Communication Fundamentals in Computer Networks

Abstract

Digital communication—that is to say, the transmission of digital messages and goods—needs computer networks. But how does information travel from one computer to another in the form of digital data? How can an email be delivered correctly? How is it possible that the right network participant can be found among all of those communicating on the global Internet—and that a million times over? This chapter introduces the basics of computer communication and helps us to understand how these technical procedures take place in a computer network such as the Internet. We learn how these multiple components interact together. The various subproblems that need to be solved before successful and efficient data transfer can take place in a computer network are bundled hierarchically. This is done with the help of a so-called layer model. The subsolutions are combined into one functioning whole. The principle of packet switching provides a successful approach to solving the problem of communication. It allows the efficient and secure communication of many communication participants via an error-free and simultaneously shared transmission medium. Communication is ensured by way of error detection and error correction procedures. These are what make reliable communication possible in the first place.

“If everyone only spoke when they had something to say people would very soon lose the use of language.”

—William Shakespeare (1564–1616).

Glossary

Bandwidth

The bandwidth of a communication path in a network is a physical quantity that is measured in Hertz (1 Hz = 1/s). In the analog domain, the bandwidth indicates the frequency range in which electrical signals are transmitted with an amplitude decrease of up to 3 dB. The greater the bandwidth, the more information can theoretically be transmitted in a unit of time. Although the term bandwidth is often used to mean the transmission of digital signals, in this case actually the transmission rate is meant. There is however, a direct relationship between the bandwidth and the transmission rate. In data transmission the attainable transmission speed is directly dependent on the bandwidth of the network. The maximum bandwidth utilization for binary signals is 2 bits per Hertz of bandwidth.

Broadcast

A broadcast transmission is the simultaneous transmission from one point to all participants. Classic broadcast applications are radio and television.

Checksum procedure

Checksums are often used in communication protocols for error detection. The sender calculates a checksum for the message to be sent and appends it to the message. Once arriving at the receiver, the same procedure is used on the received message (excluding the appended checksum) and a checksum is made. This calculated value is then compared to the checksum value appended by the sender. If both values match, it can be assumed with a large degree of certainty that the message has been transmitted correctly. Generally in a checksum procedure, the transferred bit string is interpreted as numerical values grouped into individual blocks and their sum calculated. This checksum is simply appended onto the data to be transmitted as a binary coded number. Checksum procedures are used, for example, in the IP protocol. The best known procedure is the so-called Cyclic Redundancy Check (CRC, also referred to as polynomial code).

Circuit switching

A method of message exchange via a network. An exclusive, fixed connection is set up between the communicating end devices at the beginning of the message exchange. The connection exists for the entire duration of communication. Telephone networks, for example, work according to this principle.

Code

A code is a mathematical relation that maps each character of a character string (prototype set) to a character or a string of a different character set (image set). Codes serve the representation and encryption of messages in technical systems as well as the detection and correction of errors.

Communication medium

Physical carrier used to transport communication signals between the transmitter and the receiver. For example, in direct verbal communication the sound of air is the carrier medium of the communication.

Communication protocol

A communication protocol (also called simply a protocol) is a collection of rules and regulations that fix the data format of the messages to be transmitted, as well as determining all the mechanisms and procedures for its transmission. It contains agreements regarding the establishment and disconnection of a connection between the communication partners and the manner of the data transmission.

Computer network

A computer network (or simply, network) is a communication network of autonomous computing systems connected to a single data transmission network. Each system is equipped with its own memory, own storage, own periphery and own computing capability. As all participants are linked together, the host systems of each subscriber has the possibility of contact with any of the network nodes.

Congestion

With its means of operation (transmission media, router and other intermediate system) a network can handle a certain load (communication, data transmission). However, if the load created reaches 100 % of the available capacity, an overload (congestion) occurs. The network must respond properly in order not to lose data and to avoid the breakdown of communication.

Data rate

The data rate (or transmission speed) is a measure of the speed at which data, in the form of bits per time unit, can be transferred via a transmission medium. This speed is given in bit/s or with the prefixes k (kilo = 10³), M (mega = 10⁶), G (giga = 10⁹) or T (tera = 10¹²). In bit-serial binary transmission it is equal to the step rate. Conversely, in character bit-parallel transmission the data rate of transmission is greater than the step rate. This is because with every step, multiple bits can be sent at the same time. In English technical publications, the transmission rate is specified in bps (bits per second). According to DIN 44302, the transmission speed is the product of the step rate and number of bits transmitted per step.

Data Transmission

Data transport from one computing system to another is called data transmission.

Delay

Parameter that indicates the maximum guaranteed length of time between the beginning of a data transmission and its conclusion. The delay is measured in seconds or fractions of a second and can, depending on the location of the computers communicating with each other, vary strongly. Although the end user is only interested in the total delay, at different places in the communication process different causes can occur, such as processing delay queueing delay, transmission delay and propagation delay.

Diffusion network

In a diffusion network, the sender’s signal is received directly by all of the computers connected in the network, taking into account the respective propagation delay. Every receiver must therefore determine itself whether the message is intended for it and whether to accept and processes it or not.

Error-detecting code

A code that is provided with redundancy and the capability to detect transmission errors. Simple examples for forms of error-detecting codes are appended parity bits or checksum procedures.

Error-correcting code

A code that is capable of not only detecting transmission errors but also, to a certain degree, of correcting them.

Error rate

The ratio of the defective information transmitted to the information transmitted as a whole is referred to as the rate of error. The bit error rate is, in particular, a measure for the error rate in the data network. It is calculated based on the relation between the defective bits transmitted and the total number of bits transmitted measured over a longer period of time.

Flow control

In a communication network, flow control prevents a faster sender from swamping a slower receiver with transmitted data and thus causing congestion. The receiver usually has a buffer memory where the arriving data packets can be stored until they are processed. To prevent an overflow of the intermediate storage, special protocol mechanisms must be provided. These allow the receiver to decide how long the sender has to wait until the receiving computer’s storage buffer has been processed.

Fragmentation/defragmentation

The length of the data packet that a communication protocol sends below the application layer is always limited by technical restrictions. If the length of a message to be sent is longer than the prescribed data length, the message is broken up into separate sub-messages (fragments), which adhere to the prescribed length restrictions. So that the individual fragments can be correctly reassembled (defragmented) into the form of the original message at the receiver following transmission, they must be provided with sequence numbers. This is necessary because the transmission order cannot always be guaranteed in networks.

Hamming distance

In comparing two different code words of equal length, the Hamming distance of both code words is the number of positions at which the corresponding bits are different.

Interface

An imaginary or real technically fixed transition at the boundary between two similar units (hardware or software), or between a system and its environment (external interface) with a set of precisely defined rules and procedures. In communication systems, interfaces are responsible for the transfer of data or signals (e.g.as described in DIN 44300).

Jitter

Term to describe the fluctuation of data transmission delay time in communication networks. This effect is unavoidable in packet-switched networks as the paths of individual data packets through the network are determined independent of each other. Because of this, the intermediate systems employed are utilized to different degrees.

Layer model

Complex problems can be modeled and solved when it is possible to disassemble them into hierarchically layered sub-problems. In this way, the level of abstraction increases from layer to layer. This means that a higher layer in the layer model is shielded from the problem details handled at one of the lower layers. Layer models play an important role in communication technology, but also in other areas of computer science. Such models correspond to the modified representation of the shell model. Instead of consisting of layers arranged hierarchically one on top of each other it is made up of individual shells

Message switching

A method of network communication in which the individual exchanges temporarily store the complete contents of a message before it is passed on. The sender only needs to known the path to the next exchange, which in turn sends the message it has received in the same way to the next exchange.

Multicast

A multicast transmission takes the form of a broadcast to a limited circle of participants. It is a simultaneous transfer from a single point to a dedicated subset of all network participants.

Network

A term for the group of several communication devices, the associated linking transmission medium and the intermediate systems necessary for operation of the network between systems. If the communication end devices are computers, one speaks of a computer network. Principally, networks can be separated into private networks, which are found in the private sector, and public networks, which are operated by a carrier and offered for public use.

Packet header

In a packet-switched network, the communication protocols used require that the transmitted information be fragmented into individual data packets. In order to be sure that the data packets are correctly transmitted, reach the designated receivers, and can be reassembled there into the original information again, data packets are preceded by command and control information in a so-called data packet header.

Packet switching

The main method of communication in digital networks. The message is disassembled into separate data packets of a fixed length and the packets are sent separately and independent of each other via intermediate exchanges to the receiver. A distinction is made between connection-oriented and connectionless packet switching networks (datagram networks). A virtual connection is established in the network before the beginning of actual data transmission in connection-oriented packet switching networks. There is no fixed connection path chosen in advance in connectionless packet switching networks and the data packets are each transmitted independent of each other and possibly even via different paths.

Point-to-point connection

The simplest form of a computer network architecture. Every computer in a point-to-point network is directly connected with every other computer in the network. The individual connections can be used exclusively by each of the communication partners involved and in this way allows for a high degree of communication efficiency. Point-to-point networking requires considerable effort (the square of the number of computers involved), therefore the networks are either very small or implemented in individual, dedicated wide area connections to the application.

Protocol stack

The various subproblems of network communication are each addressed by specially designed protocols in order to solve the overall problem of network communication. In order to ensure the necessary interplay, developing the network protocol software must be viewed as a comprehensive task to be solved in its entirety. A family of protocols (protocol suites) must be developed in each case to solve the sub-tasks and to integrate efficiently with each other. The entire problem of network communication can be well represented using a layer model. Because the individual protocols of the families are each assigned to a specific layer, the term protocol stack is used. The most well-known protocol stacks are the TCP/IP protocol suite of the Internet and the ISO/OSI layer model. The latter often serves as a textbook example.

Quality of Service (QoS)

Quantifies the performance of a service that is offered by a communication system. QoS describes the quality of service attributes: performance fluctuation, reliability and security. These are each specified via their own, quantifiable quality of service parameters.

Redundancy

The redundancy of a message does not contain the actual message itself. This redundant portion can often be an aid to understanding the contents of the message when the message has been transmitted in defective form.

Remote data transfer

If the computing systems between which a data transfer takes place are more than one kilometer apart, the term remote data transfer is used. This distance is however not fixed. The procedures used in remote data transmission distinguish themselves considerably from those used in data transmission systems with less distance between them.

Routing

In a WAN there are often several intermediate systems along the path between the sender and the receiver. These systems are responsible for forwarding the transmitted data to the respective receiver. The determination of the correct path between the sender and the receiver is referred to as routing. The dedicated routers thereby receive a transmitted data packet, evaluate its address information and forward it on to the next intermediate system on the path to the receiver, or to the receiver directly as the case may be.

Security

In network technology there are different security objectives summed up under the term security (quality of service parameters). These define the degree of integrity and authenticity of the transferred data. Among the most important security objectives are: confidentiality (no unauthorized third party is capable of understanding the data communication between the sender and the receiver), integrity (incorruptibility of the received data), authenticity (guarantee of the identity of the communication partner), liability (legally binding proof of that a communication was carried out) and availability (guarantee that a service offer is actually available).

Topology

A computer network’s topology refers to the geometric order of the individual computing nodes within the network. Widespread computer network topologies are the bus topology, ring topology and star topology.

Throughput

A measure for the performance of a communication system. The total data is measured within a certain time span that is processed or transmitted. Throughput is calculated from the ratio of correctly transmitted data bits and the sum of all the transmitted bit based on a specified span of time. It is expressed, for example, in bit/s or data packet/s.

Virtual connection

Packet-switched networks can implement connectionless and connection-oriented services. In order to implement a connection-oriented service, a virtual connection is established between both communication partners. This means that all of the data to be transmitted is transported through the network along this virtual connection. Message exchange between sender and receiver is carried out over so-called “bit streams” (there are also byte streams). The disassembly of the exchanged data streams is concealed from the communication partner in data packets.

Virtual Private Network (VPN)

Software, technically-realized linking of computers in a public network to a virtual network that demonstrates the properties of a secure, private network. Network management and network security are the exclusive responsibility of the “private” operator. External non-authorized computers are not able to access the VPN.