Ieee 80211 System

The IEEE 802.11a system operates in the 5-GHz band employing orthogonal frequency division multiplex modulation (OFDM) offering data rates from 6 to 54 Mbps. IEEE 802.11b specifies operation in the 2.4-GHz band providing data rates of 1 or 2 Mbps of the initial 802.11b specification and an added capability for 6.6 Mbps and 11 Mbps with CCK (complementary code keying) modulation which makes more efficient use of the radio-frequency spectrum.

The 802.11 series are commonly referred to as wireless LANs (WLANs). These WLANs can be used either to replace wired LANs or as an extension of the wired LAN infrastructure. In the WLAN arena we can have a basic service set (BSS) consisting of two or more wireless nodes or stations (STAs) which have recognized each other and have established communications. In the most basic form, stations communicate directly with each other on a peer-to-peer level sharing a given call coverage area. This type of network is often formed on a temporary basis and is commonly referred to as an ad hoc network, or independent basic service set (IBSS).

In most instances, the BSS contains an access point (AP). The main function of an AP is to form a bridge between wireless and wired LANs. The AP is analogous to a base station used in cellular phone networks. When an AP is present, stations do not communicate on a peer-to-peer basis. All communication between stations or between a station and the wired network client go through the AP. APs are not mobile, and they form part of the wired network infrastructure. A BSS in this configuration is said to be operating in the infrastructure mode.

The extended service set (ESS) illustrated in Figure 13.1 consists of a series of overlapping BSSs, each containing an AP and are connected together by means of a DS (distributed system). Although the DS could be any kind of network, it is almost invariably an Ethernet LAN. Mobile nodes can roam between APs, and seamless campus-wide coverage is possible.

The MAC layer in the IEEE 802.11 standard is a set of protocols that is responsible for maintaining order in the use of a shared medium. The 802.11 standard specifies a carrier sense multiple access with collision avoidance (CSMA/CA) protocol. In this protocol, when a node receives a packet to be transmitted, it first listens to ensure no other node is transmitting. If the channel is clear, it then transmits the packet. Otherwise, it chooses a random "backoff factor" that determines the amount of time the node must wait until it is allowed to transmit its packet. During periods in which the channel is




Figure 13.1 An ESS can provide campus-wide coverage. [From IEEE 802.11 tutorial (Ref. 5).]

clear, the transmitting node decrements its backoff counter. (When the channel is busy, it does not decrement its backoff counter.) When the backoff counter reaches zero, the node transmits the packet. Since the probability that two nodes will choose the same backoff factor is small, collisions between packets are minimized. Collision detection, as is employed in Ethernet, cannot be used for the radio-frequency transmissions of IEEE 802.11. The reason for this is that when a node is transmitting, it cannot hear any other node in the system that may be transmitting, since its own signal interferes with any others arriving at the node.

Whenever a packet is to be transmitted, the transmitting node first sends out a short ready-to-send (RTS) packet containing information on the length of the packet. If the receiving node hears the RTS, it responds with a short clear-to-send (CTS) packet. After this exchange, the transmitting node sends its packet. When the packet is received successfully, as determined by a cyclic redundancy check (CRC), the receiving node transmits an acknowledgment (ACK) packet. This back-and-forth exchange is necessary to avoid the "hidden node" problem. This may be described by considering three nodes of a network:

A, B, and C. As is often the case in these situations, node A can communicate with node

B, and node B can communicate with node C. However, node A cannot communicate node C. Thus, for instance, although node A may sense the channel to be clear, node C may in fact be transmitting to node B. The 802.11 protocol alerts node A that node B is busy, and hence it must wait before transmitting its packet.

There are seven IEEE 802.11 subset standards (802.11a through 802.11g) as follows:

802.11a—Uses orthogonal frequency division multiplex (OFDM) modulation in the 5-GHz band supporting data rates from 6 to 54 Mbps.

802.11b—Provides high-rate direct-sequence spread-spectrum (DSSS) service in the 2.4-GHz band supporting 5.5 and 11 Mbps in addition to the 1- and 2-Mbps operation. It uses complementary code keying (CCK) for more efficient use of the radio-frequency spectrum.

802.11c—Bridge operation procedure.

802.11d—Global harmonization to support widespread adoption in many countries.

802.11e—MAC enhancements for QoS.

802.11f—Inter-access point protocol, users roaming from one access point to another. 802.11g—Higher rate extensions in the 2.4-GHz band.

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