Studying 802.11 1 st lecture ~Overview of IEEE 802.11~ Tadashi YASUMOTO.

Studying 802.111st lecture

~Overview of IEEE 802.11~

Tadashi YASUMOTO

In beginning

• To understand acronyms– Overwhelming numbers

• To understand the difference with Ethernet

• To understand the 802.11 MAC layer

What IEEE 802.11 is for.

• Allow WLAN to be identical to wired LAN– With mobility– With built-in power saving operations

IEEE 802.11 defines

I. Physical layersII.MAC management protocols and serviceIII.MAC sublayer

Physical layers

• PHYs : Physical layers– Some kinds of layers

• 802.11b– HR/DSSS : a high-rate DSSS

• 802.11a– Based on OFDM; using orthogonal frequency division multiplex

• 802.11g– Using OFDM with backwards compatibility with 11b

– Common specification• CSMA/CA

– Difference with CSMA/CD

Physical layers

• CSMA/CA: Carrier Sense Multiple Access with Collision Avoidance– Dispersion Contorol (<-> Centralization Control) – Difference with CSMA/CD

• Not always detect the collision

– Features• If the other node are in use of the channel

– Wait for random delay time– And have the right to send

IEEE 802.11 defines

I. Physical layersII.MAC management protocols and serviceIII.MAC sublayer

MAC layer (1/2)

• MAC sublayer– LLC: the logical link control

• A common link layer used by lower-layer LAN tecnology

– What is LLC -> next slide

• MAC management– 802.1D

• Bridging

– 802.1Q• Virtual LAN(VLAN)

MAC layer (2/2)

• LLC: Logical Links Control

– One of the sublayer in Data Link Layer (L2 in OSI)– In 802, all of lower-layers are under LLC

802.11 Architecture

• Components• Types of Networks• Services• Other

ComponentsDS

APs

STAs

Medium

Components

• STAs: Stations• Computing devices with wireless network interface(s)• NICs familiar with in wired networks

• APs: Access points• Wireless-to-wired bridging

• Wireless medium• PHYs

• DS: Distribution system (later)• Tracking the movements of mobile STAs

Components

• Distribution Systems– Provides services to STAs– Tracking where a STA is– Delivering frames

Ethernet is a part of DS

AP has at least two interfacesone is wired, the other is wirelesswith bridging egngine

802.11 Architecture


Types of Networks(1/3)

• BSS: the basic service set– IBSS: independent BSS

• A set of STAs creating a short-lived network• Direct communication and No relay function• Sometimes referred to AD-HOC networks

– Infrastructure BSS• Using an AP• Two hops communication with relay function

– More transmission capacity than IBSS– Assist with STAs saving power

• STAs need to associate with an AP later


• ESS: Extended service area– An arbitrarily large size

• A set of BSSs • With same SSID: service set identifier

– Link with APs in layer 2


• Virtual APs– Some ESSs on same physical infrastructure

• To protect some different networks• To create parallel networks with different security levels

802.11 Architecture


Services

• STA services– Authentication– Deauthentication– Privacy– Data delivery

• Distribution services– Association– Reassociation– Disassociation– Distribution– Integration

Services

• Distribution services

Services

• Privacy (or Confidentiality)– Provide a level of protection

• WEP protocol: Wired Equivalent Privacy

802.11 Architecture


Other

• Interaction between services– After

• Mobility support– 3 types of transitions between AP s

• No transition• BSS transition• ESS transition

Other

• Interaction between services

Mobility support

• BSS transition– No need to be aware of STA's location

• the cooperation of AP s

Same ESS

Mobility Support

• ESS transition– 802.11 does not support...

ESS1 ESS2

Workshop Presentation2nd

Ryo Nakajima

• Wireless and 802.11 introduction• Time scheduling in 802.11• Frame format in 802.11• Particular situation• Frame translation between wire and wireless

802.11

• Wireless link– Many interfere

• Microwave ovens, node itself

– Need power saving mode• Mobile, battery operated

Particular system: acknowledgement, RTS/CTS, CSMA/CA

Acknowledgement

• Each frame must be acknowledged– Or the sender will re-transmit the same frame

RTS/CTS

• The Hidden Node Problem– Only node2 can know both node1 and node3

RTS/CTS

• RTS/CTS keep other node silent

CSMA/CA

• CA: collision avoidance– Like CSMA/CD

• Wait until the channel become idleDetails in Scheduling

Scheduling

• DCF (AD-HOC mode)– Basic time scheduling in 802.11– To avoid collisions, use empty time

• PCF (AP)– Rare system– To avoid collisions, AP(Access Point) gives priority

to each node in order• HCF

– Keep service quality, but now undertaking

DCF

• Carrier sensing– To find whether the channel is busy or not

• Time scheduling– To avoid collisions by simultaneous transmission

Carrier sensing

• Physical carrier sensing– Expensive electronics– Hidden nodes lurking everywhere

• Virtual carrier sensing– Use Network Allocation Vector(NAV)– NAV means time to be quiet– NAV is included almost all frames– So NAV can be heard every nodes

Time scheduling

• NAV• Interframe Spacing

– Waiting time after one frame transmitted

Data

Ack

Sender

Receiver

Interframe Spacing

Interframe Spacing

• SIFS– Short range– To higher priority:RTS+CTS, Data+Ack, Fragment

• DIFS– Long range, standard– After Data+Ack, with Backoff time

• EIFS– To detect transmit failure– SIFS+Ack+DIFS

Time Schedule Example

RTS

CTS

Sender

Receiver

SIFS

NAVNAV(RTS)

SIFS Data

SIFS ACK

NAV(CTS)

DIFS+Backoff

Data

Backoff

• Time to wait after DIFS– Random length– Each node determine by itself

• To lower collisions– Lower simultaneous beginning of transmission

Error Recovery

• No ack→retry• Short retry counter and long retry counter

– Incremented when frames are retransmitted– Can hear ack→set to 0– If reach to the limit, discard that frames

• Backoff time become long to reduce transmission– If retry counter incremented, backoff time is

doubled

General Frame format

FrameControl

Duration/ID

Address 1

Address 2

SequenceControl

Address 4

FrameBody

FCS

Octets: 2 2 6 6 2 6 0-2312 4

MAC Header

Address 3

6

Frame Control

ProtocolVersion

Type SubtypeTo DS

From DS

More Frag

RetryPwrMgt

Bits: 2 2 4 1MoreData

WEP Order

1 1 1 1 1 1 1

• Next Duration/ID

FrameControl

Duration/ID

Address 1

Address 2

SequenceControl

Address 4

FrameBody

FCS

Octets: 2 2 6 6 2 6 0-2312 4

MAC Header

Address 3

6

Duration/ID

• NAV• PS-Poll+AID(Associated ID)

– Node’s wake-up signal

– Next Address field

FrameControl

Duration/ID

Address 1

Address 2

SequenceControl

Address 4

FrameBody

FCS

Octets: 2 2 6 6 2 6 0-2312 4

MAC Header

Address 3

6

Address field

• Destination Address• Source Address• BSSID

– AP’s wireless NIC’s MAC address

• Receiver Address• Transmitter Address

• Next Sequence Control

FrameControl

Duration/ID

Address 1

Address 2

SequenceControl

Address 4

FrameBody

FCS

Octets: 2 2 6 6 2 6 0-2312 4

MAC Header

Address 3

6

Sequence Control

• 4 bits Fragment number and 12 bits Sequence number

• All fragments have the same sequence number• To detect duplicate frame

• Next FCS

FrameControl

Duration/ID

Address 1

Address 2

SequenceControl

Address 4

FrameBody

FCS

Octets: 2 2 6 6 2 6 0-2312 4

MAC Header

Address 3

6

FCS

• Frame Check Sequence• Calculate the value with hash function

Particular situation

• Group frames( broadcast multicast )• Fragment• RTS/CTS• Power saving• Multirate support

Group frames

• No acknowledgement• No fragment• No NAV set

DataData

NAV

Backoff DataDIFS

Fragment

• SIFS between fragment

RTS/CTS

• NAV=RTS+CTS+first_data+first_ack

Power saving

• Node send PS-poll to AP after wake up• AP send data to node which is buffered

– AP buffer data while node is sleeping– AP’s response: Immediate response or Deferred

response• Immediate: use SIFS• Deferred: use DIFS+Backoff

Multirate Support

• How to choose the rate depends on the vendors

• Basic rate set: every station in the system must support

• Control frame that start a frame exchange– Such as RTS/CTS– One of the rates in the basic rate set– CTS no faster than RTS

Wireless→Wire

1 integrity check2 BSSID (AP’s MAC) check3 decrypt if needed4 Fragment check5 copy address and type6 FCS recalculate

Wire→Wireless

1 Integrity check and destination node check2 prepend SNAP header3 power saving check4 set sequence number, fragment number5 encryption if needed6 copy address7 FCS recalculate

Studying 802.11 1 st lecture ~Overview of IEEE 802.11~ Tadashi YASUMOTO.

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