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IEEE 802.11 WIRELESS LAN OVERVIEW
IEEE 802 is a committee that has developed standards for a wide range of local area networks (LANs). In 1990, the IEEE 802 Committee formed a new working group, IEEE 802.11, with a charter to develop a protocol and transmission specifications for wireless LANs (WLANs). Since that time, the demand for WLANs at different frequencies and data rates has exploded. Keeping pace with this demand, the IEEE
working group has issued an ever-expanding list of standards. Table 17.1 briefly defines key terms used in the IEEE 802.11 standard.
The Wi-Fi Alliance
The first 802.11 standard to gain broad industry acceptance was 802.11b. Although 802.11b products are all based on the same standard, there is always a concern whether products from different vendors will successfully interoperate. To meet this concern, the Wireless Ethernet Compatibility Alliance (WECA), an industry con- sortium, was formed in 1999. This organization, subsequently renamed the Wi-Fi (Wireless Fidelity) Alliance, created a test suite to certify interoperability for 802.11b products. The term used for certified 802.11b products is Wi-Fi. Wi-Fi certi- fication has been extended to 802.11g products,. The Wi-Fi Alliance has also devel- oped a certification process for 802.11a products, called Wi-Fi5. The Wi-Fi Alliance is concerned with a range of market areas for WLANs, including enterprise, home, and hot spots.
More recently, the Wi-Fi Alliance has developed certification procedures for IEEE 802.11 security standards, referred to as Wi-Fi Protected Access (WPA). The most recent version of WPA, known as WPA2, incorporates all of the features of the IEEE 802.11i WLAN security specification.
IEEE 802 Protocol Architecture
Before proceeding, we need to briefly preview the IEEE 802 protocol architecture. IEEE 802.11 standards are defined within the structure of a layered set of protocols. This structure, used for all IEEE 802 standards, is illustrated in Figure 17.1.
PHYSICAL LAYER The lowest layer of the IEEE 802 reference model is the physical layer, which includes such functions as encoding/decoding of signals and bit transmission/reception. In addition, the physical layer includes a specification of the transmission medium. In the case of IEEE 802.11, the physical layer also defines frequency bands and antenna characteristics.
MEDIA ACCESS CONTROL All LANs consist of collections of devices that share the network’s transmission capacity. Some means of controlling access to the transmission medium is needed to provide an orderly and efficient use of that capacity. This is the function of a media access control (MAC) layer. The MAC layer receives data from a higher-layer protocol, typically the Logical Link Control (LLC) layer, in the form of a block of data known as the MAC service data unit (MSDU). In general, the MAC layer performs the following functions:
• On transmission, assemble data into a frame, known as a MAC protocol data unit (MPDU) with address and error-detection fields.
• On reception, disassemble frame, and perform address recognition and error detection.
• Govern access to the LAN transmission medium.
The exact format of the MPDU differs somewhat for the various MAC proto- cols in use. In general, all of the MPDUs have a format similar to that of Figure 17.2. The fields of this frame are as follows.
• MAC Control: This field contains any protocol control information needed for the functioning of the MAC protocol. For example, a priority level could be indicated here.
• Destination MAC Address: The destination physical address on the LAN for this MPDU.
• Source MAC Address: The source physical address on the LAN for this MPDU.
• MAC Service Data Unit: The data from the next higher layer.
• CRC: The cyclic redundancy check field; also known as the Frame Check Sequence (FCS) field. This is an error-detecting code, such as that which is used in other data-link control protocols. The CRC is calculated based on the bits in the entire MPDU. The sender calculates the CRC and adds it to the frame. The receiver performs the same calculation on the incoming MPDU and compares that calculation to the CRC field in that incoming MPDU. If the two values don’t match, then one or more bits have been altered in transit.
The fields preceding the MSDU field are referred to as the MAC header, and the field following the MSDU field is referred to as the MAC trailer. The header and trailer contain control information that accompany the data field and that are used by the MAC protocol.
LOGICAL LINK CONTROL In most data-link control protocols, the data-link protocol entity is responsible not only for detecting errors using the CRC, but for recovering from those errors by retransmitting damaged frames. In the LAN protocol architecture, these two functions are split between the MAC and LLC layers. The MAC layer is responsible for detecting errors and discarding any frames that contain errors. The LLC layer optionally keeps track of which frames have been successfully received and retransmits unsuccessful frames.
IEEE 802.11 Network Components and Architectural Model
Figure 17.3 illustrates the model developed by the 802.11 working group. The smallest building block of a wireless LAN is a basic service set (BSS), which consists of wireless stations executing the same MAC protocol and competing for access to the same shared wireless medium. A BSS may be isolated, or it may connect to a backbone
distribution system (DS) through an access point (AP). The AP functions as a bridge and a relay point. In a BSS, client stations do not communicate directly with one another. Rather, if one station in the BSS wants to communicate with another station in the same BSS, the MAC frame is first sent from the originating station to the AP and then from the AP to the destination station. Similarly, a MAC frame from a sta- tion in the BSS to a remote station is sent from the local station to the AP and then relayed by the AP over the DS on its way to the destination station.The BSS generally corresponds to what is referred to as a cell in the literature. The DS can be a switch, a wired network, or a wireless network.
When all the stations in the BSS are mobile stations that communicate directly with one another (not using an AP), the BSS is called an independent BSS (IBSS). An IBSS is typically an ad hoc network. In an IBSS, the stations all communicate directly, and no AP is involved.
A simple configuration is shown in Figure 17.3, in which each station belongs to a single BSS; that is, each station is within wireless range only of other stations within the same BSS. It is also possible for two BSSs to overlap geographically, so that a single station could participate in more than one BSS. Furthermore, the asso- ciation between a station and a BSS is dynamic. Stations may turn off, come within range, and go out of range.
An extended service set (ESS) consists of two or more basic service sets interconnected by a distribution system. The extended service set appears as a single logical LAN to the logical link control (LLC) level.
IEEE 802.11 Services
IEEE 802.11 defines nine services that need to be provided by the wireless LAN to achieve functionality equivalent to that which is inherent to wired LANs. Table 17.2 lists the services and indicates two ways of categorizing them.
1. The service provider can be either the station or the DS. Station services are implemented in every 802.11 station, including AP stations. Distribution ser- vices are provided between BSSs; these services may be implemented in an AP or in another special-purpose device attached to the distribution system.
2. Three of the services are used to control IEEE 802.11 LAN access and confi- dentiality. Six of the services are used to support delivery of MSDUs between stations. If the MSDU is too large to be transmitted in a single MPDU, it may be fragmented and transmitted in a series of MPDUs.
Following the IEEE 802.11 document, we next discuss the services in an order designed to clarify the operation of an IEEE 802.11 ESS network. MSDU delivery, which is the basic service, already has been mentioned. Services related to security are introduced in Section 17.2.
DISTRIBUTION OF MESSAGES WITHIN A DS The two services involved with the distribution of messages within a DS are distribution and integration. Distribution is the primary service used by stations to exchange MPDUs when the MPDUs must traverse the DS to get from a station in one BSS to a station in another BSS. For example, suppose a frame is to be sent from station 2 (STA 2) to station 7 (STA 7) in Figure 17.3. The frame is sent from STA 2 to AP 1, which is the AP for this BSS. The AP gives the frame to the DS, which has the job of directing the frame to the AP associated with STA 7 in the target BSS. AP 2 receives the frame and forwards it to STA 7. How the message is transported through the DS is beyond the scope of the IEEE 802.11 standard.
If the two stations that are communicating are within the same BSS, then the distribution service logically goes through the single AP of that BSS.
Table 17.2 IEEE 802.11 Services
The integration service enables transfer of data between a station on an IEEE
LAN and a station on an integrated IEEE 802.x LAN. The term integrated refers to a wired LAN that is physically connected to the DS and whose stations may be logically connected to an IEEE 802.11 LAN via the integration service. The integration service takes care of any address translation and media conversion logic required for the exchange of data.
ASSOCIATION-RELATED SERVICES The primary purpose of the MAC layer is to transfer MSDUs between MAC entities; this purpose is fulfilled by the distribution service. For that service to function, it requires information about stations within the ESS that is provided by the association-related services. Before the distribution service can deliver data to or accept data from a station, that station must be associated. Before looking at the concept of association, we need to describe the concept of mobility. The standard defines three transition types, based on mobility:
• No transition: A station of this type is either stationary or moves only within the direct communication range of the communicating stations of a single BSS.
• BSS transition: This is defined as a station movement from one BSS to another BSS within the same ESS. In this case, delivery of data to the station requires that the addressing capability be able to recognize the new location of the station.
• ESS transition: This is defined as a station movement from a BSS in one ESS to a BSS within another ESS. This case is supported only in the sense that the station can move. Maintenance of upper-layer connections supported by
802.11 cannot be guaranteed. In fact, disruption of service is likely to occur.
To deliver a message within a DS, the distribution service needs to know where the destination station is located. Specifically, the DS needs to know the identity of the AP to which the message should be delivered in order for that mes- sage to reach the destination station. To meet this requirement, a station must maintain an association with the AP within its current BSS. Three services relate to this requirement:
• Association: Establishes an initial association between a station and an AP. Before a station can transmit or receive frames on a wireless LAN, its identity and address must be known. For this purpose, a station must establish an asso- ciation with an AP within a particular BSS. The AP can then communicate this information to other APs within the ESS to facilitate routing and delivery of addressed frames.
• Reassociation: Enables an established association to be transferred from one AP to another, allowing a mobile station to move from one BSS to another.
• Disassociation: A notification from either a station or an AP that an existing association is terminated. A station should give this notification before leaving an ESS or shutting down. However, the MAC management facility protects itself against stations that disappear without notification.
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