Motorola T1/E1 - Manual (Page 21)

Canopy T1/E1 Multiplexer September 2004
T1/E1 Multiplexer FPGA Version 3.4
Page 21 of 73
The receiver will use these violations for synchronization. The receiver recognizes that
the transmitted bit stream containing the two AMI violations is not true data. The B8ZS is
the standard for “Clear Channel Capability” and is part of ANSI.T1.403-1989.
High Density Bipolar Order Three Encoding
High Density Bipolar Order 3 Encoding (HDB3) is a bipolar signaling technique that is
based on Alternate Mark Inversion (AMI). It inserts bipolar violations (BPVs) whenever
there is a string of 4 or more zeros. The “violation” bit has the same polarity as the last 1-
bit which was sent using AMI encoding. The receiver removes all “violation” bits.
You may select the coding for the Canopy T1/E1 Multiplexer using either the T1/E1 EMS
or the CLI manager.
1.5.3 Signaling
All voice and data channel signaling passes transparently between endpoints through the
Canopy T1/E1 Multiplexer.
1.5.4 Timing
T1/E1 circuits are synchronous. This means that the T1/E1 circuits must be synchronized
or clocked with each other. Failure to do so may result in frame slips. The consequence
for which could be a degradation of quality. The clock for a T1/E1 circuit is normally
derived from the incoming T1/E1 signal. T1/E1 circuits are typically connected in a
master-slave configuration where the master has more accuracy than the slave. A clock
in a telecommunications system or network has a quality level or stratum number
assigned to it. This number indicates the clock’s quality and position in the timing
hierarchy. This position is determined by the clock’s stability. The more accurate clocks
are listed at the top of the hierarchy. A conventional hierarchy for T1 is defined and is
used by Local Exchange Carriers (LECs). This hierarchy is called a (Building Integrated
Tuning Supply (BITS) hierarchy and the formal specifications and standards for this
hierarchy are given in the ANSI standards T1.101-1999 and T1.105.09-1996.
The highest quality clocks are called Stratum 1 clocks. These clocks have a frequency
offset of 1x10
-11
or less. This means that Stratum 1 clocks are accurate to within 1
microsecond over a 24-hour period. Stratum 1 clocks are typically Cesium-beam clocks,
but the newer GPS-referenced rubidium clocks have been shown to achieve better than
1x10
-12
frequency offset at any time. The next level of clocks is Stratum 2 clocks.
Stratum 2 clocks have a 1x10
-10
frequency offset and are typically used at toll centers.
Class 5 Central Offices (COs) use Stratum 3 clocks. Stratum 3 clocks have a frequency
offset of 3.7x10
-7 . Stratum 3 clocks have less than 255 T1 slips in 24 hours. Stratum 4
clocks have a frequency offset of 3.2x10 -6 and are found at customer premises
equipment (CPE) in PBXs, channel banks, and routers. Stratum 4 clocks have no hold-
over capability and are used to smooth out jitter and wander and provide a clean clock
source for customer data. They accomplish this through clock recovery from higher-order
T1 clocks from the Class 5 COs or through an internal clock that is running at Stratum 4.
The Canopy T1/E1 Multiplexer does not have an internal clock. Therefore, it must be set
to synchronize to an incoming T1/E1 clock source. Each network node’s (PBX, Channel
Bank, Router) clock is based on the best available clock source for it. If multiple clocks
are available with the same quality, then the source that is the closest, in number of hops,
is chosen.
Issue 3