Studying st lecture ~Overview of IEEE ~ Tadashi YASUMOTO

In beginning To understand acronyms –Overwhelming numbers To understand the difference with Ethernet To understand the MAC layer

What IEEE is for. Allow WLAN to be identical to wired LAN –With mobility –With built-in power saving operations

IEEE defines I.Physical layers II.MAC management protocols and service III.MAC sublayer

IEEE defines I.Physical layers II.MAC management protocols and service III.MAC sublayer

Physical layers PHYs : Physical layers –Some kinds of layers b –HR/DSSS: a high-rate DSSS a –Based on OFDM; using orthogonal frequency division multiplex g –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 defines I.Physical layers II.MAC management protocols and service III.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

Architecture Components Types of Networks Services Other

Architecture Components Types of Networks Services Other

Components DS 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 interfaces one is wired, the other is wireless with bridging egngine

Architecture Components Types of Networks Services Other

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

Types of Networks(2/3) ESS: Extended service area –An arbitrarily large size A set of BSSs With same SSID: service set identifier –Link with APs in layer 2

Types of Networks(3/3) Virtual APs –Some ESSs on same physical infrastructure To protect some different networks To create parallel networks with different security levels

Architecture Components Types of Networks Services Other

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

Architecture Components Types of Networks Services 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 – does not support... ESS1 ESS2

Workshop Presentation 2nd Ryo Nakajima

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

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 idle Details in Scheduling

Scheduling DCF (AD-HOC mode) –Basic time scheduling in –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

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

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

Frame Control

Next Duration/ID

Duration/ID NAV PS-Poll+AID(Associated ID) –Node’s wake-up signal

–Next Address field

Address field Destination Address Source Address BSSID –AP’s wireless NIC’s MAC address Receiver Address Transmitter Address

Next Sequence Control

Sequence Control 4 bits Fragment number and 12 bits Sequence number All fragments have the same sequence number To detect duplicate frame

Next FCS

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

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 check 2 BSSID (AP’s MAC) check 3 decrypt if needed 4 Fragment check 5 copy address and type 6 FCS recalculate

Wire→Wireless 1 Integrity check and destination node check 2 prepend SNAP header 3 power saving check 4 set sequence number, fragment number 5 encryption if needed 6 copy address 7 FCS recalculate