Chapter 8

1. 802.11 Medium Access Chapter 8

2. Outline • CSMA/CA vs. CSMA/CD • Distributed Coordination Function (DCF) • Point Coordination Function (PCF) • Hybrid Coordination Function (HCF) • Block acknowledgment (BA) • Wi-Fi Multimedia (WMM

3. CSMA/CA vs. CSMA/CD • Carrier Sense Multiple Access with Collision Detection (CSMA/CD) • Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) • The difference between CSMA/CD and CSMA/CA exists when a client wants to transmit and no other clients are presently transmitting

4. CSMA/CA vs. CSMA/CD • Carrier Sense Multiple Access with Collision Detection (CSMA/CD) • A CSMA/CD node can immediately begin transmitting. • If a collision occurs, the collision will be detected and the node will temporarily stop transmitting

5. CSMA/CA vs. CSMA/CD • Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) • When a CSMA/CA station has determined that no other stations are transmitting, the 802.11 radio will choose a random back-off value. • The station will then wait an additional period of time, based on the back-off value, before transmitting. • During this time, the station continues to monitor to make sure that no other stations begin transmitting • CSMA/CA defines a function called Distributed Coordination Function (DCF) as a medium access method that utilizes multiple checks and balances to try to minimize collisions • CSMA/CA also encompasses a Hybrid Coordination Function (HCF) that specifies advanced Quality of Service (QoS) methods

6. CSMA/CA vs. CSMA/CD • Carrier Sense determines if the medium is busy. • Multiple Access assures that every radio gets a fair shot at the medium (but only one at a time). • Collision Avoidance means only one radio gets on the medium at any given time, hopefully avoiding collisions.

7. CSMA/CA vs. CSMA/CD • Collision Detection • 802.11 radios cannot detect collisions. • Acknowledgement (ACK) frame. • If the ACK is received, then the original station knows that the frame transfer was successful. • All Unicast 802.11 frames must be ACK • Broadcast and multicast frames do not require an ACK • If the ACK is not received, then the original radio will need to retransmit the frame.

8. Distributed Coordination Function (DCF) • DCF is the mandatory access method of the 802.11 standard • InterframeSpace (IFS) • Duration/ID Field • Carrier Sense • Random Back-off Time • Distributed Coordination Function (DCF) Flowchart

9. Distributed Coordination Function (DCF) • Interframe Space (IFS) • Interframe space (IFS) is a period of time that exists between transmissions of wireless frames. • Four types of interframespaces (shortest to longest): • Short interframe space (SIFS), highest priority • PCF interframe space (PIFS), middle priority • DCF interframe space (DIFS), lowest priority • Extended interframe space (EIFS), used with retransmissions

10. Distributed Coordination Function (DCF) • SIFS and DIFS example

11. Distributed Coordination Function (DCF) • Duration/ID Field • The Duration/ID value represents the time, in microseconds, that is required to transmit the ACK plus one SIFS interval • The value of the Duration/ID field indicates how long the RF medium will be busy before another station can contend for the medium. The value of the Duration/ID field in the MAC header of the ACK frame that follows is 0 (zero).

12. Distributed Coordination Function (DCF) • Carrier Sense • Virtual carrier-sense uses a timer mechanism known as the network allocation vector (NAV) • Listening radio hears a frame transmission from another station • Determines the Duration/ID field • Set its NAV timer to this valueand use the NAV as a countdown timer, knowing that the RF medium should be busy until the countdown reaches 0

13. Distributed Coordination Function (DCF) • Carrier Sense • Virtual carrier-sense

14. Distributed Coordination Function (DCF) • Carrier Sense • Physical Carrier-Sense • The first purpose is to determine whethera frame transmission is inbound for a station to receive. If the medium is busy, the radio will attempt to synchronize with the transmission. • The second purpose is to determine whetherthe medium is busy before transmitting. This is known as the clear channel assessment (CCA). The CCA involves listening for 802.11 RF transmissions at the Physical layer. The medium must be clear before a station can transmit.

15. Distributed Coordination Function (DCF) • Carrier Sense • Virtual carrier-sense and physical carrier-sense are always happening at the same time. • Virtual carrier-sense is a layer 2 line of defense. • Physical carrier-sense is a layer 1 line of defense. • If one line of defense fails, hopefully the other will prevent collisions from occurring.

16. Distributed Coordination Function (DCF) • Random Back-off Timer • The random back-off timer is the final timer used by a station before it transmits • The station’s back-off timer begins to count down ticks of a clock known as slots. • When the back-off time is equal to 0, the client can reassess the channel and if clear, begin transmitting. • If channel is still busy, it will restart this process

17. Distributed Coordination Function (DCF) • DCF Flowchart

18. PointCoordination Function (PCF) • Point Coordination Function (PFC) • This access method is a form of polling • The access point performs the function of the point coordinator • Both the access point and the station must support PFC • The access point will alternate between PCF mode and DCF mode • PCF is an optional access method, and we do not know of any vendor that has implemented it

19. Hybrid Coordination Function (HCF) • HFC combines capabilities from both DCF and PCF and adds enhancements to them to create 2 channel methods: • Enhanced Distributed Channel Access (EHCA) • HCF Controlled Channel Access (HCCA) • HCF defines the ability for an 802.11 radio to send multiple frames when transmitting on the RF medium

20. Hybrid Coordination Function (HCF) • HCF-compliant radio contends for the medium, it receives an allotted amount of time to send frames. • This period of time is called a transmit opportunity (TXOP) • During this TXOP, an 802.11 radio may send multiple frames in what is called a frame burst • During the frame burst, a short interframe space (SIFS) is used between each frame to ensure that no other radios transmit during the frame burst

21. Hybrid Coordination Function (HCF) • Enhanced Distributed Channel Access (EDCA) • EDCA is wireless media access method that provides differentiated access for station. • EDCA is an extension of DCF • EDCA medium access method provides for the prioritization of traffic via priority tags that are identical to 802.1D priority tags. • Priority tags provide a mechanism for implementing quality of service (QoS) at MAC level

22. Hybrid Coordination Function (HCF) • Enhanced Distributed Channel Access (EDCA) • EDCA defines 4 access categories: • Background (lowest priority) • Best effort • Video • Voice (Highest priority) • Frames with the highest-priority access category have the lowest back-off values.

23. Hybrid Coordination Function (HCF) • Enhanced Distributed Channel Access (EDCA)

24. Hybrid Coordination Function (HCF) • HCF Controlled Channel Access (HCCA) • HCCA is a wireless media access method that uses a QoS-aware centralized coordinator know as a hybrid coordinator (HC) • HC is built into the access point and has a higher priority of access to the wireless medium • Using this higher priority level, it can allocate TXOPs to itself and other stations to provide a limited-duration controlled access phase (CAP)

25. Hybrid Coordination Function (HCF) • Block Acknowledgment (BA) • Block ACK improves channel efficiency by aggregating several ACKs into one single ACK frame • 2 types of Block ACK: • Immediate Block ACK • Designed for use with low-latency traffic • Delayed Block ACK • Suitable for latency-tolerant traffic • A bitmap in the Block ACK frame is used to indicate the status of all the received data frame • If only one of the frames is corrupted, only that frame will need to be transmitted • Uses of Block ACK mechanisms are further defined in the 802.11n draft

26. Hybrid Coordination Function (HCF) • Block Acknowledgment (BA) • Immediate Block ACK

27. Hybrid Coordination Function (HCF) • Wi-Fi Multimedia (WMM) • The 802.11e amendment defined the layer 2 MAC methods needed to meet the QoS requirement for time-sensitive applications over 802.11 wireless LAN • The Wi-Fi Alliance introduced the Wi-Fi Multimedia (WMM) certification as a partial mirror of 802.11e. • WMM is based on EDCA mechanism, 802.1D priority tags from the Ethernet side are used to direct traffic to four access-category priority queues

28. Hybrid Coordination Function (HCF) • Wi-Fi Multimedia (WMM)

29. Hybrid Coordination Function (HCF) • Wi-Fi Multimedia (WMM) • WMM-PS (Power Save) • 802.11e power-saving mechanism to increase the battery life via advanced power-saving mechanisms

30. The END Chapter 8