? 2.2 Spread spectrum LANs: This type of LAN makes use of spread spectrum
transmission technology. In most cases, these LANs operate in the ISM (industrial, scientific, and medical) bands so that no Federal Communications Commission (FCC) licensing is required for their use in the United States.
? 2.2.1 Configuration
Except for quite small offices, a spread spectrum wireless LAN makes use of a multiple-cell arrangement (Table 2). Adjacent cells make use of different center frequencies within the same band to avoid interference.
Within a given cell, the topology can be either hub or peer to peer. In a hub topology, the hub is typically mounted on the ceiling and connected to a backbone wired LAN to provide connectivity to stations attached to the wired LAN and to stations that are part of wireless LANs in other cells. A peer-to-peer topology is one in which there is no hub. A MAC algorithm such as CSMA is used to control access. This topology is appropriate for ad hoc LANs. ? 2.2.2 Transmission Issues
A desirable, though not necessary, characteristic of a wireless LAN is that it be usable without having to go through a licensing procedure.
? 2.3 Narrowband microwave: These LANs operate at microwave frequencies but
do not use spread spectrum. Some of these products operate at frequencies that require FCC licensing, while others use one of the unlicensed ISM bands. ? 2.3.1 Licensed Narrowband RF
Microwave radio frequencies usable for voice, data, and video transmission are licensed and coordinated within specific geographic areas to avoid potential interference between systems. The advantage is that it guarantees interference-free communication.
? 2.3.2 Unlicensed Narrowband RF
The first vendor to introduce a narrowband wireless LAN using the unlicensed ISM spectrum is the Radio LAN. It makes use of a peer-to-peer configuration and it is unstable.
Page 11 of 24
? 3. IEEE 802.11 Architecture and Service
? 3.1 IEEE 802.11 Architecture
The smallest building block of a WLAN is a basic service set (BSS), which consist of some number of stations executing the same MAC protocol and competing for access to the same shared wireless medium. A BSS may be isolated or it may connect to a backbone distribution system (DS) through an access point (AP). The MAC protocol may be fully distributed or controlled by a central coordination function housed in the AP. The BSS generally corresponds to what is referred to as a cell in the literature. The DS can be a switch, a wired network, or a wireless network. The simplest configuration is shown in Figure 3.
Page 12 of 24
? 3.2 IEEE 802.11 Services
IEEE 802.11 defines nine services that need to be provided by the wireless LAN to provide functionally equivalent to that which is inherent to wired LANs (Table 4).
? The service provider can be either the station or the DS.
? Three of the services are used to control IEEE 802.11 LAN access and
confidentiality. Six of the services are used to support delivery of MAC service data units (MSDUs) between stations.
Page 13 of 24
? 4. IEEE 802.11 Medium Access Control
? 4.1 Reliable Data Delivery
IEEE 802.11 includes a frame exchange protocol to solve the reliability of data delivery. When a station receives a data frame from another station, it returns an ACK frame to the source station. If the source does not receive an ACK within a short period of time, the source retransmits the frame. To further enhance reliability, a source first issues a Request to Send (RTS) frame to the destination, then the destination responds with a Clear to Send (CTS). After receiving the CTS, the source transmits the data frame, and the destination responds with an ACK.
? 4.2 MAC Frame
MAC Frame Types:
? Control Frames assist in the reliable delivery of data frames.
? Data Frames: There are eight data frame subtypes, organized into two groups. The
first four define frames that carry upper-level data from the source station to the destination station. The remaining four do not in fact carry any user data.
? Management Frames are used to manage communications between stations and
APs. Functions covered include management of associations (request, response, reassociation, dissociation, and authentication).
? 4.3 Access Control
The 802.11 working group considered two types of proposals for a MAC algorithm: ? Distributed Access Protocols, which, like Ethernet, distribute the decision to
transmit over all the nodes using a carrier-sense mechanism;
? Centralized Access Protocols, which involve regulation of transmission by a
centralized decision maker.
? The end result for 802.11 is a MAC algorithm called DFWMAC (distributed
foundation wireless MAC) that provides a distributed access control mechanism with an optional centralized control built on top of that. Figure 5 illustrates the architecture.
Page 14 of 24
Page 15 of 24