Wireless LAN - MAC layer

1
Wireless LAN - MAC layer

• Omer Ben-shalom

2
references

• 802.11 Wireless Networks The Definitive Guide,
  M.Gast, OReilly, 2002
• Some drawings are taken from the OReilly book
• White papers at Intersil
• Few drawings and slides borrowed from other
  lectures in the IEEE and universities

3
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

4
Lecture brief

• 802.11 terminology and challenges
• 802.11 services
• Media access coordination functions
• Distributed coordination
• Point coordination
• Frame types and formats

5
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.

6
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

7
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

C
B
A
8
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

9
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

10
An EBSS environment
DS
11
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

12
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.

13
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

14
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)

15
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)

16
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

17
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

18
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 2time slot
• PCF-IFS (PIFS) DIFS time slot for PCF
• SIFS lt PIFS lt DIFS
• Access to the media is controlled through these
  three IFS intervals

19
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

20
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

21
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

22
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

23
Collision Avoidance in 802.11
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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

25
CSMA/CA flowchart
start
NAV0 ?
yes
sense channel
random backoff
Channel Idle ?
no
yes
transmit frame
collision ?
no
?
No - success
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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

27
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

28
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

29
Timing
SIFS 16?s, PIFS 25?s, DIFS 34?s, EIFS
43?s, Slottime 9?s
Sending a single data packet
DIFS
DIFS
SIFS
SIFS
SIFS
RTS
CTS
DATA
ACK
channel sense
begin to sense channel
decide that the channel to be idle
Sending a fragmented data packet
DIFS
SIFS
SIFS
SIFS
SIFS
SIFS
SIFS
RTS
CTS
Frag 1
ACK
Frag 2
ACK
How long does it take to send an RTA, CTA, Data
or ACK? Later
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Sending back to back packets
DIFS
DIFS
SIFS
SIFS
SIFS
SIFS
RTS
CTS
Data
ACK
RTS
CTS
Wait an random backoff, i.e., random(0,CW)slottim
e) But dont increment/decrement CW.
31
RTS/CTS overhead (intersil)
32
Virtual Channel Sensing (no RTS/CTS)
33
Virtual Channel Sensing (RTS/CTS)
34
DCF without CTS/RTS
35
DCF RTS/CTS
36
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

37
Frame formats
38
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

39
802.11 Frame control field
40
802.11 frame types
41
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/extensi
  on features called information elements

42
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

43
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

44
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

45
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

46
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

47
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

48
PCF operation

• 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

49
PCF

• A station can send data to the AP in this way

50
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

51
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

52
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

53
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

54
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

55
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

56
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

57
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

58
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

59
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

60
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