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Synchronous networks for optimum performance
Today's high-speed network applications depend on the latest in high speed, synchronous transport technology for reliable operations. SONET and T1: Architectures for Digital Transport Networks gives you the big picture on SONET, the second-generation digital carrier system, placing it in the context of the widely used first-generation system, T1.
This rich resource offers detailed information on the how's and why's of managing a SONET system. SONET and T1: Architectures for Digital Transport Networks explains how to structure and maintain a high-performance network, using real-life examples of systems currently in use in a variety of business, industrial, and institutional settings. In addition, this book shows how to use SONET to interface with a variety of local and international networks, and to minimize data loss when communicating with older systems.
Key chapters explore:
Appendices provide valuable background material on the structure, function, and performance of fiber optics, and a review of ATM technology. Filled with practical and specific information, SONET and T1: Architectures for Digital Transport Networks provides a thorough introduction to the latest in digital network technology.
A technical introduction to the Synchronous Optical Network (SONET) and related T1 standard technology. Introduces and overviews SONET topology and optical issues. Explains digital transmission carrier systems, timing and synchronization, the T1 family, SONET operations, payload mapping and management, topologies and configurations. Discusses administration, operations, maintenance, and integration issues. Includes examples of AT&T and Nortel systems with snapshots of SONET manufacturers and vendors. Appendixes contain transmission media and ATM specs.
This chapter describes the T1 digital carrier technology. DSO, DS1, DS3, and DS4 signaling is emphasized, and we examine how these signals are processed with channel banks. The D generation of channel banks are described, as well as several vendors' systems. Appendix 4A provides detailed information on the bit and byte structures of the DS1C, DS2, and DS3 frames.
T1 LINE CONFIGURATIONS
Today, the majority of T1 offerings digitize the voice signal through a variety of analog-to-digital techniques. Whatever the encoding technique, once the analog images are translated to digital bit streams, the T1 system is able to time division multiplex (TDM) voice and data together in 24 user slots within each T1 frame.
Figure 4-1 shows a T1 configuration. There is no typical configuration for these systems. They can range from a simple point-to-point topology shown here wherein two T1 multiplexers operate on one link, or they can employ with digital cross-connect systems (DCS) that add, drop, and/or switch payload as necessary across multiple links.
Voice, data, and video images can use one digital "pipe." Data transmissions are terminated through a statistical time division multiplexer (STDM), which then uses the TDM to groom the traffic across the transmission line through a T1 channel service unit (CSU) or other equipment, such as a data service unit (DSU), or a combined DSU and CSU. The purpose of the CSU is to convert signals at the user device to signals acceptable to the digital line (and vice versa at the receiver). The CSU performs clocking and signal regeneration on the channels. It also performs functions such as line conditioning (equalization), which keeps the signal's performance consistent across the channel bandwidth; signal reshaping, which reconstitutes the binary pulse stream; and loop-back testing, which entails the transmission of test signals between the DSU and the network carrier's equipment.
The bandwidth of a line can be divided into various T1 subrates. For example, a video system could utilize a 768 kbit/s band, the STDM in turn could multiplex various data rates up to a 56 kbit/s rate and perhaps a CAD/CAM operation could utilize 128 kbit/s of the bandwidth.
THE DIGITAL NETWORK
The U.S. T1-based digital network has been under development for over thirty years. During this time, a hierarchy of transmission levels (low speeds to high speeds) has been implemented through time division multiplexers, channel banks, and digital cross-connects. These levels are designated by digital signal (DS) numbers ranging from DS0 to DS4.
DS1 Frame Format
Chapter 2 introduced the concept of digital signals, and Chapters 1 and 2 introduced the channel bank. In previous discussions, we learned that 24 channels (users) can be multiplexed together on one line (see Figure 4-2(a)). Each of these 24 channels is a 64 kbit/s signal. This signal is called a digital signal level 0, or DS0. The 0 means that the signal is not multiplexed (digital signal, 0 level of multiplexing). The multiplexed 24 DS0 signals are collectively called DS1 (digital signal, first level of multiplexing). Let us see how DS1 is formed.
A few simple calculations are needed at this point in the discussion in order to understand the DS1 signal. After each of the 24 channels in a channel bank (terminal) has been sampled, quantized, and encoded, the resultant pulse train (bit stream) is called a frame. A frame has a time duration of 125 microseconds (usec) (1 second/8000 samples = .000125). The bit duration is 648 nanoseconds (nsec): 125 us/193 = 648 nsec. Further, each PAM sample is encoded into an eight-bit 5.184 usec word: 648 nsec x 8 = 5.148 us.
The frame contains 24 eight-bit binary words, as depicted in Figure 4-2(a). At the end of channel 24, an additional bit (the F bit) is appended to the frame. This bit becomes the 193rd bit of a frame and is used for framing (synchronization), and a variety of operations and maintenance services.
These calculations provide an insight to the DS1 bit rate. We just learned that the pulse code modulation (PCM) terminal produces 24 eight-bit words, plus the F bit. The sampling rate of each channel in the system is 8000 times per second. Thus, 8000 x 193 bits per frame = 1,544,000 bits per second, or 1.544 Mbit/s; which is the DS1 line bit rate (see Table 4-1). This DS1 signal is transmitted onto the T1 TDM cable facilities.
Advantages of the Bipolar Code
The binary coded pulses transmitted onto the cable pair have a 50% duty cycle, which means the width of the pulses -is one-half the time slot allocated to each pulse (324 nsec). In addition, this system employs bipolar transmission, which alternates the polarity for each successive binary 1. This technique is also called alternate mark inversion or AMI (see Figure 4-2[b]).
There are several advantages to the use of the bipolar pulse pattern over unipolar transmission. First, most of the energy of the bipolar signal is concentrated at one-half the pulse repetition frequency. Thus, the maximum frequency of DS1 signals is 772 kHz (with all 1s). Second, less energy is coupled into other systems in the same transmission cable because of increased crosstalk coupling loss. Third, bipolar pulses do not have a direct current (dc) component, thus permitting simple transformer coupling at field regenerators. Fourth, the unique alternating pulse pattern can also be used for error detection because errors tend to violate an ongoing, repetitive pattern.
For example, two consecutive pulses in the same direction (positive or negative) are referred to as bipolar violations (BPVs). Test equipment can be used to check for this condition, and protection switching transfers to back-up facilities when errors exceed a threshold detected by the equipment.
INTRODUCTION TO THE D FAMILY CHANNEL BANKS
Carrier channel banks are utilized for interoffice and toll-connecting trunks at each end of a digital transmission system. Both channel banks are located in telephone company central offices. They convert speech signals into coded bits, and may pass the signals to the Digital Data System (DDS) network. They can also provide fractional T1 (FT-1) to subscribers as well as other applications.
The various generations of channel banks have brought new features and applications to the customer. The new systems offer easier maintenance, with most of the provisioning performed by software instead of physical settings. The new channel banks also come in smaller packaging, which has resulted in less power consumption.
The D1, D2, and D3 channel banks are no longer manufactured. However, understanding their development and services gives us a better idea of where (and why) we are today with the D4 and D5 channel banks. D1, D2, and D3 are designations of the Western Electric Company, but many manufacturers make compatible channel banks which they refer to as D1-, D2-, and D3-types. Nortel describes its channel banks designation with a DE, as in DE3. Western Electric's D4 and D5 channel banks are the most popular generation in use today. The D4 introduced a data port application, which is now one of the more popular features of these channel banks.
As of this writing, the D5 channel bank is the state-of-the-art system. It uses extensive software provisioning and utilizes universal channel units. The extended superframe (ESF) format and B8ZS line coding for 64 kbit/s clear channel capabilities (CCC) are two of its features, and are explained later in this chapter. Nortel offers the DE4 and DE4E (an enhancement to the DE4), which support data port applications and have
A data port application allows the customer to send a digital payload from one point to another by utilizing the carrier channel banks' interoffice application. A data port can also be an avenue for the customer to connect to the DDS network.
Universal channel units are not dedicated to a particular application, but are software programmable for one of several applications. This eliminates the need for telcos to keep many different types of channel units in stock for all the applications and service offerings that a customer may require.
B8ZS line coding for 64 kbit/s CCC. Nortel's DE4E bank provides features and applications similar to the D5 channel banks. Ericsson, Siemens, and others manufacture D-type channel banks....
|Notes for the Reader|
|Ch. 2||Digital Transmission Carrier Systems||21|
|Ch. 3||Timing and Synchronization in Digital Networks||54|
|Ch. 4||The T1 Family||90|
|Ch. 5||SONET Operations||140|
|Ch. 6||Payload Mapping & Management||159|
|Ch. 7||Topologies and Configurations||188|
|Ch. 8||Operations, Administration, and Maintenance||212|
|Ch. 9||Manufacturers' and Vendors' Systems||243|
|App. A||Transmission Media||270|
|App. B||The Asynchronous Transfer Mode (ATM)||308|
When we were planning this book, our initial intent was for it to be a SONET book, with little discussion of the T1 technology. However, we decided that the book should also include T1 because many of the SONET operations are centered around T1. In addition, as we surveyed the literature on T1, we were surprised to discover that the existing books on T1 did not cover several important aspects of the subject-omissions that we have corrected in this book.
Also, we have included material on some of the original T1 channel banks. To our knowledge, this material has not appeared in any text, and the information is essential to understanding how T1 is the way it is.
In setting out to write this book, we established two goals. First, we wish to complement the overall series, and avoid undue overlapping of the subject matter of the other books. Second, we wish to explain aspects of the subject matter that have not been provided in other reference books. We found that not much tutorial literature exists on synchronization and timing, on the Building Integrated Timing Supply (BITS), on SONET configuration (crafting) operations, and some other important subjects. This information is provided in this book.