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IEEE 802.11 AD HOC NETWORKS: PROTOCOLS, PERFORMANCE, AND OPEN ISSUES

IEEE 802.11 AD HOC NETWORKS: PROTOCOLS, PERFORMANCE, AND OPEN ISSUES. Paper by Anastasi, Conti, and Gregori. 802.X standards (working groups). IEEE 802.1Bridging (networking) and Network Management IEEE 802.2Logical link control inactive IEEE 802.3 Ethernet

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IEEE 802.11 AD HOC NETWORKS: PROTOCOLS, PERFORMANCE, AND OPEN ISSUES

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  1. IEEE 802.11 AD HOC NETWORKS: PROTOCOLS, PERFORMANCE, AND OPEN ISSUES Paper by Anastasi, Conti, and Gregori

  2. 802.X standards (working groups) • IEEE 802.1Bridging (networking) and Network Management • IEEE 802.2Logical link control inactive • IEEE 802.3 Ethernet • IEEE 802.4Token bus disbanded • IEEE 802.5Defines the MAC layer for a Token Ring inactive • IEEE 802.6Metropolitan Area Networks disbanded • IEEE 802.7Broadband LAN using Coaxial Cable disbanded • IEEE 802.8Fiber Optic TAG disbanded • IEEE 802.9Integrated Services LAN disbanded • IEEE 802.10Interoperable LAN Security disbanded • IEEE 802.11 a/b/g/n Wireless LAN & Mesh (Wi-Fi certification) • IEEE 802.11p add access in vehicular environments (WAVE), being developed • IEEE 802.12demand priority disbanded • IEEE 802.13 Not used (officially) (may be because 13 is unlucky number?)

  3. 802.X standards…. • IEEE 802.14Cable modems ,disbanded • IEEE 802.15Wireless PAN • IEEE 802.15.1Bluetooth certification • IEEE 802.15.4ZigBee certification • IEEE 802.16 Broadband Wireless Access (WiMAX certification) • IEEE 802.16e(Mobile) Broadband Wireless Access • IEEE 802.16.1 Local Multipoint Distribution Service • IEEE 802.17Resilient packet ring • IEEE 802.18Radio Regulatory TAG • IEEE 802.19Coexistence TAG • IEEE 802.20Mobile Broadband Wireless Access • IEEE 802.21Media Independent Handoff • IEEE 802.22Wireless Regional Area Network • IEEE 802.23Broadband ISDN system ,experimental • (Source : Wikipedia)

  4. Some important 802.11 standards • 802.11a : Higher speed PHY extension in 5GHz band, published as standard 802.11a-1999 • 802.11b: Higher speed PHY extension in the 2.5GHz band, published as standard 802.11b-1999 • 802.11e: Quality of service. IEEE Std 802.11e-2005 • 802.11n: Enhanced higher throughput,IEEE Std 802.11n-2007, operates in both 2.4GHz and 5GHz, 600Mbit/s. • 802.11p: Inter vehicle and vehicle-roadside communication in the licensed ITS band 5.9GHz. • 802.11s: Mesh networks. Expected to be completed in 2011

  5. IEEE 802.11 - Introduction • IEEE 802.11 standard defines two operational modes • Infrastructure-based: Commonly used to construct Wi-Fi hot spots – i.e., to provide access to Internet via an access point (AP). • Infrastructureless or adhoc: There is no concept of access point. • Any station within the transmission range of another station, can start communicating after synchronization phase. • So, this technology is good for implementing single hop adhoc networks. For implementing multihopadhoc networks, additional routing mechanisms need to be implemented at the stations for this.

  6. IEEE 802.11 Physical Layers • Issued in several stages: • First part in 1997: 802.11 • Include MAC Layer and three physical layer specifications • Two in 2.4 GHz band and one in infrared • All operating in 1 and 2Mbs • Two additional parts in 1999 • IEEE 802.11a-1999: 5GHz band, 54Mbps/20MHz channels, OFDM (this never caught on) • IEEE 802.11b-1999: 2.4GHz band, 11 Mbps/20MHz • Fourth Part: • IEEE 802.11g-2003: 2.4GHz band, 54Mbps/20MHz, OFDM • Fifth part: • IEEE 802.11n-2009: both in 2.4GHz and 5GHz, 600Mbps/40MHz channels, uses MIMO, technology

  7. IEEE 802.11 Architecture and Protocols

  8. 802.11 Architecture - Layers • The IEEE 802.11 standard specifies both the Media Access Control (MAC) layer and the Physical layer. • MAC layer offers two types of service: • Distributed Coordination function(DCF) • Point Coordination function(PCF) • Physical layer • Three technologies are specified: Infrared, Frequency hopping Spread Spectrum (FHSS) and Direct Sequence Spread Spectrum (DSSS)

  9. DCF • DCF provides the basic access method for the 802.11 MAC protocol and is based on a Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) scheme • Before transmitting a data frame, a station must sense the channel to determine whether any other station is transmitting. • If the medium is idle for an interval longer than the Distributed InterFrame Space (DIFS), the station continues with its transmission. Else, defer transmission until the end of the ongoing transmission.

  10. DCF – Back-off Timer • A Back-off time which is a random interval is selected to initialize the back-off timer. • The back-off timer is decreased for as long as the channel is idle, stopped when a transmission is detected, and re-activated when the channel is idle again for more than a DIFS. • The backoff time is slotted; i.e., the backoff time is an integer number of slots uniformly chosen in the interval (0,CW-1). CW is defined as the backoff window; intially, CW = CWmin and is doubled at each retransmission upto CWmax. For example, for the FHSS physical layer, CWmin andCWmax values are 16 and 1024.

  11. DCF - SIFS • When a collision occurs (two or more stations start transmitting simultaneously), in the CSMA/CA scheme, stations are not able to detect a collision by hearing their own transmissions. • Immediate positive acknowledgement (Ack) scheme is sent after the reception of a frame after a time interval of Short InterFrame Space (SIFS) to confirm the successful reception of a frame. If the Ack is not received by the source, a retransmission is scheduled. • Ack is not transmitted if the packet is corrupted.

  12. DCF – Source-Destination Interaction

  13. DCF – CRC and EIFS • Cyclic Redundancy Check (CRC) is used for error detection in transmission. • After an erroneous frame is detected, a station must remain idle for at least an Extended InterFrame Space (EIFS) before it re-activates the back-off algorithm. • Reception of an error-free frame during the EIFS re-synchronizes the station to the actual busy/idle state of the medium, so EIFS is terminated and normal medium access continues following reception of the frame.

  14. Problems • Two common problems in Wireless Ad Hoc Network: • The hidden-station problem • The exposed-station problem

  15. The Hidden-Station Problem • The hidden-station or node problem occurs when a station or node is visible from a wireless hub, but not from other nodes communicating with the hub. This leads to difficulties in MAC. A and B can each communicate with the hub, but are hidden from each other

  16. The Hidden-Station Solution • The hidden station problem can be alleviated by extending the DCF basic mechanism by a Virtual Carrier Sensing Mechanism (VCSM - also referred to as Floor Acquisition Mechanism or FAM) • VCSM is based on two control frames: Request To Send (RTS) and Clear To Send (CTS), respectively.

  17. Request To Send and Clear To Send • Before transmitting a data frame, the station sends a short control frame, named RTS, to the receiving station announcing the upcoming frame transmission. • Upon receiving the RTS frame, the destination station replies by a CTS frame to indicate that it is ready to receive the data frame. • Both the RTS and CTS frames contain the total duration of the transmission, i.e., the overall time interval needed to transmit the data frame and the related ACK. This information can be read by any listening station that uses this information to set up a timer called Network Allocation Vector (NAV).

  18. Network Allocation Vector • While the NAV timer is greater than zero the station must refrain from accessing the wireless medium. By using the RTS/CTS mechanism, stations may become aware of transmissions from hidden station and on how long the channel will be used for these transmissions.

  19. The Exposed-Station Problem

  20. The Exposed-Station Problem • Assume stations A and C can hear transmissions from B, but A cannot hear from C. • Further assume B is transmitting to A and C receives a frame to be transmitted to D. • According to DCF protocol, C finds the channel busy because of B’s transmission. • Therefore, the medium refrains from transmitting to C although this transmission would not cause a collision at A. • The Exposed-Station problem may cause throughput reduction.

  21. The Exposed-Station Solution • A node can identify itself as an exposed node if it hears an RTS frame but not a CTS frame from the other transmitting node. • Therefore, it concludes that it can have a simultaneous transmission . • Avoiding the reduction in throughput.

  22. IBBS As Networking Support • Ad Hoc = Independent Basic Service Set (IBBS) • IBBS enables two or more 802.11 stations to communicate to each other without interventions. Therefore, IBBS can be considered as the support provided by the 802.11 standard for mobile ad hoc networking. • To uniquely identify a IBSS, it is assigned an identification number (IBSSID – 46-bit random number) that is locally administered and used to join the ad hoc network.

  23. Timing Synchronization Function (TSF) • Due to flexibility of CSMA/CA protocol, it is sufficient for all stations within the IBSS to synchronize to a common clock. The 802.11 standard specifies a Timing Synchronization Function (TSF) to achieve this clock synchronization between stations. • In ad hoc network, clock synchronization is achieved through a distributed algorithm by transmitting special frames called beacons that contain timing information.

  24. TSF - continued • TSF requires two fundamental functionalities: • Synchronization Maintenance • Synchronization Acquirement

  25. Synchronization Maintenance • Each station has a TSF timer with modulus 264 counting in increments of micoseconds and expect to receive beacons at a rate defined by the Beacon Period parameter. • Each station uses its TSF timer to determine the beginning of beacon intervals. • The sending station sets the beacon timestamp to the value of its TSF timer at the time the beacon is transmitted. • Upon reception of a beacon, the receiving station looks at the timestamp.

  26. Synchronization Maintenance – continued • If the beacon timestamp is later than the station’s TSF timer, the TSF timer is set to the value of the received timestamp. • In other words, all stations within the IBSS synchronize their TSF timer to the fastest TSF timer.

  27. Synchronization Acquirement • This is needed for a new station to join an already existing IBSS. • If a network already exists, the scanning (tuning to different radio frequencies looking for particular control frames) will reveal the finding of the network. • If network does not exist, the new station can create a new IBSS. • Scanning procedures: active and passive.

  28. Active Scanning • Generates probe frames. • A station sends a Probe Request Frame when it needs to obtain information from another station. For example, a radio NIC would send a probe request to determine which access points are within range. • Subsequently, another station will respond with a Probe Response Frame, containing capability information, supported data rates, etc., after it receives a Probe Request Frame.

  29. Passive Scanning • A station listens to the channels for a beacon frame. • Passive scanning allows a mobile wireless NIC to find an IEEE 802.11 network while minimizing DC power consumption. In this mode, the wireless NIC listens for special frames called beacons and probe responses, while extracting information about the particular frequency channel. • Although passive scanning expends minimal power, the cost is the time spent listening for a frame on a channel that is idle or may never occur.

  30. Power Management • Mobile devices are battery powered. • Enhancement of battery life (Power-Saving Strategies or PS) enhances network lifetime. • Power states: • Awake: The station is fully powered • Doze: The station is not able to transmit or receive Power management within ad hoc network is based on the idea that AP buffers all frames for stations in Power- Saving mode.

  31. Power Saving – continued • For unicast frame, with every beacon transmitted by AP, a Traffic Indication Map (TIM) is transmitted. TIM contains the list of stations for which AP has data frames buffered. • The TSF ensures that stations wake up periodically and listen to beacon frames. If the station is within TIM, the station stays awake for transmission. For broadcast/multicast frames, AP maintains a Delivery Traffic Indication Map (DTIM) interval for sending the frames. DTIM interval is a multiple of TIM interval.

  32. Power Saving In Ad Hoc • In ado hoc networks, power management idea is to have station nodes buffer data if it wants to communicate with a power-saving station. • All stations announce a list of buffered frames using ad hoc traffic indication map ATIM after beacon period during ad hoc traffic indication map ATIM widow. • If a station receives ATIM by station, it acknowledges this ATIM and stays awake for the transmission.

  33. WiFi • Almost all wireless LANs are IEEE 802.11 based • 802.11 is also known as WiFi =“Wireless Fidelity” • Fidelity = Compatibility between wireless equipments from different manufacturers • WiFi Alliance is a non-profit organization that does the compatibility testing (WiFi.org)

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