Understanding the network.pdf - Back to Home

• The R interface acts as a point between TE2 and the TA interface to the ISDN
network. It comes in many forms, the most common being RS232 or
Centronix Parallel. The R interface is only defined by the CCITT as an
interconnect point, so the method is left up to the device manufacturer.
• The S interface is between the TA or TE1 and the NT2 device.
• The T interface is between the NT1 and the NT2. This is a logical interface for
the most part. Its purpose is to provide a communications distinction
between the NT2 and the NT1, and the NT1 and the telephone company.
• The U interface is the demarcation between the NT1 and the telephone
company's LT interface.
The U interface is only relevant in the United States because the NT1 device is
considered CPE. Most ISDN adapters sold in the United States will come with a U
type interface. ISDN equipment sold for use internationally will only come with an
S/T interface. A separate NT1 will need to be purchased to terminate the ISDN BRI
circuit. An ISDN PRI will utilize a CSU, or the NT2 will provide an integrated CSU.
The design principle behind all the different ISDN interface types is the physical
transport path. Although the BRI and PRI function the same way, conceptually they
each use different interface termination interfaces, framing, signaling, and data
paths.
ISDN Layer 1
The BRI uses two different frame formats (ITU-T I.430) to send user data: one for
network to terminal, and another from terminal to network. The frames are 48 bits
in length (32 bits for user data and 16 bits for control). The BRI data path from the
terminal adapter to the NT1 is a two-wire pair data path. The data path transports
frames at a rate of 4,000 frames per second for a total operating rate of 192Kbps.
The BRI data path up (from the TE to the U) uses a line encoding scheme called
Pseudo-Ternary Signaling (PT-S). P-TS is similar to bipolar signaling, which is used
with the TDM digital transport. P-TS uses a 750 millivolt (mv) signal that alternates
from + to -, to represent a zero bit. A one bit is represented by an absence of line
voltage.
In the United States, the NT1 represents the conversion from a two-pair data path
to a single-pair data path. The single-pair local loop data path operates as a
full-duplex link (ANSI T1.601). The NT1/U and the LT both employ an echo
cancellation scheme, which inserts a negative image of the data transmission into
the receiving circuit, which canceling out the full-duplex feedback of the
transmitter's signal. Here, the user data is converted from P-TS to 2B1Q two
binary-one quaternary signaling scheme. This line-encoding scheme uses four line
voltages to represent 4-bit symbols (called quats) that are 2 bits of actual data. The
2BIQ transport frame is 240 bits in length (216 bits of B and D channel data and 24