Mobile Systems ITU Mobile Systems The IEEE 802.11 WLAN Part I Ver 1.1

Mobile Systems ITU

Mobile Systems

The IEEE 802.11 WLAN Part I

Ver 1.1

Mobile Systems ITU

IEEE Std. 802.11-1997, 1-2 Mbit/sec.

Information Technology-Telecommunications and Information exchange betweensystems-Local and Metropolitian area networks-Specific requirements-Part 11: Wireless LAN Medium Access Control (MAC) andPhysical Layer (PHY) specifications.Sponsor:LAN MAN Standards Committee of the IEEE Computer Soc.Approved 26 June 1997.

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ABSTRACT (I)

The medium access control (MAC) and physical characteris-tics for wireless local area networks (LAN’s) are specified inthis standard, part of a series of standards for local and me-tropolitian area networks. The medium access control unit inthis standard is designed to support physical layer units asthey may be adopted dependent on the availablity of thespectrum. This standard contains three physical layer units:

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ABSTRACT (II)

• two radio units, both operating in the 2400-2500 MHz band, and • one baseband infrared unit.One radio unit employs the frequency-hopping spread spec-trum technique, and the other emplys the direct sequencespread spectrum technique.

KEYWORDS: ad hoc network, infrared, LAN, local area network, mobility, radio frequency, wireless

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The IEEE Standard 802.11 WLAN

Similaries between WLAN (Wireless Local Area Networks)and Wired Local Area Networks.

• The IEEE 802.11 WLAN is designed to look like any IEEE 802 wired LAN. The 802.11 must support all protocols and LAN management tools, that operate on a wired network.

• The IEEE 802.11 is designed to the same interface as IEEE 802.3

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Differences between WLAN (Wireless Local Area Networks)and Wired Local Area Networks.

• No wires (because air link), and mobility.- The air link: Radio or infrared.- Data carried by a WLAN is not private or protected.- Data is broadcast to all.- IEEE 802.11 Wired Equivalent Privacy (WEP), protection at the same level as wired privacy.

• Electromagnetic Propagation.- Reflection and/or attenuation (dæmpning) of the signal carrying LAN data.- Small changes in physical position => large changes in recieved signal strength.

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Differences between WLAN (Wireless Local Area Networks)and Wired Local Area Networks.

Problems introduced by mobility:• The location-based services lose the ”hook” to a user location, when network addresses are not on a physical site. The notion of ”the nearest network printer” must be re- defined. Increases the complexity of the service location provider.

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The IEEE 802.11 WLAN family

First standard for WLAN: IEEE Std. 802.11-1997, 1-2 Mbit/sec Defines:

MAC layer

PLCP sublayer

PMD sublayer

MAC: Medium Access Control layer, management protocols and services.

PHY: PHysical Layer, consisting of:PLCP: Physical Layer Convergence Procedure sublayer.PMD: Physical Medium Dependent sublayer.Three different physical layers:• Infrared (IR) baseband PHY• frequency hopping spread spectrum (FHSS) radio in the 2.4 GHz band.• direct sequence spread spectrum (DSSS) radio in the 2.4 GHz band.

Wireless Media

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IEEE Std. 802.11-1997, 1-2 Mbit/sec.

Information technology-Telecommunications and information exchange between sy-stems-Local and metropolitian area networks-Specific requirements-

Part11: Wireless LAN Medium Access Control (MAC) andPhysical Layer (PHY) specifications.

Sponsor:LAN MAN Standards Committee of the IEEE Computer SocietyApproved 26 June 1997.

Mobile Systems ITU

The IEEE 802.11 WLAN family

In 1999 two new physical layers are approved:• IEEE Std 802.11a is an Orthogonal Frequency Domaine Multiplexing

(OFDM) radio in the UNII bands delivering up till 54 Mbit/sec.

U-NII: Unlicensed national information structure (US) at 5 GHz. • IEEE Std 802.11b is an extension of the DSSS (Direct Sequence Spread Spectrum) PHY (Physical layer) in the 2.4 GHz ISM band, delivering up till 11Mbit/sec. data rates. ISM: Industrial, Scientific, and Medical band at 2.4 GHz.

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IEEE 802.11 Architecture

• Support networks with decisions in mobile stations,- thus eliminating bottlenecks of a centralized structure.

• Error tolerant in WLAN equipment.

• Flexible: Supporting - small transient networks, and - large semipermanent or permanent networks.

• Deep power saving modes to prolong battery life without losing network connectivity.

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IEEE 802.11 Architecture

• Architectural Elements:

- The Station,

- the Access Point (AP, which is a Station), the wireless medium.

- the Basic Service Set (BSS). Stations that communicate.

- The Distribution System (DS), and

- the Extended Service Set (ESS).

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The Station and The Basic Service Set

• Station: Mobile, portable or stationary.

- Other names might be: Network adapter or network interface card. - It always consists of a

MAC: Medium Access Control PHY: Physical Layer (Antenna, Radio, etc.)

• Station services:- Authentication: Prove the identity of one station to another.- Deauthentication: Eliminate a previously authorized user from acces.- Privacy: Equivalent level of protection, compared to a wired network.- Data delivery: Reliable delivery of data frames from the MAC in one station to the MAC in one or more stations.

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• Station services:

- Authentication: Prove the identity of one station to another.

- Deauthentication: Eliminate a previously authorized user from acces.

- Privacy: Equivalent level of protection, compared to a wired network.

- Data delivery: Reliable delivery of data frames from the MAC in one station to the MAC in one or more stations.

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• Basic Service Set (BSS): Set of stations communcating with one another.• Independent Basic Service Set (IBSS): All stations communicating directly with one another. Also often denoted an ad hoc network.

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The Extended Service Set (ESS)

• The access points (AP) communicate among themselves to forward traffic from one BSS to another.• The AP’s perform this communication via an abstract medium called the Distribution System (DS).

An Access Point is astation, with accessto a distribution sys-tem.

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Distribution System and Services

• One AP Communicating with another to exchange frames for stations in their BSS’s,

• forward frames to follow mobile stations from one BSS to another,

• exchange frames with wired networks.

Distribution System:

Services:• Station Services: Authentication, deauthentication, privacy, delivery of data.

• Distribution Services: Association, disassociaton, reassociation, distribution, integration.

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Station Services (I)

• Authentication:

- Prove the identity of one station to another.

- Without this, the station is not allowed to use the WLAN for data delivery.

• Deauthentication:

- Eliminate a previously authorized user from any further use of the network.

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Station Services (II)

• Privacy:

- Equivalent level of protection as that provided by a wired network with restricted physical access to the network plant.

• Delivery:

- Data delivery similar to that provided by other IEEE 802 LAN’s.

- Reliable delivery of dataframes from one MAC in one station to the MAC in one or more other stations, with minimal dublication and minimal reordering.

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Distribution Services (I)

• Association:

- Establish a logical connection between a mobile station and an AP.

- Necessary for the DS to deliver data to the mobile station.

- Invoked once, when the station enters the WLAN for the first time, after power on, or when rediscovering the WLAN after some time.

• Reassociation:

- As association, but include information about the AP, with which the mobil station was previously associated.

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Distribution Services (II)

• Disassociation:

- Mobile Station (MS) inform AP that it does not need service.- AP inform one or more MS that logical connection can no longer be provided.

• Distribution:

- Frame sent to its own basic service set (BSS) or- to another mobile station associated with another AP or- to a network outside the IEEE 802.11 WLAN.

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Distribution Services (III)

• Integration Service:

- Connect the IEEE 802.11 WLAN to other LAN’s, including one or more wired LAN’s, or other IEEE 802.11 WLAN’s.

- The integration is performed by a portal, which is an abstract architectural concept.

- The integration service translates IEEE 802.11 frames to frames that may traverse another network, and

- translate frames from other networks to frames that may be delivered by an IEEE 802.11 WLAN.

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Interaction Between Some Services (I)

The IEEE 802.11 std. requires that each station mustmaintain two variables that are dependent on

• the authentication/deauthentication service, and

• association/reassociation/disassociation services.

These two Boolean variables are • authentication state• association state.

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Interaction Between Some Services (II)

• A station may be authenticated with many different stations simultaneously.

• A station may be associated with only one other station at a time.

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Interaction Between Some Services (III)The General Frame Format

2 2 6 6 6 2 6 0-2312 4

Frame Control (2 bytes)

Duration/ID

Address 1

Address 2

Address 3

Sequence Control

Address 4

Frame Body

FCS

MAC Header

Frame body max. 18496 bits

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Interaction Between Some Services (IV)

State 1: Unauthenticated, Unassociated

State 2: Authenticated, Unassociated

State 3: Authenticated, and Associated.

Class 1Frames.

Class 1 & 2Frames

Class 1, 2& 3 Frames

Successful authentication DeAuthenticationNotfication

Disassociation, NotificationSuccessful Association or reassociation

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Interaction Between Some Services (V)A Station moving between Access Points.

a: Find AP1 and authenticate and associate. e: Disassociate stations. b: Preauthenticate with AP2, when moving. f: Find another AP3 for authen-c: Reassociate with AP2. tication and association.d: Terminate the association with AP1.

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Medium Access Control (MAC)

The IEEE 802.11 medium access control supplies the required function for

• reliable delivery mechanism for user data over • noisy, unreliable wireless media,

while providing advanced LAN services, beyond thoseof existing wired LAN’s.

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Medium Access Control (MAC)

MAC Functionality:

• Reliable data delivery service, through a frame exchange protocol.

• Fairly control access to the shared wireless medium, through two different access mechanisms:

- The basic access mechanism, called the Distributed Coordination Function (DCF), and a- centrally controlled access mechanism, called the Point Coordination Function (PCF).

• Privacy service called Wired Equivalent Privacy (WEP) for encryption.

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MAC Frame Exchange Protocol (I)

The media used by IEEE 802.11 WLAN are very noisy and unreliable.Thus the MAC implements a frame exchange protocol, which allows the

• source of a frame to determine, • when the frame has sucessfully been received at the destination.

The frame exchange protocol requires the participation of all stations in the WLAN. Every station decodes and reacts to information in the MAC header of every frame it receives.

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MAC Frame Exchange Protocol (II)

The minimal MAC frame exchange protocol consists of two frames:

• a frame sent from the source to the destination, and

• an acknowledgement from the destination of correctly received frame.

• If the source did not receive an acknowledgement it will retransmit.

A frame and its acknowledgement, constitute an atomic unit of the MAC protocol, and cannot be interripted bytransmission from any other station.

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MAC Frame Exchange Protocol (III)

If the source does not receive the acknowledgement because

• the destination did not send one, due to error in the ori- ginal frame, or because • the acknwledgement was corrupted,

the source will attempt to transmit the frame again.

Reduces the error rate of the medium, at the cost ofreduced bandwidth.

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The Hidden Node Problem (I)

Fact: Not every WLAN station can be expected to communicate directly with every other WLAN station.

Station A communicatesdirectly with B.

Station B communicatesdirectly with C.

Station A cannot communicate with station C.

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The Hidden Node Problem (II)

If station A was sending a frame to station B, theframe could be corruptedby a transmission initia-ted by station C !

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Hidden The Node Problem (III)

Announce to all station in the neighborhood of both the source and destination, the impending transmision.• Source sends: Request to Send (RTS).• Destination answers: Clear to Send (CTS).

• Then the Source sends the dataframe(s).

• The Destination sends acknowledgement.

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The Hidden Node Problem (IV)

Four frame exchange protocol, which is atomic in the MAC (Medium Access Control) protocol.They cannot be interrupted by transmissions of otherstations.

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MAC Frame Exchange Protocol (IV)

Now extend into a four frame protocol for communicating between different MAC’s:

- The source sends a Request To Send (RTS) to the destination. This is also received by other stations too.

- The destination returns a Clear To Send (CTS) to the source. This is also received by other stations too.

- The source sends the Data Frame to the destination.

- The destination sends an Acknowledgement to the source, thus completing the data transfer.

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MAC Frame Exchange Protocol (V)

Reduce the four-way frame exchange protocol to a two-wayprotocol if risk of contention is low.

The management information base (MIB):

• If length of frame > dotRTSThreshold attribute Then use the four way protocol, Else use the two way protocol.

Allow for tuning of the network, by reducing the communi-cation overhead.

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Retry Counters

Two Retry counters, when transmission fails.

• Short retry counter for frames <= dotShort11RTSThreshold, • Long retry counter for frames <= dotLong11RTSThreshold.

There is also a lifetime timer associated with each frametransmitted from a MAC.

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Basic Access Mechanism (I)

The basic access mechanism from • an Access Point (AP) to the

• Physical medium (Radio or Infrared carriers) is

- Carrier Sense Multiple Access (CSMA) with Collision Avoidance (CA) with binary exponential backoff.

This is also denoted CSMA/CA.

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Basic Access Mechanism (II)

A station will listen (Carrier Sense, CS) before beginning a transmission.

• If the medium is already carrying a transmission, Then the station that is listening will not begin its own transmission, and the station enters a deferal period.

The duration is determined by a random number which represents the amount of time, that must elapse while there are not any transmissions.

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Basic Access Mechanism (III)

The waiting time is called the Contention Window the size of which doubles with every attempt to access the medium which is deferred.

The random number for the exponential backoff algorithm is uniformly distributed in the Contention Window interval.

The window is re-initialized when a transmission is sucessfully completed.

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Basic Access Mechanism (IV)

The Network Allocation Vector (NAV):

• The amount of time, that remains, before the medium will become available.

• The NAV is updated through duration values, transmtted in all frames.

• The NAV is virtual carrier sense mechanism, which is com- bined with the physical carrier sense, into the MAC collision avoidance part of the CSMA/CD access mechanism.

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Timing Intervals (I)

The 5 timing intervals needed for control of transmission.

Short Interframe Space (SIFS), determined by PHY.

The Slot Time (ST), determind by PHY.

The Priority Interframe Space (PIFS) = SIFS + ST.

The Distributed Interframe Space (DIFS)= SIFS + 2 x ST.

The Extended Interframe Space (EIFS) >> DIFS. Used for error correc.

Time

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Timing Intervals (II)

The 5 timing intervals is used for implementing two diffe-rent control mechanisms for transmission:

• The Distributed Coordinating Function (DCF), used in Independent Basic Service Sets (IBSS) also denoted ad hoc networks.

• The Centrally Controlled Access Mechanism, used in a Poll and Response protocol to eliminate contention for the medium, thus obtaining higher bandwidth than the DCF.

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The Distributed Coordination Function (DCF)

The DCF is used in an Independent Basic Service Set (IBSS),also denoted an ad-hoc network:

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When the MAC is requested to transmit a frame, the follow-ing is carried out:

• Check the physical and virtual carrier sense mechanism (NAV) if the medium is not in use for an interval of DIFS: Distributed Interframe Space.

If the medium is in use, apply the back-off mechanism, and increment the retry counter. Decrement the back-off value if the medium is idle, by on slot time interval. When expire then transmit.

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End of previous transmission

DIFS

DIFS: Distributed Interframe Space

Contention Window

Slots

Next Transmission

time

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Centrally Controlled Access Mechanism (I)

The Centrally Controlled Access is used when• there is an Access Point (AP)

Method:

Use a poll and response protocol,to eliminate contention for themedium.

Point Coordination Function (PCF).

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Centrally Controlled Access Mechanism (II)

A point coordinator (PC) controls the PCF. A PC is always located in an Access Point (AP).The PCF operation:• A mobile requests the PC to register it on a polling list, residing in the Access Point (AP).• The PC regularly polls the stations for traffic, and• the PC delivers traffic to mobile stations.

Near-isochronous service to the stations on the polling list.www.webster.com: Isochronous: Uniform in time.

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Centrally Controlled Access Mechanism (III)

PCF is built on the DCF (Distributed Coordination Function).Both operate simultaneously.

The PCF controls and operates the • Contention Free period (CFP), where all acces is controlled by the Point Coordinator, and the DCF is not allowed access to the medium.

PCF DCF PCFAlternate:

time

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Centrally Controlled Access Mechanism (IV)

The Contention Free Period (CFP) starts when PC gains ac-cess to the medium, from the DCF.

• The PC transmits a Beacon frame containing - Time stamp- Beacon interval- Capability information.

• The PC now delivers traffic to its Basic Service Set (BSS), through- Polling mobile stations on its polling list,- Stations returning data and acknowledgements, one frame for each Contention Free Poll (CF-Poll).

Beacon: Signal for guidance.

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Centrally Controlled Access Mechanism (V)

Improve the efficiency of media utilization by:

• Piggyback both the - Acknowledgement and- CF-Poll (Contention Free Poll) onto data frames.

• The dataframe from a station to the PC, may include acknowledgement of the frame just received from the PC.

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Centrally Controlled Access Mechanism (VI)

The Point Coordinator (PC) residing in the Access Point (AP),may do the following:

• sending a frame to one station along with a CF-Poll, and

• acknowledge a frame received from a different mobile.

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Centrally Controlled Access Mechanism (VII)

PCF Timing:

Data+CF-Poll

Data + CF-Ackfrom Station 1

Data + CF-Ack + CF-Pollto Station 2

ACK fromStation 2

CF-Poll toStation n

Data + CF-PollTo Station n+1

CF-End

SIFS: Short Interframe Space.

PIFS Priority Interframe Space

time

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Centrally Controlled Access Mechanism (VIII)

During the Contention Free Period (CFP), the PC:• Ensures the interval between frames on the medium is < PIFS. PIFS: Priority Interframe Space.

Thus a station operating under DCF cannot gain access to the medium.

• Sends a frame to a station, and expect the responding frame (ack. or data) within the SIFS interval. SIFS: Short Interframe Space. If not received, the PC transmits the next frame before a PIFS expires.

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Centrally Controlled Access Mechanism (IX)

As the CFP (Contention Free Period) is not a true isochronousService, where the timing is known in advance, the PointCoordinator (PC) • announces the end of the CFP by transmitting a CF-End frame, which concludes the CFP.

Then the mobile stations, which had set their NAV (Network Allocation vector), from the initial Beacon frame, resets The NAV’s to zero.

Thus the stations are free to operate in the DCF again.

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References (I)

1 Bob O’Hara, Al Petrick, ”The IEEE 802.11 Handbook” IEEE Press, 1999. Reading material: p. 1 - 69, p. 88 (from Power Management) - 98 (not including Combining Management Tools).

2 Jennifer Bray, Charles F. Sturman, ”Bluetooth”, Prentice-Hall 2001.

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References (II)

3 IEEE Std. 802.11/1997 ”Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications”. IEEE Standards Board, June 26, 1997.

Download: www.dtv.dk -> Search literature -> Search of other literature at DTV -> IEEE/IEE Electronic Library -> Standards -> 802.11 GO

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References (III)

4 Brian P. Crow, Indra Widjaja, Jeong Geun Kim, Prescott T. Sakai ”IEEE 802.11 Wireless Local Area Networks” IEEE Communications Magazine, September 1997, pp. 116-126.

Reading material: p. 116, p. 117, p. 118 excluding the Physical Layer. P. 119, p. 120, p. 121.

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Mobile Systems ITU Mobile Systems The IEEE 802.11 WLAN Part I Ver 1.1