* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Document related concepts
Wireless System Architecture: How Wireless Works • components of a wireless network • general wireless network architectural elements • information flows through a wireless network • Wireless networks utilize components similar to wired networks. • wireless networks must convert information signals into a form suitable for transmission through the air medium. • Wireless networks directly contribute only to a portion of the overall network infrastructure, attention to all network functions is necessary to counter impairments resulting from the wireless medium. Wireless Network System Components • A wireless network consists of several components that support communications using radio or light waves propagating through an air medium. • Some of these elements overlap with those of wired networks, but special consideration is necessary for all of these components when deploying a wireless network. Users • A user can be anything that directly utilizes the wireless network. One of the most common types of user is a person. • The user initiates and terminates use of a wireless network, making the term end-user appropriate. • Users of wireless networks tend to be mobile, constantly moving throughout a facility, campus, or city. • Mobility is one of the most prominent benefits of deploying a wireless network. Computer Devices • Many types of computer devices, sometimes referred to as clients, operate on a wireless network. • Some computer devices might be specifically designed for users, whereas some computer devices are end systems. • In generally, any computer device might communicate with any other computer device on the same wireless network. • To support mobile applications, computer devices are often small, making them practical for people to carry with them at all times. • With portable and stationary applications, however, the computer devices are much larger. • Computer devices within a wireless network also include end systems such as servers, databases, and websites. • Users can adapt many existing computer devices to operate on a wireless network. • A computer device also has an operating system, such as Windows XP, LINUX, or MAC OS. NICs • The network interface card provides the interface between the computer device and the wireless network infrastructure. • The NIC fits inside the computer device, but external network adaptors are available that plug in and remain outside the computer device. • Wireless network standards define how a wireless NIC operates. • Wireless NICs also comply with a specific form factor, which defines the physical and electrical bus interface that enables the card to communicate with the computer device. Air Medium • Air provides a medium for the propagation of wireless communications signals, which is the heart of wireless networking. • Air is the conduit by which information flows between computer devices and the wireless infrastructure. • Wireless information signals cannot be heard by humans, so it's possible to amplify the signals to a higher level without disturbing human ears. • Wireless information signals cannot be heard by humans, so it's possible to amplify the signals to a higher level without disturbing human ears. • With wireless networks, the air medium supports the propagation of radio and light waves that travel from one point to another. Wireless Network Infrastructures • The infrastructure of a wireless network interconnects wireless users and end systems. • The infrastructure might consist of base stations, access controllers, application connectivity software, and a distribution system. • These components enhance wireless communications and fulfill important functions necessary for specific applications. Base Stations • The base station is a common infrastructure component that interfaces the wireless communications signals traveling through the air medium to a wired network—often referred to as a distribution system. • Base station enables users to access a wide range of network services, such as web browsing, email access, and database applications. • A base station often contains a wireless NIC that implements the same technology in operation by the user's wireless NIC. • Base stations go by different names, depending on their purpose. • An access point, for instance, represents a generic base station for a wireless LAN. A collection of access points within a wireless LAN, for example, supports roaming throughout a facility. • Residential gateways and routers are more advanced forms of base stations that enable additional network functions. • The gateway might have functions, such as access control and application connectivity, that better serve distributed, public networks. • A router would enable operation of multiple computers on a single broadband connection. • a base station might support point-to-point or point-to-multipoint communications. • Point-to-point systems enable communications signals to flow from one particular base station or computer device directly to another one. • point-to-multipoint functionality enables a base station to communicate with more than one wireless computer device or base station. • An access point within a wireless LAN implements this form of communications. • The access point represents a single point whereby many computer devices connect to and communicate with each other and systems within the wireless infrastructure. Access Controllers • Access controller-hardware that resides on the wired portion of the network between the access points and the protected side of the network. • Access controllers provide centralized intelligence behind the access points to regulate traffic between the open wireless network and important resources. • In some cases, the access point contains the access control function. • Access controller regulates access to the Internet by authenticating and authorizing users based on a subscription plan. • Access controller help a hacker sitting in the company's parking lot from getting entry to sensitive data and applications. • The use of an access controller reduces the need for smart access points. • Thin access points primarily implement the basic wireless network standard (such as IEEE 802.11), and not much more. Benefits when access controllers deployed with thin access points • Lower Costs— Access points with limited functionality cost less, which generally results in lower overall system costs. • Open Connectivity— Smart access points offer enhancements related to security and performance to the basic wireless connectivity that wireless network standards offer. • Centralized Support— An advantage of placing the smarts of the network in an access controller is that the system is easier to support, primarily because fewer touch points are in the network. Access controllers features • Authentication— Most access controllers have a built-in database for authenticating users; however, some offer external interfaces to authentication servers such as Remote Authentication Dial-In User Service (RADIUS) and Lightweight Directory Access Protocol (LDAP). • Encryption— Some access controllers provide encryption of data from the client to the server and back, using such common methods such as IPSec. • Bandwidth Management— Because users share bandwidth in a wireless network, it's important to have a mechanism to ensure specific users don't hog the bandwidth. Data Communication Terms • Data - entities that convey meaning, or information • Signals - electric or electromagnetic representations of data • Transmission - communication of data by the propagation and processing of signals TRANSMISSION FUNDAMENTALS 2-18 Examples of Analog and Digital Data • Analog – Video – Audio • Digital – Text – Integers TRANSMISSION FUNDAMENTALS 2-19 Analog Signals • A continuously varying electromagnetic wave that may be propagated over a variety of media, depending on frequency • Examples of media: – Copper wire media (twisted pair and coaxial cable) – Fiber optic cable – Atmosphere or space propagation • Analog signals can propagate analog and digital data TRANSMISSION FUNDAMENTALS 2-20 Digital Signals • A sequence of voltage pulses that may be transmitted over a copper wire medium • Generally cheaper than analog signaling • Less susceptible to noise interference • Suffer more from attenuation • Digital signals can propagate analog and digital data TRANSMISSION FUNDAMENTALS 2-21 Reasons for Choosing Data and Signal Combinations • Digital data, digital signal – Equipment for encoding is less expensive than digital-toanalog equipment • Analog data, digital signal – Conversion permits use of modern digital transmission and switching equipment • Digital data, analog signal – Some transmission media will only propagate analog signals – Examples include optical fiber and satellite • Analog data, analog signal – Analog data easily converted to analog signal TRANSMISSION FUNDAMENTALS 2-22 2.8 Analog and Digital Signaling of Analog and Digital Data TRANSMISSION FUNDAMENTALS 2-23 Analog Transmission • Transmit analog signals without regard to content • Attenuation limits length of transmission link • Cascaded amplifiers boost signal’s energy for longer distances but cause distortion – Analog data can tolerate distortion – Introduces errors in digital data TRANSMISSION FUNDAMENTALS 2-24 Digital Transmission • Concerned with the content of the signal • Attenuation endangers integrity of data • Digital Signal – Repeaters achieve greater distance – Repeaters recover the signal and retransmit • Analog signal carrying digital data – Retransmission device recovers the digital data from analog signal – Generates new, clean analog signal TRANSMISSION FUNDAMENTALS 2-25 About Channel Capacity • Impairments, such as noise, limit data rate that can be achieved • For digital data, to what extent do impairments limit data rate? • Channel Capacity – the maximum rate at which data can be transmitted over a given communication path, or channel, under given conditions TRANSMISSION FUNDAMENTALS 2-26 2.9 Effect of Noise on Digital Signal TRANSMISSION FUNDAMENTALS 2-27 Concepts Related to Channel Capacity • Data rate - rate at which data can be communicated (bps) • Bandwidth - the bandwidth of the transmitted signal as constrained by the transmitter and the nature of the transmission medium (Hertz) • Noise - average level of noise over the communications path • Error rate - rate at which errors occur – Error = transmit 1 and receive 0; transmit 0 and receive 1 TRANSMISSION FUNDAMENTALS 2-28 Classifications of Transmission Media • Transmission Medium – Physical path between transmitter and receiver • Guided Media – Waves are guided along a solid medium – E.g., copper twisted pair, copper coaxial cable, optical fiber • Unguided Media – Provides means of transmission but does not guide electromagnetic signals – Usually referred to as wireless transmission – E.g., atmosphere, outer space TRANSMISSION FUNDAMENTALS 2-29 Unguided Media • Transmission and reception are achieved by means of an antenna • Configurations for wireless transmission – Directional – Omnidirectional TRANSMISSION FUNDAMENTALS 2-30 2.10 Electromagnetic spectrum of Telecommunications TRANSMISSION FUNDAMENTALS 2-31 General Frequency Ranges • Microwave frequency range – – – – 1 GHz to 40 GHz Directional beams possible Suitable for point-to-point transmission Used for satellite communications • Radio frequency range – 30 MHz to 1 GHz – Suitable for omnidirectional applications • Infrared frequency range – Roughly, 3x1011 to 2x1014 Hz – Useful in local point-to-point multipoint applications within confined areas TRANSMISSION FUNDAMENTALS 2-32 Terrestrial Microwave • Description of common microwave antenna – – – – Parabolic "dish", 3 m in diameter Fixed rigidly and focuses a narrow beam Achieves line-of-sight transmission to receiving antenna Located at substantial heights above ground level • Applications – Long haul telecommunications service – Short point-to-point links between buildings TRANSMISSION FUNDAMENTALS 2-33 Satellite Microwave • Description of communication satellite – Microwave relay station – Used to link two or more ground-based microwave transmitter/receivers – Receives transmissions on one frequency band (uplink), amplifies or repeats the signal, and transmits it on another frequency (downlink) • Applications – Television distribution – Long-distance telephone transmission – Private business networks TRANSMISSION FUNDAMENTALS 2-34 Broadcast Radio • Description of broadcast radio antennas – Omnidirectional – Antennas not required to be dish-shaped – Antennas need not be rigidly mounted to a precise alignment • Applications – Broadcast radio • VHF and part of the UHF band; 30 MHZ to 1GHz • Covers FM radio and UHF and VHF television TRANSMISSION FUNDAMENTALS 2-35 Multiplexing • Capacity of transmission medium usually exceeds capacity required for transmission of a single signal • Multiplexing - carrying multiple signals on a single medium – More efficient use of transmission medium TRANSMISSION FUNDAMENTALS 2-36 2.11 Multiplexing TRANSMISSION FUNDAMENTALS 2-37 Reasons for Widespread Use of Multiplexing • Cost per kbps of transmission facility declines with an increase in the data rate • Cost of transmission and receiving equipment declines with increased data rate • Most individual data communicating devices require relatively modest data rate support TRANSMISSION FUNDAMENTALS 2-38 Multiplexing Techniques • Frequency-division multiplexing (FDM) – Takes advantage of the fact that the useful bandwidth of the medium exceeds the required bandwidth of a given signal • Time-division multiplexing (TDM) – Takes advantage of the fact that the achievable bit rate of the medium exceeds the required data rate of a digital signal TRANSMISSION FUNDAMENTALS 2-39 2.12 FDM and TDM TRANSMISSION FUNDAMENTALS 2-40 2.13 Synchronous TDM System TRANSMISSION FUNDAMENTALS 2-41 Key Features of a Protocol • Syntax – Concerns the format of the data blocks • Semantics – Includes control information for coordination and error handling • Timing – Includes speed matching and sequencing Protocols and the TCP/IP Suite 4-42 Agents Involved in Communication • Applications – Exchange data between computers (e.g., electronic mail) • Computers – Connected to networks • Networks – Transfers data from one computer to another Protocols and the TCP/IP Suite 4-43 4.1 TCP/IP Concepts Protocols and the TCP/IP Suite 4-44 TCP/IP Layers • • • • • Physical layer Network access layer Internet layer Host-to-host, or transport layer Application layer Protocols and the TCP/IP Suite 4-45 4.2 Protocol Data Units (PDUs) in the TCP/IP Architecture Protocols and the TCP/IP Suite 4-46 TCP/IP Physical Layer • Covers the physical interface between a data transmission device and a transmission medium or network • Physical layer specifies: – Characteristics of the transmission medium – The nature of the signals – The data rate – Other related matters Protocols and the TCP/IP Suite 4-47 TCP/IP Network Access Layer • Concerned with the exchange of data between an end system and the network to which it's attached • Software used depends on type of network – – – – Circuit switching Packet switching (e.g., X.25) LANs (e.g., Ethernet) Others Protocols and the TCP/IP Suite 4-48 T:TCP/IP Internet Layer • Uses internet protocol (IP) • Provides routing functions to allow data to traverse multiple interconnected networks • Implemented in end systems and routers Protocols and the TCP/IP Suite 4-49 TCP/IP Host-to-Host, or Transport Layer • Commonly uses transmission control protocol (tcp) • Provides reliability during data exchange – Completeness – Order Protocols and the TCP/IP Suite 4-50 TCP/IP Application Layer • Logic supports user applications • Uses separate modules that are peculiar to each different type of application Protocols and the TCP/IP Suite 4-51 Common TCP/IP Applications • Simple mail transfer protocol (SMTP) – Provides a basic electronic mail facility • File Transfer Protocol (FTP) – Allows files to be sent from one system to another • Hypertext Transfer Protocol (HTTP) – Transfers information for the World Wide Web Protocols and the TCP/IP Suite 4-52 Network functionality for OSI layers • Layer 7—Application layer: Establishes communications among users and provides basic communications services such as file transfer and e-mail. • Layer 6—Presentation layer: Negotiates data transfer syntax for the application layer and performs translations between different data formats, if necessary. • Layer 5—Session layer: Establishes, manages, and terminates sessions between applications. • Layer 4—Transport layer: Provides mechanisms for the establishment, maintenance, and orderly termination of virtual circuits, while shielding the higher layers from the network implementation details. • Layer 3—Network layer: Provides the routing of packets though a network from source to destination. • Layer 2—Data link layer: Ensures medium access, as well as synchronization and error control between two entities. • Layer 1—Physical layer: Provides the actual transmission of information through the medium. Physical layers include radio waves and infrared light. • Wireless networks directly implement only the lower layers of the model. • wireless NIC, for example, implements the data link layer and physical layer functions. • wireless middleware offer functions that the session layer implements. • Each layer of the OSI model supports the layers above it. • protocols at each layer communicate across the network to the respective peer layer. The actual transmission of data, however, occurs at the physical layer. • The architecture allows for a layering process where a particular layer embeds its protocol information into frames that are placed within frames at lower layers. • The frame that is sent by the physical layer actually contains frames from all higher layers. • At the destination, each layer passes applicable frames to higher layers to facilitate the protocol between peer layers. Spectrum considerations • Controlled by regulatory bodies – Carrier frequency – Signal Power – Multiple Access Scheme • Divide into time slots –Time Division Multiple Access (TDMA) • Divide into frequency bands – Frequency Division Multiple Access (FDMA) • Different signal encodings – Code Division Multiple Access (CDMA) Overview of Wireless 5-57 Spectrum considerations • Federal Communications Commission (FCC) in the United States regulates spectrum – – – – – – Military Broadcasting Public Safety Mobile Amateur Government exclusive, non-government exclusive, or both – Many other categories Overview of Wireless 5-58 Spectrum considerations • Industrial, Scientific, and Medical (ISM) bands – Can be used without a license – As long as power and spread spectrum regulations are followed • ISM bands are used for – WLANs – Wireless Personal Area networks – Internet of Things Overview of Wireless 5-59 Propagation Modes • Ground-wave propagation • Sky-wave propagation • Line-of-sight propagation Overview of Wireless 5-60 5.1 Wireless Propagation Modes Overview of Wireless 5-61 Ground Wave Propagation • • • • Follows contour of the earth Can propagate considerable distances Frequencies up to 2 MHz Example – AM radio Overview of Wireless 5-62 Sky Wave Propagation • Signal reflected from ionized layer of atmosphere back down to earth • Signal can travel a number of hops, back and forth between ionosphere and earth’s surface • Reflection effect caused by refraction • Examples – Amateur radio – CB radio Overview of Wireless 5-63 Line-of-Sight Propagation • Transmitting and receiving antennas must be within line of sight – Satellite communication – signal above 30 MHz not reflected by ionosphere – Ground communication – antennas within effective line of site due to refraction • Refraction – bending of microwaves by the atmosphere – Velocity of electromagnetic wave is a function of the density of the medium – When wave changes medium, speed changes – Wave bends at the boundary between mediums Overview of Wireless 5-64 Five basic propagation mechanisms 1. Free-space propagation 2. Transmission – Through a medium – Refraction occurs at boundaries 3. Reflections – Waves impinge upon surfaces that are large compared to the signal wavelength 4. Diffraction – Secondary waves behind objects with sharp edges 5. Scattering – Interactions between small objects or rough surfaces Overview of Wireless 5-65 antennas • An antenna is an electrical conductor or system of conductors – Transmission - radiates electromagnetic energy into space – Reception - collects electromagnetic energy from space • In two-way communication, the same antenna can be used for transmission and reception Overview of Wireless 5-66 Radiation Patterns • Radiation pattern – Graphical representation of radiation properties of an antenna – Depicted as two-dimensional cross section • Beam width (or half-power beam width) – Measure of directivity of antenna • Reception pattern – Receiving antenna’s equivalent to radiation pattern • Sidelobes – Extra energy in directions outside the mainlobe • Nulls – Very low energy in between mainlobe and sidelobes Overview of Wireless 5-67 5.2 Antenna Radiation Patterns Overview of Wireless 5-68 Attenuation • Strength of signal falls off with distance over transmission medium • Attenuation factors for unguided media: – Received signal must have sufficient strength so that circuitry in the receiver can interpret the signal – Signal must maintain a level sufficiently higher than noise to be received without error – Attenuation is greater at higher frequencies, causing distortion Overview of Wireless 5-69 Error Control • wireless NICs implement error control mechanisms that detect and correct bit errors. • Error control techniques highly reduce the number of transmission errors. • The two primary types of error control are automatic repeat-request (ARQ) and forward error correction (FEC). • With ARQ, which operates at the data link layer, the receiving wireless NIC detects errors and uses a feedback path to the sending wireless NIC for requesting the retransmission of frames having bit errors. • There are two main events that must occur to correct errors with ARQ. -First, a received frame must be checked at the receiver for possible errors, -and then the sender must be notified to retransmit the frames received in error. In some protocols, such as 802.11, the receiver sends an acknowledgement to the sender if the received frame has no errors. The absence of an acknowledgement indicates to the sender to retransmit the frame. • Two approaches for retransmitting unsatisfactory blocks of data exist: • Stop-and-wait ARQ • Continuous ARQ Stop-and-Wait ARQ • In the stop-and-wait method of transmission, the sending NIC transmits a block of data, then stops and waits for an acknowledgment from the receiving NIC on whether a particular frame was acceptable or not. If the sending side receives a negative acknowledgment, the previous frame will be sent again. The sending NIC will send the next frame after it receives a positive acknowledgment from the receiving NIC. The IEEE 802.11 standard specifies this form of error control. Continuous ARQ • the transmitter sends data blocks continuously until the receiving NIC detects an error. The sending NIC is usually capable of transmitting a certain number of frames and keeps a log of which frames have been sent. Once the receiving side detects a bad block, it will send a signal back to the sending NIC requesting that the bad frame be sent over again. When the receiver gets the signal to retransmit a certain frame, several subsequent frames might have already been sent because of propagation delays between the sender and receiver. Go-back-n technique • The go-back-n technique is useful in applications where the receiver has little memory space because all that is needed is a receiver window size of one (ability to store one frame), assuming frames do not need to be delivered in order. • When the receiving NIC rejects an erred frame —sends a negative acknowledgment—it does not need to store any subsequent frames for possible reordering while it is waiting for the retransmission. • It need not wait because all subsequent frames will also be resent. Selective Repeat Approach • Selective repeat is obviously better than continuous go-back-n in terms of throughput because the sending NIC only transmits the erred data block; • The receiver must be capable of storing a number of data frames if they are to be processed in order. • The receiver needs to buffer data that have been received after an erred frame was requested for retransmission as only the damaged frame will be resent. FEC • As an alternative to ARQ, FEC automatically corrects as many errors as it can within the physical layer at the receiving NIC without referring to the sending NIC. • This is possible because the sending NIC includes enough redundant bits in case some are lost because of errors. • This makes FEC well suited for simplex communications links, and cases where a return path to the sending NIC is not feasible. Connecting with the Wireless Network Infrastructure • The base station, such as an access point, includes both a wireless and wired NIC, as well as software that interfaces the two networks. • When a wireless user communicates with another wireless user, the base station might simply resend the data frame received from one user so that the other user is able to receive it. • In this case, the base station is acting as a repeater. • Alternatively, the base station might forward the data to the wired side of the base station if the destination is located somewhere on the wired side of the network. • Upon receiving a data frame, the wireless NIC within the base station converts the analog radio wave or light signal into a digital signal and performs error detection to ensure that the resulting data frame does not have any bit errors. • The error control mechanism will cause the sending wireless NIC to retransmit the data frame if errors are present. • After taking care of erred frames, the wireless NIC within the base station will either resend the frame or forward the frame to the wired side of the base station. Summary • Wireless networks include components that make mobile and portable application possible. • Users are end points of the wireless network and utilize computer devices designed for a particular application. • Wireless NICs and base stations are key components that communicate over the air medium. • To provide roaming throughout a facility or city, a distribution system such as Ethernet interconnects base stations and interfaces users to servers and applications located on the wired network. • The seven-layer OSI reference model depicts functions necessary for a network, but wireless networks implement only functions defined by the bottom two layers—the physical and data link layer. • These functions include medium access, error control, and formation of radio and light signals for propagation through the medium. • When deploying wireless networks, however, it's important to ensure that protocols operating at higher layers have features that counter impairments found in wireless networks.