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40-400-00001, Rev. A
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 syn-
chronizes quickly. All ISDN-compatible user devices, or
Type 1 Terminating Equip-
ment (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 conver-
sion 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 se-
quenced 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 three logical links
or destinations within a device. SAPI 0 addresses Call Control functions, and SAPI
16 addresses the
Packet Handling Function (PHF) associated with X.25 LAP-B user
data.
SAPI 63 addresses the
Signaling Access Controller (SAC), which negotiates and
assigns the second packet address number, called a
Terminal Endpoint Identifier or
TEI. The TEI is a unique number for each Terminating Equipment on a particular
line. For example, if a packet addressed to SAPI 0 and TEI 3 is sent to four ISDN
devices connected to a single line, only the Call Control function (SAPI 0) of the
device assigned TEI 3 will acknowledge and process the packet.