Data Communications

Technology is advancing at a frantic pace. Data communications is no longer restrained by the minute bandwidth of a telecommunications voice channel (less than 4 kHz). Eventually, when optical fiber is fed all the way to the customer premises, bandwidth problems will disappear. In the meantime, bandwidth is always an important consideration. Also, data transmission discussions go beyond the bounds of the transmission media and terminal equipment, and it is necessary to have a feel for the network as a whole.

As the convergence of telecommunications and data communications accelerates, the term multimedia is used to describe applications that incorporate a combination of voice, data, and video. For example, workstation teleconferencing, image retrieval or transfer, and voice electronic mail are multimedia applications that require a wide range of bit rates.

The increased acceptance of the latest generation of packet switching instead of conventional circuit switching is central to the convergence of data communications with telecommunications. Some of the advantages of packet switching are (1) variable bit-rate service capability, (2) multipoint-to-multipoint operation, (3) service integration, and (4) resource sharing. These facilities result in a lower switching cost per customer.

The high-bit-rate local, metropolitan, and wide area networks (i.e., LANs, and WANs), which make up the Internet, are expanding their horizons by becoming broadband multimedia networks instead of just data networks. This has necessitated the development of fast packet-switching protocols.

A protocol is a set of rules that control a sequence of events which take place between equipment or layers on the same level. The asynchronous transfer mode (ATM) is now being used in this respect to allow voice, data, and video multimedia operation. These protocols speed up packet-switching by eliminating or limiting error control and flow control overheads. Even such protocols will probably merge with the Internet protocol.

When one first encounters the subject of data communications, there are to be a plethora of standards, and some believe there are too many. In literature, it seems that references are made to ISO, ANSI, or IEEE Standards or ITU-T Recommendations in every sentence. As one becomes more familiar data communications at the engineering or technician level, the need ft stringent standards quickly becomes apparent.

With so many manufacturing the business, it is absolutely essential that equipment is fabricated to be compatibile with all the other equipment with which it is required to interface within the network. In the early days of personal computers it was all impossible to interface one computer with another. Each manufacturer was trying to establish its own de facto standard and therefore corner a major share in the market. With powerful software, this problem is now largely overcome. necessity for standardization is very important to the consumer, and telecomm administrations are adopting international standards to make life easier though many engineers might not appreciate that fact.

Standards

Standards aim to specify technical characteristics that allow compatibility! interoperability of equipment made by many different manufacturers.;

The International Standards Organization (ISO),; has its headquarters in Geneva, Switzerland, has become a dominant leader in the quest for global standardization of data communications. Together with the ITU-T section of the ITU, the ISO has developed a significant number of standards that are gradually receiving global acceptance.

The ISO (with ITU-T collaboration) has established a seven-layer network architecture as illustrated in Fig. 10.1. Each layer can be develop independently, provided the interface with its adjacent layers mi requirements.

Layer 1: The physical level.

This layer is responsible for the electrical characteristics, modulation schemes, and general bit transmission details. For example ITU-T Recommendations V.24 and V.35 specify interfaces for analog modem and X.21 specifies the digital interface between data terminal Equipment in synchronous mode circuit-terminating equipment.

Layer 2: The link level.

This level of protocol supplements the Layer 1 rate transfer service by including extra block formatting information to enable such features as error detection and correction, flow control, etc…

Layer 3: The network control level.

The function of this layer is to provide addressing information to guide the data through the network from the sender terminal to the receiver location, in other words, call routing. ITU-T Recommendations X.21 and X.25 are the protocols involved. X.21 refers to a dedicated circuit-switched network, and deals with the signaling and data transmission from terminals to switches. The X.25 protocol refers to network signaling, call routing, logical channel multiplexing, and flow control for terminals operating in the packet mode and connected to public data networks by dedicated circuits.

Layer 4: The transport level.

The fourth to seventh layers are devoted to network architecture. The transport layer is concerned with end-to-end message transport across the network. It takes into account the need to interface terminals with different networks (circuit switched or packet switched). Further error protection might be included at this layer to give a specific quality of service. Details can be found in ITU-T Recommendations X.214 and X.224 and ISO 8072. Layers 1 to 4 relate to the complete communication service.

Layer 5: The session level.

Layers 5, 6, and 7 relate to applications. Layer 5 protocols are concerned with establishing the commencement (log-on) and completion (log-off) of a "session" between applications. It can establish the type of link to be set up, such as a one-way, two-way alternate, or two-way simultaneous link. ITU-T Recommendations X.215 and X.225 and ISO -provide details of these protocols.

Layer 6: The presentation level.

This layer is necessary to en subscriber views the incoming information in a set format, regardless 01 manufacturer supplying the equipment (e.g., ASCII representation characters). Also, screen presentation of size, color, number of lines, etc. have uniformity from one supplier to another. ITU-T Recommends and X.226 and ISO 8823 refer to these protocols.

Layer 7: The application level.

Finally, Layer 7 is concerned with the interface between the network and the application. For example, the application be a printer, a terminal, file transfer, etc. In this respect, ITU-T Recommendation X.400 relates to message-handling services and ISO 8571 to file transfer and management.

These seven layers might seem somewhat academic on first they are of value. They are what is known as the Open Systems Interconnection (OSI). It must be emphasized that OSI is not a protocol and does not contain protocols. OSI defines a complete architecture that has seven layers. It defines a consistent language and boundaries establishing protocols.

Systems conforming to these protocols should be "open" to each other, enabling communication. OSI is nothing new per se. It just standardizes the manner in which communication is viewed. In fact, the protocols at all seven level can actually be applied to any type of communication regardless of whether it is data or speech. For example, one can make an analogy between the seven levels and the processing of a conventional telephone call, as follows:

1. Physical layer. Concerns the sounds spoken into the telephone mouthpiece and heard in the receiver earpiece.

2. Link layer. Involves speaking when required and listening when necessary. A repeat is requested if something is misunderstood. The person at the other end is told to slow down if talking too fast.

3. Network layer. Concerns dialing the number and listening for the connection to be made. If the busy signal is given, dial again. On completion of the call, disconnect by replacing the handset.

4. Transport layer. Involves deciding which is the most cost-effect make the call (which carrier to use).

5. Session layer. Concerns deciding if one call will suffice or whether it will be necessary. Will more than one person need to be included will control a subsequent conference type of call? Who will set again if cut off?

6. Presentation layer. Establishes whether or not the two partis are speaking the same language.

Bandwidth problems

Ever since the start of data communication, perhaps the greatest impediment to progress has been the constant battle to solve the problems associated with operating with very limited bandwidth. The public telephone network was not designed to deal with isolated, individual pulses, so it is even less able to cope with high-speed bit streams. Transmitting pulses through the analog telephone bandwidth of 300 to 3400 Hz causes severe pulse distortion because of (1) loss of high-frequency components, (2) no dc component, and (3) amplitude and phase variation with frequency.

Ultimately, the bandwidth problem will simply disappear when optical fiber is taken to every home. Because that situation is probably at least 10 to 15 years away, what can be done in the meantime? Packet-switching networks are springing up and special leased lines are available, but the fact remains that in a mixed analog/digital environment, some temporary solutions are needed. As many Internet users will confirm, retrieving data can be painfully slow, depending on the data throughput. The Internet itself will grind to a halt if increased bandwidth is not made available at a rate that can cope with the rapidly expanding Internet population.

To explore the fascinating world of data communications it is instructive to start by considering the translation of digital signals to a condition suitable for transmission over analog voiceband telephone circuits. This is performed by the famous device called the modem. Although these devices will become a dying breed by the time broadband ISDN is universal, they are worthy of a brief mention as they are still very much alive today. The asymmetric digital subscriber line (ADSL) equipment is also often referred to as a modem.