All You Wanted to Know About T1 but Were Afraid to Ask

7/29/2019 All You Wanted to Know About T1 but Were Afraid to Ask

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All You Wanted to Know About T1 But Were Afraid to

Ask

by

Bob Wachtel

Introduction

We are experiencing a breakneck growth in the interconnection of personal computers,terminals and telephones in the business environment. T1 technology is proving to be acost-effective means of linking voice and data, both inter-office and intra-office, andserves as an alternative to high speed modems for data transport. There is significantdiscussion these days about "T1 Gateways" and "T1 trunks" as the cost from the variousphone companies of these services goes down. Users are discovering that it costs less tohave a T1 trunk than a series of leased telephone lines in a point-to-point topology. Thisincrease in the use of T1 requires a fundamental understanding of the technology.

DCB Manufacturers a complete line of data communications equipment including

T1 Channel Banks,T1 DSU/CSUs, 56/64Kbps DSU/CSUs, multiplexers, andvoice/data DSUs. T1 equipment includes the T-Extender for extending in-house 4-wire T1, the 4200 Access unit/DACS, the 4300 mini-DACS, the FT fractionalDSU/CSU, the 2500 DSU/CSU

Background

T1 is a high speed digital network (1.544 mbps) developed by AT&T in 1957 andimplemented in the early 1960's to support long-haul pulse-code modulation (PCM) voicetransmission. The primary innovation of T1 was to introduce "digitized" voice and tocreate 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 CarrierSystem referred to as "ACCUNET T1.5". It is described as a "two-point, dedicated, highcapacity, digital service provided on terrestrial digital facilities capable of transmitting1.544 Mb/s. The interface to the customer can be either a T1 carrier or a higher order
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multiplexed facility such as those used to provide access from (fiber optic) and radiosystems."

So in the basic definition there is the discussion that there is a "higher order" or hierarchyof T1. There is T1 which is, as we have discussed, a network that has a speed of 1.544Mbps 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 at6.312 Mbps, which was implemented in the early 1970's to carry one Picturephonechannel or 96 voice channels.

There is T-3, operating at 44.736 Mbps and T-4, operating at 274.176 Mbps. These areknown as "supergroups" and their operating speeds are generally referred to as 45 Mbpsand 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


DS3C 89.472 Mbps 56 T-1 1344 Channels



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Chart 1 - T1 Hierarchy

For mathematical reasons, a voice channel was selected to be at 64 Kbps. 24 of thesechannels is a composite of 1.536 Mbps, not 1.544 Mbps! Why is there a difference? Thereason 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 sentand 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 ofT1C and higher signals are not related in any technical way to the T1 stream which istreated simply as a string of bits. The simplistic diagram in Figure 1 is correct from anorganizational 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


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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 (InternationalStandards Organization) OSI(Open System Interconnect) model: the Physical andLogical layers. The Physical layer focuses on the electrical characteristics such as signalshape, voltage levels, etc. The logical layer deals primarily with the format issue - how isthe data extracted from the low-level protocol?

The designation "DS" in Figure 3 refers to "Digital Signals" and describes the physicallayer. The designation "T" refers to the type of carrier that is being used. Often these areused 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 CEPTand is a CCITT standard. As a point of interest, the CEPT standard is at 2.048 Mbps anddoes 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 tointerconnect a T1 network provided by MCI to a T1 network provided by Sprint. Thisrequires 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 maymean 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 severalframing formats (D4, ESF, etc.). More likely than not, they mean all three but theirconcentration may be on only one of these items. The confusion in the user community isa result of the interchangeability of words and the confusing requirements for connectionto the AT&T system.

Services and Quality

AT&T through ACCUNET T1.5 offers several services besides the already mentionedpoint-to-point service. There are four "transfer arrangements" that can be purchased:


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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 individualswitched and non-switched services offered by AT&T.

3. M44 Multiplexing allowing the user the capability to combine 2 T-1 lines, eachcarrying up to 22 channels to 1 T1 line using Bit Compression Multiplexing (BCM).

4. Customer Controlled Reconfiguration (CCR) allowing the customer to dynamicallyallocate circuits without AT&T assistance.

These services allow the user to have T1 trunks in several cities and allow data transfer toeach. 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 alsosuperior to leased lines. By specification, AT&T states that the performance objective is95% Error Free Seconds (EFS) on a daily basis and the availability objective is 99.7% ona yearly basis.

Channel Banks and Formats

A digital source, or terminal, is the equipment that generates digital signals fortransmission through the digital network. The large majority of digital sources nowproduce a DS-1 signal. The D4 Channel Bank is an example, although it can producesignals at other rates as well.

The reference to the term "Channel Bank" is made quite often in the T-1 language. Thetype 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 ofmultiplexing and demultiplexing the 24 voice channels (DS0). The D-type Channel Bankis used for digital signals. There are five kinds of Channel Banks that are used in theSystem: D1, D2, D3, D4, and DCT (Digital Carrier Trunk).


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A transmitting portion of a Channel Bank digitally encodes the 24 analog channels, addssignalling information into each channel, and multiplexes the digital stream onto thetransmission 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 iscompanded and expanded through the use of CODECs. D1 banks (later called D1A) werefirst installed in 1962 and their success led to modifications of D1B and D1C. Theoriginal D1A,B, and C banks used 7 bits for each voice sample and one bit in each codeword for carrying the signalling (off hook, ring, etc). When it became desirable toconnect several T1 transmission spans together, the performance was not too good. Inaddition, it was realized that providing signaling information in every code word waswasteful since 8,000 bits per second was not required to provide the signalinginformation 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 newD2 channel bank were developed. The D2 bank uses all eight bits of every time slot toencode the analog signal except for selected frames. Supervisory and signallinginformation is sent by using the least significant bit from the code word in each channelevery sixth frame. The companding characteristic also was changed to give betterperformance. The D2 bank increased the packing density to 96 channels in the samespace as the 72 channels for a D1 bank.

D3 and D4 banks were motivated by advances in ICs, allowing packaging of 144channels in a single bay. Following the D4 bank, advances in technology resulted in thedevelopment of the Digital Carrier Trunk unit, or DCT. It was developed by the BellSystem 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 bitposition. It was assumed that random data would not contain this pattern, in bits spacedexactly 193 bits apart, for any significant length of time. The receiving device would findthe 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 availableICs for building complex logic functions, and elementary designs cost less. Thedisadvantages of this technique were rapidly uncovered when equipment was installed inactual 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 analternating one and zero pattern every 193 bits in one or more voice channels. It waspossible for the terminal to lock up on the incorrect pattern. This condition, affecting all


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24 channels, could last until the test tone was removed. The 1000 Hz tone has beenchanged to a 1004 Hz test tone.

By the time this problem became apparent, it had been decided to use T-carrier for tollquality telephony, which required more precise coding techniques. D1 channel banksused seven bit encoding for voice signals, and an eighth bit for signalling. The newformat provided for eight bit coding most of the time (5/6 frames) and seven bits only inone frame out of six. This is known as 7 5/6 coding with "robbed bit" signaling and wasfirst implemented in the D2 channel bank (D1D is a retrofit of D1 channel banks with D2capability).

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, findthe sixth and 12th frame in a 12-frame sequence. The time required to find the proper bitsequence 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 framingpattern (odd numbered frame bits) and the superframe alignment pattern (even numberedframe bits). Finding the 193rd bit position was still based on an alternating 1's and 0'spattern, 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 themaximum average reframe time (MART) would be about 200 milliseconds. This was toomuch time to be out of service so new algorithms were developed that decreased the timeto 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 ismost often referred to as the D4 frame format even though it began with D2. Thissequence defines a "superframe" consisting of two interleaved patterns. The terminalframing pattern ("F" bit) is a repeating ones and zeros in odd numbered frames and thesuperframe alignment pattern ("S" bit) is "001110" in the even numbered frames. Thisresults in a 12-bit superframe pattern of:

Odd Six Bits Even Six Bits Combined Twelve Bits101010 001110 100011011100

The D4 Format is shown in Figure 4 below. Notice that the "F" bit and the "S" bit are allcalled "S bits". While this is confusing, it is a terminology remnant of the time whenthere were only "S" bits (vis-a-vis D1 format).


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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 onits T1 circuits in order to provide in-service diagnostic capabilities as well as improvedfalse 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. Framesynchronization 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 D4superframe. "False frame" sensitivity is eliminated by using the CRC-6 error checkingbits 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 (all4632 bits) and obtains a 6-bit (hence its name) coded "signature" for those data bits. TheFDL may be used for any purpose, but is ideally suited for communicating ESFperformance information from local, remote, and intermediate equipment along a facilityand for sending control commands for protection switching, network and remoteequipment configuration, etc. In essence it is a 4 Kbps channel embedded in the T1format. Bellcore documement TR-TSY-000194 (Extended Superframe Format InterfaceSpecification - December 1987), ANSI T1.403-1989, and AT&T Publication 54016describes how this channel may be used. This includes the format of the messages ,


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commands, and responses. Most CSU's today interpret these commands and execute theappropriate 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. Asmost equipment today is either D4 or ESF, the data for D1 and D2 is displayed only forcompleteness.

Time Slots D1D D2 D3,D4,ESF

1 1 12 1


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2 13 13 2

3 2 1 3

4 14 17 4

5 3 5 5

6 15 21 67 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) wherecabling between system components is connected. Each digital signal is defined for andhandled 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 Publication43801 describes the requirement of this pulse to drive from 0 to 655 feet of 22 gaugeABAM cable between the channel bank and the DSX-1. The maximum time of reframetime is defined at 50 msec. Actually the DS-1 pulse is a slightly relaxed version the DSX-


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1 pulse mask. Figure 7 shows the specification (less template) of the DSX-1 signal andhow 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


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Notice that in the specification in Figure 7, there is reference to the "MaximumSuccessive Zeros". One of the requirements of the coding sequence and hence the signalshape 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 issometimes used for signalling so it couldn't be universally used. The human ear wouldnever 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 fortransport yields disastrous consequences. Thus a mechanism had to be developed for dataonly applications.

The easiest approach and a technique still in use in DDS is to make every bit 8 a 1 and touse only the lower 7 bits. This 7/8 mode yields 56Kbps instead of the standard DS0 rateof 64 Kbps. This technique also disallowed the use of signalling bits.

An improvement to this technique was developed known as B8ZS with stands for BinaryEight 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 codeword. If the pulse preceding the inserted code is transmitted as a positive pulse (+), theinserted code is 000+-0-+ (BPV's in position 4 and 7). If the pulse preceding the insertedcode is transmitted as a negative pulse (-), the inserted code is 000-+0+- (again BPV's inposition 4 and 7).

Figure 9 shows how B8ZS works.


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Figure 9 - B8ZS

This is the standard for "Clear Channel Capability". AT&T references it in Publication62411 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 cablethat has unshielded twisted pairs with a wire size of 22 AWG. Some authorities suggestthat it is pulp insulated while others suggest that it is plastic insulated. 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 orlevels. Category/Level 2 cable is adequate for the T1 data rate and has the followingcharacteristics:

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 perRS-568 is listed in Figure 10.

LEVEL SERVICE TYPE SPEED


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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 T1line 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 differencebetween "de facto" standards, things used in products, and specification. AT&T specifythat the Network Interface (NI) should be a subminiature 15-pin female connector withthe 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
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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 tobe met, an 8 pin mini-modular connector is recommended" with the following pin-out:

1 Transmit (ring)

2 Not Used

3 Not Used4 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 fourUniversal Service Ordering Code (USOC) connectors (RJ48C, RJ48X, RJ48M, andRJ48H)" 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 howcurrently available hardware is configured.

Applications


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Well, then, what do we do with these DS-1/DSX-1/T-1 signals? There are severalapplications 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 thesame but the constraints on the equipment are more stringent using the AT&Tconnection.

DACS (Digital Access Cross-Connect)

There are three levels of DACS compatibility. The first level is DS-1 and is at the full T1rate. The second level is "bundled" or 1/4 T1 level. This allows the customer to utilizeCustomer Controlled Reconfiguration or "fanout" at the CO (central office). The thirdlevel is at the 64 Kbps or DS-0 level. What happens is a single T1 signal is generatedusing 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 thisfunction is called a DACS. DACS may also be configured with a topology such as a ringtopology. If one of the trunks goes down, the data will be reconfigured to go over thestandby trunk. In the past, almost all DACS are owned by the telcos; now, manycommunications users are using DAC functionality on their own networks. DCB cansupply aDACS or mini-DACS!

D4 Channel Bank

As we mentioned the T1 signal must somehow be split into the 24 separate and distinctvoice 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 subscriberloops. Again, most Channel Banks tend to be owned and operated at the CO's (CentralOffices). Since deregulation in the 1980's, more T1's are owned by users, as telephonecarriers continue to reduce the cost of the local loop (the wires from the central office tothe customer premise).DCB can supply a full featuredchannel bankor full-feature DSU/CSU for full orfractional T1 termination.
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PBX (Private Branch Exchange)

Clearly the intended use of T1 was to bring in as many telephone lines using voice aspossible through a digitized technique (PCM Pulse Code Modulation). Tie lines betweenPBXs 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 T1trunk. In the latter mode, the T1 line acts as an "extension cord". The primary way inwhich 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 thecustomer. This line must be given the proper termination, line protection (vis-a-vis FCCPart 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). Theoutput of the CSU is the DSX-1 signal. The most common CSU is found in a T1 Muxhowever they can stand alone with various added functionality.

The bipolar output of the CSU can be connected to a DSU (Digital Service Unit) whichconverts the bipolar signals to unipolar and vice versa at the data rate gleaned from thebipolar 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 aCSU/DSU is quite possible. AT&T Pub 62411 requires that a CSU perform the followingfunctions:

regeneration loopback keep alive

The regeneration part is part of the T-Driver functionality. Loopback is commanded fromthe 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 toincorporate the appropriate responses to the command structure of the loopback from thecarrier. The interface is already surge protected and meets FCC Part 68. The conclusion isthat we have with relatively small impact an "ESF CSU" in the T-Driver product that canconnect directly to the carrier. To incorporate an "SF CSU" which is still quite prevalent


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in use with D4 channel banks, would be a more significant undertaking requiringhardware and software changes.

As a matter of note, DDS (Digital Data Service) also requires a CSU but most units aresold as a CSU/DSU with a V.35 or RS-530 connector right on the device. DCB'sT1 andfractional T1 CSU/DSUs are examples.

T1-MUX

This is actually a family of devices dedicated for customer use. They are normally T1 orfractional T1 TDMs which comply with format constraints , DACS interfaces, and oftenhave an optional CSU. Their purpose, depending on the number of ports, is to allowtransmission of data, image, and voice form many different sources of a single networklink.

Many T1 Muxes are also Subrate Data Muxes (SRDMs). By this identification they areable 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 isalso compatible with the devices. They also comply with the D4 channel bankrequirements of bit density, zero density, and the provision of clear channel. FT1 is likeSRDM 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. Theseare 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 DCBproducts. Check the Product Indexor 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 takedata at a configured speed via an RS-530/V.35 interface and convert the data to a T-1data stream. The format of the data is can be D-4 or ESF. The transmitter is configuredwith a selectable signal attenuator (LBO) of 0, 7dB, and 15 dB per AT&T spec. The FTseries is available in a single channel units (FT-1), two channel unit (FT-2) and a 4channel unit (FT-4). Each port can be configured to use from 1 to 24 of the DS-0's (56 or64 Kbps each DS-0). The FT-2 and FT-4 units also have drop and insert capability.
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T-Extender

T-Extender is a T1 repeater designed to AT&T specifications. This device takes a DS-1signal 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. Forexample, 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 theRepeater Specification, AT&T Publication TA24/CB113 and was done to simplify thecircuit. The product has a robust receiver and therefore should have no difficulty in goingrepeater 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. Dependingon 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 ofthem, (c) as a digital cross-connect system (DACS), (d) as sets of ICSU combined in onebox, and (e) as a channel bank. As a CSU/DSU, data from the V.35 or X.21 port canoccupy any fraction of a E1 or T1 port As an E1 to T1 converter, A to law and signalingconversion are correctly handled. For both E1 and T1 ports, continuous error checking,performance polling, and in-service diagnostics are provided. In any of the abovecombinations, full time slot interchange (TSI) among the ports are possible, making the V4200 a small DACS (digital access cross-connect system). The ports can further be usedin pairs as ICSUs (intelligent CSU) at lower cost and smaller space than individualICSUs. Lastly, the V-4200 can be configured as a channel bank. By using high speedcards, 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 andring. For example...

If there is a 0.5 volt positive voltage on tip and a 0.5 volt negative voltage on ring...


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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 next1 will have the voltages reversed.

Many specifications give the "pulse amplitude" rather the dBdsx. This parameter is thepositive 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 necessarilysymetric. AT&T Pub 62411 states that the maximum + voltage is defined as 3.0 +/- 0.3volts while the maximum - voltage is its absolute value (without sign) and must be within0.20 volts of the + voltage but no less than 2.7 volts or greater than 3.3 volts.

All You Wanted to Know About T1 but Were Afraid to Ask

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