Wireless networks and the subsequent mobile communication are growing by leaps and bounds in the past years and the demand for connection without cables is certainly high. Nowadays, wireless networks are quite common and can be found on university campuses, corporate offices and in public places like hotels, airports, coffee shops and so forth. Not only are mobile devices getting smaller and cheaper, they are also becoming more efficient and powerful, capable of running applications and network services. This is causing the uncontrollable growth of mobile computing as we are witnessing today. Among the many number of applications and services that are executed by mobile devices, network and data services are in high demand. Brief descriptions of some selective wireless technologies that help mobile computing, like IEEE 802.11 networks (with infrastructure mode and ad-hoc mode), Bluetooth, HomeRF, WiMAX and cellular technologies are given below.
Ieee 802.11 Infrastructure Network
Wireless local area network (WLAN) which is also known as Wi-Fi (Wireless Fidelity) networks, requires an infrastructure network that could provide the services of accessing other networks, along with forwarding functions and medium access control. The Institute of Electrical and Electronics Engineers (IEEE) in 1997 initiated the first WLAN standard and they called it 802.11. But, 802.11 only supported a maximum bandwidth of 2 Mbps, which is quite slow for most applications. The IEEE 802.11 family consists of different standards. The initial standard was approved in 1997 and it backed wireless LAN Medium Access Control (MAC) and Physical layer (PHY) specifications that supported 1 Mbps and 2 Mbps data rate over the 2.4 GHz ISM band.
In a wireless infrastructure setup, there are two basic components, access points and wireless stations. An access point or base station functions as a bridge by connecting to a wired LAN (Local Area Network) through Ethernet cables. It receives data, buffers and transmits data between the wireless and the wired network infrastructure. A single access point supports on average 20 users and has a coverage varying from 20 meters in areas with obstacles like walls, stairways, elevators, and so forth, and up to 100 meters in areas where there is clear line of sight. The design of infrastructure-based wireless network is rather simpler as most of the network functionality lies within the access point. Transmission and reception of wireless communication can happen in different channels.
A building may require several access points to provide complete coverage and allow users to roam seamlessly between access points. A wireless network adapter connects users via an access point to the rest of the LAN. A wireless station can be using a PC card in a laptop, an ISA or PCI adapter in a desktop computer, or can be fully integrated within a handheld device. Security of a WLAN is of great concern with WEP (Wired Equivalent Privacy) encryption as static WEP keys could be easily recovered because of a design flaw (Stubblefield, Ionnidis, & Rubin, 2002). RADIUS Server authentication which uses EAP protocol (Extensible Authentication Protocol) with TKIP (Temporal Key Integrity Protocol) encryption is proposed as interim solution in WPA (Wi-Fi protected Access) standard, with AES encryption option being looked into as long term solution (Gast, 2002). Figure 1 shows a simple wireless network that uses RADIUS server authentication.
The wireless network in an organization showing mobile laptops
There are different wireless LAN technologies that the IEEE 802.11 standard supports in the unlicensed bands of 2.4 and 5 GHz. They share the same MAC (Medium Access Control) over two PHY layer specifications: Direct-Sequence Spread Spectrum (DSSS) and Frequency-Hopping Spread Spectrum (FHSS) technologies. Infrared technology though supported, is not accepted by any manufacturer. Data rates of up to 2 Mbps were achieved initially by IEEE 802.11 systems operating at the 2.4 GHz band. Their wide acceptance initiated new versions and enhancements of the specification. The different extensions to the 802.11 standard use the radio frequency band differently. Popular 802.11 standards like 802.11a, 802.11b and 802.11g are listed in table 1.
Key Terms in this Chapter
Wireless Transmission: Electromagnetic transmission through air, vacuum or water by means of antenna.
Encryption: To convert plain text or data into unreadable form by means of a reversible mathematical computation.
Scatternet: A scatternet is collection of piconets connected through sharing devices.
Direct Sequence Spread Spectrum: A form of spread spectrum in which each bit in the original signal is represented by multiple bits in the transmitted signal, using a spreading code.
Medium Access Control (MAC): For a communication network, the method of determining which station has access to the transmission medium at any time.
Access Point: The central or master device through which an infrastructure wireless node makes a connection to the local area network. It acts more like a bridge between wireless node and LAN.
HomeRF: Short form for “home radio frequency.” It is designed specifically for wireless networks in homes, in contrast to 802.11, which was created for use in businesses.
Frequency Hopping Spread Spectrum: It is a spread-spectrum method of transmitting radio signals by rapidly switching a carrier among many frequency channels, using a pseudorandom sequence known to both transmitter and receiver.
Bluetooth: It is an industrial specification and standard for wireless personal area networks (W-PANs).
Piconet: A small network of communication devices connected in an ad-hoc fashion using Bluetooth technology.
WiMAX: Short form for Worldwide Interoperability for Microwave Access, to provide wireless data over long distances from point to point links to full mobile cellular type access.