Transmission - North American Transmission Systems
August 19, 1997
Table of Contents
1.0 Summary ......................................................................................................................................................... 3
2.0 Introduction .................................................................................................................................................... 3
3.0 Frame Formats................................................................................................................................................ 4
3.1 SUPERFRAME................................................................................................................................................. 4
3.1.1 Superframe Summary ............................................................................................................................ 6
3.2 EXTENDED SUPERFRAME .............................................................................................................................. 6
3.2.1 Extended Superframe Summary ............................................................................................................ 9
4.0 Bit-Robbed Signalling .................................................................................................................................... 9
4.1 SF FORMAT ................................................................................................................................................. 10
4.2 ESF FORMAT............................................................................................................................................... 10
5.0 Method of Research ...................................................................................................................................... 10
6.0 Conclusion ..................................................................................................................................................... 12
Author: Michael Martin
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Table of Figures
Figure 1 - Table of DSn Formats ........................................................................................ 4
Figure 2 - Frame format of frames 1 - 6 for Superframe T1 ............................................. 5
Figure 3 - Frame format of frames 7 - 12 for Superframe T1 ........................................... 5
Figure 4 - Frame format of frames 1 - 6 in ESF T1 ............................................................ 7
Figure 5 - Frame format of frames 7 - 12 in ESF T1 .......................................................... 7
Figure 6 - Frame format of frames 13 - 18 in ESF T1 ........................................................ 8
Figure 7 - Frame format of frames 19 - 24 in ESF T1 ........................................................ 8
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1.0 Summary
The aim of this assignment is to discuss and compare the North American DS1 Super
Frame (SF) and Extended Superframe (ESF) formats. The assignment itself is split into two
parts concentrating on specific areas of interests.
The first part of the assignment will
provide an overview comparison of the two frame formats; listing and explaining the
additional features provided by the ESF format. The second part of the assignment will look
into the principle of operation of the North American T1 bit robbed signalling process;
particular questions answered will be: why this process does not unduly affect the quality of
each voice channel and show the difference between the bit robbed signalling employed in
the SF and ESF DS1 formats.
2.0 Introduction
T1 is a high speed digital network (1.544MBs) developed by AT&T in 1957 and implemented
in the early 1960's to support long-haul pulse-code modulation (PCM) voice transmission.
The primary innovation of T1 was to introduce "digitised" voice and to create a network fully
capable of digitally representing what was up until then, a fully analogue telephone system.
Perhaps the best formal definition of T1 is as follows:
T1 is a "two-point, dedicated, high capacity, digital service provided on
terrestrial digital facilities capable of transmitting 1.544 Mb/s. The interface to
the customer can be either a T1 carrier or a higher order multiplexed facility
such as those used to provide access from (fibre optic) and radio systems."
So in the basic definition there is the discussion that there is a "higher order" or hierarchy of
T1. There is T1 that is, as said, a network that has a speed of 1.544MBs and was designed
for voice circuits or "channels" (24 per each T1 line or "trunk"). There is also T-2, operating
at 6.312MBs, which was implemented in the early 1970's to carry one Picturephone channel
or 96 voice channels. There is T-3, operating at 44.736MBs and T-4, operating at
274.176MBs. These are known as "supergroups" and their operating speeds are generally
referred to as 45MBs and 274Mbs respectively.
The general T-Carrier hierarchy is shown in the following table:
Format
Speed
# Of T1's
# Of Channels
DS0
64Kbps
1/24
1
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DS1
DS1C
DS2
DS3
DS3C
DS4
1.544MBs
3.152 MBs
6.312MBs
44.736MBs
89.472MBs
274.176MBs
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1
2
4
28
56
168
24
48
96
672
1344
4032
Figure 1 - Table of DSn Formats
It has been my personal experience and from the research done, that the 'C' formats as
shown in the table above are rarely used in practice.
This assignment is based around the DS1 format (T1) which, as described, takes in 24 64Kbs signals and multiplexes them into a single 1.544MBs output for transmission.
3.0 Frame Formats
This section of the report will provide the reader with an overview of the two frame formats
used in the North American system by having a brief discussion of each in turn.
3.1 Superframe
The origins of the superframe format came from AT&T as far back as 1962 with their
original implementation of the D1 format used for T1 transmission. Without going into too
much detail this method of framing was extremely inefficient as, in order to maintain 1'sdensity for synchronisation purposes for the repeater technology of that time, a 1 was sent
at the end of every timeslot (only 7-bits used for information). This framing method was
advanced and superseded eventually by the superframe format of framing (D4).
The T-carrier was designed to carry 24 independent digitised voice channels, each channel
encoded as a 64Kbs data stream. The D4 frame format evolved principally to carry voice
traffic. The frame consists of 193 bits, with the last bit always being a framing bit. The first
192 bits correspond to 24 conversations, or channels, that have been sampled with the PCM
type methods generating 8-bit words. The combined signal is "byte-interleaved", providing a
single frame.
A superframe is a repeating signal made up of 12 of the above-discussed frames. In order
to keep track of the of the frame structure, at least 1 bit in 15 bits of the combined stream
must be a 1, and at least 3 bits in 24 bits of the stream must be 1's. The bandwidth used on
voice frequency signalling is minimised by putting signalling information only in the LSB in
the sixth and twelfth frames. This frame format is shown following:
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Figure 2 - Frame format of frames 1 - 6 for Superframe T1
Figure 3 - Frame format of frames 7 - 12 for Superframe T1
The above figures show two types of frame bits; signal framing (Fs) and terminal framing
(Ft).
We can also see the voice/data signalling on frames 6 and 12 (A and B bits
respectively) which use the bit robbing method to be discussed later.
The 12-bit framing word is used for synchronisation and for identifying frames number 6 and
12 which contain the channel signalling bits. For 5 consecutive frames bit 8 will contain
voice bits, and on the sixth, it will contain a signalling bit; again on the twelfth frame bit 8 will
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contain a signalling bit. The sixth bit is referred to as the 'A' bit and the twelfth bit is referred
to as the 'B' bit. These combinations of bits allow the end-user station to carry out its
signalling protocol, which involves indicating such states as idle, busy, ringing etc.; as there
are only two bits however the number of states is limited to 4 only.
For data applications this "bit robbing" can be of major concern, this has lend to an
"unwritten" standard of 56Kbs data circuits in the U.S. vs. 64Kbs on superframe systems.
3.1.1 Superframe Summary
The following paragraph is an attempt at summarising the format of the frame structure for
the T1 superframe format:
The standard frame is 193 bits long (1 Framing bit + 24 8-bit timeslots). Each timeslot is
scanned at a rate of 8000 times per second. Therefore, in one second, there are 8000 * 8
bits/TS * 24 TS = 1,536,000 Bits of "Payload" data transmitted. There are: 8000 * 1 = 8,000
Bits of synchronisation bits transmitted within a one second interval. Therefore, the total
aggregate rate of the T1 signal is 1,544,000 BPS (1.544 MBPS).
3.2 Extended Superframe
The extended superframe format is not wholly unlike the superframe format (as the name
suggests), it basically provides added features to the T1 transmission system. The main
changes are as follows:

Number of frames increased to 24 - provides extra bits for signalling ('C' and 'D'
bits.

Only 6 bits in the 24 frames are used for synchronisation versus 12 out of 12 in
the superframe.
The main reason that ESF has come about is the advances in VLSI technology, which
means that fewer bits are required to keep a system synchronised.
The frame format for the extended superframe system is shown following:
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Figure 4 - Frame format of frames 1 - 6 in ESF T1
Figure 5 - Frame format of frames 7 - 12 in ESF T1
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Figure 6 - Frame format of frames 13 - 18 in ESF T1
Figure 7 - Frame format of frames 19 - 24 in ESF T1
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The 4 signalling bits employed in the ESF format allows for a maximum of 16-state
signalling (although most systems allow this to be selectable).
As early as 1979, AT&T proposed the Extended Superframe Format be implemented on its
T1 circuits in order to provide in-service diagnostic capabilities as well as improved false
frame protection. With ESF, the 193rd bit is now time-shared by three functions: frame
synchronisation bits; CRC-6 bits; and Facility Data Link (FDL) bits. Frame synchronisation
bits are carried in six of the 24 bit positions provided by the 193rd bit. These are in the 4th,
8th, 12th, 16th, 20th, and 24th positions and the pattern is "001011". This simple six-bit
pattern performs both the "F bit" and "S bit" functions of the D4 superframe. "False frame"
sensitivity is eliminated by using the CRC-6 error checking bits to determine which of several
"candidates" for the frame bit are the actual 193rd bit. CRC-6 uses a mathematical algorithm
to check the contents of the entire superframe (all 4632 bits) and obtains a 6-bit (hence its
name) coded "signature" for those data bits. The FDL may be used for any purpose, but is
ideally suited for communicating ESF performance information from local, remote, and
intermediate equipment along a facility and for sending control commands for protection
switching, network and remote equipment configuration, etc. In essence it is a 4 Kbs
channel embedded in the T1 format.
3.2.1 Extended Superframe Summary
The following paragraph is an attempt at summarising the format of the frame structure for
the T1 superframe format:
The standard frame is 193 bits long (1 Framing bit + 24 8-bit timeslots). Each timeslot is
scanned at a rate of 8000 times per second (as in D4/SF). The line rate is 1.544 MBPS
supporting a data "payload" of 1.536 MBPS.
There are three types of framing bits; Frame Pattern Sync (FPS), Datalink (DL), and Cyclic
Redundancy Check (CRC) bits. Of the 8 KBPS framing bit bandwidth:

4 KBPS is allocated to the Datalink

2 KBPS is allocated to the CRC-6 character

2 KBPS is used for synchronisation purposes
An Extended Superframe consists of twenty-four 193-bit frames.
4.0 Bit-Robbed Signalling
As the reader should be aware of already from reading the previous text, telephone channel
signalling is achieved in both the SF and ESF frames by using up a number of bits that
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would normally have been allocated as information bits. The formal name for this method of
signalling is "Bit Robbed Signalling". The main points were discussed previously as well, but
will be elaborated on slightly here for clarity.
Bit-robbed signalling effectively provides 7-5/6 bits for encoding of voice data (vs. 8-bits).
This is more than sufficient for voice and no degradation of the signal will be noticeable to
the human ear.
There is a problem when trying to send data (at speeds above 56Kbs). On these type of
frame formats and what is normally employed is some form of programming which will tell
the equipment that signalling is not required for a particular channel and that the whole
64Kbs can be used for data; this is often referred to as 64Kbs "Clear Channel" operation. It
has been my experience in the past that when commissioning lease (data) circuits on T1
links, this option (disable bit robbing) is missed and can lead to a frustrating fault finding
process as to why the circuit is not functioning.
4.1 SF Format
The transport of signalling states is required in Switched voice or data (Switched
56Kservice). Signalling is accomplished through a "Robbed Bit" method where bit 8 of each
channel's timeslot is "robbed" to indicate a signalling state in the 6th and 12th frames.
Effective throughput for the 'A' signalling bit (Frame 6) is 666.66 BPS. Effective throughput
for the 'B' signalling bit (Frame 12) is the same (666.66 BPS).
4.2 ESF Format
Again, as the transport of signalling states is required in Switched voice or data (Switched
56K service). Signalling is accomplished through a "Robbed Bit" method where bit 8 of each
channel's timeslot is "robbed" to indicate a signalling state in the 6th, 12th, 18th, and 24th
frames. Effective throughput for the 'A' signalling bit (Frame 6) is 333.33 BPS. Effective
throughput for the 'B', 'C' and 'D' bits is the same (333.33 BPS).
5.0 Method of Research
The information for this report came from a number of sources. The basic process used in
researching this report was to start off with the relevant ITU-T recommendations which
refreshed my mind as to the subject matter. Next, using the library OPAC system as a
guide, I found a number of books that provided detailed information that needed to be
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understood and summarised for inclusion in the report. My final area of research was the
use of the Internet, which can be chaotic at best, I did find some useful information,
however, including the diagrams of the frame formats used in this report.
Follows is a list of the reference material used for this report:
Internet Web Site:

Datacoms for Business - T1 Primer
http://www.dcbnet.com
Reference Books:

Enterprise Networking
Daniel Minoli

ITU-T G.703 Recommendations

Intermediate Transmission Course Notes
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6.0 Conclusion
The most obvious conclusion that I have come to while researching this assignment is that
even the newest and fanciest T1 systems are based around technology designed in the
1960's. We have reached the envelope where no more enhancements can be made to the
existing T1 (and E1 for that matter) systems and as such have been literally forced into new
methods of transmission. T1 systems were designed based upon the fact that bandwidth
was a rare commodity and as such very little overheads are on these systems; with today's
technology, bandwidth is much less of a concern, and network manageability and reliability
are more pressing issues. This is the reason that we see technologies such as SDH coming
to a fore more and more.
I believe that T1 and E1 systems are here to stay for quite some time, but they are being
pushed further and further back into customer premises; one time ago (not an overly long
period) a T1 was considered a big "pipe" but now it is being utilised really only as customer
interfaces. As customers require more and more bandwidth we shall see a gradual phasing
out of T1 and E1 technologies (therefore PDH) altogether.
From a personal point of view I found this assignment interesting for the same reason as
mentioned above; I have always thought a T1 to be a relatively advanced technology; to find
it has really little changed since the 1970's is quite amazing.
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