Wireless LAN - MAC layer

Omer Ben-shalom

references 802.11 Wireless Networks: The Definitive

Guide, M.Gast, O’Reilly, 2002 Some drawings are taken from the O’Reilly book

White papers at Intersil Few drawings and slides borrowed from other

lectures in the IEEE and universities

Introduction The 802.11 MAC is common to all 802.11

flavors provides transmission of user data into the air Uses Carrier sense multiple access (CSMA) Uses Collision avoidance (CA) rather than

detection Uses a distributed access function like Ethernet

with no central controller but has a controller based mode

Lecture brief 802.11 terminology and challenges 802.11 services Media access coordination functions

Distributed coordination Point coordination

Frame types and formats

802.11 Vs 802.3 (Ethernet) 802.11 is an 802 (i.e. Ethernet) protocol for

use in a wireless environment The protocol has to deal with some significant

differences from wired Ethernet Power management – Common devices are

mobile, battery life is of utmost important Bandwidth The ISM spread spectrum do not offer

BW similar to the wired options and is shared Security – The wireless signal can be picket up

without direct attachment removing the option of physical security.

802.11 Vs 802.3 (Ethernet) Addressing -The topology of a wireless network

is dynamic since the stations are free to move around. The protocol is expected to allow and support such roaming

Noise - Radio networks are very noisy Narrowband transmissions Microwaves and other Multi path fading

Media sense The transmitter cannot listen while talking The Hidden node problem - Not all the users are

guarantied to hear each other, unlike Ethernet

Hidden node problem not everyone hears everyone

Distance Physical barriers (walls etc) A traffic to B can collide with C traffic to B without

A or C being in the know

CBA

Basic 802.11 terminology AP – Access Point. A central controller that can

extend the range of the service set stations in the BSS talk through a central controller (AP) The AP sets configurable parameters that all must match Those are carried in special packets called beacons

BSS – Basic service set (‘cell’) Group of stations using the same media and in a Basic Set

Area (BSA) Stations communicate directly or using an AP If no central controller exist this is an iBSS

Membership in a BSS is defined by the Service Set Identifier (SSID) and the BSSID (Normally controlled by the AP) Multiple APs per SSID. Potentially multiple SSID per AP

Basic 802.11 terminology Different APs connect through a distribution

system (DS). Normally a wired backbone All the APs connected on the DS and their

BSS form the ESS - Extended service set The ESS is a single L2 environment

/broadcast domain Stations send packets other stations in the same

ESS ‘directly’ Stations can freely move within the ESS

An EBSS environment

DS

802.11 MAC layer details 802.11 is a “listen before talk” protocol with two

basic modes of operation: Basic access – send whenever media is free RTS/CTS – asking request for sending

Based on a series of timers governing the sending of frames (Interframe spaces)

Uses ARQ Scheme based on positive acknowledgment of packets (ACK) for unicast No ACK mechanism for broadcast/multicast

Distributes the resources ‘fairly’ among clients In spite of using ‘Collision avoidance’ collisions can

and do occur

Associations and Mobility There are three kinds of mobility:

No AP transition: A wireless station is either stationary or moves only within a

single BSS. Nothing special is needed. BSS transition:

The wireless station is moving from one BSS to another BSS inside the same ESS. Uses the reassociation service to support the move. No packets should be lost

ESS transition: Requires a disassociation and a new association in the new

ESS. Usually involves change of IP address and sessions are broken unless using mobile IP or similar.

Power save modes 802.11 allows for a power save mode Clients go to ‘sleep’ for intervals set by the AP AP buffers frames to the client for that period When client wakes up it will retrieve missing frames

by sending a DS-poll to the AP The AP can respond in two ways

Immediately send the packet Send a simple ACK on the request with no data. Station

than has to stay awake until data is delivered and the AP beacon specifies it has no waiting data

Defined 802.11 services – station services MAC Services Data Unit (MSDU) delivery

This is the service of passing a data unit from sender to receiver in the same BSS

WEP/security services Authentication: supplying identity to the other

station in order to be allowed to for relationship De-authentication: informing the other side that

you are terminating the relationship Privacy includes the use of WEP for encryption

Those are the only services allowed in an iBSS (no AP)

Defined 802.11 services – distribution services

Distribution Data delivery service from any station to any other station in the

ESS though the AP For this to work any AP has to know all the stations associated to

it and be a proxy for them on the DS Association services

Association: The act of joining a BSS and registering in the AP for distribution to work (initiated by station)

Disassociation: The act of terminating the relations between AP and station (initiated by both sides)

Reassociation: The act of letting an AP know you are joining his BSS from another BSS and specifying the last AP. New AP can instruct the old AP to void the station registration

Integration Allows stations on an ESS to talk to devices on different kinds of

LAN (Ethernet for example)

802.11 distribution services

There are two basic types of distribution services define controlling how stations can access the medium Distributed coordination function (DCF) – not

using a central controller. More similar to normal Ethernet. Mandatory Two sub implementations with and without RTS/CTS

Point coordination function (PCF) – access is regulated by a central controller (the AP). Not mandatory and usually not implemented Will be discussed at the end of the lecture only

DCF (Distributed coordination function)

Fundamental channel access method in 802.11

Used by asynchronous data services implements explicit Acknowledgements Does not use a central controller Based on CSMA/CA (Collision Avoidance) Collision detection is not used, because a

station cannot listen to the (air) channel for collisions when transmitting Uses Collision Avoidance (CA) with timers

Contention function timers Inter frame space (IFS): Time interval

between transmission of frames Three IFS values are specified

Time slot is defined to 9 microseconds Short-IFS (SIFS) defined to 16 microseconds DCF-IFS (DIFS) = SIFS + 2*time slot PCF-IFS (PIFS) = DIFS + time slot for PCF SIFS < PIFS < DIFS

Access to the media is controlled through these three IFS intervals

Media sense DCF implements two different kinds of media

sense Physical Carrier sense/Clear Channel

assessment is carried out in the physical layer and is based on energy levels and/or 802.11 protocol activity detection

network allocation vector (NAV) or Logical Carrier sense – indicates amount of time that must elapse before channel can be tested again for idle. For simple DCF the NAV is Initiated by hearing the duration field of a data frame

Collision Avoidance in DFS If a node wants to broadcast, it checks if the channel

is idle for a little while (DIFS microseconds). DIFS is the distributed inter frame spacing

If the channel is idle, it broadcasts. When the receiver gets the frame, it check the CRC and if all is ok, it transmits an ACK after a shorter (SIFS microseconds) interval. Else source will resent. Means ACK has preference over any other frame transmission

Essentially collision detection is performed by not receiving an ACK

Fragmentation support The short IFS (SIFS) was created to support

fragmentation and resending of corrupted packets That is the real difference from Ethernet

Another fragment or a resent packet only have to wait SIFS microseconds and not DIFS So resent/fragment has preference over new

packets and the same preference as the ACK

Exponential backoff algorithm

If the channel is busy, waits until it is clear + DIFS interval

Allows more fragments or ACKs After DIFS add a random interval between 0

and the CW (contention window) time slots CW is started at 31 and decremented by 1 while

medium is free If medium becomes busy, the timer is frozen

Collision Avoidance in 802.11

More collision avoidance Having different counters does not guarantee

that transmissions will not collide When two stations transmit simultaneously a

collision will occur The collision is resolved as by both stations

doubling the CW and restarting the random access process again Exponential backoff algorithm

CSMA/CA flowchartstart

NAV=0?

sense channel

yes

ChannelIdle

?

transmit frame

yes

collision?

No - success

random backoff

no

no?

Limitation of simple DCF Assumes stations can hear each other susceptible to the ‘Hidden node’ problem Sender cannot detect a collision during

transmission Wasteful if collision happen for a long frame

Solution – RTS/CTS RTS/CTS allows a station to seize the channel for a

short time avoiding collisions A requests to send to B (RTS) for duration X after contending

for the channel If B senses does not know of a conflict will clear A to send

(CTS) with same timer after waiting SIFS microseconds A will send the packet after waiting just SIFS microseconds If C hears the CTS it will not transmit itself

All stations in the BSS, read the RTS frame and adjust their NAV accordingly

RTS/CTS frames are very short (20 bytes), so collision is unlikely and if it happens less BW is wasted

Implications of CTS/RTS CTS/RTS has overhead

Need to send both for each packet send Used only for packets over a certain length

threshold (XXX bytes by default) Taken into consideration when implementing

logical carrier sense The NAV can be now set by the duration fields in

CTS/RTS Solves the hidden node problem because every

‘hidden’ node will hear RTS or CTS

TimingSIFS = 16s, PIFS = 25s, DIFS = 34s, EIFS = 43s, Slottime = 9s

begin to sense

channel

DIFS

decide that the channel to be idle

RTS CTS SIFS DATA ACK

How long does it take to send an RTA, CTA, Data or ACK? Later

SIFS

channel sense

Sending a single data packetSIFS DIFS

DIFS RTS CTS SIFS Frag 1 ACKSIFS SIFS SIFS Frag 2 ACKSIFS SIFS

Sending a fragmented data packet

DIFS RTS CTS SIFS Data ACKSIFS SIFS

Sending back to back packets

DIFS

Wait an random backoff, i.e., random(0,CW)*slottime)But don’t increment/decrement CW.

RTS CTSSIFS

RTS/CTS overhead (intersil)

Virtual Channel Sensing (no RTS/CTS)

Virtual Channel Sensing (RTS/CTS)

DCF –without CTS/RTS

DCF – RTS/CTS

802.11 MAC Frame types Management Frames: Used for

Station association, dissociation, timing and synchronization, authentication and more

Control Frames: Used for controlling medium access Handshaking during contention periods (RTS/CTS) ACK frames during contention period

Data Frames: Used for Sending data

Frame formats

Address fields The address fields are used differently for

different frame types Normally 3 addresses are used:

Source Address Destination Address BSSID – network identifier. May be the AP MAC

The 4 address format is only used with WLAN bridges Source/address bridges Source/address of original packet

802.11 Frame control field

ProtocolVersion

Type SubTypeToDS

RetryPwrMgt

MoreData

WEP Rsvd

Frame Control Field

Bits: 2 2 4 1 1 1 1 1 1 1 1

DSFrom More

Frag

802.11 frame types

Management frames Management frames carry in the MSDU a

payload made of information elements and fixed fields

Are very versatile and contain two types of fields Fixed length fields defined by the standard Variable length fields that can be extended in the

future by vendors. Support proprietary/extension features called information elements

Fixed fields Fixed fields are used for the various

management operations and include: Authentication details Beacon interval Capacity information AP address Listen interval Time stamp Reason and status codes for authentication and

association

Information elements Information elements are variable length

components. Each has the ID, length and data. New ones can be created as needed Examples include: SSID Supported rates Traffic indication map (TIM) – an indication of

waiting traffic for stations coming up from a sleep period

Main management frame types Beacon – Sent by the AP to coordinate

Allow finding and identifying networks Includes the SSID and the BSSID Set timers and other parameters for the cell Has the traffic indication map (TIM) for all stations

Probe request/response A request to get service for a specific SSID and

transmission rates. Candidate APs will answer

Association/Authentication request, responses and the relevant de-association/de-authentication

PCF

PCF is an optional capability which is connection oriented and provides contention free frame transfer

PCF is based on a central coordinator (PC which is usually at the AP

The PC arbitrates the media using polling. Polling interval is not standardized and left to implementers

Polled stations are allowed to transmit data sequentially, thus removing contention

PCF PCF sits of top of DCF (shown earlier) PCF and DCF times alternate

PCF uses the contention free period (CFP) DCF uses the contention period (CP) A CFP followed by a CP form a superframe

CFP_Rate is parameter used to determine the frequency with which CFP occurs

A limit is set on the duration of CFP so that the DCF traffic is not starved. It has to allow for a minimum of one maximum size frame

PCF operation AP initiates the PCF by sending a beacon

frame announcing the CFP and its duration Beacon is sent every target beacon transmission

time (TBTT) The CFP is ended by the PC sending an CF-End

management frame All clients must honor the CFP, if they do not

implement the PCF they are simply not able to transmit during the CFP and wait for the CP

In any case PCF has priority over DCF since a sending station only has to wait a shorter time (PIFS) to transmit

Stations register for the CFP in the AP and are on a polled station vector

Once CFP starts, the PC polls the stations in its polling vector

SIFS interval after the beacon frame, the PC sends a CF-Poll frame sequentially to each station that required service

A station on receiving this, sends a CF-ACK (no data) or a CF-ACK + Data frame, after SIFS duration

PCF operation

PCF

A station can send data to the AP in this way

Problems with PCF The beacon starting the CFP is subject to the

DCF contention as so its timing is not guarantied in spite of using PIFS (Deferred beacon problem)

The duration of transmission from a station is not really under the control of the PC

WLAN QoS WLAN QoS deals with two main contention types

Priority between packets internal to the host Priority in media access between hosts

Another consideration is the direction of the traffic Downlink QoS – from the AP to all stations much simpler since it is normally done on a single station –

AP Uplink QoS – from each station to the AP

Requires application awareness to register for the queues Arbitrates access to the media from multiple stations Therefore much more problematic

802.11e The 802.11e is a working group charged with

making changes to the MAC layer to allow for QoS (Quality of service) in WLAN Formally: “The purpose of Task Group E is to: Enhance the

current 802.11 MAC to expand support for applications with Quality of Service requirements, and in the capabilities and efficiency of the protocol.”

The standard is in a late draft (draft 13 at this time) form and expected to be rectified this year.

Implements two main methods of QoS control Extended DCF – also implemented as WMM HCF – Hybrid Coordination function – uses PCF functions

and only available as part of the final 802.11e spec

802.11e EDCF/WMM queues WMM is an interim spec on the way to 802.11e

implementing only eDCF EDCF is based on using different contention

parameters (CW) to differentiate queues WMM has 4 priority levels and queues:

Audio/real time Video Best effort Background

EDCF will support 8 priority levels but still 4 queues

Contention with eDCF Packets from different traffic categories (TCs).

Traffic compete unevenly in two ways: The CW for each TC is different so the time a station has to

wait to access the media is different The time available for transmission when the media is

taken (transmit opportunity or TXOP) is different between classes and may allow more than a single packet to be sent

The access is still statistical due to the randomness of the backoff part of the DCF Very good for providing different BW over time Far from optimal for time sensitive traffic

eDCF backoff algorithm details During the PC a station in each TC waits a

different timer called AIFS (Arbitrated Inter Frame space) AIFS uses backoff starting from CWmin and

growing for each For each collision as in DCF However the starting CWmin depends on the TC

and the increase after collision is also different as is newCW [TC] = (oldCW [TC] +1) * PF [TC] . 1

PF is the ‘persistence factor’

HCF There is a Hybrid Coordinator (HC) usually in the AP

like the PCF PC The Hybrid Contention function is active in both CFP

and CP with a similar frame exchange but different access rules. A station can get TXOP in both times The length of TXOP for each TC is published in the beacon During CP a station gets TXOP either by using eDCF or

getting a CF-Poll from the HC During CFP the TXOP are defined by the HC in the CF-poll.

Stations cannot transmit without getting a CF-Poll. The HC may allocate time to itself waiting only the

PIFS (shorter than any DIFS/AIFS) CFP ends with CF-End or the expiration of the timer

set in the beacon

HCF (cont) There are new acknowledge rules

The HC can use two new ack rules block acknowledge (optional) is more efficient. It is negotiated and allows the

sending station to use the TXOP to send multiple frames seperated only by SIFS and the HC will acknowledge all together.

no acknowledges is useful for VOIP and other traffic types were retransmit makes no sense

This is a per-station not per station x TC priority scheme ! There are also new frame definitions

The HC can send any combination of data, ack and poll in a single frame The TXOP granted is defined in the poll frame

HCF controlled contention To guarantee the ability of high priority stations to

request sending data the HC also create periods of ‘controlled contention’ in which only certain stations are allowed to send.

This is used to send a resource request to ask for a TXOP

The controlled contention is a number of windows separated by SIPS

Each station allowed (vector) chooses one and tries to request resources

The HC will Ack each request so the station knows if it succeeded in requesting or collision occurred

Client resource request Clients request resources through special

request frames called TSPECs The HC may or may not accept a TSPEC,

this is implementing access control The AP may offer an optional TSPEC

If a TSPEC is denied the sending station has to do with a lower priority level

TSPECs are not used to gain access to the best effort and background queues

Other optional features in 802.11e Direct link protocol (DLP) – allows stations to send

traffic directly to each other without the AP Very useful for things like WLAN projectors and such

Automatic power save delivery (APSD) – allows setting up scheduled delivery of packets The station now does not have to wake up for every

beacon Time offset in the beacon interval can be specified so

stations wake up in different parts of the beacon interval to listen