IBM Wireless RF LAN design and architecture

IBM Systems Journal,  Sept, 1995  by F.J. Bauchot,  F. Lanne

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During the last few decades the complexity of data communications has significantly increased. The amount and the diversity of networking solutions available in the marketplace are far more important today than during the 1960s when mainframe-based networks were almost the only solutions offered to users. Today the customer can choose from among various subnetworking options (local area network [LAN], metropolitan area network [MAN], wide area network [WAN], etc.), network protocol stacks (Systems Network Architecture [SNA], Advanced Peer-to-Peer Networking(*) [APPN(*)], Transmission Control Protocol/Internet Protocol [TCP/IP], Sequenced Packet Exchange [IPX(**)], Internetwork Packet Exchange [IPX(**)], etc. ), link controls (Ethernet, token ring, Fiber Distributed Data Interface [FDDI], asynchronous transfer mode [ATM], etc.), and so on. Moreover, all these alternatives can be mixed and matched, resulting in heterogeneous network solutions.

When a new data communications product is launched in such a complex environment, many different product design and system aspects have to be considered. This paper addresses such aspects for the IBM Wireless Radio Frequency (RF) LAN. The focus is on two major characteristics of the wireless LAN product: the Medium Access Control (MAC) protocol, which governs how multiple users can access the wireless channel, and network integration, which allows this new product to interconnect various other networks for resource-sharing purposes.

Next, the section on MAC protocol describes how a wireless LAN product can capitalize on existing MAC protocols within the constraints of a wireless channel. The section on network integration then describes how the new wireless LAN products fit into various data communication environments.

Wireless LAN MAC protocol

The evolution of LAN networking: A brief history. In the late 1960s, when teleprocessing was young, the means used to interconnect computers with remote computers and data I/O devices were primarily telephone lines, characterized by a limited bandwidth and poor reliability. They provided limited throughput and led to new data link control (DLC) protocols such as binary synchronous communication (BSC) and high-level data link control (HDLC) to provide upper protocol layers with reliable communication services on the low-quality channels.[1]

Later during the 1970s, new communication channels appeared, providing much better reliability. Although the bit error rate (BER) of terrestrial telephone lines was commonly in the range of [10.sup.-3] to [10.sup.-5], it became possible to achieve BERS in the range of [10.sup.-7] to [10.sup.-9] or better.[2] Moreover, these new communication channels, which were designed to carry data rather than voice, offered much more bandwidth, but reduced the range between the communicating devices. These new communication channels, introduced as LANs, became pervasive in the 1980s. The availability of LANs has driven the paradigm shift to client/server and peer-to-peer network architectures from large main-frame-based teleprocessing.

During the early 1990s, technological innovations in electronics and packaging led to the widespread use of portable computers. Their commercial success is mainly because they can be used anywhere. They are no longer tied to a communication line, provided that the data required to run applications have been previously loaded on a fixed disk. Thus it remains necessary to provide some telecommunication line attachment (using modems or LAN adapters, for instance) to upload files from or download files to these portable computers.

The next paradigm shift in teleprocessing is to avoid use of any cable, allowing connectivity even if the computer is not cable-attached. This last shift has resulted in the wireless LAN (WLAN) products available today on the market. The end-user computer has become mobile without precluding on-line attachment to a server, mainframe, or peer device.

Such connectivity relies on wireless communications using electromagnetic waves. Different wireless channels are available, along with different spectrum-management techniques: infrared channels or radio channels, possibly using spread-spectrum techniques. The choice of such a channel or spectrum-management technique has been discussed in Reference 3. Whatever the choice is, the communication channel and the wireless devices are characterized by the following attributes:

* The communication range is limited (either due to regulatory constraints or the laws of physics) but sufficient to accommodate regular LAN-type connectivity.

* The available bandwidth is sufficient to sustain LAN operation but remains limited in comparison with state-of-the-art wired LANs.

* Carrier-sensing of electromagnetic waves is far less reliable than for wired channels and cannot be achieved (at an affordable product cost) while transmitting.

* Wireless devices commonly are battery-powered portable computers; it is thus desirable to allow efficient access to the channel with low power consumption.