1 / 25

Chapter 2 Wireless LANS and PANS

Chapter 2 Wireless LANS and PANS. Jang Ping Sheu. Differences Between Wireless and Wired Transmission. Address is not equivalent to physical location Dynamical topology and restricted connectivity Medium boundaries are not well-defined Error-prone medium. IEEE 802.11 Standard.

sherlyn
Download Presentation

Chapter 2 Wireless LANS and PANS

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Chapter 2 Wireless LANS and PANS Jang Ping Sheu

  2. Differences Between Wireless and Wired Transmission • Address is not equivalent to physical location • Dynamical topology and restricted connectivity • Medium boundaries are not well-defined • Error-prone medium J. P. Sheu

  3. IEEE 802.11 Standard • Design Goals • Operational simplicity • Power-efficient operation • License-free operation • Tolerance to interference • Global usability • Security • Safety requirements • QoS requirements • Compatibility with other technologies and applications J. P. Sheu

  4. Characteristics • Destination address .NEQ. destination location • Air Media Impacts: • shared medium, unprotected from outside signals • lack full connectivity, STA may hidden from each other. • dynamic topologies • less reliable • Mobility of Stations • Interaction with other 802 Layers • 802.11 consists of only PHY and MAC layers. • 802.11 should appear the same to higher-layer (LLC) 802-style LAN. So mobility should be handled within the MAC layer. J. P. Sheu

  5. MAC Protocol Overview (1/2) • MAC should be developed independent of the physical • Authentication • link-level authentication process • not intended to provide end-to-end, or user-to-user authentication • MAC Traffic: • Asynchronous data service: in a best-effort basis • Time-bound service: as connection-based data transfer J. P. Sheu

  6. MAC Protocol Overview (2/2) • CSMA/CA: carrier sense multiple access with collision avoidance • a station wishing to send must sense the medium • mandate a minimum gap between continuous frames • collision avoidance: a random back-off after the medium is sensed idle • only decrement the back-off interval while the medium is free • all non-broadcast uses immediate ACK • if no ACK is received, the frame is repeated immediately J. P. Sheu

  7. CD vs. CA • CD = collision detection • CA = collision avoidance • CD will not function properly because the STA may not be able to detect the collision while transmitting. J. P. Sheu

  8. D B B A C C A (a) (b) Fig. 1: (a) the hidden terminal problem, (b) the exposed terminal problem Hidden-Terminal and Exposed-Terminal Problems J. P. Sheu

  9. Solution RTS D • RTS-CTS exchange: • RTS = Request To Send • CTS = Clear To Send • Problem: high overhead for short frames B C A CTS CTS D B C A data D B C A J. P. Sheu

  10. Hardware Architecture (1/2) ESS • STA: • any device that contains an 802.11-conformed MAC and PHY interface to wireless medium • Basic Service Set (BSS): • A set of STAs controlled by a single CF (Co-ordination Function). • The member STAs in a BSS can communicate with each other directly. BSS 1 BSS 2 STA 1 STA 4 STA 2 STA 3 AP AP DS 分散式系統 J. P. Sheu

  11. Hardware Architecture (2/2) • Extended Service Set (ESS): • A set of interconnect BSSs and integrated LANs as a single BSS. • The ESS network appears the same to an LLC layer as an independent BSS network. • Stations within an ESS can communicate with each other and mobile stations may move from one BSS to another transparently to LLC. • Distribution System (DS): A system used to be interconnect a set of BSSs and integrated LANs to create an ESS. J. P. Sheu

  12. The Service of IEEE 802.11 Station Services (SS) • Authentication • Deauthentication • Privacy • Data delivery • AP Services • Distribution • Integration • Association • Disassociation • Reassociation J. P. Sheu

  13. MAC Frame Formats • Each frame consists of three basic components: • MAC Header (control information, addressing, sequencing fragmentation identification, duration, etc.) • Frame Body (0-2304 bytes) • IEEE 32-bit CRC (Frame Check Sequence) MAC Header 2 2 6 6 6 2 6 0 - 2312 4 位元組 Sequence Control Frame Control data Duration/ID Addr 3 FCS Addr 1 Addr 2 Addr 4 Frame Control Power Mang. From DS To DS More Flag Protocol Version More Data Retry WEP Order Type Subtype 2 2 4 1 1 1 1 1 1 1 1 位元 J. P. Sheu

  14. MAC Architecture Contention Free Period Contention Period Point Coordination Function (PCF) MAC Extent Distributed Coordination Function (DCF) J. P. Sheu

  15. MAC Architecture • Distributed Coordination Function (DCF) • The fundamental access method for the 802.11 MAC, known as Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA). • Shall be implemented in ALL stations and APs. • Used within both ad hoc and infrastructure configurations. • Point Coordination Function (PCF) • An alternative access method • Shall be implemented on top of the DCF • A point coordinator (polling master) is used to determine which station currently has the right to transmit. • Shall be built up from the DCF through the use of an access priority mechanism. J. P. Sheu

  16. Network Types • Infrastructure Mode CFP CP Execute PCF Beacon Execute DCF AP Centralized Control For new Station Authentication and association with AP • Ad Hoc Mode CP Execute DCF Sta communicates with any other Sta J. P. Sheu

  17. Distributed Coordination Function (DCF) (1/2) • Allows sharing of medium between PHYs through • CSMA/CA and, • random back-off following a busy medium • All packets should be acknowledged (through ACK frame) immediately and positively • Retransmission should be scheduled immediately if no ACK is received J. P. Sheu

  18. Distributed Coordination Function (DCF) (2/2) • Carrier Sense shall be performed through 2 ways: • physical carrier sensing: provided by the PHY • virtual carrier sensing: provided by MAC • by sending medium reservation through RTS and CTS frames • duration field in these frames • The use of RTS/CTS is under control of RTS_Threshold. • An NAV (Net Allocation Vector) is calculated to estimate the amount of medium busy time in the future. • Requirements on STAs: • can receive any frame transmitted on a given set of rates • can transmit in at least one of these rates • This assures that the virtual carrier sense mechanism work on multiple-rate environments. J. P. Sheu

  19. Priority Scheme in MAC • Priorities of frames are distinguished by the IFS (inter-frame spacing) incurred between two consecutive frames. • 4 IFS's: • SIFS: the highest priority • ACK, CTS, the second or subsequent MPDU of a fragmented burst, and to respond to a poll from the PCF. • PIFS (PCF-IFS): 2nd highest • by PCF to send any of the Contention Free Period frames. • DIFS (DCF-IFS): 3nd highest • by the DCF to transmit MPDUs or MMPDUs • EIFS (Extended-IFS): lowest • by the DCF to retransmit a frame J. P. Sheu

  20. The Random Back-off Time (1/2) • Before transmitting asynchronous MPDUs, a STA shall use the CS function to determine the medium state. • If idle, the STA • defer a DIFS gap • transmit MPDU (MAC Packet Data Unit) • If busy, the STA • defer a DIFS gap • then generate a random back-off period (within the contention window CW) for an additional deferral time to resolve contention. J. P. Sheu

  21. The Random Back-off Time Back-off time = CW* Random() * Slot time where CW = starts at CWmin, and doubles after each failure until reaching CWmax and remains there in all remaining retries (e.g., CWmin = 7, CWmax = 255) Random() = (0, 1) Slot Time = Transmitter turn-on delay + medium propagation delay + medium busy detect response time CWmax 255 255 127 8 63 31 CWmin 15 7 第三次重送 初始值 第二次重送 第一次重送 J. P. Sheu

  22. DIFS Immediate access when medium is free >= DIFS PIFS Contention Window DIFS SIFS Back-off-Window Slot time Busy Medium Next Frame Defer Access DCF Access Procedure • A STA can try to send when: • no PCF detected • or, Contention Period of a Superframe when using a PCF. • Basic Access • A STA with a pending MPDU may transmit when it detects a free medium for  DIFS time. • But when a Data, Poll, Request, or Response MPDU is to be sent, the Backoff procedure shall be followed. J. P. Sheu

  23. DIFS Fragment Burst SIFS SIFS SIFS SIFS PIFS Fragment 1 Fragment 2 SIFS SIFS Ack 1 Ack 2 Fragment 3 Backoff-Window Src Ack 3 Dst Control of the Channel • Once a station has contended for the channel, it will continue to send fragments until • all fragments of a MSDU (MAC Service Data Unit) have been sent, • Fragmentation_Threshold: to determine to fragment or not. • an ack is not received: • It will attempt to retransmit the fragment at a later time (according to the backoff algorithm) and go through the contention procedure again. • a dwell time boundary is reached • The SIFS is used to guarantee its priority. J. P. Sheu

  24. DIFS PIFS SIFS Backoff-Window NAV(RTS) NAV(Fragment 1) NAV(Fragment 2) Other NAV(CTS) NAV(ACK 1) NAV(ACK 2) SIFS SIFS SIFS SIFS SIFS SIFS SIFS Fragment 1 Fragment 2 Fragment 3 RTS Src CTS Ack 1 Ack 2 Ack 3 Dst Duration Reservation Strategy • The RTS/CTS frames define the duration of the first frame and ACK. • Each Fragment and ACK acts as a “virtual” RTS and CTS for the next fragment. • The duration field in the data and ACK specifies the total duration of the next fragment and ACK. • The last fragment and ACK will have the duration set to zero. J. P. Sheu

  25. Exercises • 1, 5, 6, 7, 10, 13 J. P. Sheu

More Related