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 Introduction to Digital Transmission The Information Factory is providing you the entire contents of Section 1 of our video training course "Introduction to Digital Transmission" so that you may get a feel for the content and technical level of the course. SECTION 1 Introduction Introduction to Digital Transmission is one of the most exciting courses that the Information Factory has produced. This course will take you into the vastly changing world of telephone and data communications as it is moving from its old analog transmission systems to modern digital transmission. Today's world of telecommunications is quickly changing from telephone based services into a global network of all digital services. This course will allow you to learn about and participate in the digital revolution. The difference between analog and digital transmission will be explained later in this course. The purpose of this section is to give you an overview of what effects the conversion to digital transmission will have on future telephone service. Telephone service has been one of the primary methods of communications in the Twentieth Century. Although the telephone network was designed to transmit voice signals, pressure from other technologies forced it to be adapted to handle data transmission, facsimile and video. When computers began sending data electronically the telephone network provided the logical framework for the connections between systems. The telephone network was ideal for computer communications because telephone service provided connections into most businesses and homes. The only problem with the telephone network was converting the non-voice signals so that they could be carried on voice circuits. Since computers communicated in a digital format and the telephone network was analog, the transmitted signals had to be converted by devices called modems. Technology has now shown it is easier to create a network for digital transmission and convert the voice signals to move them. In actual usage voice, data and video can all be digitized and moved through the same network. The main challenge seen today is the majority of the existing public telephone network is set up to carry voice signals. We are also seeing a growth in communications requirements due to new services like videoconferencing and Local Area Networks. These new services require transmission speeds ranging from 100 kilobits per second up to a megabit or more. Modern transmission technology came to the rescue by showing that it is easier and more cost effective to build a network that transmits digital signals. All we have to do is convert everything to digital prior to putting it into the network for transmission. In actual usage, the computers already use digital information, and only the voice and video need to be digitized. For the past twenty years telephone companies around the world have been installing new transmission equipment in their internal networks to carry all signals in digital format. Now they are extending the digital signals all of the way into businesses and homes by converting the telephone networks from "telephone service" to "digital service." These new networks will provide you with end to end connections of binary or digital bit streams at various speeds. You will then select and use the bit stream at the proper speed to encode your voice, video or data information on the circuit. In essence the telephone companies will become "digital companies" providing a service called Integrated Services Digital Network (ISDN). The purpose of this training course is to provide an introduction to digital transmission techniques and systems. The course is divided into five parts that take you from the basic concepts of the digital network through to the customer interface and use of the network. Analog Transmission Using analog transmission techniques, such as those found on the existing telephone networks has presented a problem as newer services are brought into the network. This section will explore some of those problems and contrast them to the advantages of providing digital services. To begin, we must understand that for most of its first one hundred years the worldwide telephone network was concerned with only carrying voices. This meant that the network was built to connect telephones across the world and carry a voice between two points. Although the network architecture worked well for voice transmission, the newer services like computer data, facsimile and video transmission have been constrained by the analog voice network design.
Looking at Figure 1.1 (above), we can see how the existing telephone network is used to provide a variety of office communications. This drawing shows one key problem with the telephone network. Each of the various lines for telephone or data communications requires a separate line or circuit. The telephone systems have tie-lines and trunks to the outside world, computers use high speed analog or digital circuits and video systems use high speed or broadband circuits. Each type of circuit has its own problems related to installation, support and maintenance. In many cases the telephone companies had different service organizations to handle each service creating confusion for the customers. Telephone companies also found problems with voice quality as it was transmitted across long distances. Analog transmission and amplification had limits, since noise was usually amplified along with the voice signal. This meant there was a limit to how many times an analog signal could be amplified and still be recognized. Analog transmission systems were also difficult to test and service. Usually, the telephone company technicians had to take an analog transmission system out of service before doing any testing or maintenance. In addition to the analog transmission problems, telephone companies were faced with increased pressure from customers for more services and greater bandwidths. Growth in telephone usage in the 1950's and 1960's also added to the telephone companies requirements to provide more voice circuits globally. Digital Transmission In the early 1960's the telephone companies began using a new form of transmission called "digital." This system allowed them to accept standard analog voice signals and convert them to digital signals for transmission across their long distance and toll networks. This service was called T-Carrier in the North America and E-Carrier in Europe (or sometimes just "digital transmission systems"). This system was a great advantage for telephone companies because it reduced their transmission and maintenance costs. The disadvantage was that it did not extend directly to the customer's office. Up until the 1980's the telephone company still accepted and switched the majority of telephone circuits using analog techniques. The only benefit the customer received from this arrangement was better quality sound on long distance telephone calls. There was also an anomaly since great efforts were being made to build modems that translated computer digital signals to analog signals for transmission across telephone circuits. So while companies were paying money for modems which translated computer digital signals into analog transmission, the telephone company was transporting that analog signal on a digital network. Taking the digital signal and converting it into analog and then converting that analog signal into a digital transmission format was a very inefficient way to transmit information. In fact the modems had upper speed limits of 19,200 bits per second, while the signal was actually being carried on a 64,000 bit per second digital carrier. In the 1980's the telephone companies began extending the digital service to the customer allowing them to do their own analog to digital conversion. By moving the digital conversion into the customers office, the telephone company can provide one type of link and the customer can differentiate the various services. This means that the customer can have one link into the telephone company and the telephone company has one "easy to maintain" network.
As shown in Figure 1.2 (above) the use of digital services allows the one link to provide all of the voice and data connections (contrast this drawing to Figure 1.1 where each service had its own links back to the telephone company). As digital networks evolve, everything can be connected to a common communications link. First, the computer systems are connected to the PBX or some other type of digital communications controller. This PBX or controller can then be connected to one or more high speed digital circuits which go into the telephone company. Whether the incoming transmission is voice, data, video or images it will be translated into common digital format and moved through a common digital network worldwide. It's easy to understand that if we convert everything to digital it makes communications easier because we only need one type of circuit for all of our communications. You should understand though that through the use of all digital communications both the telephone company and the end user are winners:
The world's telephone companies are responding to the economic, competitive and customer pressures to install networks that are fully digital. Eventually this will mean that the ability to provide telephone connections, data transmission, video links or other services will be done by the customer at their site through a common digital controller. The organization that we know today as the Telephone Company or PTT will become only a "DIGITAL COMPANY" providing one of the new services like ISDN (Integrated Services Digital Network) or ATM (Asynchronous Transfer Mode). The Future Of Digital Communications Within today's office the use of communications has advanced far beyond the simple telephone on a desk. The revolution begins with personal computers on desks tied together on a Local Area Network as shown in Figure 1.3 (below). Next, the modern PBX or telephone system is a sophisticated, computer controlled system that switches voice and data in a digital format. Next, new uses for digital technologies like videoconferencing allows companies to hold meetings between offices across the globe. And finally there are still central computer systems providing support for core applications such as accounts payable, accounts receivable and payroll. Today, most of these systems can be connected over common high speed digital channels to communicate with other systems worldwide on private or public networks.
Figure 1.3 Modern Office Communications In reality though the use of digital communications is not that prolific and as a result we will still see two types of switching systems and networks supported for many years as shown in Figure 1.4. The older network will continue to provide common telephone services to areas that still use the older equipment. The new all digital network will support the switching and transmission of digital signals along with modern digital telephones and other services. Both networks will allow telephone calls and certain other existing services to cross between them and maintain compatibility.
Figure 1.4 The Future Networks Although there will be bridging to support telephone calls and other common service between the two networks, some of the newer services will only be available to all digital users. Again looking Figure 1.4 (above), the new all digital network will be simply providing digital bandwidth. Therefore in order to compete the telephone companies will have to provide access to other services in order to maintain their market shares. Rather than using separate lines to connect to these information services, you could simply connect through the digital company and receive the charges as a part of your regular telephone billing. The types of services customers will see through the future all digital networks include:
Remember that any of these services will be as easy as dialing a telephone call. Most of the digital circuits in use today are available either as special services from the telephone company or provided through private networks of microwave, fiber optic or satellite links. The concept of a single common digital public network has been emerging over the past ten years under the "Integrated Services Digital Network" or the acronym ISDN. ISDN is not a theory but, is actually available and in use in several industrialized countries including Japan, Singapore, France, the United States and Great Britain to name a few. Private Digital Networking Today It is also possible that a company may want to build a private digital network and bypass the telephone company. Digital services are making this easier than ever because you only need to meet one common interface regardless of the type of information you transmit. In today's market, the end user has a variety of private digital transmission systems to choose from as shown in Figure 1.5 (below).
Figure 1.5 Private Digital Link Options Options for private digital networks today include:
Private networks can also be built around dedicated services purchased from telephone companies. Summary Section 1 reviewed analog transmission and explained how digital networks are seen as the common transmission systems for all future types of communications. Or, rather than using separate communications circuits for each type of transmission we can simplify our networks by making all communications appear as digital or binary information streams and moving them across common circuits. The important thing to understand at this part of the course is that the world's communications are moving toward a common global network and away from separate networks for voice, data, video or other services. Section 2 of this course will explain how signals are converted into digital formats. In Section 3 the various standard digital interfaces will be explained along with their various formats. Section 4 will describe the various standard transmission systems for digital transmission including microwave systems, satellite transmission and fiber optics. And, finally Section 5 will explain how digital transmission can be used in networking various office systems. Terms to Review Section 1 Analog - Electrical signals that contain a large continuous range of values are referred to as analog. The most common form of analog signals usually follow a sinusoidal wave form. Analog Transmission - Transmission techniques where the signal is an analog form of the information being transmitted. Examples of this would be amplitude or frequency modulation to represent the human voice. Digital - Digital signals are representations of information in a binary format (or other numbering system). Digital Company - When future transmission services become all digital, the use of the name "Telephone Company" will be replaced by a term like "Digital Company" which will not differentiate between voice, data or other services. Digital Service - A digital service provides communications for the digital or binary transmission of information. Digital Transmission - This system transmits information using pulses to represent binary numbers. E-Carrier - This is the name of the digital transmission hierarchy standardized by the CEPT in Europe. Integrated Services Digital Network - see "ISDN." ISDN - This is the name for the current all digital network replacing the standard telephone network across the world. T-Carrier - This is the name for the digital transmission hierarchy used in North America. Telephone Company - This is a generic name for a company that provides basic telephone services. In some parts of the world the term used is PTT which is an acronym for Postal Telephone and Telegraph. PTT usually refers to a government owned telephone and/or telegraph communications company. Telephone Service - Telephone service refers to the existing networks which are designed to transport and switch voice communications.
Questions For Review Section 1 1. Some of the communications problems with modern office communications has been the need to? 2. Telephone companies are attracted to digital communications because? 3. The intent of digital communications is to provide? 4. Eventually the telephone companies will move away from telephone networks to become? 5. The emerging network for future communications is called?
Answers For Review Section 1 1. Some of the communications problems with modern office communications has been the need to? Provide separate circuits and support for each service connected to the telephone company.
2. Telephone companies are attracted to digital communications because? Digital transmission has economies based on a single service. With a single service, the telephone company only needs one type of technician and a common technical support system.
3. The intent of digital communications is to provide? A single access for the customer to all communications services.
4. Eventually the telephone companies will move away from telephone networks to become? "Digital Companies" providing one type of link for all of a customer's needs.
5. The emerging network for future communications is called? ISDN or Integrated Service Digital Network. End Of Section 1 This completes Section 1 of our course Introduction to Digital Transmission. If you are having trouble with the material, we suggest that you review the videotape for Section 1. If you feel confident with the subject matter, go on to Section 2. |
