Historical T-1 Generations and E-1 Comparison
page 1
Southern Methodist University
Electrical Engineering Department.
Graduate Telecommunications Program
T-1 Generations Summary Chart (and E-1 Comparison)
By Prof. Richard Levine
Issued Fall 2003
Several aspects of the technology used in the North American T-1 digital multiplexer have gone
through more than one generation of technological change. The historical information
summarized on this chart summarizes the major differences. Although the historically obsolete
methods are no longer used, they help to explain how we arrived at the present technology. T-1
systems use a 1.544 Mb/s bit stream carrying 24 physical channels of 64 kb/s each, numbered 1
through 24 in documents, plus a single framing bit in each such frame.
Original D-1 generation of T-1, issued 1961 and later officially obsolete (replaced by later D-2
through D-4 generations).
Aspect
Description
Frame
The Framing bit was set alternately to binary
Synchronization 1 and then binary zero in each successive
frame.
Mu-law digital
Of the 8 bits used for each sample, 7 bits were
coding
used for sign-magnitude description of the
voltage sample. One sign bit and 6 magnitude
bits. (The 8th bit was used for supervision.) A
mu-law approximately logarithmic
compression method was used, but with the
parameter mu equal to 100, The 6 magnitude
bits permitted only encoding 64 distinct
positive voltage steps and 63 negative voltage
steps (127 steps total due to positive and
negative zero each using a separate code).
Avoiding all
zeros.
Since there is a low voltage close to zero
frequently during a voice call, the digital code
produced internally in the Mu-law digital
coder frequently contains many binary zeros.
This bit pattern is reversed (all zeros changed
to ones, and all ones changed to zeros) before
the next step, thus producing lots of binary
Comments
The historical multi-frame
size was thus 2 frames.
Repeated analog-digital
conversion that occurred on a
signal path that used chained
analog and digital sections
produced degraded quantizing (round-off) noise and
perceptible degradation in
noise level. This is called
“trans-coding loss” (loss of
quality). A temporary fix for
this problem was a plan to
limit the number of analogdigital conversions in any
path to four or less, but this
was difficult to enforce when
the network was growing and
new dynamic call routing
methods were introduced.
In the 1960s there was no
plan to allow subscribers to
transmit digital data via the
digital T-1 links. Digitally
coded voice was the only
application.
Historical T-1 Generations and E-1 Comparison
Supervision
Line Coding
ones in most frames. Furthermore, any voice
call in progress always has a binary 1 in the
8th bit position because of the supervision bit
described in the next row of this chart.
The 8th bit was not used for part of the digital
sample coding, but was set to binary 1 in
every frame to indicate that channel is in use,
or zero in every frame when that channel is
idle.
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This method reports the
supervision status much more
frequently (one report for
each 125 microsecond
interval) than is really needed
for electromechanical
switching devices.
Alternate Mark Inversion (AMI) coding
produced either positive or negative 3 volt
pulses for each binary 1, and zero volts for
each binary zero. A simple 1-bit memory was
used to ensure that each 3 volt pulse had the
opposite polarity of the previous 3 volt pulse,
even when one or more zero pulses occurred
in between them.
Three later generations D-2 through D-4 share these properties.
Aspect
Description
Frame
The Framing bits are changed in each
Synchronization successive frame so they cycle through the 12
bit pattern 100011011100. (D-4 systems also
support extended super frame – ESF -described below.)
Mu-law digital
All 8 bits are used for each sample: one sign
coding
bit and 7 bits for magnitude description of the
voltage sample. (When “robbed bit” signaling
is used, the 8th bit of certain frames is used for
supervision, thus causing a small error in the
voltage magnitude in some frames.) A mu-law
approximately logarithmic compression
method is used with the parameter mu equal
to 255, The 7 magnitude bits permit encoding
128 distinct positive voltage steps and 127
negative voltage steps (255 steps total due to
positive and negative zero each using a
separate code).
Avoiding all
For voice and modem uses, two methods are
zeros.
used together. 1. all binary bits coming from
the mu-law digital coding are reversed (zero
and one values are replaced by each other). 2.
The binary code from digitizing the largest
Comments
This is necessary so the 6th
and 12th frame can be
identified to select the
supervision signal bit.
Trans-coding loss of quality
was reduced sufficiently so
that no special limitation on
the number of analog-digital
conversions in any path is
required.
Since the output of the digital
encoding often produces
small loudness values close
to zero, the inversion
frequently produces digital
Historical T-1 Generations and E-1 Comparison
magnitude negative voltage sample, namely
11111111, is automatically replaced by
11111101. For subscriber data transmission,
to achieve clear channel unrestricted error-free
digital transmission, either ZBTSI or B8ZS
must be used.
Supervision
Line Coding
Data
Transmission
Limitations
page 3
values containing many
binary 1s. The distortion due
to the 11111101 subsitution
is both a numerically small
change in the voltage sample,
and only occurs for the
infrequent case of an
extremely loud sound.
Two choices: 1. Common channel signaling.
These special robbed bits are
2. Robbed bit signaling: In the 6th and 12th
called the A bit and the B bit
th
frames of the multi-frame sequence, the 8 bit respectively. They only
of the voice channel set to binary 1 to indicate convey the supervision
that channel is in use, or zero when that
status. Dialing and other call
channel is idle.
control information is
typically transmitted via
audio tones or other signals
in the voice channel in these
cases.
Three choices in recently manufactured D-4
ZBTSI is currently only used
systems: 1. Alternate Mark Inversion (AMI).
by the Qwest (US West)
2. Binary 8-zero Substitution (B8ZS) in which operating company in the
a special pattern of positive and negative
US. Operating companies
polarity pulses and zero pulses is substituted
using B8ZS line coding
for each string of 8 zeros found in the bit
require test equipment that
stream to be transmitted. 3. Zero Byte Time
“recognizes” the special
Slot Interchange (ZBTSI) in which the binary substitution pulse pattern and
data bits are re-arranged in time sequence with does not mistakenly identify
non-zero binary value pointers used to
it as a bipolar violation
indicate where zero byte values were before
(BPV) error. ZBTSI requires
rearrangement. The resulting binary bit stream use of ESF as a prerequisite
is transmitted using AMI. (See course
(see explanation below).
lectures for further details on B8ZS and
ZBTSI.)
Digital end-to-end subscriber service (such as Level 1 works with
ISDN) is available in three levels of bit rate:
“ordinary” T-1 transmission
1. 56 kb/s unrestricted, where the subscriber
methods, using any method
can transmit 7 bits in each channel time slot,
of supervision or line coding.
but the 8th bit is set in every frame to a binary Levels 2 and 3 require
1 for compatibility with robbed bit signaling.
common channel signaling.
2. Restricted 64 kb/s data: The subscriber can Level 3 requires B8ZS or
transmit 8 bits in each channel time slot, but
ZBTSI line coding as well.
the subscriber’s equipment must pre-treat the
data (for example, by means of certain types
of “bit stuffing” modification of the bit
stream) so the case of 8 binary zeros never
occurs in the same time slot. 3. Unrestricted
Historical T-1 Generations and E-1 Comparison
Optional
Features
64 kb/s, where the subscriber can transmit 8
bits in every channel time slot, and any bit
pattern including all zeros is permitted.
ESF (described below).may be optionally
used instead of the traditional 12-frame multiframe method.
Channels in a T-1 link can be used together in
a non-channelized configuration, supporting
up to 1536 kb/s.
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ISDN configurations also
allow groups of 6 or more
channels to be used in a nonchannelized configuration for
various digital bit rates.
Extended Super Frame (ESF) is widely used since the late 1980s. Its main properties and benefits
are summarized in the following chart:
Property
Framing bit sequence cycles
through the following 24 bit
pattern: m e1 m 0 m e2 m 0 m e3 m
1 m e4 m 0 m e5 m 1 m e6 m 1
Any type of signaling and
supervision may be used with
ESF.
Any type of line coding may
be used with ESF.
Explanation and Benefit
Note only 6 of the 24 framing
bit values are fixed: 001011.
The six e bits are used for a
continuous error detection
process (see lecture notes).
The 12 m bits are used as a 4
kb/s signaling channel.
Comments
The most important benefit of
ESF is the continuous error
detection afforded by the e
bits. Bit errors can be detected
without taking any payload
channels out of service. Other
benefits are also significant:
the m bits permit operations,
administration, test and
maintenance signaling.
When robbed bit signaling is
Use of the C and D bits only
used, the least significant bit
occurs in certain proprietary
th
th
of each channel in the 6 , 12 , private tie line systems or for
18th and 24th frames can be
rarely occurring connection to
used independently to provide certain R2 signaling systems
four separate supervisionused in Latin America.
signaling control bits called A,
B, C and D bits. In most
PSTN installations, the ABCD
bits all indicate the same thing
(channel busy vs. idle).
AMI, B8ZS or ZBTSI may be Note that ESF is a preused with ESF.
requisite for ZBTSI.
Comparison to E-1 digital multiplexing systems. (E-1 is also called Conference of European
Post and Telegraph – CEPT – primary rate multiplexing, or Modulation par Impulsions et
Codage – MIC, a French-language translation of the English term “Pulse Code Modulation.”) E1 has only one basic generation instead of different symbolic uses of the bit stream. E-1 systems
use a 2.048 Mb/s bit stream carrying 32 physical channels of 64 kb/s each, numbered 0 through
31 in documents. Two of these channels, carried in time slots numbered 0 and 16 respectively,
are not used for payload traffic but are used for frame synchronization and limited alarm
Historical T-1 Generations and E-1 Comparison
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signaling in slot 0, and for call processing signaling in time slot 16. The major properties that
distinguish E-1 from T-1 are summarized in the following chart:
Aspect
Description
Frame
Most of the bits in channel 0 are used to carry
Synchronization a fixed signaling pattern
A-law digital
coding
All 8 bits are used for each sample: One sign
bit and 7 bits for magnitude describe the
voltage sample. A-law approximately
logarithmic compression method is used.
The 7 magnitude bits permit encoding 128
distinct positive voltage steps and 127
negative voltage steps (255 steps total due to
positive and negative zero each using a
separate code).
Almost avoiding For voice and modem uses, even-numbered
all zeros in the
binary bits (bits 2,4,6 and 8) coming from the
binary internal
A-law digital coding are reversed or inverted
bit stream.
(zero is changed to one and one is changed to
zero). This still leaves the possibility of the Alaw coder occasionally producing the binary
code 01010101, which will then be changed to
00000000. This could indeed occur, but the
use of HD3B coding (explained below)
produces intentional line coding bi-polar
violations (BPVs) in a pre-designated pattern
for the case of all zeros. HD3B line coding
patterns are automatically converted back
onto binary all-zero strings.
Signaling and
Two choices: 1. “Channel Associated
Supervision
Signaling” (CAS) logically associates a
distinct 4-bit pattern with each of the 32
physical channels. Two of these 4-bit patterns
are transmitted during time slot 16. The 32
separate physical channels each correspond
respectively to one of the 4-bit patterns that
occur in a repetitive cycle in a sequence of 16
frames. The bit patterns corresponding to
channels 0 and 16 here are “dummy” patterns.
2. Common Channel Number 7 signaling
(CCS7 or other abbreviations), which uses
packet messages to describe commands or
status changes for various subscriber
channels, using a channel identification code
Comments
Use of more than one bit per
frame allows E-1 systems to
acquire synchronization more
rapidly than T-1 systems.
For international calls
between A-law and Mu-law
national telephone systems,
the Mu-law country
equipment (typically the US,
Canada or Japan) must
perform the conversion of
each code sample value.
Since the output of the digital
encoding often produces
small loudness values close
to or equal to zero, the even
bit inversion frequently
produces digital values
containing many binary 1s,
which is desirable to keep the
repeaters synchronized.
These four CAS bits are
called the A, B, C and D bits
respectively. When they were
historically used, they were
typically used in
conformance with a signaling
code called R2 signaling. R2
signaling has many national
variants, and is mostly being
replaced by CCS7. (Note that
some people use the name
and abbreviation CAS for
other types of signaling such
as “robbed bit” signaling as
well.)
Historical T-1 Generations and E-1 Comparison
Line Coding
Data
Transmission
Limitations
END
(CIC) to cross-reference the relevant channel.
Note that “robbed bit” signaling is never used
with E-1 systems.
All E-1 systems use HD3B line coding, which
produces a special line coding pulse pattern
containing a special pattern of BPVs when
four consecutive binary zeros are transmitted.
The receiving device at the end of the E-1 link
converts this special pulse pattern back into
four binary zeros.
When ISDN was first proposed, E-1 already
had the capability to transmit unrestricted 64
kb/s, where the subscriber can transmit 8 bits
in every channel time slot, and any bit pattern
including all zeros is permitted.
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.When a 56 or 64 kb/s data
call is set up in an ISDN
system, a special parameter is
used in the call setup (IAM)
message so that the A-law/
Mu-law conversion used for
voice or modem calls is not
performed on international
connections between Mu-law
and A-law national systems.