Wireless Local Area Network (WLAN)

Wireless Local Area Network (WLAN)
The most common fixed wireless application is the popular wireless local area network (WLAN) replacing common connections previously made by cable. Cable provides an excellent transmission media and supports data rates in the tens and hundreds of millions of bits per second. There are applications, however, that cannot use cable or are prohibitively expensive if cable is used.

Figure 1 shows product that are typically used in a WLAN system. This WLAN system includes radio access ports and extension ports. The extension ports shown in figure 1 are PCMCIA cards that plug into a laptop computer. These extension ports communicate via radio-to-radio access ports. The radio access ports convert the WLAN radio signal back into computer network signals (such as Ethernet or token ring).

Figure 1: Wireless Local Area Network (WLAN)

Other WLAN applications, like point-of-sale terminals in the ever-changing retail environment make wireless access more cost-effective than cabled access. Mobile inventory scanning in warehouses tie WLANs to a wireless scanner. Some building architectures make cable installation prohibitively expensive, WLANs are well suited for these types of applications.

Often, Infrared (IR) light energy is used for point-to-point computer connections, because IR cannot pass through walls, ceilings, or floors. This is considered an advantage because it enhances the security of a WLAN link and decreases interference between other nearby WLANs.

Wireless Networks - Radios

Wireless networks are composed of radios, radio towers or base stations, interconnection systems, and network management and information systems.

Radios may be fixed in location (such as a television) or may be mobile (such as a cellular telephone). Some radios may only communicate in one direction (typically a receiver) or may have two-way capability. When a single radio has both a transmitter and receiver contained in the same unit, it is called a transceiver.

Figure 1 shows a block diagram of a mobile radio transceiver. In this diagram, sound is converted to an electrical signal by a microphone. The audio signal is processed (filtered and adjusted) and is sent to a modulator. The modulator creates a modulated RF signal using the audio signal. The modulated signal is supplied to an RF amplifier that increases the level of the RF signal and supplies it to the antenna for radio transmission. This mobile radio simultaneously receives another RF signal on a different frequency to allow the listening of the other person while talking. The received RF signal is then boosted by the receiver to a level acceptable for the demodulator assembly. The demodulator extracts the audio signal and the audio signal is amplified so it can create sound from the speaker.

Figure 1: Mobile Radio Block Diagram

Radio Towers and Transmitter Equipment
Radio towers are poles, guided towers, or free standing constructed grids that raise one or more antennas to a height that increases the range of a transmitted signal. Radio towers can vary in height from about 20 feet to more than 300 feet. A single radio tower may host several antenna systems that include paging, microwave, or cellular systems. Radio towers are located strategically around the city to provide radio signal coverage to specific areas. At the base of the towers are electronic control rooms that contain the components to operate the radio portion of the communications system.

Radio towers and their associated radio equipment (e.g., base station) may include one or more antennas, transmitters, receivers (for two-way systems), system controllers, communication links, and power supplies. Transmitters provide the high level RF power that is supplied to the antenna. For broadcast systems, the amount of transmitter power can exceed 50,000 Watts. Receivers boost and demodulate incoming RF signals from mobile radios. If a base station contains receivers, it is typical to use one or more different antennas for the receivers. Controllers coordinate the overall operation of the base station and coordinate the alarm monitoring of electronic assemblies. Communication links allow a command location (such as a television studio or a telephone switching center) to control and exchange information with the base station. Base station radio equipment requires power supplies. Most base stations contain primary and backup power supplies. A battery typically maintains operation when primary power is interrupted. A generator may also be included to allow operation during extended power outages.

Figure 2 shows a typical radio base station block diagram that is used in a mobile telephone system. This diagram shows that the base station holds the radio transceiver (transmitter and receiver assemblies) that is part of the radio tower (cell site). This diagram also shows that one antenna is used for transmitting and two antennas are used for receiving (for improved reception). This base station also contains a backup battery that is maintained at full charge so radio communications will not be interrupted in the event AC power is lost.

Figure 2: Radio Tower and Base Station Equipment

Switching Facilities
Switching facilities are typically used in two-way mobile communication systems to allow the connection of mobile radios to other radios in the system or to the public telephone network. When used in a cellular system, the switching system is typically called a mobile switching center (MSC). The MSC, just like a local telephone company, processes requests for service from mobile radios (subscribers) and routes the calls to other destinations.

Figure 3 illustrates a wireless switching system basic functional components. These include: communication line interfaces, a switch, a customer database, system and communication controllers, primary and backup (batteries) power, and the software to interface and control the radio tower’s and base station (BS) it is connected to.

Figure 3: Wireless Switching System Block Diagram

Interconnection to Other Networks
Wireless systems may be connected to other networks. Broadcast wireless systems are connected to media sources (such as audio or video programs) via satellite links while cellular networks may be interconnected to the public telephone network. Interconnection involves the physical and software connection of network equipment or communications systems to the facilities of another network such as the public telephone network. Government agencies such as the Federal Communications Commission (FCC) or Department of Communications (DOC) regulate interconnection of wireless systems to the public telephone networks to ensure reliable operation.

Customer Databases
Customer databases are computer storage devices (typically a computer hard disk) that hold service authorization and feature preferences of customers. For wireless systems that allow the customer to operate in other territories, a home (local) database is used. Each wireless subscriber has a real-time user profile in the database that is typically called the home location register (HLR). The HLR identifies the current location of the mobile radio, the most likely place for the mobile to be, or the last location the subscriber was active. The MSC system controller uses this information to route calls to the appropriate radio tower for call completion. If the wireless user is not in a predetermined “home” range of the MSC, the mobile will register back through to the home signaling system to its home location register (HLR) for profile information.

When customers use the wireless services of systems outside of their home area, their information is transferred to a database in that system called the visitor location register (VLR). The VLR is part of a wireless network (typically cellular or PCS) that holds the subscription and other information about visiting subscribers that are authorized to use the wireless network.

System Security
In some wireless networks, access to system services requires validation of the customer’s identity. These systems may use an authentication center (AUC) to store and process secret data to stop fraudulent calls or prohibit access to other paid for subscription services.

Wireless phones transmit some of their identification information over the public airwaves when they attempt to access the system. Thieves may try and intercept this information and copy (clone) the identification information that would allow them to make phone calls that would be billed to the other telephone. To prevent this unauthorized duplication of identification information, an authentication process can be used that uses secret keys to validate access information.

During the authentication process, code keys are created from secret codes that are stored in both the mobile radio and in the system. Along with basic identification information, these keys are exchanged during each system access attempt. The secret codes are not transmitted. Because the system and the mobile radio have the secret keys, both the mobile phone and the system can validate that the code information is correct. If the codes do not match, the system should not allow the call to be processed. New codes are created during each access attempt to prevent copying of the codes and immediately attempting access.

Wireless Networks: Radio Frequency (RF)

Radio Frequency (RF)
The radio frequency spectrum is divided into frequency bands that are authorized for use in specific geographic regions. Globally, the International Telecommunications Union (ITU) specifies the typical use for radio frequency bands. Within each country, government agencies create and enforce the rules for which specific types of systems and services are used in specific frequency bands and which companies will be able (will be licensed) to own and operate these systems.

The national government is responsible for dividing the available frequency bands for licensing to users and regulates what the frequencies may be used for. The legal right-to-use of this public resource is controlled by rules and licensing of very specific frequencies, a range of frequencies or a block of sub-divided channels at a given frequency or frequency range.

For example, the frequencies allocated for FM radio must be used for the purpose licensed; that is a combination of music or news and public information. FM radio stations are not licensed to broadcast a secret “Morse-code” to a following of undercover militia! Neither can a “Paging Service” use one or all of their frequency channels to broadcast radio. However, with the recent deregulation of telecommunications services, wireless service providers are now permitted to offer many new types of services provided they can fulfill their basic licensing requirements.

To prevent unwanted interference from radio devices, the reckless use of transmitting energy or information on our public airwaves according to publicly published rules or licenses will violate federal law. Such transmissions are subject to prosecution or suspension of the radio operator’s license.

Frequency Allocation Charting
There are thousands of wireless applications that are assigned to many different frequency bands. The selection of the assigned frequency bands is determined by a variety of factors including the radio propagation characteristics and the availability of radio channel frequencies at the time.

Because most of the frequencies have already been assigned to licensees, a new assignment of frequencies typically requires existing licensees or users to stop using a band. These users are typically shifted to another band. This process is called re-allocation.

Historically, major re-allocations are done in the higher frequencies to avoid congestion. This has advantages and disadvantages. The radio frequency (RF) devices employed within the newer systems are subject to more loss based on distance. This requires closer distances, increasing the total number radio sites to cover the same area previously covered by radio devices at a lower frequency. However, the higher frequencies tend to penetrate buildings more readily and the antennas involved are physically smaller - both important attributes for systems that seek to reach 100% of the available population.

RF Channels and Bandwidth
An RF channel is a communication link that use radio signals to transfer information between two (or more) points. To transfer this information, a radio wave (typically called a radio carrier) is modulated (modified) within an authorized frequency band to carry the information. The modulation of the radio wave forces the radio frequency to shift above and below the reference (center) frequency. Typically, the more the modification of frequency, the more information can be carried on the radio wave. This results in RF channels typically defined by their frequency and bandwidth allocation.

Bandwidth allocation is the frequency width of a radio channel in Hertz (high and low limits) that can be modulated to transfer information. The amount type of information being sent determines the amount of bandwidth used and the method of modulation used to impose the information on the radio signal.

A government regulation agency (the FCC in the United States) defines a total frequency range (upper and lower frequency limits) that a radio service provider can use to transmit information. In some systems (such as AM or FM radio station broadcasting), this is a single radio channel. For other systems (such as cellular, PCS, or PCN), this is a range of frequencies that can be sub divided into smaller radio channels as determined by the radio carrier. When the allocated frequency range is further subdivided into smaller allowable bands, these subdivided areas are referred to as channels.

Mobility and Fixed Wireless
Most applications use wireless to allow mobile service. However, many fixed applications of wireless are practical. There is a general data transmission rate tradeoff between mobile and fixed wireless systems. Mobile wireless systems have a relatively low data transfer rate (typically below 28 kbps) while fixed wireless systems can have data transfer rates that exceed 45 Mbps. The primary advantage of fixed wireless service is the ability to focus radio transmissions to a particular direction or region. This typically reduces interference to and from other radios and increases the capacity (data transfer rate) available to the fixed wireless device. The basic types of fixed wireless systems in use include wireless computer networks, competing wireless television systems, and wireless local telephone service.

Future Enhancements: 10 Gigabit Ethernet (10 GE), LAN Telephony, Storage Area Networks (SANs)

Future Enhancements
Future enhancements for data communication networks include increased data transmission speed, LAN telephony, and storage area networks (SANs).

10 Gigabit Ethernet (10 GE)

10 Gigabit Ethernet (10 GE) is a data communication system that combines Ethernet technology with fiberoptic cable transmission to provide data communication transmission at 10 Gbps (10,000 Mbps). The specifications for 10 GE are being developed by the Gigabit Ethernet Alliance. The Gigabit Ethernet Alliance is a group of companies that was formed in January 2000.

LAN Telephony
LAN telephony (sometimes called TeLANophy) use LAN systems to transport voice communications. LAN telephone technology is a merging of packetized voice with the high-speed data transmission ability of LAN systems. The ability to share data networks with voice systems offers significant cost reduction for telephone services.

LAN telephone system consists of LAN telephones, a data network, a LAN call processing system, and a voice gateway to the PSTN. LAN telephones convert audio into digitized packets that are transferred on the LAN to the call processing computer (CTI system). Each LAN telephone has its own network data address that is related to its telephone number or extension number.

LAN telephones can be integrated into computers or they can be standard along telephones that use LAN protocols that communicate with the systems. In 2001, there were several manufacturers producing IP telephones.

Storage Area Networks (SANs)
Storage area networks (SANs) distribute data and other information to multiple storage devices that are interconnected by data networks. SANs allow for the sharing of resources and pooling of information in the form of shared files at both the server level and the client (individual PC) level. Storage area networks (SAN’s) provide fault-tolerant operation through the use redundant data storage in multiple locations. If a failure occurs in one data storage device, other redundant data storage devices may automatically be used as the backup source of information.

Services : Internet Service Provider (ISP) & Virtual Private Networks (VPN’s)

Some of the key services provided by data network operators include Internet service provider (ISP) and virtual private networks (VPNs).

Internet Service Provider (ISP)
Internet service provider is a company that provides an end user with data communication service that allows them to connect to the Internet. An ISP purchases a high-speed link to the Internet and divides up the data transmission to allow many more users to connect to the Internet. Internet service providers provide a gateway between end-users and the Internet. For this service, an ISP usually charges a monthly access fee and may charge for the amount of time or amount of data transferred during the billing period.

Virtual Private Networks (VPN’s)
Virtual private networks (VPN) network operators provide data connections to companies to allow interconnection of data networks. Companies use VPN to create MANs or WANs.

The best examples of VPN’s today are ATM and frame relay networks that connect multiple client sites on what appears to be dedicated circuits. In these networks, data is routed through the VPN network using routing algorithms that transfer data based on congestion and priorities. Because of the speed and fault-tolerance of the VPN provider network, the client company operates as if the inter-site connections were dedicated circuits.

Data Communications Systems: ATM 25, Phoneline Networking, Universal Serial Bus (USB),

Asynchronous Transfer Mode 25 (ATM 25)
ATM 25 is a 25 Mbps version of the asynchronous transfer mode (ATM) system. ATM technology is relatively complex when compared to Ethernet and token ring systems. As a result, the use of standard ATM technology in LAN systems has been limited. However, a 25 Mbps version of the ATM standard was developed for PDN LANs. The capability of ATM systems to simultaneously provide multiple communication channels with varying levels of quality of service (QoS) make it advantageous for use in multimedia systems. ATM 25 technology is used to provide digital video and Internet access through the use of ATM in digital subscriber line (DSL) and cable modem systems.

Phoneline Networking
In the late 1990’s, the home phoneline network alliance (HomePNA) developed a specification that allows home computers and data devices (such as network printers) to interconnect via standard home telephone wiring. In the first generation of phoneline networking, data rates of 1 Mbps were achieved but recently data transmission rates of 10 Mbps have been demonstrated. The Phoneline Network uses special NIC’s that send and receive high frequency signals that do not interfere with standard telephone service. To connect a phoneline network to a DSL connection, a phoneline bridge must be used.

Universal Serial Bus (USB)
Universal serial bus (USB) is a short distance data communication interface (typically, only a few meters) that now comes standard on most personal computers. The USB was designed to replace the older slower UART data communications port. USB ports permit data transmission speeds up to 12 Mbps. Most computers that were manufactured in 2001 included a universal serial bus (USB) connector. The USB data bus can also connect up to 10 devices to the same bus using a low cost hub device. USB lines can only extend for a few feet from the computer.

FireWire is a short distance data communications interface (up to approximately 5 meters) that is based on industry standard IEEE-1394. FireWire can transmit at speeds up to 400 Mbps and can support up to 63 devices per bus. Firewire provides for isochronous (repetitive streaming data format) that allows it to transfer audio and video signals.

Data Communications Systems : Internet

The Internet is a public data network that interconnects private and government computers. The Internet transfers data from point-to-point by packets that use Internet protocol (IP). Each transmitted packet in the Internet finds its way through the network switching through nodes (computers). Each node in the Internet forwards received packets to another location (another node) that is closer to its destination. Each node contains routing tables that provide packet-forwarding information. The Internet was designed to allow continuous data communication in the event some parts of the network were disabled. The world wide web (WWW) is an application on the Internet that allows users to graphically navigate through computers that are connected to the Internet.

The Internet is a network of networks. Although these networks communicate with each other using many different languages (protocols), they all agree to transport data within their network according to a common Internet communication language called transmission control protocol/Internet protocol (TCP/IP). TCP/IP is a set of protocols developed by the U.S. Department of Defense (US DOC) that facilitate the interconnection of dissimilar computer systems across networks. The TCP protocol coordinates the overall flow of data during a data communication session between points (nodes) in the Internet.

IP is an addressing structure that allows packets of data to be routed (re-directed) as they migrate through different networks to reach their ultimate destination. Each network receives packets of data in a format that is compatible with the Internet (IP address followed by control and data information) and they encapsulate (place the whole Internet data message into their own data packet format (including the IP address and control information). This allows IP data packets (called “datagrams”) to be sent through the network regardless of their actual length or format.

Figure 1 shows that the Internet is the network of networks and it communicates using the universal protocol language TCP/IP. This diagram shows a user who is sending email through the Internet. In this diagram, the application is email. The data from the email is divided into packets and given sequence number by TCP protocol. The destination address is appended to each packet by the IP layer. The IP packets are then sent through an Ethernet LAN by encapsulating the IP datagram within the Ethernet data packet. When the data packet is extracted from the Ethernet, it is placed on the E1 transmission line. When the IP data packet reaches the ATM network, it is subdivided into very small 53 byte data packets that travel through the ATM network. When the ATM packets reach their destination in the ATM network, the original IP datagram is recreated and transferred via the T1 communication line. The T1 communication line interfaces to another Ethernet data network. This Ethernet data network encapsulates the IP datagram and forwards it on to the NIC of the receiving computer. The NIC of the receiving computer removes the IP address and reassembles the IP data packets to form the original email message.

Figure 1: Internet Data Routing

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