ISDN Demonstrator User’s Manual
40-400-00011, Rev. B
8
cancellation, and an
Embedded Operations Channel
(EOC) for testing and
monitoring.
Network Termination-1 (NT-1)
The U Interface terminates at a device called a
Network Termination-1
, or
NT-1. This device converts the U Interface to an S/T Interface. It also permits
loop back and testing by the telephone company and a means of routing
local power to devices previously designed to receive power from the line,
such as a telephone.
S/T Reference Point
This interface is a 192 Kbs, four-wire path; two wires are used for
transmission, two for reception. The encoding scheme is ASI bipolar. Unlike
the U Interface, it synchronizes quickly. All ISDN-compatible user devices, or
Type 1 Terminating Equipment
(TE-1), are connected directly to the S/T
interface.
Terminal Adapters
To accommodate a variety of communications equipment, the S/T interface
may be connected to a
Terminal Adapter
(TA). This device provides any
necessary conversion from an older or proprietary interface, such as RS-232
or a Local Area Network (LAN) protocol, to an ISDN-BRI S/T Interface.
Devices that connect to a TA are called
Type 2 Terminating Equipment
(TE-
2).
Layer 2 - Data Link Operation
Layer 2 of the OSI model establishes a method of transporting binary
information via data links. The ISDN data link layer is a variant of X.25
packet exchange called
LAP-D
. All signaling and switching information
(Layer 3), and optionally X.25 LAP-B user data, are transported on the D
Channel using this protocol. LAP-D packets are of variable length, and are
bounded by a flag pattern and terminated with a cyclic redundancy
checksum.
There are three categories of packet. Unnumbered Information (UI) packets
carry LAP-D protocol commands. These commands establish addresses,
initiate sequenced packet exchanges, or negotiate various protocol
parameters. Numbered Information (I) packets carry Layer 3 Call Control
messages or X.25 LAP-B user data. These packets are given sequence
numbers, referred to as Next Send (NS) and Next Receive (NR), which
ensure delivery in correct order. Supervisory (S) packets acknowledge or
reject sequenced I packets.
Packet Addressing
Each packet is given an address consisting of two numbers. The first
number, called a
Service Access Point Identifier
or SAPI, addresses one of
