Timing synchronization function (TSF) is specified in IEEE 802.11 wireless local area network (WLAN) standard to fulfill timing synchronization among users. A TSF keeps the timers for all stations in the same basic service set (BSS) synchronized. All stations shall maintain a local TSF timer. Each mobile host maintains a TSF timer with modulus 264 counting in increments of microseconds. The TSF is based on a 1-MHz clock and "ticks" in microseconds. On a commercial level, industry vendors assume the 802.11 TSF's synchronization to be within 25 microseconds[ citation needed ].
IEEE 802.11 is part of the IEEE 802 set of LAN protocols, and specifies the set of media access control (MAC) and physical layer (PHY) protocols for implementing wireless local area network (WLAN) Wi-Fi computer communication in various frequencies, including but not limited to 2.4, 5, and 60 GHz frequency bands.
In IEEE 802.11 (Wi-Fi) terminology, a station is a device that has the capability to use the 802.11 protocol. For example, a station may be a laptop, a desktop PC, PDA, access point or Wi-Fi phone. An STA may be fixed, mobile or portable. Generally in wireless networking terminology, a station, wireless client and node are often used interchangeably, with no strict distinction existing between these terms. A station may also be referred to as a transmitter or receiver based on its transmission characteristics. IEEE 802.11-2007 formally defines station as: Any device that contains an IEEE 802.11-conformant media access control (MAC) and physical layer (PHY) interface to the wireless medium (WM).
Timing synchronization is achieved by stations periodically exchanging timing information through beacon frames. In (infra) BSS, the AP sends the TSF information in the beacons. In Independent Basic Service Set (IBSS, ad-hoc), each station competes to send the beacon.
Each station maintains a TSF timer counting in increments of microseconds (μs). Stations adopt a received timing if it is later than the station’s own TSF timer.
All stations in the IBSS adopt a common value, aBeaconPeriod, that defines the length of beacon intervals or periods. This value, established by the station that initiates the IBSS, defines a series of target beacon transmission times (TBTTs) exactly aBeaconPeriod time units apart. Time zero is defined to be a TBTT.
All stations in the IBSS compete for beacon transmission every aBeaconPeriod time units. This time period is called a beacon period (BP). At the beginning of each BP, there is a beacon generation window consisting of w + 1 slots each of length aSlotTime. Each station calculates a random delay uniformly distributed in [0, w] and is scheduled to transmit a beacon when the delay timer expires. If a beacon arrives before the random delay timer has expired, the station cancels the pending beacon transmission and the remaining random delay. Upon receiving a beacon, a station sets its TSF timer to the timestamp of the beacon if the value of the timestamp is later than the station’s TSF timer.
Time-division multiple access (TDMA) is a channel access method for shared-medium networks. It allows several users to share the same frequency channel by dividing the signal into different time slots. The users transmit in rapid succession, one after the other, each using its own time slot. This allows multiple stations to share the same transmission medium while using only a part of its channel capacity. TDMA is used in the digital 2G cellular systems such as Global System for Mobile Communications (GSM), IS-136, Personal Digital Cellular (PDC) and iDEN, and in the Digital Enhanced Cordless Telecommunications (DECT) standard for portable phones. It is also used extensively in satellite systems, combat-net radio systems, and passive optical network (PON) networks for upstream traffic from premises to the operator. For usage of Dynamic TDMA packet mode communication, see below.
A wireless LAN (WLAN) is a wireless computer network that links two or more devices using wireless communication to form a local area network (LAN) within a limited area such as a home, school, computer laboratory, campus, office building etc. This gives users the ability to move around within the area and yet still be connected to the network. Through a gateway, a WLAN can also provide a connection to the wider Internet.
Carrier-sense multiple access with collision detection (CSMA/CD) is a media access control method used most notably in early Ethernet technology for local area networking. It uses carrier-sensing to defer transmissions until no other stations are transmitting. This is used in combination with collision detection in which a transmitting station detects collisions by sensing transmissions from other stations while it is transmitting a frame. When this collision condition is detected, the station stops transmitting that frame, transmits a jam signal, and then waits for a random time interval before trying to resend the frame.
IEEE 802.11e-2005 or 802.11e is an approved amendment to the IEEE 802.11 standard that defines a set of quality of service (QoS) enhancements for wireless LAN applications through modifications to the media access control (MAC) layer. The standard is considered of critical importance for delay-sensitive applications, such as Voice over Wireless LAN and streaming multimedia. The amendment has been incorporated into the published IEEE 802.11-2007 standard.
IEEE 802.15.4 is a technical standard which defines the operation of low-rate wireless personal area networks (LR-WPANs). It specifies the physical layer and media access control for LR-WPANs, and is maintained by the IEEE 802.15 working group, which defined the standard in 2003. It is the basis for the Zigbee, ISA100.11a, WirelessHART, MiWi, 6LoWPAN, Thread and SNAP specifications, each of which further extends the standard by developing the upper layers which are not defined in IEEE 802.15.4. In particular, 6LoWPAN defines a binding for the IPv6 version of the Internet Protocol (IP) over WPANs, and is itself used by upper layers like Thread.
In IEEE 802.11 wireless local area networking standards, a service set is a group of wireless network devices that are operating with the same networking parameters.
Clock synchronization is a topic in computer science and engineering that aims to coordinate otherwise independent clocks. Even when initially set accurately, real clocks will differ after some amount of time due to clock drift, caused by clocks counting time at slightly different rates. There are several problems that occur as a result of clock rate differences and several solutions, some being more appropriate than others in certain contexts.
Beacon frame is one of the management frames in IEEE 802.11 based WLANs. It contains all the information about the network. Beacon frames are transmitted periodically, they serve to announce the presence of a wireless LAN and to synchronise the members of the service set. Beacon frames are transmitted by the access point (AP) in an infrastructure basic service set (BSS). In IBSS network beacon generation is distributed among the stations. For the 2.4 GHz spectrum, having more than 15 SSIDs on overlapping channels and beacon frames start to consume significant amount of air time and degrade performance even when most of the networks are idle.
In the GSM cellular mobile phone standard, timing advance value corresponds to the length of time a signal takes to reach the base station from a mobile phone. GSM uses TDMA technology in the radio interface to share a single frequency between several users, assigning sequential timeslots to the individual users sharing a frequency. Each user transmits periodically for less than one-eighth of the time within one of the eight timeslots. Since the users are at various distances from the base station and radio waves travel at the finite speed of light, the precise arrival-time within the slot can be used by the base station to determine the distance to the mobile phone. The time at which the phone is allowed to transmit a burst of traffic within a timeslot must be adjusted accordingly to prevent collisions with adjacent users. Timing Advance (TA) is the variable controlling this adjustment.
The Precision Time Protocol (PTP) is a protocol used to synchronize clocks throughout a computer network. On a local area network, it achieves clock accuracy in the sub-microsecond range, making it suitable for measurement and control systems. PTP is currently employed to synchronize financial transactions, mobile phone tower transmissions, sub-sea acoustic arrays, and networks that require precise timing but lack access to satellite navigation signals.
IEEE 802.11p is an approved amendment to the IEEE 802.11 standard to add wireless access in vehicular environments (WAVE), a vehicular communication system. It defines enhancements to 802.11 required to support Intelligent Transportation Systems (ITS) applications. This includes data exchange between high-speed vehicles and between the vehicles and the roadside infrastructure, so called V2X communication, in the licensed ITS band of 5.9 GHz (5.85-5.925 GHz). IEEE 1609 is a higher layer standard based on the IEEE 802.11p. It is also the basis of a European standard for vehicular communication known as ETSI ITS-G5.
IEEE 802.11w-2009 is an approved amendment to the IEEE 802.11 standard to increase the security of its management frames.
Audio-to-video synchronization refers to the relative timing of audio (sound) and video (image) parts during creation, post-production (mixing), transmission, reception and play-back processing. AV synchronization can be an issue in television, videoconferencing, or film.
Short Interframe Space (SIFS), is the amount of time in microseconds required for a wireless interface to process a received frame and to respond with a response frame. It is the difference in time between the first symbol of the response frame in the air and the last symbol of the received frame in the air. A SIFS time consists of the delay in receiver RF, PLCP delay and the MAC processing delay, which depends on the physical layer used. In IEEE 802.11 networks, SIFS is the interframe spacing prior to transmission of an acknowledgment, a Clear To Send (CTS) frame, a block ack frame that is an immediate response to either a block ack request frame or an A-MPDU, the second or subsequent MPDU of a fragment burst, a station responding to any polling a by point coordination function and during contention free periods of point coordination function.
Audio Video Bridging (AVB) is a common name for the set of technical standards developed by the Institute of Electrical and Electronics Engineers (IEEE) Audio Video Bridging Task Group of the IEEE 802.1 standards committee. This task group was renamed to Time-Sensitive Networking Task Group in November 2012 to reflect the expanded scope of work.
IEEE 802.11ah is a wireless networking protocol published in 2017 to be called Wi-Fi HaLow as an amendment of the IEEE 802.11-2007 wireless networking standard. It uses 900 MHz license exempt bands to provide extended range Wi-Fi networks, compared to conventional Wi-Fi networks operating in the 2.4 GHz and 5 GHz bands. It also benefits from lower energy consumption, allowing the creation of large groups of stations or sensors that cooperate to share signals, supporting the concept of the Internet of Things (IoT). The protocol's low power consumption competes with Bluetooth and has the added benefit of higher data rates and wider coverage range.
Time-Sensitive Networking (TSN) is a set of standards under development by the Time-Sensitive Networking task group of the IEEE 802.1 working group. The TSN task group was formed in November 2012 by renaming the existing Audio Video Bridging Task Group and continuing its work. The name changed as a result of extension of the working area of the standardization group. The standards define mechanisms for the time-sensitive transmission of data over Ethernet networks.
The Reference Broadcast Infrastructure Synchronization (RBIS) protocol is a master/slave synchronization protocol. RBIS, as the Reference Broadcast Time Synchronization (RBS), is a receiver/receiver synchronization protocol, as a consequence timestamps used for clock regulation are acquired only on the receiving of synchronization events. RBIS is specifically tailored to be used in IEEE 802.11 Wi-Fi networks configured in infrastructure mode. Such a kind of networks are based on an access point that coordinates the communication between the wireless nodes, and they are very common.