Proposal for Fermilab remote access via ISDN (Ver. 1.0) Page: 7 of 32
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line has 2B+D, a PRI line, 23B+D. Each B channel can be individually accessed and controlled.
In this paper we will concentrate on BRI ISDN. BRI lines are delivered in two forms, T (4-wire)
or U (2-wire). Unless the installation is within 2000 wire feet of the central office, a U delivery
form is used. Since almost all ISDN customer-premises equipment (CPE) is made to use a T inter-
face, an NTL must be used to provide the 2-wire to 4-wire conversion, isolation, and loop-back.
The NT1 is necessary because the ISDN U interface can be as long as 18,000 wire feet (30,000
with a repeater) and the NT1 terminates and provides a standard interface for ISDN. Actually the
NT! has the capacity for some network administration, loopback testing, maintenance, and per-
formance monitoring. From the customer's point of view, the common-carrier network boundary
is the user side of the NT-1, the T reference point.
In the US, many of the trunks between telephone central offices are still configured to use in-band
signalling these situations one bit out of eight is "robbed" from the bearer data stream and used
for call control. With ISDN, however, all signalling is done out-of-band so the full 64 Kbps of the
channel can be used for bearer data. Since not all facilities have been upgraded to support ISDN,
many times an ISDN call will be sent across a trunk using in-band signalling. The call will not
fail, but the sender must not use the eighth bit since it will be overwritten by the central office. All
equipment sold in the US can meet this requirement through a scheme known as rate adaption
which reduces the available bandwidth to 56 Kbps per B channel.
ISDN terminal equipment is often referred to as a "modem" although since the transmission is all-
digital, there is no modulation/demodulation being performed. A more accurate term is ISDN ter-
minal adapter. A terminal adapter typically has a basic rate ISDN interface and a serial port such
as V.35 or RS-232. The terminal adapter places calls, handles necessary rate adaption and makes
the B-channel bandwidth available on the serial port. The serial port may either be asynchronous
(usually at speeds up to 19.2 Kbps) or synchronous (usually at speeds up to 128 Kbps).
3. ACCESS SCHEMES
There are many ways of using ISDN to provide remote access. One is to use an ISDN line and a
terminal adapter as a direct replacement for an analog line and modem. This would increase the
link bitrate to 128 Kbps, but few personal computers drive their serial port at 128 Kbps and most
terminal software is not capable of taking advantage of the increased speed. Finally, few terminal
adapters run at an asynchronous speed of more than 38.4 Kbps. A variation on this scheme is to
use a terminal adapter and a asynchronous line protocol like ARAP, PPP, or asynchronous DEC-
net with the protocol stack running on the remote machine. However, this variation suffers from
the same problem, lack of support on personal computers or terminal adapters for very high asyn-
chronous speeds, as well as loading down the remote computer with packet serialization.
Another scheme is to place a multi-protocol router at each remote site and use a terminal adapter
to connect the router's V.35 port to an ISDN line. The problem with the router scheme is that rout-
ers are expensive and would need to support a wide variety of protocols if the same type of router
was used at every remote location. In addition, both a router and a terminal adapter would have to
be purchased and managed for every location.Proposal for Fermilab Remote Access via ISDN (Ver. 1.0)
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Lidinsky, W.P. & Martin, D.E. Proposal for Fermilab remote access via ISDN (Ver. 1.0), report, July 2, 1993; Batavia, Illinois. (https://digital.library.unt.edu/ark:/67531/metadc793454/m1/7/: accessed May 4, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.