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Ubiquitous Networks WLAN

Ubiquitous Networks WLAN. Lynn Choi Korea University. Portal. Distribution System. 802.11 - Infrastructure Mode. Terminology Station (STA) Wireless terminals Basic Service Area (BSA) Coverage area of one access point Basic Service Set (BSS) Group of stations controlled by the same AP

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Ubiquitous Networks WLAN

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  1. Ubiquitous NetworksWLAN Lynn Choi Korea University

  2. Portal Distribution System 802.11 - Infrastructure Mode • Terminology • Station (STA) • Wireless terminals • Basic Service Area (BSA) • Coverage area of one access point • Basic Service Set (BSS) • Group of stations controlled by the same AP • Distribution System (DS) • Fixed infrastructure used to connect several BSS to create an Extended Service Set (ESS) • Portal • Bridge to other (wired) networks • Infrastructure mode • Every transmission is with AP • No peer to peer communication • Multiple BSSs form an ESS 802.11 LAN 802.x LAN STA1 BSS1 Access Point Access Point ESS BSS2 STA2 STA3 802.11 LAN

  3. 802.11 – Ad Hoc mode • Multiple stations and no AP • Peer to peer communication only • Independent BSS -> IBSS 802.11 LAN STA1 STA3 BSS1 STA2 BSS2 STA5 STA4 802.11 LAN

  4. Two Coordination Functions • DCF (Distributed Coordination Function) • Mandatory function • For both infrastructure mode and ad hoc mode • 99% of WLAN implements DCF only • For distributed contention-based channel access • PCF (Point Coordination Function) • Optional • For infrastructure mode only • For centralized contention-free channel access • Polling-based • Seldom-used

  5. Distributed Coordination Function (DCF) Contention window DIFS PIFS DIFS SIFS Busy medium Next frame Time Wait for reattempt time Defer access • Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) • Contention-based: all stations contend for access to medium • Stations wait as long as channel is busy • When channel becomes idle, • All stations must wait Interframe Space (IFS) • SIFS (Short IFS), PIFS (PCF IFS), DIFS (DCF IFS) • Use random backoff to avoid collision slot time

  6. CSMA (Carrier Sense Multiple Access) • ALOHA • Transmit whenever they have data. • On a collision, wait a random amount of time and retransmit • Pure ALOHA, slotted ALOHA • CSMA • Listen before transmit • If channel sensed idle, transmit entire frame • If channel sensed busy, defer transmission • Don’t interrupt others!

  7. CSMA (Carrier Sense Multiple Access) • CSMA/CA (Collision Avoidance) • Transmit after a random backoff even if an idle channel is detected • A variation of this called CSMA-CA is used in WLAN • CSMA-CD • Abort transmission as soon as they detect a collision • Used in Ethernet (802.3) • Exponential backoff • Backoff counter is randomly selected from [0, CW], where CW is contention window • For each unsuccessful transmission, CW doubles to reduce the collision probability

  8. Hidden Terminal Problem RA RC transmission Node A Node B Node C • Node A is not aware that node B is currently busy receiving from node C, and therefore may start its own transmission, causing a collision.

  9. Exposed Node Problem RD RA transmission Node D Node A Node B Node C • Node B wants to transmit to node C but mistakenly thinks that this will interfere with A’s transmission to D, so B refrains from transmitting (loss in efficiency).

  10. RTS-CTS Exchange CTS-Range AP STA STA Stations do not hear each other. But they hear the AP RTS-Range • A way to handle hidden terminal problem • RTS (Request to Send) / CTS (Clear to Send) to reserve medium • Work with virtual carrier sensing

  11. Net Allocation Vector This is called Virtual Carrier Sensing.. DIFS RTS data sender SIFS SIFS SIFS CTS ACK receiver DIFS NAV (RTS) Next frame other stations NAV (CTS) t defer access backoff The RTS packet has a duration field, which consists of information about the length of data packet. Other stations hear the RTS packet set their NAV accordingly. The CTS packet also has the duration field. Other stations hear the CTS packet set their NAV accordingly.

  12. PHY Evolution History • 11b • Published in 1999, market introduction in 1999 • WLAN became popular due to 11b • Up to 11Mbps (unregulated 2.4GHz band) • 11a • Published in 1999, market introduction in 2002 • Never have been popular • Up to 54Mbps (OFDM-based) by using (5GHz band) • 11g • Published in 2003 (OFDM-based) to combine the best of 11a &11b • Market introduction in 2003 • Emerging as a dominating form • Backward compatible with 11b • Up to 54Mbps by using 2.4GHz band • 11n • Published in October 2009 (OFDM-based) to combine the best of 11a, 11b &11g • Add MIMOand 40MHz channel (compared to 20MHz channels in 802.11b) to the physical layer and frame aggregation to the MAC layer • High data rate (54~600Mbps by using 2.4GHz band and lesser used 5GHz band) • Backward compatible with 11g

  13. Ubiquitous Networks802.15: WPAN Lynn Choi Korea University

  14. 802.15 • IEEE 802.15 • IEEE 802.15 is the 15th working group of the IEEE 802 and specializes in Wireless PAN (Personal Area Network) standards (PHY and MAC layer). • 7 Task groups • IEEE 802.15.1 (Bluetooth) • IEEE 802.15.2 (Coexistence): WPAN coexistence with other wireless devices operating in unlicensed frequency bands such as WLAN • IEEE 802.15.3 (High Rate WPAN) • IEEE 802.15.4 (Low Rate WPAN) • IEEE 802.15.5 (Mesh Networking): mesh networking with WPANs • Network initialization, addressing, multi-hop routing, multicast, etc. for 15.3/4 • IEEE 802.15.6 (BAN): body area network, short-range wireless standards • IEEE 802.15.7 (VLC): visible light communication

  15. Body Area Network (802.15.6) • Wireless communication for wearable computing devices • Communication between several small body sensor units (BSU) and a single body central unit (BCU) worn at the human body by using WPAN technologies • Applications • Healthcare • Monitoring and logging vital parameters such as diabetes, asthma, and heart attacks • Sports, Military, and Security • Sensors • Vital sign monitoring sensors, motion detectors • ECG, SpO2, blood pressure, EEG • Physical • 40MHz channel allocated for 2.4GHz band for medical applications

  16. 802.15 • IEEE 802.15.1 • IEEE 802.15.1 is the WPAN standard based on the Bluetooth specifications. It includes a media access control and physical layer specification. • Bluetooth is an open wireless technology standard for exchanging data over short distances (using short wavelength radio transmissions) from fixed and mobile devices, creating personal area networks (PANs) with high levels of security. • Created by telecoms vendor Ericsson in 1994, it was originally conceived as a wireless alternative to RS-232 data cables. It can connect several devices, overcoming problems of synchronization. • IEEE 802.15.3 • MAC and PHY standard for high-rate (11 to 55 Mbit/s) WPANs. • IEEE 802.15.4 • MAC and PHY standard for low data rate but very long battery life (months or even years) and very low complexity. The first edition of the 802.15.4 standard was released in May 2003.

  17. 802.15.4 • IEEE 802.15.4 • IEEE 802.15.4-2006 is a standard which specifies the physical layer and media access control for low-rate wireless personal area networks (LR-WPANs). • It is the basis for the ZigBee specification. • Can be used with 6LoWPAN to build a Wireless Embedded Internet. • ZigBee • A specification for a suite of high level communication protocols using small, low-power digital radios based on the IEEE 802.15.4-2003 standard. • For wireless home area networks (WHANs), such as wireless light switches, electrical meters with in-home-displays, consumer electronics equipment. • Simpler and less expensive than other WPANs, such as Bluetooth. • 6LoWPAN • 6lowpan (IETF working group) is an acronym of IPv6 over Low power WPAN • 6lowpan group has defined encapsulation and header compression mechanisms that allow IPv6 packets to be sent /received from over IEEE 802.15.4 networks. • Applications: wireless embedded internet, smart grid

  18. Zigbee Protocol Stack

  19. 802.15.4 PHY/MAC • PHY layer • Use unlicensed frequency bands • 868MHz (Europe) / 915 MHz (North America): 20~40 kbps up to 100~250kbps • 2.4GHz (World): 250kbps • Modulation • DSSS (Direct Sequence Spread Spectrum) • Binary or offset quadraturephase shift keying, amplitude shift keying • Direct sequence UWB (<1GHz, 1~3GHz, 6~10GHz), CSS (Chirp Spread Spectrum, 2.4GHz) • MAC layer • MAC allows the transmission of MAC frames through the use of the physical channel. It manages access to the physical channel through network beaconing, controls frame validation, guarantees time slots and handles node associations.

  20. 802.15.4 Node Type • Node type • Full-function device (FFD) serve as the coordinator of a personal area network just as it may function as a common node. It implements a general model of communication which allows it to talk to any other device: it may also relay messages, in which case it is dubbed a coordinator (PAN coordinator ) • Reduced-function devices (RFD) extremely simple devices with very modest resource and communication requirements; due to this, they can only communicate with FFD's and can never act as coordinators.

  21. 802.15.4 Topology • Topology • Either peer-to-peer or star networks • P2P: ad-hoc multi-hop network • Cluster Tree • Mesh network • Star network • A FFD declare itself its coordinator. After that, other devices can join. • Every network needs at least one FFD to work as the coordinator of the network. Network coordinator = Root of the cluster tree

  22. 802.15.4 MAC • Frames • Frames are the basic unit of data transport, of which there are four fundamental types (data, acknowledgment, beacon and MAC command frames) • Superframe • Each superframe consists of 16 equally sized slots and is bounded by network beacons, which are periodically broadcast by a designated coordinator device. • During the contention access period, the slotted mode of the CSMA-CA algorithm is used, while transmissions in the contention free period take place according to pre-assigned guaranteed timeslots. Up to 7 slots can be allocated by a coordinator upon device’s request

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