The IEEE 802.11 Wireless Standard


The main standard for wireless LANs is the IEEE 802.11 group of standards, which were in definition as early as 1990 and have gone through several drafts since then. This standard includes definitions for the Physical layer (PHY) and the Media Access Control (MAC) layer protocols for wireless networkingboth of which are covered later in this chapter. If you are familiar with Ethernet, you know that the MAC address, which is a unique address burned into a network adapter when it is manufactured, is used on a local area network to communicate with other members of the same LAN.

The standard envisions two kinds of clients: ad hoc and client/server. The ad hoc client method involves a peer-to-peer network between clients located close to each other. The client/server method uses an AP device to mediate network communications and possibly provide a connection to a wired network. Note that the first draft of the IEEE 802.11 standard provided for only transmission rates of 1Mbps and 2Mbps, but the newest standards (802.11a, b, and g) provide for much faster data transmission rates. The next few chapters discuss the three main wireless technologies. Before getting into those, however, let's take a look at the PHY and MAC portions that are the basics of these standards.

The Physical Layer

The Physical layer of a network involves the mechanisms used to actually transmit the signal on the network medium. In this case, the medium is infrared, Frequency Hopping Spread Spectrum (FHSS), or Direct Sequence Spread Spectrum (DSSS). In the original IEEE 802.11 standard using FHSS, the data rate is 1Mbps. For DSSS, the original standard defines both 1Mbps and 2Mbps techniques. Infrared communications (it's really wireless, after all) also is supported at both 1Mbps and 2Mbps data rates. Because few, if any, infrared devices are on the market today, they aren't discussed in this chapter. The most important devices are those using radio frequencies (FHSS and DSSS), which you'll find for sale by many vendors.

Note

Although the standard defines both spread-spectrum and infrared methods at the Physical layer, wireless clients using different Physical layer components may or may not interoperate. In other words, an infrared-equipped client can't communicate with another computer that uses the radio frequency spectrum for transmissions. Yet there are Access Points that can handle a combination of 802.11a, 802.11b, and 802.11g at the same time. These specifications all involve similar technologies, but operate in different frequencies or use different transmission techniques.


The MAC Layer

For the MAC layer, the standard is similar to the 802.3 standard for traditional Ethernet networks. Before a network node (or station, as they're usually called in the wireless world) can transmit, it must first determine whether the radio frequency channel is available, and a mechanism must be used to determine whether a transmission was successfully received by the destination station.

Carrier Sense Multiple Access/Collision Avoidance

Chapter 13, "Ethernet: The Universal Standard," explains the mechanism Ethernet network adapters use to gain access to the network medium (a copper wire, for example): Carrier Sense Multiple Access/Collision Detection (CSMA/CD). The network adapter that wants to transmit a frame of data first listens to the wire. If the wire is silent, the card transmits a frame. It then listens to make sure that no other network adapter tried to transmit at the same time (a collision). If a collision occurs, each of the nodes that caused the collision backs off for a random interval before attempting another transmission. This random back-off mechanism is intended to keep multiple network adapters from trying to transmit at the same time again.

This works well when you have a network adapter card that transmits and receives at the same time, which is exactly how Ethernet network adapters operate. However, when you're dealing with the airwaves, a wireless adapter can either transmit or receive, but not both at the same time. So another method for gaining access to the medium (the radio frequency on which the network is based) is used. It's called Carrier Sense Multiple Access/Collision Avoidance, or CSMA/CA for short.

Using this principle, the wireless network adapter card first listens to determine whether any other station is transmitting, just like a traditional Ethernet card does. If the frequency is not being used, the station can transmit a frame. However, because it can't listen at the same time to determine whether a collision has occurred, another method is used. If the destination of the transmission receives the frame intact, it sends back an acknowledgment packet (ACK). The standard provides for a higher priority for transmission of ACK packets so that they are transmitted before other stations can transmit.

After a wireless adapter has transmitted a frame, it waits to see whether an ACK is sent back. If some other station also has transmitted a frame during the same time, then the receiving end of both communications attempts will not receive an intact frame, no ACKs will be sent back, and thus both stations know they must retransmit the frame. Just as with traditional Ethernet cards, the stations that do not receive an ACK in response to a transmission assume that a collision has occurred, and wait for a random time interval before again listening to the airwaves to determine whether they can retransmit.

Another term used in the specifications for this media access method is Distributed Coordination Function (DCF); all stations based on IEEE 802.11 must implement this method.

Virtual Carrier SenseRTS/CTS

As stated earlier in this chapter, it's always possible that two wireless-equipped computers can begin to transmit at the same time. However, what happens when you use an AP and two stations that are so far apart they can't hear the transmission of the other computer? This is referred to as the hidden node problem. In this situation, another means must be employed to ensure that only one station on the wireless network is transmitting at a given time.

To solve this problem, wireless computers that want to transmit a frame must first make a request for air time. Instead of just listening to the radio frequency and starting a frame transmission if the frequency is not being used, the network adapter instead transmits an RTS (request to send) frame to the AP. This frame contains information that identifies the station that wants to transmit, as well as the duration of time it wants to reserve for the transmission.

If the RTS frame is received by the AP (that is, no collision occurred due to another station also trying to transmit an RTS frame), the AP transmits a CTS (clear to send) frame that grants permission to the original computer to begin its transmission. This frame also contains the ID field of the computer that is being given permission to transmit, as well as the amount of time granted to it. Because all stations participating in the wireless network controlled by the AP can hear the CTS frame, they know they can't begin a transmission and they also know how long they must wait before making an attempt to send an RTS frame.

This method of accessing the transmission media also is referred to as Point Coordination Function (PCF). Although DCF is required by the IEEE 802.11 standard, PCF is not. It can be implemented but is optional according to the standards.

Other Services Performed at the MAC Layer

The MAC layer also provides other services, such as association and reassociation. Remember that an AP and its clients make up a BSS in the network. A client is associated with a particular BSS. When a client moves from one BSS to another, reassociation takes place. Although the 802.11 standard provides for the concept of reassociation, the actual mechanism for this function is not specified in the standard.




Upgrading and Repairing Networks
Upgrading and Repairing Networks (5th Edition)
ISBN: 078973530X
EAN: 2147483647
Year: 2006
Pages: 411

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