HP 200 Series Services And Applications page 228

ISDN Wide Area Network Design: Dry Creek Joint Elem. School District
ISDN
ISDN
Traditionally, the point-to-point network depicted in figure 1 would be
implemented with leased 56-Kbit/s circuits; however, the low cost of ISDN
required a thorough investigation to see whether this option was appropriate
for Dry Creek Schools.
ISDN is a switched technology provided as a basic-rate service or a
primary-rate service. The basic-rate service, called 2B + D, provides two
64-Kbit/s "B" ("bearer") channels. The primary-rate service, in North
America called 23B + D (or in Europe 31B + D), provides 23 channels for a
variety of voice, video, and data connection options. Each service includes
one control or messaging "D" channel (16-Kbit/s for basic service). Basic-
rate service (2B + D) is all that Dry Creek School District needs. The
implementation considerations for basic-rate ISDN only are discussed in this
article.
When comparing cost and speed to digital 56-Kbit/s services, ISDN seems to
be the better choice, but ISDN has potential problems. The terminal adapter
equipment is currently more costly than DSU/CSU equipment, and ISDN
services are not always available in all areas of a particular local exchange
carrier or between carriers. In many cases, a digital link may be available
between two end points, but the call may start out as ISDN with out-of-band
signaling, be converted to Switched 56 with in-band signaling, and be
converted back to ISDN. In this example, there would be a loss of through-
put because of the Switched 56 in-band signaling. Signaling System 7 is a
standard network service that will eliminate this problem when it is
universally implemented by all carriers.
Another design consideration is that there may be measured charges
incurred for an ISDN data call, depending on the geography of the sites.
Usage charges are regulated and will be applied to carrier network services,
depending on how the customer is set up as a business group with a particu-
lar carrier. Extended LAN connections regardless of protocol tend to estab-
lish their connections for long periods. A good rule of thumb is that if there
are toll charges or measured-business-unit charges for voice connections
between two end points, there probably will be such charges for ISDN data.
Obviously, all ISDN charges must be considered when comparing the costs
of ISDN to other digital services.
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