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multimedia data Although all the details have not yet been nalized, nevertheless the basic elements have been selected Speci cally, the e standard de nes two main access modes that are backward compatible with DCF and PCF: 1 The enhanced DCF (EDCF) that extends the DCF to provide service differentiation 2 The hybrid coordination function (HCF) that modi es the PCF to have more ef cient polling4 This method is under the control of the hybrid coordinator (HC) that is typically located in the access point Similarly to 80211, the channel access is subdivided into contention periods (CPs) and contention free periods (CFPs) EDCF is used during CPs only, while the HCF can operate in both periods 14441 Enhanced DCF EDCF enhances the DCF access scheme to support service differentiation by introducing, in each station, four access categories (ACs) that are used to support user traf c with eight different priorities Each AC operates as a virtual station following the DCF-like rules with its own parameters for interframe spacing and contention parameters The values of these parameters are selected to give the highest access probability to the traf c with the highest requirements As each access category corresponds to a separate virtual station, a station with multiple ACs will have an internal contention among its ACs (besides the access contention with the other stations) Contentions among ACs within the same station are locally managed: the highest AC performs the transmission attempt (that may collide with the other stations), while the other ACs behave as when a collision occurs (virtual collision) As in DCF, a real collision occurs if more stations start transmitting at the same time 14442 Hybrid coordination function Some types of interactive and synchronous traf c may require QoS guarantees (eg bounded delays) more stringent than those provided by EDCF service differentiation To manage these classes of traf c, the HCF access mode is included in the 80211e standard The HCF access mode uses a coordinator (hybrid coordinator) generally located in the access point that is in charge of starting, when necessary, controlled access periods in the channel access The HC performs bandwidth management by allocating the transmission opportunities (TXOPs) to 80211e stations A TXOP de nes the time interval in which a station can transmit, and it is characterized by a starting time and a maximum duration The HCF access method operates during both contention periods (CPs) and contention free periods (CFPs) by initiating and managing time intervals during which only the hybrid coordinator and the polled stations can access the channel During CPs, the hybrid coordinator can start controlled access periods in which bursts of frames are transmitted using the polling-based channel access mechanism This also provides guaranteed services under heavy load conditions A signaling protocol is used by HC to learn which stations must be polled Speci cally, the stations signal to the hybrid
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PCF is optional, HCF is an 80211e mandatory mechanism
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coordinator the requirements of their traf c streams (eg required bandwidth, maximum delay, etc) during controlled contention periods that are started on the channel by the HC sending a special control frame By using this information, the coordinator determines the time instants at which stations must be polled
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Several Task Groups are currently active inside the 80211 Working Group to x technical and regulatory issues, and to promote the worldwide use of this technology [20] For example, Task Group 80211h is working to comply with European regulations for 5 GHz WLANs, while 80211d is developing an autocon guration protocol through which an 80211 device receives the regulatory information related to the country in which it is operating
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14451 Security issues The activities currently ongoing in Task Group 80211i deserve special attention This Task Group is working to improve security in 80211 networks Wireless communications obviously generate potential security problems, as an intruder does not need physical access to the traditional wired network to gain access to data communications The IEEE 80211 Working Group addressed security issues by de ning an optional extension to 80211: wired equivalent privacy (WEP) [13] WEP is a form of encryption that should provide privacy comparable to that of a traditional wired network If the wireless network has information that should be secure, then WEP should be used WEP supports both data encryption and integrity The security is based on a 40-bit secret key The secret key can either be a default key shared by all the devices of a WLAN, or a pair-wise secret key shared only by two communicating devices Since WEP does not provide any support for the exchange of secret keys, the secret key must be manually installed on each device Cryptographers have identi ed many aws and weaknesses in WEP, pointing out that WEP is only useful for preventing casual traf c capture Challenges for wireless security To address these security issues, the 80211i Task Group has been set up It will apply to 80211 physical standards a, b and g by providing new encryption methods and authentication procedures IEEE 8021x forms a key part of 80211i 8021x is based on the IETF Extensible Authentication Protocol (EAP) [21] The computing power of today s handheld devices can be compared to that of their desktop counterparts of one generation earlier This phenomenon, while driving more and more functionality into handheld wireless Internet-enabled services, is also driving the security risks that we have today in desktop computing into wireless devices [13] Consequently, basic security requirements, such as network services availability, entity authentication, data con dentiality, data integrity and nonrepudiation, which have been advocated for Internet-based communication and computing, are directly advocated for wireless e-applications as well In fact, in addition to presenting the usual Internet security threats in online applications, wireless devices introduce new challenging issues speci c to their mobility the shared communication medium and the lack of interoperability while roaming between different security domains Additionally, in ad hoc networks, we need to consider also the lack of assistance from a xed infrastructure and from a central authority [22] The
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