1 / 31

Medium Access Control in

Medium Access Control in. IEEE 802.11. Introduction. This presentation deals with the principles of Medium Access Control in wireless LAN under standards IEEE 802.11. Medium Access Control MAC. Methods of Access Control: Polling from a central control:

munin
Télécharger la présentation

Medium Access Control in

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. Medium Access Controlin IEEE 802.11

  2. Introduction • This presentation deals with the principles of Medium Access Control in wireless LAN under standards IEEE 802.11

  3. Medium Access Control MAC • Methods of Access Control: • Polling from a central control: • Controller coordinates the access of devices to the medium • Token Passing: • The access to the medium is determined by the reservation of a token frame • Contention: • Devices fight, with equal rights, for the access to the medium

  4. Contention for the Medium Access Control • Contention: • Devices fight, with equal rights and rules, for the access to the medium Medium

  5. Contention Mechanism • Detect if medium is free to be used • By sensing the carrier signal in the medium • If medium is idle, then attempt to transmit • Otherwise, wait and keep listening to the medium until there is an opportunity to transmit Medium

  6. Carrier Sense and Multiple Access • CSMA I can read the signals in the medium And I can access the medium Medium I can read the signals in the medium And I can access the medium I can read the signals in the medium And I can access the medium

  7. CSMA Two ways: • CSMA with Collision Detection • CSMA with Collision Avoidance

  8. Collision Avoidance in 802.11 • The radio frequency RF devices can either transmit or receive, but not both at the same time • They are half-duplex by design since a channel is being used for both transmit and receive • Consequently, they can not directly detect (by listening) if the collision of a frame has happened • A system such as Collision Detection is not the right one in 802.11 • Instead, Collision Avoidance is implemented RF Medium

  9. CSMA CSMA with Collision Detection CSMA with Collision Avoidance Listen to the medium If idle, wait for a certain time There are ways to notify this time • Listen to the medium • If idle, attempt to transmit • If collision happens, back-off with a random time

  10. Access method for 802.11 • Distributed Coordination Function (DCF) • DCF is the access method defined in the standard 802.11 • The method has components that seek out that only one device is transmitting at any given time • There are four components in the method: • Inter frame space (time gap actually) • Virtual Carrier Sense • Physical carrier sense • Random back-off timer

  11. Inter Frame Space • Wireless PC-A sends a unicast frame to be received by wireless AP Unicast Frame PC-A

  12. Inter Frame Space • Wireless PC-A sends a unicast frame to be received by wireless AP • AP receives the frame and waits for a SIFS time Unicast Frame PC-A

  13. Inter Frame Space • Wireless PC-A sends a unicast frame to be received by wireless AP • AP receives the frame and waits for a SIFS time • AP replies back to PC-A with an Acknowledgement frame ACK Frame PC-A

  14. Inter Frame Space • Wireless PC-A sends a unicast frame to be received by wireless AP • AP receives the frame and waits for a SIFS time • AP replies back to PC-A with an Acknowledgement frame ACK Frame PC-A

  15. Inter Frame Space • The total time that has elapsed is the frame time plus the inter frame space gap (SIFS) plus the acknowledgement frame time. • All of this process has been observed by PC-B. • PC-B is waiting for its own opportunity to transmit ACK Frame PC-B PC-A Listening

  16. For how long? • For how long does PC-B have to wait? • How does PC-B know the time that it has to wait for? ACK Frame PC-B PC-A Listening

  17. For how long? • For how long does PC-B have to wait? • How does PC-B know the time that it has to wait for? Unicast Frame ACK Frame IFS gap PC-B Listening

  18. For how long? • For how long does PC-B have to wait? • Frame time plus SIFS plus Ack time • How does PC-B know the time that it has to wait for? • When PC-A transmitted the frame; it set a field to a value from 0 to 32,767 micro-seconds • Such field is the DURATION/ID field and it tells the time Unicast Frame ACK Frame IFS gap PC-B Listening

  19. Inter Frame Space IFS • There are rules for what kind of frame will be following a specific IFS • There are 5 types of IFS: • SIFS short IFS. • The highest priority and shortest IFS. • Frames like ACK and CTS follow SIFS • PIFS • Second best priority • DIFS • Lowest priority. • Most frames are preceded by DIFS • AIFS • Arbitration for QoS • EIFS • Extended IFS for retransmissions

  20. Carrier Sense in 802.11 • Virtual: • Wireless nodes listen to wireless frames Frame

  21. Virtual carrier-sense mechanism • Provided by the MAC sub-layer. • Referred to as the network allocation vector (NAV). • RTS/CTS frames are exchanged before the actual exchange of data. • The NAV is set based on the information announced by the RTS/CTS exchange. • The NAV maintains a prediction of future traffic on the medium.

  22. Virtual Carrier Sense in 802.11 • Request to Send • Clear to Send RTS CTS I see

  23. Virtual Carrier Sense in 802.11 • Wireless nodes listen to wireless frames • The FRAME has a field called DURATION/ID • If the field is set to DURATION, it also indicates a value in micro-seconds • The listening PC will set its Network Allocation Vector (a timer) to such value • Wireless PC starts counting down • Wireless PC can content for use of medium when timer NAV reaches 0 Frame

  24. Physical Carrier Sense in 802.11 • Stations might have just missed the Duration field of a frame when they start listening to the medium • They listen to RF signals anyways • They perform a CCA clear channel assessment

  25. Contention Window Calculation * • A wireless station STA shall invoke the carrier-sense mechanism to determine the busy/idle state of the medium. • If the medium is busy, the STA shall defer until the medium is idle without interruption for a period of time equal to DIFS when the last frame detected on the medium was received correctly, or after the medium is determined to be idle without interruption for a period of time equal to EIFS when the last frame detected on the medium was not received correctly. • After this DIFS or EIFS medium idle time, the STA shall then generate a random back-off period for an additional deferral time before transmitting • This process minimizes collisions during contention between multiple STAs that have been deferring to the same event. * Source: IEEE 802.11 Original Standard Document

  26. Other Medium Access Systems • Beside Distributed Coordination Function (DCF) there are other methods defined: • Point Coordination Function (PCF) • Hybrid Coordination Function (HCF) • Enhanced Distributed Channel Access (EDCA)

  27. Point Coordination Function (PCF) • This is different system of access control based in Polling the wireless stations STA • In this case, the Access Point AP acts as a Point Coordinator • PCF implements “managed” access to the medium • PCF is a contention free method • PCF, even if defined, is not implemented Polling Frame Polling Frame Polling Frame

  28. Hybrid Coordination Function (HCF) Transmit Opportunity TXOP • In this system the HFC client receives an allotted amount of time to send a burst of frames continuously • This is for Quality of Service purposes • Defined in 802.11e for QoS amendment • Process: • Transmit a frame • Short IFS wait • Transmit a frame • Short IFS wait Frame SIFS Frame SIFS Frame

  29. Hybrid Coordination Function (HCF) • Block Acknowledgement: • Rather than acknowledging every frame, the AP can ACK a range or block of frames • Obviously, there must be a field in the frames (sequence number) that can be used as a reference index to point out to the beginning and end of the block of frames Transmit Opportunity TXOP BLOCK ACK Frames 1 to 3 Frame 1 SIFS Frame 2 SIFS Frame3

  30. Enhanced Distributed Channel Access (EDCA) • Method that provides differentiated service access for stations using 8 user priority levels • 23 = 8 tags or levels or service • The lowest value is the less preferred • The highest value is the most preferred • Defined in 802.11D priority tags Voice over IP tags: 7 and 6 IP Video tags: 5 and 4 Best effort: 0 and 3 Background: 2 and 1

  31. Summary • The Medium Access Control in WLAN standard 802.11 is based in the avoidance of collisions • This main principle is implemented using a somewhat complex system of timers and frames of different formats • In short, the choice of a avoidance control forces the layer 2 access control to be more complex than its relative protocol 802.3 used in wired LANs

More Related