Wireless LAN Technologies
802.11 and HiperLAN/2 are the most common standards for wireless LANs. Examine each of these standards.
The IEEE 802.11 standard specifies a common medium access control (MAC) and several physical layers for wireless LANs. The initial 802.11 standard became available in 1997, but wireless LANs didn't begin to proliferate on a large scale until 2001, when prices fell dramatically. The IEEE 802.11 working group actively continues to enhance the standard to improve the performance and security of wireless LANs.
802.11 MAC Layer
The 802.11 standard specifies a single MAC Layer, which provides a variety of functions that support the operation of 802.11-based wireless LANs. The MAC Layer manages and maintains communications between 802.11 stations (radio network cards and access points) by coordinating access to the shared air medium. Often viewed as the brains of the network, the 802.11 MAC Layer directs a particular 802.11 Physical layer, such as 802.11a, 802.11b, or 802.11g, to perform the tasks of sensing the medium, transmission, and receiving of 802.11 frames.
Before transmitting frames, a station must first gain access to the medium, which is a radio channel that stations share. The 802.11 standard defines two forms of medium access: distributed coordination function (DCF) and point coordination function (PCF). DCF is mandatory and based on the CSMA/CA (carrier sense multiple access with collision avoidance) protocol. With DCF, 802.11 stations contend for access and attempt to send frames when there is no other station transmitting. (See Figure 5-9.) If another station is sending a frame, stations wait until the channel is free.
Figure 5-9. DCF Offers a Distributed Form of Medium Access
As a condition to accessing the medium (see Figure 5-9), the MAC Layer checks the value of its network allocation vector (NAV), which is a counter resident at each station that represents the amount of time that the previous frame needs to be sent. The NAV must be 0 before a station can attempt to send a frame. Prior to transmitting a frame, a station calculates the amount of time necessary to send the frame based on the its length and data rate. The station places a value representing this time in the duration field in the header of the frame. When stations receive the frame, they examine this duration field value and use it as the basis for setting their corresponding NAVs. This process reserves the medium for the sending station.
An important aspect of the DCF is a random back-off timer that a station uses if it detects a busy medium. If the channel is in use, the station must wait a random period of time before attempting to access the medium again. This ensures that multiple stations wanting to send data don't transmit at the same time. The random delay causes stations to wait different periods of time and avoids all of them sensing the medium at exactly the same time, finding the channel idle, transmitting, and colliding with each other. The back-off timer significantly reduces the number of collisions and corresponding retransmissions, especially when the number of active users increases.
With radio-based LANs, a transmitting station can't listen for collisions while sending data, mainly because the station can't have its receiver on while transmitting the frame. As a result, the receiving station needs to send an acknowledgement (ACK) if it detects no errors in the received frame. If the sending station doesn't receive an ACK after a specified period of time, the sending station assumes that there was a collision (or RF interference) and retransmits the frame.
For supporting time-bounded delivery of data frames such as video, the 802.11 standard defines the optional PCF where the access point grants access to an individual station to the medium by polling the station during the contention-free period. Stations can't transmit frames unless the access point polls them first. The period of time for PCF-based data traffic (if enabled) occurs alternately between contention periods.
An access point polls stations according to a polling list, then switches to a contention period when stations use DCF. This process enables support for both synchronous and asynchronous modes of operation. No known wireless NICs or access points on the market today, however, implement PCF.
An issue with PCF is that not many vendors support it in their products. As a result, it's generally not an option available to users. Future products, however, might support PCF in order to offer quality of service (QoS) mechanisms.
The following sections summarize primary 802.11 MAC functions.
The 802.11 standard defines both passive and active scanning, whereby a radio NIC searches for access points. Passive scanning is mandatory where each NIC scans individual channels to find the best access-point signal. Periodically, access points broadcast a beacon, and the radio NIC receives these beacons while scanning and takes note of the corresponding signal strengths. The beacons contain information about the access point, including SSID and supported data rates. The radio NIC can use this information along with the signal strength to compare access points and decide on which one to use.
Optional active scanning is similar, except the radio NIC initiates the process by broadcasting a probe frame, and all access points within range respond with a probe response. Active scanning enables a radio NIC to receive immediate response from access points, without waiting for a beacon transmission. The issue, however, is that active scanning imposes additional overhead on the network because of the transmission of probe and corresponding response frames.
Stations set to ad hoc mode form are what the 802.11 standard refers to as an independent basic service set. In this mode, one of the stations always sends a beacon, which alerts new stations of the network presence. The responsibility of transmitting the beacon is based upon each station waiting for a beacon interval to expire and an additional random time. A station transmits a beacon if after the beacon interval and random time the station does not receive a beacon from another station. This distributes the responsibility for sending beacons among all stations.
Authentication is the process of proving identity, and the 802.11 standard specifies two forms: open system authentication and shared key authentication. Open system authentication is mandatory, and it's a two-step process. A radio NIC initiates the process by sending an authentication request frame to the access point. The access point replies with an authentication response frame containing approval or disapproval of authentication indicated in the status code field in the frame body.
Shared key authentication is an optional four-step process that bases authentication on whether the authenticating device has the correct WEP key. The radio NIC starts by sending an authentication request frame to the access point. The access point then places challenge text into the frame body of a response frame and sends it to the radio NIC. The radio NIC uses its WEP key to encrypt the challenge text and then sends it back to the access point in another authentication frame. The access point decrypts the challenge text and compares it to the initial text. If the text is equivalent, the access point assumes that the radio NIC has the correct key. The access point finishes the sequence by sending an authentication frame to the radio NIC with the approval or disapproval. Many hackers know how to break through shared key authentication, however, so it's not a good idea to depend on it for a high level of security.
When authenticated, the radio NIC must associate with the access point before sending data frames. Association is necessary to synchronize the radio NIC and access point with important information, such as supported data rates. The radio NIC initiates the association by sending an association request frame containing elements such as SSID and supported data rates. The access point responds by sending an association response frame containing an association ID along with other information regarding the access point. Once the radio NIC and access point complete the association process, they can send data frames to each other.
With the optional WEP enabled, the wireless NIC encrypts the body (not the header) of each frame before transmission using a common key; and the receiving station decrypts the frame upon receipt using the common key. The 802.11 standard does not specify a key distribution method, which makes 802.11 wireless LANs vulnerable to eavesdroppers. The 802.11i version of the standard, however, is improving 802.11 security by incorporating 802.1x and stronger encryption into the standard.
The optional request-to-send and clear-to-send (RTS/CTS) function allows the access point to control use of the medium for stations activating RTS/CTS. With most radio NICs, users can set a maximum frame-length threshold for when the radio NIC activates RTS/CTS. For example, a frame length of 1,000 bytes triggers RTS/CTS for all frames larger than 1,000 bytes. The use of RTS/CTS alleviates hidden node problems (where two or more radio NICs can't hear each other and they are associated with the same access point).
If the radio NIC activates RTS/CTS, it first sends an RTS frame to an access point before sending a data frame. The access point then responds with a CTS frame, indicating that the radio NIC can send the data frame. With the CTS frame, the access point provides a value in the duration field of the frame header that holds off other stations from transmitting until after the radio NIC initiating the RTS can send its data frame. This avoids collisions between hidden nodes. The RTS/CTS handshake continues for each frame, as long as the frame size exceeds the threshold set in the corresponding radio NIC.
Power Save Mode
The optional power save mode that a user can turn on enables the radio NIC to conserve battery power when there is no need to send data. With power save mode on, the radio NIC indicates its desire to enter a sleep state to the access point through a status bit located in the header of each frame. The access point takes note of each radio NIC wanting to enter power save mode and buffers packets corresponding to the sleeping station.
In order to still receive data frames, the sleeping NIC must wake up periodically (at the right time) to receive regular beacon transmissions coming from the access point. These beacons identify whether sleeping stations have frames buffered at the access point and are waiting for delivery to their respective destinations. The radio NICs having awaiting frames will request them from the access point. After receiving the frames, the radio NIC can go back to sleep.
The optional fragmentation function enables an 802.11 station to divide data packets into smaller frames. This is to avoid needing to retransmit large frames in the presence of RF interference. The bit errors resulting from RF interference are likely to affect a single frame, and it requires less overhead to retransmit a smaller frame rather than a larger one. As with RTS/CTS, users can set a maximum frame-length threshold for when the radio NIC activates fragmentation. If the frame size is larger than the threshold, the radio NIC breaks the packet into multiple frames, with each frame no larger than the threshold value.
802.11 Physical Layers
Several 802.11 Physical layers satisfy a variety of application requirements. The following sections provide a summary of each of the 802.11 Physical layers.
The initial 802.11 standard ratified in 1997 includes frequency hopping spread spectrum (FHSS) and direct sequence spread spectrum (DSSS) physical layers operating in the 2.4-GHz band with data rates of up to 2 Mbps. FHSS transmits a wideband signal that spans the entire 2.4-GHz band. It's possible to tune FHSS access points to as many as 15 different hopping patterns that don't interfere with each other, which enables up to 15 FHSS access points to effectively operate in the same area.
Because the current version of 802.11 FHSS has only a maximum data rate of 2 Mbps, not many companies sell FHSS solutions for an indoor wireless LAN. Much faster 802.11a, 802.11b, and 802.11g wireless LANs are now available. Also, FHSS doesn't interoperate with any of the other 802.11 physical layers. FHSS does, however, provide a very solution for outdoor, point-to-multipoint systems. This is because FHSS is more resilient to the RF interference that might be present in outdoor environments.
802.11 DSSS also operates only up to 2 Mbps, but it interoperates with the newer 802.11b physical layer. As a result, a user having an 802.11 DSSS radio NIC in her laptop can interface with an 802.11b access point. This situation is unlikely, however, because 802.11 DSSS radio NICs are not sold anymore.
Toward the end of 1999, the IEEE released 802.11a, which defines operation in the 5-GHz band using Orthogonal Frequency Division Multiplexing (OFDM) with data rates up to 54 Mbps. Products, however, didn't become available until 2000, primarily because of the difficulties in developing circuitry in the 5-GHz band.
802.11a operates up to 54 Mbps in the 5-GHz band using OFDM with a range up to 100 feet depending on the actual data rate. 802.11a access points and radio NICs just became available in late 2001; therefore, the installed base of 802.11a wireless LANs today is relatively small as compared to 802.11b. Because of this, carefully consider interoperability issues that might result if you choose to deploy 802.11a networks.
A strong advantage of 802.11a is that it offers the highest capacity because of 12 separate, non-overlapping channels. This makes it a good choice for supporting a high concentration of users and higher-performance applications, such as video streaming. In addition to outperforming 802.11b systems, 802.11a has greater capacity than 802.11g.
Another advantage of 802.11a is that the 5-GHz band is uncrowded, which enables users to achieve higher levels of performance. Most interfering devices, such as microwave ovens and cordless phones, operate in the 2.4-GHz band. With less potential for RF interference, the deployment of a wireless LAN is less risky.
A potential issue of 802.11a is limited range, mainly because of operation in the higher frequency 5-GHz band. At 54 Mbps, you'll have a range less than 100 feet in most facilities. This requires a greater number of access points to fully cover a facility, as compared to an 802.11b system.
If you compare the operation of 802.11b and 802.11a, however, an 802.11a user has a higher data rate at the same range as an 802.11b user until the 802.11a user loses connectivity. The 802.11b user, however, can continue operating at lower data rates?1 or 2 Mbps, for example?at longer ranges than 802.11a.
A definite problem is that 802.11a and 802.11b/g are not compatible. For example, a user equipped with an 802.11b radio card can't associate with an 802.11a access point. The opposite scenario also applies. Vendors are solving this problem by introducing multimode radio cards that implement both 802.11a and 802.11b.
An 802.11a modulator converts the binary signal into an analog waveform through the use of different modulation types, depending on which data rate is chosen. With 6-Mbps operation, for example, the PMD uses binary phase shift keying (BPSK), which shifts the phase of the transmit center frequency to represent different data bit patterns. The higher data rates, such as 54 Mbps, employ quadrature amplitude modulation (QAM) to represent data bits by varying the transmit center frequency with different amplitude levels in addition to phase shifts.
Along with 802.11a, IEEE ratified 802.11b, which is a higher-rate extension to the initial direct sequence standard in the 2.4-GHz band?with data rates up to 11 Mbps. 802.11b access points and radio NICs have been available since 1999; therefore, most wireless LANs installed today are 802.11b compliant.
A significant advantage of 802.11b is its relatively long-range properties. With 802.11b, you can achieve a range of 300 feet in most indoor facilities. The superior range allows the deployment of wireless LANs with fewer access points to cover a facility as compared to 802.11a.
A disadvantage of 802.11b is that you're limited to three non-overlapping channels in the 2.4 GHz band. The 802.11 standard specifies 14 channels (only channels 1 through 11 are available in the U.S.) for configuring access points; but each channel occupies roughly one third of the overall 2.4-GHz band while transmitting a signal. Most companies utilize only channels 1, 6, and 11 to ensure access points don't interfere with each other. This limits overall capacity of 802.11b, which makes it most suitable for supporting medium performance applications, such as e-mail and web surfing.
Another disadvantage of 802.11b is the potential for RF interference from other radio devices. For example, a 2.4-GHz cordless phone severely interferes with an 802.11b wireless LAN, which significantly reduces the performance for users. Microwave ovens and other devices operating in the 2.4-GHz band can also cause interference.
802.11b uses DSSS to disperse the data frame signal over a 22-MHz portion of the 2.4-GHz frequency band. This results in greater immunity to RF interference as compared to narrowband signaling, which is why the FCC deems the operation of spread spectrum systems license free.
The 802.11b modulator converts the spread binary signal into an analog waveform through the use of different modulation types, depending on which data rate is chosen. For example with 1-Mbps operation, the PMD uses differential binary phase shift keying (DBPSK). This isn't really as complex as it sounds. The modulator merely shifts the phase of the center transmit frequency to distinguish a binary 1 from a binary 0 within the data stream.
For 2-Mbps transmission, the PMD uses differential quadrature phase shift keying (DQPSK), which is similar to DBPSK except four possible phase shifts that represents every two data bits. This is a clever process that enables the data stream to be sent at 2 Mbps while using the same amount of bandwidth as the one sent at 1 Mbps. The modulator uses similar methods for the higher 5.5-Mbps and 11-Mbps data rates.
IEEE ratified the 802.11g standard in 2003, which is compatible with 802.11b and increases performance up to 54 Mbps in the 2.4-GHz band using OFDM.
A strong advantage of 802.11g is that it's backward compatible with 802.11b. Companies with existing 802.11b networks can generally upgrade their access points to become 802.11g compliant through simple firmware upgrades. This provides an effective migration path for wireless LANs. An issue, however, is that the presence of 802.11b client devices within an 802.11g environment requires protection mechanisms that limit the performance of the overall wireless LAN. The problem is that 802.11b devices can't understand when 802.11g devices are transmitting because of a difference in modulation types. As a result, both types of devices must announce their impending use of the medium using a commonly understood modulation type.
The disadvantages of 802.11b, such as potential for RF interference and limit of three non-overlapping channels, still apply to 802.11g because of operation in the 2.4-GHz band. As a result, 802.11g networks have capacity constraints as compared to 802.11a.
The Wi-Fi Alliance, which began its work known as the Wireless Ethernet Compatibility Alliance (WECA), is an international nonprofit organization focusing on the marketing and interoperability of 802.11 wireless LAN products. The Wi-Fi Alliance is the group that pushes the term Wi-Fi to cover all forms of 802.11-based wireless networking, such as 802.11a, 802.11b, 802.11g, or whatever becomes available in the future. Wi-Fi Alliance is also behind Wi-Fi Protected Access (WPA), the stepping stone between the much-criticized WEP and the 802.11i security standard.
The Wi-Fi Alliance has three main goals:
What Wi-Fi Means
Wi-Fi certification is a process that assures interoperability between 802.11 wireless LAN equipment, including access points and radio cards complying with a variety of form factors. In order to qualify for obtaining Wi-Fi certification for products, a company must become a member of the Wi-Fi Alliance.
The Wi-Fi Alliance follows an established testing program to certify that products are interoperable with other Wi-Fi certified products. After a product successfully passes every test, the manufacturer is granted the right to use the Wi-Fi Certified logo on that particular product and its corresponding packaging and manuals.
Wi-Fi certification is meant to give consumers confidence that they are purchasing wireless LAN products that have met multivendor interoperability requirements. A Wi-Fi logo on the product means that it has met interoperability testing requirements and definitely works with other vendors' Wi-Fi?certified products.
Wi-Fi Protected Access
802.11 WEP doesn't provide enough security for most enterprise wireless LAN applications. Because of static key usage, it's fairly easy to crack WEP with off-the-shelf tools. This motivates IT managers to use stronger and more dynamic forms of WEP.
The problem to date, however, is that these enhanced security mechanisms are proprietary, making it difficult to support multivendor client devices. As a result, the Wi-Fi Alliance took a bold step forward to expedite the availability of effective standardized wireless LAN security by defining WPA while promoting interoperability. With WPA, an environment having many different types of 802.11 radio NICs?such as public hotspots?can benefit from enhanced forms of encryption.
WPA 1.0 is a snapshot of the initial, unratified version of 802.11i, which includes Temporal Key Integrity Protocol (TKIP) and 802.1x mechanisms. The combination of these two mechanisms provides dynamic key encryption and mutual authentication, something much needed in wireless LANs.
For authentication, WPA 1.0 uses a combination of open system and 802.1x authentication. Initially, the wireless client authenticates with the access points, which authorizes the client to send frames to the access point. Next, WPA performs user-level authentication with 802.1x. During this, WPA 1.0 interfaces to an authentication server in an enterprise environment. WPA 1.0 is also capable of operating in what's known as pre-shared key mode, if no external authentication server is available, such as in homes and small offices.
The 802.11i standard is backward compatible with WPA 1.0; however, 802.11i also includes an optional Advanced Encryption Standard (AES) encryption. AES requires coprocessors not found in most access points today, which makes AES more suitable for new wireless LAN installations. The newer WPA 2.0 includes AES.
HiperLAN/2, which stands for High Performance Radio LAN, is a wireless LAN standard developed by the Broadband Radio Access Networks (BRAN) division of the European Telecommunications Standards Institute (ETSI). HiperLAN/2 defines an efficient, high-speed wireless LAN technology that fully meets the requirements of Europe's spectrum regulations.
HiperLAN/2 has a physical layer that is similar to IEEE's 802.11a, which operates at up to 54 Mbps in the 5-GHz band using OFDM. A major difference with HiperLAN/2 is the use of a connection-oriented protocol with time division multiplexing as the basis for supporting data transfer between users. This method of transmission is efficient for multimedia applications including voice and video.
The similarities between 802.11a and HiperLAN/2, however, stop at the MAC Layer. While 802.11a uses CSMA/CA to transmit packets, HiperLAN/2 uses Time Division Multiple Access (TDMA). A problem is that CSMA/CA causes stations to wait for an indefinite period of time, which is referred to as asynchronous access. With this mode of operation, there are not any regular time relationships associated with medium access. As a result, there's no guarantee of when a particular station will be able to send a packet. The lack of regular access to the medium draws down the efficiency of the system, which is not good for supporting voice and video information.
The use of TDMA in HiperLAN/2, however, offers a regular time relationship for network access. TDMA systems dynamically assign each station a time slot based on the station's need for throughput. The stations then transmit at regular intervals during their respective time slots, making more efficient use of the medium and improving support of voice and video applications.
HiperLAN/2 has a number of attractive features as compared to 802.11. The first, and probably most important, is higher throughput. Both 802.11a and HiperLAN/2 boast maximum data rates of 54 Mbps, but this doesn't represent the actual rate that information flows between the station and the access point.
The true usable maximum throughput of HiperLAN/2 is 42 Mbps, while the maximum usable throughput of 802.11a is only around 18 Mbps. This puts HiperLAN/2 well ahead of 802.11a in terms of performance of each access point.
A unique feature of HiperLAN/2 technology is the ability to interface with other high-speed networks, including 3G cellular, asynchronous transfer mode (ATM), and other Internet protocol-based networks. This can be a real advantage when integrating wireless LANs with cellular systems and WANs.
Is HiperLAN/2 a Threat to 802.11?
Despite bold predictions of mass production and deployment of HiperLAN/2 products during the second half of 2002, not many, if any HiperLAN/2 products are currently available for consumer purchase. In fact, exhaustive searches on the Internet reveal no HiperLAN/2 products available to consumers. HiperLAN/2 doesn't seem to be moving forward at any discernable pace.
Much of this has to do with regulatory issues and big supporters pulling out of the HiperLAN/2 movement. In addition, the 802.11h Task Group has been working on revisions to 802.11 that make it more suitable for deployment in Europe, which is where HiperLAN/2 could dominate if anywhere.
Essentially 802.11h is 802.11a with two additional European features. The first of these is Transmit Power Control (TPC), which enables automatic controls for keeping transmissions from interfering with other nearby systems. The second feature is Dynamic Frequency Selection (DFS), which allows the station to listen to the airspace before picking a channel. This is also an interference avoidance mechanism that the ETSI requires for operation within Europe.
802.11 currently has a definite lead in the worldwide market as the top choice for wireless LAN deployments. This makes 802.11 the only alternative for wireless LAN deployments today. Combined with the absence of HiperLAN/2 products, it's doubtful that HiperLAN/2 will catch up and become the dominant player in the wireless LAN market.