2.2 Understanding 802.11 Speak

In this section, I discuss some of the terminologies that you will often come across when you set up your wireless network.

2.2.1 WEP

By default, encryption is not enabled for wireless networks. Encryption is important because malicious hackers equipped with the necessary software can sniff the packets transmitted by the wireless network, thereby compromising your data. Wired Equivalent Privacy (WEP) is a protocol used for encrypting packets on a wireless network. It uses a 64-bit (or 256-bit, depending on the equipment) shared key algorithm. Although it is far from perfect, WEP increases the protection of your data, but in doing so reduces your effective data rates.

Chapter 4 talks about WEP in more detail. More secure techniques used for securing wireless networks such as WPA (Wi-Fi Protected Access) and 802.1X are discussed there.

2.2.2 SSID

The Service Set Identifier (SSID) acts as a name for a wireless network. All devices participating in a particular wireless network must specify this SSID. The wireless devices will not be able to participate in this network if the SSID is not specified (or not stated correctly). For example, Linksys products use the default SSID "linksys" (D-Link products use "default"). If you are concerned about unauthorized users connecting to your access point, you should change this to something else and disable SSID broadcast in the access point's configuration (see Chapter 4). This makes it harder for unauthorized users to find your access point. Use WEP, or if your hardware supports it, 802.1X to allow only authorized users to connect.

2.2.3 Channels

The 802.11b standard defines 14 channels. A channel is a particular frequency that is selected so that a Wi-Fi adapter and an access point can communicate at an agreed frequency. Table 2-3 list the frequencies of the 14 channels.

Table 2-3. The frequencies for the 14 channels in 802.11b


Frequency (GHz)





























Not all channels are used universally. In the U.S., only channels 1 to 11 are allowed. In Europe, an additional two channels are allowed, so channels 1 to 13 are usable. In Japan, all fourteen channels are used.

A channel represents the center frequency used by both the radio card and the wireless access point. Each channel occupies 25 MHz of the spectrum (for example, channel 1 ranges from 2.3995 GHz to 2.4245 GHz, a range of 0.025 GHz, or 25 MHz). But this range overlaps with other channels, particularly channels 2 and 3. If you use multiple access points within your network, you need to set the channels of each access point so that they do not overlap, or else they will cause interferences. Figure 2-4 illustrates that the frequency difference between each channel is 5 MHz, but each channel width is 25 MHz. So effectively, to avoid interference, you should use channels 1, 6, and 11 if you are using multiple access points that are within range of each other.

Figure 2-4. The nonoverlapping channels in 802.11b

In infrastructure mode, the access point determines the channel (the access point will come preconfigured to use a particular channel, commonly channel 6); clients are not required to configure the channel number, since they will automatically select the correct channel needed to connect to the access point. For ad-hoc mode, the clients have to select the channel to be used.

In contrast with 802.11b and 802.11g (which has the same three nonoverlapping channels limitation), 802.11a has a total of eight nonoverlapping channels that are 20 MHz wide. Each of these channels is subdivided into 52 subcarriers. And each channel is approximately 300 KHz wide.

2.2.4 Link Quality and Signal Strength

Most access points indicate the link quality and signal strength of a wireless network. Link quality is a measure of the ability of the client to communicate with the access point. The signal strength indicator indicates the signals of all received packets. In general, the higher the link quality and signal strength, the better throughput you will get from the wireless network.

2.2.5 MAC Address

In order to uniquely identify each node on the network, every network adapter is assigned a hardware address known as the Media Access Control (MAC) address. This address is assigned by the manufacturer and stored in the card's firmware. A MAC address is a 48-bit number that uniquely identifies a network device. An Ethernet card has a MAC address, as does an 802.11 wireless card. A wireless access point also has a MAC address. For an explanation of how MAC addresses relate to IP addresses, see Chapter 1.

Locking MAC Addresses

Some ISPs use a technique called MAC address locking to prevent unauthorized access to the network. This is how it works: when you subscribe to a broadband service, the ISP will enter the MAC address of your modem into their database. When your modem requests an IP address, the ISP's router checks the MAC address of your modem against its database. If it is not found, no IP address will be issued and you will not be able to get onto the network. Some ISPs also lock the MAC address of your network adapter.

A wireless router has two MAC addresses ? one for the local network and one for the WAN (the ISP's network that is connected to the Internet). For those ISPs that lock your network adapter's MAC address, you may have problems in using a wireless router to connect to the Internet. In this case, you can try one of the following:

  • In some cases, the MAC address locking is done dynamically. The cable or DSL modem remembers the first MAC address it encounters and won't let any more machines connect unless you reboot it. So, try power-cycling the modem and then doing the same with your access point.

  • Give your ISP a call and tell them the MAC address of your wireless access point. Most ISPs are friendly to home networking, but be sure to check the terms of your service agreement before you call.

  • Use the Clone MAC address feature, available in most wireless routers, to replace the MAC address of the wireless router with that of the network adapter (whose MAC address is locked by the ISP). All devices connected to the wireless router will now use that particular MAC address to connect to the Internet.

In Windows XP, to find the MAC address of a network device, simply use the ipconfig command:

c:\> ipconfig /all

You should see something like the following:

Ethernet adapter Wireless Network Connection 9:

        Connection-specific DNS Suffix  . : ict.np.edu.sg
        Description . . . . . . . . . . . : Cisco Systems 350 Series
                                            Wireless LAN Adapter
        Physical Address. . . . . . . . . : 00-40-96-40-7E-F9
        Dhcp Enabled. . . . . . . . . . . : Yes
        Autoconfiguration Enabled . . . . : Yes
        IP Address. . . . . . . . . . . . :
        Subnet Mask . . . . . . . . . . . :
        Default Gateway . . . . . . . . . :
        DHCP Server . . . . . . . . . . . :
        DNS Servers . . . . . . . . . . . :
        Lease Obtained. . . . . . . . . . : Wednesday, April 30, 2003 
                                            3:05:37 AM

        Lease Expires . . . . . . . . . . : Wednesday, May 07, 2003 
                                            3:05:37 AM

Most wireless cards also have their MAC address printed on the card itself. A typical MAC address looks like this: 00-40-96-40-7E-F9.