Introduction to IEEE 802.11 In 1997 the IEEE adopted IEEE Std. 802.11-1997, the first wireless LAN (WLAN) standard. This standard defines the media access control (MAC) and physical (PHY) layers for a LAN with wireless connectivity. It addresses local area networking where the connected devices communicate over the air to other devices that are within close proximity to each other. This paper provides an overview of the 802.11 architecture and the different topologies incorporated to accomodate the unique characteristics of the IEEE 802.11 wireless LAN standard. The standard is similar in most respects to the IEEE 802.3 Ethernet standard. Specifically, the 802.11 standard addresses: Functions required for an 802.11 compliant device to operate either in a peer-to- peer fashion or integrated with an existing wired LAN Operation of the 802.11 device within possibly overlapping 802.11 wireless LANs and the mobility of this device between multiple wireless LANs MAC level access control and data delivery services to allow upper layers of the 802.11 network Several physical layer signaling techniques and interfaces Privacy and security of user data being transferred over the wireless media Figure 1 - IEEE 802.11 standards mapped to the OSI reference model. What makes a Wireless LAN unique? There are a number of characteristics that are unique to the wireless environment (as compared to a wired LAN) that the 802.11 standard must take into consideration. The physical characteristics of a wireless LAN introduce range limitations and unreliable media, dynamic topologies where stations move about, interference from outside sources, and lack of the ability for every device to "hear" every other device within the WLAN. These limitations force the WLAN standard to create fundamental definitions for short- range LANs made up of components that are within close proximity of each other. Larger geographic coverage is handled by building larger LANs from the smaller fundamental building blocks or by integrating the smaller WLANs with an existing wired network. More on mobility Mobility of wireless stations may be the most important feature of a wireless LAN. A WLAN would not serve much purpose if stations were not able to move about freely from location to location either within a specific WLAN or between different WLAN "segments". For compatibility purposes, the 802.11 MAC must appear to the upper layers of the network as a "standard" 802 LAN. The 802.11 MAC layer is forced to handle station mobility in a fashion that is transparent to the upper layers of the 802 LAN stack. This forces functionality into the 802.11 MAC layer that is typically handled by upper layers. The IEEE 802.11 Wireless LAN Architecture The 802.11 architecture is comprised of several components and services that interact to provide station mobility transparent to the higher layers of the network stack. Wireless LAN Station The station (STA) is the most basic component of the wireless network. A station is any device that contains the functionality of the 802.11 protocol, that being MAC, PHY, and a connection to the wireless media. Typically the 802.11 functions are implemented in the hardware and software of a network interface card (NIC). A station could be a laptop PC, handheld device, or an Access Point. Stations may be mobile, portable, or stationary and all stations support the 802.11 station services of authentication, de-authentication, privacy, and data delivery. Basic Service Set (BSS) 802.11 defines the Basic Service Set (BSS) as the basic building block of an 802.11 wireless LAN. The BSS consists of a group of any number of stations. The BSS is not a very interesting topic until we take the topology of the WLAN into consideration. 802.11 Topologies Independent Basic Service Set (IBSS) The most basic wireless LAN topology is a set of stations, which have recognized each other and are connected via the wireless media in a peer-to-peer fashion. This form of network topology is referred to as an Independent Basic Service Set (IBSS) or an Ad-hoc network. In an IBSS, the mobile stations communicate directly with each other. Every mobile station may not be able to communicate with every other station due to the range limitations. There are no relay functions in an IBSS therefore all stations need to be within range of each other and communicate directly. BSS Figure 2 - Independent Basic Service Set (IBSS) Infrastructure Basic Service Set An Infrastructure Basic Service Set is a BSS with a component called an Access Point (AP). The access point provides a local relay function for the BSS. All stations in the BSS communicate with the access point and no longer communicate directly. All frames are relayed between stations by the access point. This local relay function effectively doubles the range of the IBSS. The access point may also provide connection to a distribution system. Distribution System Figure 3 - Infrastructure Basic Service Set Distribution System (DS) The distribution system (DS) is the means by which an access point communicates with another access point to exchange frames for stations in their respective BSSs, forward frames to follow mobile stations as they move from one BSS to another, and exchange frames with a wired network. As IEEE 802.11 describes it, the distribution system is not necessarily a network nor does the standard place any restrictions on how the distribution system is implemented, only on the services it must provide. Thus the distribution system may be a wired network like 803.2 or a special purpose box that interconnects the access points and provides the required distribution services. Extending coverage via an Extended Service Set (ESS) 802.11 extends the range of mobility to an arbitrary range through the Extended Service Set (ESS). An extended service set is a set of infrastructure BSS's, where the access points communicate amongst themselves to forward traffic from one BSS to another to facilitate movement of stations between BSS's. The access point performs this communication through the distribution system. The distribution system is the backbone of the wireless LAN and may be constructed of either a wired LAN or wireless network. Typically the distribution system is a thin layer in each access point that determines the destination for traffic received from a BSS. The distribution system determines if traffic should be relayed back to a destination in the same BSS, forwarded on the distribution system to another access point, or sent into the wired network to a destination not in the extended service set. Communications received by an access point from the distribution system are transmitted to the BSS to be received by the destination mobile station. Network equipment outside of the extended service set views the ESS and all of its mobile stations as a single MAC-layer network where all stations are physically stationary. Thus, the ESS hides the mobility of the mobile stations from everything outside the ESS. This level of indirection provided by the 802.11 architecture allows existing network protocols that have no concept of mobility to operate correctly with a wireless LAN where there is mobility. Figure 4 - Extended Service Set (ESS) Station Services The 802.11 standard defines services for providing functions among stations. Station services are implemented within all stations on an 802.11 WLAN (including access points). The main thrust behind station services is to provide security and data delivery services for the WLAN. Authentication Because wireless LANs have limited physical security to prevent unauthorized access, 802.11 defines authentication services to control access to the WLAN. The goal of authentication service is to provide access control equal to a wired LAN. The authentication service provides a mechanism for one station to identify another station. Without this proof of identity, the station is not allowed to use the WLAN for data delivery. All 802.11 stations, whether they are part of an independent BSS or ESS network, must use the authentication service prior to communicating with another station. IEEE 802.11 defines two types of authentication services. Open system authentication This is the default authentication method, which is a very simple, two-step process. First the station wanting to authenticate with another station sends an authentication management frame containing the sending station's identity. The receiving station then sends back a frame alerting whether it recognizes the identity of the authenticating station. Shared key authentication This type of authentication assumes that each station has received a secret shared key through a secure channel independent of the 802.11 network. Stations authenticate through shared knowledge of the secret key. Use of shared key authentication requires implementation of encryption via the Wired Equivalent Privacy or WEP algorithm. De-authentication The de-authentication service is used to eliminate a previously authorized user from any further use of the network. Once a station is de-authenticated, that station is no longer able to access the WLAN without performing the authentication function again. De-authentication is a notification and cannot be refused. For example, when a station wishes to be removed from a BSS, it can send a de-authentication management frame to the associated access point to notify the access point of the removal from the network. An access point could also de-authenticate a station by sending a de-authentication frame to the station. Privacy The privacy service of IEEE 802.11 is designed to provide an equivalent level of protection for data on the WLAN as that provided by a wired network with restricted physical access. This service protects that data only as it traverses the wireless medium. It is not designed to provide complete protection of data between applications running over a mixed network. With a wireless network, all stations and other devices can "hear" data traffic tacking place within range on the network, seriously impacting the security level of a wireless link. IEEE 802.11 counters this problem by offering a privacy service option that raises the security of the 802.11 network to that of a wired network. The privacy service, applying to all data frames and some authentication management frames, is an encryption algorithm based on the 802.11 Wired Equivalent Privacy (WEP) algorithm. Data Delivery Data delivery service is similar to that provided by all other IEEE 802 LANs. The data delivery service provides reliable delivery of data frames from the MAC in one station to the MAC in one or more other stations, with minimal duplication and reordering of frames. Distribution Services Distribution services provide functionality across a distribution system. Typically, access points provide distribution services. The five distribution services and functions detailed below include: association, disassociation, re-association, distribution, and integration. Association The association service is used to make a logical connection between a mobile station and an access point. Each station must become associated with an access point before it is allowed to send data through the access point onto the distribution system. The connection is necessary in order for the distribution system to know where and how to deliver data to the mobile station. The mobile station invokes the association service once and only once, typically when the station enters the BSS. Each station can associate with one access point though an access point can associate with multiple stations. Disassociation The disassociation service is used either to force a mobile station to eliminate an association with an access point or for a mobile station to inform an access point that it no longer requires the services of the distribution system. When a station becomes disassociated, it must begin a new association to communicate with an access point again. An access point may force a station or stations to disassociate because of resource restraints, the access point is shutting down or being removed from the network for a variety of reasons. When a mobile station is aware that it will no longer require the services of an access point, it may invoke the disassociation service to notify the access point that the logical connection to the services of the access point from this mobile station is no longer required. Stations should disassociate when they leave a network, though there is nothing in the architecture to assure this happens. Disassociation is a notification and can be invoked by either associated party. Neither party can refuse termination of the association. Re-association Re-Association enables a station to change its current association with an access point. The re-association service is similar to the association service, with the exception that it includes information about the access point with which a mobile station has been previously associated. A mobile station will use the re-association service repeatedly as it moves through out the ESS, loses contact with the access point with which it is associated, and needs to become associated with a new access point. Buy using the re-association service, a mobile station provides information to the access point to which it will be associated and information pertaining to the access point which it will be disassociated. This allows the newly associated access point to contact the previously associated access point to obtain frames that may be waiting there for delivery to the mobile station as well as other information that may be relevant to the new association. The mobile station always initiates re-association. Distribution Distribution is the primary service used by an 802.11 station. A station uses the distribution service every time it sends MAC frames across the distribution system. The distribution service provides the distribution with only enough information to determine the proper destination BSS for the MAC frame. The three association services (association, re-association, and disassociation) provide the necessary information for the distribution service to operate. Distribution within the distribution system does not necessarily involve any additional features outside of the association services, though a station must be associated with an access point for the distribution service to forward frames properly. Integration The integration service connects the 802.11 WLAN to other LANs, including one or more wired LANs or 802.11 WLANs. A portal performs the integration service. The portal is an abstract architectural concept that typically resides in an access point though it could be part of a separate network component entirely. The integration service translates 802.11 frames to frames that may traverse another network, and vice versa as well as translates frames from other networks to frames that may be delivered by an 802.11 WLAN. 802.11 Media Access Control The 802.11 MAC layer provides functionality to allow reliable data delivery for the upper layers over the wireless PHY media. The data delivery itself is based on an asynchronous, best-effort, connectionless delivery of MAC layer data. There is no guarantee that the frames will be delivered successfully. The 802.11 MAC provides a controlled access method to the shared wireless media called Carrier-Sense Multiple Access with Collision Avoidance (CSMA/CA). CSMA/CA is similar to the collision detection access method deployed by 802.3 Ethernet LANs. The third function of the 802.11 MAC is to protect the data being delivered by providing security and privacy services. Security is provided by the authentication services and by Wireless Equivalent Privacy (WEP), which is an encryption service for data delivered on the WLAN. More on CSMA/CA The fundamental access method of 802.11 is Carrier Sense Multiple Access with Collision Avoidance or CSMA/CA. CSMA/CA works by a "listen before talk scheme". This means that a station wishing to transmit must first sense the radio channel to determine if another station is transmitting. If the medium is not busy, the transmission may proceed. The CSMA/CA protocol avoids collisions among stations sharing the medium by utilizing a random backoff time if the station's physical or logical sensing mechanism indicates a busy medium. The period of time immediately following a busy medium is the highest probability of collisions occurring, especially under high utilization. The CSMA/CA scheme implements a minimum time gap between frames from a given user. Once a frame has been sent from a given transmitting station, that station must wait until the time gap is up to try to transmit again. Once the time has passed, the station selects a random amount of time (the backoff interval) to wait before "listening" again to verify a clear channel on which to transmit. If the channel is still busy, another backoff interval is selected that is less than the first. This process is repeated until the waiting time approaches zero and the station is allowed to transmit. This type of multiple access ensures judicious channel sharing while avoiding collisions. 802.11 Physical Layer (PHY) The 802.11 physical layer (PHY) is the interface between the MAC and the wireless media where frames are transmitted and received. The PHY provides three functions. First, the PHY provides an interface to exchange frames with the upper MAC layer for transmission and reception of data. Secondly, the PHY uses signal carrier and spread spectrum modulation to transmit data frames over the media. Thirdly, the PHY provides a carrier sense indication back to the MAC to verify activity on the media. 802.11 provides three different PHY definitions: Both Frequency Hopping Spread Spectrum (FHSS) and Direct Sequence Spread Spectrum (DSSS) support 1 and 2 Mbps data rates. An extension to the 802.11 architecture (802.11a) defines different multiplexing techniques that can achieve data rates up to 54 Mbps. Another extension to the standard (802.11b) defines 11 Mbps and 5.5 Mbps data rates (in addition to the 1 and 2Mbps rates) utilizing an extension to DSSS called High Rate DSSS (HR/DSSS). 802.11b also defines a rate shifting technique where 11 Mbps networks may fall back to 5.5 Mbps, 2 Mbps, or 1 Mps under noisy conditions or to inter-operate with legacy 802.11 PHY layers. Spread Spectrum Spread spectrum is a technique trading bandwidth for reliability. The goal is to use more bandwidth than the system really needs for transmission to reduce the impact of localized interference on the media. Spread spectrum spreads the transmitted bandwidth of the resulting signal, reducing the peak power but keeping total power the same. Frequency Hopping Spread Spectrum (FHSS) Frequency Hopping utilizes a set of narrow channels and "hops" through all of them in a predetermined sequence. For example, the 2.4 GHz frequency band is divided into 70 channels of 1 MHz each. Every 20 to 400 msec the system "hops" to a new channel following a predetermined cyclic pattern. The 802.11 Frequency Hopping Spread Spectrum (FHSS) PHY uses the 2.4 GHz radio frequency band, operating with at 1 or 2 Mbps data rate. Direct Sequence Spread Spectrum (DSSS) The principle of Direct Sequence is to spread a signal on a larger frequency band by multiplexing it with a signature or code to minimize localized interference and background noise. To spread the signal, each bit is modulated by a code. In the receiver, the original signal is recovered by receiving the whole spread channel and demodulating with the same code used by the transmitter. The 802.11 Direct Sequence Spread Spectrum (DSSS) PHY also uses the 2.4 GHz radio frequency band. Infrared (IR) The Infrared PHY utilizes infrared light to transmit binary data either at 1 Mbps (basic access rate) or 2 Mbps (enhanced access rate) using a specific modulation technique for each. For 1 Mbps, the infrared PHY uses a 16-pulse position modulation (PPM). The concept of PPM is to vary the position of a pulse to represent different binary symbols. Infrared transmission at 2 Mbps utilizes a 4 PPM modulation technique. Conclusion Wireless networking has a promising future with 802.11 leading the way as the standard for adoption in local networking environments. 802.11 addresses mobility, security, reliability, and the dynamic nature of wireless LANS while keeping compatibility with 802-type legacy networks. Expect to see availability of 802.11 products increase dramatically in the near future as businesses discover the increased productivity provided by "untethered" networks.