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Über dieses Buch

Computer communications is one of the most rapidly developing technologies and it is a subject with which everyone in the computer systems profession should be familiar. Computer communications and networks is an introduction to communications technology and system design for practising and aspiring computer professionals. The subject is described from the computer system designer's point of view rather than from the communications engineer's viewpoint. The presentation is suitable for introductory reading as well as for reference. The emphasis is on practical, rather than theoretical, aspects and on technology which will become more important in the future. The majority of the subject matter applies to civil and military communications but some aspects which are unique to military applications have been included where considered signifi­ cant. Computer communications is a rapidly changing and highly complex subject. Sufficient practical knowledge of the subject is not usually gained at university or college but is generally developed over a period of several years by trial and error, attending courses, reading reference books and journals; this book attempts to simplify and speed up the process by bringing together a body of information which is otherwise distributed throughout many books and journals. The information is presented in a framework which makes a wider understanding of the subject possible. Basic knowledge of communications is assumed, a general famil­ iarity with computer systems is anticipated in later chapters, and, where relevant, theory is explained.

Inhaltsverzeichnis

Frontmatter

1. Data communication concepts and alternatives

Abstract
The first binary data communication system was demonstrated by Cooke and Wheatstone in 1839. This used six parallel wires to control the position of five needles which pointed to the letters of the alphabet. The first machine to use serial binary data communication was the teleprinter, developed by Emil Baudot in 1874. The Baudot Code uses five bits to represent the letters of the alphabet and a figures shift character, after which code combinations are interpreted as numbers.
John R. Freer

2. Communications media

Abstract
The most basic consideration in data transmission is the media through which the transmission will take place. The recurring cost of the media is frequently the most costly part of long-distance communications but for shorter-distance connections the data communications equipment cost may be dominant. When designing a communications network it is therefore necessary to give much thought to the selection of the combination of data transmission media used and the cost change trends which are always present.
John R. Freer

3. Modems and multiplexers

Abstract
Long-distance data communication over analog telephone lines requires a device called a MOdulator-DEModulator or MODEM to convert the digital signals into analog signals with frequencies within the bandwidth of the telephone line. Early data communications depended on dedicated or public switched telephone lines because there were few alternatives.
John R. Freer

4. Network topologies, switching and access control

Abstract
A communication network is a shared resource used to exchange information between users. A computer network is a distributed collection of computers, viewed by the user as one large computer system which allocates jobs without user intervention. A computer communication network is viewed as a collection of several computer systems from which the user can select the service required and communicate with any computer as a local user.
John R. Freer

5. Layered network architectures

Abstract
A network architecture is the set of rules which govern the connection and interaction of the network components; it includes the data formats, protocols and logical structures for the functions which provide effective communication between data processing systems connected to the network. Layering separates the functions into distinct levels which communicate individually with like levels in distant nodes.
John R. Freer

6. Interface standards

Abstract
The communication media and the physical aspects of interfacing were the first to be standardised. Standardisation at this level is essential in any communications application because without an agreement on the electrical (or optical) signal characteristics, communication of any form is impossible. Without an agreed physical interface specification, manufacturers of communications equipment, such as modems, cannot make their equipment operate with a wide range of computer and terminal equipment and terminal manufacturers would be unable to connect their products to a wide range of computers.
John R. Freer

7. Local area networks

Abstract
A Local Area Network is a data communication system which allows a number of independent devices in moderate geographic proximity to communicate with each other.
John R. Freer

8. Wide area network standards

Abstract
A wide area network may be arbitrarily defined as a network which interconnects computers, terminals and local area networks at a national or international level. The most significant standards which apply to wide area networks are concerned with:
a)
The media (modems, lines, etc.).
 
b)
The physical interfaces.
 
c)
The data link control protocols.
 
d)
The network architecture standards.
 
John R. Freer

9. Performance prediction

Abstract
A point-to-point communication link may be procured with a fair amount of confidence that the link circuitry and protocol will meet the design objectives. To estimate the line throughput (number of information bits correctly received divided by the time taken for transmission) several factors must be taken into account including:
a)
Data transmission rate.
 
b)
Transmission block size.
 
c)
Block overheads and other protocol factors.
 
d)
Line error rate.
 
e)
Number of blocks which must be retransmitted when an error occurs.
 
f)
Propagation delay.
 
g)
Modem turnaround and throughput delays.
 
h)
Delays and limitations within the computer/terminal interface circuits.
 
i)
Software-induced delays within the host computers.
 
j)
Transaction characteristics.
 
John R. Freer

10. Computing and software issues

Abstract
The evolution of data communications technology and network design is closely allied to the evolution of data processing. The ability to transmit data at high speed over long distances first made remote computer terminals possible and later led to distributed data processing. The needs of distributed data processing systems with satellite processors determined the architecture of many communications networks.
John R. Freer

11. Threats to communications security

Abstract
An autonomous computer, with no external communications links and with all its terminals and peripherals housed within a secure, screened computer room, is vulnerable only to the entry of unauthorised users and users gaining access to information which they are not authorised to receive. These security risks may be reduced by an efficient means of user identification and control on access to the computer room, plus a system of access controls for information within the computer. These aspects are outside the main theme of this chapter because they are computer and physical security issues. This chapter is concerned with security threats which arise when the computer is connected to external terminals or peripherals and to communications networks which provide remote access or file transfers with other computers. Computer and communications security are considered in greater detail in reference 11.1.
John R. Freer

12. Network implementation

Abstract
Before embarking upon any communications system design it is necessary to establish a definition of requirements which is agreed by the sponsoring authority or by user representatives. Information systems are often extremely complex and the communications system may be a relatively small part. The more complex systems may not be controlled by one department so it may not be possible for one department to take authority for the definition of the system requirements.
John R. Freer

Backmatter

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