The 802.11 standard defines the protocols that govern all Ethernet-based wireless traffic. However, within this standard exist several sub-standards that compete with each other for a place in the market. This section outlines the major sub-standards of 802.11.
Long before wireless networks climbed out from the primordial ooze of Ethernet, the Institute of Electrical and Electronics Engineers (IEEE) had set up a system by which new technologies could become certified. The IEEE's certification ensures that a technology can be compatible with other products using the same certified technology.
One of the many technologies to go through this reviewing and certification process was that of Local Area Networks (LANs) . A LAN is simply a local group of connected computers and the supporting hardware and software to facilitate communication between the computers. However, there are a number of rules and specifications that are required in order for a product to be deemed LAN-compliant. Thus, to handle this specific technology sector, the IEEE created the 802 group , which is responsible for reviewing both old and new networking technologies to ensure that they are reliable and conflict-free. If a new technology is submitted for certification, it is intensely scrutinized by this group, and it will undergo many tests before it is deemed worthy.
The 802 certification includes many subsets , which represent different facets of networking. For example, 802.3 is the standard that defines how Ethernet works. If a product is to be considered "Ethernet" (see Figure 2.2), it must meet all the requirements specified in 802.3. This leads us to "wireless Ethernet," which is classified and controlled by 802.11.
Figure 2.2. An 802.3 wired network.
In addition to the previously mentioned categorization, 802.11 is further broken down into more specific certifications, such as 802.11a, 802.11b, and 802.11g. Each of these defines a different method for providing wireless Ethernet. Each protocol specifies various aspects of data transfer that distinguish them from the other certifications. This chapter discusses the key 802.11 certifications.
One of the most popular standards set by the 802 group was the 802.3 standard. This is the certification used by Ethernet-ready devices. For example, an Ethernet device must support a technology known as Carrier Sense Multiple Access/Collision Detection (CSMA/CD) . Now CSMA/CD might seem like a mouthful, but you use the same type of rules when sitting in a classroom. If we break down the acronym, the meaning becomes clear. CS is carrier sense , which basically means only one person (or device) can talk at a time. Imagine the confusion if everyone in the classroom talked at the same time!
The next part is multiple access , which is a technical way of saying there is more than one person listening to the conversation. In a class, everyone hears the words spoken by the instructor or another student. However, if the instructor is talking to one specific student only, the information being passed is irrelevant to the others and can be ignored by the rest of the class. The same applies to an Ethernet network.
The last part is collision detection , which is another way of saying every Ethernet can determine whether two devices have started talking at the same time. When humans do this, we simply stop and then one person starts talking again. In an Ethernet environment, as in humans , the devices will stop and wait a random amount of time. The device that has the lower random time gets to talk first.
Why is this relevant? The answer is found in the fact that 802.11 uses CSMA/CA, or CSMA/ Collision Avoidance , which is the alternative to CSMA/CD. It does this by broadcasting a message of intention to talk. In other words, this is like "calling" the ball when playing volleyball. If everyone knows who intends to go for the ball, people will not run into each other trying to return a volley. However, this type of communication does have some extra overhead, as each network device must send data out over the network before transmission starts. Although the individual amount of this data seems small, the cumulative amount can become a serious issue in an already overloaded network. 802.11 is not the only standard that uses CA; in fact, AppleTalk, used by Mac computers, also use CA in their data networks.
802.11 is a series of standards that defines wireless methods of transmitting Ethernet traffic. Commonly referred to as Wi-Fi or WLAN traffic, this technology is tested and marketed by the WECA (Wireless Ethernet Compatibility Alliance).
This standard has several substandards important to your understanding of WLANs ” 802.11a, 802.11b, and 802.11g. Each of these is based on a different physical layer, and has its own benefits and disadvantages.
Pre-Standard/Non-Standard Wireless LANs and the ISM
Prior to the ratification of the 802.11 standard by the IEEE, several other technologies were developed that used various forms of spectrum hopping to facilitate wireless data transfer. These technologies were proprietary, and were typically slower than the finalized standards, with speeds at 1 “2Mbps and frequencies in the 900MHz and 2.4GHz ranges.
All of the 802.11 x standards use the ISM (Industrial, Science, and Medical) frequencies, as defined by the FCC. These frequencies ”900MHz, 2.4GHz, and 5GHz ”are all open ranges that can be used by anyone for testing or consumer goods.
After the IEEE ratified 802.11 in 1997, three main frequency technologies became the main methods of data transmission: DSSS, FHSS, and IrDA. Of the three, DSSS and FHSS (discussed later) showed the most promise, and were eventually incorporated into most WLAN technology.
We will discuss 802.11b first because it is the standard found in most every wireless device, and is by far the most prevalent . An 802.11b device operates by sending a wireless signal using direct sequence spread spectrum in the 2.4GHz range.
It should be noted that the current implementation of 802.11b supports the 1 “2Mbps speed of older WLAN products, providing they use DSSS in the 2.4GHz range. The current standard added 5.5 and 11Mbps to the operational ranges.
The 2.4GHz range is an open frequency in which many devices operate , including phones and microwaves . The FCC opened this range to allow vendors to create wireless devices that did not require specific FCC approval. In other words, anyone can make a 2.4GHz device and use it without fear of breaking into the range of a regulated frequency, such as the 911 frequency. This is why an organizing body needs to monitor such usage ” otherwise total chaos would reign. Imagine trying to send a distress signal, but having it scrambled by someone else's cell phone.
Although setting aside the 2.4GHz range was a good idea, the concept of "too much of a good thing" is now causing WLAN users some problems. Because so many other devices use the 2.4GHz range, it is likely that some interference will occur. For example, have you ever heard someone else's conversation on your wireless house phone? This is because they are on the same frequency as you. The same can happen to a WLAN device. Although the interference is not totally destructive to a signal, it can impede it to the point where an 11Mbps signal can be reduced to a 1Mbps signal.
DSSS (direct-sequence spread spectrum) , illustrated in Figure 2.3, helps prevent interference by spreading the signal out over several frequencies at one time. In other words, DSSS takes a byte of data, splits it up into several chunks, and sends the chunks out at the same time by multiplexing them onto different frequencies. As the next byte is selected it is then split up and sent out over another set of frequencies. This helps increase bandwidth, and allows for multiple devices to operate on one WLAN. As long as the time and frequency domains don't collide, the data will remain intact.
Figure 2.3. Frequency hopping using DSSS.
Inside the 802.11a Standard
802.11a was the first officially ratified wireless Ethernet standard. However, it was not rapidly accepted because it was based on new technologies, and used a different nature of data transmission. Because of this, 802.11b made it to the market first and became the standard most WLAN products use.
Ironically, 802.11a is the fastest of the current 802.11 standards. It is capable of speeds up to 54Mbps, which is roughly five times faster than the more common 802.11b that operates at 11Mbps. 802.11a also operates in a different frequency (5GHz) and uses a different method of transmission (OFDM), which has several advantages as described in the following sections.
The 5GHz Frequency
As mentioned previously, the 2.4GHz range is becoming saturated by the many devices rushing to cash in on wireless technologies. However, the 5GHz range is still mostly free of this problem. In addition, a 5GHz signal means more data transfer at the same time. A gigahertz means there are 1 billion cycles per second; therefore, 5GHz as compared to 2.4GHz is more than twice as fast. This added speed, in combination with a different type of frequency control, makes 802.11a five times faster than its predecessor 802.11b.
802.11a uses Orthogonal Frequency Division Multiplexing to take the 5GHz frequency and split it up into multiple overlapping frequencies. In other words, OFDM can get many more times the data passing during one cycle. In some aspects, 802.11a passes data at a frequency of over 15GHz. As you can see in Figure 2.4, the signals overlap each other. The actual 5GHz frequency range is represented by the darker line. The lighter lines are the results of using OFDM to split the larger frequency into numerous smaller frequencies, each allowing its own data transmission. This not only speeds up data transmission, but also allows for multiple frequencies, and thus reduces collision with other wireless device transmissions. Note that there is only one half of the full frequency curve.
Figure 2.4. Example of OFDM signalling.