Notes: Transmitted polarities are reversed if last ONE received was negative.
Bipolar violations always occur at 4th and 7th bit positions.
Figure 1-3. B8ZS Encoding
Framing Formats
Two framing formats are used in today’s T1 networks: D4 and ESF. Both are
based on the “frame” described above: 192 information bits preceded by a
framing/overhead bit. Thus, 8 kbps of the 1.544 Mbps is reserved for overhead,
leaving 1.536 Mbps for user information.
In the older D4 or Superframe (SF) format, a Superframe is defined as twelve
193-bit frames (2316 bits total). Figure 1-4 illustrates the D4 framing format. All
12 overhead (F) bits in each Superframe are used for frame synchronization.
As more T1 networks were deployed, a primary deficiency in D4—the inability
to monitor live data—became significant. To monitor performance on a D4
circuit, live traffic must be halted and test signals transmitted on the line. When
improvements in synchronizing circuits reduced the requirements for framing
synchronization to considerably less than 8 kbps, the Extended Superframe
(ESF) format was developed to allow performance monitoring of live data
without loss of information bandwidth.
Transmitted Bits
Received Bits
Transmitted Signal
1
0
0
0
+1
-1
0
-1
+1
1
0
0
0
0
0
0
0
0
8 consecutive ZEROS
Chapter 1
ACST-0351-005
Introduction
June 1996
1-8
