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Wireless Networking Handbook
(Publisher: Macmillan Computer Publishing)
Author(s): Jim Geier
ISBN: 156205631x
Publication Date: 09/01/96

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Operating radios can use spread spectrum within the same frequency band and not interfere, assuming they each use a different hopping pattern. While one radio is transmitting at one particular frequency, the other radio(s) uses a different frequency. A set of hopping codes that never use the same frequencies at the same time are considered orthogonal. Some vendors allow the user to choose the channel (a particular hopping code) through software that the radio will operate on, all users within the same local network, however, have to use the same code. This does give you the ability, though, to have wireless LANs within close proximity to each other operate within the same band and not interfere with each other, as long as you assign them orthogonal hopping codes. The FCC’s requirement for the number of different transmission frequencies allows frequency-hopping radios to have many non-interfering channels.


Figure 2.6  A frequency hopping spread spectrum.

Radio-based Wireless LAN Components

The components of a radio-based LAN consist mainly of a wireless NIC and a wireless local bridge, which is sometimes referred to as an access point.

Wireless NICs

The wireless NIC interfaces the computer to the wireless network by modulating the data signal with the spreading sequence and implementing a carrier sense access protocol. If the computer needs to send data onto the network, the NIC will listen for other transmissions. If the NIC doesn’t sense other transmissions, it will transmit a frame of data. Other stations, constantly listening for incoming data, capture the transmitted frame and check whether its address matches the destination address in the frame’s header. If there is a match, the receiving station will process the frame. If not, the station will discard the frame. Most radio-based wireless LAN products can maintain bit error rates better than 10-8.

The wireless NIC plugs into the ISA bus of a desktop computer or a small credit card-sized adapter inserted into the PCMCIA (Personal Computer Memory Card International Association) slot of a portable computer. Wireless radio NICs have an external antenna that you can attach to a wall or office partition. The radio NIC interfaces the users’ computer to the wireless network and is comparable to an ethernet board, except the wireless products are more expensive. The radio-based cards range in price from $300 to $700 each, depending on their form factor (the PCMCIA format is generally most expensive).

There are many companies that produce radio-based wireless LAN NICs. The following sections discuss several examples:


NOTE:  

Some manufacturers of devices such as power meter readers, medical instruments, point-of-sale devices, and order entry equipment, have interest in incorporating wireless data communications into their product lines. Many of the wireless network companies can support this need by offering Original Equipment Manufacture (OEM) versions of their products. Proxim, for example, has RangeLAN2 6300 Mini-ISA that measures under four inches long. RangeLAN2 6300 is an integrated hardware/software OEM package that enables developers to easily incorporate wireless networking capabilities into their products. It is especially well suited for integration into portable computer platforms for mobile data applications. Another company, Digital Wireless Corporation, offers an OEM product called WIT2400 Frequency Hopping Transceiver. The WIT2400 operates over the 2.4 GHz ISM band at up to 2000 feet with data rates of 250 Kbps.


Proxim’s RangeLAN

Proxim, Inc. has a full family of radio-based wireless LAN products. In 1993, Proxim became the first company to ship a spread spectrum PCMCIA wireless LAN adapter, the RangeLAN2 7200 PCMCIA. The RangeLAN2 7200 is a high-performance wireless LAN adapter for PCMCIA Type II or III equipped portables. RangeLAN2 7200 uses frequency-hopping spread spectrum radio signals in the 2.4–2.4835 GHz band, delivering a data rate of 1.6 Mbps at up to 3,000 feet. RangeLAN2 7200 operates at a distance of up to 500 feet in normal office environments and up to 1000 feet in open spaces. Through a multi-channel approach, RangeLAN2 7200 enables 15 independent wireless LANs to operate within the same physical space. Proxim also sells RangeLAN2 7100 ISA for AT bus computers, and it has the same performance characteristics as the PCMCIA version.

Lucent’s WaveLAN

Lucent has several versions of their wireless LAN product WaveLAN. You can purchase WaveLAN for ISA or PCMCIA formats at either 902 MHz or 2.4 GHz. WaveLAN’s range in open environments is 600–800 feet for the 902 MHz version and 12–180 feet for the 2.4 MHz model. The entire WaveLAN family uses direct sequence spread spectrum. For higher levels of security, WaveLAN will support data encryption. This product has undergone several ownership changes. The initial product was developed by NCR, which was acquired by AT&T and renamed AT&T Global Information Solutions. AT&T then split into three parts: the service groups still belong to AT&T, the computer section is with NCR again, and WaveLAN is part of Lucent Technologies.

Windata’s FreePort

Windata’s FreePort Wireless Ethernet system consists of two main hardware components: FreePort Wireless Hub and Wireless Transceiver. The system operates at 5.7 Mbps throughput and has a range of up to 260 feet. The FreePort Wireless Hub receives and retransmits data packets from Wireless Transceivers in its coverage area. The Wireless Hub can interface directly to IEEE 802.3 ethernet, acting as a wireless bridge.

AeroComm’s GoPrint Wireless Printer Sharing

GoPrint features a 2.4 GHz spread spectrum radio that enables a computer to interface with a printer at distances of 3,000 feet unobstructed and up to 200–500 feet where office walls are present. GoPrint transmits at speeds up to 1 Mbps, which is 5–10 times the data throughput of wired printer sharing solutions. GoPrint adapters plug directly into the parallel port of computers and the Centronics port of printers. Multiple computers can output to a single printer or up to eight printers in the same office. GoPrint will work in both server-oriented and non-server environments.


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