T3 Fundamentals

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Digital Link Presents:

T3 Fundamentals

DS1

DS2

DS2

DS2

DS3DS1

DS1

T3 Technology

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i Copyright © 1996, Digital Link Digital Link CorporationWorld copyright reserved. No part of this publication may be stored in a retrieval system, transmitted, orreproduced in any way, including but not limited to photocopy, photograph, magnetic, chemical, or other record,without the prior agreement and written permission of Digital Link Corporation.Digital Link Corporation 217 Humboldt Court Sunnyvale, California 94089 (408) 745-6200 FAX (408) 745-6250

T3 Technology

This course book section is designed to assist the user of T3 technology understand thebasic principles and concepts that are associated with DS3 networks. This book is designed toaccompany an instructor presentation, but can also be used as a source of reference.

T3 is an upcoming technology that is being driven by the increase of the needs of customersto support higher bandwidth applications. T3 is commonly seen as the next step up from a T1network for a user to get better response times for data networks as well as consolidation forlarge voice networks.

In this course we will discuss the topics that are important in the setup and understanding oftypical T3 networks. We will start out with a brief history of T3 and the driving forces for T3networks. We will then discuss in detail the construction of the T3 frame, discern thedifferences between various framing methods, what line coding method is used on T3s, mainapplications that are used, and how to troubleshoot T3 networks.

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Course Objectives:

Upon completion of this course the student will be able to:

• Understand the basic T3 transmission concepts

• Understand how the T3 frame is constructed• Discern the differences between M13 and Cbit framing formats• List the main benefits of the Cbit framing format

• Understand the functions of the Cbits in the Cbit framing format• Describe T3 alarm states (Blue, Yellow, Red)

• Understand the wiring that is used on T3 lines• Understand the line coding methodology of T3 (B3ZS)• Understand T3 troubleshooting techniques and equipment

T3-ServiceT3-Service

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History of T3

T3 History

T3 initially was designed to assist carrier offices in consolidating multiple lower speedconnections(T1) into a single high speed communication path. The carriers moved to T3technology in the late 1970’s. T3 was initially implemented based on AT&T Accunet T45services that were essentially designed for point-point services, with a multiplexed format of28 T1s that could be individually identified.

T1 and T3 were not initially designed to provide service directly to the end user. It wasperceived that while the carriers would need high bandwidth paths to consolidate many slowerlinks, the end users wouldn’t need such high bandwidth solutions.

Users started to need higher bandwidth solutions in the late 1980’s to implement very largefile transfers, high bandwidth graphics applications as well as consolidate WAN and LANnetworks.

Today more and more technological advances are being made by utilizing T3 technology asthe physical interface between multiple sites. Packet and cell based technologies are alreadyimplemented over T3 and Fiber links. Smaller companies are finding T3 more affordable astariff rate prices drop on T3 lines due to high demands into higher bandwidth solutions such asfiber.

We will now look into the benefits and features that make T3 so attractive.

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History of T3

T3 History

• Late 1970’s to Early 1980’s

• Initially used only at Carrier Offices

• Drive to T3 reflection of newer servicesand increase technology of customers

• Large file transfers, WAN, LAN,Graphics

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T3 Features/Benefits


Todays user applications have changed significantly from the applications of yester year.Todays user isn’t satisfied with applications that are limited by the particular system he/she ison. Nor do they want to have to purchase several expensive pieces of equipment that does thesame thing only because the employees are spread out to several regional areas. Todayscustomer wants access to all software and devices on multiple LANs (Local Area Networks)by utilizing WAN(Wide Area Network) technology to interconnect several or just a few mainsites together. By doing this the user conserves vital resources by being able to share devicessuch as mainframes, printers, and disk space. With this move to interconnecting of multiplesites as well as the ability to send and run large batch jobs/ applications to various locations,T3 is looking to be a cost feasible solution to get higher bandwidth for better response times onthe network and lower costs.

Today the break even point (cost equivalent point) between lower speed T1 services and T3services are approximately 5-7 T1s(depending on locations). T3 also offers good networkperformance monitoring through error checking and terminal data link connection to statistics(available in CBit only). By using T3 Customers are able to run a lot of bandwidth extensiveapplications for instance Video, graphics(MPEG), as well as voice.

We will now take a look at an overview of T3, then take a few steps back and review somefundamentals concepts that make T3 easier to understand.

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T3 Features/Benefits

• Increase in bandwidth requirements• Break even point of T1 to T3 is between

five and seven T1s• Improved network performance• Multiple protocol (M13, Cbit, Syntran)• Expanded range of applications

– Mixes a variety of different services (video,graphics, host-to-host, multimedia and voicecommunications)


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What is T3?

Concepts Behind T3

If someone were to ask what T3 was in a few words here are some things that you couldsay. You could say that it is a two cable channelized or non-channelized service that is fullduplex in nature and is a 44.736 Mbps digital service. In addition it can be a multiplexing of28 T1’s and is provided by most major carriers. Let’s look at what this means in layman’sterms.

Two Cable:

T3 commonly is seen on Coaxial Cables that have an impedance of 75Ohms(particularly aRG59 type). One cable is used for transmit of information and the other is used for receive.

Full Duplex: By utilizing 2 cables(one for TX, one for RX) T3 operates in a full duplexenvironment. This means that you can both TX and RX data simultaneously.

Channelized or Non-Channelized: This means that the T3 payload data can either be used inindividual groupings of the whole T3 data(channelized) or as just one large group containingthe entire T3 bandwidth. This is important in todays market as by having the versatility to beeither the customer can customer fit a solution for him/her. Channelized T3 is especiallyhelpful for sharing the cost of T3 with several different media(be it video, voice, data..)

44.736 Digital Service: Keep in mind that this is the line data rate for T3, not the actual userthroughput.

Multiplexing of 28 T1’s: This was one of the first applications of T3 used in the carriers toconsolidate many lines. This was accomplished through the use of a device called an M28multiplexer.

DS3 Vs T3: It is important to note that you will here these two terms used synonymously.DS3 is actually the contract with you and the carrier and defines the service at what cost youwill receive. T3 is a DS3 signal that is sent over terrestrial lines.

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• Multiplexed stream of 28 T1s– additional 1.5 Mbps of overhead

• Full Duplex 2 coax cables for TX and RX

• 44.736 Mbps Digital Service

• Channelized or Non-channelized Service– Channelized for 64kbps to 1.544Mbps

applications

– Non-channelized for high speed dataapplications

• All major carriers provide service

• DS3 Vs. T3

What is T3?

Concepts Behind T3

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T1 Fundamentals Review

Concepts Behind T3

T1 is a high speed digital service. A T1 frame consists of 193 bits which are sampled at a rate of 8000frames per second yielding a T1 line data rate of 1.544Mbps. The T1 frame is segmented into 24 64Kbpschannels (also known as DS0s) which when concatenated together yield a data line rate of 1.536Mbps.The remaining 8Kbps are used for framing.

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T1 Review

T1 Fundamentals Review

What is T1?• High Speed Digital Service

• Time Division Multiplexed Signal

• 24 8 Bit Channels (DS0s)

• 193 Bits per Frame

• 8000 Frames per Second

Concepts Behind T3

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T1 Fundamentals Review: Components

Concepts Behind T3

The main components that are required to make a decision to buy or use a T1 service are as follows:Framing, Coding, Diagnostic information.

Framing: There are two types of T1 framing methods, namely ESF and D4. The D4 frame format uses 12 T1frames to form a superframe. The 8 Kbps of overhead in the superframe is used to maintain linesynchronization as well as detect framing errors.

The ESF frame format is built off of the D4 framing format, but uses a grouping of 24 T1 frames andcalls them an Extended Superframe. The extended superframe uses the 8Kbps to provide linesynchronization, CRC checking and a FDL link. If possible, the user should always choose the ESFformat as it is more featured than D4 and can detect logic as well as format errors on a T1.

Coding: There are two types of line coding used on T1s. The AMI line code is a standard bipolar signal thatdoes not take into account line ones density requirements. As such, when using AMI line coding the usermay need to rob bits from the payload data to ensure that 1s density is met. This would result in a reducedpayload data rate (1.344Mbps).

B8ZS line coding accounts for 1s density requirements. B8ZS will insert intentional violations in the4th and 7th positions of a DTE word with a string of bits that exceed 1s density requirements. As a B8ZSline takes into account 1s density, a full payload data rate of 1.536Mbps can be achieved by the DTEdevice.

Errors: There are different types of errors that can occur on a T1. Framing errors and BPV are termed asFormat errors as they violate T1 framing and data transmission rules. CRC errors are logic errors that canbe detected by a T1 line using the ESF frame format. Logic errors are pulses that are inserted or deletedfrom a data stream which may or may not cause format errors.

There are three main alarms generated and declared by T1 devices. These are Red, Yellow and Blue.The red alarm indicates a LOS or LOF. Yellow alarms represent that the remote end device has received ared or blue alarm. The Blue alarm is a Keep Alive alarm that is generated when a T1 device notices a lossof DTE CSU signal or Yellow alarm.

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Framing• D4 or ESF

Coding• AMI- Needs to be checked for 1s density

• B8ZS- Corrects for 1s density problems

Types of Errors• Framing, CRC, BPV

• Alarms: Red, Yellow, Blue

T1 Fundamentals Review: Components

Concepts Behind T3

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Concepts Behind T3

T3 Concepts: T1 Frame The next slide shows what the previous slide said in words. Another reason that this picture is here isthat we may refer back to it to show bit interleaving up to higher data rates.

This is the review, now let’s look closer at how T3 was recreated and what steps need to be taken.

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Concepts Behind T3

T3 Concepts: T1 Frame

. . . .1 2 3 4 24 F

T1 Frame

1 0 1 0 0 0 0 0

DS 0Occurs at

8000 Times/Sec

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T3 Concepts

Concepts Behind T3

The first question that usually pops into the mind when someone mentions the line data rate of T3 iswhy? There is a lot of history behind T3, that led AT&T to choose 44.736Mbps. Really a lot of thereasons are outside the realm of this class, but it has to do with the old radio systems and L carriers. Canthe T3 data rate be easily obtained based on how many T1’s are multiplexed? The answer is no. Thereason is that the overhead on T3s are two fold. One from going from T1 to T2. Then T2 to T3. One ofthe dilemmas that arose in creating M28 multiplexers for a DS3 signals is that all the T1s need to beoperating at the exact same rate so that they can be recovered at the remote end of a network.

T1’s are synchronous in nature. That is that they are dependent on timing, and that timing is recoveredfrom the the data bits themselves as there is no clocking wire on a T1 network. Each T1 has it’s owntiming source and as such may be asynchronous to one another (or no common clocking reference). To getby this hurdle, bit stuffing was created so that T1s that were not running at the exact same rate can have abits added to make all the incoming T1s into the multiplexer operate at the same rate. As such sometimesbits had to be added to the T1 streams and sometimes not. This leads the T3 frame needing to have theability to indicate in which T1 or T2 the bit stuffing occurred and more overhead in the T3 frame.

It is much easier to walk backwards by assuming that the DS3 data rate is 44.736Mbps and find out whatyou need from that direction.

In constructing the M13 frame format, bit interleaving is done. Bit interleaving basically means that 1bit from input 1 is sent, followed by 1 bit from input 2 etc....

One other problem with T3s is that it is very difficult to get individual T1 and T2 information by lookingjust at the T3 frame. Since it is interleaved and stuffed(to get common input to multiplexer rates the same)to look at the information in the frame with out taking it apart is impossible. That was one of the reasonsSyntran was created which is synchronous T3.

In conclusion, the T3 line data rate is 44.736Mbps is due to a two step multiplexing process in which bitinterleaving and bit stuffing occur. The bit stuffing is what makes the T3 data rate not be the exact as themultiples of T1. The reason for that is that bit stuffing messes up the sampling rate of 8000times per secper frame (for T1), and makes it into 9398 times per sec(T3).

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Why is the T3 data line rate 44.736Mbps?

• Multiplexing of 28 T1s to higher carrier rate

• Not directly based on T1 line data rate

– Each T1 is synchronous to itself

– May not be synchronous to one another

– Need scheme in ensuring T1 signal can berecovered

• Stuffing

• 4 T1 are made into a T2 using bit interleaving

• 7 T2 are made into a T3 using bit interleaving

One or Two step process

• M13 or M12-M23

T3 Concepts

Concepts Behind T3

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T-Carrier Hierarchy


The T1-T3 digital T-Carrier hierarchy is shown on the adjoining page. Note that 4 T1s make up a T2.Seven T2s make a single T3. Thus 28 T1s can be multiplexed to make a T3 signal. The multiplexer to gofrom T1-T2 is called and M12 multiplexer. From T2-T3 is called an M23 Multiplexer. A device that goesstraight from 28 T1s to T3 is called an M13 multiplexer or M28. M13 (pronounced “M” “1” “3”) meansgoing from T “1” to T “3”. M28 is used to note the number of T1s. The basic frame format used on T3s iscalled M13 and is the building block to other enhancements like C-Bit.

Next we will look into the T2 frame format and construction, as the method is the same in going to T3frame construction.

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T-Carrier Hierarchy

Transmission Levels T1 T2 T3 ---------------------------------------

Designation DS1 DS2 DS3Rate Mbps 1.544 6.312 44.736T1 equiv. 1 4 28

Cable Type 22Awg 18-22Awg RG-59 ABAM ABAM Coax


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DS0

ChannelBankor DSU/CSU

.

.

.

.

1T1 T2

M12 Mux1

2

3

4

T31234567

M23 Mux

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Concepts Behind T3

T3 Concepts:T2 Frame The T2 frame is composed of 4 T1 frames. To the T2 multiplexer all the T1 information(payload andOH or all 193 bits per T1) is looked on as payload. The T2 multipelxer has four input ports, one for eachT1 input. The information is then bit interleaved into the T2 frame.

The T2 frame is broken down to 4 subframes which is further broken down into 6 blocks ofinformation. Four subframes were chosen so that the T2 multiplexer could identify in which T1 bitstuffing had occurred(since 4 T1s are multiplexed = 4 subframes) If the OH(over head) bits were set toone in a certain manner in subframe 1, it would mean that stuffing occurred in T1 #1. This would let thefar end know how to handle it without distorting the data. I will go more into how this works when we talkabout T3. The division of 6 blocks was done to ensure sub-frame alignment and T2 frame error checking.

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Concepts Behind T3

T3 Concepts:T2 Frame

• 4 T1 frames make a T2 frame

• T1 frames are bit interleaved onto the T2

• Each T2 is subdivided into 4 T2 subframes

• Each sub-frame is divided into 6 blocks

• Each block contains 48 bits payload + 1 bit overhead

• T2 Line Data Rate= 6.312Mbps

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Concepts Behind T3


The adjoining slide shows a representation of a single T2 frame. Notice that f represents a single bitper each T1. Each of the 6 T2 blocks within each subframe are 48 bits in length. This works out to be 12bits of information per each T1 input. A T2 frame consists of 1176 bits or information of which 1152 bitsis for payload data. The line data rate of a T2 is 6.312Mbps. As such 5367 T2 frames pass by everysecond. T2 also uses a different coding scheme than either T1 or T3. T1 uses AMI or B8ZS. T2 usesB6ZS(will not be discussed). The overhead bits have different functions to indicate stuffing, framealignment(subframe and multiframe) and error checking capabilities.

We will in the next slides discuss T3 frames. You’ll notice in T3 each block similarly uses 12 bits fromeach T2, just like the T2 block uses 12 bits per each T1. In addition more bits are used for subframealignment, but functions of the overhead for T3 will be similar to T2.

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Concepts Behind T3


f1-N represent information from DS1 number 1-N.

Sub-frame 1

OH OH OH OH OH OH

f1 f 2f 3f4 . . . . . . . .

48 bits

Subframe 2

OH OH OH OH OH OH

f1 f 2f 3f4 . . . . . . . .

48 bits

Subframe 3

OH OH OH OH OH OH

f1 f 2f 3f4 . . . . . . . .

48 bits

Subframe 4

OH OH OH OH OH OH

f1 f 2f 3f4 . . . . . . . .

48 bits

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T3 Issues

T3 ComponentsT3 Components

In the next few pages we will discuss some of the critical information that is required in understandingT3. We will discuss and show what the T3 frame looks like, what the overhead information bits are usedfor and where they are located. In addition we will look at various uses for the M13 format, talk about theline coding technique used as well as discuss the various alarms and statistics required on a T3 line.

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T3 Issues


• Building the M13 frame

• Framing formats

• Line Coding

• Errors and alarms

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T3 Framing: M Frame


The M-Frame format is 4760 bits long. It is subdivided into 7 subframes(chosen for 7 T2 inputs),which are divided further into 8 blocks of 85bits each. We will look at the frame format pictorially shortly.Each block starts off with an overhead bit, which are followed by 84 bits of payload data(84 bits werechosen like in T2 with 12 bits from every T2 (7T2 * 12bits=84).

Each block starts off with an overhead bit and there are 8 blocks per subframe with 7 subframes per M-Frame. This means that there are 56 overhead bits per M-Frame. We will discuss the meaning of each ofthe overhead bits shortly.

Here is how to calculate your throughput rate:

44.736 M bits/sec T3 line data rate

44.736 Mbits/sec * 1M-frame/4760 bits= approx. 9398 M-frame/sec How many M frames/sec

9398 M-Frame/Sec * 4704 bits/M-Frame= approx. 44.21Mb/sec Pay load Throughput

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T3 Framing: M Frame

• 4,760 bit frame• Each frame is divided into subframes

– seven (7) subframes of 680 bits

• Subframes are divided into 8 blocks– 85 bit blocks

• Blocks: 1 overhead bit and 84 data bits• 56 overhead bits per DS3 (T3) frame• Total payload data rate equals 44.21 Mbps


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M13 Framing Format


The adjoining diagram shows the M-frame broken down into just the overhead bits. Note that eachlabeled square represents both the overhead bit as well as the 84 bits of payload information. We willnow discuss why the frame is drawn this way and the function of each of the bits.

The adjoining picture is a helpful way to understand the M-frame. In reality, the M-frame would betransmitted serially (one bit after another). By looking at the M-frame in the spreadsheet format thatfollows it helps the student identify the subframe for stuffing, as well as the general requirements foroverhead.

X Bits: Used for an alarm indication channel to the remote unit. Both X bits must be the same binaryvalue (i.e. 1 or 0). If both X bits are 1, then the network is ok. If both X bits are 0, this means that thefar end device that transmitted the X bits is having a problem(this will cause a Yellow alarm on thereceiving unit). One of the reasons of having 2 bits to do the same function is to assist in seeing errorsin the M-Frame overhead bits themselves.

P Bits: These bits are used as a parity check for the previous M-Frame. Possible values are 11 or 00.The parity method is an indicator of the sum of the previous 4704 payload bits in the previous M-Framebeing odd or even.

F Bits: These four bits per subframe are used as a subframe alignment signal. The repetitive pattern is1001, and helps identify each of the overhead bit positions.

C Bits: Used for different applications depending on M-Frame format for M13 or C-Bit Paritymethods. We will look into the individual purposes of these bits shortly. For M13 these bits identifythe subframes where bit stuffing occurred in the 1st bit of the 8th block of each subframe. Rememberthat stuffing in this case is where T2’s had bits inserted prior to multiplexing to make them all operateat the same data rate.

M Bits: Repeating 010 pattern to assist identifying multiframe alignment and identification of the 7 T2subframes.

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M13 Framing Format

X1 F1 C11 F0 C12 F0 C13 F1X2 F1 C21 F0 C22 F0 C23 F1P1 F1 C31 F0 C32 F0 C33 F1P1 F1 C41 F0 C42 F0 C43 F1M0 F1 C51 F0 C52 F0 C53 F1M1 F1 C61 F0 C62 F0 C63 F1M0 F1 C71 F0 C72 F0 C73 F1

Block 1 Block 8Frame

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5

6

7

• X= Remote alarm indication X1=X2– X1=X2=1 (net ok) X1-X2=0 (Net problem)

• P= Parity check; either both 1 or 0. Indicates odd or even pulses.

• F= Sub-Frame Alignment; 1001; F1=1, F0=0 Identifies OH bits• C= Pulse stuffing indicators depends on DS3 application

• M=Multi-frame alignment; 010; Allows identification ofsubframes.


Stuffing Block

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M13 Framing FormatAsynchronous


The M Frame format was originally designed for the multiplexing of 28 T1s. Today however theM13 format can be used for channelized or non-chanelized data. Channelized data can either be datafrom muliplexed T1s or data that is subdivided to provide multiple or single high speed data channelsthat are a fraction of the T3 payload data (from the 84 payload data chunks that accompanies eachblock.)

We have already discussed the use of the overhead bits previously. Note that for the C bits welabeled them per subframe as C11, C12, C13, C21, C22, C23... C73. This was done to assist theuser/student identify the subframe in question for each of the three C bits. Normally the carrierspecifications identify the C bits by counting up starting with the first C bit in the first subframe up tothe last C bit in the seventh subframe (I.E. C1, C2, ...C21.) The C bits are normally used to identifystuffing that has occurred in each of the individual subframes. For instance all three C bits in subframe1 = 0 would mean that no stuffing occurred. If the three C bits = 1 would identify that stuffingoccurred on T2 input #1.

Why three C bits? The reason for 3 C bits are used as further error checking within the M frame. Ifall three C bits are the same value this would indicate that an error occurred in the overhead bits. Thisis important since the parity check only identifies errors in the payload data.

Digital Link uses the C bits in a proprietary function. Since no T1-T2 and T2-T3 multiplexingoccurs on the DS3 products, no stuffing bit indicators are necessary. Therefore Digital Link uses the Cbits in there own way in the M13 framing format. Digital Link uses the C bits to provide an in-bandcommunications link to the far end device. Digital Link uses the C bits in the 2nd and 7th sub-framesto provide the communications path. Since there are 6 bits that are used and the M frame occurs at9398 bits per second, Digital Link provides a 56.388Kbps channel to manage the far end DS3 product.

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M13 Framing FormatAsynchronous

• Originally used for multiplexing 28 DS1s

• The 56 overhead bits are divided as follows:– 1. 28 bits are used reserved for control (F-bits)

(sub frame alignment) 1001– 2. 2 for low-speed signals between ends (X-bits) either

both 0 or both 1. Used for alarm messages.– 3. 2 bits are used for parity checks on the user data

stream (P-bits). checks 4704 bits.

– 4. 3 bits for frame alignment (M-bits) 010 identifiessubframes

– 5. 21 bits are used for synchronization (C-bits)

Adapted from DS2 to DS3 multiplexing. Indicatesstuffing on per Sub-frame. I.e.. C11, C12, C13DS2 input 1

* The Digital Link T3 product family “M13” parity setting usessome of the C-bits for remote unit communication (end-to-end). The C-bits are not used for stuffing because the M13frame is not actually multiplexing DS2s from differentsources.


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C-Bit Framing Format


The C Bit Parity framing format is an enhancement of the original M13 application. The maindifference is the C Bit Framing format always stuffs the 1st bit of the 8th block in each subframe. Thismeans that the use of the C bits for their original stuffing function would be redundant. Therefore, in Cbit format the C bits can be used for greater management and performance functions on the M frame.The new functions are as follows.

FID: Format Id. This identifies the M frame as a C Bit parity application.

FEAC: Far End Alarm and Control Channel. This function allows the user to send loop up and downcodes as well as performance information to the far end device. We will go over some of the maincodes and alarms that can be sent on this channel in the troubleshooting section of this guide.

TDL: Terminal Data Link. This link is used to send equipment and line information to the remote endunit. Some of the information that is sent is the Equipment Identification Code, the FrameIdentification Code, and Location Code. In addition this channel does allow for a communications linkthat to the remote unit. Note the purpose of this channel is to send information to the remote unit formanagement and maintenance purposes. Also this link for communications to remote unit will be slowas the remote link connection will only be a shared 28.2Kbps channel (in contrast to the proprietary56.388Kbps channel).

CP bits: C bit Parity Bits. These bits serve the same functions of the P bits in the M13 frame formatand are set to be the same value of the 2 P bits. This allows for greater error checking of errors thatoccur in the M frame itself.

FEBE: Far End Block Errors. Are used to indicate that CP bit parity errors were received by thetransmitting device and the transmitting devices is notifying the remote device that it received CP biterrors. Again for enhance error performance of the M frame. Indicates CP bit errors in the M Frame.

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X1 F1 FID F0 ? F0 FEAC F1X2 F1 ? F0 ? F0 ? F1P1 F1 CP F0 CP F0 CP F1P1 F1 FEBE F0 FEBE F0 FEBE F1M0 F1 TDL F0 TDL F0 TDL F1M1 F1 ? F0 ? F0 ? F1M0 F1 ? F0 ? F0 ? F1

Block 1 Block 8Frame

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5

6

7

• FID= Format ID. Set to 1 to indicate Cbit Parity• FEAC= Far End Alarm and Control Channel: These

can be used to send and receive loop up codes andinformation from the far end T3 device.

• TDL= Terminal Data Link; This is to allow terminalcommunications. LAPD format. Equip. ID.

• CP= Cbit parity bits. These are set to the same value asthe P bits to get better analysis of parity information in MFrame

• FEBE= Far End Block Errors: Are used to indicateframing or CP bit parity errors from the transmitter.

• X bits=alarm channel


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• AT&T were the initiators of the C bit parity format. C bit parity framing format was introduced asan enhancement of the M13 framing format. The nicest attribute of C bit parity is that it is only the Cbit functions that change. The next page shows the C bit locations for the new functions. Note the allthe recommended unused or future use C bits should be set to a value of one for standard compliance toC Bit Parity framing format.

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• AT&T proposed for maintenance anddiagnostics

• F-bits, X-bits, P-bits and M-bits are used inthe same manner as M13 Framing

• C-bits are reassigned as follows:Bit No. Definition

C11 Tells whether frame is for C-Bit(1), M13, Syntran

C12 not used

C13 far-end alarm channel

C3x C-bit parity error detection=Pbits in value

C4x far end block error (FEBE) conditions

111=no problem, else cpbit

C5x 28.2 kbps data link (tdl) applic. link equip status

C2x,6x,7x Not used (Digital Link C2x, 7x are used by DigitalLink to provide terminal connection to remote)


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T3 Framing: M13 Vs Cbit


The two main framing formats that are found on T3 lines are M13 and Cbit parity. C bit parityoffers all the same functions as M13 with further enhancement in the ways of a remote communicationslink, better error performance methods, and local transmissions to remote of errors and loopbacks viathe FEBE, TDL and FEAC functions. All in all if given a choice, the user should choose the C bitparity format.

Another item of note between the two formats, is that the C bit parity format uses stuffing all thetime, where the M13 framing format uses the C bits to identify stuffing in particular T2’s.

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T3 Framing: M13 Vs Cbit


• CBit allows for more network diagnostics– Remote Terminal Data links– Better error checking

» FEBE, FEAC, CPbits

• Throughput– Cbit parity for true M13 has mandatory stuffing– M13 uses stuffing c bits for indicators if stuffing has

occurred

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T3 Line Coding: B3ZS


In T1, there are two coding methods that are in use today namely AMI and B8ZS. The mainprinciple in deciding which coding method is best for you (the user) was how best to meet the onesdensity rule. The 1s density rule states that you must have at least one 1 in every 8 bits of informationfor a T1. Due to faster data rates and smaller pulses (.3-.8Vp for T3 Vs. 3 Vp for T1) in T3 anothercoding method is used.

The ones density rule for T3 is that there can be no more that 3 consecutive 0’s in the data stream.This stringent requirement is needed so that timing and jitter problems can be decrease across the T3network. Keep in mind that in T3, like T1, timing for the frame is carried within the data bitsthemselves. As such it is important that there is a sufficient number of 1’s to determine the clockingfrequency of the clocking source. Also note, that like T1, T3 transmits a 0 value as no voltage.

The coding method that is used is called B3ZS(Bipolar 3 Zero Substitution). This is a method bywhich a group of three 0’s is transmitted by the DSU as a substituted word. All devices on a T3 willrecognize the substituted pattern. The general rule for B3ZS is as follows:

There must be an odd number of pulses(ones) between intentional violations.

This means that there are two possible substitutions to be used.

For last pulse before the substitution being:

Odd: Substitute a 00V

Even: Substitute a B0V

(V means intentional bipolar violation (same polarity of last pulse))

(B means a regular bipoloar pulse that follows bipolar rules (of opposite polarity of last pulse))

The reason for the B0V substitution is to make an even pulse sequence into an odd. The reason for the00V substitution is to keep the substitution odd.

39

T3 Line Coding: B3ZS

• T3 requirement for greater one’s density– keep synchronization– reduce timing jitter– 1 pulse in every 3 bits

• B3ZS Substitution Rules:– Always an odd number of ones between two intentional

BPV’s.– Below are the possible substituted words.

Odd Even

0 0 - + 0 +

0 0 + - 0 -


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B3ZS examples


In the adjoining example of B3ZS note that substitutions are done to replace strings of 3 consecutivezeroes. Unlike T1, where information is viewed by 8 bit windows to determine whether or not asubstitution is to occur, T3 replace any string of 3 consecutive zeros.

In the first substitution the last pulse transmitted after intentional violation is odd (given at top ofpage). Then two more pulses happen before 3 zeros. This means that the number of pulses beforesubstitution is still odd (odd + 2 is odd). This means that we will use the substitution of 00V for the 3zeros.

The second substitution has two pulses before another substitution is needed. Two is even, thereforeto make the number of pulses between violations odd we must substitute B0V. The same is true forsubstitution 3.

The fourth substitution has 1 pulse before another substitution is needed. This means we use the00V substitution because the number of pulses between violations needs to be odd.

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B3ZS examples

First violationEven # of pulsesbetweenviolations

Odd pulses Between Violations

Signal to be transmitted (last pulse was odd):

1 0 1 0 0 0 1 1 0 0 0 0 0 0 0 0 1 0 0 0

voltage Levels

+ 0 - 0 0 - + - + 0 + - 0 - 0 0 + 0 0 +

Odd 0 0 V 1 0 0 V

Even 1 1 B 0 V B 0 V


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Application:Point-Point Data


The adjoining picture shows a point to point data application. In this example the router could besending high speed data applications across the T3 link via Digital Link’s DL3100 T3 access device.Another application could be using multiple (up to three) router connections or FDDI routerconnections to best utilize the full T3 bandwidth.

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Application:Point-Point Data


EFS 100 PERCENT

EFS 100 PERCENT

T3

Router

Router

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Application:Packet and Cell Based Technology


The adjoining picture shows T3 being used as a physical transport for frame relay, ATM and SMDS.Note that the DL3100 does not support ATM or SMDS and will only support frame relay if there isanother DL3100 before the frame relay switch. This is because the Digital Link product uses aproprietary means of segmenting the T3 payload data and T3 overhead information. In addition theDL3100s can be used as a transparent data path for ATM and SMDS data from the router device, aslong as it is understood that the DL3100 will not do segmentation and reassembly of the cells. Again aDL3100 will be needed on both the customer site and the switch side of the circuit.

Frame relay is a packet based technology that is optimally suited for transmitting data. Frame relayis a connection oriented protocol, which means a call setup procedure must be followed before data canbe sent. Some limitations to frame relay stem from frame relay packets being variable length and thusmore difficult to process. Usually only data is sent across frame relay networks.

SMDS (Switched Multimegabit Digital Service) is a cell based technology that is a connectionlessprotocol. SMDS uses fixed cell lengths (53 bytes) that are easier to process in the switch, but due tobeing connectionless, no call setup is required. This poses delay issues in the reassembly of theinformation as the cells are received out of order. SMDS is usually good for data and video services.

ATM (Asynchronous Transfer Mode) is a cell based technology that uses the best of both framerelay and SMDS. ATM uses both fixed length cells (53 byte) and connection oriented technology.This means that not only are the packet easy to process at the switches, but are received in the sameorder as sent at the remote ATM device. Therefore ATM can support voice, data and videoapplications across a T3 network.

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Application:Packet and Cell Based Technology


DL3100

Router

TokenRing

Router

DL3100 Router

DL3100

TokenRing

Router

Video

Video

DL3100

DL3100

DL3100

Frame RelayATM

SMDS

T3

T3

T3

** Note that the DL3100 would only be applicable in this application for Frame Relay. Special units are required forCell based Technologies.

T3

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T3 TroubleshootingT3 Troubleshooting

T3 Troubleshooting Tips When troubleshooting T3s it is important to follow an orderly procedure to isolate networkproblems. Since T3s carry an abundance of data it is important the customer limit the actual down timeneeded to test the circuit. Therefore the steps shown on the adjoining page should be a guide to testinga T3 network that may not necessarily be down, but experiencing troubles.

First look at the alarm states posted by the unit, either on the terminal menus or via the front panel(on the DL3100). The possible alarm states are Red, Yellow and Blue. We will discuss what each ofthese alarm states are in the next few pages.

After checking out the alarms, if problems like framing or scrambling of data seem to be the issue,start off by checking out the configuration. Start with the network configuration items for distance andframing, then move on to the data port configuration options.

Once you have verified the configuration is correct, start checking out the statistics to help identifywhat types of errors are being seen on the line.

If you are seeing errors on the T3, you can further determine where the problem lies by usingexternal test equipment in conjunction with available loopacks to find just where the problem lies.

Another cause of weird problems could be that an improper cable is being used. Be sure to use 75ohm coaxial cables (RJ59) so that line reflections due to impedance mismatching don’t occur.

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T3 TroubleshootingT3 Troubleshooting

T3 Troubleshooting Tips

• Look at alarm states

• Check configuration

• Statistics

• Loopbacks

• Wiring- 75 Ohm, COAX for TX and RX

• External Test Equipment

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Troubleshooting: Alarm States

T3 Troubleshooting

The alarm states on a T3 are similar to T1 in operation. A red or blue alarm will cause a yellowalarm indication to be transmitted to the remote.

Red Alarm(CFA): The red alarm is caused by either a LOS (Loss of Signal) or OOF (Out of Frame)conditions. Both these indicators are processed by the network receive on a T3 DSU. LOS is reported,same as in T1, on the occurrence of 175 pulse positions without a pulse. OOF is reported when 3/16 For M bits are in error. CFA=Carrier Failure Alarm

Yellow Alarm(RAI): A yellow alarm condition will be declared when the X bits in the M frame areset to 0. A yellow alarm condition identifies problems on either the declaring units transmit or thereceiving units receive. Can be caused by a Red Alarm or Blue Alarm. RAI=Remote Alarm Indication.

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Troubleshooting: Alarm States

T3 Troubleshooting

• Red Alarm-– LOS- 175 connect pulse position without a 1

– OOF- 3 out of 16 consecutive F bits (Failure declared after2 seconds, cleared within 20 sec(ANSI T1M1)

• Yellow Alarm– Generated when Red alarm occurs. X bits are set to 0

Digital Link

EFS 100 PERCENT

T1 DIGITAL SERVICE MULTIPLEXER Encore Digital Link

EFS 100 PERCENT

T1 DIGITAL SERVICE MULTIPLEXER Encore

IN RED ALARMIN YELLOW ALARM

YELLOW ALARM PATTERN

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T3 Troubleshooting

A Blue (AIS) alarm is caused by a failure between regenerators in a carrier network.

Yellow Alarm(RAI): A yellow alarm condition will be declared when the X bits in the M frame areset to 0. A yellow alarm condition identifies problems on either the declaring units transmit or thereceiving units receive. Can be caused by a Red Alarm or Blue Alarm. RAI=Remote Alarm Indication.

Blue Alarm(AIS): A blue alarm condition will be declared when the framing bits are all normal andthere is an alternated 101010… pattern in the payload data. A blue alarm condition will cause a yellowalarm to be sent from the blue alarm declared unit. AIS= Alarm Indication Signal.

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T3 Alarm States(AIS)

Regenerator Regenerator

Carrier Cloud

RouterDSU/CSU RouterDSU/CSU

TX

RX TX

RXXTX

RX

AIS Sent

AIS/Blue DeclaredYellow Alarm Sent

Yellow Sent

Yellow Sent

Yellow Sent

Yellow Alarm Declared

• Blue Alarm– Caused by break in line between carrier

regenerators. Xbits=1 payload alternating 10pattern

• Yellow Alarm - X bits are set to 0

T3 Troubleshooting

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Troubleshooting: Configuration

T3 Troubleshooting

When verifying the configuration, her e are some things to look out for:

Verify correct network framing format (M13 or Cbit): If there is a mismatch in setting the unitswill show constantly incrementing CP bit errors. This would be a good clue of a framing mismatch.The local unit would also show incrementing FEBE errors. Since these errors would be continuousthey would trigger 10 consecutive SES and make the T3 go out of service.

Make sure LBO setting is correct for cable distance: Since a T3 multiplexer is a DSU, it is distancesensitive on how far it can drive a signal. Therefore the user must know how far the T3 access deviceis from the Digital Service Crossconnect Level 3 point (DSX-3).

V.35 config- Make sure SCTE is used for applications >3Mbps: Keep in mind that when usingV.35 for higher bandwidth, SCTE must be used to loop the SCT timing source back to keep the data inphase with the timing. If this is not done the user may see constantly running bit errors on the dataport.

Scrambler- Use scrambling to make sure inadvertent user codes don’t cause an alarm or loop onfiber equipment: Fiber equipment is especially sensitive to loopup and alarm codes in the payloaddata. In order to avoid problems, enable the scrambler on Both locations of the T3 multiplexers. Ifscrambler is enabled on one side and not the other, all data will be lost.

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Troubleshooting: Configuration

T3 Troubleshooting

• Verify correct framing format (M13 or Cbit)

• Make sure LBO setting is correct for cabledistance.

• V.35 config- Make sure SCTE is used forapplications >3Mbps

• Scrambler- Use scrambling to make sureinadvertent user codes don’t cause an alarm or loopon fiber equipment..

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Troubleshooting: Statistics

T3 Troubleshooting

Statistics can be a valuable tool to find out how your T3 network is performing. Below is a list ofthe required statistics and what type of error they indicate.

BPV- Bipolar violation. Possible problems: Noisy line (bit errors)

FE(Frame bit error)- Error in F or M bits. Bit errors not caught by P bit errors. Excessive errors thatcan cause OOF (out of frame) condition. Can be cause by faulty equipment. Not caused by mis-configuration.

P-Bit Parity error- P-bit parity error in DS3 frame. Either P bits don’t match or payload data sumdoes not match what the transmitting device sent. Remember P Bits are a calculation on previous Mframes’ data.

CP bit Parity error-(C-Bit only) CPbit error. (P bits value doesn’t match CP bit values)

FEBE(Far end block error) (C-Bit only)- FEBE bits in subframe4 aren’t all ones. Caused by a CPbit error. Means that CPbit error occurred on remote unit.

Severely Errored Frame(SEF)- OOF 3/16 consecutive F in error. Cause by errors on line.

LOS (Loss of Signal)- 175 consecutive positions no pulses.

ES(Errored seconds)- 1 or more P,CP-bit,BPV,LOS or OOF errors

FS(Unavail. Seconds)- 10 SES.

SES(Severely Error Seconds)- 44 or more error events.

ES-L(Errored Second Line)- Is number secs. with BPV or excess 0’s or LOS or OOF.

ES-P(Errored Second Parity)- one or more P or CP bit error. Reported by unit that receives it.

Scrambler- Use scrambling to make sure inadvertent user codes don’t cause an alarm or loop.

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Troubleshooting: Statistics

T3 Troubleshooting

• BPV- Consecutive pulse of same polarity as previous. Not B3ZS

• FE(Frame bit error)- Error in F or M bits

• P-Bit Parity error- P-bit parity error in DS3 frame.

• CP bit Parity error-(C-Bit only) CPbit error. (No match P bits)

• FEBE(Far end block error)- Not all ones condition on FEBE bits.

– Means that CPbit error occurred on remote unit.• Severely Errored Frame(SEF)- OOF 3/16 consecutive F or M bits.

• LOS (Loss of Signal)- 175 consecutive positions no pulses.

• ES(Errored seconds)- 1 or more P,CP-bit,BPV,LOS or OOF errors

• FS(Unavail. Seconds)- 10 SES.

• SES(Severely Error Seconds)- 44 or more error events.

• ES-L(Errored Second Line)- Is number secs. with BPV, LOS orOOF

• ES-P(Errored Second Parity)- one or more P or CP bit error.

• Scrambler- Use scrambling to make sure inadvertent user codesdon’t cause an alarm or loop.

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Troubleshooting:FEAC Alarms(C-Bit Parity)

T3 Troubleshooting

The FEAC (Fare End Alarm Channel) codes for alarms and loopbacks are shown on the next page.FEAC alarms are sent from the remote end device towards the near end device by means of the C bit inSubframe 1 C bit location 3. When a failure is declared on the remote end unit this is how it notifiesthe near end unit.

FEAC loopcodes are activated by sending the command code on what is going to occur (either loopup or loop down) 10 times before the loopback code that is desired is sent. Once the command isrecognized after 10 repetitions the specific loopback code will be sent. On the adjoining page only theDS3 line loopback codes is listed. This is because all the other loopback codes available pertain only toapplications using multiplexing for multiple T1’s to T2’s and T2’s to T3. Since the Digital Linkproducts do not support this, only the DS3 line loopback code is shown.

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Troubleshooting:FEAC Alarms(C-Bit Parity)

T3 Troubleshooting

• Format= 0xxxxxx0 11111111xxxxxx

DS3 Equip. Failure(Service Affecting) 011001

DS3 LOS 001110

DS3 OOF 000000

DS3 AIS RCV 010110

DS3 IDLE RCV 011010

DS3 EQUIQ FAIL(NON-SERVICE AFFECT) 001111

COMMON EQUIP. FAIL (NSA) 011101

MULTIPLE DS1 LOS 010101

DS1 EQUIP FAIL 000101

SINGLE DS1 LOS 011110

DS1 EQUIP FAIL(NON-SERVICE AFFECT)` 000011

Loopbacks:Line loopback activate: 000111

Line loopback de-activate: 011100

DS3 Line: 011011

(There are more, but are for individual 28 T1 loopbacks.)

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Troubleshooting:Loopbacks

T3 Troubleshooting

The only mandatory loopbacks that are required are those loopbacks actuated through the FEACloop codes. However most vendors also provide user configurable loopbacks as well. These are shownon the adjoining page.

DTE Loopback: On the DL3100 the DTE loopback is actuated on the T3MB Tests menu. The DTEloopback will test the DTE ports, attached cables to DTE device, and T3 Framer of the T3 DL3100.

Network Loopback: On the DL3100 the Network loopback is used to initiate a Network loopbacktowards the T3 network on the local unit that the loopback is configured on. This loopback is used totest the T3 network. The Network loopback is actuated from the T3MB tests menu.

FEAC Loopback: The FEAC loopback is a code that is sent across the FEAC C bit in the C bit parityformat. It will actuate a network loopback on the remote end T3 device. The only part of the DL3100tested is the T3 regenerator. Note that the FEAC loopback is used to test the quality of the T3 line.

DTE/Network Loopback: On the DL3100 this loopback is used to initiate a bi-directional loopbackon the local unit that initiated the test. This test is initiated on the HSDB(High speed daughter board)or HVDB (High speed V.35 daughter board). The DTE/Network loopback loops the data from theDTE device, back towards the DTE device. It also loops the data from the T3MB back towards theT3MB and network, thus testing not only the timeslots allocated to the data port, but the link betweenthe daughter board and T3MB. This test is best to test individual data ports as it doesn’t hinder theother data ports.

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T3MBT3MB T3MB T3MB

FEAC(CBit)

Troubleshooting:Loopbacks

T3 Troubleshooting

L i n e L o o p b a c k ( N e t ) - A c t u a t e w i t h F E A C ( C B i t )

Loopup and down

T3MBT3MB T3MBT3MB

D a u g h t e r B o a r d s

D T E / N E T ( B i - D i r e c t i o n a l )

DTE Loopback

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Test Equipment

T3 Troubleshooting

External test equipment is very useful in troubleshooting T3 networks. There are a few tools thatare necessary to troubleshoot a T3 effectively and these are listed below. Note that external testequipment gives the user the ability to view system quality without relying on the units or servicesbeing tested. Gauging the correct operation of a suspected bad unit with that bad unit is always a badidea.

T-Berd 310: The T-Berd is made by TTC (Telecommunications Techniques Corporation). It is ahigh quality test set used to analyze the signals sent to and received on a T3 network. It gauges signallevels, error levels and general system quality. Note that the T-Berd 310 has the ability to sendcomplicated test patterns in a DS3 framing format. This allows the tester to send a complicated patternthat is expected to be received back in the same manner it was sent. This allows the user to do end-endtesting as well as loopback testing. If the pattern is not the same errors are reported.

Fireberd 6000: T h e F i r e b e r d d e v i c e i s u s e d o n t h e D T E s i d e o f t h e e q u i p m e n t t o p r o v i d e a H S S I o r

V . 3 5 / R S 4 4 9 e l e c t r i c a l i n t e r f a c e t o t a k e t h e p l a c e o f a s u s p e c t b a d D T E d e v i c e . B y s e n d i n g c o m p l i c a t e d

p a t t e r n s t o t h e p o r t o n t h e D C E d e v i c e ( o r D T E ) i t w i l l e x p e c t e d t o s e e t h e s a m e p a t t e r n b a c k . I f t h e

p a t t e r n i s a l t e r e d t h i s i s r e c o r d e d a s a n e r r o r . T h i s w a s t h e u s e r c a n t e s t t h e D T E c a b l i n g a s w e l l a s t h e

T 3 n e t w o r k ( b y u s i n g a n e n d - e n d t e s t ( e n d - e n d t e s t i n g m e a n s t h a t a s i m i l a r p i e c e o f t e s t i n g e q u i p m e n t

i s s e n d i n g t h e s a m e c o m p l i c a t e d p a t t e r n b a c k t o t h e s e n d i n g u n i t , t h u s t e s t i n g t h e w h o l e c i r c u i t a n d

a t t a c h e d d e v i c e s ) ) .

IDS (International Data Sciences): IDS tests sets are used to do the same function as the F.B.6000,but does not support a HSSI interface. Therefore the IDS units are used for lower speed functionaltesting, for instance V.35 electrical interfaces.

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• T-Berd 310• Fireberd 6000• IDS equivalents(Low Speed)

Test Equipment

T3 Troubleshooting

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• Understand the basic T3 transmission concepts• Understand how the T3 frame is constructed• Discern the differences between M13 and Cbit framing formats

• List the main benefits of the Cbit framing format• Understand the functions of the Cbits in the Cbit framing format• Describe T3 alarm states (Blue, Yellow, Red)

• Understand the wiring that is used on T3 lines• Understand the line coding methodology of T3 (B3ZS)

• Understand T3 troubleshooting techniques and equipment

ConclusionConclusion

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References• Telecommunications Transmission Engineering Vol 2 Facilities

– Published by Bell Telephone Laboratories

• AT&T TR 54014 June 1987– Accunet T45 Service Description and Interface Specifications

• ANSI T1M1/91-056 June 1991– Layer 1 In-Service Digital Transmission Performance Monitoring

• ANSI T1.404 1989– Carrier-Customer installation- DS3 metallic interface specification

• Bellcore TA-TSY-000342 March 1990– High Capacity Digital Special Access Devices and Interface Combinations

• ANSI T1.107A 1990– Digital Hierarchy- Supplement to Format Specifications (DS3 Format Applications)

• Bell Communications Research TR-TSY-000009 May 1986– Asynchronous Digital Multiplexers; Requirements and Objectives

• The Fundamentals of DS3: Parts 1 and 2

– Data Communications Magazine. August-September 1989 Pg 67-75

• Bud Bates 1992– Introduction to T1/T3 networking

• Gilbert Held:– Digital Networking and T-Carrier Multiplexing

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Notes

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Notes

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Notes

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Notes

T3 Fundamentals

Documents

Transcript of T3 Fundamentals