Describing Wireless Standards

Wireless networks operating according to the 802.11 standard are becoming more and more common. You will need to understand not only how to incorporate them into the larger network design but also the components involved, the terminology used, and the operation of the contention mechanism that is used. This section explains all of those topics.

Identifying Standards Bodies

Several organizations have an impact on the wireless world. Some of the organizations create regulations that must be followed, while others simply create standards that are voluntary. The following are the main bodies in the United States:

Institute of Electrical and Electronics Engineers (IEEE) The IEEE is a professional association dedicated to advancing technological innovation and excellence. A part of what it does is to create standards for networking. The IEEE created the standards you learned about in Chapter 5, “Physical and Logical Topologies,” for Ethernet (802.3) and Token Ring (802.5). It created the original 802.11 standard and has since amended the standard a number of times to account for technological advances. These standards are entirely voluntary but are usually followed by the major manufacturers to ensure interoperability with other equipment.

Federal Communications Commission (FCC) Whereas the standards created by the IEEE are voluntary, the regulations created and enforced by the FCC are not. This organization controls the use of the radio spectrum. Some parts of this spectrum are licensed, and some parts are not. The range of the spectrum that is used by 802.11 is unlicensed.

Wi-Fi Alliance The Wi-Fi Alliance is an industry group that encourages cooperation and standardization among its members. One of its larger contributions was the introduction of a security solution called Wi-Fi Protected Access (WPA), which addressed security weaknesses that were hampering the adoption of 802.11 in the enterprise. WPA served as a temporary solution while the IEEE completed work on the 802.11i security standard. The Wi-Fi Alliance logo is used to indicate that a piece of equipment is interoperable with other equipment bearing the same logo.

Understanding 802.11 Amendments

In 1997, the 802.11 standard was adopted by the IEEE. It describes a standard that uses either of two technologies: direct-sequence spread spectrum (DSSS) or frequency-hopping spread spectrum (FHSS). Starting with the 802.11a amendment and going forward, FHSS is no longer a part of the standard. DSSS operates on a fixed frequency, while FHSS changes frequencies in a pattern known by the transmitter and receiver. It operates in the 2.4 GHz frequency and is capable of 1 and 2 Mbps.

As time went by and technical advancements occurred, many amendments were made to the standard. These amendments are indicated by letters added to the right of the 802.11 name. The major amendments and their main characteristics are listed here:

802.11a The 802.11a amendment was not widely adopted when it was initially released as it operated in a different frequency, necessitating a hardware upgrade. Many thought that the extra performance gained by upgrading would not be worth the extra expense. Operating in the 5.0 GHz frequency it was inoperable with the current 802.11 devices, which operated in the 2.4 GHz frequency. Later, after the spread of 802.11g, it became more widely accepted. 802.11a operates in the 5.0 GHz frequency and supports up to 56 Mbps.

802.11b The 802.11b amendment was pretty much an upgrade of the 802.11 standard in that it uses the same frequency, is backward compatible with 802.11, but supported up to 11 Mbps. Although not as fast as 802.11a, 802.11b was more widely embraced because it required no hardware upgrades as did 802.11a.

802.11g By departing from the use of DSSS as its modulation technique and using orthogonal frequency-division multiplexing (OFDM), 802.11g is able to achieve 56 Mbps while still operating in the 2.4 GHz frequency to maintain compatibility with both 802.11 and 802.11b.

802.11n 802.11n uses multiple antennas, which is not new, but the way in which it uses them is. Multiple antennas had been used before to prevent a behavior called multipath, whereby the signal reflects off an object and arrives slightly out of sequence with the main signal, corrupting the main signal. By using two antennas and constantly sampling each for the cleanest signal, this could be avoided. When multiple antennas are used in this fashion, it is called antenna diversity.

802.11n uses the multiple antennas (as many as eight) working together in a process called multiple-input multiple-output (MIMO) to transmit multiple frames at once that are then sequenced after transmission. 802.11n also uses a 40 MHz channel, which is double that of the other 802.11 standards and thereby doubles the speed. Finally, changes to the CSMA/CA contention method allow blocks of frames to be acknowledged (instead of individual frames, as the other 802.11 standards require). You will learn more about CSMA/CA later in this section.

Understanding Wireless LAN Components and Terminology

You must be able to identify the main components of wireless LANs (WLANs), understand how they work together, and use correct terminology when discussing WLANs. This section first identifies the components, then covers some terminology, and finally explains the manner in which the parts communicate.

Access points Wireless access points (APs) and wireless routers transmit and receive signals and are a required piece in all network types except ad hoc networks (covered later in the “Service Sets” list item). The access point is the point of connection to all the wireless stations and usually connects them to the wired LAN. When stations communicate with one another, they do not do so directly. The transmission is relayed through the AP.

This relationship is depicted in Figure 9.9. In the figure, laptop C is transmitting to laptop B wirelessly through the AP, and laptop A is transmitting wirelessly through the AP back to WS 1 on the wired network. The transmissions of all the wireless stations are relayed through the AP.

FIGURE 9.9 WLAN

The AP is also responsible for announcing the presence of the network, if desired, in frames called beacon frames. In a secured network, the AP controls access to the network by authenticating stations before allowing them to connect.

Wireless Stations Wireless stations can be any devices capable of using 802.11. This could include laptops, barcode scanners, PDAs, tablet computers, and smartphones.

Service Sets The service set defines the wireless objects that are a part of the same wireless network. The three types of service sets are as follows:

• Basic Service Set (BSS) This includes a single AP and its associated stations. This is the type of set illustrated in Figure 9.9; the set includes the AP and laptops A, B, and C.
• Independent Basic Service Set (IBSS) This is also called an ad hoc or peer-to-peer network. It has no AP, and the wireless devices communicate directly with one another. An IBSS requires the first station to create the network and others to join. It is shown in Figure 9.10.

  

  FIGURE 9.10 IBSS

• Extended Service Set (ESS) An extended service set is one that has a single SSID but multiple APs. It may use the multiple APs in a small space to provide more bandwidth, or they may be placed apart from one another to extend the range of the wireless network. When a station moves out the range of one of the APs, it will scan for APs with the same SSID in the area and “roam” to the next AP in the ESS. This is shown in Figure 9.11.

  

  FIGURE 9.11 ESS

Describing CSMA/CA Operation

Because it is impossible for wireless stations to detect collisions, another contention method is required to arbitrate access to the network. The method is called Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA). It requires a more involved process of checking for existing wireless traffic before a frame can be transmitted wirelessly. The stations (including the AP) must also acknowledge all frames. The steps in the process are as follows:

1. Laptop A has a frame to send to laptop B. Before sending, laptop A must check for traffic in two ways. First, it performs carrier sense, which means it listens to see whether any radio waves are being received on its transmitter.
2. If the channel is not clear (traffic is being transmitted), laptop A will decrement an internal countdown mechanism called the random back-off algorithm. This counter will have started counting down after the last time this station was allowed to transmit. All stations will be counting down their own individual timers. When a station's timer expires, it is allowed to send.
3. If laptop A checks for carrier sense and there is no traffic and its timer hits zero, it will send the frame.
4. The frame goes to the AP.
5. The AP sends an acknowledgment back to laptop A. Until that acknowledgment is received by laptop A, all other stations must remain silent. The AP will cache the frame, where it already may have other cached frames that need to be relayed to other stations. Each frame that AP needs to relay must wait its turn to send using the same mechanism as the stations.
6. When the frame's turn comes up in the cache queue, the frame from laptop A will be relayed to laptop B.
7. Laptop B sends an acknowledgment back to the AP. Until that acknowledgment is received by the AP, all other stations must remain silent.

When you consider that this process has to occur for every single frame and that there are many other frame types used by the AP to manage other functions of the network that also create competition for air time, it is no wonder that actual throughput on a wireless LAN is at best about half the advertised rate.

For example, if two wireless stations were the only wireless clients and they were using 802.11g, which is capable of 56 Mbps, the very best throughput experienced would be about 25–28 Mbps. Moreover, as soon as a third station arrives, throughput will go down again because the stations are dividing the air time by 3 instead of 2. Add a fourth, and its gets even worse! Such is the challenge of achieving throughput on a wireless LAN.