Pete <speed@advcomm.co.nz> wrote:
>Do ISPs use an E1 to terminate ISDN calls though? Or is it a PRI (or is that
>BRI, I can never remember). Or are they one and the same?
ISDN primary rate (PRI), in NZ at least, runs over E1 (2 Mbps) lines. E1
is used for lots of things; it is usually divided into 32 64 kbps
channels (or timeslots), with one timeslot (0) reserved for framing. Many
protocols, use timeslot 16 for higher level control; in ISDN PRI, this is
the 64kbps "D" channel, which carries call control and optionally packet
data, although I don't believe anyone uses this for IP.
Note that you don't have to use all the timeslots, but if you do use
"fractional" E1, the raw bit rate of the line is the same; it just goes
quiet during the free timeslots. Wideband DDS is like that; if you have
256k WDDS, the timeslot usage is 0 framing, 1-4 data, 5-15 free, 16
reserved, 17-31 free.
>I know when we terminated ISDN calls on a Max, Telecom called it a "SDP"
>(Service Delivery Point), which was basically 4Mbits inwards over which we
>could accept 48 (could be 64, but only paying for 48) modem calls or B1
>channels of ISDN.
The SDP is basically the place where any E1 or bigger telco circuits
come in. When you order an E1 based service (ISDN PRI, digital phone
trunks, DDS >128k, point-to-point E1 trunks), each site is charged for
an SDP. After that, you pay for each service. So if you have 40 ISDN
channels into a site, you pay 1 SDP, two PRI (E1) lines, plus 40
activated channels. When you get over 60 channels, you need a third
PRI, but no new SDP.
The SDP charge is somewhat decoupled from the actual hardware required;
in the bad old days you got this odd rack full of gear; then they went
to smaller boxes that could feed two E1s, so your third E1 got you a new
termination box, but you didn't pay a new SDP. Each E1 was its own
discrete pair; the box just held the cards to convert the signal to a
G.703 interface for plugging into your PABX, WDDS NTU, PRI socket or
whatever. When you wanted enough E1 lines, they'd run a fibre and
enough multiplexing to get you the number of lines you needed. Still no
new SDP.
Note that this is the Telecom model, not other carriers. The technology
is similar, but the charging models differ, at least in detail.
-- don
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Received on Fri Oct 6 13:19:06 2000