802.11ac: A Survival Guide

Glossary

ACK

Abbreviation for “acknowledgement.” ACKs are used extensively in 802.11 to provide reliable data transfers over an unreliable medium. For more details, see “Contention-Based Data Service” in Chapter 3 of 802.11 Wireless Networks: The Definitive Guide.

See Also Block ACK, Implicit feedback.

AES

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

AP

Access Point. A bridge-like device that attaches wireless 802.11 stations to a wired backbone network. For more information on the general structure of an access point, see Chapter 20 of 802.11 Wireless Networks: The Definitive Guide.

AS

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

Basic Block ACK

The original block acknowledgement specification in the 802.11e amendment allowed a receiver of a group of frames to selectively acknowledge individual 802.11 fragments. Extensions in 802.11n make the protocol more efficient for use with 802.11n networks.

See Also Compressed Block ACK.

Basic service set

See BSS.

Beamforming

A method of using precise phase shifts on an antenna array that focuses the resulting transmission in a particular direction. Sending beamformed transmissions may require an exchange of control information to set up the antenna array.

Beamformee

The receiver of a beamformed transmission. The beamformee may need to transmit some packets in a beamforming setup exchange, but the main purpose of the beamforming exchange is to receive a directional transmission.

Beamformer

The sender of a beamformed transmission. The beamformer may need to receive some packets in a beamforming setup exchange, but the main purpose of such an exchange is to send a directional transmission.

Block ACK

A mechanism that allows the recipient of a series of frames to transmit one acknowledgement for the entire series. It enables selective acknowledgement of each frame in the series. By transmitting just one umbrella ACK frame, it makes substantially more efficient use of airtime than the traditional positive ACK transmitted in response to a single frame.

Block ACK Request

The Block ACK Request (BAR) frame is sent prior to a series of frames that the transmitter would like to be acknowledged. Without a block ACK request, the receiver cannot send a block ACK.

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 one another.

BSSID

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

Code rate

In the context of a forward error correcting code, the code rate describes the fraction of bits devoted to error correction, and is typically symbolized by R. For example, an R=1/2 code takes the input data stream and encodes every payload bit as two bits. Codes can be described as conservative, or able to correct large errors. Conversely, a code rate may be aggressive, meaning that error correction capacity is being sacrificed for efficiency. The lower the code rate, the more conservative a code is; coding at R=1/2 enables more error recovery than coding at R=5/6.

Compressed Block ACK

A new block ACK extension defined by 802.11n. The “compression” referred to in the name refers to the fact that the compressed block ACK mechanism can only acknowledge nonfragmented frames. 802.11n uses such large aggregate frames that fragmentation is not commonly used, and the block ACK window can be made substantially more efficient by acknowledging at the frame level instead of the fragment level.

See Also Block ACK, Basic Block ACK.

Constellation

A set of points that describes a precise phase shift and amplitude. By transmitting a carrier wave with a given phase shift and amplitude, the sender conveys a symbol to the receiver.

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 in wireless LANs for security.

CCMP

Counter Mode with CBC-MAC Protocol. 802.11i-2004 defined the use of AES with the CCM mode of operation as CCMP. It is the strongest encryption protocol available for use with wireless LANs, and the only security protocol allowed for use with 802.11n.

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 (collisions) 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.

Delayed Block ACK

A method of transmitting a block ACK some time after the last data frame in the burst to be acknowledged has been successfully received.

DFS

Dynamic Frequency Selection. A spectrum management service required by European radio regulations (European Commission decisions 2005/513/EC and 2007/90/EC, along with ETSI EN 301 893) to avoid interfering with 5 GHz radar systems, as well as to spread power across all available channels. DFS was also key to the FCC decision to open up the harmonized frequency band in the US.

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 connect 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.

EAP

Extensible Authentication Protocol. An authentication framework that is frequently used in wireless networks; it supports multiple authentication methods

ESS

Extended Service Set. A logical collection of access points all tied together. Link-layer roaming is possible throughout an ESS, provided all the stations are configured to recognize each other.

ETSI

European Telecommunications Standards Institute. ETSI is a multinational standardization body with regulatory and standardization authority over much of Europe. GSM standardization took place under the auspices of ETSI.

Explicit feedback

When used with beamforming, this refers to a beamforming method that requires frames to be sent between the two parties to a beamformed transmission. The beamformee must send frames that help the beamformer calibrate future transmissions.

FEC

Forward Error Correction. A type of code in which the transmitter takes the payload for transmission and encodes it with redundant bits to enable the receiver to correct errors. There are two main types: convolutional codes that work on arbitrary-length streams of data, and block codes that work on fixed-length blocks.

FCC

Federal Communications Commission. The regulatory agency for the United States. The FCC Rules in Title 47 of the Code of Federal Regulations govern telecommunications in the United States. Wireless LANs must comply with Part 15 of the FCC rules, which are written specifically for RF devices.

FCS

Frame Check Sequence. A checksum appended to frames on IEEE 802 networks to detect corruption. If the receiver calculates a different FCS than the FCS in the frame, it is assumed to have been corrupted in transit and is discarded.

FIPS

Federal Information Processing Standard. Public standards used by nonmilitary agencies of the United States federal government and its contractors.

Four-way handshake

The key exchange defined in 802.11i that expands a pairwise master key into the full key hierarchy. The four-way handshake allows a supplicant and an authenticator to agree on dynamically derived encryption keys.

GCMP

Galois-Counter Mode Protocol. A combination of the well-known counter mode with Galois field multiplication for authentication. It provides similar security to CCMP with significantly higher performance.

GMK

Group Master Key. The key used by an authenticator to derive the group transient key.

GTK

Group Transient Key. Derived by combining the group master key with the group random number, the GTK is used to derive the group key hierarchy, which includes keys used to protect broadcast and multicast data.

HR/DSSS

High-Rate Direct-Sequence Spread Spectrum. The abbreviation for signals transmitted by 802.11b equipment. Although similar to the earlier 2 Mbps transmissions in many respects, advanced encoding enables a higher data rate.

HT

High Throughput. The official name of the 802.11n PHY, and a common abbreviation that is used colloquially to mean “802.11n.”

IEEE

Institute of Electrical and Electronics Engineers. The professional body that has standardized the ubiquitous IEEE 802 networks.

Immediate Block ACK

A style of block ACK in which the Block ACK frame is sent immediately following the frames that it is acknowledging.

Implicit feedback

A method of beamforming where no explicit communication takes place between the beamformer and beamformee. Implicit feedback often uses the received frames themselves to estimate the required channel calibration. It does not produce as effective a steering matrix, but it does not require software support at both ends of the link.

ISM

Industrial, Scientific, and Medical. Part 15 of the FCC Rules sets aside certain frequency bands in the United States for use by unlicensed ISM equipment. The 2.4 GHz ISM band was initially set aside for microwave ovens so that home users of microwave ovens would not be required to go through the burdensome FCC licensing process simply to reheat leftover food quickly. Because it is unlicensed, though, many devices operate in the band, including 802.11 wireless LANs.

ITU

International Telecommunications Union. The successor to the Consultative Committee for International Telephony and Telegraphy (CCITT). Technically speaking, the ITU issues recommendations, not regulations or standards. However, many countries give ITU recommendations the force of law.

LDPC

Low-Density Parity Check. A block error-correction code that can optionally be used in 802.11.

LLC

Logical Link Control. An IEEE specification that allows further protocol multiplexing over Ethernet. 802.11 frames carry LLC-encapsulated data units.

MAC

Medium Access Control. The function in IEEE networks that arbitrates use of the network capacity and determines which stations are allowed to use the medium for transmission.

MCS

Modulation and Coding Set. A number that describes both the modulation and the forward error correcting code used.

MIMO

Multiple-Input/Multiple-Output. An antenna configuration that uses more than one transmission antenna and more than one receiver antenna to transmit multiple data streams. MIMO antenna configurations are often described with the shorthand “Y×Z,” where Y and Z are integers, used to refer to the number of transmitter antennas and the number of receiver antennas, respectively.

MPDU

MAC Protocol Data Unit. A fancy name for frame. The MPDU does not, however, include PLCP headers.

MRC

Maximal Ratio Combining. A method of combining the signals from multiple antennas in an antenna array to boost the signal-to-noise ratio of a received frame. MRC uses the “extra” radio chains in an antenna array to provide additional information.

MSDU

MAC Service Data Unit. The data accepted by the MAC for delivery to another MAC on the network. MSDUs are composed of higher-level data only. For example, an 802.11 management frame does not contain an MSDU.

Multi-user

In 802.11ac, a multi-user transmission is a transmission that sends distinct frames for each member of a set of receivers. In 802.11ac, up to four receivers can be designated for a multi-user transmission.

MU-MIMO

Multi-User MIMO. The application of MIMO techniques to send different transmissions to multiple users simultaneously.

NAV

Network Allocation Vector. The NAV is used to implement the virtual carrier-sensing function. Stations will defer access to the medium if it is busy. For robustness, 802.11 includes two carrier-sensing functions. One is a physical function, which is based on energy thresholds, whether a station is decoding a legal 802.11 signal, and similar things that require a physical measurement. The second is a virtual carrier sense function, which is based on the NAV. Most frames include a nonzero number in the NAV field, which is used to ask all stations to politely defer from accessing the medium for a certain number of microseconds after the current frame is transmitted. Any receiving stations will process the NAV and defer access, which prevents collisions. For more detail on how the NAV is used, see “Contention-Based Data Service” in Chapter 3 of 802.11 Wireless Networks: The Definitive Guide.

Noise floor

The level of ambient background “static” in an area. Transmissions must rise above the noise floor in order to be received. A good analogy for the noise floor is the burble of conversations within a room where a party is being held. In order to hear and understand a single voice, you have to be able to concentrate on it so you can hear it over the background level.

OBSS

Overlapping BSS. Refers to another network installed in the same physical space on the same channel, whether it is part of the same ESS or not. If two access points were installed next to each other on channel 6, each would be an OBSS of the other.

OFDM

Orthogonal Frequency Division Multiplexing. A technique that splits a wide frequency band into a number of narrow frequency bands and inverse-multiplexes data across the subchannels. 802.11a and 802.11g are based on OFDM. 802.11n uses MIMO to transmit multiple OFDM data streams.

PDU

Layers communicate with each other using protocol data units. For example, the IP protocol data unit is the familiar IP packet. IP implementations communicate with each other using IP packets.

See Also SDU.

PHY

Common IEEE abbreviation for the physical layer.

PMK

Pairwise Master Key. The root of all keying data between a supplicant and an authenticator. It may be derived from an Extensible Authentication Protocol (EAP) method during authentication, or supplied directly as a preshared key.

PPDU

PLCP Protocol Data Unit. The complete PLCP frame, including PLCP headers, MAC headers, the MAC data field, and the MAC and PLCP trailers.

protocol data unit

See PDU.

PS

Power Save. Used as a generic prefix for power-saving operations in 802.11.

PSDU

PLCP Service Data Unit. The data the PLCP is responsible for delivering. Typically it will be one frame from the MAC, with headers. In 802.11, however, the PSDU may consist of an aggregate of several MAC service data units.

PSK

Pre-Shared Key. In 802.11i, this refers to an authentication method depending on a statically configured authentication key that must be distributed manually. Also called WPA-PSK.

PSMP

Power-Save Multi-Poll. A power-saving system specific to 802.11n that improves both power efficiency and airtime efficiency by scheduling transmissions to associated clients.

QAM

Quadrature Amplitude Modulation. A modulation method that varies both the amplitude and phase simultaneously to represent a symbol of several bits. 802.11n uses both 16-QAM and 64-QAM at higher transmission rates.

QPSK

Quadrature Phase Shift Keying. A modulation method that encodes bits as phase shifts. One of four phase shifts can be selected to encode two bits.

RA

Receiver Address. The MAC address of the station that will receive the frame. The RA may also be the destination address of a frame, but is not always. In infrastructure networks, for example, a frame destined for the distribution system is received by an access point.

RADIUS

Remote Authenticated Dial-In User Service. A protocol used to authenticate dial-in users that has become more widely used because of 802.1X authentication. The most common type of authentication server used in 802.1X systems.

RLAN

Radio LAN. A term used by European radio regulations to refer to any wireless network built on radio technology. Although 802.11 is the most popular, others do exist. One of the better known alternative radio network technologies is ETSI’S HIPERLAN.

RF

Radio Frequency. Used as an adjective to indicate that something pertains to the radio interface (“RF modulator,” “RF energy,” and so on).

RIFS

Reduced Interframe Space. A shortened frame separator that allows better use of available airtime when two HT devices are communicating with each other.

RSN

Robust Security Network. A network that uses the security methods originally defined 802.11i-2004 and does not provide any support for the use of WEP.

RSSI

Received Signal Strength Indication. This is a value reported for the strength of a frame that has been received; it acts much like a “volume” indicator for the transmission. The RSSI may be reported in many different ways, but a common method is in dBm.

RTS

Request to Send. The frame type used to begin the RTS/CTS clearing exchange. RTS frames are used when the frame that will be transmitted is larger than the RTS threshold.

SA

Source Address; as disinct from TA. The station that generated the frame. Different when the frame originates on the distribution system and goes to the wireless segment.

SDU

When a protocol layer receives data from the next highest layer, it is sending a service data unit. For example, an IP service data unit can be composed of the data in the TCP segment plus the TCP header. Protocol layers access service data units, add the appropriate header, and push them down to the next layer.

See Also PDU.

Service Data Unit

See SDU.

SIFS

Short Interframe Space. The shortest of the four interframe spaces. The SIFS is used between frames in an atomic frame exchange.

Spatial stream

MIMO techniques are sometimes called spatial reuse because a MIMO system will send multiple independent data streams between the transmitter and the receiver. Each data stream is called a spatial stream because it takes a different path through space between the transmitter and receiver. An 802.11n device may have up to four spatial streams. For any given transmission, the maximum number of spatial streams is defined by the lower number.

Single user

A single-user transmission is a frame that is sent to one recipient. Contrast with multi-user.

SSID

Service Set Identifier. A string used to identify an extended service set. Typically, the SSID is a recognizable character string for the benefit of users.

STBC

Space-Time Block Coding. A method of transmitting a single data stream across multiple antennas for additional transmission redundancy.

TA

Transmitter Address. The station that actually put the frame in the air. Often the access point in infrastructure networks.

TIM

Traffic Indication Map. A field transmitted in Beacon frames used to inform associated stations that the access point has buffered. Bits are used to indicate buffered unicast frames for each associated station as well as the presence of buffered multicast frames.

TK

Temporal Key. 802.11i key hierarchies derive a temporal key to be used for authentication protocols. The temporal key is the main input to link-layer encryption protocols such as TKIP or CCMP.

TKIP

Temporal Key Integrity Protocol. One of the improved encryption protocols in 802.11i, TKIP uses the fundamental operations of WEP with new keying and integrity check mechanisms to offer additional security. 802.11n clearly forbids the use of TKIP with 802.11n frames.

WEP

Wired Equivalent Privacy; derided as “Wiretap Equivalence Protocol"” by its critics. A standard for ciphering individual data frames. It was intended to provide minimal privacy and has succeeded in this respect. In August 2001, WEP was soundly defeated, and public code was released. WEP is not supported by 802.11n devices.

Wi-Fi

An umbrella term used to refer to wireless LANs in general, and a testament to the strength of the Wi-Fi Alliance’s branding activities. “Wi-Fi” is often used interchangeably with “wireless LAN” or “802.11.”

Wi-Fi Alliance

The Wi-Fi Alliance (formerly the Wireless Ethernet Compatibility Alliance) started the Wi-Fi certification program to test interoperability of 802.11 implementation. Originally, the term was applied to devices that complied with 802.11b (11 Mbps HR/DSSS), but further programs have extended PHY interoperability testing to include 802.11a, 802.11g, 802.11n and 802.11ac, as well as security.

Wi-Fi CERTIFIED

Trademark of the Wi-Fi Alliance used to indicate that a particular device has passed an interoperability test. Once certified, a product’s capabilities are published in the Wi-Fi Alliance certification database, and an interoperability certificate lists certified capabilities.

WPA and WPA2

Wi-Fi Protected Access. A security standard based on 802.11i draft 3. The Wi-Fi Alliance took 802.11i draft 3 and began certifying compliance with early TKIP implementations to accelerate adoption of 802.11 security protocols. WPA2 is based on the full ratified version of 802.11i-2004. Products certified with 802.11n are only allowed to use CCMP to encrypt high-speed 802.11n frames.