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IEEE 802.11

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802 Project started by IEEE for setting standard for LAN. This project started in (1980, February), Name given to project is year and month 80 2 IEEE Project 802 define network standards for the physical components of a network (the interface card and the cabling) that are accounted for in the physical and data-link layers of the OSI reference model.

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802 IEEE802 The 802 specifications define the ways NICs access and transfer data over physical media. These include connecting, maintaining, and disconnecting network devices.

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MAC Sublayer PCF Point Coordination Function (controlled access) DCF Distributed Coordination Function (random access)

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802-Categories 802.1Sets Internetworking standards related to network management. 802.2Defines the general standard for the data-link layer. The IEEE divides this layer into two sublayers: the LLC and MAC layers (discussed in the previous lesson). The MAC layer varies with different network types and is defined by standard IEEE 802.3. 802.3Defines the MAC layer for bus networks that use Carrier-Sense Multiple Access with Collision Detection (CSMA/CD). This is the Ethernet Standard. 802.4Defines the MAC layer for bus networks that use a token-passing mechanism (Token Bus LAN). 802.5Defines the MAC layer for token ring networks (Token Ring LAN). 802.11Defines wireless network standards.

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802.11-Configuration Access-Point (AP) Architecture : Stations select an Access-Point and associate with it Access-Points : Support roaming Provide time synchronization functions (beaconing) Provide Power Management support Traffic typically flows through Access-Point

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BSS(Basic Service Set) BSS A set of stations controlled by a single Coordination Function (=the logical function that determines when a station can transmit or receive)

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IBSS Independent Basic Service Set (IBSS): A Basic Service Set (BSS) which forms a self-contained network in which no access to a Distribution System is available A BSS without an Access-Point One of the stations in the IBSS can be configured to initiate the network and assume the Coordination Function

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IBSS

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802 Extended Service Set (ESS): A set of one or more Basic Service Sets interconnected by a Distribution System (DS) Traffic always flows via Access-Point Distribution System (DS): A system to interconnect a set of Basic Service Sets Integrated; A single Access-Point in a standalone network Wired; Using cable to interconnect the Access-Points Wireless; Using wireless to interconnect the Access-Points

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BSS Distribution System Extended Service Set( With wired distribution system)

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BSS Distribution System Extended Service Set( With wireless distribution system)

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IEEE 802.11 defines two MAC sublayers: the distributed coordination function (DCF). and point coordination function (PCF). MAC Sublayer

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Distributed Coordination Function (DCF) Distributed access protocol Contention-Based Makes use of CSMA/CA rather than CSMA/CD for the following reasons: Wireless LANs cannot implement CSMA/CD for three reasons: 1. For collision detection a station must be able to send data and receive collision signals at the same time( costly stations and increased bandwidth requirements). 2. Collision may not be detected because of the hidden station problem. 3. The distance between stations may result in Signal fading which prevent a station at one end from hearing a collision at the other end. Suited for ad hoc network and ordinary asynchronous traffic MAC Sublayer

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Wireless Medium Access-CSMA/CA RTS/CTS Mechanism Node wants to send data, first request to receiver for sending data. Send A RTS packet. Receiver reply by sending CTS(Clear to Send) packet. This packet include duration of communication. Other node receiving CTS will defer their transmission for duration of communication.

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CSMA/CA Sender Receiver RTS CTS DATA

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CSMA/CA-Hidden Terminal Problem A B C A wants to send data to B. A will Sense channel, it finds channel free and start sending data. C also wants to send data to B. C will sense channel and finds channel free and start sending. Both signal reach Node B and collide. Collision not detected.

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CSMA/CA-Hidden Terminal Problem A B C A wants to send data to B. A will send RTS to B. RTS

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CSMA/CA-Hidden Terminal Problem A B C A wants to send data to B. A will send RTS to B. B will received RTS and Reply to A that you can transmit data by sending CTS. CTS packet also include duration of this communication. CTS CTS packet reach A and C both. C check CTS packet and learn that B going to start communication with A for Duration mentioned in packet.

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CSMA/CA-Hidden Terminal Problem A B C A wants to send data to B. A will send RTS to B. B will received RTS and Reply to A that you can transmit data by sending CTS. CTS packet also include duration of this communication. Data CTS packet reach A and C both. C check CTS packet and learn that B going to start communication with A for Duration mentioned in packet. A receive CTS and finds that it can transmit data.

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Point Coordination Function (PCF) an optional access method on top of DCF Implemented in an infrastructure network (not in an ad hoc network). Contention-Free mostly for time-sensitive transmission services like voice or multimedia. The AP performs polling stations one after another, sending any data they have to the AP. MAC Sublayer

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To give priority to PCF over DCF, another set of interframe spaces has been defined: SIFS - Short Inter Frame Spacing Used for immediate response actions e.g ACK, CTS PIFS - Point Inter Frame Spacing PIFS (PCF IFS) is shorter than the DIFS. if, at the same time, a station wants to use only DCF and an AP wants to use PCF, the AP has priority. MAC Sublayer

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Repetition interval has been designed to cover both contention-free (PCF) and contention-based (DCF) traffic to allow DCF accessing the media. The repetition interval starts with control frame, called a beacon frame. When the stations hear the beacon frame, they start their NAV for the duration of the contention-free period of the repetition interval. MAC Sublayer

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repetition interval used by the PC (point controller) stations. At the end of the contention-free period, the PC sends a CF end (contention-free end) frame to allow the contention- based stations to use the medium. MAC Sublayer

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WLAN Terminology-SSID Service Set Identifier (SSID): Network name 32 octets long One network (ESS or IBSS) has one SSID

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BSSID Basic Service Set Identifier (BSSID) cell identifier 6 octets long (MAC address format) One BSS has one SSID Value of BSSID is the same as the MAC address of the radio in the Access-Point

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Frame Formats MAC Header format differs per Type: Control Frames (several fields are omitted) Management Frames Data Frames Frame Control Duration ID Addr 1Addr 2Addr 3Addr 4 Sequence Control CRC Frame Body 22666620-23124 802.11 MAC Header Bytes: Protocol Version TypeSubType To DS Retry Pwr Mgt More Data WEPRsvd Frame Control Field Bits: 22411111111 DS FromMore Frag Frame Format

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Address Field Description Address 1 always the address of the next device. Address 2 is always the address of the previous device. Address 3 is the address of the final destination station if it is not defined by address1. Address 4 is the address of the original source if it is not same as address 2. Protocol Version TypeSubType To DS Retry Pwr Mgt More Data WEPRsvd Frame Control Field Bits: 22411111111 DS FromMore Frag To DS 0 0 1 1 From DS 0 1 0 1 Address 1 DA BSSID RA Address 2 SA BSSID SA TA Address 3 BSSID SA DA Address 4 N/A SA Address Fields Description

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Address mechanism

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Type field descriptions Type and subtype identify the function of the frame: Type=00Management Frame Beacon (Re)Association Probe (De)Authentication Power Management Type=01Control Frame RTS/CTS ACK Type=10Data Frame Protocol Version TypeSubType To DS Retry Pwr Mgt More Data WEPRsvd Frame Control Field Bits: 22411111111 DS FromMore Frag Type Field Description

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DIFS Contention Window Slot time Defer Access Backoff-Window Next Frame Select Slot and Decrement Backoff as long as medium is idle. SIFS PIFS DIFS Free access when medium is free longer than DIFS Busy Medium Inter frame spacing required for MAC protocol traffic SIFS = Short interframe space PIFS = PCF interframe space DIFS = DCF interframe space Back-off timer expressed in terms of number of time slots Inter-Frame Spacing

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Ack Data Next MPDU Src Dest Other Contention Window Defer Access Backoff after Defer DIFS SIFS DIFS Acknowledgment are to arrive at within the SIFS The DCF interframe space is observed before medium is considered free for use Data Frames and Their ACK

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MAC Management Frames Beacon Timestamp, Beacon Interval, Capabilities, SSID, Supported Rates, parameters Traffic Indication Map Probe SSID, Capabilities, Supported Rates Probe Response Timestamp, Beacon Interval, Capabilities, SSID, Supported Rates, parameters same for Beacon except for TIM

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MAC Management Frames (contd) Association Request Capability, Listen Interval, SSID, Supported Rates Association Response Capability, Status Code, Station ID, Supported Rates Re-association Request Capability, Listen Interval, SSID, Supported Rates, Current AP Address Re-association Response Capability, Status Code, Station ID, Supported Rates

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MAC Management Frames (contd) Dis-association Reason code Authentication Algorithm, Sequence, Status, Challenge Text De-authentication Reason

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Assignment 1.Prepare a comparative study on 802.11a/b/g 2.Explain exposed terminal and its solution. 3.Compare DSR, AODV and DSDV 4.

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802

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