User guide

38 RealCT Direct API Developer Guide
Chapter 3: T1 Networking
Understanding T1 Trunks
T1 trunks provide digital communications in North America,
Japan, and Hong Kong. A T1 trunk carries 24, 64 Kb/s lines. An
additional 8 Kb/s signal provides synchronization information.
The combination of the 24 lines plus the synchronization
information yields a 1.544 Mb/s signal:
(24 X 64 Kb/s) + 8 Kb/s = 1,544 Kb/s or 1.544 Mb/s
The T1 board provides two 24-line T1 trunks for a total of
48 T1 lines. The T1 board only provides T1 line resources; RDSP
or Vantage series boards provide voice processing resources
through the MVIP bus.
Transmitting Digital Data
Digital data is composed of zeros and ones that the CO and CPE
transmit as an electrical signal. The waveform used to indicate
zeros and ones is called line coding. The CO and CPE must use
the same line coding method in order to communicate properly.
There are three basic types of line codes: Binary, Polar, and
Bipolar.
The Binary signal, also called unipolar, uses two voltage
levels to represent zero and one bits. In Figure 6, 0V
represents zero and +3V represents one. This line coding
method is called unipolar because it is not symmetrical
around 0V.
The Polar signal uses the positive and negative voltage of a
certain value to represent zero and one. In Figure 6, –1.5V
represents zero and +1.5V represents one. This code is also
called a non-return to zero (NRZ).
In polar and binary signals, the signal level remains
constant throughout the time that the bits are transmitted
on the line. For example, if four ones are transmitted, the
signal remains at some positive voltage for the duration of
the ones transmission.