Wireless networks have become widespread and are found in both public and commercial settings. As a matter of fact, it is now possible to find wireless networks in many public spaces like coffee shops, malls, airports, and hotels. To that end, the entry level technician should know about the various wireless network components and their installation factors.
Wireless Network Components
Wireless networks are a little less complex than their wired counterparts. They require fewer components to operate properly. There are two main devices that can be found in a small wireless network: a wireless access point and a wireless NIC. In order to understand proper wireless network installation, you should understand the basics of these two components.
1.Wireless Access Points (WAPs)
For a majority of wired networks, there is a central component, like a hub or a switch, that connects the nodes together and allows them to communicate. Wireless networks are similar in that they have a component that connects all wireless devices together. That device is known as a wireless access point (WAP). Its function is to operate as a hub of sorts for the wireless devices.
It has at least one antenna (sometimes two for better reception) and a port to connect the wireless AP to a wired network. Figure 6.4 shows an example of a wireless access point.
A wireless access point
One way of thinking of a WAP is as a bridge between the wireless clients and the wired network. In fact, an WAP can be used as a wireless bridge (depending on the settings) to bridge two wired network segments together.
In addition to the stand-alone WAP, there is a WAP that includes a built-in router that can be used to connect both wired and wireless clients to the Internet. This device is usually known as a wireless router. Wireless routers usually act as Network Address Translation (NAT) servers by using the one ISP-provided global IP address to multiplex multiple local IP addresses (often handed out to inside clients by the wireless router from a pool in the 192.168.x.x range). Therefore, the subscriber need not change their service with the ISP in order to increase the number of devices that can simultaneously access the Internet.
Every station that wants to connect to a wireless network will need a wireless network interface card (NIC). In most respects, a wireless NIC does the same job as a traditional NIC, but instead of having a socket to plug some cable into, the wireless NIC will have a radio antenna. In addition to the different types of wireless networking (discussed in the next section), wireless NICs (like other NICs) can also differ in which type of connection they use to connect to the host computer.
A wireless NIC
There are wireless adapters that are not NICs. For example, Linksys makes an external USB wireless adapter for notebooks. It is not a NIC because it isn’t an expansion card (the C in NIC), so they are generally referred to as “adapters.” Additionally, NICs also come in the form of PC cards, generally for laptops, not just conventional expansion cards.
Wireless Antenna Characteristics
Wireless antennas act as both transmitters and receivers. There are two broad classes of antennas on the market, omni directional (Omni, or point-to-multipoint) and directional (Yagi or point-topoint). As a general rule, Yagi antennas have greater range than Omni antennas of equivalent gain because Yagis focus all their power in a single direction whereas Omnis must disperse the same power in all directions at once. The drawback of using a directional antenna, though, is that more care must be taken to align communication points, generally making Yagi a good choice only for point-to-point bridging of access points. Most WAPs use Omnis because clients and other APs could be in any direction at any given moment. A non-networking example of an Omni antenna is the FM antenna on your automobile. The orientation of your car does not affect the reception of the signal. The television aerials that some of us are old enough to remember rotating into a specific direction for a certain channel (how many of you labeled your set-top antenna dial for the actual TV stations you could receive?) are examples of Yagi antennas.
Omnis and Yagis are both rated according to their signal gain with respect to an actual or theoretical laboratory reference antenna. These ratings are relative indicators of the corresponding production antenna’s range. Range is also affected by the bit rate of the underlying technology, with higher bit rates extending shorter distances. Remember, a Yagi will always have a longer range than an equivalently rated Omni, but the straight-line Yagi will be limited in coverage area.
Manufacturers rate these antennas in units of decibel isotropic (dBi) or decibel dipole (dBd), based on the type of reference antenna (isotropic or dipole) of equivalent frequency operation used to rate the production antenna. A positive value for either unit of measure represents a gain in signal strength with respect to the reference antenna. Webster’s defines isotropic as “exhibiting properties (as velocity of light transmission) with the same values when measured along axes in all directions.” Isotropic antennas are not able to be produced in reality, but their properties can be engineered from antenna theory for reference purposes. As a practical example, consider Cisco Systems’s series of Aironet Access Point (indoor) and Bridge (outdoor) antennas. Table 6.1 illustrates the effect gain ratings and attempted bit rates have on range limitations.
The rule of thumb is that antennas operating with frequencies below 1GHz are measured in dBd while those operating above 1GHz are measured in dBi. As this is not always the case, you may find the need to compare the strength of one antenna, measured in dBd, with another, measured in numerically equivalent dBi, in order to determine which is stronger. That’s why it’s important to know that a particular numerical magnitude of dBd is more powerful than the same numerical magnitude of dBi. The good news is that the relationship between the two is linear, making the conversion quite simple. At the same operating frequency, a dipole antenna has about 2.2dB gain over a 0dBi theoretical isotropic antenna. Therefore, you can easily convert from dBd to dBi by adding 2.2 to the dBd rating. Conversely, subtract 2.2 from the dBi rating to produce the equivalent dBd rating.
Taking into account what you’ve learned about the difference between Omni and Yagi antennas and the difference between dBd and dBi gain ratings, you should be able to compare the relative range of transmission of one antenna with respect to another based on a combination ofthese characteristics. By way of example, the following four antenna ratings are given in relative order from greatest to least range:
7dBd Yagi (equivalent to a 9.2dBi Yagi)
7dBi Yagi (longer range than 7dBi Omni)
4.8dBd Omni (equivalent to a 7dBi Omni)
4.8dBi Omni (equivalent to a 2.6dBd Omni)
Wireless Network Installation
Now that you understand the basic components involved in a wireless network, it’s time to learn about their actual installation. Although we’ve stated earlier that wireless networks contain fewer components and are less complex, there are several major factors that figure into a wireless network installation:
1.Wireless LAN Standards
Although wireless LANs have been around for only a relatively short time (in networking terms), there are many standards that have been ratified that deal with them. The majority of the technology in use today for wireless LANs is based on the IEEE 802.11 series of standards, although a slightly misaligned niche market exists for infrared and Bluetooth networking as well. More suited to LAN networking than infrared, Bluetooth, and the original 802.11 standard, the three most commonly used 802.11 standards today are as follows:
All three of these wireless versions are technically subgroups of the 802.11 working group. Even though they are in the same group, they are fundamentally different, as you will see.
One type of wireless networking that doesn’t receive much attention is infrared wireless. Infrared wireless uses the same basic transmission method as many television remote controls, infrared technology. Infrared is used primarily for short distance, point-to-point communications, like those between a peripheral and a PC. The largest use of infrared wireless is for peripherals using the IrDA standard.
A little-known fact about infrared is that the original IEEE 802.11 wireless standard specified a somewhat limited baseband infrared medium in addition to the more common Direct Sequence Spread Spectrum (DSSS) and Frequency Hopping Spread Spectrum (FHSS) modulation techniques. IrDA stands for Infrared Data Association, which is the standards body that develops the IrDA standard for point-to-point, peer-to-peer communications over infrared radiation. Infrared equipment that uses the IrDA standard can be found in many places, including cell phones, handheld PDAs and computers, keyboards, and so on.
The standard specifies a data transmission rate of 16Mbps (that will soon be increased to over 100Mbps with updates to the standard) and a maximum range of about 1 meter (1m). As you can see, although it possesses significant throughput, the range is lacking for a wireless LAN standard for large LANs.
One of the newest wireless standards is the wireless networking standard known as Bluetooth. It was designed to replace the myriad cords on an average computer user’s desk. Cords for things like keyboards, mice, and headphones can all be eliminated. The standard allows for these many different types of peripherals to all be able to communicate wirelessly with a host device, like a computer. For example, a popular Bluetooth accessory is the wireless headset for cellular phones. It’s battery powered and will communicate directly with the phone wirelessly.
Bluetooth has a total maximum throughput of 1Mbps. It isn’t a speed demon as far as throughput is concerned, but it is still more than enough for peripheral communications like mice, keyboards, and headphones, and it is possible for two Bluetooth devices to network to each other in a peer-to-peer fashion. But, as with infrared, it is impractical to build an entire multistation wireless LAN using the Bluetooth technology.
The original 802.11 standard specified a somewhat impractical recommendation, in terms of data rates, with regard to the bandwidth-hungry mentality of its contemporary LANs. In 1997, IEEE specified what is now referred to as 802.11-1997, a wireless LAN standard with a bandwidth of 2Mbps (with the ability to fall back to 1Mbps in noisy environments) when using DSSS modulation and a bandwidth of 1Mbps when using FHSS modulation. Even when using FHSS, the standard allows for possible 2Mbps operation in environments in which the noise level is below an acceptable threshold. Both the DSSS and FHSS methods operate in the unlicensed 2.4GHz frequency range. 802.11-1997 has since been updated by 802.11-1999, the supplements to which have given rise to the newer, more common standards of 802.11a, 802.11b, and 802.11g.
The IEEE 802.11a standard is an extension to the IEEE 802.11 standard that specifies a wireless radio frequency LAN technology that provides for up to 54Mbps of available throughput.It uses the 5GHz radio frequencies (regulated) and OFDM for data encoding. It has a maximum range of 250ft (76m) indoors and approximately 1000ft (305m) outdoors. The IEEE 802.11a standard was released at approximately the same time as 802.11b. However, 802.11b received more attention because 802.11a equipment was released approximately two years after the introduction of the 802.11b equipment and because of 802.11b’s lower equipment cost. Plus, 802.11a has shorter range due to its higher frequency (higher frequencies attenuate sooner), and also due to the higher frequency, its signal is interfered with more easily. But, on the plus side, because it uses regulated frequencies, there is less chance of standard devices like microwaves and such interfering with the wireless signal.
The IEEE 802.11b standard has been given credit for the explosion of wireless networking. The equipment is cheap (and getting cheaper) and provides for decent network access speeds. It’s easy to set up and use and is readily available. 802.11a and 802.11b were created at approximately the same time, but the 11b standard got the spotlight as the preferred LAN standard (primarily because of cost and the late introduction of 11a equipment).
The IEEE 802.11b standard specifies a wireless radio frequency LAN technology that provides for up to 11Mbps of available throughput. It uses the 2.4GHz radio frequencies (unregulated) and Direct Sequence Spread Spectrum (DSSS) for data encoding. It has a maximum range of 300ft (91m) indoors and about 1500ft (457m) outdoors. Even though they are subsets of the same standard, IEEE 802.11a and 802.11b are incompatible.
The most recent player in the 802.11 standards game is the IEEE 802.11g standard. It is kind of a “best of both worlds” standard. It includes the high data rate (54Mbps) of 802.11a with the stability and wide product base of 802.11b. Plus, it is backward compatible with 802.11b (alas, not so with 802.11a).
The IEEE 802.11g standard specifies a wireless radio frequency LAN technology that provides for up to 54Mbps of available throughput. It uses the 2.4GHz radio frequencies (unregulated) and both DSSS and OFDM for data encoding. It has a maximum range of 300ft (91m) indoors and about 1500ft (457m) outdoors
It is important to note that most 802.11g devices are compatible with 802.11b devices. For example, a 802.11b NIC will work with an 802.11g access point (at the lower, 802.11b speed, of course) and vice versa.
Let’s say you just bought a wireless NIC for your laptop and a WAP. What can you do with them? Well, that all depends on the type of installation you are going to do with these devices. There are two major installation types: ad-hoc and infrastructure mode. Each 802.11 wireless network device is capable of being installed in one of these two modes.
The simplest installation type for wireless 802.11 devices is ad-hoc mode. In this mode, the wireless NICs (or other devices) can communicate directly without the need for a WAP. A good example of this is two laptops with wireless NICs installed. If both cards were set up for ad-hoc mode, they could connect and transfer files (assuming the other network settings, such as protocols, were set up correctly).
To set up a basic ad-hoc wireless network, all you need are two wireless NICs and two computers. Install the cards into the computers according to the manufacturer’s directions. During the installation of the software, you will be asked at some point if you want to set up the NIC in ad-hoc mode or infrastructure mode. For an ad-hoc network, choose the ad-hoc mode setting. Then bring the computers within range (90–100m) of each other.The computers will “see” each other and you will be able to connect to each other. In order to transfer files, both computers will need to have security settings that will allow it.
The most common use for wireless networking equipment is to provide the wireless equivalent of a wired network. To do this, all 802.11 wireless equipment has the ability to operate in what is known as infrastructure mode. In this mode, NICs will only communicate with an access point (instead of each other as in ad-hoc mode). The access point will facilitate communication between the wireless nodes as well as communication with a wired network (if present). In this mode, wireless clients appear to the rest of the network as standard, wired nodes.
A wireless network in infrastructure mode
When configuring a client for wireless infrastructure mode, you need to understand a couple of basic wireless concepts: SSID and security. The SSID (short for Security Set Identifier) is the unique 32-character identifier that represents a particular wireless network. All devices participating in a particular wireless network must be configured with the same SSID. If a wireless network is to have more than one access point that provides access to the same wireless network, the access points must all have the exact same SSID.
Multiple access points with the same SSID spread over a large area allow a user to move around that area while maintaining a connection to the wireless network. This process is called roaming. Because most access points are configured by default to broadcast their SSID so wireless clients can browse and find them, and because wireless signals can travel long distances (even outside of a building), security is extremely important on wireless LANs. To that end, most access points have one or more of the following security measures in place:
WEP Short for Wired Equivalent Privacy, this protocol, when enabled, requires that both access point and workstation are configured with the same 64-bit, 128-bit, 152-bit, or 256-bit encryption key in order to communicate. This key is manually configured by the network administrator and usually comprises a string of alphanumeric or hexadecimal characters. You may also see WEP referred to as the Wired Equivalency Protocol, although that is not the original term for which the acronym was used.
MAC List Some WAPs are capable of restricting which clients can connect to the AP by keeping track of authorized MAC addresses. The administrator configures the AP with the list of all the MAC addresses of wireless NICs that are authorized to connect to that AP. If a NIC with a MAC address not on the AP’s MAC list tries to connect, it will be rejected.
Disabling SSID Broadcast By default, WAPs broadcast their SSID to make it easier for clients to find them. For example, Windows XP has a built-in utility that allows users to browse for WAPs. However, you can turn this feature off. You then must configure each client with the SSID of the WAP that client will connect to.
Another factor to consider when installing a wireless network is signal degradation. Because the 802.11 wireless protocols use radio frequencies, the signal strength varies according to many factors. The weaker the signal, the less reliable the network connection will be, and thus the less usable as well. These factors are included in the following list:
Distance This one should be fairly obvious. The farther away from the WAP you get, the weaker the signal. Most APs have a very limited maximum range (less than 100m for most systems). To some degree, this can be extended using amplifiers or repeaters or using different antennas.
Walls The more walls a wireless signal has to pass through, the more attenuated (reduced) the signal becomes. Also, the thicker the wall, the more it interrupts the signal. In an indoor office area with lots of walls, the range of wireless could be a low as 25m.
Protocols Used Another factor that determines the range of wireless LAN is the protocol used. The various wireless 802.11 protocols have different maximum ranges. As discussed earlier in Table 6.2, you can see that the maximum effective range varies with the 802.11 protocol used.
Interference The final factor that affects wireless performance is outside interference. Since 802.11 wireless protocols operate in the 900 MHz, 2.4GHz, 5GHz range, interference can come from several sources, including other wireless devices, such as Bluetooth, cordless telephones, microwave ovens (a huge adversary of 802.11b and 802.11g), cell phones, other wireless LANs, and any other device that transmits radio frequency (RF) near the frequency bands that the 802.11 protocols use.
The installation of 802.11 equipment is fairly simple. There are really two main types of components in 802.11 networks: WAPs and NICs. Wireless NIC installation is just like installing any other network card (which you will learn later in this chapter). But, once it’s installed, you must connect to a WAP.
WAP installation is fairly simple as well. Take it out of the box, connect the antenna(e), if necessary, and power, and place the WAP where it can reach the most clients. This last part is probably the trickiest. You must place the WAP in such a way that it is servicing the most clients. This will involve a little common sense and a little trial and error. Knowing that walls obstruct the signal, wide open spaces are better indoors. Also, it should be placed away from sources of RF interference, so right near all the other office equipment is probably not the best place for an AP. You might have to move the AP around a bit to get the most signal strength for all the clients that need to use it.
Once you have the hardware installed, it is time to configure it properly.
Now that you have both the AP and NIC installed, you must configure them to work together. This isn’t as tricky as it sounds. Most wireless equipment is designed to work almost without configuration. The only things you need to configure are customization settings (name, network address, etc.) and security settings.
Windows XP includes software to automatically configure a wireless connection and it installs this software automatically when you install a wireless NIC. The first time you reboot after the installation of the NIC, you will see a screen like the one shown in Figure 6.8. This is the Windows wireless configuration screen. From this screen, you can see any available wireless networks and configure how a computer connects to them. You can also configure several of the properties for how this wireless NIC connects to a particular wireless network:
Use Windows to Configure My Wireless Settings This check box determines whether or not Windows XP will configure the wireless settings. When it’s unchecked, Windows XP will need an external program to configure how it connects to a wireless network, as is the case with some wireless NICs that have their own software program for this purpose. It is usually best to let Windows XP manage your wireless settings.
Available Networks This list shows of all the wireless networks within range. The networks are listed by their SSID. From this list, you can choose which network you wish to connect to, and you can configure how your workstation connects by clicking the Configure button. If you don’t see the wireless network you are looking for, and you are in range, click the Refresh button.
Preferred Networks This list details any wireless networks you have connected to before and want to connect to again automatically. If there is more than one wireless network in range, this list determines the order in which the workstation will try to connect to them. You can change this order using the Move Up and Move Down buttons.
In addition to the general configuration, you may have to configure the encryption for the connection (if the wireless connection you are using requires it). To set up how your workstation uses encryption for a particular connection, click the SSID of the wireless network you want to configure, and then click Configure.
From this screen, you can configure several parameters for the specific connection:
Network Name If for some reason the SSID of the WAP changes, you can change the name of the WAP you are connecting to in this field. Just delete the old one and type in the new name.
Wireless Network Key (WEP) This section contains all the parameters for configuring encryption for this connection. If the network you are connecting to uses WEP encryption, this is the section where you will click the check boxes and configure how the wireless connection uses WEP, the key it uses, and what type of key it is. The following parameters are in this section:
Data Encryption (WEP Enabled) If the network uses a key to encrypt data sent over the network, you should make sure this box is checked (it is checked by default). You will then need to specify the key in the box labeled Network Key. You will also need to specify what type of key it is (ASCII or hex) by selecting the appropriate item from the drop-down list.
Network Authentication (Shared Mode) If your WAP uses shared mode authentication, you must check this box to ensure that your workstation will authenticate to the WAP using the shared key. Often, the key is provided automatically by the WAP during the response to the initial request. If this is the case, you must check the checkbox labeled The Key Is Provided for Me Automatically (the default). Otherwise, uncheck it and enter the key and related information in the appropriate boxes.
This Computer Is a Computer-to-Computer (Ad Hoc) Network Check this check box if you are connecting to another computer instead of an access point.
Once you have changed any settings you need to, click OK to save the changes and finish the configuration.
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