Some details about Medium Access Mechanism in Wi-Fi

1. IEEE 802.11 – Medium Access

2. IEEE 802.11 MAC
Medium Access - DCF vs PCF (Distributed vs Point Coordination Function)
When DCF is used:

There isn't any Centralized - Coordinator

All of associated STAs and the AP in a BSS share the medium by using “CCA –
Clear Channel Assessment” mechanism, in that, to assess if the channel is free, they
use :

Energy Detection and

Carrier Sense – or Wi-Fi preamble Detection).

3. IEEE 802.11 MAC (cont.)
Medium Access - DCF vs PCF (Distributed vs Point Coordination Function)
When DCF is used: (cont.)

In some cases, the AP and the STAs can use NAV (Network Allocation Vector - be
carried in MAC header) to reserve the medium for some consecutive frames.
RTS/CTS mechanism is one of those cases. The mechanism in that NAV is used
also known as Virtual Carrier Sense mechanism.

4. 802.11 MAC (cont.)
Medium Access - DCF vs PCF (Distributed vs Point Coordination Function)
When PCF is used:

There is a Point Coordinator (reside in the AP) that has the right to allocate the medium
to the PCF-based STAs by polling frames.

The Point Coordinator poll the STAs one-by-one to send and receive data, hence there
is not internal contention in a BSS. This period is so called “CFP - contention-free
Period”.

PCF-based STAs, in fact, are the STAs which can reply to the polling frames of the AP.
AP use NAV(sent in beacon) to prevent all of STAs transmit data except which one is
polled.

5. 802.11 MAC (cont.)
Medium Access - DCF vs PCF (Distributed vs Point Coordination Function)
When the AP want to provide PCF:

NAV is used to prevent DCF-based STAs (or normal STAs) to access to the medium.

Due to the priority of PCF over DCF, the CFP is not provided full-time. It alternate
with the period where the standard DCF-based services are provided. In that case,
the first period after Beacon Frames are always CFP. A CFP-end frame will be send
to explicitly terminate the CFP.
The PCF is not widely implemented, it's restricted to infrastructure networks and Wi-Fi
Alliance does not include yet PCF functionality in their interoperability standard.

6. IEEE 802.11e MAC
QoS for 802.11 – part of 802.11 from 2007

802.11e-2005 is an approved amendment to the IEEE 802.11 standard that defines a set of quality
of service (QoS) enhancements for wireless LAN applications through modifications to the MAC
layer.

Wireless Multimedia Extensions (WME), also known as Wi-Fi Multimedia (WMM), is a Wi-Fi Alliance
interoperability certification, based on the IEEE 802.11e standard.

The 802.11e enhances the DCF and the PCF, through a new coordination function: the hybrid
coordination function (HCF).

Within the HCF, there are two methods of channel access, similar to those defined in the legacy
802.11 MAC:

HCCA - HCF Controlled Channel Access – works a lot like PCF – and:

EDCA - Enhanced Distributed Channel Access – uses classified IFS for different categories of traffic
and TXOP (Transmit Opportunity) to provide contention-free periods to QSTAs (QoS-STAs – STAs
in 802.11e) to access the channel in DCF period.
Both EDCA and HCCA define Traffic Categories (TCs). For example,

emails could be assigned to a low priority class, and

Video or Voice over Wireless LAN could be assigned to a high priority class.
HCCA is generally considered the most advanced (and complex) coordination function. Implementing
the HCCA on end stations uses the existing DCF mechanism for channel access (no change to DCF
or EDCA operation is needed). Stations only need to be able to respond to poll messages!

7. IEEE 802.11e MAC (cont.)
HCCA - HCF Controlled Channel Access
With HCCA:
Similar to PCF: Same polling mechanism to assign TXOP to QSTA, but:

Polling can be issued in both CFP and CP (in Controlled Access Periods)

HC Polling is scheduled according to Transmit specifications (TSPECs)

HC grants a polled TXOP to one QSTA, which restricts the duration of the
QSTA’s access to the medium.

8. IEEE 802.11e MAC (cont.)
EDCA - Enhanced Distributed Channel Access
With EDCA:

Traffic is classified into 4 categories (AC-access categories).

Background – AC_BK (lowest priority)

Best Effort – AC_BE

Video – AC_VI

Voice – AC_VO (highest priority)
High-priority traffic has a higher chance of being sent than low-priority traffic because of
it has shorter inter-frame space (IFS) and back-off time.

AP can provide contention-free access period called a Transmit Opportunity (TXOP) –
a bounded time interval during which a QSTA can send multiple frames without
contention (within their BSS).

TXOP is assigned to each QSTA, specified per TS – Traffic Stream in that QSTA. Each
QSTA may have upto 4 TSs classified as mention above.

A QSTA can use a TXOP to transmit multiple frames within an access category – as
many frames as possible – as long as the duration of the transmissions does not extend
beyond the maximum duration specified of a TXOP.

If a frame is too large to be transmitted in a single TXOP, it should be fragmented into
smaller frames and send them in consecutives TXOP.

9. IEEE 802.11e MAC (cont.)
EDCA - Enhanced Distributed Channel Access
EDCA (cont.)
If there are many TSs in a QSTA, they have to be resolved within that QSTA as like as
“virtual collision” or “internal collision” with a internal “collision handler”.

10. IEEE 802.11e MAC (cont.)
EDCA - Enhanced Distributed Channel Access
EDCA (cont.)
Arbitration Inter-Frame Space (AIFS) and CW in 802.11e for Traffic Categories of
QSTAs

11. IEEE 802.11 – Power Saving Modes

12. IEEE 802.11 MAC
Power Saving Mode
802.11 stations (STAs) can use “Power Save Mode – PS-mode” to save their batteries
lifes, in that mode, they can go to sleep and wake up in listen mode to monitor Beacon
frames. The AP will buffer all of PS-STA's incomming messages, notify them on every
Beacon frame, up until they wake up and get their informations (or timeout !).
For using PS mode:

During association, the STA tells the AP that it will use PS-mode, and it's “listen
interval” (or “wake up cycle”, this interval may be longer than few beacon intervals). In
PS-mode, the STA has to listen periodically Beacon frames.

The AP will buffer incomming frames of the PS-mode STAs, notify them by the TIM
(Traffic Indication Map) – associated in every beacon frame – whether the frames for
each PS-mode STA.

For Multi-cast or Broad-cast data, AP use TIM to notify whether the frames like that,
and use DTIM (Delivery TIM) for more detail. Every DTIM-interval (= beacon-
interval x n), TIM is periodically replaced by DTIM in beacon frame, AP also uses TIM to
notify STAs when DTIM will be used instead of TIM. After the beacon frame with DTIM,
AP will transmit Multi-cast and Broad-cast data frames before any individually
addressed frames.

If an STA get relevant notice, it should stay awake to retrieve it's data.

13. IEEE 802.11 MAC
Power Saving Mode (cont.)
In PS-mode, STAs retrieve their data in the following ways:
For Uni-cast data:

In CFP: STAs have to wait for the AP transmit the polling frames (CF-poll) to the
pollable STAs (CPF-based STAs), and then send them their data, one-by-one.

In non-CFP (where DCF is used): STAs transmit a PS-poll frame, the AP either

transmits the information frames, or

send ACKs the PS-poll frame and transmits the information frames later
For Group data (Multi-cast and Broad-cast data):
AP send group data frames, if there are any, after the beacon frames with DTIM, any
STAs should keep awake to receive DTIM (if there was a notice in previous TIMs)
and these group data.

14. IEEE 802.11 MAC
Power Saving Mode (cont.)
Two STAs listen TIM and retrieve their uni-cast data in PS-mode (when DCF is used)

15. IEEE 802.11 MAC
Power Saving Mode (cont.)
TIM CF-Poll TIM TIM
Beacon_
Interval
AP
STA 2 in
PS mode
PS-poll
STA 1 in
PS mode
DataACK
PS-poll
Data
ACK
CF-Poll
Two STAs listen TIM and retrieve their uni-cast data in PS-mode (when PCF is
used)

16. IEEE 802.11 MAC
Power Saving Mode (cont.)
Two STAs listen DTIM and retrieve broad-cast data in PS-mode (DCF & PCF)
DTIM
Broadcast Data
TIM TIM
Beacon_
Interval
AP
STA 2 in
PS mode
STA 1 in
PS mode

17. IEEE 802.11 MAC
Power Saving Mode (cont.)
What a STA has to do in PS mode:

Gets beacon, during association, notifies that “I shall be in PS-mode, I shall wake-up
every N beacon interval”. If the AP accept, STA can switch to sleep mode.

Wakes up prior every N beacon interval (estimate by a local timer), listen to the
beacon, if :
1.Do not heard the beacon: continue to wait for a beacon, sometime the beacons are
delayed due to the busyness of the media.
2.A beacon with DTIM is received, stays awake to receive group data, continues to
awake to get next beacon with TIM, if TIM indicates uni-cast data buffered, send PS-
poll to retrieve data, sleeps again when finish or there is not any data.
3.A beacon with TIM is received:
a)Sends PS-poll to retrieve data if TIM indicates uni-cast data buffered.
b)Stays awake to get DTIM (if it is indicated by TIM).
c)Sleeps again when finish or not (a) and (b) aboved.
Note: after retrieves uni-cast data, STA has to stay awake to receive one more beacon
to make sure that the relevant indicator bit is reset.

18. IEEE 802.11e MAC
Power Saving Modes
APSD – Automatic Power-Save Delivery – Enhancing Power-Saving mode in QoS
Basic Service Set
QAP (QoS_AP – AP in 802.11e) automatically delivers downlink frames, which
belong to some specified Access Category, to PS-STAs.
PS-STAs do not need to listen for beacon (to receive TIM or DTIM).
Two types of delivery mechanism

Unscheduled APSD (U-APSD)

Scheduled APSD (S-APSD)

19. IEEE 802.11e MAC
Power Saving Modes (cont.)
Unscheduled APSD (U-APSD)
1. Power-saving QSTA wakes up and send a “trigger”dataframe belonging to “trigger-
enabled” AC to QAP
2. After receiving “trigger” frame, a Service Period (SP) is started
3. QAP send frames belonging to “delivery-enabled” AC to QSTA
U-APSD is available only for EDCA

20. IEEE 802.11e MAC
Power Saving Modes (cont.)
Scheduled APSD (S-APSD)
1. QSTA negotiates a APSD Schedule with QAP
2. QAP start transmitting the frames of the specified Traffic Stream at Service Start
Time and the following periods
3. QSTA must wake up at Service Start Time and the following periods to receive
frames
S-APSD is available for both channel access mechanisms: EDCA and HCCA

21. IEEE 802.11 MAC
Some issues in 802.11 MAC layer:

The PS-STAs have to get both DTIM and TIM to have buffering information about all
kinds of data (broad-cast, multi-cast and uni-cast).

After retrieves uni-cast data, STA has to receive TIM in next beacon to make sure
that the relevant indicator bit was reset, if next beacon is associated with DTIM, it has
to receive one more beacon.

When there is a large number of PS-STAs in a network, the length of the beacon
frame could become extremely long due to the excessive length of the partial virtual
bitmap in TIM (up to 253 bytes). In addition, if the amount of the buffered traffic is too
heavy to be accommodated within a beacon interval, some PS-STAs inevitably stay
awake in a long time to complete the receptions of their buffered packets.

Maximal value of Idle-time is 18.64 hours, somes time it's not enough, for e.g., some
alarm sensors send a packet once a month only to confirm that it's “alive” for save its
battery.

…..................................

22. Annex A
Closely related standards
802.11af: White-Fi
802.11s: mesh networking
802.11ax: MIMO-OFDM
802.11u: interworking with external networks
802.11e: QoS for 802.11

23. Annex B
Legacy 802.11's beacon contents

Beacon Interval

Timestamp: Timing Synchronization Function (TSF) keeps the timers for all stations -
counting in increments of microseconds with modulus 264
- in the same Basic Service Set
(BSS) - are synchronized. Upon receiving a beacon, a station sets its TSF timer to the
timestamp of the beacon if the value of the timestamp is later than the station’s TSF timer.
On a commercial level, the vendors assume the TSF's differences in a BSS will be within 25
microseconds.

SSID – service set ID

Supported rates

Parameters Sets: parameter of signaling method, i.e. DS, FH, OFDM,...

Capability information: signifies requirements to associate (WEP, WPS,...)

Traffic Indication Map (TIM or DTIM – Delivery TIM) (if there is any associated PS-STA).
In ad hoc networks, there are no access points. As a result, one of peer stations assumes the
responsibility for sending the beacon. After receiving a beacon frame, each station waits for
the beacon interval and then sends a beacon if no other station does so after a random time
delay. This ensures that at least one station will send a beacon, and the random delay
rotates the responsibility for sending beacons.

24. Annex C
802.11e – some more key advantages
Block acknowledgments Improves channel efficiency by aggregating several acknowledgments
into one frame. (More details in Annex D).
NoAck:In QoS mode, service class for frames to send can have two values: QosAck and
QosNoAck. Frames with QosNoAck are not acknowledged. This avoids retransmission of
highly time-critical data.
Direct Link Setup: allows direct station-to-station frame transfer within a basic service set. This is
designed primarily for consumer use, where station-to-station transfer is more commonly
used. For example, when streaming video to a television across the living room, or printing to
a wireless printer in the same room.

25. Annex D
802.11e - Block Acknowledgement
Block Acknowledgement (BA) was initially defined in IEEE 802.11e as an optional
scheme to improve the MAC efficiency. Recently ratified amendment 802.11n
enhances this BA mechanism then made it as mandatory to support by all 802.11n-
capable devices (formally known as HT - High Throughput devices).

Instead of transmitting an individual ACK for every MPDU (i.e. frame), multiple
MPDUs can be acknowledged together using a single BA frame.

Block Ack (BA) contains bitmap size of 64*16 bits. These 16 bits accounts the
fragment number of the MPDUs to be acknowledged. Each bit of this bitmap
represent the status (success/failure) of a MPDU.

Block acknowledgement consist of a setup and tear-down phases.

In the setup phase, capability information such as buffer size and BA policy are
negotiated with the receiver.

Once the setup phase completed, the transmitter can send frames without waiting for
ACK frame.

Finally the BA agreement is torn down with a so-called DELBA frame.

26. Annex D
802.11e - Block Acknowledgement (cont.)
There are 2 types of Block Ack mechanisms: immediate and delayed
Delayed ACK is provided to support low-performance STA that are unable to
immediately calculate the ACK.