a fractional
T1 data circuit and voice). The OCU, DACS, and MUX neither know nor care
whether your data bits have arrived in one piece -- or even whether it's
data or voice on the line. To them, it's just signal. Therefore, a DSU is
needed only at the other end of the circuit (your remote site's CSU-DSU on a
point-to-point, or a CSU-DSU connected to a port on a frame relay switch).
HTH,
doctorcisco
Silicon ... just fancy sand.
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