Inter-Tel 3000 T1 Technical Training Self-Study Course

(1)

T1 Technical Training

Self-Study Course

(2)

(3)

About This Course

This course is provided as a supplement to basic Inter-Tel 3000 training. It is assumed that those taking this course are already familiar with basic telephony and the Inter-Tel 3000 system.

After completing this course you should be able to: • Understand ISDN, T1, and PRI services • Install the T1/PRI Module

• Program the Inter-Tel 3000 T1 features

Taking the Course

Study all the provided material carefully.

Also refer to the Inter-Tel 3000 documentation as your study guide. It is essential to understanding the Inter-Tel 3000 system.

Taking the Exam

The Inter-Tel 3000 T1 exam is available on your training CD or online at: www.inter-tel3000.com

The exam questions were developed to test your full understanding of T1/PRI on the Inter-Tel 3000. After studying the material, complete the exam. Answer each question, referring to this course to find the best answer.

If you receive a passing score of 80% or higher, you will be directed to fax your results to Inter-Tel University so that your certification can be added to Inter-Tel’s computerized certification list.

If you do not pass the exam, you will be given an opportunity to repeat it and improve your test score.

Being certified on Inter-Tel 3000 T1 entitles you to call Inter-Tel’s Technical Support department when you need assistance. Your certification will be verified at the start of your call.

IMPORTANT

You must complete this course and pass the exam to receive Inter-Tel 3000 T1 certification. Without Inter-Tel 3000 T1 certification you will not be able to receive Technical Support on T1 matters.

(6)

(7)

Introduction to Inter-Tel 3000 T1

Inter-Tel 3000 versions 3.0 and higher support T1 Primary Rate Interface (PRI) services using

t

he Inter-Tel 3000 T1/PRI Module. That means that you can choose digital trunk connection (T1 or PRI) instead of analog trunk connection. Features and benefits of Inter-Tel 3000 T1/PRI Module:

• Additional bandwidth on network connections enabling richer and faster out-of-band signaling for voice and/or data.

• Combined voice and/or data to provide cost savings and streamline resource management.

Digital voice and data trunk connection can be established on Inter-Tel 3000 one of three ways:

• T1 Connection: If you purchased a voice-only T1 service from the ISP, the T1/PRI Module can be connected to provide voice connection without an external CSU.

• PRI ISDN: If you purchased PRI service from your Internet Service Provider (ISP), the T1/PRI Module can be used to provide voice connection, and may be used to provide data connection on a dial-up basis.

• External Channel Service Unit (CSU): If you purchased a T1 service for voice and data from your ISP, the ISP will provide a CSU that splits the connection into separate voice and data elements. An Inter-Tel 3000 CO Module can be connected to analog ports on the CSU using loop start for voice connection.

(8)

(9)

Understanding ISDN, T1, and PRI

This section will provide the basic knowledge for installing and maintaining the central office services of T1 and Primary Rate Interface (PRI).

Although the T1/PRI information contained here is applicable to any T1/PRI service, this document specifically addresses T1/PRI and their implementation in the Inter-Tel 3000 system. The Inter-Tel 3000 system requires at least version 3.0 system software and the installation of the Inter-Tel 3000 T1/PRI Module to support T1/PRI service. Please refer to the applicable Inter-Tel 3000 manual(s) for further information regarding the proper configuration of components, and for information on wiring and physical installation.

T1 Overview

A typical T-carrier system consists of a transmission component, a user interface, and termination equipment, as well as multiplexing schemes and signal

hierarchies. Termination Equipment Termination Equipment Digital Transmission Media Channel Service Unit Channel Service Unit Copper Coaxial Fiber Microwave Radio Channel Bank T1 Multiplexer Transcoder Digital Cross-Connect System

Termination Equipment Termination Equipment Digital Transmission Media Channel Service Unit Channel Service Unit Copper Coaxial Fiber Microwave Radio Channel Bank T1 Multiplexer Transcoder Digital Cross-Connect System

Termination Equipment

Several types of terminal equipment other than the basic switch provide digital connectivity. The equipment can be grouped into three general categories:

• Terminals (channel banks and transcoders): Terminals take analog input and transform it into a digital stream.

• Digital Cross-Connect Systems (DCSs): Digital cross-connects are the interconnection points for terminals, multiplexers, and transmission facilities.

(10)

Channel Banks

A channel bank performs the first stop of call handling. Channel banks are a bridge between the analog and digital worlds, and have two basic functions.

• They convert analog voice to digital code, and digital code to analog voice.

• They combine, or multiplex, the resulting digital streams from several active sessions (voice or data) onto a single stream. They are a combination of the digital terminals and the digital multiplex function. A channel bank is a Time Division Multiplexer used in T1 networks, which also includes the CODECS (Coders/Decoders) for each channel. The channel bank accepts 24 analog voice connections, digitizes the signals, and combines them into a 1,544,000 bps T1 aggregate called a DS1, or digital signal level 1. The CODEC samples each analog voice signal 8,000 times per second and produces an eight-bit digital representation of each sample called Pulse Code Modulation (PCM. The result is a 64,000 bps stream of digital signals called a DS0, or digital signal level zero.

Time Division Multiplexer CODEC CODEC 24 24 Analo g 64 Kbps DIGITAL CHANNELS T1 Digital Link Aggregate 1.544 Mbps Analog

Time Division Multiplexer CODEC CODEC 24 24 Analo g 64 Kbps DIGITAL CHANNELS T1 Digital Link Aggregate 1.544 Mbps Time Division Multiplexer

CODEC CODEC 24 24 Analo g 64 Kbps DIGITAL CHANNELS T1 Digital Link Aggregate 1.544 Mbps Time Division Multiplexer

CODEC CODEC 24 24 Analo g 64 Kbps DIGITAL CHANNELS CODEC CODEC 24 24 Analo g 64 Kbps DIGITAL CHANNELS CODEC CODEC 24 24 Analo g 64 Kbps DIGITAL CHANNELS Analo g 64 Kbps DIGITAL CHANNELS T1 Digital Link Aggregate 1.544 Mbps Analog 4 1 5 3 2 Aggregate

Chan 1 Chan 2 Chan 3 Chan 4

4 1 5 3 2 4 1 5 3 2 4 1 5 3 2 1 5 3 2 Aggregate

Aggregate

(11)

A Time Division Multiplexer (TDM) divides the T-carrier into equal time slices, and assigns one time slot to each channel. The TDM takes the information from each channel, in sequence, and places it into a time slot on the T-carrier. Another TDM at the other end of the link receives the aggregate stream and sorts it out into the original channels.

The T1 multiplexer consists of fundamental elements:

• Channel Interface Units (CSUs): These ports provide the physical level connection of equipment, either voice or data, to the T1 multiplexer. Channel interface units are available for synchronous data and asynchronous data using several types of interfaces or connectors. • Buffer: There is a bulk data memory in the T1 multiplexer. It provides

data bit storage that can be read and written between the channels and the aggregate Time Division Multiplexed link.

• Frame Builder/Multiplexer: The frame builder, sometimes called the Time Slot Interchanger (TSI), multiplexes the information from the channel interface units into an aggregate for transmission over the T1 link. It also demultiplexes the received aggregate into separate channels. The most common framing format is D4 format. This module usually is referred to as a T1 Garage Card.

• T1 Line Interface: The primary function of the T1 line interface is to convert the aggregate stream, from the frame builder/multiplexer, to a format suitable for T1 transmission. This includes converting the signals from unipolar to bipolar format, controlling link transmit and receive functions, framing control, and ensuring sufficient ones density.

• Other Elements: Other elements usually include a timing reference and a controller module that sets up the configurations and routing tables on user command. The controller module may be attached to a supervisory terminal that can be located at the same site or at a remote site.

(12)

Pulse Code Modulation

Current channel banks utilize a standard arrangement of channel assignments called the D4 format. Each eight bits of PCM (Pulse Code Modulation) coded information is inserted into a TDM (Time Division Multiplexing) time slot. The 24-channel structure was carried forward from the analog FDM (Frequency Division Multiplexing) multiplexers for compatibility. Channel banks that conform to the standard are known as D4 Channel banks, or D-banks.

D-banks have equipment to provide proper voice frequency and signalization interfaces to the central office. They also provide filters to limit the input frequency (300 to 3000 Hz voice frequency), so that Pulse Code Modulation techniques employing 8,000 samples per second provide consistent voice reproductions, and provide a means for controlling the timing and

synchronization.

The DS1 (1,544,999 bps) D4 format is used to transmit 24 separate channels of PCM voice or digital data. Each channel is transmitted 8 bits at a time. All 24 channels are grouped together to form a group of 192 bits (24 channels times 8 bits). For synchronization of both end span equipment, every group of 192 bits is preceded by a framing bit (F-bit). Together, all 193 bits make up a FRAME.

1 Framing Bit 192 Bits – Free Form

DS 1 Frame = Total of 193 bits transmitted every 125 Microseconds 125 Microseconds

D4 Frame

1 Framing Bit 192 Bits – Free Form

DS 1 Frame = Total of 193 bits transmitted every 125 Microseconds 125 Microseconds

D4 Frame

125 Microseconds

(13)

D4 Frame

In the D4 format, 24 channels are transmitted with 8 bits per channel, plus one framing bit for synchronization every 125 microseconds (one frame every 125 microseconds). This is repeated 8,000 times per second (1 second = 8,000 125-microsecond intervals) to produce 1,544,000 total bps (193 bits x 8,000 times per second = 1,544,000 bps). 1 Framing Bit Channel 1 8 Bits Channel 2 8 Bits Channel 3 8 Bits Channel 4 8 Bits Channels 5 through 24 8 Bits per Channel

1 Time Slot

D4 Format = 8 Bits Per Channel x 24 Channels, + 1 Framing Bit = 193 Bits every 125 Microseconds

125 Microseconds D4 Frame 1 Framing Bit Channel 1 8 Bits Channel 2 8 Bits Channel 3 8 Bits Channel 4 8 Bits Channels 5 through 24 8 Bits per Channel 1 Framing Bit Channel 1 8 Bits Channel 2 8 Bits Channel 3 8 Bits Channel 4 8 Bits Channels 5 through 24 8 Bits per Channel

1 Time Slot

D4 Format = 8 Bits Per Channel x 24 Channels, + 1 Framing Bit = 193 Bits every 125 Microseconds

125 Microseconds

D4 Frame

There’s more to voice transmission than carrying conversations. When a caller picks up the phone (goes off hook) the request for service must be carried to the central office, or PBX. Likewise, a digitally multiplexed voice channel between PBX’s must allow one side to seize the line, carry dial pulses or DTMF, respond with a busy signal, etc. Collectively, these functions constitute signaling. The presence of a specific signaling condition is coded and multiplexed.

PCM channel banks have a standard way to transmit signaling. A small portion of the channel can be used for signaling with no apparent effect on voice quality. In North America, the least significant bit in every sixth sample per voice channel is devoted to signaling. This is referred to as “bit robbing.” These bit positions aren’t available for voice or data transmission.

Frame 1 193 Bits Frame 2 193 Bits Frame 3 193 Bits Frame 4 193 Bits Frame 5 193 Bits Frames 6 through 8,000 193 Bits Per Frame

24 Channels of 8 Bits each = 193 Bits (8 Bits = 1 PCM Sample)

D4 Format =8,000 Frames of 193 Bits each, transmitted every second = 1,544,000 bps

1 Second D4 Format Frame 1 193 Bits Frame 2 193 Bits Frame 3 193 Bits Frame 4 193 Bits Frame 5 193 Bits Frames 6 through 8,000 193 Bits Per Frame Frame 1 193 Bits Frame 2 193 Bits Frame 3 193 Bits Frame 4 193 Bits Frame 5 193 Bits Frames 6 through 8,000 193 Bits Per Frame

24 Channels of 8 Bits each = 193 Bits (8 Bits = 1 PCM Sample)

D4 Format =8,000 Frames of 193 Bits each, transmitted every second = 1,544,000 bps

1 Second

D4 Format

(14)

The existing worldwide standard for digital voice is Pulse Code Modulation (PCM). The incoming analog signal, representing the loudness of a voice, is sampled 8,000 times per second. The modulator uses the sample to create a very narrow pulse whose voltage (height) is the same as the analog signal. The height of the pulse is then converted to an 8-bit word representing the analog input at the time of the sample. The two-step process converts an analog signal to a digital stream of 64,000 bps (8 bits x 8,000 samples per second = 64,000 bps). PCM Sample 1 8 Bits Information PCM Sample 2 9 Bits Information PCM Sample 3 8 Bits Information PCM Sample 4 8 Bits Information PCM Sample 5 8 Bits Information PCM Sample 6 7 Bits Information 1 Signaling Bit

PCM Sample = 8,000 Samples of 8 bits each/every second = 64,000 bps

1 Second PCM Sampling PCM Sample 1 8 Bits Information PCM Sample 2 9 Bits Information PCM Sample 3 8 Bits Information PCM Sample 4 8 Bits Information PCM Sample 5 8 Bits Information PCM Sample 6 7 Bits Information 1 Signaling Bit PCM Sample 1 8 Bits Information PCM Sample 2 9 Bits Information PCM Sample 3 8 Bits Information PCM Sample 4 8 Bits Information PCM Sample 5 8 Bits Information PCM Sample 6 7 Bits Information 1 Signaling Bit

PCM Sample = 8,000 Samples of 8 bits each/every second = 64,000 bps

1 Second

PCM Sampling

Of the 8,000 samples per second, 6,667 samples are 8-bits of information, 1,333 samples are 7 bits of information, and 1signaling bit (6,667 samples of 8 bits = 64,000 bits of information + 1,333 samples of 7 bits = 9,331 bits of information + 1,333 signaling bits = 64,000 bps).

Notice that 24 channels multiplied by 64,000 bps doesn’t equal the 1,544,000 bps speed of the T1 channel (24 x 64,000 = 1,536,000). The remaining 8,000 bps are the framing bits, used by the channel bank for synchronization.

Synchronization keeps the multiplexers at each end of the link channel-locked with each other so that what goes in at one end on a specific channel comes out the same at the other end.

The “robbed bit” method puts control information in-band (signaling is carried along the same circuit as the talk path), so an individual voice channel can be switched easily. In meeting the framing requirement, 1 bit in 193 is provided by the multiplexer, leaving 1,536,000 bps available to the customer for transmission of voice and data.

NOTE: A T1 span uses in-band signaling. An ISDN Primary Rate Interface (PRI)

uses out-of-band signaling. The PRI consists of 23 B-Channels (Bearer or Information) and 1 D-Channel (Digital or Signaling). The customer has access of 23 channels carrying 64,000 bits of information per second. The D-Channel is used to carry signaling and control information at 64,000 bits of information per second. The D-Channel is used to carry signaling and control information at 64,000 bps.

(15)

Ordering T1

Sample T1 order forms are included in the back of this book. When ordering T1 spans, you must specify the framing scheme to be used on the span. There are two framing schemes available:

• D4 Superframe

• Extended Superframe.

D4 Superframe

A superframe is a repeating sequence of 12 frames and thus contains 12 framing/signal bits (the 193rd bit of each frame). One frame corresponds to 125 microseconds; one superframe is thus 1.5 milliseconds in duration. Each frame contains one synchronization bit to allow the receiving equipment to decode, demultiplex, and allocate the incoming bits to the appropriate channels. Each superframe thus contains a 12-bit word, comprising individual bits from each of the 12 frames. The framing bits are called BFf, and the signaling bits are BFs. BFfs are the odd-numbered framing bits, and BFs are the even-numbered bits. The 12-bit word is used for synchronization and for identifying frame numbers 6 and 12, which contain channel-signaling bits.

Each frame contains a BFf or BFs framing/signaling bit on the 193rd position. The channel bank will “rob” or “share” the 8th bit from the user data stream. For five frames, bit 8 will contain voice bits. On the sixth, it will contain a signaling bit. These combinations of bits allow the end-user termination equipment to carry out its signaling protocol, which involves indicating such states as idle, busy, ringing, no-ringing, loop open, etc.

D4 Superframe Format Frame 1 Frame 2 Frame 3 Frame 4 Frame 5 Frame 6 Frame 7 Frame 8 Frame 9 Frame 10 Frame 11 Frame 12 Superframe Frame 12 With Signaling Frame 12 With Signaling

Chan 1 Chan 2 Chan 3 Chan 22 Chan 23 Chan 24

BFs BFt

B1 B2 B3 B4 B5 B6 B7

BF B8

Signalling Bit = “Robbed Bit”

Frame 6 = A Bit, Frame 12 = B Bit

Framing Bit Speech Bits or Data Bits D4 Superframe Format Frame 1 Frame 2 Frame 3 Frame 4 Frame 5 Frame 6 Frame 7 Frame 8 Frame 9 Frame 10 Frame 11 Frame 12 Superframe Frame 12 With Signaling Frame 12 With Signaling

BFs BFt B1 B2 B3 B4 B5 B6 B7 BF B8 D4 Superframe Format Frame 1 Frame 2 Frame 3 Frame 4 Frame 5 Frame 6 Frame 7 Frame 8 Frame 9 Frame 10 Frame 11 Frame 12 Superframe Frame 12 With Signaling Frame 12 With Signaling

BFs BFt D4 Superframe Format Frame 1 Frame 2 Frame 3 Frame 4 Frame 5 Frame 6 Frame 7 Frame 8 Frame 9 Frame 10 Frame 11 Frame 12 Superframe Frame 12 With Signaling Frame 12 With Signaling D4 Superframe Format Frame 1 Frame 2 Frame 3 Frame 4 Frame 5 Frame 6 Frame 7 Frame 8 Frame 9 Frame 10 Frame 11 Frame 12 Superframe Frame 12 With Signaling Frame 12 With Signaling D4 Superframe Format Frame 1 Frame 2 Frame 3 Frame 4 Frame 5 Frame 6 Frame 7 Frame 8 Frame 9 Frame 10 Frame 11 Frame 12 Superframe Frame 12 With Signaling Frame 12 With Signaling

Chan 1 Chan 2 Chan 3 Chan 22 Chan 23 Chan 24 Chan 1 Chan 2 Chan 3 Chan 22 Chan 23 Chan 24 Chan 1 Chan 2 Chan 3 Chan 22 Chan 23 Chan 24

BFs BFt

B1 B2 B3 B4 B5 B6 B7 BF B1 B2 B3 B4 B5 B6 B7 B8

BF B8

Signalling Bit = “Robbed Bit”

Frame 6 = A Bit, Frame 12 = B Bit

Framing Bit Speech Bits

or Data Bits

(16)

Extended Superframe Format (ESF)

The Extended Superframe format is an extension of the D4 format that was announced in 1981. As old AT&T equipment is replaced, new equipment supporting the ESF is installed, and users are offered the ESF. In addition to benefiting end users indirectly by offering a more reliable digital service, ESF allows the use of the bandwidth to provide more advanced services. Unlike the D4 Superframe, which requires 8,000 bps for housekeeping, ESF requires only 2,000 bps. This implies that the other 6,000 bps become available for other service-related purposes. Among these services might be the user’s ability to reconfigure their networks in real-time from a data terminal.

The ESF has 24 frames in its definition of a Superframe, but only six bits in its framing pattern. Rather than re synchronize every 1.5 milliseconds in the regular format, it only needs to re synchronize every 3 milliseconds.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 3 Milliseconds Extended Superframe Chan 1 Chan 2 Chan 3 Chan 22 Chan 23 Chan 24 Signaling Frames 6, 12, 18 & 24

Channels per Frame

Frame 6 = A Signal Frame 12 = B Signal Frame 18 = C Signal Frame 24 = D Signal B1 B2 B3 B4 B5 B6 B7 B8 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 3 Milliseconds Extended Superframe Chan 1 Chan 2 Chan 3 Chan 22 Chan 23 Chan 24 Signaling Frames 6, 12, 18 & 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 3 Milliseconds Extended Superframe 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 3 Milliseconds Extended Superframe Chan 1 Chan 2 Chan 3 Chan 22 Chan 23 Chan 24 Signaling Frames 6, 12, 18 & 24

Chan 1 Chan 2 Chan 3 Chan 22 Chan 23 Chan 24 Chan 1 Chan 2 Chan 3 Chan 22 Chan 23 Chan 24 Signaling Frames 6, 12, 18 & 24

Channels per Frame

Frame 6 = A Signal Frame 12 = B Signal Frame 18 = C Signal Frame 24 = D Signal B1 B2 B3 B4 B5 B6 B7 B8 B1 B2 B3 B4 B5 B6 B7 B8 Remember that the framing bits are the 193rd bit of each frame. Since the ESF uses 24 frames, the framing bits are seen as a 24-bit word.

The framing bit positions of the extended superframe are partitioned for three separate uses:

1. Six of the framing bits are used for frame synchronization.

2. Six of the framing bits are block-check bits and are used for cyclical redundancy checking (CRC6).

(17)

The CRC6 and the DL bits are features of the ESF that provide capabilities for network maintenance of the circuits.

Framing Bit Summary

D C1 D 0 D C2 D 0 D C3 D 1 D C4 D 0 D C5 D 1 D C6 D 1

Frame Synchronization

6 Bits Data Link Bits _{12 Bits} _CheckingError 6 Bits

Framing Bit Summary

D C1 D 0 D C2 D 0 D C3 D 1 D C4 D 0 D C5 D 1 D C6 D 1

Frame Synchronization

6 Bits Data Link Bits _{12 Bits} _CheckingError 6 Bits

Using the Extended Superframe Format, 2,000 bits per second are framing bits. The cyclical redundancy check (CRC) uses 2,000 bits per second and detects approximately 98.4% of all single and multiple errors. The CRC-6 also provides false-frame protection. The data link (also called the Embedded Operations Channel) uses 4,000 bps for maintenance information, supervisory control, and other future needs. The user has the benefit of using the 6,000 bits for network maintenance and error checking not available with the D4 Superframe format, as all 8,000 bps are required for framing and signaling.

(18)

Coding Methods

When ordering a T1 span, the user must specify the coding method to be used on the span. There are two coding methods available:

• Alternate Mark Inversion (AMI) • Bipolar 8th Zero Suppression (B8ZS)

In the early 1960s, considerable study was given to the choice of a coding method for placing the information signal onto the transmission carrier. A bipolar encoding scheme was selected to prepare the signal for direct application to a T1 copper facility. The choice of bipolar was largely determined by the

characteristics of the copper medium. In bipolar coding, alternating positive and negative pulses represent one state (a binary 1). Absence of bipolar coding pulses represents the other state (a binary 0).

T1 information is transmitted as positive and negative marks (ones) that alternate. A logical 0 is coded as a “zero excursion.” This coding technique is called bipolar return to zero, or Alternate Mark Inversion (AMI).

Bipolar Pulse Stream

-3 Volts +3 Volts

Binary 1 Binary 1 Binary 0 Binary 0

-3 Volts +3 Volts

In this example, the first binary one is +3 Volts. The second binary one is -3 Volts. The absence of any pulse on the line (0 Volts) represents two binary zeros.

(19)

Bipolar Encoding

Bipolar encoding was selected for T1 spans because it possesses the following properties:

• Clocking need not be absolutely perfect for the bit to be decoded, and the sampling need not be done exactly at the maximum signal level (+3 Volts). Any time that voltage is present, a binary 1 is read, even if the voltage is below the specified level (-3 Volts). If no energy is present, then a 0 is read.

• Any single-bit error can be detached, which is useful since copper facilities are subject to a variety of natural (e.g. lightening) and

mechanical (induction motors, elevators, power lines, etc.) noises. With bipolar inversion, the polarity of any pulse must be opposite to the polarity of the preceding pulse. The pulse is normally 3 volts in absolute

amplitude. If an error occurs in a 1-bit position, thereby converting it to a 0, adjacent 1s will be of identical polarity that is easily detectable since it violates the polarity rule. If an error occurs in a 0, converting it to a 1, there will also be two consecutive 1s of identical polarity, which also violates the polarity rule. These are called “bipolar violations,” or “BVP.” • Remote maintenance margin testing. At least 50% of the stream should

be 0s.

Bipolar Violations

Bipolar violations (BPV), as described above, are important in T1 transmission. The main limitation of Alternate Mark Inversion (AMI) coding is the absence of timing information when the message signal consists of a long sequence of binary zeros. This can be overcome by imposing restrictions on the message signal. In North America, this is referred to as the “ones density requirement” to maintain synchronization at least 12.5% (one-eighth), on the average, must be binary ones and there can be no more than 15 consecutive binary zeros.

(20)

Bipolar 8th Zero Suppression (B8ZS) Coding

Traditionally, in meeting the ones density requirement, a further 1 bit in 8 is required to be permanently set to a one, leaving a maximum of 1,344,000 bps available to the customer equipment. Recent introduction of B8ZS encoding (bipolar with eight zero substitution) of the T1 data eliminates the requirement of ones insertion, leaving 1,536,000 bps of “clear channel” available. Not all central office equipment currently supports B8ZS coding.

Bipolar 8th Zero Suppression (B8Zs) is a zero code suppression scheme. The ones density condition (12.5% [one-eighth] on the average must be 1’s and no more than 15 consecutive zeros can occur) can be met using the B8ZS

algorithm. (B8ZS is a favorite of ISDN for provision of the Clear 64 service.) With B8ZS, deliberate bipolar violations in the frame (192 bits of PCM plus one

framing bit) are substituted whenever eight consecutive zeros occur in the bit stream from which the frame signal is derived.

1 0 0 0 0 0 0 0 0

Frame Signal without B8ZS Coding = Transmission of a Binary 1 and Eight Binary Zeros

Bipolar 8th_{Zero Suppression}

1 0 0 0 1 0 1 1

Frame Signal with B8ZS Coding =Two Bipolar Violations Replace the String of 8 Zeros 1

1 0 0 0 0 0 0 0 0

1 0 0 0 1 0 1 1

1 0 0 0 0 0 0 0 0

1 0 0 0 1 0 1 1

The “Bipolar 8th Zero Suppression” figure above illustrates the impact of B8ZS coding. The top portion of the diagram shows the coded output of a channel bank without B8ZS. The bottom portion of the diagram shows the effect of B8ZS - two bipolar violations (two contiguous pulses of the same voltage polarity) are deliberately inserted, replacing the string of eight zeros. The specific pattern described is recognized as a legal indication of eight successive zeros and is not a bipolar violation.

(21)

Channel Service Unit

So far we’ve talked about the terminal equipment, the channel bank, the framing format, D4 Superframe or Extended Superframe, and the coding scheme,

Alternate Mark Inversion or B8Zs. There is another key piece of CPE supplied by the user - the Channel Service Unit.

Termination Equipment Channel Service Unit Channel Service Unit Termination Equipment Digital Transmission Media Copper Coaxial Fiber Microwave Radio Channel Bank T1 Multiplexer Transcoder Digital Cross-Connect System Termination Equipment Channel Service Unit Channel Service Unit Termination Equipment Digital Transmission Media Copper Coaxial Fiber Microwave Radio Channel Bank T1 Multiplexer Transcoder Digital Cross-Connect System

The Channel Service Unit (CSU) interfaces the transmission facility (T1 span) to the user’s termination equipment (channel bank, T1 multiplexer, a digital cross-connect system, or a PBX). In digital transmission, precise synchronization is essential, and the CSU is a key element of this synchronization process.

Basically, the CSU ensures a high-quality digital signal is sustained into and out of the network.

The CSU can transmit and receive. In transmit mode, the device regenerates the digital signal received from the user’s equipment, checks for bit stream errors, and applies the regenerated signal to the transmission facility (T1 span). In receive mode, the CSU regenerates the signal received from the network, checks for remote loop back codes, and applies the signal to the customer’s equipment. The FCC Rules, Part 68, require the CSU to provide certain network functions. In the event of termination equipment failure, the CSU must send a continuous stream of one pulses, called “Network Keep Alive,” to the T1 span and central office.

CPE Channel Service

Unit

T1 Data T1 Data

Repeater C.O.

Unit

T1 Data T1 Data

Repeater C.O.

Unit

T1 Data T1 Data

(22)

Typically, CSUs are transparent to the data stream, but they must provide a ones-density monitor that will force the mandated ones density requirement (12.5% on the average must be binary ones) onto the data stream if it does not comply. Unfortunately, it doesn’t recover the inserted ones at the far end, implying data errors if the customer data stream doesn’t meet the ones density requirement. The network was designed for voice traffic where occasional data bit errors by random ones insertion are inaudible.

T1 network performance is optimized when facilities exist to detect and isolate line and equipment problems. In-band and out-of-band signals at the network interface enable maintenance personnel to conduct local and remote span diagnostics. Front-panel switches on the CSU enable users to generate line-loop back and test-loop back signals to diagnose local equipment problems. (Loop back is the return of information back to the source at any one of a number of locations in the network.) The CSU occupies a critical position in the network and is used to aid in the detection and isolation of problems.

Channel Service

Unit

Repeater Central

Office

Digital Loop Back

Channel Service Unit Repeater Central Office Channel Service Unit Repeater Central Office

Digital Loop Back

Some of the tests performed by a CSU interrupt normal operations. Loop back tests, for example, affect service. More sophisticated CSUs can monitor

performance of the T1 link on a real-time basis, and concurrently support traffic. These CSUs can also activate alarms based on selectable thresholds and report them to a central location. Some of the faulty conditions reported by a CSU are signal level, all “1s” condition, loss of synchronization, framing error, bipolar violation, jitter, bit error and errored seconds rate.

Channel Services Units can store and display facility performance parameters, including errored seconds, failed seconds, and bit error rate. If properly

engineered, the CSU can provide conversion between D4 alternate mark inversion and ESF signals, thus allowing end users to retain the existing

(23)

Repeaters

The next point in the network is the repeater. When a signal is transmitted down a cable pair, it is attenuated (loses strength) and distorted (noise is introduced) by the characteristics of the cable. However, in digital transmission, the signal intelligence is defined by the presence or absence of pulses, not by the shape as in an analog carrier. Since the repeaters detect the presence or absence of pulses and restore them to their original form and amplitude (height), the signal arrives at the far end as a nearly exact replica of the transmitted signal. Since noise and distortion are removed from the signal each time it is restored

(regenerated), the signal can be transmitted over long distances and arrive at its destination in a zero-loss state.

^^^^ CSU Last Line Repeater Repeater Repeater Last Line Repeater Office Repeater

Fault Locate Pair T1 Data Maintenance Order Wire 3,000 Ft. Maximum 1 Mile Maximum Up to 40 Repeaters 1 Mile Maximum 3,000 Ft. Maximum Customer

Premise Central _Office

^^^^ ^^^^ ^^^^ ^^^^ CSU Last Line Repeater Repeater Repeater Last Line Repeater Office Repeater

^^^^ ^^^^ CSU Last Line Repeater Repeater Repeater Last Line Repeater Office Repeater

CSU Last Line Repeater Repeater Repeater Last Line Repeater Office Repeater CSU Last Line Repeater Repeater Repeater Last Line Repeater Office Repeater

^^^^

^^^^ ^^^^

The T1 span will typically have many repeaters up to a practical maximum of 40, and they are separated by distances which are determined primarily by the cable attenuation (signal loss) and the ability of the repeaters to filter out pulse jitter (timing loss).

(24)

Regenerative Repeaters

Repeaters for digital signals are called “regenerative repeaters” since they create brand new pulses to send along the line. The repeaters examine each incoming “smeared” pulse and decide whether the pulse is binary one or zero. Based on the decision made, either a brand new one or zero is created and sent along the line. Noise or other impairments are not amplified and also sent along as with analog transmissions. Repeaters are usually placed about one mile apart to be able to reliably regenerate the 1,544,000 pulses per second.

Repeater

New Pulse Starts Out

Regenerated Signal

Pulse Travels Over Cable Pulse Gets “Smeared”

Repeater

Regenerated Signal

Repeater

Regenerated Signal

Independent transmit and receive paths are provided to and from the central office, using four wires (two for transmission in each direction). The repeaters are bi-directional. Historically, the central office has provided power for the span repeaters. This is slowly changing as fiber optics are being implemented. Fiber cables cannot carry the power.

Repeater Office Repeater Central Office Switch Control T1 Data T1 Data T1 Span Repeater Office Repeater Central Office Switch Control T1 Data T1 Data T1 Span Repeater Office Repeater Central Office Switch Control T1 Data T1 Data T1 Span

(25)

Network Timing

T1 networks are designed to be synchronous networks. That is, data clocked in at one point in the network has a fixed timing relationship to the point in the network at which the data is clocked out. This means that the speeds at both points are the same, and there is always a fixed frequency relationship between clocks that transmit and receive the data. This is usually referred to as “frequency locked.”

The North American T1 network is derived from a series of network multiplexer unsynchronized clocks using a technique called “pulse suffering” to overcome clock inaccuracies and fluctuation. However, this does not eliminate network synchronization at the T1 level. Synchronization of the network at the T1 level is achieved by framing (D4 or ESF in North America) the data streams and

frequency locking the node and network clocks. Loss of synchronization results in “frame slips.” A frame slip is a condition in which framing is momentarily lost, as well as network information, generally resulting in data loss.

There are a number of ways to synchronize a digital network consisting of TDM nodes and T1 transmission links:

• Plesiochronous

• Network master clocking • Master-slave clocking • Network-wide pulse stuffing

In a T1 network, the bipolar pulse transmission technique is used because clocking information is embedded in the data. Each pulse must last at least 50% of the clock interval. There are 1,544,000 clock intervals per second, 650

nanoseconds each. Any delays in the pulse stream result in equal delays of the clock and data. During transmission, clock and data are essentially locked together so distribution of the T1 aggregate links provides a source of network timing transmission.

Plesiochronous

A plesiochronous (pronounced please-e-ock-ronus) doesn’t synchronize the network but uses highly accurate clock at each node so the slip rate between nodes is acceptably low. These clocks are very expensive and would be needed at every node. This method was not chosen for implementation in North America.

Network Master Clocking

This method involves selecting one node as the master clock reference node. The master clock is distributed to outlying nodes, enabling them to lock to the common master reference. This is a simple technique, with the outlying nodes operating in a loop-timed node. Reliability considerations are an issue because

(26)

Master-Slave Clocking

The major consideration in configuring a network master synchronized network is the need for reliable network transmission. The distributed nature of the network topology can be used to distribute a master reference by way of the T1 links themselves.

In this typical configuration, the network master reference frequency is

transmitted to selected slave nodes. These nodes synchronize their clocks to the reference and pass on to lower level nodes the network timing by way of the T1 transmission links. The process of passing the reference downward from one level to another is referenced as master-slave synchronization.

S

M

Slave Node Slave Node Slave Node Master Clock Reference Node

S

M

Slave Node Slave Node Slave Node Master Clock Reference Node

With master-slave synchronization, all nodes are either directly or indirectly synchronized and all run at the same nominal clock frequency. If the master node or one of the master node level 1 or level 2 links should fail, it’s necessary to rearrange the network clocking hierarchy. It would be possible to lock levels 2 and 3 together as independent “timing islands” by assigning one of the level 2 branch nodes as the alternate network master.

(27)

Channel Pulse Stuffing

Another method to prevent frame slips is called “pulse stuffing,” or justification. This method avoids both slips and the requirement for clock synchronization. Pulse stuffing can be used at both the channel level to correct for data terminal equipment timing differences and at the T1 level to overcome network timing discrepancies.

Channel pulse stuffing uses a data channel between end points of a network that runs at a slightly higher rate than the input channel speed. The data channel can carry all input data plus some variable number of bits or “stuff bits.” These bits are inserted to “pad” the input data stream to a higher rate. This technique is used for T-3 and T-4 networks when the incoming lower level signals (T1s) are not synchronized to each other. Pulse stuffing is usually used in satellite applications where the T1 network operates on a timing source independent of the attached channels.

Jitter and Timing Inaccuracies

No matter how stable the clocks are at both ends of a digital transmission

system, certain amounts of instability occur in the received signal. Typical causes are path length changes, receiver nose and repeater regenerator inaccuracies. There is a simple way of viewing hitter on a T1 circuit. Jitter implies that the start and end times of the bit are not always exactly where they should be at the same time. The signal actually jitters back and forth with respect to the rise and fall time of the signal. This degradation is also called “phase jitter.”

Data Stream Jitter

(Clock Interval is 650 Nanoseconds) Data Stream

Jitter

(28)

Testing Network Systems

As a corporation’s communications traffic is increasingly put through a small set of separate and distinct facilities, the potential liability from either catastrophic failure of the communications vehicle (termination equipment of lines) or from a deterioration of the service due to a partial impairment of some of these facilities increases. This raises the critical issue of testing and diagnostic procedures and equipment that must be put in place by a user of the T1 facilities to monitor, prevent, or resolve any problems.

In-service tests can be performed while the facility is carrying actual traffic data. Service-disruptive testing, for example, loop-around and direct bit error rate (BER) testing means that the link must be taken out of service.

In-Service Testing

The access tester is a basic type of testing equipment that provides access to the individual channels and to the A/B signaling bits, and may also provide

monitoring for error performance, integrity checking, and bit-stream pattern synthesis. Access testers are ideal for voice applications.

Noise and cross-talk can affect the bipolar signal to the point where a no-pulse condition is interpreted as a pulse condition. The resulting bipolar violations can be counted. When the BVP violation rate is high, it is generally indicative of some inherent problem. An instrument that measures this data can be useful in

isolating the problem and is ideal for interLATA and intraLATA facilities, and less useful for non-telco facilities.

Framing errors occur when the receiving equipment or a repeater is incapable of recovering the clock. Excess zeros in the bit stream can cause the repeaters in the network to shut down and put the facility out of service. Instrumentation capable of detecting excess zeros can help prevent or resolve problems. The T1 signal must satisfy certain defined electrical levels (+3/-3 Volts) at the repeater, the CSU and the termination equipment. Signals exceeding the specifications will cause cross-talk on other circuits (e.g. circuits going through the same repeater). Low signals will have noise.

Jitter occurs when timing doesn’t meet the specification of within 50 parts per million. Jitter implies timing slips and data errors. Jitter control and the capability of the various T1 components to tolerate the hitter are important in establishing a reliable network. Jitter measurement is a sophisticated and fairly technical in-service test, and some detailed knowledge of the various jitter specifications is required.

(29)

Service-Disruptive Testing

There are times when out-of-service testing is the only recourse available. These types of tests may involve test equipment at each end, or an instrument

connected through a loop-around arrangement. In-service testing will detect only 50% of the signal errors. Two methods are used for this type of testing. The first involves accessing the CRC data provided by ESF. The second method involves out-of-service bit error rate (BER) testing.

(30)

T1 Networks

T1 networks have evolved considerably over recent years. Advances in

technology have improved efficiency, flexibility and reliability. Software control of the equipment has enabled many complex and tedious functions to be

automated. Network management software can monitor the network and take corrective action when problems occur, sometimes without the users even being aware of the problem.

Advances within the network have enabled new services to be offered to meet a wide variety of applications and reduce communications costs. In this section we will discuss the most commonly used T1 network topologies.

Point-to-Point T1 Networks

The most basic form of T1 network is the point-to-point link. One T1 multiplexer is at point A, a single 1.544 Mbps communications link, and another T1 multiplexer is at point B. The channels, or ports, can be connected in any combination to voice, data, facsimile, or video applications, based on he capability of the termination equipment. T1 Multiplexer Chandler, AZ T1 Multiplexer Phoenix Branch T1 Link 1.544 Mbps Fax ^^^ ^^^ ^^^ ^^^ 64 KBps Channels Host Voice T1 Multiplexer Chandler, AZ T1 Multiplexer Phoenix Branch T1 Link 1.544 Mbps Fax ^^^ ^^^ ^^^ ^^^ 64 KBps Channels Host T1 Multiplexer Chandler, AZ T1 Multiplexer Phoenix Branch T1 Link 1.544 Mbps Fax ^^^ ^^^ ^^^ ^^^ 64 KBps Channels T1 Multiplexer Chandler, AZ T1 Multiplexer Phoenix Branch T1 Link 1.544 Mbps Fax ^^^ ^^^ T1 Multiplexer Chandler, AZ T1 Multiplexer Phoenix Branch T1 Link 1.544 Mbps T1 Multiplexer Chandler, AZ T1 Multiplexer Phoenix Branch T1 Link 1.544 Mbps T1 Multiplexer Chandler, AZ T1 Multiplexer Phoenix Branch T1 Link 1.544 Mbps Fax ^^^ ^^^ ^^^ ^^^ 64 KBps Channels Host Host Voice

(31)

Point-to-Point Multiple Links

Point-to-point applications can also operate with multiple T1 links. Multiplexers with more than one link can operate in several ways. One link may be used as the primary link, and the second as a backup. If the primary fails, the multiplexer will automatically switch to the backup.

Multiple aggregate multiplexers are the functional equivalent of two or more multiplexers in one cabinet, to provide additional capacity. The links are sometimes set up in a “load sharing” configuration, where the traffic is routed over both links more or less equally. This arrangement also provides an automatic backup for when one link fails; all traffic is sent over the remaining operational links. The links are usually routed to the distant location over different paths to avoid common points of failure. This is called “diverse routing.” The carriers can do this quite easily for the long haul network, but the local loops from the customer premise to the central office are more difficult to diversify.

Printer PC Voice T1 Link 1.544 Mbps Fax ^^^ ^^^ ^^^ ^^^ 64 KBps Channels Host Multiple Aggregate T1 Multiplexer Chandler, AZ Multiple Aggregate T1 Multiplexer Phoenix Branch T1 Link 1.544 Mbps Printer PC Voice T1 Link 1.544 Mbps Fax ^^^ ^^^ ^^^ ^^^ 64 KBps Channels Host Multiple Aggregate T1 Multiplexer Chandler, AZ Multiple Aggregate T1 Multiplexer Phoenix Branch T1 Link 1.544 Mbps T1 Link 1.544 Mbps Fax ^^^ ^^^ ^^^ ^^^ 64 KBps Channels Host T1 Link 1.544 Mbps Fax ^^^ ^^^ ^^^ ^^^ 64 KBps Channels T1 Link 1.544 Mbps Fax ^^^

^^^ _{T1 Link 1.544 Mbps}_{T1 Link 1.544 Mbps}_{T1 Link 1.544 Mbps}

Fax ^^^ ^^^ ^^^ ^^^ 64 KBps Channels Host Host Multiple Aggregate T1 Multiplexer Chandler, AZ Multiple Aggregate T1 Multiplexer Phoenix Branch T1 Link 1.544 Mbps

(32)

Point-to-Multipoint Networks

When the full 24-channel capacity of a T1 link is not needed at some of the remote locations, a point-to-multipoint network can be the most cost effective configuration. These networks are similar to multidrop or multipoint links used in analog modem networks. The T1 links are routed from point A to point B, from B to C, and so on. Point B Point C Point A A to B T1 Link B to C T1 Link 24 Channels Channels 1-12 Channels 13-24 Point B Point C Point A A to B T1 Link B to C T1 Link 24 Channels Point B Point C Point A A to B T1 Link B to C T1 Link Point B Point C Point A A to B T1 Link B to C T1 Link Point B Point C Point A A to B T1 Link B to C T1 Link Point B Point C Point A A to B T1 Link B to C T1 Link 24 Channels Channels 1-12 Channels 13-24

The multiplexer at point B must be able to selectively remove and insert traffic from the appropriate channels (drop and insert) and pass the remainder along to point C (bypass). Individual channels are demultiplexed at a local port in the multiplexer and patched, via cable, to a port on a second multiplexer. (This channel bypass is not the same thing as bypassing the local telephone company to reach a long distance carrier.)

CSU CSU T1 Point A Point B CSU CSU T1 Point B Point C Channels “Dropped” at Destination B Insert CSU CSU T1 Point A Point B CSU CSU T1 Point B Point C CSU CSU T1 Point A Point B CSU CSU T1 CSU CSU T1 Point A Point B CSU CSU T1 Point B Point C CSU CSU T1 CSU CSU T1 CSU CSU T1 Point B Point C Channels “Dropped” at Destination B Insert

(33)

Star Networks

When many point-to-point links terminate at a central location, the network is called a star network. The multiplexers may be single aggregate or multiple aggregate, but they are all operating on simple point-to-point links. The star topology is one of the most expensive in terms of line costs when long distances to the remote locations are involved. When many point-to-point links terminate at a central location, the network is called a star network.

Central Location Phoenix Mesa Chandler Scottsdale Tempe Central Location Phoenix Mesa Chandler Scottsdale Tempe

Ring Networks

Multiple aggregate T1 multiplexers can also be arranged to form ring networks. The ring topology provides “built-in” link redundancy. When one T1 link fails in the ring, the traffic can be rerouted around the ring in the reverse direction and still get to the destination. Ring networks also utilize the drop and insert and bypass features to remove and place traffic onto the network.

Dual Aggregate Dual Aggregate Dual Aggregate T1 Multiplexer T1 Multiplexer T1 Multiplexer T1 Link T1 Link T1 Link T1 Link Dual Aggregate Dual Aggregate Dual Aggregate T1 Multiplexer T1 Multiplexer T1 Multiplexer T1 Link T1 Link T1 Link T1 Link

(34)

Mesh Networks

Mesh networks are similar to ring networks, but have additional T1 links to interconnect each T1 multiplexer directly with every other multiplexer. The mesh topology is the ultimate in redundancy and offers the added benefit of minimum transmit time for information to flow through the network. This is done to ensure the connection is given the shortest available path as network loading changes.

Dual Aggregate Dual Aggregate Dual Aggregate Dual Aggregate T1 Multiplexer T1 Multiplexer T1 Multiplexer T1 Multiplexer T1 Link T1 Link T1 Link T1 Link T1 Link T1 Link Dual Aggregate Dual Aggregate Dual Aggregate Dual Aggregate T1 Multiplexer T1 Multiplexer T1 Multiplexer T1 Multiplexer T1 Link T1 Link T1 Link T1 Link Dual Aggregate Dual Aggregate Dual Aggregate Dual Aggregate T1 Multiplexer T1 Multiplexer T1 Multiplexer T1 Multiplexer T1 Link T1 Link T1 Link T1 Link T1 Link T1 Link

(35)

T1 Services

The companies offering long distance T1 services are called Interexchange Carriers (IEC or IXC) as they provide service between central office exchanges.

• Local T1 services are provided by the Local Exchange Carrier (LEC). The most common LECs are the Regional Bell Operating Companies

(RBOCs).

• T1 services are also available from the domestic satellite carriers

(DOMSAT) that include AT&T, American Satellite, SGS, GTE, Spacenet and RCA Americom.

LATAs

As part of deregulation, the United States was divided into geographical areas called Local Access and Transport Areas (LATAs). The local telephone company, which can be an RBOC or an independent, provides service only within the LATA. The service can be an end-to-end link if both ends are within the LATA. They also provide the portion of the link from the customer’s premise to the interexchange carrier point-of-presence (POP) for service between LATAs. The interexchange carriers such as AT&T and Sprint, as well as other long distance carriers for long-haul circuits provide inter-LATA service. The RBOC can also provide inter-LATA service if the LATAs are within the RBOC serving area. For example, a Miami, FL to Jacksonville, FL link crosses several LATA

boundaries, but all are within Southern Bell operating territory.

The interexchange carrier will generally take the responsibility for coordinating the ordering, installation and maintenance between the LEC and IXC, and provide billing for the entire link.

CO CO POP POP Customer Customer CO LATA 2 LATA 1 Interexchange Carrier CO CO POP POP Customer Customer CO LATA 2 LATA 1 Interexchange Carrier CO CO POP POP Customer Customer CO LATA 2 LATA 1 Interexchange Carrier

(36)

Method of Access

The method of access determines the cost per minute for long distance services. There are two methods for accessing the interexchange carrier - switched access and dedicated access.

Switched Access

The access is “switched” when the local operating company (local exchange carrier) routes the long distance access to the interexchange carrier, through the central office on a call-by-call basis.

1 0 X X X Long Distance Carrier Central Office Business Line Monthly Rate Local Exchange Carrier Computer Switched 1 0 X X X Long Distance Carrier Central Office Business Line Monthly Rate Local Exchange Carrier Computer Switched

The local exchange carrier charges for switching the call through the local central office. This carrier access charge (CAC) is billed per minute on each long

distance call routed through the central office to the interexchange carrier.

Dedicated Access

Dedicated access eliminates the switching through the central office and eliminates the CAC per minute charged by the local exchange carrier.

Long Distance Carrier T1 Link Local Exchange Carrier Dedicated Access Central Office Long Distance Carrier T1 Link Local Exchange Carrier Dedicated Access Long Distance Carrier T1 Link Local Exchange Carrier Dedicated Access Central Office

Dedicated access costs less per minute since the CAC is eliminated. However, dedicated access requires a volume of lines to justify the monthly rate for the T1 circuit. NetSolutions recommends that customers who spend between $3,500.00 and $4,000.00 per month in switched long distance will be at the break-even point for dedicated T1 access.

(37)

Benefits of T1 Access

With T1 access, the customer dials a long distance number and the call goes directly to the long distance company. The user is charged a flat monthly rate for access. Since the LEC is not switching the calls, that percentage is dropped from the price of the call charged to the user. If the calling volume warrants paying the monthly access charge, the user can save on long distance calls. NetSolutions has found that $3,500.00 a month in long distance is an average break-even point.

The user also benefits with T1 access because the call set-up time is shortened. The user is flagged as a priority customer because they are directly connected to the carrier. With T1 being totally digital, the calls are clearer and user data has better protection. The user has the latest technology right to his doorstep. Digital allows the customer to use features like special routing for 800 numbers, Dialed Number Identification Service (DNIS) and real-time Automatic Number

Identification (ANI).

Configurations and lines can be changed quickly. Unused lines can be “turned up” very quickly. T1 spans are easier to troubleshoot than analog lines.

Dedicated access allows the long distance carrier to troubleshoot the entire network. Most newer T1 multiplexers are software configurable. Through a terminal, the user can add or delete channels, change configurations, initiate diagnostic tests, monitor network performance, and check the operating status of remote units from a central location.

Many software-based multiplexers offer additional flexibility, such as the ability to assign priority to channels or automatically reconfigure the system at

predetermined times to accommodate different traffic patterns.

Fractional T1 Service

Fractional T1 (FT1) service provides users with cost-efficient alternatives to some of the older private line services (e.g. analog private lines, digital data service, and full T1 circuits).

FT1 service is most attractive to low-to-medium volume users. It benefits small organizations requiring more bandwidth than is provided by standard private line digital service offerings, but do not need or cannot afford full T1 links. FT1 might prove to be most beneficial to users who are geographically confined, such as branch banks, state governments and universities.

(38)

ISDN Overview

Integrated Services Digital Network (ISDN) is a worldwide standard for

transporting information digitally across the public switched telephone network. It defines a structure for the network and incorporates flexibility to include new technologies and services as they are developed.

ISDN voice and data are carried by bearer channels (B channels) that use a bandwidth of 64 kb/s (bits per second). A data channel (D channel) handles signaling at 16 kb/s or 64 kb/s, depending on the service type.

There are two basic types of ISDN service:

• Basic Rate Interface (BRI): BRI consists of two 64 kb/s B channels and one 16 kb/s D channel for a total of 144 kb/s. This basic service is

intended to meet the needs of most individual users. (This is not currently supported by the Inter-Tel 3000.)

• Primary Rate Interface (PRI): PRI is intended for users with greater capacity requirements. Typically the channel structure is 23 B channels plus one 64 kb/s D channel for a total of 1536 kb/s. In Europe, PRI consists of 30 B channels plus one 64 kb/s D channel for a total of 1984 kb/s. It is also possible to support multiple PRI lines with one 64 kb/s D channel using Non-Facility Associated Signaling (NFAS).

There are three primary local ISDN services:

• ISDN Centrex: Support for BRI Services as part of a digital Centrex offering. ISDN Centrex can also be configured with PRI access to Interchange Carriers (IXCs) if required.

• ISDN Single Line Service: ISDN single line service is essentially BRI service for either business or residential customers. The targets for this service are business customers who need some of the ISDN capabilities (e.g. ISDN BRI for video teleconferencing and work-at-home

applications).

• ISDN Primary Rate Service: ISDN Primary Rate Service provides

connection to the local ISDN for PBX, ACD, and Calling Line Identification features.

(39)

Primary Rate Interface (PRI)

PRI is intended for multi-user systems like PBXs, Automatic Call Distributors (ACDs), or higher bit rate terminals. The 23 B channels available can support a mix of voice and data connections.

High bit rate services can be provided over a PRI by grouping multiple B channel connections to form 384K and 1536 kbps channels.

In the United States, the PRI has two configurations: • 23 B + D (64 kbps)

• 24 B (24B is only available with a service called “Non-Facility Associated Signaling” (NFAS) using “D channel backup,” which allows control signaling on a D channel equipped on another PRI span. This configuration is not available on the Inter-Tel 3000 system).

PRI operates at the standard North American DS-1 transmission rate (1.544 mbps) and uses the same transmission specification as a T1 Carrier Span. The transmission channel must support Extended Superframe Format (ESF) and Bipolar 8 Zero Substitution (B8ZS).

The PRI can operate on any digital transmission system supporting DS-1. PRI can be carried on:

• T-Span (2 copper pairs with repeaters)

• DS-1 Channel provided on a digital microwave • DS-1 Channel provided on a fiber optic system • DS-1 Channel provided on a SONET system

(40)

ISDN Hardware Definitions

• Terminal Equipment 1 (TE1): This is an end-user type of device directly compatible with BRI or PRI. Examples include ISDN compatible

telephones, fax machines and P C cards.

• Terminal Equipment 2 (TE2): Analog devices that don’t support an ISDN interface. Examples include Single line telephones, keysets, RS-232 devices, modems, two-wire analog (Tip and Ring) service, RS-232/RS-449 and V.24/V.35.

• Terminal Adapter (TA): A device used to connect an ISDN network to a non-ISDN (TE2) compatible device

• Network Termination 1 (NT1): This is a customer-provided hardware device on the customer’s premise that connects to the local telephone company’s loop facility. The NT 1 for PRI is the same Channel Service Unit (CSU) currently used for T1/DS-1 rate services. The NT1 functions to:

o Isolate the customer equipment from the network o Provide four-to-two-wire conversion for BRI o Facilitate interface sharing (BRI passive bus) o Allow maintenance testing

o Provide power for the downstream (customer side) equipment • Network Termination 2 (NT2): A device used to connect ISDN and

non-ISDN devices to an non-ISDN interface. Examples include: PBX Systems, Automatic Call Distributors (ACDs), Terminal Controllers and Gateways.

TE2 (I.e. a PC)

TE2

(I.e. a FAX machine) _TA (Terminal Adapter:

Converts TE2 Device to work On ISDN line)

TE1 (I.e. ISDN phone)

NT1 CO

TE2 (I.e. a PC)

TE2

NT1 CO

TE2 (I.e. a PC)

TE2

(41)

ISDN Reference Points

The ISDN reference points are junctions at which two ISDN compatible devices are connected. The 5 main reference points are explained below.

• R Reference Point: This is the connection between a non-ISDN terminal (PC w/modem, fax, credit card reader, etc.) and a Terminal Adapter (TA). • S Reference Point: This is the connection between Terminal Equipment

1 (TE1) or a Terminal Adapter (TA) and a Network Termination 2 (NT2). • T Reference Point: This is the connection between the Network

Termination 1 (NT1) and the ISDN Terminal (TE1).

• U Reference Point: The connection between the Network Termination 1 (NT1) and the telephone company’s line, normally referred to as the “demarc,” or “demarcation point.”

• R Reference Point: This is the connection between a non-ISDN terminal (PC w/modem, fax, credit card reader, etc.) and a Terminal Adapter (TA). • V Reference Point: This is the connection in the central office between

the ISDN line card and the line termination. The V reference point is inside the central office and in most cases, is of no concern to the customer.

TE2 (I.e. a PC)

TE2

NT1 CO

R Interface S/T Interface U Interface

TE2 (I.e. a PC)

TE2

NT1 CO

TE2 (I.e. a PC)

TE2

NT1 CO

(42)

D Channel Signaling

For both PRI, the D channel controls the activity on the B channels. The D channel is a dedicated communication link between the Central Office and the customer site equipment.

The protocol for ISDN D channel signaling is designed in a layered format to provide flexibility in the design. This feature facilitates the incorporation of enhancements to ISDN services without redesigning the signaling protocol D channel signaling is established and interpreted using the Open Systems Interconnection (OSI) Reference Model. This model defines protocols for communication between systems, allowing systems made by different

manufacturers to communicate and providing an open network where any ISDN device can communicate with any other device.

The OSI Reference Model defines seven protocol layers. These layers do not necessarily define physical equipment. The standard representation of the OSI Reference Model is shown below. The entire group of protocols is called a “stack.” The function of each layer is described below.

LAYER FUNCTION DESCRIPTION

Layer 7 Application Provides functions for particular services such as file transfers, virtual terminals, etc.

Layer 6 Presentation Used to specify format and coding, including encryption if necessary, of communication packets from the Application Layer.

Layer 5 Session Allows the presentation packets (see Layer 6) to organize and synchronize their data exchange.

Layer 4 Transport Is responsible for connection between the end points of the systems.

Layer 3 Network Is for routing data, including network addressing, routing, switching, acknowledgments, etc. This layer is responsible for decoding the address of the message, providing error recovery for incorrect addressing, and determining the message destination.

Layer 2 Data Link Defines formats for data transmission.

Layer 1 Physical Defines the mechanical and electrical signaling standards for data transmission.

Inter-Tel 3000 T1 Technical Training Self-Study Course

T1 Technical Training

Self-Study Course

Table of Contents

About This Course

Taking the Course

Taking the Exam

IMPORTANT

Introduction to Inter-Tel 3000 T1

t

Understanding ISDN, T1, and PRI

T1 Overview

Termination Equipment

Channel Banks

Pulse Code Modulation

D4 Frame

Ordering T1

D4 Superframe

Extended Superframe Format (ESF)

Coding Methods

Channel Service Unit

Digital Loop Back

Digital Loop Back

Repeaters

Network Timing

S

S

S

M

S

S

S

S

S

S

M

M

Channel Pulse Stuffing

Jitter and Timing Inaccuracies

Testing Network Systems

T1 Networks

Point-to-Point T1 Networks

Point-to-Point Multiple Links

Point-to-Multipoint Networks

Star Networks

Ring Networks

Mesh Networks

T1 Services

LATAs

Method of Access

ISDN Overview

Primary Rate Interface (PRI)

ISDN Hardware Definitions

ISDN Reference Points

D Channel Signaling