Wide Area Networks Ch. 7.pptx

Wide Area Networks Ch. 7 Wide Area Networks – This chapter discusses communications over a large geographical area ) where the media and/or infrastructure is not owned by the entity doing the communicating – This network may include the public switched network or leased portions of the network for...

Wide Area Networks Ch. 7 Wide Area Networks – This chapter discusses communications over a large geographical area ) where the media and/or infrastructure is not owned by the entity doing the communicating – This network may include the public switched network or leased portions of the network for private lines – Because business drivers and purposes have changed but media characteristics haven’t—a twisted copper line pair has the same impedance regardless of the network type, although the impedance may have a greater or lesser tolerance Wide Area Networks – Although the modems and digital lines described here may be part of your immediate network, when a LAN wants to communicate with the outside world , it will do so using one of the wide area technologies described in this chapter—or, at a minimum, it will employ a means that can trace its parentage back to one of these technologies – In discussing WANs, this chapter proceeds in an almost chronological fashion, beginning with wireline and wireline modems and working up to the digital line offerings – New students of data communications may wonder why wireline modems were first used and why they were so slow, particularly if they remember their own experience with the Internet and 56 Kbps modems Wide Area Networks – Keep in mind that industrial control systems use LANs and are not normally run over a WAN Wireline Transmission – This section describes WAN devices that fit into the first two layers of the ISO OSI model, as do many of the WAN technologies: Layer 1, the Physical layer, and Layer 2, the Data Link layer – Layer 1, the Physical layer, provides the physical connection—the electrical and mechanical means to establish, maintain, and end physical connections between Data Link points – Since the length of the line introduces distortions and line losses to electrical signals that have a large direct current component , an alternating current is used as a carrier for the data Wireline Transmission – This is accomplished through the process of modulation and demodulation, which is a method for translating frequencies by taking the DC signaling rate and transforming it into a higher frequency range, thus eliminating the DC component – Modulation is a term used to describe the impression of intelligence on a carrier – In other words, modulation consists of modifying some sort of carrier with some sort of data – Since almost all electrical communications over any great distance or at a high rate of speed use one or more modulation techniques, grasping them is essential to understanding industrial applications in which data is normally transferred some distance Carrier Concepts – Why is modulation needed? – Although there are many reasons, for our purposes here there is only one: to translate a given signal’s data to a different frequency – The reason that this is necessary will become evident in the discussion that follows Wireline Effects on a DC Signal People are normally inclined to think of data communications through a wire as being instantaneous This is because their observations are usually based on short lengths of wire and are focused on such effects as turning on a car’s headlights, which seem to come on immediately Figure 7-1 illustrates the waveforms, both ideal and real, for a simple series circuit that consists of a load , a source , a switch, and a relatively long length of connecting wire To see the reason for this output change, we must analyze the step voltage and understand that any conductor, such as a wire, has a series inductance and a parallel capacitance to some other conductive body Wireline Effects on a DC Signal – The state of being lumped is represented by the schematic symbol for an inductor whose value represents the total inductance throughout the wire; the same is true for the capacitor, a single symbol representing the lumped capacitance of a segment of line – The level state is made up of the lowerfrequency components—primarily the fundamental sine wave – Developing a square wave from the fundamental sine wave and its odd harmonics is performed by Fourier analysis techniques ; all that is necessary to know is that the resultant square wave shape can be modeled by its fundamental frequency and odd harmonics Wireline Effects on a DC Signal – Figure 7-3 is an example of modeling a square wave; the figure on the left is using the fundamental and the 3rd harmonic, the one on the right, the 3rd, 5th, 7th, 9th, 11th, 13th, and 15th harmonic Sine Wave as a Carrier – The sine wave is used as a carrier because a sine wave cannot be integrated or differentiated mathematically – Long wireline processes affect a square wave exactly as they affect the ID functions of a proportional-integral-derivative controller – That a sine wave cannot be integrated or differentiated by linear or passive devices, that a square wave is made up of a fundamental sine wave and its odd harmonics, and that an electrical line acts as a low–pass filter are physical phenomena; we may not necessarily know why, but we can use the rules Modulation – Modulation is the process whereby a carrier’s characteristic waveform is modified to contain the data that is to be transmitted – The sine wave has three characteristics that may be modified: Amplitude Frequency Amplitude is the magnitude of a signal – Frequency and phase are collectively considered angular momentum; a change in frequency comes about from a continuing change in phase – Over time, the on-off modulation of the carrier evolved into another form of modulation called frequency shift keying, a form of amplitude modulation – Amplitude modulation and frequency modulation, known as AM and FM, respectively, are familiar to most people through their radios and home stereo system receivers Modulatio n An American analog broadcast television signal has a 6 MHz baseband whose range covers 0 to 6 MHz At the height of analog TV transmission, even more features were put into the baseband signal, such as text and stereo sound The same information is now transmitted digitally and four channels will fit into one analog channel Along with the DVD/Blu-Ray component, the modern digital TV is quite probably one of the most technically sophisticated devices in the average household and we haven’t even approached high definition TV , which brings yet another layer of complexity Amplitude Modulatio n Amplitude modulation is the process whereby the amplitude of a carrier wave is varied according to the data that is to be transmitted AM involves more than simply turning the carrier on or off, although that in itself is a legitimate form of modulation To better explain amplitude modulation, we will use the classic case in which the carrier is a sine wave at some frequency many times higher than the frequency of the modulating signal, which is also a sine wave Amplitude Modulation – The output actually consists of four different frequencies after modulation: the carrier frequency, the modulating frequency, and two new frequencies that are the result of modulation: The instantaneous sum of the carrier and the modulating frequency The instantaneous difference of the carrier and the modulating frequency Figure 7-4 shows the results of a carrier at 1 MHz and a modulating frequency of 1 KHz – The change in the carrier’s amplitude at the modulating frequency rate causes the side frequencies which are referred to as sidebands – In this context, bandwidth means the span of frequency, measured in hertz; a signal occupies and is determined by the difference between the highest- and lowest-frequency components that are significant DoubleSideband AM The sidebands of a double-sideband amplitude modulation signal are redundant In the data case, the carrier frequency is usually just above the signaling rate, rather than a number of magnitudes greater, as is the AM radio example DSBAM can be used for data transmission VestigialSideband AM – The scheme most often used for AM data transmission is vestigial-sideband amplitude modulation – This form is used because the bandwidth it requires is a little more than one-half that required for a DSBAM signal – In VSBAM, the carrier is not fully suppressed and a portion of the upper sideband is transmitted SingleSideband AM – Many people are familiar with single-sideband AM from its use in amateur and Citizens Band radios – During modulation, the carrier signal is totally removed and one sideband is eliminated along with the carrier, then all of the transmit power is employed in only one sideband to send a single data stream – SSBAM is seldom used as the sole modulation method for digital signals Independe ntSideband AM Independent-sideband AM is a singlesideband type of signal used in telecommunications It is similar to the SSBAM signal except that both the upper and lower sidebands are transmitted with the carrier suppressed An important characteristic of ISBAM is that when the upper sideband is used for one data channel and a lower sideband is used for another channel, they cannot demodulate each other Frequency Shift Keying – One of the most popular and oldest methods of frequency modulation for digital data is frequency shift keying – This method has two tones or carrier frequencies in the audio range: a tone – A rule-of-thumb requirement for this method is that the tones’ frequencies should be separated by approximately the same number of hertz as the signal’s bps rate – Actual practice allows less separation, but inter-symbol noise will increase – Frequency shift keying is employed because it is an easy process and it fits in a binary digital transmission scheme – However, FSK occupies two cycles of bandwidth for every bit per second, which requires double the bandwidth required for other methods; in this method we are trading bandwidth for simplicity Frequency Shift Keying A 1,200 bps signal requires 2,400 Hz of bandwidth Nonetheless, this is the method used in all low-speed modems Frequency Modulatio n In frequency modulation , the modulating signal varies the carrier’s frequency change from rest according to the modulating signal’s amplitude Moreover, the carrier’s frequency changes vary at the modulating frequency An important fact to remember about FM is that the output carrier’s amplitude remains constant; only the frequency changes Most noise involves amplitude; that is, it tends to ride on signals above and below those of the average carrier An important fact to remember about FM is that the deviation—the amount by which the carrier frequency is varied—depends only on the modulating signal’s amplitude Frequency Modulation – Their relationship to the carrier is such that each sideband is an integral multiple of the modulating frequency away from the carrier Phase Modulation – Phase modulation is essentially like frequency modulation, except that the amplitude of the modulating signal causes a shift in the reference carrier phase – The basic difference between FM and PM is this: for a particular modulating signal value of amplitude, the phase modulator’s output amplitude is a constant and the phase modulator’s output phase varies only with the modulating signal’s amplitude and not the modulating signal’s frequency – The output of the FM modulator, on the other hand, varies directly with the modulating signal’s amplitude but inversely with the modulating signal’s frequency Encoding Data – In our prior discussions, we determined that the baud rate was the maximum signal change rate the medium could support – The rate is specified in baud and is called the baud rate, line rate, or line signaling rate – The baud rate and the data rate can be quite different – We also discussed the attributes of transmission line lumped characteristics and determined that any given line will suffer from signal degradation as the signaling frequency increases – In 1948, Claude Shannon stated that the maximum data rate possible on a communication channel is proportional to the bandwidth of that channel – To increase the data rate for a particular modulation scheme without increasing the line’s baud rate requirements, the digital data may be encoded Encoding Data – This type of phase modulation is called continuous phase modulation or continuous phase shift keying and the resulting four-state modulation is referred to as quaternary phase shift keying – The detection process is differentially coherent detection, which means that the last transmitted phase becomes the reference for the next transmitted phase – A long string of 1s or 0s could cause a continuous change in phase resulting in a change in the transmit frequency Modulation Summary – Frequency modulation and phase modulation are usually lumped together as angle modulation – Amplitude modulation changes the carrier’s amplitude according to the modulating signal’s amplitude and the changes in the carrier are at the modulating signal’s frequency – Regardless of which modulation scheme is selected , the following rule applies: encoding the data effectively makes it possible for transmission lines to support higher data rates while still staying within the available bandwidth Wireline Modems – Serial modems originated in the 1960s with point-topoint serial data transmission, which, at the time, was the most economical way to transmit data over long distances – Networks were composed of interconnected, point-topoint transmission media – At that time, most loops were local , though several could operate through modems for a dedicated pointto-point line – The modem’s parameters were expressed in terms of the AT&T specifications – Long-distance data communication, although more rapid than four decades ago, still uses some of the same specifications, as well as all the old technical jargon – Synchronous – Synchronous generally means tied to a common clock, the clock signal is transmitted along with the data Wireline Modems – Data rate, in bits per second, is the transmission speed the device is capable of transmitting or receiving – The bit rate ) of a modem depends on the baud rate, the condition of the line, the packetizing method used, the amount of data compression, and the encoding scheme used – Baud per second describes a change in transmission bandwidth, not a data rate Modem Types – Wireline modems are for use over dialup or leased telephone-type lines and are divided into two classes: Bell1 and International Telecommunication Union2 , which issues the V and X standards – Most modern modems are synchronous between the modems, using Link Access Protocol – Modem packets and may have a synchronous or an asynchronous interface to the DTE – The Bell 212 is capable of identifying what type of modem it is talking to and adjusting itself to become that type Faster Modems Prior to the Bell 212 modem, the Bell 202 modem operated at 1,200 bps using FSK, with one carrier at 1,200 Hz and the other at 2,200 Hz The Bell 202 could actually be called the originator of the asymmetrical data line, in which data in one direction proceeded at a much higher line rate than data in the other direction The 212 modem used di-bit-encoded, quaternary phase shift keying with coherent detection and synchronous modulation/demodulation because it has noise advantages over a four-level AM signal The 212 modem had a unique answer tone that allowed for the convenient identification of modem speed when the called end answered Modem Data Compression and Error Detection – Since strings of repetitive ones and zeros can be encoded in fewer characters , throughput increases – Modem manufacturers offer different data compression, as well as error detection and correction schemes – Two sets of standardized schemes are in general use: the V.42 international standard, which in the original and bis versions allows data compression at two to one and four to one , and the Microcom Networking Protocol , which is discussed below Other Modern Modem Features – The MNP comes in various flavors, the first five have been released to the public domain – MNP-1 and MNP-2 are generally concerned with packetizing data, while MNPs 3 through 5 address a range of parameters, including losing the start and stop bits , for a 20 percent gain in throughput, and by packetizing the data – MNPs 6 through 10 are still in the proprietary domain, meaning that it is necessary to have modems that have the appropriate level of MNP software at both ends of the data transmission – V.42 employs the LAP-M frame and uses code lists to represent strings of characters, replacing the strings with the shorter code lists to perform data compression – Part of modern modem operation is a process called training or negotiation Other Modern Modem Features – On the way to the current modem standards, a variety of techniques were developed to permit highspeed data transmission over a 3 KHz wireline – This scheme is the basis for the discrete multi-tone process used in DSL lines Modems Summary In this section we have discussed modems, from low-speed to high-speed, that operate over the typical telephone wireline Data speeds increased from the high speed of the early 1970s to an order of magnitude in just four decades Modems are used for frequency translation, and wireline modems comprise only a small segment of those devices WAN Digital Lines A WAN is commonly understood to be one that serves geographically separated areas One of the best and largest examples is the Internet, but there are other wide area data networks Intercity links can be considered as WANs and usually operate at 1.544 Mbps, the T1 carrier data rate; T1 has become the standard telco channel Telephone Lines as Media One of the original WANs was the landline telephone network, in particular the direct distance dialing network that existed for many years prior to digital networks Since the 1990s, the DDD has been a digital network with the exception of the subscriber lines; these are still analog In digital transmission, phase distortion problems are corrected as much as possible by applying conditioning—using features that may be focused in the line, in the modem, or possibly in both For years, telephone operating companies have been promoting digital lines to customers by leasing a line, a data service unit , and a channel service unit Telephone Lines as Media Although the public switched telephone network began life as analog, it started becoming digital in the 1960s and today is totally digital, except for the final mile connection to your home The phone company digitizes your voice using an 8 kHz sampling rate and 8-bit analog-to-digital circuit This produces a continuous data rate of 64 Kbps , which the phone company has to continuously deliver from your land-line phone to the land-line phone at the other end Public Switched Telephone System – The DDD system is still used for calls outside the so-called local access and transport area – In North America, the ten-digit LATA number includes a three-digit area-code prefix to the normal number, which is a three-digit central office and a four-digit station number – At the rapid rate that mobile devices are being used, land line services are becoming a historical novelty; so it really makes little difference anymore what a land line is or is not because DSL, cable modems, satellite links, voice over IP , 3G and 4G smart phones, and Ethernet everywhere generally command the lion’s share of technical attention Public Switched Telephone System – For land line data transmission, the home computer transmission is digital to the modem, converted to analog to go over the subscriber line to the central office , and converted to digital for long lines – Although the telephone company continues to deliver analog subscriber lines to homes that need multiple phone lines and Internet access, businesses are more likely to get one or more T1s from the phone company and hook them to a digital private branch exchange that interfaces via Ethernet LAN with VoIP phones in each office and the company email/web servers Public Switched Telephone System – The DDD system is still used for calls outside the so-called local access and transport area – In North America, the ten-digit LATA number includes a three-digit area-code prefix to the normal number, which is a three-digit central office and a four-digit station number – Calls in the LATA or inter-LATA can be routed in many ways: by radio link, satellite, wireline, microwave, or fiber optics Public Switched Telephone System – At the rapid rate that mobile devices are being used, land line services are becoming a historical novelty; so it really makes little difference anymore what a land line is or is not because DSL, cable modems, satellite links, voice over IP , 3G and 4G smart phones, and Ethernet everywhere generally command the lion’s share of technical attention – For land line data transmission, the home computer transmission is digital to the modem, converted to analog to go over the subscriber line to the central office , and converted to digital for long lines Public Switched Telephone System – Some long lines are still analog and connect to trunk lines where the circuit is converted from a two-wire to a four-wire circuit – To deal with this problem, an echo suppressor is placed in both lines – One person talks and when he or she falls silent, providing an opening, the other person talks Packet Switching – Once data is packetized , the packet assembly/disassembly device begins the process of breaking up a digital data stream into packets or fragments of packets – Each packet includes its destination and source address, perhaps with packet control and data as well so the packet may be switched rather than routed – CCITT ) Recommendation X.25 provides a packetswitching scheme to be used over the public network – The scheme provides the technique used by the PAD to create and add the headers to each of the small packets – A network cloud is a mechanism or symbology allowing one to see the connection interface but the mechanics in the cloud are not observable, only data at the interface Packet Switching – To review, in connection-oriented transmissions, a destination is set up and the packets are routed to that source in the same sequence in which they were generated, even if it means the packets must be stored at one point while they wait for an available path – Connectionless service allows the packets to be routed by any available path and they may arrive at the destination out of sequence Integrated Services Digital Network – The integrated services digital network service was designed to offer voice, data, and video, all integrated into one service and generally distributed throughout the local area, as long as the on-premises equipment was not more than 3900 m from the central office – The objective of ISDN was point-to-point digital connectivity and it was aimed at the small office, home office market – ISDN was conceived based on the sometimesanalog public telephone network – Somewhere else on the phone system network the ISDN data traffic would be delivered, usually to a corporate mainframe or another SOHO termination – Although the technology to implement ISDN has been around over 35 years, the implementation has lagged Integrated Services Digital Network – Since many foreign governments have a monopoly over telecommunications and can dictate standards, many countries had previously established standards for ISDN – Some of these standards clash with the technology and rationale of the competitive systems found in the United States – ISDN did achieve a degree of popularity in the United States, not as an integrated service medium but for relatively fast Internet access from 1990 to – One of the key features of ISDN is out-of-band signaling – The original intention was that the B channels could carry either digitized voice or data on either channel and that the D channel would provide control signals and meet other low-speed signaling requirements Frame Relay – Frame relay is the connectionless version of X.25; instead of using LCN, it uses a data link connection identifier – Frame relay speeds typically start out at 64 Kbps and can go as high as you have the pocket money to afford – Frame relay may be used to connect corporate LANs, and the present-day cost per single channel is low enough for industrial use, particularly if the frame relay Digital Data Service is used – Frame relay DDS provides 56 Kbps with no CIR, using the DDS-type tariffs , and it provides this service typically at the monthly cost of a business telephone line Frame Relay – Many industrial users of SCADA technology, particularly in urban areas, have paid for leased analog phone lines from the local telephone company in order to communicate with their RTUs T1 Carrier – A T1 carrier is a 24 digital-voice channel , time-division multiplexed signal – It multiplexes the outputs of 24 DS0 channels onto one 1.544 Mbps line, generally a coaxial cable but increasingly fiber optic cable – Each of the 24 channels that make up the T1 has a data rate of 64 Kbps; however, in most cases, the customer actually only gets 56 Kbps, since the provider uses bits from each DS0 for control T3 Carrier – The T3, however, is used here – A T3, which is also an international standard known as DS3, consists of 28 T1 lines – T3 is only an option for large corporations and government organizations since its cost per month is high Dataphone Digital Service – The two dataphone digital service terminations—a digital service unit and its corresponding channel service unit —offer digital transmission without the need for modems – These two units perform services analogous to a modem on an analog line – DDS was originally a leased-line service that offered data rates from 300 to 1,200 bps, as AT&T Digital Data Service it offers bit rates from 2.4 KBs and up Fractional T1 – Fractional T1 is a line that consists of one or more DS0 channels – Multiplexers are required to put the data on a T1 line – Various fractional T1 combinations provide different amounts of bandwidth: Most fractional T1 lines run at 384 Kbps , 512 Kbps , or 768 Kbps Functions of Basic Telco Digital Services – With most of these services, the assumption is that the user has multiple locations to network together and doesn’t want to build or maintain the necessary infrastructure or have message traffic going over the Internet and opening corporate sites up for attacks from across the Internet – In some cases, a company may want to build its own WAN and can do so using the same equipment and technology as the phone company – The following WAN technologies are, and have been, used by the various service providers and can also be used to build private WANs Fiber Distributed Data Interface – The original high-speed data bus, fiber distributed data interface , is a fiber optic token-passing ring – FDDI consists of a dual counter-rotating fiber ring where data may circulate in opposite directions – FDDI does allow a 100-km network span with up to 2 km between any two nodes – FDDI was much faster than the original 10 Mbps that Ethernet allowed when it was originally introduced, however Fast Ethernet which came about soon after, was as fast and subsequent iterations of Ethernet are much faster – FDDI has been modified from its original packetswitching format to be able to handle circuit switching for voice and video transmission if necessary Metropolit an Area Network The metropolitan area network discussed here is described in IEEE 802 MANs are currently operated at 155 Mbps, with accommodation for Switched Multi-megabit Data Service with speeds up to 600 Mbps when SMDS is standardized The typical industrial user will probably not be concerned with a MAN, and if he or she were, it would be through a DSU-like device and at the data rate of the LAN Asynchronous Transfer Mode – Asynchronous transfer mode , also known as cell relay, is primarily a fiber optic transmission system and is highly suitable for that medium, as the small transmission packet, called a cell, is Layer 2 only , which requires it to use hardware switching and error detection – ATM was originally scheduled to have a 155Mbps data rate, although there are now different rates to suit differing applications, from 25 Mbps through 2.3 Gbps – When it was introduced, ATM quickly became the technology of choice for carriers because it handles voice, video, and data over WANs or LANs with a specified quality of service Asynchron ous Transfer Mode ATM switches can be configured in a range of topologies, including trees, stars, meshes, and rings, and can support automatic re-routing around failed nodes and circuits ATM technology is perfect for highbandwidth applications, such as multisite video conferencing ATM is more of a technology for telecommunication carriers and service providers than a LAN technology; however, now that 40 GbE and 100 GbE are standardized , carrier companies have displayed a great interest in moving services toward Ethernet Synchronous Optical Network – A Synchronous Optical Network network can be configured in differing topologies , but it is most often used as a dual counter-rotating ring topology, similar to FDDI as shown in figure 7 – SONET will encapsulate into its payload whatever data appears at the interface – SONET networks have been proposed as a fiber-to-the-curb infrastructure in which copper will carry the transmission into individual residences —something like having a central office in every neighborhood Digital Subscriber Line Using the same copper pairs that bring the venerable voicegrade wireline to your residence or business, digital subscriber line technology, xDSL, has gained wide acceptance It is considered a broadband technology because you can have concurrent telephone, video, and data services on the line Unlike the digital connection services described previously , DSL does not specifically connect a user to the telephone system network, except for the existing analog phone service; it connects the user’s home computer devices to the Internet and the phone company is the Internet service provider It should be noted that DSL, as offered, is not just a connection medium and it must be considered as a connection to the Internet Digital Subscriber Line – Among the many kinds of DSL are these: ADSL – asymmetric DSL VDSL – very-high-bit-rate DSL Here, we will only discuss the most often encountered type: ADSL – These data rates are tariffed quite reasonably, with an ISP providing residential service with a cost of about $15—$24 a month Cable Modems The modem typically provides a 512 Kbps to 6 Mbps data rate to customers when the cable segment is not loaded; data rates drop as loading goes up because all customers are using the same shared media Cable modems have brought one key benefit: they gave telcos the impetus to push for wider implementation of DSL Several wireless schemes are proposed for remote rural customers but the direct satellite link is already here and, with the exception of occasional weather-related outages, satellite can supply DSL data rates or better Wireless WAN Technologies – The main reason industrial users with a single large facility would consider wireless is that nearly half the cost of any large networked installation, both initial and life-cycle costs, is in the wiring – The benefits of wireless can be great, although whether this is truly a wireless WAN or merely wireless LAN technology may be up for debate and depends on plant/facility size – Wireless WAN technologies can be useful for establishing communications throughout a geographically-large plant or industrial facility, for linking geographically separated facilities , and for establishing communications where wires may be impractical Worldwide Interoperability for Microwave Access – Worldwide Interoperability for Microwave Access is a wireless communications standard ) designed to provide 30 to 40 Mbps data rates to both fixed and mobile users with the 2011 update providing up to 1 Gbps for fixed clients or stations – The name WiMAX was created by the WiMAX Forum, which was formed in June 2001 to promote conformity and interoperability of the standard – WiMAX was originally seen as a means for providing wireless broadband Internet services in rural areas that did not have cable TV or DSL infrastructure due to distances and low housing density Worldwide Interoperability for Microwave Access – WiMAX base stations can provide both service to local mobile and fixed subscribers, as well as make point-to-point interstation connections in order to provide a high bandwidth trunk channel to connect to the Internet and wired telephone system Wireless Mesh Networks – It is often handy to have wired Ethernet available around a large plant or industrial facility but it can be messy and even dangerous to run cable and install a wired Ethernet infrastructure – Another option is to create a wireless mesh network that allows Wi-Fi access points to be placed anywhere desired and to use a second radio in the access point to establish point-to-point links to other access points – Mesh technology provides wireless communication with multiple pathways that can address line-of-sight obstructions and single points of failure, while providing builtin redundancy and self-healing for network reliability Wireless Mesh Networks – Wireless Ethernet mesh networks are built with wireless mesh routers that use multiple radios and an integral computer that handles routing, message storage, and path failure recovery SCADA Applications – Wireless networks have historically been a perfect solution for supervisory control and data acquisition systems – One of the largest changes coming to the classic SCADA network is the requirement for higher security at remote locations – Network security in an industrial mesh network must be a priority today and the WPA2/IEEE 802.11i wireless security standards discussed in chapter 9 of this book are essential for such networks Digital Microwave – In the case of point-to-point radio links, antennas are placed on a tower or another tall structure at sufficient height to provide a direct, unobstructed LOS path between the transmitter and receiver sites – An LOS microwave is used for both shortand long-haul telecommunications to complement wired media, such as optical transmission systems – Early applications of an LOS microwave were based on analog modulation techniques but today’s microwave systems use digital modulation schemes, where trellis modulation is used for increased capacity and performance Satellite – While both 4G cellular and WiMAX continue to make inroads into far suburbia and rural areas, the primary option for rural areas that want high-speed communications access is still going to be satellite service – Corporations can lease transponder bandwidth on commercial satellites for point-to-point and point-to-multipoint networking – Residential and commercial users can contract with providers that offer low to moderatespeed, asymmetric uplink and downlink satellite service, providing a bi-directional connection to the Internet – The majority of satellites used to provide communication services are in geosynchronous orbit about 37,000 km above the equator Mobile Telephony If there is to be a publicly available, high-speed data service to outlying areas in the future, it will most likely be provided by satellite or in the form of cellular telephony services One form of wireless is the Universal Mobile Telecommunications Service , which is a mobile technology that has a data rate of up to 2 Mbps At one time, the most promising wireless technology for fixed customers was the Local Multipoint Distribution System , which had a data rate of 155 Mbps Summary – Our brief discussion of WANs has hopefully familiarized you with some of the services and terminologies – A variety of WANs are in service today— many of them packet-switched digital networks that use different protocols – Many of the concepts presented here are already working themselves into the industrial area and, with the advent of corporate intranets spread over a large geographical area, adoption of WAN technologies in the process control and automation industries seems a foregone conclusion