ISDN is an
international communications standard
for sending voice, video, and data
over digital telephone lines or normal telephone wires. ISDN allows
data to be transmitted simultaneously across the world using end-to-end digital
connectivity. With ISDN, voice and data are carried by bearer channels (B
channels) occupying a bandwidth of 64 kb/s (bits per second). There are two basic types of ISDN service: Basic
Rate Interface (BRI) and Primary Rate Interface (PRI).
PRI is intended for users with greater capacity requirements. Typically the
channel structure is 23 B channels plus one 64 kb/s D channel for a total of
1536 kb/s.
To access BRI service, it is necessary to subscribe to an
ISDN phone line. Customer must be within 18000 feet (about 3.4 miles or 5.5 km)
of the telephone company central office for BRI service; beyond that, expensive
repeater devices are required, or ISDN service may not be available at all.
Customers will also need special equipment to communicate with the phone
company switch and with other ISDN devices. These devices include ISDN Terminal
Adapters and ISDN Routers.
The Central Office will provide the customer with a U
interface. The U interface is a two-wire (single pair) interface from the
phone switch, the same physical interface provided for POTS lines. It supports
full-duplex data transfer over a single pair of wires, therefore only a single
device can be connected to a U interface. This device is called an Network
Termination 1 (NT-1). The NT-1 is a
relatively simple device that converts the 2-wire U interface into the 4-wire S/T
interface. The S/T interface supports multiple devices (up to 7 devices can
be placed on the S/T bus) because, while it is still a full-duplex interface,
there is now a pair of wires for received data, and another for transmit
data. Technically, ISDN devices must go
through an Network Termination 2 (NT-2) device, which converts the T
interface into the S interface. Virtually all ISDN devices include an NT-2 in
their design. The NT-2 communicates with terminal equipment, and handles the
Layer 2 and 3 ISDN protocols. Devices that connect to the S/T (or S) interface
include ISDN capable telephones and FAX machines, video teleconferencing
equipment, bridge/routers, and terminal adapters. All devices that are designed
for ISDN are designated Terminal Equipment 1 (TE1). All other
communication devices that are not ISDN capable, but have a POTS
telephone interface (also called the R interface), including ordinary
analog telephones, FAX machines, and modems, are designated Terminal
Equipment 2 (TE2). A Terminal Adapter (TA) connects a TE2 to an ISDN
S/T bus. The diagram is shown below.
During installation, the
central company will connect its end of the wire pair to digital equipment at
the central office. The signaling conventions on the U-interface are designed
to transfer data over a fairly long distance (up to 18000 feet), at a fairly
high rate, on one pair of wires, in both directions at once. The company have
to program the central office switch with the SPIDs that represent
telephone number and type of service (data, voice, or both).
AN ISDN router connects to an
Ethernet instead of a COM port. This device will serve as the connection from
the school to the central office. The
PC requires an Ethernet adapter card. A router filters out messages and only
sends real data over the ISDN line. An external LAN-attached ISDN router
provides a simple way for several computers to share a single ISDN line. There
should also be a specific router support at the central site. The central site
sees a single phone line and traffic from a single machine.
