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Autor: anton • November 24, 2010 • 3,229 Words (13 Pages) • 512 Views
A Brief History
The history of wireless networking stretches farther back than you might think. It was over fifty years ago, during World War II, when the United States Army first used radio signals for data transmission. They developed a radio data transmission technology, which was heavily encrypted. It was used quite extensively throughout the campaign with the US and her allies. This inspired a group of researchers in 1971 at the University of Hawaii to create the first packet based radio communications network. ALOHNET, as it was named, was essentially the very first wireless local area network (WLAN). This first WLAN consisted of 7 computers that communicated in a bi-directional star topology (see http://www.its.bldrdoc.gov/fs-1037/ and http://www.webopedia.com/ -- both are excellent sources of computer and telecommunication terms and definitions) that spanned four of the Hawaiian Islands, with the central computer based on Oahu Island. With this, wireless networking was born.
While wired LANs have wholly dominated the networking market, the last few years show a rise in wireless networking usage. This can best be seen in academic circles (i.e. University campuses), health-care, manufacturing, and warehousing. All the while, the technology is improving, making it easier and cheaper from companies to go wireless
Wireless Network Topologies
Topology: The physical (real) or logical (virtual) arrangement of elements.
In our case, this refers to the arrangement of nodes (i.e. computers, network printers, servers, etc.) in which the network is connected. There are five major topologies in use today in wired networks: Bus, Ring, Star, Tree, and Mesh, but only two make sense in a wireless environment. These include the star and mesh topologies.
The star topology, which happens to be in widest use today, describes a network in which there is one central base station or Access Point (AP) for communication. The information packets transmitted by the originating node and are received by the central station and routed to the proper wireless destination node.
This station can then be a bridge to a wired LAN giving access to other wired clients, the Internet, other network devices, and etc. From our review system, Compex's SoftBridge program provides a "software bridge" to wired clients and services without specialized hardware or AP. With this software, any computer that is connected to the wired network and has a wireless Network Interface Card (NIC) can act as the bridge.
The mesh topology is a slightly different type of network architecture than the star topology, except that there is no centralized base station. Each node that is in range of one another can communicate freely.
IEEE 802.11, 802.11a, and 802.11b
In order for WLANs to be widely accepted, there needed to be an industry standard devised to ensure the compatibility and reliability among all manufacturers of the devices. The Institute of Electrical and Electronics Engineers (IEEE) has provided just that. The original standard IEEE 802.11 was defined as a standard in 1997 followed by IEEE 802.11a and IEEE 802.11b in September of 1999. The original standard operates at a radio frequency (RF) band that surrounds 2.4GHz and provides for data rates of 1Mbps and 2Mbps and a set of fundamental signaling methods and services. The IEEE 802.11a and IEEE 802.11b standards are defined at bands of 5.8GHz and 2.4GHz, respectively. The two additions also define new Physical (PHY) layers for data rates from 5Mbps, 11Mbps, to 54Mbps with IEEE 802.11a. These standards operate in what is known as the Industrial, Scientific, and Medical (ISM) frequency bands. The typical bands are 902-928MHz (26MHz available bandwidth), 2.4-2.4835 GHz (83.5 MHz available), and 5.725-5.850 GHz (125MHz available), with the latter allowing for IEEE 802.11a's higher data rate.
The standard defines the PHY and Media Access Control (MAC) layers for the wireless communication. A layer is simply a group of related functions that are separate from another layer of related functions. The layers in our wireless networking scenario can be best understood in the following analogy. Consider moving a book (representing a data packet) from a shelf on one side of the room to the desk on the other. Well, the MAC layer can be thought of as how one picks up the book and the PHY layer is how you walk across the room.
The PHY layer as defined by the standard includes two different types of radio frequency (RF) communication modulation schemes: Direct Sequence Spread Spectrum (DSSS) and Frequency Hopping Spread Spectrum (FHSS). Both types were designed by the military for reliability, integrity, and security. Both types have their own unique way of transmitting data.
FHSS works by splitting the available frequency band into several channels. It uses a narrow band carrier wave that continuously changes in a 2-4 level Gaussian Frequency Shift Keying (GFSK) sequence. In other words, the frequency of transmission changes in a pseudorandom manner that is known by the sending and receiving nodes. This builds into the layer a decent bit of security. A hacker would generally not know the next frequency to switch to receive the entire signal. One advantage to FHSS is that it allows for multiple networks to coexist in the same physical space.
IEEE 802.11, 802.11a, and 802.11b, Continued
DSSS works in a different manner altogether. DSSS combines the data stream with a higher speed digital code. Each data bit is mapped into a common pattern of bits known only to the transmitter and the intended receiver. This bit pattern is called a chipping code. This code is a random sequence of high and low signals that signify the actual bit. This chipping code is inverted to represent the opposite bit in the data sequence. This frequency modulation, if the transmission is properly synchronized, offers it's own error correction, and thusly has a higher tolerance for interference.
The MAC layer defines a way of accessing the physical layer and also controls the services related to the mobility management and the radio resource. It is similar to the wired Ethernet standard for data transmission. The differences arise in the way data collisions are handled. In the wired standard, data packets are sent out to the network indiscriminately. Only when two packets in a sense "collide"