Background
T1 is a high speed digital network (1.544 mbps) 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
"digitized" voice and to create a network fully
capable of digitally representing what was up until then, a
fully analog telephone system.
Perhaps the way to really begin this discussion is to
discuss the AT&T Digital Carrier System referred to as
"ACCUNET T1.5". It is described as 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 (fiber 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 which is, as we have discussed, a network that has a
speed of 1.544 Mbps and was designed for voice circuits or
"channels" (24 per each T1 line or
"trunk"). In addition, there is T1-C which operates
at 3.152 Mbps. There is also T-2, operating at 6.312 Mbps,
which was implemented in the early 1970's to carry one
Picturephone channel or 96 voice channels.
There is T-3, operating at 44.736 Mbps and T-4, operating
at 274.176 Mbps. These are known as "supergroups"
and their operating speeds are generally referred to as 45
Mbps and 274 Mbps respectively.
The general T-Carrier hierarchy appears in Figure
1 and is detailed in Chart 1.
Figure 1 - T-Carrier Hierarchy
DS0 | 64Kbps | 1/24 of T-1 | 1 Channel
| DS1 | 1.544Mbps | 1 T-1 | 24 Channels
| DS1C | 3.152 Mbps | 2 T-1 | 48 Channels
| DS2 | 6.312 Mbps | 4 T-1 | 96 Channels
| DS3 | 44.736 Mbps | 28 T-1 | 672 Channels
| DS3C | 89.472 Mbps | 56 T-1 | 1344 Channels
| DS4 | 274.176 Mbps | 168 T-1 | 4032 Channels
|
Chart 1 - T1 Hierarchy
For mathematical reasons, a voice channel was selected to
be at 64 Kbps. 24 of these channels is a composite of 1.536
Mbps, not 1.544 Mbps! Why is there a difference? The reason
is that after a byte (8 bits) of data is sent from each
channel (24 * 8 = 192 bits) there is an extra bit used for
synchronizing called a Frame bit - hence 193 bits are sent
and this increase of 1 bit per 192 causes the speed to
increase to 1.544 Mbps.
The fundamental frame of T1 is shown in Figure 2.
Figure 2 - Frame Organization
Well, you might ask, 1.544*2 = 3.088 Mbps and
not 3.152 Mbps for T1C, how come? Well, the answer is that
the T1C frame is made up of 1272 bits and is quite different
from the 193 bit frame of the T1 data stream. It should be
pointed out that the frame length of T1C and higher signals
are not related in any technical way to the T1 stream which
is treated simply as a string of bits. The simplistic diagram
in Figure 1 is correct from an organizational point of view
and does not show the relationship of the formatted data.
Now I have been using the term "T1 data
stream". To be consistent with AT&T parlance, a
"T1 data stream" is called a "DS1".
Equally, a T1C stream is referred to as "DS1C",
etc. Another summary chart to show the relationship is in
Figure 3:
Sig. Lvl |
Carrier |
# of T1's |
# Voice Ckts |
Speed Mbps |
DS-0
|
--
|
1/24
|
1
|
.064 |
DS-1
|
T1
|
1
|
24
|
1.544 |
DS-1C
|
T1C
|
2
|
24
|
3.152 |
DS-2
|
T2
|
4
|
96
|
6.312 |
DS-3
|
T3
|
28
|
672
|
44.736 |
DS-4
|
T4
|
168
|
4032
|
274.760 |
Figure 3 - T1 Hierarchy Summary Chart
A convenient way to think of T1 is from the
first two layers of the ISO (International Standards
Organization) OSI(Open System Interconnect) model: the
Physical and Logical layers. The Physical layer focuses on
the electrical characteristics such as signal shape, voltage
levels, etc. The logical layer deals primarily with the
format issue - how is the data extracted from the low-level
protocol?
The designation "DS" in Figure 3 refers to
"Digital Signals" and describes the physical layer.
The designation "T" refers to the type of carrier
that is being used. Often these are used interchangeably but
that technically is not correct.
On the topic of standards, T1 has been specified first by
AT&T and second, by ANSI (American National Standards
Institute). The European equivalent of T1 is called CEPT and
is a CCITT standard. As a point of interest, the CEPT
standard is at 2.048 Mbps and does not use a "master
clock". In the U.S., the three major carriers each have
a single "master T1 clock" from which all the
others are derived. In the U.S., all T1 clocks are
"slave" to this master clock. The problem that
occurs is when someone wants to interconnect a T1 network
provided by MCI to a T1 network provided by Sprint. This
requires what is known as an elastic buffer and this is built
into most T1 devices.
When someone says they are running T1, they may be saying
several different things: The may mean that they have a
network that is passing data at 1.544 Mbps; they may mean
that they have a network that conforms to the T1 electrical
interface specification (DSX-1), or that they have a network
that passes data that conforms to one of the several framing
formats (D4, ESF, etc.). More likely than not, they mean all
three but their concentration may be on only one of these
items. The confusion in the user community is a result of the
interchangeability of words and the confusing requirements
for connection to the AT&T system.
Services and Quality
AT&T through ACCUNET T1.5 offers several services
besides the already mentioned point-to-point service. There
are four "transfer arrangements" that can be
purchased:
1. Customer ability to change terminating location of T1
link with AT&T assistance (either signal or dial)
2. M24 Multiplexing allowing the user to connect up to 24
channels to individual switched and non-switched services
offered by AT&T.
3. M44 Multiplexing allowing the user the capability to
combine 2 T-1 lines, each carrying up to 22 channels to 1 T1
line using Bit Compression Multiplexing (BCM).
4. Customer Controlled Reconfiguration (CCR) allowing the
customer to dynamically allocate circuits without AT&T
assistance.
These services allow the user to have T1 trunks in
several cities and allow data transfer to each. This along
with the T1-Mux (to be discussed later) forms the modern T-1
network.
Associated with the lower costs of T1, the guaranteed
quality of the network is also superior to leased lines. By
specification, AT&T states that the performance objective
is 95% Error Free Seconds (EFS) on a daily basis and the
availability objective is 99.7% on a yearly basis.
Channel Banks and Formats
A digital source, or terminal, is the equipment that
generates digital signals for transmission through the
digital network. The large majority of digital sources now
produce a DS-1 signal. The D4 Channel Bank is an example,
although it can produce signals at other rates as well.
The reference to the term "Channel Bank" is
made quite often in the T-1 language. The type of Channel
Bank is important since it defines the type of formatting
that is required. For example, a D4 Channel Bank must have a
DS-1 signal with data formatted in accordance with the D4
format.
The purpose of a Channel Bank in the telephone company is
to form the foundation of multiplexing and demultiplexing the
24 voice channels (DS0). The D-type Channel Bank is used for
digital signals. There are five kinds of Channel Banks that
are used in the System: D1, D2, D3, D4, and DCT (Digital
Carrier Trunk).
A transmitting portion of a Channel Bank digitally
encodes the 24 analog channels, adds signalling information
into each channel, and multiplexes the digital stream onto
the transmission medium. The receiving portion reverses the
process. As these were designed as voice circuits, the
assumption is that the digital data is PCM voice and that the
voice is companded and expanded through the use of CODECs. D1
banks (later called D1A) were first installed in 1962 and
their success led to modifications of D1B and D1C. The
original D1A,B, and C banks used 7 bits for each voice sample
and one bit in each code word for carrying the signalling
(off hook, ring, etc). When it became desirable to connect
several T1 transmission spans together, the performance was
not too good. In addition, it was realized that providing
signaling information in every code word was wasteful since
8,000 bits per second was not required to provide the
signaling information for a channel; the signalling
information simply did not change that quickly.
As a result of these conditions, another modification to
the D1 series (D1D) and the new D2 channel bank were
developed. The D2 bank uses all eight bits of every time slot
to encode the analog signal except for selected frames.
Supervisory and signalling information is sent by using the
least significant bit from the code word in each channel
every sixth frame. The companding characteristic also was
changed to give better performance. The D2 bank increased the
packing density to 96 channels in the same space as the 72
channels for a D1 bank.
D3 and D4 banks were motivated by advances in ICs,
allowing packaging of 144 channels in a single bay. Following
the D4 bank, advances in technology resulted in the
development of the Digital Carrier Trunk unit, or DCT. It was
developed by the Bell System to be smaller, lower cost, and
easier to maintain than the D4 channel bank.
The D1 type channel bank (D1A,B,C) placed alternate 1's
and 0's in the 193rd bit position. It was assumed that random
data would not contain this pattern, in bits spaced exactly
193 bits apart, for any significant length of time. The
receiving device would find the 193rd bit by using a simple
search technique. This algorithm had the advantages of
circuit simplicity and speed. In the early 1960's, there were
few commercially available ICs for building complex logic
functions, and elementary designs cost less. The
disadvantages of this technique were rapidly uncovered when
equipment was installed in actual customer sites. Certain
standard analog tones, such as the 1000 Hz test tone, applied
to one or more voice channels and digitized by Channel Bank,
created an alternating one and zero pattern every 193 bits in
one or more voice channels. It was possible for the terminal
to lock up on the incorrect pattern. This condition,
affecting all 24 channels, could last until the test tone was
removed. The 1000 Hz tone has been changed to a 1004 Hz test
tone.
By the time this problem became apparent, it had been
decided to use T-carrier for toll quality telephony, which
required more precise coding techniques. D1 channel banks
used seven bit encoding for voice signals, and an eighth bit
for signalling. The new format provided for eight bit coding
most of the time (5/6 frames) and seven bits only in one
frame out of six. This is known as 7 5/6 coding with
"robbed bit" signaling and was first implemented in
the D2 channel bank (D1D is a retrofit of D1 channel banks
with D2 capability).
Besides the "false frame" problem, D2 bank
designers were faced with a new set of problems. The new
format required two steps; first, find the 193rd bit, and
second, find the sixth and 12th frame in a 12-frame sequence.
The time required to find the proper bit sequence rises
exponentially as the number of bit positions between frame
bits increases. Although we still use every 193rd bit, it is
time-shared between the terminal framing pattern (odd
numbered frame bits) and the superframe alignment pattern
(even numbered frame bits). Finding the 193rd bit position
was still based on an alternating 1's and 0's pattern, but
now it only appeared in every other 193rd bit.
The new technique provided for increased "false
frame" protection. The downside of the technique was
that the time to reframe was much longer. With the D2 format
the maximum average reframe time (MART) would be about 200
milliseconds. This was too much time to be out of service so
new algorithms were developed that decreased the time to 50
msec which is now the specification standard. Succeeding
channel bank equipment (D3 and D4) used the same framing
sequence as D2. In fact, the Superframe Format is most often
referred to as the D4 frame format even though it began with
D2. This sequence defines a "superframe" consisting
of two interleaved patterns. The terminal framing pattern
("F" bit) is a repeating ones and zeros in odd
numbered frames and the superframe alignment pattern
("S" bit) is "001110" in the even
numbered frames. This results in a 12-bit superframe pattern
of:
Odd Six Bits |
Even Six Bits |
Combined Twelve Bits |
101010 |
001110 |
100011011100 |
---|
The D4 Format is shown in Figure 4 below. Notice that the
"F" bit and the "S" bit are all called
"S bits". While this is confusing, it is a
terminology remnant of the time when there were only
"S" bits (vis-a-vis D1 format).
Frame # |
S-bit terminal Framing (Ft) |
S-bit signal Framing (Fs) |
Information bits |
Signalling bit |
Signalling Channel |
1 |
1 |
- |
1-8 |
- |
|
2 |
- |
0 |
1-8 |
-
|
|
3 |
0 |
- |
1-8 |
-
|
|
4 |
- |
0 |
1-8 |
-
|
|
5 |
1 |
- |
1-8 |
-
|
|
6 |
- |
1 |
1-7 |
8 |
A |
7 |
0 |
- |
1-8 |
-
|
|
8 |
- |
1 |
1-8 |
-
|
|
9 |
1 |
- |
1-8 |
-
|
|
10 |
- |
1 |
1-8 |
-
|
|
11 |
0 |
- |
1-8 |
-
|
|
12 |
- |
0 |
1-7 |
8 |
B |
---|
Figure 4 - The D4 Format
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 synchronization
bits; CRC-6 bits; and Facility Data Link (FDL) bits. Frame
synchronization 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 Kbps
channel embedded in the T1 format. Bellcore documement
TR-TSY-000194 (Extended Superframe Format Interface
Specification - December 1987), ANSI T1.403-1989, and
AT&T Publication 54016 describes how this channel may be
used. This includes the format of the messages , commands,
and responses. Most CSU's today interpret these commands and
execute the appropriate responses. The ESF Format is shown is
Figure 5.
Frame # |
Fe bit |
DL bit |
CRC-6 |
Info bits |
Signalling bit |
Signalling channel |
1 |
- |
m |
|
1-8 |
- |
|
2 |
- |
- |
C1 |
1-8 |
-
|
|
3 |
- |
m
|
|
1-8 |
-
|
|
4 |
0 |
-
|
|
1-8 |
-
|
|
5 |
- |
m
|
|
1-8 |
-
|
|
6 |
- |
- |
C2 |
1-7 |
8 |
A |
7 |
- |
m
|
|
1-8 |
-
|
|
8 |
0 |
-
|
|
1-8 |
-
|
|
9 |
- |
m
|
|
1-8 |
-
|
|
10 |
- |
- |
C3 |
1-8 |
-
|
|
11 |
- |
m
|
|
1-8 |
-
|
|
12 |
1 |
-
|
|
1-7 |
8 |
B |
13 |
- |
m
|
|
1-8 |
-
|
|
14 |
- |
- |
C4 |
1-8 |
-
|
|
15 |
- |
m
|
|
1-8 |
-
|
|
16 |
0 |
-
|
|
1-8 |
-
|
|
17 |
- |
m
|
|
1-8 |
-
|
|
18 |
- |
- |
C5 |
1-7 |
8 |
C |
19 |
- |
m
|
|
1-8 |
-
|
|
20 |
1 |
-
|
|
1-8 |
-
|
|
21 |
- |
m
|
|
1-8 |
-
|
|
22 |
- |
- |
C6 |
1-8 |
-
|
|
23 |
- |
m
|
|
1-8 |
-
|
|
24 |
1 |
- |
|
1-7 |
8 |
D |
---|
Figure 5 - The ESF Format
The chart shown in Figure 6 shows the
differences between D1 through ESF formats. As most equipment
today is either D4 or ESF, the data for D1 and D2 is
displayed only for completeness.
Time Slots |
D1D |
D2 |
D3,D4,ESF |
1 |
1 |
12 |
1 |
2 |
13 |
13 |
2 |
3 |
2 |
1 |
3 |
4 |
14 |
17 |
4 |
5 |
3 |
5 |
5 |
6 |
15 |
21 |
6 |
7 |
4 |
9 |
7 |
8 |
16 |
15 |
8 |
9 |
5 |
3 |
9 |
10 |
17 |
19 |
10 |
11 |
6 |
7 |
11 |
12 |
18 |
23 |
12 |
13 |
7 |
11 |
13 |
14 |
19 |
14 |
14 |
15 |
8 |
2 |
15 |
16 |
20 |
18 |
16 |
17 |
9 |
6 |
17 |
18 |
21 |
22 |
18 |
19 |
10 |
10 |
19 |
20 |
22 |
16 |
20 |
21 |
11 |
4 |
21 |
22 |
23 |
20 |
22 |
23 |
12 |
8 |
23 |
24 |
24 |
24 |
24 |
---|
Figure 6 - Channel & Time Slot Number
Assignments
Signal Shapes and Codes
A Digital Cross-connect (DSX) consists of equipment
frames (patch panels) where cabling between system components
is connected. Each digital signal is defined for and handled
by its own cross-connect. Thus, for example, DSX-1 is used to
interconnect equipment operating with DS1 signals.
The pulse shape of a DS1 pulse is defined at the DSX-1
cross connect. AT&T Publication 43801 describes the
requirement of this pulse to drive from 0 to 655 feet of 22
gauge ABAM cable between the channel bank and the DSX-1. The
maximum time of reframe time is defined at 50 msec. Actually
the DS-1 pulse is a slightly relaxed version the DSX-1 pulse
mask. Figure 7 shows the specification (less template) of the
DSX-1 signal and how it compares to the DS-1 signal
specification.
Functions |
DSX-1 |
DS-1 |
Line Rate |
1.54 Mhz +/- 200 Hz |
1.544 Mhz +/- 75 Hz |
Cable Length at DSX
point |
ABAM/655 ft. |
6000 ft. |
Pulse Amplitude |
2.4 to 3.6 v. |
2.7 to 3.3 v. |
Receive
Attenuation |
<10 dB |
15 to 22.5 dB |
Line Build Out |
Yes |
0.0, 7.5, 15 dB |
Max Successive
Zeros |
15 (or B8ZS) |
15 (or B8ZS) |
---|
Figure 7 - Comparison of DSX-1 Signals and DS-1
Signals
The ANSI standard T1.403-1989 is different yet
again. Fundamentally the signals and the templates (signal
shapes) are pretty much the same. Modern IC manufacturers
have insured that their products meet all of the specs. When
we are communicating to the CO or to the carrier we are using
DS-1; when we are regenerating the signal after the demarc,
we are using DSX-1.
It is important to note that the template of the DS-1
signal is bipolar. This means that a plus voltage, a zero
voltage, and a minus voltage are important to the coding of
the signal. The code which is used in T1 is call AMI for
Alternate Mark Inversion. This means that if a "1"
or Mark is coded as a positive voltage, the very next
"1" must be a minus voltage or the result will be a
Bipolar Violation (BPV).
Figure 8 shows a valid AMI sequence and a sequence with a
BPV.
Figure 8 - Two AMI sequences
Notice that in the specification in Figure 7, there
is reference to the "Maximum Successive Zeros". One
of the requirements of the coding sequence and hence the
signal shape of the DS-1 is that a "1" bit is sent
in order to maintain the timing synchronization. For example,
a signal that was sending all 0's would be a constant zero
voltage line. Eventually the timing of the system would be
lost.
The requirement is that no more than 15 0's can be sent
before a "1" must be transmitted. In telephone
applications that was accomplished with bit 7. Remember, bit
8 is sometimes used for signalling so it couldn't be
universally used. The human ear would never detect these
slight variances in the lower order bits. In the case of
sending data, using bit 7 and bit 8 for other than faithfully
representing the data being presented for transport yields
disastrous consequences. Thus a mechanism had to be developed
for data only applications.
The easiest approach and a technique still in use in DDS
is to make every bit 8 a 1 and to use only the lower 7 bits.
This 7/8 mode yields 56Kbps instead of the standard DS0 rate
of 64 Kbps. This technique also disallowed the use of
signalling bits.
An improvement to this technique was developed known as
B8ZS with stands for Binary Eight Zero Substitution. This
technique takes advantage of BPV's in the data stream to be
decoded as a signal.
With B8ZS coding, each block of 8 consecutive zeros is
replaced with the B8ZS code word. If the pulse preceding the
inserted code is transmitted as a positive pulse (+), the
inserted code is 000+-0-+ (BPV's in position 4 and 7). If the
pulse preceding the inserted code is transmitted as a
negative pulse (-), the inserted code is 000-+0+- (again
BPV's in position 4 and 7).
Figure 9 shows how B8ZS works.
Figure 9 - B8ZS
This is the standard for "Clear Channel
Capability". AT&T references it in Publication 62411
in Appendix B as CB144. It is part of the ANSI T1.403-1989
standard as well.
Cabling
Now for some discussion on ABAM cable. This is the cable
that is called out in the DSX-1 spec and is a physical cable
that was manufactured by AT&T. Generally it is a cable
that has unshielded twisted pairs with a wire size of 22 AWG.
Some authorities suggest that it is pulp insulated while
others suggest that it is plastic insulted. In any event,
ABAM cabling, per se, is no longer available. Modern cable
manufacturers, however, especially those active in EIA-568,
have developed cables with specific categories or levels.
Category/Level 2 cable is adequate for the T1 data rate and
has the following characteristics:
- 24 AWG
- 2 pairs
- 100 ohms impedance @ .772 MHz
- 7 dB attenuation/ 1000 ft @ .772 MHz
- 41 dB crosst
all @ 1000 ft.
Several manufacturers make this cable type. A summary of
the Category/Level types per RS-568 is listed in Figure
10.
LEVEL |
SERVICE TYPE |
SPEED |
1 |
POTS (plain old telepnone
service) |
n/a |
|
RS-232/RS-562 |
19.2 to 115.2 Kbps |
|
T1, Fractional T1 |
64 Kbps increments |
|
ISDN Basic Rate |
144 Kbps |
|
RS-422 |
up to 1.0 Mbps |
2 |
IEEE 802.3 1BaseT |
1.0 Mbps |
|
IBM System 3x/AS400 |
1.0 Mbps |
|
T1 |
1.544 Mbps |
|
ISDN Primary Rate |
1.54 Mbps |
|
IBM 370 |
2.36 Mbps |
|
IEEE 802.5 |
4.0 Mbps |
3 |
Wang |
4.3 Mbps |
|
IEEE 802.5 10BaseT |
10.0 Mbps |
|
IEEE 802.5 Token Ring |
16.0 Mbps |
4 |
IEEE 802.5 Token Ring |
16.0 Mbps |
|
New Arcnet |
20.0 Mbps |
5 |
X3T9.5 TPDDI |
100.0 Mbps |
---|
Figure 10 - New Cable Types (Proposed
EIA-568)
DCB Manufacturers the T-extender, a simple T1 repeater that allows the length of a T1 line to be up to 5,000 ft. It's easy to install, having no switches or settings, and inexpensive at $495.
Connectors
The discussion of connectors sometimes becomes confusing
as there is a difference between "de facto"
standards, things used in products, and specification.
AT&T specify that the Network Interface (NI) should be a
subminiature 15-pin female connector with the following
pin-out:
1 | Send Data (tip)
| 2 | Reserved for network
| 3 | Receive Data (tip)
| 4 | Reserved for network
| 5 | Not defined
| 6 | Not defined
| 7 | Not defined
| 8 | Not Defined
| 9 | Send Data (ring)
| 10 | No connect
| 11 | Receive Data (ring)
| 12 | No connect
| 13 | No connect
| 14 | No connect
| 15 | No connect
|
AT&T Publication 62411 further states that "in
such cases where ISDN standards need to be met, an 8 pin
mini-modular connector is recommended" with the
following pin-out:
1 | Transmit (ring)
| 2 | Not Used
| 3 | Not Used
| 4 | Receive (ring)
| 5 | Receive (tip)
| 6 | Not Used
| 7 | Not Used
| 8 | Transmit (tip)
|
To complicate the matter, ANSI T1-403-1989 specification
calls out for "one of four Universal Service Ordering
Code (USOC) connectors (RJ48C, RJ48X, RJ48M, and RJ48H)"
with pin assignments as follows:
1 | Receive (ring)
| 2 | Receive (tip)
| 3 | Not Used
| 4 | Transmit (ring)
| 5 | Transmit (tip)
| 6 | Not Used
| 7 | Not Used
| 8 | Not Used
|
As it goes, the above pin-out and connectors is also the
"de facto" standard vis-a-vis how currently
available hardware is configured.
Applications
Well, then, what do we do with these DS-1/DSX-1/T-1
signals? There are several applications and specific
equipment that can be applied.
- DACS
- D4 Channel Bank
- PBX
- CSU
- T1 Muxes
- SRDM (Subrate Data Mux)
- Fractional T1
The most important issue to see is that there can be T1
networks that are customer owned and T1 networks that use the
AT&T Accunet T1.5 system. The applications will be the
same but the constraints on the equipment are more stringent
using the AT&T connection.
DACS (Digital Access Cross-Connect)
There are three levels of DACS compatibility. The first
level is DS-1 and is at the full T1 rate. The second level is
"bundled" or 1/4 T1 level. This allows the customer
to utilize Customer Controlled Reconfiguration or
"fanout" at the CO (central office). The third
level is at the 64 Kbps or DS-0 level. What happens is a
single T1 signal is generated using channels a and b and goes
to the CO. The CO splits this into two T1 trunks one carrying
channel a and the other carrying channel b. The device the
performs this function is called a DACS. DACS may also be
configured with a topology such as a ring topology. If one of
the trunks goes down, the data will be reconfigured to go
over the standby trunk. In the past, almost all DACS are owned by the
telcos; now, many communications users are using DAC functionality on their own networks.
DCB can supply a DACS or mini-DACS!
D4 Channel Bank
As we mentioned the T1 signal must somehow be split into
the 24 separate and distinct voice channels. When this is
done, it is still in the digital form. The codecs must then
convert the digital signal (per channel) into analog signals
to be sent on the subscriber loops. Again, most Channel Banks
tend to be owned and operated at the CO's (Central Offices).
Since deregulation in the 1980's, more T1's are owned by
users, as telephone carriers continue to reduce the cost of
the local loop (the wires from the central office to the
customer premise).
DCB can supply a full featured channel bank or full-feature DSU/CSU for full or fractional T1 termination.
PBX (Private Branch Exchange)
Clearly the intended use of T1 was to bring in as many
telephone lines using voice as possible through a digitized
technique (PCM Pulse Code Modulation). Tie lines between PBXs
account for many private T-1 network applications. This is
supported through 2 and 4 wire E & M (Ear and Mouth)
signalling techniques through the T1 Mux. A 2w FXS (Foreign
Exchange Subscriber) function (dedicated line to a distant
CO) and 2w FXO (Foreign Exchange Office) function (the CO
version) can also be supported by the T1 trunk. In the latter
mode, the T1 line acts as an "extension cord". The
primary way in which customers use this function is through
the T1 Multiplexor.
CSU (Channel Service Unit)
This may be the easiest to explain. A DS-1 comes from the
phone company to the customer. This line must be given the
proper termination, line protection (vis-a-vis FCC Part 68),
and message handling capability. In the old days, the phone
company supplied this equipment but today this probably will
be CPE (Customer Premise Equipment). The output of the CSU is
the DSX-1 signal. The most common CSU is found in a T1 Mux
however they can stand alone with various added
functionality.
The bipolar output of the CSU can be connected to a DSU
(Digital Service Unit) which converts the bipolar signals to
unipolar and vice versa at the data rate gleaned from the
bipolar signals.
The DCB T-Driver, for example, is a
DSU. It takes unipolar data from the terminal and coverts it
to a DS-1 signal. In many ways it also acts as a CSU and its
transition to a CSU/DSU is quite possible. AT&T Pub 62411
requires that a CSU perform the following functions:
- regeneration
- loopback
- keep alive
The regeneration part is part of the T-Driver
functionality. Loopback is commanded from the Carrier in one
of two ways:
- in line data pattern with D4 (SF) formatting
- using the FDL with ESF formatting
As the FDL is already being used in T-Driver, it would be
rather straightforward to incorporate the appropriate
responses to the command structure of the loopback from the
carrier. The interface is already surge protected and meets
FCC Part 68. The conclusion is that we have with relatively
small impact an "ESF CSU" in the T-Driver product
that can connect directly to the carrier. To incorporate an
"SF CSU" which is still quite prevalent in use with
D4 channel banks, would be a more significant undertaking
requiring hardware and software changes.
As a matter of note, DDS (Digital Data Service) also
requires a CSU but most units are sold as a CSU/DSU with a
V.35 or RS-530 connector right on the device. DCB's T1 and fractional T1 CSU/DSUs are examples.
T1-MUX
This is actually a family of devices dedicated for
customer use. They are normally T1 or fractional T1 TDMs
which comply with format constraints , DACS interfaces, and
often have an optional CSU. Their purpose, depending on the
number of ports, is to allow transmission of data, image, and
voice form many different sources of a single network
link.
Many T1 Muxes are also Subrate Data Muxes (SRDMs). By
this identification they are able to accommodate synchronous
data rates of 2.4, 4.8, 9.6, and 19.2 Kbps. Asynchronous data
rates are also allowed in some devices. SDRM operates per
DS0.
Since T1 muxes are also DACS compatible at the DS0 level,
Fractional T-1 service is also compatible with the devices.
They also comply with the D4 channel bank requirements of bit
density, zero density, and the provision of clear channel.
FT1 is like SRDM only at the DS1 level. Hence, data may be at
multiples of 64Kbps.
Also many T1 Muxes allow for the integration of the
AT&T Switched 56 service. These are important month-end
transfers, CAD/CAM files and teleconferencing.
DCB Products
Data sheets and application notes are available from the DCB web site for all DCB products. Check the Product Index or the Education Section for direct links.
FT Series Fractional T1 DSU/CSU
The FT DSU/CSU's have a DS-1 output signal, and are FCC
registered DSU's. They take data at a configured speed via an
RS-530/V.35 interface and convert the data to a T-1 data
stream. The format of the data is can be D-4 or ESF. The
transmitter is configured with a selectable signal attenuator
(LBO) of 0, 7dB, and 15 dB per AT&T spec. The FT series
is available in a single channel units (FT-1), two channel
unit (FT-2) and a 4 channel unit (FT-4). Each port can be
configured to use from 1 to 24 of the DS-0's (56 or 64 Kbps
each DS-0). The FT-2 and FT-4 units also have drop and insert
capability.
T-Extender
T-Extender is a T1 repeater designed to AT&T
specifications. This device takes a DS-1 signal and
regenerates it as a DS-1 signal. T-Extender can have the
DSX-1 output of T-Lan as an input signal and T-Lan will also
accept and decode the output of the T-Extender. T-Extender,
being a signal repeater, is not constrained by any formating.
For example, a BPV is passed through just a readily as a
normal signal. The output of T-Extender is -4 dBdsx and is
fixed. This is -4db from the allowable power as defined in
the Repeater Specification, AT&T Publication TA24/CB113
and was done to simplify the circuit. The product has a
robust receiver and therefore should have no difficulty in
going repeater to repeater nearly 6000 feet on 22AWG solid,
shielded twisted pairs.
DACS
The V 4200 is a versatile 9 or 28 slot integrated T1/T3/OC-3 access device. Depending on the plug-in cards selected, this unit can be configured (a) as a CSU/DSU with drop and insert and voice capabilities, (b) as a multiple E1 to T1 converter or fractions of them, (c) as a digital cross-connect system (DACS), (d) as sets of ICSU combined in one box, and (e) as a channel bank. As a CSU/DSU, data from the V.35 or X.21 port can occupy any fraction of a E1 or T1 port As an E1 to T1 converter, A to law and signaling conversion are correctly handled. For both E1 and T1 ports, continuous error checking, performance polling, and in-service diagnostics are provided. In any of the above combinations, full time slot interchange (TSI) among the ports are possible,
making the V 4200 a small DACS (digital access cross-connect system). The ports can further be used in pairs as
ICSUs (intelligent CSU) at lower cost and smaller space than individual ICSUs. Lastly, the V-4200 can be configured as
a channel bank. By using high speed cards, it can also interface to up to two OC-3 lines.
Appendix A
Definition of dBdsx
A simplified equation for the definition of
dBdsx is the following:
dBdsx = 20 X log (.167 Vp-p measured)
where "Vp-p measured" is the peak-to-peak
measurement of the voltage between tip and ring. For
example...
If there is a 0.5 volt positive voltage on tip and a 0.5 volt negative voltage on ring...
The peak-to-peak voltage measurement is 1.0 volts. Using
the equation,
dBdxs = 20 * log (.167 X 1.0)
= 20 * (-.777)
= -15.5
Notice that tip and ring signals are inverted. When a 1 is
sent one line (tip, for example) will be a positive voltage and the other (ring, for example) will be a negative
voltage. When 0's are begin sent, both lines are at 0 volts. Since T1 is AMI or alternating, the next 1 will have
the voltages reversed.
Many specifications give the "pulse amplitude"
rather the dBdsx. This parameter is the positive voltage,
measured from zero, of a 1 being sent. In other words, it is
half of the peak-to-peak voltage. As a note of interest, the
T1 pulse is not specified as necessarily symetric. AT&T
Pub 62411 states that the maximum + voltage is defined as 3.0
+/- 0.3 volts while the maximum - voltage is its absolute
value (without sign) and must be within 0.20 volts of the +
voltage but no less than 2.7 volts or greater than 3.3
volts.
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