Electrical computers and digital processing systems: multicomput – Network computer configuring
Reexamination Certificate
2000-01-05
2004-05-04
An, Meng-Al T. (Department: 2154)
Electrical computers and digital processing systems: multicomput
Network computer configuring
C370S320000, C455S450000, C455S452200
Reexamination Certificate
active
06732163
ABSTRACT:
FIELD OF INVENTION
The present invention relates generally to a wireless communication system, and more particularly to a system for automatically selecting communication frequencies for wireless communication devices.
BACKGROUND OF THE INVENTION
The use of wireless networks has become prevalent throughout the modern workplace. For example, retail stores and warehouses may use a wireless local area network (LAN) to track inventory and replenish stock and office environments may use a wireless LAN to share computer peripherals. A wireless LAN offers several advantages over regular LANs. For example, users are not confined to locations previously wired for network access, wireless work stations are relatively easy to link with an existing LAN without the expense of additional cabling or technical support; and wireless LANs provide excellent alternatives for mobile or temporary working environments.
In general there are two types of wireless LANs, independent and infrastructure wireless LANs. The independent, or peer-to-peer, wireless LAN is the simplest configuration and connects a set of personal computers with wireless adapters. Any time two or more wireless adapters are within range of each other, they can set up an independent network. In infrastructure wireless LANs, multiple base stations link the wireless LAN to the wired network and allow users to efficiently share network resources. The base stations not only provide communication with the wired network, but also mediate wireless network traffic in the immediate neighborhood. Both of these network types are discussed extensively in the IEEE 802.11 standard for wireless LANs.
In the majority of applications, wireless LANs are of the infrastructure type. That is, the wireless LAN typically includes a number of fixed base stations, also known as access points, interconnected by a cable medium to form a hardwired network. The hardwired network is often referred to as a system backbone and may include many distinct types of nodes, such as, host computers, mass storage media, and communications ports. Also included in the typical wireless LAN are intermediate base stations which are not directly connected to the hardwired network.
These intermediate base stations, often referred to as wireless base stations, increase the area within which base stations connected to the hardwired network can communicate with mobile terminals. Associated with each base station is a geographical cell. A cell is a geographic area in which a base station has sufficient signal strength to transmit data to and receive data from a mobile terminal with an acceptable error rate. Unless otherwise indicated, the term base station, will hereinafter refer to both base stations hardwired to the network and wireless base stations. Typically, the base station connects to the wired network from a fixed location using standard Ethernet cable, although in some case the base station may function as a repeater and have no direct link to the cable medium. Minimally, the base station receives, buffers, and transmits data between the wireless local area network (WLAN) and the wired network infrastructure. A single base station can support a small group of users and can function within a predetermined range.
In general, end users access the wireless LAN through wireless LAN adapters, which are implemented as PC cards in notebook computers, ISA or PCI cards in desktop computers, or fully integrated devices within hand-held computers. Wireless LAN adapters provide an interface between the client network operating system and the airwaves. The nature of the wireless connection is transparent to the network operating system.
In general operation, when a mobile terminal is powered up, it “associates” with a base station through which the mobile terminal can maintain wireless communication with the network. In order to associate, the mobile terminal must be within the cell range of the base station and the base station must likewise be situated within the effective range of the mobile terminal. Upon association, the mobile unit is effectively linked to the entire LAN via the base station. As the location of the mobile terminal changes, the base station with which the mobile terminal was originally associated may fall outside the range of the mobile terminal. Therefore, the mobile terminal may “de-associate” with the base station it was originally associated to and associate with another base station which is within its communication range. Accordingly, wireless LAN topologies must allow the cells for a given base station to overlap geographically with cells from other base stations to allow seamless transition from one base station to another.
Most wireless LANs, as described above, use spread spectrum technology. Spread spectrum technology is a wideband radio frequency technique developed by the military for use in reliable, secure, mission-critical communication systems. A spread spectrum communication system is one in which the transmitted frequency spectrum or bandwidth is much wider than absolutely necessary. Spread spectrum is designed to trade off bandwidth efficiency for reliability, integrity, and security. That is, more bandwidth is consumed than in the case of narrowband transmission, but the tradeoff produces a signal that is, in effect, louder and thus easier to detect, provided that the receiver knows the parameters of the spread spectrum signal being broadcast. If a receiver is not tuned to the right frequency, a spread spectrum signal looks like background noise.
In practice, there are two types of spread spectrum architectures: frequency hopping (FH) and direct sequence (DS). Both architectures are defined for operation in the 2.4 GHz industrial, scientific, and medical (ISM) frequency band. Each occupies 83 MHz of bandwidth ranging from 2.400 GHz to 2.483 GHz. Wideband frequency modulation is an example of an analog spread spectrum communication system.
In frequency hopping spread spectrum systems the modulation process contains the following two steps: 1) the original message modulates the carrier, thus generating a narrow band signal; 2) the frequency of the carrier is periodically modified (hopped) following a specific spreading code. In frequency hopping spread spectrum systems, the spreading code is a list of frequencies to be used for the carrier signal. The amount of time spent on each hop is known as dwell time. Redundancy is achieved in FHSS systems by the possibility to execute re-transmissions on frequencies (hops) not affected by noise.
Direct sequence is a form of digital spread spectrum. With regard to direct sequence spread spectrum (“DSSS”), the transmission bandwidth required by the baseband modulation of a digital signal is expanded to a wider bandwidth by using a much faster switching rate than used to represent the original bit period. In operation, prior to transmission, each original data bit to be transmitted is converted or coded to a sequence of a “sub bits” often referred to as “chips” (having logic values of zero or one) in accordance with a conversion algorithm. The coding algorithm is usually termed a spreading function. Depending on the spreading function, the original data bit may be converted to a sequence of five, ten, or more chips. The rate of transmission of chips by a transmitter is defined as the “chipping rate.”
As previously stated, a spread spectrum communication system transmits chips at a wider signal bandwidth (broadband signal) and a lower signal amplitude than the corresponding original data would have been transmitted at baseband. At the receiver, a despreading function and a demodulator are employed to convert or decode the transmitted chip code sequence back to the original data on baseband. The receiver, of course, must receive the broadband signal at the transmitter chipping rate.
The coding scheme of a spread spectrum communication system utilizes a pseudo-random binary sequence (“PRSB”). In a DSSS system, coding is achieved by converting each original data bit (zero or one) to a predetermined repetitive
An Meng-Al T.
Cisco Technology Inc.
Hu Jinsong
Tucker Ellis & West LLP
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