Communication

Section II

 

The Electromagnetic Spectrum: The electromagnetic spectrum is currently used for many types of analog and digital communication. Electromagnetic waves include radio, micro, light, infrared, x rays, and gamma rays as shown below:


z

These waves have the following characteristic in a vacuum: C = f w In this equation, C is the velocity of light; f is the frequency; and w is the wave length. The communication capacity of an electromagnetic wave is twice its frequency, and over the electromagnetic spectrum the frequency varies by a factor of 10 raised to the 15th power. [ This number is a million times a million times a thousand.] THIS MEANS THAT THE POTENTIAL CHANNEL CAPACITY OR THE POTENTIAL VOLUME OF MESSAGES VARIES BY THE SAME FACTOR OF 10 TO THE 15TH. The allocation of the spectrum between gadgets( including cordless phones, baby monitors and garage-door openers), CB radio, radio ( AM, FM and shortwave), television, cellular phone, and satellite communications is an economic/political problem controlled in the US by the FCC. As new types of communications devices are developed, turf battles occur over the use of the electromagnetic spectrum. The FCC has auctioned off portions of the electromagnetic spectrum. Electromagnetic Spectrum:

Wired Communication:

Currently the three main types of wired communication are:

  1. Wired telephone
  2. Cable TV
  3. Wired computer networks.

Wired Telephone:

Circuit Switched network: The wired telephone made tremendous advances before the advent of the integrated circuit. Communication engineers developed the analog telephone that transmits the waveform of human voice into a worldwide system displacing the telegraph. The evolution of the phone system has been driven by providing humans the type of service they desire. Humans want their conversations over phones to be a close approximation of live conversations between two people, that is with no delays in transmission. Consequently, the phone system was developed as a circuit switched network where channel capacity is dedicated in a path from the initiator phone to the destination phone until the conversation is terminated. Why? With a circuit switched phone network the parties communicate through a continuously open channel in which there is no delay in transmitting the phone conversation.

Exchanges (Switches): Also, as most individuals use their phones at infrequent intervals, the phone system has been designed on the premise that most phones will be idle at any one instance. This fact enables the phone companies to connect phones through exchanges (switches). If the phone companies had simply hardwired every phone to every other phone, the number of connections would have increased as the square of the number of phones. Just imagine the number of wires there would be entering your home or apartment. Your phone only requires a single line to the switch where it is connected to other phones as requested by dialing.

The system is designed with a tradeoff between the number of switches and the number of calls which can be processed at one time. For example, if you have 100 telephones and you hook each one with every other phone, you would need 10,000 connections. If you create a switch with 10 input and 10 output lines, ten telephone conversations are possible at one time between any 10 pairs of phones. If the switch has 20 input and output lines, 20 pairs of phones can be linked at any one time. To design systems which meet the estimated demand for phone conversations almost all the time, telephone analysts developed a branch of operations research called queuing theory.

To initiate a phone conversation, the two parties must be connected in a phone channel through the switch, which remains continuously open until disconnected by one of the parties. At first, phone calls were manually placed by human telephone operators manning the first type of telephone exchanges (switches). Later, communication engineers developed automatic switchboards which could connect phones by dialing. The reason for developing switches is a simple matter of economics in that creating automatic switches to replace manual switches greatly reduces the need for labor to operate the phone system.

Analog to Digital: Currently, the telephone system has shifted from analog to digital. The shift to digital communications has been promoted by the advances in microelectronics. These advances in integrated circuit technology can be used to build ever cheaper and more powerful communication devices. For example, today telephone switchboards are simply special purpose computers with software to provide services such as call waiting. Also, with digital communications, errors in communication can be more easily controlled than with analog communication. The telephone system is a mix between analog and digital technologies. Currently, phone conversation to an exchange is analog. At the exchange it is converted to digital and then sent through the telephone networks to the final exchange where it is converted back to analog and sent to the destination phone.

Multiplex: Among switches, either within a city or between cities, it is not efficient to string a separate phone line for each phone conversation. Instead, phone conversations are multiplexed, that is combined, so that many phone conversations can be carried in a single higher frequency of the electromagnetic spectrum. Over time, the demand for communication capacity for phone traffic, data communications, and now image communication, such as pictures and now video teleconferencing, has constantly increased. Consequently, communication engineers have moved to harness higher and higher frequencies in the electromagnetic spectrum. As part of the circuit switch strategy to create a dedicated path from the originator to destination phone, each conversation that is multiplexed is given a dedicated range of frequencies in the multiplexed signal until it is terminated when its dedicated range can be assigned to a new phone conversation.

Light: The latest is laser light in optical fibers, which represents a 1,000,000 increase in capacity of over microwave communications. Because these optical fibers can fit in existing copper cable ducts, they have a much greater capacity than the copper, and they are no more expensive than copper, their use is expanding very rapidly. Through the utilization of railroad rightaways, several optical fiber networks have been constructed to connect the various major communication centers.

A laser light-optical fiber communication system consists of encoding and decoding devices, a transmitter, silica fibers, repeaters, and a receiver. Through applied research the capacity of such systems is increasing 10 fold every four years, and by 1992 it was possible to transmit 100,000 billion bits of information 10 kilometers per second. A basic problem in any communication system is attenuation, which is the disintegration of the signal as it travels down the communication channel. To restore the signal to its original strength, amplifiers (repeaters) must be placed at regular intervals in the system. The increase in capacity has been obtained by (1) increasing the purity of the optical fiber, (2) developing optical repeaters, (3) developing better laser generators, and (4) experimenting with various pure and mixed frequencies of laser light. For example, an optical repeater is created by doping the optical fiber with the rare earth element erbium. This is a major advance over previous repeaters which necessitated converting the light beam from light to electricity, amplifying the electrical signal, and then converting the electrical signal back to light.

Finally, the phone systems have had to develop elaborate billing systems to charge individual customers for each long distance call. Telephone

Cable TV: Cable TV started as high capacity analog signal. Since each station needs a frequency of 6M, 50 cable channels are multiplexed into a signal of 300M hertz. Starting in 1989 cable systems switched from analog to digital. With MPEG compression the learned to pack 10 vidoe channels into 6M. To transmit a HDTV channel more bandwith is needed that a standard TV channel. Cable systems frequently use optical fiber to junction boxes and then use coaxial cables into residences. These coaxial cables have a much higher capacity than the twisted copper wire phone system, but currently cable systems are one-way broadcast communication systems. There are a few experimental interactive systems with a limited amount of interaction. Because cable systems broadcast a fixed set of channels into each home, they have no need for exchanges found in telephone systems. Also, because each household has a fixed number of channels, cable companies do not need an elaborate billing system, such as the one for long distance telephone traffic. Cable TV

Computer Communication:

 

Communication networks for computer communication generally connect office machines or factory machines. It is completely digital. The three aspects of this communication we need to discuss are:

  1. Packets
  2. Routers
  3. Networks

Packets: Computer communication is broken down into packets, whose size in terms of bits depends on the communication protocol. One advantage of packet communication is that it provides a mechanism to correct errors, or if this can not be accomplished, resend the packet until it is communicated without error. One of the first data protocols was x.25, which is very robust in noisy channels. With less noisy channels, the protocol, Frame Relay sends data more quickly because it spends less time checking for errors.

Routers: In computer communication networks, routers serve a similar role as exchanges in telephone networks. They direct the flow of packets through the computer communication networks. Each packet has a header that enables the router to send the packet towards its destination. Packet communication in computer networks is very different from continuous voice or video communication, which as was pointed out, requires dedicated channel capacity. In contrast, in packet communication there is no dedicated channel capacity and each packet could travel a different route through the communication network. For example, in sending a long text file from New York City to Los Angeles on the Internet some packets could travel from New York to Los Angeles through Chicago and some through New Orleans. When they arrive, all the packets are assembled into the original long text file. With a voice or video phone, you either have the capacity to transmit the message in real time or you don't. With a dedicated channel there is no noticeable time delay on the earth. With data packets, the higher the channel capacity the more quickly the packets can be sent through the system. This should be readily apparent to you if you download videos or movies.

 

Computer Networks:

We will discuss computer networks in order of the area the cover. We shall consider three:

  1. LAN (Local Area Network)
  2. WAN (Wide Area Network)
  3. Internet

LAN: These small networks are now everywhere in corporations and public institutions. In large organizations like UT they connect departments, such as the economics department. We have a LAN that connects all faculty offices to servers and printers. A common type of LAN is a loop in which messages from the sender travel until they reach the receiver station. The loop is analogous to a party line of telephone. But, as the number of stations on the loop increases, the amount of traffic can greatly slow down communication. For this reason in larger organizations, networks are subdivided into LANs connected by bridges. For example, the UT economics department LAN is connected to the larger UT collection of networks for the various departments and colleges.

WAN: These large networks for large institutions such as transnationals can cover a large geographical area. Firms lease commuication bandwidth from the telephone firms in capacities that can range up to Gbps. (Gigabits per second). In designing WANs the designer must consider the delay caused by transmission of a message through a large number of routers, each of which must take some time processess each packet.

 

Internet:

Development: Data traffic for long distance is sent over many types of networks. For example, telephone networks and private computer networks carry great volumes of data traffic. Currently the computer network that connects almost all computer networks is known as the Internet In 1964 the RAND Corporation issued a report calling for a military network that had no central authority that would survive a nuclear attack. The Defense Department Advanced Research Projects Agency set up the ARPANET in 1969. This network supported researchers associated with military research. In 1986 the National Science Foundation, NSF, created the NSFNET connected to ARPANET to connect supercomputer centers to research universities. This network gradually connected most higher education centers. There was a great expansion in traffic as the combined network was used for all types of research. The third phase in the growth of the Internet started in the 1990s when ARPANET and NSFNET were replaced by six commercial networks. The current rapid expansion in Internet usage is due to exploding commercial applications and general use. How fast is the Internet growing? If it keeps growing at the same pace everyone on the planet will be a user by 2003.


Protocols: The protocols behind the Internet are the TCP/IP protocols. TCP covers the packets and IP covers the addresses. In 1991 the adoption of OSI, open systems interconnection made it possible to connect almost any type of computer network to the Internet even if the connecting network was not based on TCP/IP. What this means is that almost all computer networks can and have been connected to the Internet.

 

Email: The growth of the Internet is associated with the type of services that can be provided through the Internet Tom Truscott and Jim Ellis, two graduate students at Duke created USENET in 1979 based on the Unix UUCP program. USENET has a very large number of discussion groups on practically any topic imaginable. USENET was a separate network from ARPANET. In 1972 Ray Tomlinson created the first E-mail program. At first E-mail users were generally researchers communicating with other researchers. Currently most people who use computers use E-mail. As all of you are well aware, E-mail is free, fast and generally more effective than playing voice tag. At first there were numerous unconnected E-mail systems in information utilities such as CompuServe, Prodigy and Internet. The use of E-mail is exploding in the 1990s because of the adoption of the world standard, x.400 and the creation of E-mail directories under the x.500 standard. The adoption of these standards and OSI means interinstitution E-mail communication has become as easy as intrainstitution E-mail

 

WEB: The service that led to the explosive growth of the Internet was the creation of the WWW, generally called the WEB, by CERN in 1992 and the creation of the first WWW browser MOSAIC in 1993 that was displaced over time by Microsoft Explorer, Firefox, Safari and others.. WWW browsers reduce the knowledge requirement to use the Internet to basic computer literacy in point and click operations. In addition, browsers folded services such as USENET and E-mail in the WWW browser as additional options. Thus using a modern browser the user has access to all Internet type services in an easy to use form. The fact that browsers cover all the Internet services in an easy to use form is just one aspect of the explosion in Internet use. The other is that browsers are created such that some variation of the browser runs on all types of operating systems whether Unix, Window, or MacOS. This means that WWW applications are independent of the machine or operation system. A firm can use the WEB to create firmwide applications regardless of the fact that the divisions of the firm might have very different computer equipment and operating systems. These two factors mean that more and more different types of activities will go through the WEB. Also, the capabilities of the WEB will keep on expanding to include voice and video.

 

Browser Tags: Pages in browsers were first composed in a language called HTML. This language had tags that controlled the layout of a page. The number of tags was about 6000 and they resided in the browser. The limited number of tags limited what could be displayed on a page. For example, for math there were superscript and subscipt tags, but little else. To overcome the limitations of HTML a new browser page representation language has been devloped and deployed called XM, which is gradually replacing HTML. In XML the tags are unlimited and are communicated to the browser to interpret the page being interpreted. XML encourages groups to create special purpose collections of tags for special purposes. For example, a set of math tags in XML has been created so that math can be displayed on a browser page with the same detail as a math textbook.

 

Internet Traffic: Internet traffic is growing by a factor of about 4 each year; whereas phone traffic is growing about 10% each year. To accommodate this growth communication firms have strong economic incentives to make the maximum use out of optical fiber networks. The current trend is dense wavelength division multiplex (DWDM). This buzz word means sending many optical wavelengths down an optical fiber at one time. The equipment cost to do this is only 40% the cost of laying a new fiber. In 1999 the state of the art is 80-120 wavelengths each carrying 10 gigabits of data. Because of interference there is a limit to how many wavelengths. My guess is that the more wavelengths the closer you have to have the repeaters (amplifiers) so the cost goes up. Every 100 miles or so the optical signal must be amplified. Previously to do this the signal had to be converted to electricity, amplified, and then converted back to light. Research and development has developed optical switches that can route traffic optically the way electrical switches currently do.

 

Voice over the Internet: The latest development in phone is the expansion of telephone conversations over the Internet using VOIP, a standard for IP phone. VOIP is more efficient than circuit switched phone because it more effectively uses channel capacity, but it has the potential for delays in transmission.  Computer Communications

Wireless Communication:

Radio and TV: Wireless radio was invented by Marconi at the turn of the century. In the twenties the AM radio broadcast industry was created. Later the higher quality FM radio was introduced. Interactive radio communication is used by ships, airplanes, remote stations, and ham operators. The commercial broadcast TV industry was created after World War II. These industries are all allocated portions of the electromagnetic spectrum for their activities. By 2010 TV will shift to high definition digital.

Wireless

Radio and Television

Satellites:

22,000 mile orbits: By placing a satellite in an orbit with a radius of 22,000 miles, the satellite goes around the earth once a day. If the orbit is about the equator the satellite appears to be stationary with respect to the earth. To minimize interference you must keep these satellites separate (no more than 90 in this orbit). Such satellites are used for broadcast TV and multiplexed phone conversations. Communication from earth to satellite uses microwave communications, which travels through clouds. Low-orbit satellites can also be used for communications systems; however, such a system is much more complicated because the antennas must follow a moving target and shift between satellites as they go over the horizon. Satellites make excellent communication relay stations because you do not need any ground network. Corporations establishing private communication systems outside the telephone system are big users of satellite communications. Third world countries such as India also make extensive use of satellite communications because it is much cheaper to reach the millions of villages by satellite than try to string copper or fiber cable. The emerging communication system will be a mixture of optical fibers between major nodes and of satellite communications in rural areas.

GPS: The orbits for the global positioning system are 12,000 miles. To accurately determine the position of an object on the earth requires triangulation from three satellites. The system was created for the military, but gradually the number of civilian uses has grown and grown. For example, truck companies can know the exact position of all their vehicles and hikers can know exactly where they are. Europe is puting up its own system to compete with the US.

Low Earth Orbits: The disadvantage of satellites at 22,000 miles for voice and video communication is that there is a 1/2 second delay in the communication of the messages because of the distances involved. Motorola has created a low earth satellite system, Iridium with 77 satellites (Iridium has 77 electrons; however, the number of satellites has been reduced to 66) which will eliminate the 1/2 second time delay. The cost of service was to high and Iridium went bankrupt. But, quess what? Because the Iridium system is an excelent miltitary communication device for ships and sea and soldiers in places like Afganistan, the military is now supporting Iridium. [The Iridium system was the only system that worked at ground zero on 11 Sep 01.] They use encrypted transmissions. It may be that this system will slowly gain a commerical market for out of the way places and emergencies when ground communications fail such as hurricanes.  In 1999 Sprint's Nextel entered a contract with Iridium so that the Nextel cell phone service offers low earth orbit communication as part of the service.

Satellite Communications

Wireless Voice and Data Networks: (Revised 6-7 Oct 08 by A. Norman and Griffin Seilfried) There were two types of wireless communication networks-voice and data, but these are currently being integrated. Almost all cell phone traffic goes through towers on the ground.  In order to minimize interference and send as many messages as possible wireless networks are organized into low power cells, which cover about 10 square miles, such that nonadjacent cells can use the same frequencies. If everyone owning a wireless phone would try to call at the same time the system would be completely overloaded.  Cell phone networks are much cheaper to install than landline networks.  Consequently, third world countries are developing cell phone networks without creating land line networks.  Some examples here

There are three factors to consider in cell phone technology:

  1. The technology: FDMA, TDMA, or CDMA
  2. The bandwidth used to send messages: 1G, 2G, 2.5G, or 3G
  3. The protocol: GSM or IS-95

Technology:  As is the case with wired networks, voice has a continuously open circuit when calling, and data sends packets without a continuously open circuit. There are three approaches to transmitting wireless messages-- (1) FDMA that assigns each phone a different frequency. This approach is used for analog voice wireless; (2) TDMA splits each frequency into time slots. This is three times as efficient for digital voice; (3) CDMA provides each phone a unique code to filter the message spread over the entire frequency domain. This approach is more efficient than TDMA.
Bandwidth:  The frequency used in cell phone communication determines the bandwidth. The first standards, 1G were analog with varying voice quality. The second generation, 2G, were digital in the range of 9.6-14.4kbps and expanded the number of users and offered more services such as e-mail and the internet The 3G raises the communication rate to 2Mbps in buildings, 384kbps in urban areas and 144kbps in wide area networks. This is enough bandwidth to offer multimedia services such as streaming video Internet. An interim standard 2.5G with a transmission rate up to 153Kbps was proposed. We are in the process worldwide of moving to the 3G standard.
Protocols:  The two main protocols are GSM, which has about 80-85% of the world market, and IS-95 that has about 10-15%.  For 3G both protocols are using the CDMA approach to cell phone communication
Wireless Business Data: Likewise data wireless networks have evolved. Earlier standards were the ARDIS, RAM Mobile Data and CDPD standards. ARDIS was started by Motorola and IBM and later sold to the current Motient data communications firm that runs the DataTAC network for business with up to 19.2Kbps worldwide in metropolitan areas.   With the introduction of 3G, wireless data is being integrated into voice, text, Internet, and video into a seamless network so that a Blackberry or other cell phone/PDA can send business data messages such as invoices.
Cell Phones in Less Developed Countries
In both rural and urban areas in India cell phones are leapfrogging landline technology.  For rural Indian villages it is much easier to use cell phone towers to cover vast tracks of land than to lay down line.  In urban areas, cell phone service extends into slums where landlines are very difficult to maintain. Mobile phones are much more useful to individuals than landlines because (surprisingly) they are mobile.  Indian fishermen use mobile phones to call shore to find out where the price of fish is highest and Indian businesses are now able to expand into even the poorest and unconnected areas because cell phones provide a reliable means of communication that landlines do not.
With the expansion of mobile Internet, the growth in the use of cell phones in developing economies will continue to expand.  In China many consumers who cannot afford computers, will instead opt for a “smart phone” to connect to the Internet. Basic data transfer services and the various Internet computing features beginning to be offered by Google often times make next generation handsets like the iPhone and Nokia’s Nseries more useful than a traditional computer to students and professionals in developing economies. 
Evolution of the cell phone:
Next generation “smart phones” like the iPhone and Blackberry integrate various services and features onto one device.  Handsets now make phone calls, access email, take and send pictures, and play music and videos.  More importantly handsets are becoming more or less “windows to the Internet,” where they can tap into vast cloud computing capabilities.  Apple uses third party developers to make thousands of applications that range from diet software to games, to budget calculators, to calendars. Safaricom, a mobile operator has developed software that allows users to deposit and withdrawal money via text message.  South Korea allows costumers to pay for goods and services with similar software.  Currently, Mastercard is pushing mobile phone based credit services that would allow consumers to simply pass their phones over a sensor to pay, rather than swiping a credit card.   The users’ checking, savings, and credit balance would be conveniently displayed and budget software would track spending and saving trends.
Wireless Internet: The wireless standard for the Internet is Wi-FI, a broadband standard for the interior of buildings with a large bandwidth of 54Mbps. There are several Wi-Fi standards and they are evolving. City governments are creating citywide Wi-Fi networks to provide an Internet infrastructure that will advance their city. These standards will be linked so that wireless phones can be used over Wi-Fi.