Understanding Wireless Networking
No area of networking has seen as rapid an ascent as wireless networking has over the last few years. What used to be slow and unreliable is now fast and pretty stable, not to mention convenient. It seems like everywhere you go these days there are Internet cafés or fast food restaurants with wireless hotspots. Even many mobile phones sold today have Internet capabilities. No matter where you go, you’re likely just seconds away from being connected to the Grid.
The most common term you’ll hear thrown around referring to wireless networking today is WiFi. While the term was originally coined as a marketing name for 802.11b, it’s now used as a nicknamereferring to the family of IEEE 802.11 standards. That family comprises the primary wireless networking technology in use today, but there are other wireless technologies out there too. You might hear about Bluetooth, cellular, infrared, or WiMax. Each of these standards has its strengths and weaknesses and fills a role in computing. The A+ exam covers only 802.11 though, so that’s what we’ll primarily focus on here.
As a technician, it will fall to you to provide users with access to the Grid. You must make sure that their computers and mobile devices can connect and they can get their email and that downtime is something that resides only in history books. To be able to make that a reality, you must understand as much as you can about networking and the topics discussed in the following sections, where we’ll take an in-depth look at the 802.11 standards. After that, we’ll spend some time on wireless security features as well.
802.11 Networking Standards
In the United States, wireless LAN (WLAN) standards are created and managed by the Institute of Electrical and Electronics Engineers (IEEE). The most commonly used WLAN standards used today are in the IEEE 802.11 family. Eventually, 802.11 will likely be made obsolete by newer standards such as 802.16 or 802.20, but that is some time off. IEEE 802.11 was ratified in 1997 and was the first standardized WLAN implementation. There are over 20 802.11 standards defined, but you will only see a few in common operation: 802.11a, b, g, and n. As mentioned in the introduction, there are several wireless technologies on the market, but 802.11 is the one currently best suited for WLANs.
In concept, an 802.11 network is similar to an Ethernet network, only wireless. At the center of Ethernet networks is a connectivity device such as a hub, switch, or router, and all computers connect to it. Wireless networks are configured in a similar fashion, except they use a wireless router or wireless access point instead of a wired connectivity device. In order to connect to the wireless hub or router,the client needs to know the service-set identifier (SSID) of the device. Wireless access points may connect to other wireless access points, but eventually they connect back to a wired connection with the rest of the network.
802.11 networks use the Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) access method instead of Ethernet’s Carrier Sense Multiple Access/Collision Detection (CSMA/CD). Packet collisions are generally avoided, but when they do happen, the sender will need to wait a random period of time (called a back-off time) before transmitting again.
Since the original 802.11 standard was published in 1997, there have been several upgrades and extensions of the standard released.
802.11
The original 802.11 standard defines WLANs transmitting at 1Mbps or 2Mbps bandwidths using the 2.4GHz frequency spectrum and using either frequency-hopping spread spectrum (FHSS) or direct-sequence spread spectrum (DSSS) for data encoding.
802.11a
The 802.11a standard provides WLAN bandwidth of up to 54Mbps in the 5GHz frequency spectrum. The 802.11a standard also uses a more efficient encoding system, orthogonal frequency division multiplexing (OFDM), rather than FHSS or DSSS.
This standard was ratified in 1999, but devices didn’t hit the market until 2001. Thanks to its encoding system, it was significantly faster than 802.11b (discussed next), but never gained widespread popularity. They were ratified as standards right around the same time, but 802.11b devices beat it to market and were significantly cheaper. Today you will see very few 802.11a devices out in the wild.
802.11b
The 802.11b standard was ratified in 1999 as well, but device makers were much quicker to market, making this standard the de facto wireless networking standard for several years. 802.11b provides for bandwidths of up to 11Mbps (with fallback rates of 5.5, 2, and 1Mbps) in the 2.4GHz range. The 802.11b standard uses DSSS for data encoding.
802.11g
Ratified in 2003, the 802.11g standard provides for bandwidths of 54Mbps in the 2.4GHz frequency spectrum using OFDM or DSSS encoding. Because it operates in the same frequency and can use the same modulation as 802.11b, the two standards are compatible. Because of the backward compatibility and speed upgrades, 802.11g is the most common standard you’ll see in use today.
As we mentioned, 802.11g devices are backward compatible with legacy 802.11b devices, and both can be used on the same network. That was initially a huge selling point for 802.11g hardware and helped it gain popularity very quickly. However, you should know that there are some interoperability concerns to be aware of. 802.11b devices are not capable of understanding OFDM transmissions; therefore, they are not able to tell when the 802.11g access point is free or busy. To counteract this problem, when an 802.11b device is associated with an 802.11g access point, the access point reverts back to DSSS modulation to provide backward compatibility. This means that all devices connected to that access point will run at a maximum of 11Mbps. To optimize performance, you should upgrade to all 802.11g devices and set the access point to G-only.
One additional concept you need to know about when working with 2.4GHz wireless networking is channels. We’ve said before that b/g works in the 2.4GHz range. Within this range, the FCC has defined 14 different 22MHz communication channels. An illustration of this is shown in Figure 8-1.
Although 14 channels have been defined, you’re only allowed to configure your wireless networking devices to the first 11. When you install a wireless access point and wireless NICs, they will all auto-configure their channel and this will probably work okay for you. If you are experiencing interference, changing the channel might help. And if you have multiple, overlapping wireless access points, you will need to have non-overlapping channels. (We’ll talk about this more in the section “Installing and Configuring SOHO Networks” later in this chapter.) The three non-overlapping channels are 1, 6, and 11.
Figure 8-1: 2.4GHz communication channels
802.11n
The most recent standard to hit the market is 802.11n, which was ratified in 2010. The standard claims to provide for bandwidth up to 600Mbps, which sounds pretty fast. It works in both the 2.4GHz and 5GHz ranges.
802.11n achieves faster throughput a couple of ways. Some of the enhancements include the use of 40MHz channels, multiple-input multiple-output (MIMO), and channel bonding. Remember how 802.11g uses 22MHz channels? 802.11n combines two channels to (basically) double the throughput. MIMO means using multiple antennas rather than a single antenna to communicate information. (802.11n devices can support up to eight antennas.) Channel bonding allows the device to simultaneously communicate at 2.4GHz and 5GHz and bond the data streams, which increases throughput.
One big advantage to 802.11n is that it is backward compatible with 802.11a/b/g. This is because 802.11n is capable of simultaneously servicing 802.11b/g/n clients operating in the 2.4GHz range as well as 802.11a/n clients operating in the 5GHz range.
Table 8-1 summarizes the 802.11 standards we discussed here.
Table 8-1: 802.11 standards
Also keep in mind that when discussing ranges, the further away from the WAP you get, the lower your connection speed will be. For example, to get 54Mbps out of your 802.11g router, you need to be within about 100 feet of it. At the far end of its range, your throughput will be only about 6Mbps.
Modulation Techniques
We have mentioned three signal modulation techniques used in the 802.11 standards. Here is how the three in common use today work:
The mathematics and theories of these transmission technologies are beyond the scope of this book and far beyond the scope of this exam.
802.11 Devices
If you think about a standard wired network and the devices required on such a network, you can easily determine what types of devices are available for 802.11 networks. Network cards come in a variety of shapes and sizes, including USB and PCMCIA Type II models and wireless print servers for your printers. As for connectivity devices, the most common are wireless routers (as shown in Figure 8-2) and a type of hub called a wireless access point (WAP). WAPs look nearly identical to wireless routers and provide central connectivity like wireless routers, but they don’t have nearly as many features. The main one most people worry about is Internet connection sharing. You can share an Internet connection using a wireless router but not with a WAP.
Figure 8-2: Wireless router
Most wireless routers and WAPs also have wired ports for RJ-45 connectors. The router shown in Figure 8-2 has four wired connections, but they are on the back side of the device (meaning you can’t see them in the figure).
Wireless Encryption Methods
The growth of wireless systems has created several opportunities for attackers. These systems are relatively new, they use well-established communications mechanisms, and they’re easily intercepted. Wireless controllers such as 802.11 routers use SSIDs to allow communications with a specific access point. The SSID is basically the network name. Because by default wireless routers will broadcast their SSID, all someone with a wireless client needs to do is search for an available signal. If it’s not secured, they can connect within a few seconds.
You can configure the router to not broadcast and then manually set up your clients with the SSID of the device. But using this type of SSID configuration doesn’t necessarily prevent your wireless network from being compromised.
A more effective way of securing your network is to use one of the several encryption methods available. Examples of these are WEP, WPA, and WPA2, which we discuss next.
WEP
Wired Equivalency Protocol (WEP) was one of the first security standards for wireless devices. WEP encrypts data to provide data security. It uses a static key; the client needs to know the right key to gain communication through a WEP-enabled device. The keys are commonly 10, 26, or 58 hexadecimal characters long.
The protocol has always been under scrutiny for not being as secure as initially intended. WEP is vulnerable due to the nature of static keys and weaknesses in the encryption algorithms. These weaknesses allow the algorithm to potentially be cracked in a very short amount of time—no more than two or three minutes. This makes WEP one of the more vulnerable protocols available for security.
Because of security weaknesses and the availability of newer protocols, WEP is not used widely. It’s still better than nothing though, and it does an adequate job of keeping casual snoops at bay. If you were setting up a home network and wanted a quick and easy security method, it works just fine.
WPA
WiFi Protected Access (WPA) is an improvement on WEP that was first available in 1999 but did not see widespread acceptance until around 2003. Once it became widely available, the WiFi Alliance recommended that networks no longer use WEP in favor of WPA.
This standard was the first to implement some of the features defined in the IEEE 802.11i security specification. Most notably among them was the use of the Temporal Key Integrity Protocol (TKIP). Whereas WEP used a static 40- or 128-bit key, TKIP uses a 128-bit dynamic per-packet key. It generates a new key for each packet sent. WPA also introduced message integrity checking.
When WPA was introduced to the market, it was intended to be a temporary solution to wireless security. The provisions of 802.11i had already been drafted, and a standard that employed all of the security recommendations was in development. The upgraded standard would eventually be known as WPA2.
WPA2
Even though their names might make you assume that WPA and WPA2 are very similar, they are quite different in structure. WiFi Protected Access 2 (WPA2) is a huge improvement over WEP and WPA. As mentioned earlier, it implements all of the required elements of the 802.11i security standard. Most notably, it uses Counter Mode CBC-MAC Protocol (CCMP), which is a protocol based on the Advanced Encryption Standard (AES) security algorithm. CCMP was created to address the shortcomings of TKIP, so consequently it’s much stronger than TKIP.
Since 2006, wireless devices have been required to support WPA2 to be certified as WiFi compliant. Of the wireless security options available today, it provides the strongest encryption and data protection.