Inter-Tel 3000 T1 Technical Training Self-Study Course

Transcription

1 Inter-Tel 3000 T1 Technical Training Self-Study Course

2 Inter-Tel, Inc All rights reserved.

3 Table of Contents ABOUT THIS COURSE...1 Taking the Course...1 Taking the Exam...1 INTRODUCTION TO INTER-TEL 3000 T1...3 UNDERSTANDING ISDN, T1, AND PRI...5 T1 Overview...5 Termination Equipment...5 Channel Banks...6 Pulse Code Modulation...8 D4 Frame...9 Ordering T D4 Superframe...11 Extended Superframe Format (ESF)...12 Coding Methods...14 Channel Service Unit...17 Repeaters...19 Network Timing...21 Channel Pulse Stuffing...23 Jitter and Timing Inaccuracies...23 Testing Network Systems...24 T1 Networks...26 Point-to-Point T1 Networks...26 Point-to-Point Multiple Links...27 Point-to-Multipoint Networks...28 Star Networks...29 Ring Networks...29 Mesh Networks...30 T1 Services...31 LATAs...31 Method of Access...32 ISDN Overview...34 Primary Rate Interface (PRI)...35 ISDN Hardware Definitions...36 ISDN Reference Points...37 D Channel Signaling...38 INSTALLING T1 ON INTER-TEL Supported Switch Types...41 System Installation Steps...42 i

4 PROGRAMMING T1 ON THE INTER-TEL T1 Channel Programming...45 PRI Channel Programming...47 Line Programming for T Equipped Lines...48 Outgoing Groups...48 GLOSSARY OF TERMS...49 PRACTICE EXAM QUESTIONS...57 SAMPLE T1 ORDER FORMS...63 ii

5 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: The exam questions were developed to test your full understanding of T1/PRI on the Inter-Tel 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. 1

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7 Introduction to Inter-Tel 3000 T1 Inter-Tel 3000 versions 3.0 and higher support T1 Primary Rate Interface (PRI) services using the 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. 3

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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 Channel Service Unit Digital Transmission Media Channel Service Unit Termination Equipment 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. Multiplexers: Digital multiplexers and transcoders are the interfaces between the different bit rates in the digital network. They stack blocks of data on top of each other before they enter a high-capacity transmission media. 5

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. CODEC Time Division Multiplexer T1 Digital Link Aggregate Mbps CODEC Analo Analog g 64 Kbps DIGITAL CHANNELS Chan 1 Chan 2 Chan 3 Chan 4 Aggregate 24 Time Division Multiplexer 6

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. Full Duplex: T1 links always operate in full duplex mode, where data traffic flows in both directions, as shown in the above point-to-point multiplexer configuration. 7

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. D4 Frame 125 Microseconds DS 1 Frame = Total of 193 bits transmitted every 125 Microseconds 1 Framing Bit 192 Bits Free Form 8

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, microsecond intervals) to produce 1,544,000 total bps (193 bits x 8,000 times per second = 1,544,000 bps). D4 Frame 125 Microseconds D4 Format = 8 Bits Per Channel x 24 Channels, + 1 Framing Bit = 193 Bits every 125 Microseconds 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 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. D4 Format 1 Second D4 Format =8,000 Frames of 193 Bits each, transmitted every second = 1,544,000 bps Frame Bits Frame Bits Frame Bits Frame Bits Frame Bits Frames 6 through 8, Bits Per Frame 24 Channels of 8 Bits each = 193 Bits (8 Bits = 1 PCM Sample) 9

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 Sampling 1 Second PCM Sample = 8,000 Samples of 8 bits each/every 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 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. 10

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 Superframe Frame 1 Frame 2 Frame 3 Frame 4 Frame 5 Frame 6 Frame 7 Frame 8 Frame 9 Frame 10 Frame 11 Frame 12 BFs Frame 12 With Signaling BFt Frame 12 With Signaling Chan 1 Chan 2 Chan 3 Chan 22 Chan 23 Chan 24 BF B1 B2 B3 B4 B5 B6 B7 B8 Speech Bits or Data Bits Signalling Bit = Robbed Bit Frame 6 = A Bit, Frame 12 = B Bit Framing Bit The signaling channels are divided into an A and B subchannel, with the A subchannel information sent in the sixth frame, and the B subchannel signaling sent in the twelfth frame of every superframe. 11 ON HOOK Signaling Bits A = 0 B = 0 OFF HOOK Signaling Bits A = 1 B = 1

16 Extended Superframe Format (ESF) The Extended Superframe format is an extension of the D4 format that was announced in 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. Extended Superframe 3 Milliseconds Signaling Frames 6, 12, 18 & 24 Channels per Frame Chan 1 Chan 2 Chan 3 Chan 22 Chan 23 Chan 24 B1 B2 B3 B4 B5 B6 B7 B8 Frame 6 = A Signal Frame 12 = B Signal Frame 18 = C Signal Frame 24 = D Signal 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). 3. The remaining 12 bits are data link (DL) bits and are used to provide a 4 Kbps data channel. 12

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 Error Checking 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. 13

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 Binary 1 Binary 1 Binary 0 Binary 0-3 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. 14

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. 15

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. Bipolar 8 th Zero Suppression Frame Signal without B8ZS Coding = Transmission of a Binary 1 and Eight Binary Zeros Frame Signal with B8ZS Coding =Two Bipolar Violations Replace the String of 8 Zeros 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. 16

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 Digital Transmission Media Channel Service Unit Termination Equipment 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 crossconnect 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. T1 Data Repeater C.O. CPE T1 Data Channel Service Unit 17

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 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 equipment while taking advantage of the monitoring features available with ESF. 18

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. Customer Premise Central Office CSU Last Line Repeater Repeater ^^^^ ^^^^ ^^^^ ^^^^ Repeater Last Line Repeater Office Repeater 3,000 Ft. Maximum 1 Mile Maximum Up to 40 Repeaters 1 Mile Maximum 3,000 Ft. Maximum Fault Locate Pair T1 Data Maintenance Order Wire 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). 19

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. New Pulse Starts Out Regenerated Signal Pulse Travels Over Cable Pulse Gets Smeared Repeater 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 T1 Span T1 Data T1 Data Office Repeater Central Office Switch Control Office Repeater The T1 span is terminated at the central office with an Office Repeater (OR). The OR regenerates the signal for routing and switching within the central office. The office repeater ensures that the T1 data stream complies with Mbps pulse shape requirements, as defined in AT&T Publication This publication also defines the ones density and framing requirements. The OR incorporates circuitry to enforce ones density and detect bipolar violations. 20

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 failure of the master node results in loss of the entire network. 21

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. Master Clock Reference Node S M Slave Node S S Slave Node Slave 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. 22

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) 23

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 inservice test, and some detailed knowledge of the various jitter specifications is required. All-ones signal patterns are transmitted to keep the repeaters synchronized even when no real data is being transmitted. Detection of this alarm condition will indicate a failure of some network component. The ESF format allows more sophisticated in-service tests to be carried out. Some of the newer tests are based on a cyclical redundancy check (CRC) code embedded in the bit stream formed by the accumulation of the 193rd bit. The CRC can provide measurement of the line s quality. 24

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. 25

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 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. Fax Voice ^^^ ^^^ T1 Multiplexer Chandler, AZ T1 Link Mbps T1 Multiplexer Phoenix Branch 64 KBps Channels ^^^ ^^^ Host 26

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. Fax Voice Printer ^^^ ^^^ Multiple Aggregate T1 Multiplexer Chandler, AZ T1 Link Mbps T1 Link Mbps Multiple Aggregate T1 Multiplexer Phoenix Branch ^^^ ^^^ 64 KBps Channels PC Host 27

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. Channels 24 Point A A to B T1 Link B to C T1 Link Point B Point C Channels 1-12 Channels 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.) Insert Point B CSU T1 CSU Point C Point A CSU T1 CSU Point B Channels Dropped at Destination B Drop and insert has some drawbacks that become more serious as network size increases. The cost and complexity of input/output ports and cables increase, space and power requirements grow, control and management get more difficult and conversion from digital to analog and back to digital adds quantizing noises to voice channels. Channel bypass handles the routing of channels from destination A to destinations B and C somewhat differently. Channels earmarked for B are demultiplexed at the B-site multiplexer, while channels destined for C bypass the demultiplexing function at site B and are routed directly to destination C. 28

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. Mesa Scottsdale Central Location Phoenix Chandler 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 T1 Multiplexer T1 Link T1 Link Dual Aggregate T1 Multiplexer Dual Aggregate T1 Multiplexer T1 Link T1 Link Dual Aggregate T1 Multiplexer 29

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 T1 Multiplexer T1 Link T1 Link T1 Link Dual Aggregate T1 Multiplexer T1 Link Dual Aggregate T1 Multiplexer T1 Link T1 Link Dual Aggregate T1 Multiplexer 30

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. LATA 1 POP Customer CO CO Interexchange Carrier CO LATA 2 POP Customer CO 31

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. Business Line Monthly Rate 1 0 X X Central X Office Local Exchange Carrier Computer Switched Long Distance Carrier 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. T1 Link Central Office Long Distance Carrier Local Exchange Carrier Dedicated Access 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, and $4, per month in switched long distance will be at the break-even point for dedicated T1 access. Dedicated vs. Switched Access With switched access, when the customer picks up the phone and dials a long distance number, the LEC takes the call and sends it to the long distance carrier. They switch the call. The long distance company is charged a percentage for switching the call and that charge is built into the cost per minute to the user. 32

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, 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. 33