Industrial Data Communication Theoretical and General Applications Westermo Handbook 5.
First edition published December 1994. © Westermo, Sweden 1994. Second edition published 1996. © Westermo, Sweden 1996. Edition 2.1 published 1997. © Westermo, Sweden 1997. Edition 3.0 published 1998. © Westermo, Sweden, 1998. Edition 4.0 published 2001. © Westermo, Sweden, 2001. Edition 5.0 published 2004. © Westermo, Sweden, 2005. Production: Westermo Teleindustri AB, Sweden. Illustrations:Visual Information Sweden AB, Eskilstuna. Photo: bildN,Västerås, Sweden.
Dear Reader You are holding in your hand the fifth edition of the Westermo Handbook. The first edition of the Handbook was printed ten years ago in 1994 and has over the years become a tool used by engineers and others who have an interest in data communication. As in the previous editions our goal has been to give not only an in-depth presentation of the Westermo product range, but also a comprehensive overview of the most common theoretical aspects of data communication.
Contents Data communication – not just cables and connectors ................................................................................... 10–13 Industrial data communication ......................................................................................................................................................................... 10 The industrial IT revolution ................................................................................................................................
Data communication is extremely important in order to increase productivity ......................................................................................................................................................... 24–55 Interface ...........................................................................................................................................................................................................................................
Types of copper cables .................................................................................................................................................................................. 43–44 Twisted pair wire ......................................................................................................................................................................................................... 43 Coaxial cable ...................................................................
Connection ......................................................................................................................................................................................................................... 70 Telecom modem language ............................................................................................................................................................................ 70 Error correction and compression ................................................
ISDN components/interface ....................................................................................................................................................................... 98 Physical layer ...................................................................................................................................................................................................................... 99 Frame format of the S-interface ..................................................
Router ..................................................................................................................................................................................................................... 124–125 Brouter .................................................................................................................................................................................................................................. 125 Hub ........................................
Data communication – not just cables and connectors Industrial data communication The industrial IT revolution Competitive advantages can be achieved through creating new and efficient information channels in a company’s processes. Shorter delivery times, faster product development, customer-focused production and shorter changeover times, are just a few of the key expressions pertaining to the industrial IT wave. Like fast access to information and the possibility to control the processes.
What is industrial data communication to us? No downtime All equipment must be designed so that communication interference and downtime are eliminated. We achieve this by using high quality components, for example, capacitors with a long life and through validating designs in environments exposed to interference. No maintenance Our products are developed to withstand the harshest of environments without maintenance or service.
Galvanic isolation One of the most common causes of communication errors is the problem with potential differences between interconnected equipment. This is eliminated with galvanic isolation of the interface, this is one of the basic functions in our products. Transient suppression Industrial equipment is often exposed to interference generated by, e.g. high power cables, reactive loads and different forms of transients. Products from Westermo are designed to withstand these types of interference.
Determinism When using equipment in real time applications it is important to have different degrees of prioritisation. Our range of switches feature integrated functions and queues that guarantee the transfer of prioritised data. Approval Our equipment is installed in different applications throughout the world. In order to conform to local safety requirements, requirements governing electrical immunity/ emissions and mechanics, we design and produce based on international standards and requirements. www.
General technical data Environmental and mechanical conditions Factor Temperature Operating Temperature Storage & transport Relative humidity Operating Relative humidity Storage & transport Airborne contaminants severity level Requirement Severity Standard +5 to +55°C (+41 to 131°F) IEC 721-3-3 –25 to +70°C* (–13 to 158°F *) –25 to +70°C (–13 to 158°F) IEC 721-3-1/2 5 to 95%, non-condensing IEC 721-3-3 5 to 95% condensation allowed outside packaging G2 (1000 Å=0.1 µm) Moderate IEC 721-3-1/2 ISA 71.
Electrical conditions Factor Emission Immunity Power supply (LV) Rated voltage range Operating voltage range Power supply (HV) Rated voltage range Operating voltage range Rated Power supply frequency range Reverse polarity protection Short circuit protection TNV-3 TNV-1 SELV Requirement Standard EN 55022 class B EN 61000-4-2 EN 61000-4-3 EN 61000-4-4 EN 61000-4-5 EN 61000-4-6 EN 61000-4-8 EN 61000-4-11 Information Technology EN 55024 Equipment Severity EN 61000-6-3 Residential EN 61000-6-2 Industrial
1.1 General emissions EN 61000-6-3 EMC – Generic standards – Emission standard for residential, commercial and light-industrial environments. Emission Immunity Immunity = Tolerance against environmental effects. Emission = Influence on environment (emanated energy). The emission level is approx. 100 000 times lower than what our equipment handles in terms of immunity. Maximum levels for radio interference generated by equipment connected to the public network or DC-power source.
1.4 General immunity EN 61000-6-2 Electromagnetic compatibility (EMC). Generic standards. Immunity standard for industrial environments. Radio* Surge • 50 Hz • Magnetic field power and pulse ESD Electrical Fast Transient (Burst) * Radiated field immunity. Conducted radiofrequency fields. Test requirement on equipment connected to networks in industrial environments for immunity to continuous and transient, conducted and radiated disturbances (including electrostatic discharges).
EMC severity levels in different environments Test Residential Residential, commercial and light-industrial environments. Port Emission Radiated Enclosure Conducted AC Power Industrial Immunity for industrial environments. DC Power Railway Railway applications – Signalling and telecommunications apparatus. Substation Communication networks and systems in electrical substations. Westermo A combination of residential, industrial, railway, added with experiences from installed Westermo products.
Test Port Level Emission Radiated Enclosure Conducted AC Power DC Power Immunity ESD Radiated field immunity Electrical Fast Transient Surge Conducted radiofrequency field Encl. contact Encl. air Enclosure Signal AC Power DC Power Signal L-E Signal L-L AC Pow. L-E AC Pow.
EN 61000-4-3 Electromagnetic compatibility (EMC). Testing and measurement techniques. Radiated, radio-frequency, electromagnetic field immunity test. … Method for testing the immunity of electrical equipment against radiated, radio frequency, electromagnetic fields. States a number of test levels and test methods. EN 61000-4-4 Electromagnetic compatibility (EMC). Testing and measurement techniques. Electrical fast transient/burst immunity test.
Safety conditions Factor Electrical safety Service life Supply connection Accessibility Maintenance Isolation Circuit Supply Supply HV SELV TNV-1 TNV-1 TNV-3 Requirement Severity Standard Information technology EN 60 950 equipment 10 years Permanently connected Restricted access location Comments Reference See 1.6 Access, by service personnel and by tool No To Circuit(s) All other All other TNV-1, TNV-3 TNV-3 TNV-1 TNV-3 Electric strength ≥1 kVAC 3 kVAC 1 kVAC 1 kVAC 1 kVAC 1 kVAC See 2.3 See 2.
1.6 Electrical safety EN 60950 Information technology equipment. Safety. General requirements. … ITE safety standard that defines the requirements to reduce the risk of fire, electric shock or injury to the user and those coming into contact with the equipment as well as service personnel. Applicable to mains connected and battery fed ITE as well as ITE intended for direct connection to the telephone network, irrespective of feeding source.
1.8 Flammability UL 94 The Standard for Flammability of Plastic Materials for Parts in Devices and Appliances … Methods to measure and describe the characteristics of specimen materials relating to flammability, when exposed to heat and flames under controlled forms in a laboratory environment. 2 Definitions 2.1 Rated voltage range … Voltage range specified by the manufacturer. 2.
Data communication... ...is extremely important in order to increase productivity Increases in automation also place demands on reliable data communications between units and the systems that control and those producing and measuring. Data communication is the nervous system that forms the basis of increased efficiency and competitiveness. Irrespective of whether it concerns manufacturing, installation, transport or healthcare.
Signals in V.24/RS-232-C Pin 9/25 V.
Cable configuration How the connection between 9-/25-pos. D-sub connectors is made for all combinations with DTE and DCE units is shown below.
Key to the most important signals Explanation of the most important signals GND Protective Ground Pin no. 1 is reserved for protective ground between the devices. SG Signal Ground Signal ground is a signal reference and must always be connected to pin 7 (25-pin) pin 5 (9-pin) in V.24. TD Transmitted Data This signal transmits data from a DTE to a DCE. RD Received Data This signal is the data that a modem or a DCE transmits to a DTE.
ASCII ASCII is an abbreviation for American Standard Code for Information Interchange. The ASCII code is available in different versions for different languages and in an Extended ASCII where the 8th data bit is utilised.
Industrial interfaces Meter 10 000 RS-422 RS-422 is an ideal standard for industry as the interface is created to build data buses, typically multidrop, between central computers and a number of substations. The interface is balanced and relatively insensitive to interference. The interface switches polarity on the wire pair depending on whether it is a one or a zero being transferred.
+5 V R+ R- 0V Termination and Fail-Safe The line should be terminated using a resistor that has the same value as the characteristic impedance for the line. This resistance should be approximately 120 ohm. Termination should be applied as shown in the diagrams on page 29. Termination should be made at each end of the bus. Termination prevents reflections in the cable. “Fail-safe” is a resistance from each wire to the + supply on the one hand, and to the 0V on the other.
Stop bit Data bits Start bit General recommendations for installation … Twisted pair wire should be used. … Star networks are not permitted and distance from the bus to the device must be a maximum of 30 cm (1ft). … Receivers at the end of the bus are to be terminated with a 120 ohm resistor. … The RS-232/V.24 connection should not be longer than 15 metres (50 ft). … RS-422/485-supports transmission ranges up to 1200 m (4000 ft) at 100 kbit/s.
10 mA balanced current loop (W1) Westermo has developed its own transmission technology for short-haul modems that ensures communications over greater distances and in environments with a high level of interference. The technology is based on converting the signals to a ±10 mA balanced current loop, where the current direction is shifted on the wire pair, depending on whether it is a high or low signal from RS-232/V.24.
Network The local network’s breakthrough came during the eighties, initially via centrally located mainframe or minicomputers with terminals connected in a star. The establishment of these networks also resulted in a need for reliable and secure data communication. Transmission requires: A transmitter, a receiver, a medium, information and a protocol. The transmitter, receiver and media require a specification for the physical devices (how to connect to a network, etc).
The quality of a network depends, among others on: … Speed, which in turn depends on the number of simultaneous users, media, hardware and software. … How the transmission takes place, whether it reaches the right receiver and only the right receiver. … The quality of the data, minimization of communication disturbances. … Speed of the network. … Reliability, how well the network is protected against transients, earth currents and other phenomenon that can disturb communications.
Topology The expression topology refers to how a network is structured; the physical or logical placement of the nodes. There are five basic topologies: point to point, ring, star, bus and combined network. The choice of topology is important, as it is a long term infrastructure that shall manage and transport important data without downtime. In addition, it must be possible to adapt and expand the network as and when conditions change. Serial point to point Point to point data communications, i.e.
Bus network In principle a bus network consists of a main line where all units are connected as nodes. All data traffic is sent out via the bus to the receiver. A bus network must have regulations for how a transmitting device checks whether the line is free and how it should act in the event of a transmission colliding with other data traffic, for example through delayed retransmission. The bus network is easy to install, Node expand and extend. Ethernet and AppleTalk are common examples of a bus network.
The Problem of Interference Unfortunately not everything is resolved just because we have succeeded in finding the right transmission methods and the right interface. The largest irritant to data communications still remains-Interference. Outside disturbances that result in data loss, transmission errors and in the worst possible scenario knock out equipment. Computer development has resulted in smaller circuits and components being driven by less power.
It is not just storms that create external transients. Your lamps may also flash when a neighbouring industry starts or shuts down its machinery, this also causes transients and voltage peaks on the network. As a rule most transients are created within your own premises. Machines, equipment and fluorescent lamps cause voltage pulses on the network. A fluorescent lamp that is switched off can, for example, emit stored energy in the form of a transient of up to 3000 V.
There is a large selection of overvoltage protection for signal/telecommunication lines available on the market as well as for telecom modems, RS-232, 4–20 mA, RS-485 and other typical signals. The protection consists of primary protection and secondary protection, where the secondary protection is adapted to the communication method. The protection is usually maintenance free, when a transient is taken care of the protection returns to its original state.
Reducing Interference In any system, electronic signals are always prone to interference. Analogue signals tend to be more prone due to the fact that all points on the signal carry informationi.e. amplitude and frequency. Small disturbance to the signal will cause the receiving system to interpret the signal differently to that of the original transmitted signal and give an incorrect output. Digital signals are less prone to interference as there are only two basic states; high or low.
Ground networks The very best overall method to minimise disturbances is for the system to have an equipotential design. This means that buildings, electronics, fieldbuses and field devices all have the same ground potential. This is very difficult to achieve in practice, you can obtain a uniform potential with the help of special ground conductors and ground wire networks.
Telecom modems and interference When telecom modems are used within industry you must remember that these are extra sensitive to interference, despite isolation and signal codes. Communication can be disturbed and component faults can result when the cable is not protected carefully. Cabling for telecommunications must be separated from process cabling. Combination protection can provide increased protection in harsh industrial environments.
Types of copper cables The physical cable is often the weak link in data communication. It is the cable that handles the interference sensitive analogue signal. It is the cable, through its design, installation and length together with the surrounding electrical effects, which determines the rate and quality of communications. Twisted pair wire Twisted pair wire is the simplest, cheapest and most common cable. Usually as a twisted pair 4-wire cable.
Copper conductor Shielding Dielectric material Coaxial cable Coaxial cable consists of a single copper conductor surrounded by a screen. In order to maintain the distance constant the gap is filled with an insulating plastic dielectric layer. The screen is used as protection and for the return signals. Coaxial cable has good electrical properties and is suitable for communication at high transmission rates.
Calculation of resistance When you do not know the resistance of the cable you can use this formula: Q = R x A/I Where Q = resistivity for the material to be used. For copper you can use 0.017 µ Ωm, or 0.017 x 10-6. R = the resistance in the cable, A = cable cross section and l = length. The formula is easy to use with solid conductors. With multicore the cross section of the conductor is multiplied by the number of conductors. Cross section = radius x radius x pi.
Colour codes DIN 47100 for LiYY and LiYCY data cables. Conductor no. and colour: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 46 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 Cable coding The Swedish Standard for cable marking is set out in SEN 241701 and a common international standard has been formulated in CENELEC.
Fibre Optic Communications The greatest advantage of fibre cable is that it is completely insensitive to electrical and magnetic disturbances. It is therefore ideal for harsh industrial environments. It provides reliable transmission and has a very high data transfer capacity. Fibre cable can be used on specific sensitive sections of networks and be combined using a modem with, for example, 4-wire cable in a system.
Material The material used for the core and cladding differ on different types of fibre. The most common material used is glass. The glass used for these is extremely pure, silicon dioxide (silica). Other types of cable are PCS (Plastic-Clad Silica) with a core of glass and an outer cladding of plastic, or a plastic fibre cable with both the core and outer cladding of plastic. Glass cable gives the best performance, but is more complicated to terminate.
Attenuation in singlemode fibre A singlemode fibre has such a fine core that it can only support one mode, which means that the transmitted light pulse is not distorted whilst travelling through the cable. Transmitted light pulse Light path in singlemode fibre Received light pulse Wave length The attenuation in a cable is also dependent on the wavelength of the light produced by the transmitter. Wavelengths with low attenuation are 820 nm, 1300 nm and 1550 nm.
Light Attenuation in Glass Fibre at different wavelengths 100 Rayleigh Scattering IR losses due to heat 10 Attenuation dB/km 1 Wave length nm 850 nm 1300 nm 1500 nm Summary of fibre types Material Plastic Type Core/Outer casing Multimode Step index Glass (silicon) Multimode core plastic Step index Glass Multimode Step index Glass Multimode Graded index Glass Singelmode 50 200-600/450-1000 um Attenuation (dB/km) 330-1000 200-600/350-900 um 4-15 50-400/125-440 um 4-15 30-100/100-140 um 2-1
Termination There are many different ways to terminate fibre cable. With glass fibre, multimode cable termination is the easiest to learn. One simple procedure called “crimp and cleave” is to crimp the connector on the fibre, this requires special pliers and then cleave the fibre carefully. Another more reliable procedure is to use an epoxy to bond the fibre into the connector; connectors are available with the adhesive already in the connector.
Loss Budget Calculation The communication range of a system is dependent on the transmission output, the sensitivity of the receiver and the loss that arises in terminations and cable splices. In order to calculate this range a fibre budget is stated, which is the difference between the transmitter output power and receiver sensitivity, both these values have a typical value and a minimum level. We have chosen to document both these values for most products.
OSI model In order for systems to communicate with each other a structured framework is necessary that makes it possible to connect together solutions from individual suppliers. This was the reason for the creation of the OSI-model (Open System Interconnection). The OSI-model was developed by ISO and explains how the communication between any two systems works. As the name implies the purpose is to make systems open and with that supplier independent.
Network independent layer Interface Application layers Using the European V.24 standard as an example, this is a logical specification that is specified by the physical layer. It only defines the task of the lines: control, data and possible transmission rates. Hence the V.24 standard is supplemented with an electronic specification known as V.28, which is also a subset of the physical layer. V.24 and V.
A comparison In order to give a clearer image of OSI we can make a comparison with an everyday telephone call. … The physical layer is the telephone network and definitions of the signals that are transferred. … The data link layer’s logical link control (LLC) corresponds to the telephone’s speaker and microphone.
Local communication Fieldbuses Today, each part of a modern automation system must have the capacity to communicate and have uniform communication paths. Data communication requirements are increasing all the time, both horizontally on a field level and vertically through more hierarchical levels. A fully integrated data communication solution for industry usually involves all these elements. This applies to everything: sensor signals, which in turn are connected to instruments, valves, motors etc.
Different Fieldbuses A number of different media are used within industrial communications such as: copper cable, fibre optics, infrared transfer or radio technology. Fieldbus technology has been developed with the intention of replacing the earlier systems with standardised solutions.
PROFIBUS PROFIBUS is an open uniform digital communication system for a broad range of applications, especially within engineering and process automation. PROFIBUS is ideal for both fast time critical applications and for complex communication applications. PROFIBUS communication is rooted in the international standards IEC 61158 and IEC 61784 and with that satisfies the requirements of fieldbus users of being open and manufacturer independent.
ed in applications exposed to electromagnetic interference, between installation sites with different ground potential and to bridge large distances. Network topology PROFIBUS As the basic interface is RS-485 devices should be connected in a bus structure. Up to 32 stations can be connected to a segment. Active bus termination is connected at the beginning and end of each segment as in the figure below. Both bus terminations must have a permanent voltage supply to give error free communication.
PROFIBUS DP Represents basic, fast, cyclic and deterministic process data exchange between a bus master and its assigned slaves. Communication between the master and slave is regulated and controlled by the Master. A master is normally the central programmable control system such as a PLC or industrial PC.
Modbus Modbus ASCII and Modbus RTU Modbus ASCII and Modbus RTU are protocols that have become the de-facto standard in many applications. The protocol was developed at the end of the seventies by Modicon. Communication is based on multidrop with a master and slaves. Modbus was not just intended for industrial applications. It is used universally where there is a need to control a process or the flow of information. Master Termination Slave Slave Slave Max 1000 m (0.
Modbus Plus Modbus Plus is an industrial application network that utilises token exchange, peer-topeer communication. Token exchange and peer-to-peer involves communicating over a logical ring where all nodes can initiate communication, however, a node can not send until it has obtained the token. The transfer rate is 1 Mbit/s over screened twisted pair cable.
LON®WORKS The Echelon® Corporation has, through the introduction of LONWORKS® technology, made available a complete platform to develop open distributed control systems based on an intelligent network architecture. A LONWORKS® system usually consists of a number of intelligent devices, called nodes, where each node manages a specific task, for example, measuring a temperature or controlling a valve. The nodes exchange essential information with each other via the network.
same type of transceiver and be able to send network variables. Nodes also need to understand the contents of the network variables. For example, nodes must know whether a temperature is stated in Fahrenheit or Celsius or whether a flow is stated in litre/second or millilitres/second. Thus standards for how the content of these data packets should be interpreted are necessary. Within LONWORKS® standardisation is managed by an organisation called the LONMARK® Association.
5 4 3 2 1 BV 40° • 30° • 50° • 1 2 3 4 5 6 7 8 9 1485460 Router 0 Open • 20° • 10° Twisted pair 78 kbit/s Radio 1.25 Mbit/s twisted pair back-bone Power line Link power twisted pair 78 kb LonTalk® data in different ways from one medium to another. It is commonplace for channels with a slow medium to be connected to a backbone with a faster medium. This results in logical and physical segmentation of the network, which gives improved performance and security.
Remote Connections PSTN Dial-up lines Data communication over the telephone network Remote communication is an important supplement to local data communication. That is to say, the possibility to connect to remote data sources to search for information about for example markets, prices quoted on the stock exchange or public registers. The number of data sources has increased significantly and they are often linked via global networks.
0 0 1 1 0 1 0 0 0 1 0 Amplitude modulation Frequency modulation Phase modulation Modulation Modem is a composite of the word modulation, i.e. signal conversion, and demodulation, which is the regeneration of the original signal. The data signals must be converted and adapted in order to be transferred over different types of cable. The digital signal levels (ones and zeros) are converted to readable transformations for the chosen cable. There are three basic types of modulation.
Is bit/s the same as baud? The transfer rate of a telecom modem is described both in terms of bit/s (Bit rate) and in Baud (Baud rate). This has resulted in some confusion, which is why an explanation is called for.
Some standards Standard V.21 V.22 V.22bis V.23 V.32 V.32bis V.34 V.
Connection When a modem connection is established handshaking occurs where the data transfer rate and level of error correction are negotiated. The specification below shows the connection times between two modems for different protocol settings. This measurement illustrates that the fastest data rate is not always the most effective. The connection time is the key factor when you need to call several devices and only transfer a small amount of data. Protocol V.32 bis error corrected V.32 bis V.
Tomorrow’s highways Intensive work is in progress to create international standards and affect the expansion of what is known as tomorrow’s highways for communiComp cation. Fast digital high-speed networks, such as broadband, that can transfer vast amounts of information including data, audio and video across continents. The high capacity of the cable television network can also be a new resource Comp for faster data traffic.
V.23 on a leased line V.23 is an old standard that initially was designed for leased lines. Data transfer rates are standardised to 600 and 1200 baud. Modems that follow the V.23 standard usually have at least the following functions: … Modulation speeds up to 600 or 1200 baud. … Frequency modulation (FSK) Two different frequency modulations are used as follows: … Mode 1: 600 baud 1300 Hz–1700 Hz … Mode 2: 1200 baud 1300 Hz–2100 Hz V.23 normally permits up to 6 drop points on a 2-wire cable.
Using HyperTerminal To configure a modem serial emulation software is often required, one of the most frequently used applications is HyperTerminal in Windows, this example shows Windows XP. The following is a guide as to how to use HyperTerminal to communicate with a modem: 1. Connect the modem using a modem cable to the serial port on the computer, in this example, Com 1. A straight through 9-pos. cable is used as the computer is DTE and the modem DCE (see page 26). 2.
4. From the drop down list select the communications port that is connected to the modem. … In this example we choose COM1. … When COM1 is selected the fields for country, area code and telephone number are deactivated (dimmed). … Click on OK. 5. Define the properties for the communication port, i.e. the communication rate, number of data bits, parity, number of stop bits and flow control.
6. Once these settings have been made HyperTerminal is configured. Further settings can be made from the File menu, Properties. Here you can select, among others to emulate different terminals, i.e. VT100. Using the ASCII-settings button it is possible to set the conditions for character, line feed and local echo. Com 1 9600 8N1 - Hyper Terminal File Edit Wiew Call Transfer Help New Connection Open... Save Save As... Page Setup... Print... Properties Exit Alt+F4 7.
TDtool One configuration option for our modems is the TDtool utility, this is an application that automatically reads which modem is connected and then facilitates its configuration. The application reads the configuration parameters for the connected modem. This applies to the current settings as well as possible configuration options. These can be found under the Configuration and Advanced tabs. TDtool can be downloaded from our website. 76 Theoretical and general applications www.westermo.
In the examples we have connected TDtool to two alternative modems, the screen shots show how the application adapts according to which modem is connected. Option 1) A TD-33 Option 2) A TD-34, a modem that, among other features, can send SMS messages. Under the Advanced tab for TD-33, you can enter the telephone number for call-back and the password to use.
AT-commands A telecom modem works in two modes: … Command mode. … Communication mode. In command mode you can configure your modem so that it works with your application. Communication mode is the mode when the modem is connected to another modem and is exchanging data. As previously mentioned, Hayes Microcomputer Products developed a command set that has become the de-facto standard, a.k.a. the Hayes®-commands. These commands are used partly to configure the modem and partly to initiate a connection.
ATQn – Quiet Result Code Control The result codes sent from the modem are activated or deactivated, some applications require the modem to be set so no characters are sent. ATEn – Echo on/off Turns echo on/off to a connected terminal. This is required by some applications and can also cause confusion when attempting to enter commands.
The screen shot showing the content of the modem’s registers, a complete specification of the registers can be found in the manual for the modem. The example below describes some of the functions in the S-registers Register S00 S01 S02 S03 S04 S05 S07 S10 80 Functionality The content of the register tells the modem after how many ring signals the modem should answer. In this example the modem answers on the second ring signal as the value is set to 002. Counts the number of incoming ring signals.
Higher speeds xDSL xDSL is a collective name for a family of technologies where digital modems are used on a standard telephone or fixed line. The type of digital system sent over the line is described by the letter that replaces the x. Examples of designations are: ADSL, SDSL, SHDSL and VDSL. These technologies suit different applications. For example, VDSL can reach transfer rates up to 52 Mbit yet only over about 300 m (984 ft), SHDSL supports a maximum of 2.3 Mbit up to 3 km (1.
SDSL SDSL (Symmetric Digital Subscriber Loop) and G.SHDSL are symmetrical xDSL technologies. A distinguishing feature of these is that they have similar uploading and downloading rates, thus the name symmetrical. Using SDSL the user attains a maximum of 2.3 Mbit/s in both directions. Symmetrical SDSL can be used in Back to Back mode, which involves interconnecting two modems using copper cable. SDSL is a proprietary technology mainly installed in North America.
G.703 The ITU standard G.703, describes the electrical and physical properties and a number of transfer rates. There are three basic physical types of the interface, codirectional, contradirectional and centralised interfaces. The standard specifies speeds from 64 kbit/s to 155 520 kbit/s. The standard was originally created to carry speech over a PCM-link. The transmission medium can either be a 120 ohm balanced pair or an unbalanced 75 ohm coaxial cable.
GSM GSM, GPRS, UMTS what do all of these expressions mean and what possibilities are there for data communication? Technical descriptions often contain abbreviations and acronyms. We have chosen to use the technical designations and abbreviations, which although are usually in English have become industry standard. The history of GSM At the beginning of the eighties there were numerous analogue systems in use within Europe of varying quality.
There are many advantages of digital transmission over analogue technology on mobile networks, these include: … Improved quality of the telephone connection. … Higher transmission rates. … Improved utilisation of the bandwidth, which brings an increase in the number of subscribers on the network. … New services and functions are possible such as, data, text and fax. … Possibility of data encryption for greater security.
Components in the network ME Represents Mobile Equipment . This is equipment adapted for use on the GSM network. Each ME unit has a unique identification (IMEI-number), International Mobile Equipment Identity. This makes it possible for the network operator to block the use of a unit, e.g. when a ME unit has been stolen. SIM Stands for Subscriber Identity Module, this is a card used together with the MEunit.
Cell structures Base stations are positioned to give maximal coverage. The area covered by a base station is called a cell. The entire GSM network is organised with cells of varying sizes. A cell can D2 cover areas with a radius of 200 metre (656 ft) up to areas with a radius of ~30 km (18.64 mi). This depends on where the base station is located and E21 the surrounding environment.
MHz Channel 1 Frequency 1 Channel 2 Frequency 2 A limitation in bandwidth has resulted in the use of techniques so that a maximum number of simultaneous users can be supported. This is achieved through a combination of TDM, Time Division Multiplexing and FDM, Frequency Division Multiplexing. Frequency division (FDM) means the available 25 MHz band is divided into 200 KHz bands. In the above description of frequency utilisation between cells, A1, B2, B3, etc are examples of frequency division.
Services on the GSM network Speed 2400 bit/s 4800 bit/s 9600 bit/s 14400 bit/s 2400 bit/s 4800 bit/s 9600 bit/s 14400 bit/s There are a number of services available via GSM such as: … Telephony … CSD (Circuit Switched Data, data transfer). … SMS (Short Message Service). … MMS (Multimedia Message Service). … FAX. … GPRS (General Packet Radio Service). Protocol V.22 bis V.32 V.32 V.32 bis V.110 V.110 V.110 V.110 Telephony The most common GSM service, which has contributed towards its global usage.
SMS The most used service after telephony. An SMS message utilises the signal channel to transfer text messages. SMS has become popular for both private and professional use on account of its simplicity. In summary the service offers: … A message may be up to 160 characters in length. … Transfer cannot be guaranteed as the receiver may be switched off or outside of the coverage area.
GPRS GPRS is an extension to the GSM network where packet switching data traffic is supported. This is different to the circuit switching data traffic that is supported in GSM. With GPRS each channel that is not busy with call traffic is available for packet switching data traffic. Packets from several different users can be mixed within the same channel, which results in efficient sharing of available network resources. GPRS permits even higher transfer rates as it uses several time slots for the transfer.
Information packet 1 Information packet 2 A packet switching network is a network where the traffic is divided up into small packages which are sent over the network. This means that others can utilise the network at the same time. If you compare a circuit switching network with a telephone call, you can compare a packet switching network with a haulage contractor or the post office. Several persons can send a lot of packages at the same time.
Differences between GSM and GPRS CSD Circuit Switched Data TDM Time Division Multiplexing 1 2 3 4 5 6 7 GPRS General Packet Radio Service TDM Time Division Multiplexing 8 One timeslot is used which gives a maximum throughput of 14.4 kbit/s. The running cost is based on how long the connection is made regardless of the amount of data sent. 1 2 3 4 5 6 7 8 By using four timeslots and Coding Scheme 4 the maximum throughput will be 85,6 kbit/s.
Communication with GPRS uses another procedure. GPRS is based on IP communication and the connected unit must provide an IP address before a connection can be established. This is done by: … Connecting to the GPRS network. … A dynamic address is assigned. … The exchange of data can take place. 1 2 GPRS 3 1 GPRS attached 2 Dynamic IP addresses alloted 3 Transmitted/Received data At the present moment in time not all operators can offer subscriptions with static address allocation.
The problem occurs when a unit, for example, a computer wants to communicate with peripheral equipment and the computer generates the connection. No one knows the IP address that the computer should connect to, as these are assigned dynamically. ? GPRS ISP Internet Another application where the same type of problem occurs is when two devices need to communicate and none of them is the master. The modem can not initiate IP communication as it does not know which address will be assigned.
GPRS classes GPRS equipment is available in three categories, these are defined as Class A, B and C. Class A Class B Class C Supports simultaneous GSM and GPRS operations Supports GSM and GPRS operations, but not simultaneously. The connection only supports GPRS or GSM data. When switching is necessary between GPRS and GSM you must reconnect the connection. Multislot classes with 1 to 4 time slots.
ISDN What is ISDN ISDN (Integrated Services Digital Network) is a digital equivalent to the standard PSTN telephone network (Public Switched Telephone Network). The ISDN technology is standardised according to the recommendations from the International Telecommunications Union (ITU). Signalling Instead of the telephone company activating the ring signal in your telephone (“In-Band signal”), a digital packet is sent on a separate channel (“Out-of-Band signal”).
ISDN components/interface ISDN components include terminals, Terminal Adapters TA, Network-Termination devices NT, Line Termination equipment LT, and Exchange-Termination equipment CLA. Two terminal types are used in ISDN. Specialised ISDN-terminals with an ISDNinterface, Terminal equipment type 1 TE1, and terminals with an interface other than ISDN, units with a V.24 interface. These are referred to as Terminal Equipment type 2 TE2.
ISDN equipment that can connect directly to ISDN line Network Termination.
Frame structure U-Frame when 2B1 Q coding 240 bits, 1.5 ms 12 words, 216 bits S O/M W12 W11 W2 W1 S B1 D S = Syncronisation pattern 18 bits O/M = Operation and Maintenance 6 bits B2 8 bits 8 bits 2 bits Frame format of the S-interface The S-interface’s frames use 48 bits of which 36 are used for data transfer; the bit rate in the S-interface is 192 kbit/s. The internal structure of the frames differs slightly depending on in which direction the frames where sent.
Layer 2 – Data link layer The data link layer for ISDN is specified by ITU Q.920 to Q.923 standards. The Dchannel’s signalling is defined in Q.921. Link Access Procedure – D channel (LAP-D) is the protocol used in the data link layer. The LAP-D is nearly identical to X.25 LAP-B and both are based on HDLC. The structure of the frames used by LAP-D are shown below: Flag Address Control Information CRC Flag Flag (1 octet) Start flag always 7E16 (0111 11102).
SAPI The Service Access Point Identifier (SAPI) is a 6-bits field that allows the specification of up to 64 different service functions that layer 2 supplies to layer 3. SAPI value 0 1–11 12 13–15 16 17–31 63 All others Related layer 3 or management entity Call control procedures Reserved for future standardization Teleaction communication Reserved for future standardization Packet communication conforming to X.
Layer 3 – Network layer The network layer for ISDN is specified by ITU in Q.930 to Q.939. Layer 3 has functions to establish, maintain, and terminate a logical connection between two devices. The structure of the information field on layer 3 has a variable length and the different fields are specified by Q.
CAPI COMMON-ISDN-API (CAPI) provides a standardised interface to design software applications that utilises ISDN Signing up to the CAPI-standard gives applications that can communicate over ISDN without the need of considering manufacturer specific implementations of ISDN. At present, work with the standard has virtually stopped and most telephone operators provide ISDN based on Q931/ETSI 300 102, CAPI version 2.0 developed to support the protocol based on Q 931.
Radio Radio communication Wireless data communications via a radio modem provide a means of maintaining communications with: … remote units. … measuring stations. … external buildings and unmanned installations. … temporary or mobile sites. The purpose may be that of gathering test readings, controlling or regulating equipment or recording various kinds of alarms.
Attenuation and noise A propagated radio wave is affected by both the ground and the air layers through which it passes. In the frequency bands in which radio modems operate, with wavelengths of around 1 metre (3.28 ft), there are many objects such as hills and buildings that can cause a radio shadow (cf. Mobile telephony). This is in addition to intermittent interference from other equipment.
Antennas Terminology When discussing radio communications and antenna it is vital to understand a few basic terms and expressions. The first basic formula to remember relates frequency (f) to wavelength (l) by the equation: l [m] = 300 / f [MHz]. The radiation pattern is the three dimensional radiation characteristics of an antenna in 2 planes, the electric field (E) and magnetic field (H).
Example of Yagi aerial Example of Dipole aerial 108 Types of antennas Dipoles and dipole arrays are constructed of one or multiple dipole antennas and power splitters combining the antennas. These are typically omnidirectional or off-set pattern antennas. Yagi and Yagi arrays are constructed of one or multiple yagi antennas and power splitters combining the antennas. These are always directional antennas.
Radio network A radio link budget calculation should be performed to see if enough power and margin is left at the receiver end of the radio link after propagation. In radio link calculations everything is expressed in dB, plus or minus, and added together. Radio link budget calculation parameters are distance, frequency, terrain, antenna height, transmitter output power, receiver sensitivity, feeder loss, antenna gain and propagation loss.
Industrial Ethernet As a communication standard, Ethernet has existed for many years and today forms the basis of most networks throughout the world. Despite many claims over the years that Ethernet will be replaced, it continues to be developed and offers the properties that users have requested. In recent years Ethernet has also won approval in the industrial market. IEEE 802.
Ethernet Address & Packets All Ethernet hardware has an address that uniquely identifies each node in a network. This address is programmed into the device by the manufacturer, for example, a network adapter card. This can not be changed by the user or by software, which means there is not (should not be) two network adapter cards with the same address. This address is often refered to as the MAC Media Access Control Address.
Collision domain A collision domain is a segment where connected equipment must be capable of detecting and managing collisions (as several devices send simultaneously). Data that collides does not disappear automatically, but CSMA/CD neatly and tidily ensures the data is retransmitted. The number of retransmission attempts can be limited to 16, and it is not until then that data can be lost.
… Assume that A intends to send a packet to B. … The network includes a certain amount of equipment that has an internal delay (t). … A continuously empties its send buffer, when no collision is discovered. … A collision occurs on the outermost node on the network (E). … All data (D) is not received, which results in (B) not being able to interpret it. … The collision signal (F) is sent back to the transmitter (A).
Byte 1 2 3 4 192 . 168 . 3 . 23 Addressing in a network Before we describe how an IP address is built up we need to explain a few concepts: … An IP address consists of four bytes. … One byte is 8 data bits, for example, 11000000, which corresponds to the decimal value 192, see byte 1 in the example opposite. … In turn, addresses are allocated in different classes (A, B, C, D and E) where the class describes an address interval.
Private and public addresses There may be cases where you can not use or do not want to use public IP addresses on your internal network, instead you can use private IP addresses (RFC1918). These IP addresses will not work on an Internet connection, the solution is then to use NAT (Network Address Translation). Internal network with private IP addresses 10.01.4 60.20.10.10 Internet 10.0.1.1 10.0.1.2 10.0.1.
Ipv4 and Ipv6 IPv6 is version 6 of the Internet-protocol, the new version was drawn up at the end of the 1990s to replace the current, IPv4 (version 4), mainly because the IP addresses are starting to come to an end. The greatest difference between IPv6 and IPv4 is that the address length has been increased from 32 bits to 128 bits. This means the number of possible addresses has been increased from 4 billion to a real astronomical number.
The following standard netmasks (i.e. a without subnetwork) apply to the address classes A, B and C: Address class Netmask Binary value Binary value Binary value Binary value Byte 1 Byte 2 Byte 3 Byte 4 A 255.0.0.0 11111111 00000000 00000000 00000000 B 255.255.0.0 11111111 11111111 00000000 00000000 C 255.255.255.
Ports An application receives data on a special port number that identifies communication with this application. For example, a computer can be both a web server, E-mail server and DNS server running at the same time. In order for the traffic to the different applications not to collide, it must be divided up, this is done by predefining the port number to the application. Port numbers between 1 and 1024 are known port numbers and must not be used by applications other than those specified.
This is why devices A and B have their own ARP-table of IP addresses and associated MAC-addresses. ARP Address Resolution Protocol, manages a dynamic update of the ARP-tables so that the association between IP and MAC-addresses is always known. … Assume that computer (A) wants to communicate with the PLC (B). Computer (A) already knows (B’s) IP address (can e.g. have been manually configured by an operator) but (B’s) MAC address is unknown to (A).
PAP works similarly to when a user logs in using a terminal, you state your user name and password. Authentication only takes place once when the connection is being established, never while communication is in progress. … The PAP-procedure starts by one of the parties sending an AuthenticateRequest, containing name and password. This packet is repeated until the opposite party responds. … When the name and password are accepted the recipient answers with an Authenticate-Ack.
TCP/IP and UDP/IP In the OSI model each layer is responsible for the data that passes through it. The transport layer bears responsibility for the transfer of data and there are two alternative protocols available for this, TCP and UDP. Windows Sockets Applications Telnet, FTP NetBios Applications Sockets NetBios NetBios over TCP/IP TCP UDP UDP UDP (User Datagram Protocol) is usually classified ICPM IGMP as a connectionless protocol.
A Application SYN Establishing a TCP connection A connection is established using a handshaking procedure comprising of three steps: … The client A sends a connection request with the SYN-bit enabled. This allows the client to synchronise a sequence number with the B Server (B). … Server (B) acknowledges (ACK) the client with its SYN-bit enabled and with that the server has also synchronised its sequence number with the client. Application … Finally the client acknowledges with (ACK).
Building a network Devices in a network Repeaters A repeater can be compared to an amplifier, it has no intelligence it only recreates signals. Signals are attenuated depending on the length of the medium and the frequency of the signal, which results in a network having a limited range. Using a repeater you can extend a medium by recreating the signal, thus the signal is identical to its initial state with regard to strength and appearance.
Router A 1.2 2.1 Address 1.1 Router A 124 Router The word route means to select or find the right path. A router is a device, or in some cases software in a computer, that determines where a packet should be sent on its way to the end destination (the router is the end destination from a LAN’s perspective). Subsequently, the router is a network device that links together two or more logically separate networks.
dynamic or static. A dynamic routing table is updated automatically based on the structure of the surroundings. How the traffic should be routed is controlled by a routing protocol, e.g. RIP (Routing Information Protocol) or OSPF (Open Shortest Path First). Brouter There are many standards on the market, the most common are Ethernet, Token ring and FDDI. All these use different communication techniques and formats, but addressing is common and standardised by IEEE.
Switch Switch A switch is similar to a hub in that it is the central connection point for the network. The difference is that the switch keeps track of which devices are connected to its respective ports. When data is sent to a device in the network, the recipient address is checked by the switch and data is only sent to the port where the device is connected (switched network). In this way the network is not overloaded with unnecessary traffic.
Hub or Switch Why is a switch so much better than a hub and what is the difference between these products? We have already ascertained that it was the hub that made the installation of star coupled networks possible, and together with Ethernet, made structured cable systems popular. The hub does not have an advanced design, everything sent to one port is transferred to the other ports. This means that everyone hears what everyone sends and everyone is in the same collision domain.
Different types of switches Depending on the application and installation requirements there are a number of different switches. First we differentiate between the interfaces, where there are TX (copper) and FX (fibre). Other variants are unmanaged/managed switches, this means you either have or do not have the possibility of communicating with and monitoring the switch using SNMP.
Ringswitch Our ringswitches are available in two variants, for basic ring networks and for bridged ring networks. The models feature different software for reconfiguration FRNT0 and FRNT1. FRNT0 There are always two alternative directions for traffic in a ring, a right-handed or lefthanded circuit. A ringswitch utilises this and in doing so eliminates network errors. Should an error occur the switch, which is configured as the focal point, is notified.
Time switches Ethernet through design is not deterministic, i.e. you can not guarantee the transfer time of a data packet from one occasion to another. This previously made it impossible to use Ethernet for real time applications, such as monitoring transformer stations or controlling complex machinery, but these limitations no longer exist.
Switch functions Prioritisation (QoS, Quality of Service) Switches that support prioritisation have two or more queues connected to respective ports to handle data (QoS). Prioritisation can take place on different levels and using different techniques. There are a number of techniques, the switch can send a predetermined number of packets from a high priority queue before is sends a low priority packet (Round-robin).
Layer 3 priority Using a layer 3 switch you can partly prioritise data on the MAC-level (layer 2) as above, or together with an IP “header level” i.e. as a router. Each packet is given priority based on the content of the field, Type of Service (ToS). Layer 3 IP header MAC Version IHL IP Type of service Identification Total length Fragment offset Time to live Header checksum Source IP address Destination IP address Options 132 Theoretical and general applications www.westermo.
Head of Line blocking prevention Incoming and outgoing data is buffered in a switch (queue handling), this is normally based on FIFO handling, i.e. first in -first out. When the received data needs to be sent to several ports and one of these is overloaded, it is necessary to wait until the overloaded buffer can receive data again. The function is called Head of Line (HoL) blocking. If a switch has several queues for low and high priority data, a high priority packet can be delayed due to HoL.
A1 B2 VLAN VLAN or Virtual LAN is a technique that permits grouping of equipment in a common network. There are several options, on a port level or on a MAC-address level. Furthermore, there are supplier specific solutions. Historically companies and organisations have used routers to segment large networks. This segmentation can also be done using VLAN. A network with installed equipment forms a common “broadcast” domain for all connected devices.
IGMP/IGMP snooping Internet Group Management Protocol (IGMP) is a protocol used by IP hosts to report membership in Multicast groups to the closest multicast routers. Multicast routers periodically send out a “Host Membership Query message” to remain updated about group membership for the local network. The hosts on the local network then answer with a Report-datagram. The hosts only respond to the request for the groups they belong to.
Time synchronised networks Up until now distributed real time systems have usually been based on fieldbuses, but switched Ethernet is now an option. This is partly due to features such as: bandwidth, possibility of prioritisation and industrial specification of network equipment. However, also because Ethernet equipment prices have dropped. Variable delay (latency) in a switched network means that data sent from nodes can be affected by different delays.
SNTP/NTP RFC 2030 Simple Network Time Protocol (SNTP), RFC 1305 Network Time Protocol (NTP) and P1588 are established protocols for time synchronised IP traffic. SNTP is a subset of NTP. The SNTP/NTP server handles the system clock, which in turn can be based on GPS or the internal clock. The time information is then distributed either through unicast or multicast. 1. Updating via unicast, updating is initiated by the client after which the server returns an answer.
SNMP SNMP stands for Simple Network Management Protocol. SNMP makes it possible to manage devices on a network. A device that can be monitored is called an agent. A master system sends an enquiry message to the agents and requests data, this can be done using special applications or using Telnet. Using SNMP you can: … Monitor trends. … Monitor events for analysis. … Monitor devices in the network and their status. … Monitor an especially important connection.
SNMP software Software used to communicate with the agent is called Network Management Solution (NMS). The exchange of data with the agents is similar to communication between a master and slaves, i.e. communication with the underlying devices takes place through polling. The manager can request information from or perform an action on the agent, this responds to the enquiries or actions requested. Another option is for the agent to set a “trap” i.e.
SNMP, SNMPv2 and SNMPv3 There are three versions of SNMP. The original version of SNMPv1 has a multi security mechanism, which is a password. In version 1 you can not identify the sender of a message with all certainty. This makes SNMP open, which allows the reconfiguration of devices in the network. As a consequence of this many equipment manufacturers have chosen not to implement all the functions in the standard.
MIB Each agent in the network has a set of MIBs (Management Information Base), a MIB is an object that can be called by a manager. Information can either be standard information such as port status or port state, or company specific MIBs (private) for example the temperature inside the device. MIBs are structured tables made up of the different objects that can be called. The structure can be compared to a tree with a root and underlying directories.
… OPC eXtensible Markup Language (commonly known as OPC XML) HTML based language for information exchange between applications. In order to illustrate the problems, assume that three applications need to exchange information between two PLCs and an operator panel (HMI). Each supplier has his own specific application with its drivers. The drivers need to download data from respective PLCs and HMI, here this results in nine integration points. 142 Theoretical and general applications www.westermo.
OPC simplifies this by using standard tools. The development of OPC is the result of collaboration between leading automation suppliers and Microsoft. Technically, Microsoft’s COM (Component Object Model) and DCOM (Distributed Component Object Model) are used for the communication between applications. Consequently, in this example each PLC and HMI only has one connection point, which in turn leads to simpler and more cost effective implementation of the entire system.
Ethernet on the cable 10 Mbit/s Ethernet Signals sent over all 10 Mbit/s media systems uses Manchester encoding. Manchester encoding combines data and clock into bit symbols, which provide a clock transition in the middle of each bit. A logical zero (0) is defined as a signal that is high for the first half of the bit period and low for the second half, i.e. a negative signal transition. A logical (1) is defined as a positive signal transition in the middle of the bit period.
VTX V D A TA 0 0 1 IDLE 1 10Base-T 0 2.5 0 t ns 250 ns -2.5 16 ms 50 ns Fibre Transmitter D A TA 0 0 1 10Base-FL IDLE 1 0 On t ns Off 50 ns 500 ns VTX V D A TA 0 0 1 IDLE 1 10Base2 0 t ns 0 -1 -2 50 ns VTX V D A TA 0 0 1 IDLE 1 1 100Base-TX 0 1 t ns 0 -1 8 ns 8 ns Fibre Transmitter D A TA 0 0 1 IDLE 1 1 100Base-FX 0 On t ns Off 8 ns 8 ns www.westermo.
Glossary 146 10Base2 An Ethernet wiring standard that uses thin coaxial cable as the network medium. A maximum of 185 metres (616 ft) is possible per network segment. Devices can be connected directly onto the LAN by daisy-chaining. 10Base5 An Ethernet wiring standard that uses thick, double shielded coaxial cable as the network medium. A maximum of 500 metres (1666 ft) is possible per network segment.
Auto-Negotiate The IEEE802.3u standard specifies a MAC sub-layer for the identification of the speed and duplex mode of connection being supported by a device. Support of this feature is optional. Auto-Sense The ability of a 10/100 Ethernet device to interpret the speed and duplex mode of the attached device. It will automatically configure itself to match the required configuration. Baud Defines the speed of the number of “packets” transferred per second. With local data communication baud = bit/s.
Capacitance Ability to absorb an electrical charge. F = 1 µF Measured in microfarad = 10–6 nanofarad = 10–9 F = 1 nF picofarad = 10–12 F = 1 pF CAT5 A copper twisted pair cable that supports bandwith up to 100 MHz or 1000 MHz when using all four pairs. Common data rates are 100 Mbit/s or 1000 Mbit/s. CAT5e Enhanced Cat 5 standard provides noice immunity.This is the most common in new installations. CHAP The Challenge Handshake Authentication Protocol is far more secure than PAP.
Data bits See bit. Databus Several parallel cables for the transfer of data internally in equipment. Datagram A self-contained sequence of data that carries sufficient information so that it can be routed from source to destination without any other or earlier type of interaction between these two devices.This type of connection is commonly referred to as connectionless based communication. DC Direct Current. DCE Data Communication Equipment. DDS1 European standard for ISDN connections.
Faxmodem A modem that can send and receive data information (text, images) in fax format. FDDI Fibre Distributed Data Interface: A standard for fibre-optic networks. Fibre optics Modulated laser light or laser beams from light emitting diodes through thin glass or plastic fibre, normally between 800–1300 nm (nanometre). Bunches of fibre cable can transfer vast amounts of information. Fieldbus A defined standard for industrial data networks, for example, PROFIBUS.
Ground currents Current that flows in the ground conductors between two systems with different ground potential. GSM Global System for Mobile communication, a standard for digital wireless communication. Half duplex Two way communication. Handover Name for switching between base transceiver stations when communicating via the GSM network. Handshaking Confirmation and status signals sent between communicating equipment to check the data flow.
IP Address The IP address is a 32-bit number that identifies a network device.The IP address is made up of two parts. Firstly, the identifier of a particular network and secondly an identifier of the particular device on that network. Due to the finite number of IP addresses with a 32-bit number a new IPv6 address method is now being implemented. ISDN Integrated Services Digital Network, standard concerning digital networks for telecommunication, data, fax, video and video telephony.
MAU Media Attachment Unit. Enables a device to tap into the LAN Medium. Commonly the LAN medium used with this type of interface is coaxial cable.This type of cable is referred to as to as Thicknet or Thinnet. MDI Medium Dependant Interface. An Ethernet port that allows connection to other Data Communication Equipments (Switches, Hubs etc) without the need of a null modem coaxial cable or cross over cable.These can be referred to as uplink ports.
Network General designation of communication links between two or more pieces of equipment. OPC Open Process Control. (Formally OLE Process Control). An open standard that enables devices to openly communicate with each other regardless of manufacturer. Optocoupler Signal transmission via light, for example, light emitting diodes and photo-transistors. An optocoupler does not conduct electrical current and in thus provides galvanic isolation. Optoplexor Multiplexer for fibre cable.
Phase Modulation Affects the signals position during the period, phase angle, to encode data bits. Phase modulation is primarily used in digital transmisions. Pin Terminal in e.g. a D-sub connector and on circuits intended for mounting/ soldering. PLC Programmable Logic Controller. Polling Connected units are asked, polled, by the main computer whether they have information to send. POTS Plain Old Telephone System, same as PSTN PPP Point to Point Protocol.
Ring network A series connected network where all units are connected in a closed ring and all communications pass through all units. RJ-45 8-pos. modular connector according to ISO standard 8877. RLP Radio Link Protocol. Error correction protocol used in GSM. RMON Remote Monitoring. Is a standard MIB that provides diagnostic data for networks. Roaming Possibility to use GSM equipment on several different operators’ network.
Star network A network built from a central unit, with direct lines to the connected units. Start bit Denotes the beginning of data transfer. With asynchronous transfer each character is preceded by a start bit. Status signal Reports the status of the connected equipment, for example, switched on, ready to receive, ready to send. Stop bit One or more stop bits denote that the character is finished. Switch Switch, manually or software controlled that redirects the data traffic.
UDP User Datagram Protocol is responsible for delivering data from one device to another. UDP usually uses IP to pass data but unlike TCP does not enable the message to be broken down into packets that can be correctly re-assembled at the destination. Therefore, the application using UDP must have the ability to detect that the message or data has been correctly received. However, UDP has an advantage of passing data faster and with less overhead when compared with TCP.
