Glossary For WLAN (802.11)

AAA: Authentication, Authorization, and Accounting. A protocol or system which enables users to prove identity, obtain access to resources, and collect usage statistics. RADIUS is the most common AAA protocol in use with 802.11 networks.

ACK: Abbreviation for "Acknowledgment." ACKs are used extensively in 802.11 to provide reliable data transfers over an unreliable medium.

ad hoc: A network characterized by temporary, short-lived relationships between nodes.

AES: Advanced Encryption Standard. A cipher selected by NIST to replace the older Data Encryption Standard (DES) in 2001 after a five-year evaluation. AES is a 128-bit block cipher which uses either 128-, 192-or 256-bit keys. It has been widely adopted by many protocols requiring the use of a block cipher, including 802.11i's CCMP, though CCMP uses only 128-bit keys. AES is specified in FIPS Publication 197.

AID: Association Identifier. A number that identifies data structures in an access point allocated for a specific mobile node.

AKM: Authentication and Key Management. A set of protocols used to establish user identity and keys. The two currently defined on wireless networks are 802.1X and pre-shared keys.

AP: Access Point. Bridge-like device that attaches wireless 802.11 stations to a wired backbone network.

AS: Authentication Server. The network service that validates user credentials. Usually RADIUS in 802.11 networks.

ASN: Abstract Syntax Notation. The formal description of the grammar used to write MIB files.

ATIM: Announcement Traffic Indication Message. ATIMs are used in ad hoc (independent) 802.11 networks to announce the existence of buffered frames.

BER: Bit Error Rate. The number of bits received in error. Usually, the number is quite low and expressed as a ratio in scientific notation. 10-2 means one bit in 100 is received in error.

BPSK: Binary Phase Shift Keying. A modulation method that encodes bits as phase shifts. One of two phase shifts can be selected to encode a single bit.

BSS: Basic Service Set. The building block of 802.11 networks. A BSS is a set of stations that are logically associated with each other.

BSSID: Basic Service Set Identifier. A 48-bit identifier used by all stations in a BSS in frame headers.

CCITT: Comité Consultatif International Télé-graphique et Téléphonique. A UN body responsible for telephone standardization. Due to a reorganization, it is now called the International Telecommunication Union-Telecommunication Standardization Sector (ITU-T).

CCK: Complementary Code Keying. A modulation scheme that transforms data blocks into complex codes and is capable of encoding several bits per block.

CCM: Counter Mode with CBC-MAC. An authenticated block cipher mode defined in RFC 3610. It can be used with any 128-bit block cipher, but is commonly used with AES.

CCMP: Counter Mode with CBC-MAC Protocol. 802.11i defines the use of AES with the CCM mode of operation as CCMP. It is the strongest encryption protocol available for use with wireless LANs.

CF: Contention Free. Services that do not involve contention for the medium are contention-free services. Such services are implemented by a Point Coordinator (PC) through the use of the Point Coordination Function (PCF). Contention-free services are not widely implemented.

CFP: Contention-Free Period. Even when 802.11 provides contention-free services, some contention-based access to the wireless medium is allowed. Periods controlled by a central authority are called contention-free periods (CFP).

CRC: Cyclic Redundancy Check. A mathematical checksum that can be used to detect data corruption in transmitted frames. The CRC is a linear hash function, and should not be used for data security assurance.

CSMA: Carrier Sense Multiple Access. A "listen before talk" scheme used to mediate the access to a transmission resource. All stations are allowed to access the resource (multiple access) but are required to make sure the resource is not in use before transmitting (carrier sense).

CSMA/CA: Carrier Sense Multiple Access with Collision Avoidance. A CSMA method that tries to avoid simultaneous access (collsions) by deferring access to the medium. 802.11 and AppleTalk's LocalTalk are two protocols that use CSMA/CA.

CTS: Clear to Send. The frame type used to acknowledge receipt of a Request to Send and the second component used in the RTS-CTS clearing exchange used to prevent interference from hidden nodes.

DA: Destination Address. The MAC address of the station the frame should be processed by. Frequently, the destination address is the receiver address. In infrastructure networks, however, frames bridged from the wireless side to the wired side will have a destination address on the wired network and a receiver address of the wireless interface in the access point.

DBPSK: Differential Binary Phase Shift Keying. A modulation method in which bits are encoded as phase shift differences between successive symbol periods. Two phase shifts are possible for an encoding rate of one data bit per symbol.

DCF: Distributed Coordination Function. The rules for contention-based access to the wireless medium in 802.11. The DCF is based on exponentially increasing backoffs in the presence of contention as well as rules for deferring access, frame acknowledgment, and when certain types of frame exchanges or fragmentation may be required.

DFS: Dynamic Frequency Selection. A spectrum management service required by European radio regulations to avoid interfering with 5 GHz radar systems, as well as spread power across all available channels. DFS was also key to the FCC decision to open up the harmonized frequency band in the U.S.

DHCP: Dynamic Host Configuration Protocol. An IETF standard used by network administrators to automatically configure hosts. Hosts needing configuration information may broadcast a request that is responded to by a DHCP server. DHCP was the Internet community's admission that the Internet was growing so fast that network administrators had lost control over what was plugged into networks.

DIFS: Distributed Inter-Frame Space. The interframe space used to separate atomic exchanges in contention-based services. See also DCF.

DQPSK: Differential Quadrature Phase Shift Keying. A modulation method in which bits are encoded as phase shift differences between successive symbol periods. Four phase shifts are possible for an encoding rate of two data bits per symbol.

DS: Distribution System. The set of services that connects access points together. Logically composed of the wired backbone network plus the bridging functions in most commercial access points.

DSSS: Direct-Sequence Spread Spectrum. A transmission technique that spreads a signal over a wide frequency band for transmission. At the receiver, the widespread signal is correlated into a stronger signal; meanwhile, any narrowband noise is spread widely. Most of the 802.11-installed base at 2 Mbps and 11 Mbps is composed of direct-sequence interfaces.

DTIM: Delivery Traffic Indication Map. Beacon frames may contain the DTIM element, which is used to indicate that broadcast and multicast frames buffered by the access point will be delivered shortly.