Wireless Communications 159 complements the Cisco Aironet 1200 Series 802.11a Access Point, providing a solution that combines performance and mobility with the security and manageability that enterprises require. It will be possible to achieve data rates of greater than 20 Mbps in this frequency range. The drawback of the 5-GHz frequency, however, is its limited range. The typical range for 5 GHz inside is about 50 feet; outside poses a limitation of approximately 2500 feet. Spread-Spectrum Technology Just as the radio in your car has AM and FM bands, other radios use certain bands, frequencies, and types of modulation. Spread spectrum (SS) is a modulation technique developed in the 1940s that spreads a transmission signal over a broad band of radio frequencies. The term spread spectrum describes a modulation technique that sacrifices bandwidth to gain signal-to-noise performance. This technique is ideal for data com- munications because it is less susceptible to radio noise and creates little interference. Spread spectrum, as illustrated in Figure 3-42, is a system in which the transmitted sig- nal is spread over a frequency much wider than the minimum bandwidth required to send the signal. The fundamental premise is that in channels with narrowband inter- ference increasing the transmitted signal bandwidth results in an increased probability that the received information is correct. Figure 3-42 Spread-Spectrum Technology To use the unlicensed radio bands, you have to use spread-spectrum techniques. Fre- quency-hopping spread spectrum (FHSS) and direct-sequence spread spectrum (DSSS) are two ways of doing spread spectrum. These spread-spectrum techniques spread the RF energy over the available band. The next sections describe FHSS and DSSS in more detail. FHSS Versus DSSS As modulation techniques, both frequency-hopping spread spectrum (FHSS) and direct-sequence spread spectrum (DSSS) have advantages and limitations. NOTE Narrowband interfer- ence occurs when two signals are broadcast- ing at the same fre- quency in the same geographic area. The term band refers to a grouping of fre- quencies; narrow- band would mean a relatively smaller range of frequencies. Narrowband noise might disrupt certain channels or spread- spectrum components. Narrowband Information Signal (Before Spreading) Spread-Spectrum Signal (After Spreading) Power Frequency 1102.book Page 159 Tuesday, May 20, 2003 2:53 PM 160 Chapter 3: Networking Media With FHSS technology, transmissions hop from one frequency to another in random pat- terns. Figure 3-43 illustrates an example of a FHSS. In this example, the transmission hops from C (2.42 GHz), to A (2.40 GHz), to D (2.43 GHz), then to B (2.41 GHz), and finally to E (2.44 GHz). This technique enables the transmissions to hop around narrow- band interference, resulting in a clearer signal and higher reliability of the transmission. However, FHSS technology is slower, and the receiver must use the same pattern to decode. Figure 3-43 Frequency-Hopping Spread Spectrum DSSS technology transmissions, as illustrated in Figure 3-44, are more reliable because each bit (1 or 0) is represented by a string of 1s and 0s called a chipping sequence. Even if up to 40 percent of the string is lost, the original transmission can be reconstructed. DSSS technology also enables high throughput of data and longer-range access. Figure 3-44 Direct-Sequence Spread Spectrum Time 5 4 3 2 1 Hopping Pattern: C A D B E Freq. (GHz) 2.40 2.41 2.42 2.43 2.44 2.45 B A E D C Time Frequency (GHz) 1 5 4 3 2 2.40 2.41 2.42 2.43 2.44 1102.book Page 160 Tuesday, May 20, 2003 2:53 PM Wireless Networking 161 Limited to a 2-Mbps data-transfer rate, FHSS is recommended for only very specific applications such as for certain types of watercraft. For all other wireless LAN applica- tions, DSSS is the better choice. The recently released evolution of the IEEE standard, 802.11b, provides for a full Ethernet-like data rate of 11 Mbps over DSSS. FHSS does not support data rates greater than 2 Mbps. Wireless Networking When the computer was first introduced to the world, it was affordable by only large corporations, governments, and universities. From the first building-sized devices with minimal computing power to those that fit in the palm of a person’s hand, huge leaps in technology have occurred. The same is true on the connectivity side of the industry. The various types of networking discussed earlier in this chapter have all involved physical connectivity. The advantages are speed, reliability, and to a certain extent con- venience. Physical connectivity allows an increase in productivity by allowing the shar- ing of printers, servers, and software. However, networked systems require that the workstation remain stationary, permitting moves only within the limits of the media and office area. The introduction of wireless technology removes these restraints and brings true port- ability to the computing world. While the current state of wireless technology does not provide the high-speed transfers of cabled networks nor the security and uptime reli- ability, the flexibility justifies the trade off. When considering the installation of a network in an existing facility, wireless is at the top of many an administrator’s lists of options. A simple wireless network can be up and running in just a few minutes after the workstations are turned on. Connectivity to the Internet is provided through a wired connection, router, cable modem, or Digital Subscriber Line (DSL) modem, and a wireless access point that acts as a hub for the wireless nodes. In a residential or small office environment these devices might be com- bined into a single unit. Wireless LAN Organization and Standards An understanding of the regulations and standards that apply to wireless technology ensures that deployed networks are interoperable and in compliance. Just as in cabled networks, IEEE is the prime issuer of standards for wireless networks. The standards have been created within the framework of the regulations set forth by the FCC. A key technology contained within the IEEE 802.11 standard is DSSS. DSSS applies to wireless devices operating within a 1 to 2 Mbps range. A DSSS system can operate at 1102.book Page 161 Tuesday, May 20, 2003 2:53 PM 162 Chapter 3: Networking Media up to 11 Mbps but is not considered compliant above 2 Mbps. The next standard approved was IEEE 802.11b, which increased transmission capabilities to 11 Mbps. Even though DSSS WLANs are able to interoperate with the FHSS WLANs, problems developed prompting design changes by the manufacturers. In this case, IEEE’s task was simply to create a standard that matched the manufacturer’s solution. IEEE 802.11b, called Wi-Fi or high-speed wireless, refers to DSSS systems that operate at 1, 2, 5.5, and 11 Mbps. All 802.11b systems are backward-compliant in that they also support 802.11 for 1- and 2-Mbps data rates for DSSS only. This backward com- patibility is extremely important because it allows upgrading of the wireless network without replacing the network interface cards (NICs) or access points. IEEE 802.11b devices achieve the higher data throughput rate by using a different cod- ing technique from 802.11, allowing for a greater amount of data to be transferred in the same time frame. The majority of 802.11b devices still fail to match the 10 Mbps throughput of wired Ethernet and generally function in the 2–4 Mbps range. 802.11a covers WLAN devices operating in the 5-GHz transmission band. Using the 5-GHz range disallows interoperability of 802.11b devices as they operate within 2.4 GHz. 802.11a is capable of supplying data throughput of 54 Mpbs and with pro- prietary technology known as rate doubling has achieved 108 Mbps. In production networks a more standard rating is 20 to 26 Mbps. 802.11g provides the same throughout as 802.11a but with backwards compatibility for 802.11g devices using Othogonal Frequency Division Multiplexing (OFDM) mod- ulation technology. Cisco has developed an access point that permits 802.11b and 802.11a devices to coexist on the same WLAN. The access point supplies gateway services allowing these otherwise incompatible devices to communicate. Wireless Devices and Topologies A wireless network can consist of as few as two devices, two nodes with wireless NICs. Figure 3-45 shows an internal wireless NIC, and Figure 3-46 shows an external USB wireless NIC. The nodes can be desktop workstations or notebook computers. Equipped with wireless NICs, an ad hoc network can be established that equates to a peer-to- peer wired network. Both devices act as servers and clients in this environment, and although it does provide connectivity, security is at a minimum along with throughput. Another problem with this type of network is compatibility; oftentimes, NICs from different manufacturers do not interoperate. 1102.book Page 162 Tuesday, May 20, 2003 2:53 PM Wireless Networking 163 Figure 3-45 Internal Wireless NIC Figure 3-46 External USB Wireless NIC 1102.book Page 163 Tuesday, May 20, 2003 2:53 PM 164 Chapter 3: Networking Media More commonly, an access point (AP), as shown in Figure 3-47, is installed acting as a central hub for the WLAN infrastructure mode. The AP is hard wired to the cabled LAN to provide Internet access and connectivity to the wired network. APs are equipped with antennae and provide wireless connectivity over a specified area referred to as a cell. Figure 3-47 Access Point Depending on the structural composition of the location in which the AP is installed and the size and gain of the antennae, the size of the cell can range from a few dozen feet to 25 miles. More commonly the range is from 300 to 500 feet. To service larger areas multiple APs can be installed with a degree of overlap, permitting roaming between cells, as illustrated in Figure 3-48. This roaming is very similar to the services provided by cellular phone companies. Overlap on multiple AP networks is critical to allow for movement of devices within the WLAN, and although it is not addressed in the IEEE standards, a 20–30 percent overlap is desirable. This rate of overlap permits roaming between cells, allowing for the disconnect/reconnect activity to occur seam- lessly without service interruption. When a client is activated within the WLAN, it starts listening for a compatible device with which to associate. This process is referred to as scanning and can be active or passive. Active scanning causes a probe request to be sent from the wireless node seeking to join the network. The probe request contains the Service Set Identifier (SSID) of the network it wants to join. When an AP with the same SSID is found, the AP issues a probe response, and the authentication and association steps are completed. 1102.book Page 164 Tuesday, May 20, 2003 2:53 PM Wireless Networking 165 Figure 3-48 Roaming Passive scanning nodes listen for beacon management frames (beacons), which are transmitted by the AP (infrastructure mode) or peer nodes (ad hoc). When a node receives a beacon that contains the SSID of the network it is trying to join, an attempt is made to join the network. Passive scanning is a continuous process and nodes can associate or disassociate with APs as signal strength changes. How Wireless LANs Communicate After establishing connectivity to the WLAN, a node passes frames similarly to any other 802 network. WLANs do not use a standard 802.3 frame. Therefore, using the term wireless Ethernet is misleading. There are three types of frames: control, manage- ment, and data. The following lists the frames that are included in each type of frame: ■ Management frames — Association request frame — Association response frame — Probe request frame — Probe response frame — Beacon frame — Authentication frame 1102.book Page 165 Tuesday, May 20, 2003 2:53 PM 166 Chapter 3: Networking Media ■ Control frames — Request to send (RTS) — Clear to send (CTS) — Acknowledgment ■ Data frames Only the data frame type is similar to 802.3 frames. However, the payload of wireless and 802.3 frames is 1500 bytes, and an Ethernet frame cannot exceed 1518 bytes. On the other hand, a wireless frame can be as large as 2346 bytes. Usually the WLAN frame size is limited to 1518 bytes because it is most commonly connected to a wired Ether- net network. Because RF is a shared medium, collisions can occur just as they do on wired shared medium. The significant difference is that there is no method by which the source node is able to detect that a collision has occurred. In view of this, WLANs use carrier sense multiple access with collision avoidance (CSMA/CA). This feature is somewhat like Ethernet carrier sense multiple access collision detect (CSMA/CD). Chapter 5, “Ether- net Fundamentals,” discusses CSMA/CD in greater detail. When a source node sends a frame, the receiving node returns a positive acknowledg- ment (ACK); which can consequently cause consumption of 50 percent of the available bandwidth. This overhead, when combined with the collision avoidance protocol overhead, reduces the actual data throughput to a maximum of 5.0 to 5.5 Mbps on an IEEE 802.11b wireless LAN rated at 11 Mbps. Performance of the network will also be affected by signal strength and degradation in signal quality due to distance or interference. As the signal becomes weaker, Adaptive Rate Selection (ARS) can be invoked, and the transmitting unit drops the data rate from 11 Mbps to 5.5 Mbps, from 5.5 Mbps to 2 Mbps, or 2 Mbps to 1 Mbps, as illustrated in Figure 3-49. Authentication and Association WLAN authentication occurs at Layer 2 and is the process of authenticating the device, not the user. This point is a critical one to remember when considering WLAN secu- rity, troubleshooting, and overall management. Authentication might be a null process, as in the case of a new AP and NIC with default configurations in place. The client sends an authentication request frame to the AP, and the frame is accepted or rejected by the AP. The client is notified of either course of action via an authentication response frame. The AP might also be configured to hand off the authentication task to an authentication server, which performs a more thorough credentialing process. 1102.book Page 166 Tuesday, May 20, 2003 2:53 PM Wireless Networking 167 Figure 3-49 Adaptive Rate Selection Association, performed after authentication, is the state that permits a client to use the AP’s services to transfer data. Authentication and Association Types The authentication and association types are as follows: ■ Unauthenticated and unassociated—The node is disconnected from the network and not associated to an access point. ■ Authenticated and unassociated—The node has been authenticated on the net- work but has not yet associated with the access point. ■ Authenticated and associated—The node is connected to the network and able to transmit and receive data through the access point. Methods of Authentication IEEE 802.11 lists two types of authentication processes: ■ Open system—This process is an open connectivity standard in which only the SSID must match. It can be used in a secure or non-secure environment, although the ability of low-level network sniffers to ascertain the SSID of the WLAN is fairly high. 1102.book Page 167 Tuesday, May 20, 2003 2:53 PM 168 Chapter 3: Networking Media ■ Shared key—This process requires the use of Wired Equivalent Privacy (WEP) encryption. WEP is a fairly simple algorithm using 64- and 128-bit keys. The AP is configured with an encrypted key, and nodes attempting to access the network through the AP must have a matching key. Statically assigned WEP keys provide a higher level of security than the open system but are definitely not hack proof. The susceptibility to unauthorized entry into WLANs is being addressed by a number of emerging security solution technologies. The Radio Wave/Microwave Spectrum Computers send data signals electronically. Radio transmitters convert these electrical signals to radio waves. The radio waves are generated by changing electric currents in a transmitter’s antenna. These radio waves radiate out in straight lines from the antenna. However, radio waves weaken (attenuate) as they move out from the transmitting antenna. In a WLAN, a radio signal measured at a distance of just 10 meters (30 feet) from the transmitting antenna is only 1/100th of its original strength. Like light, radio waves can be absorbed by some materials and reflected by others. When passing from one material like air into another material like a plaster wall, radio waves are refracted (bent). Radio waves are also scattered and absorbed by water droplets in the air. These qualities of radio waves are important to remember when a WLAN is being planned for a building or for a campus. The process of evaluating a location for the installation of a WLAN is called making a site survey. Because radio signals weaken as they travel away from the transmitter, the receiver must also be equipped with an antenna. When radio waves hit a receiver’s antenna, weak electric currents are generated in that antenna. These electric currents, caused by the received radio waves, are equal to the currents that originally generated the radio waves in the transmitter’s antenna. The receiver amplifies the strength of these weak electrical signals. In a transmitter, the electrical (data) signals from a computer or a LAN are not sent directly into the transmitter’s antenna. Rather these data signals are used to alter a second, strong signal called the carrier signal. A receiver demodulates the carrier signal that arrives from its antenna. The receiver interprets the phase changes of the carrier signal and reconstructs from it the original electrical data signal. 1102.book Page 168 Tuesday, May 20, 2003 2:53 PM . E Freq. (GHz) 2. 40 2. 41 2. 42 2.43 2. 44 2. 45 B A E D C Time Frequency (GHz) 1 5 4 3 2 2.40 2. 41 2. 42 2.43 2. 44 11 02. book Page 16 0 Tuesday, May 20 , 20 03 2: 53 PM Wireless Networking 16 1 Limited to a 2- Mbps data-transfer rate, FHSS is recommended. interoperate. 11 02. book Page 16 2 Tuesday, May 20 , 20 03 2: 53 PM Wireless Networking 16 3 Figure 3-45 Internal Wireless NIC Figure 3-46 External USB Wireless NIC 11 02. book Page 16 3 Tuesday, May 20 , 20 03 2: 53. within a 1 to 2 Mbps range. A DSSS system can operate at 11 02. book Page 16 1 Tuesday, May 20 , 20 03 2: 53 PM 16 2 Chapter 3: Networking Media up to 11 Mbps but is not considered compliant above 2 Mbps.