Wi-Fi MAC basics: IEEE 802.11(2)

WLAN technical chanllenges and design issues

Problems making WLAN design a complicated task:

• Address is not a physcial location
• Dynamically changed topology
• Medium boundaries are soft
• Erroneous medium
• Hidden and exposed terminal problems

—> Reliable network using unreliable channels

Criteria to be met

• Operational Simplicity
• Power efficient opeartion
• License-free operation
• Tolerance to interference
• Security
• Compatibility
• Global usability, safety, QoS

Overview of IEEE 802.11

• 802.11x specifies the physical and the medium access conrol (MAC) layers only
• interfaces and higher layer is the same as those in IEEE 802.x standards
• MAC layers should be able to work with multiple physical layers

Task Group

802.11 Task group	description
802.11 WG	develop MAC layer and physical layer specifications, 1997
802.11a WG	**WLAN operations in 5Ghz freq.band , 1999 (54Mbps), **, (first appeared on the market)
802.11b WG	2.4GHz (ISM) frequency band, 1999,11Mbps, refer to Wifreless Fidelity (Wi-Fi) (most successful among family )
802.11c WG	bridging and access points operations, 1998
802.11d WG	802.11 operation in different countries,2001
802.11e WG	QoS provision extension
802.11f WG	inter access point protocols for operation in ESS, 2003
802.11g WG	54Mbps, compatible with 802.11b;
802.11h WG	MAC layer to be in compliance with European standards, 2003
802.11i WG	security extensions for 802.11
802.11j WG	4.9GHz band in Japan
802.11n WG	**MAC layer to achieve very high data rates (up to 600Mbps) **, is backward compatible with 802.11b/g

Development and layered structure of IEEE 802.11

• Wireless connection management
• link reliablity management
• power and security management

Physical Layer

• IEEE has three options for medium to be used for communcation
  • one is based on infrared
  • two tohers based on radio
• Physical Layer divided into
  • PMD(Physical medium-dependent sublayer)（物理媒體相關子層）
    • Handles functions related to medium adaptation
      • encoding
      • decoding
      • modulation
    • FHSS PMD (Frequency Hopping Spread Spectrum PMD)
      • 2.4GHz ISM band
      • use -2-level GFSK for 1Mbps and 4-level GFSK for 2Mbps
        • Gauss frequency Shift Keying
    • DSSS PMD (Direct Sequence Spread Spectrum)
      • operates in 2.4Ghz ISM band
      • use DBPSK for 1Mbps and DQPSK for 2Mbps
    • INFRARED PMD
      • operates in 850-950nm range
      • provides data rates of 1Mbps and 2Mbps using PPM(Pulse Position Modulation)
  • PLCP(Physical layer convergence protocol)（物理層匯聚過程子層）
    • Abstract the functionality of PMD providing
      • Service Access Point (SAP)
      • Clear Channel Assessment (CCA) Carrier sense signal

EXTENSTIONS FOR IEEE 802.11 DEFINE the following PMDs

IEEE WLAN standards
IEEE 802.11b	2.4Ghz ISM band, DSSS with CCK provide up to 11Mbps
IEEE 802.11a	5Ghz, OFDM to provide up to 54Mbps
IEEE 802.11g	2.4Ghz , OFDM 20~54Mbps (DSSS with CCK if < 20Mbps)
IEEE 802.11n	2.4Ghz OFDM+MIMO spatial streams, wider 40Mhz channel

MAC Layer Mechanisms

Main function of MAC Layers

• to arbitrate transmission requests of wireless stations opearting in the area
• to multiplex transmission requests
• to provide roaming support
• to authentication wireless stations;
• to conserve power consumption

Service Support

• Asynchronous data service is mandatory
• Real-time service is optional

Medium Access Method Defined

• The Distributed Coordination Function (DCF)
  • primary access method defined in IEEE 802.11
  • based on CSMA/CA that use RTS-CTS mechanism
• Point Coordination Function (PCF)
  • is implemented on top of DCF to provide real-time service
  • AP controls medium access avoiding simultaneous transmissions

Interframe Spacing: Priorities in frame transmission

Inter-frame Spacing (訊框間隔)
Shortest inter-frame spacing (SIFS)	the shortest ISF, highest priority, used for RTS/CTS and ACKs
PCF inter-frame spacing (PIFS)	used by PCF in contention-free operation
DCF inter-frame spacing (DIFS)	used by staions in DCF mode(asynchronous data)
Extended inter-frame spacing(EIFS)	used when there is an error in frame transmission

Choices for shared medium access

• CSMA-CD
  • used in IEEE 802.3 networks
  • DISADV :collisions is wireless channels are harder to detect
  • DISADV: collisions leads to usage of bandwidth which is scarce
• CSMA/CA was used

Carrier Sensing

• Physical Carrier sensing
  • direct sensing of the PHY
  • expensive provided by the physcial layer, complexity depends on the PHY.
• Virtual carrier sensing:
  • provided by the Network Allocation Vector (NAV)
  • NAV Indicates how long the medium is reserved
  • NAV is set acccrdoing to fields (Duration ) indiated in most frames

How CSMA/CA Performs

• if hte medium is sensed to be free for DIFS, the node access medium for transmission
• if the medium is busy, the node back off for a contention time,
• when back off time expires , the station can access the medium
  • -during back off, if the node detect a channel busy, it freezes the CW, where is the integer multiply of slot times
  • CW is resumed when the channel is sensed to be free for DIFS

Contention Window Size Setting

• if CW is small in size
  • values are close to each other at different MTs
  • increases in the numbers of collision on the shared medium
• if CW is very large
  • unneccessary delay is introduced
• contention window in set to a random value between (0,$CW_{min}$);
• Collision occurs : CW doubles up to $CW_{max}$

Acknowledgements

WHY ACKs

• frequently frames are incorrectly received

HOW ACKS

• if a packet is correctly received , the priority transmission is organized for ACK(SIFS)
• the receiver accesses the medium after waiting for a SIFS and sends ACKS

Error Detection

• CRC code is used
• if no ACK is recevied by the sender, frame is retransmitted
• the number of retransmissioned is limited
• if the limit is exceedd , the rror to higher layer is reported

RTS-CTS

• to solve Hidden Terminal Problem

RTS-CTS Mechanism

• the senders sends an RTS packet to the recevier including

  • the intended receiver of the datapacket

• the whole expected duration of transmission

• RTS packet is received by all MTs in one-hop neighborhood of the sender

  • they set their NAV
  • NAV specifies the earliest time when the station is permitted to attempt transmission

• The intended receiver of packet does :

  • wait for SIFS
  • repsonse with CTS packet
  • CTS contains the duration field

• CTS packets is received by all Mts in one-hop neightborhood of the receiver

  • set NAV
  • if the set of stations receiving RTS and CTS are different, hidden terminal exist.

• All stations are informed and the medium is resreved for one sender exclusively

• The sender starts its transmission after waiting for SIFS

• The receiver receives packets waits for SIFS and repsonds with ACK

  The NAV in each node marks the medium as free

Shortcoming	Advantage
significant overhead and sometimes is not performed	remove the hidden terminal problem
	Perforamnce well in overload network

• Sometimes RTS-CTS is not performed
  • depends on RTS threshold
    • ”>” RTS threshold , RTS-CTS-DATA-ACK
    • ”<” RTS threshold, DATA-ACK
  • perform well in moderately loaded network but not in overload network

Fragmentaiton and reassembly

Purpose

• to decraese the number of incorrectly received frame due to bit errors
• the length of the fragments are equal to each other within a single packet
• the length of final fragment can be less
• fragments contains information nedded to resemble the initial packet

Other Mac functions

• Point Coordination Function(PCF)
  • provides QoS parametres : max access delay, minimum transmission bandwidth
• Synchronization
  • each statiion has a clock ,all clocks have to be sychrnoized
• Power management
• Qos in IEEE 802.11 environment
  • Hybrid coordination Function (HCF)
• Support for roaming
  • AP has a range of up to several hundreds metres
  • when the stations begins to experience a poor single quality it scans for a new AP

Scanning Method

• Active Scanning
  • sends a probe on each channel and waits for responsive
• Passive scanning
  • listneing to the medium to find other networks

	IEEE 802.11b	IEEE 802.11a	IEEE 802 .11g	who wins
Power efficiency	DSSS direct-sequence spread spectrum	OFDM Orthogonal frequency-division multiplexing	OFDM Orthogonal frequency-division multiplexing	802.11b more powe efficient
Frequency	2.4Ghz ISM, highly overloaded	5Ghz,higher absorbtion rate	2.4Ghz ISM, highly overloaded	poor performance in overloaded environment
Communication range	~150m	~50m	50~150m	802.11b
Data-rate	11Mbps	54Mbps	up to 54Mbps	higher than b ,lower than a
Cost efficiency	well-established manufacturing	components are more expensive	expensive than 802.11b, less expensive than a	802.11b is cheaper
Compatibility	first	not compatible with 802.11b	compatible with 802.11b;	802.11g is good
Number of users		More users, higher bandwidth and more channels	same number of channel as in 802.11b	capacity of 802.11a is higher

System design for networking in IEEE802.11

Types of network based on IEEE 802.11

Baic Concept in 802.11 System

• Basic Service Set (BSS): a set of stations communicating with each other
• Basic Service Area(BSA): area in which stations communicate

Two modes in operation

• Independent BSS: IBSS (Ad-hoc mode)

  • MT communicates directly with other Mts with APs

  

• Infrastructure BSS (Infracstruture mode)

  • MTs communicatie via AP

  • Less complex configuration

  • Ap assists station in power savings

• Extended Service Set (ESS)
  • created by linking BSSs using backbone network
  • provide larger service areas
  • does not specify pariticular technology
  • just requires backbone to provide a specific set of service

Components in infrastructure BSS/ESS

Compoents

• Distribution system : backbone network
  • capabilities of bridging APs
  • relay (接力) of frames within ESS
  • Inter AP protocoal (IAPP)
• Access points
• Wireless medium
• Station

Network Services

Provided by

AP

Network Sevices provided by AP
Association	The address of MT must be known by AP before communication. This is done via association.
Reassociation	The established association is transferred from one MT to another using reassociation.
Disassociation	When node leaves AP or shuts down it enforces disassociation.
Distribution	This refers to the distribution of traffic within, in and out the network.
Integration	This service is evoked when transmission via non-IEEE 802.11 network is required

MT

Network Sevices provided by MT
Authentication	This is used in order to establish the identity of stations to each other. Authentication implementation may range from insecure handshake procedures, to public key encryption schemes.
Deauthentication	terminate existing authentication
Privacy	The contents of messages os sometimes encrypted using a certain protocol to prevent unauthorized reading.
Data delivery	IEEE 802.11 networks provide a way to transmit and receive data. The transmission is not guaranteed to be completely reliable.