Lecture 7 - 802.11 WLAN Architecture

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Lecture 7 - 802.11 WLAN Architecture February 2005 Copyright 2005 All Righs Reserved 1802.11 Network Architecture(1 September, 2006)February 2005 Copyright 2005 All Righs Reserved 2●Define the different types of service sets that can be configured●Explain what beacons are and what information they provide●Describe authentication and association and how they allow users to gain access to the wireless LAN●Define the power management features available with wireless LANs●Explain Dynamic Rate ShiftingObjectivesUpon completion of this lecture you will be able to:February 2005 Copyright 2005 All Righs Reserved 3 802.11 WLAN Topology and ArchitectureFebruary 2005 Copyright 2005 All Righs Reserved 4● Distribution System (DS).✔ A wired/wireless medium which connect the Access Points to one another.✔ The backbone network used to relay frames between access points● Access Points (AP).✔ APs form a bridge between wired and wireless medium.✔ Coordinate the connection of wireless stations to the DS● Wireless Medium.✔ The Radio Frequency spectrum used to transfer frames between the wireless station and the AP or between wireless stations.● Wireless Stations.✔ Computing devices with wireless network interfaces.✔ Typically battery operated laptops or handheld computers. Major WLAN ComponentsFebruary 2005 Copyright 2005 All Righs Reserved 5● Independent Basic Service Set (IBSS).✔ No Access Point ✔An ad-hoc group of wireless nodes.✔ Peer-to-peer transmission✔ One node is elected to act as a proxy to perform the functions of the AP. WLAN TypesFebruary 2005 Copyright 2005 All Righs Reserved 6 WLAN Types Contd Access Point Network Wireline Network● Basic Service Set (BSS).✔ A single Access Point ✔ The AP acts as a bridge between clients and the wireline.✔ Roaming is limited to the single radio cell✔ All clients operate on the same channel.✔ A BSS connected to a wired network is called an Infrastructure BSS.✔ The clients must request to join and be accepted to associate with the AP before they can send data.✔ A BSS is identified by a 48 bit hex value called the BSS identifier – BSSID. This is the MAC address of the APFebruary 2005 Copyright 2005 All Righs Reserved 7● Extended Service Set (ESS).✔ Multiple Access Points that communicate through the DS.✔ The APs share the same Service Set Identifier (SSID) – an Extended SSID or ESSID.✔ The AP acts as a bridge between clients and the wireline.✔ Each AP forms a radio cell that overlap. Each AP is assigned a different channel✔ All clients operate on the same channel in the same cell but can communicate through the DS.✔ The APs interconnected through the Wireline constitute a Distributed System. Access Point Access Point Wireline Network WLAN Types ContdFebruary 2005 Copyright 2005 All Righs Reserved 8● Extended Service Set (ESS) Contd.✔ The SSID is used to control APs with which the stations can associate.✔ Clients may also associate with an AP using a special "null" SSID value which indicates they would like to associate with any AP within range regardless of the assigned APs SSID.✔AP can be configured to reject this "null" value.✔The BSSID identifies a single BS.✔The ESSID denotes a group of APs sharing a common SSID within which a client can roam. Access Point Access Point Wireline Network WLAN Types ContdFebruary 2005 Copyright 2005 All Righs Reserved 9● Station Services✔ Authentication - The client identifies itself to the AP in order to form an Association. This can be done by:☛ Service Set Identifier (SSID)☛ MAC Filtering.✔ De-authentication - Destroys a previously known station identity- terminates the current Association.☛ The device shuts down.☛ Out of AP range 802.11 Network ServicesFebruary 2005 Copyright 2005 All Righs Reserved 10● Distribution Services.✔ Association - establish a logical connection between the client and the AP, i.e., A station registers with an AP.☛ Determines the location of the client for the DS.☛ Determines the path to reach the DS needs to reach the client. ☛ A client can be authenticated to multiple APs but Associated with only one AP. ✔ Reassociation - Retains network session information when the wireless client passes from one AP to another AP.☛ This information tells the new AP the identify of the last AP. ☛ This allows the old AP to forward any remaining frames to the new AP for delivery via the DS. 802.11 Architecture Contd[...]... 6 6 2 0-2 ,312 4 Frame Body Timestamp Beacon Interval Capability Info SSID FH Param Set TIM 8 2 2 Var 7 2 8 4 Var DS Param Set CF Param Set IBSS Param Set✔ Timestamp - Synchronization between stations.✔ Beacon Interval - Announce existence of 802.11 network.✔ Capability Info - Advertisement of network's capabilities.✔ Service Set Identity (SSID) - The... Set - Specifies such things a Dwell time, Hop Set and Hop Pattern.✔ DS Parameter Set - Specifies the DS channel number. February 2005 Copyright 2005 All Righs Reserved 35 802.11 Authentication and Association - Joining a Wireless LAN- February 2005 Copyright 2005 All Righs Reserved 36Authentication & Association● There are two separate sub-processes for connecting a client to a WLAN. ... 2 6 6 6 2 0-2 ,312 4 Frame Body Timestamp Beacon Interval Capability Info SSID FH Param Set 8 2 2 Var 7 2 8 4 DS Param Set CF Param Set IBSS Param Set● Timestamp - Synchronization between stations.● Beacon Interval - Announce existence of 802.11 network.● Capability Info - Advertisement of network's capabilities.● Service Set Identity (SSID) - The identity... The Probe Response contains:✔ Service Set Identity (SSID) - The identity of the service set.✔ Supported Rates - Data rates supported by the 802.11 network. February 2005 Copyright 2005 All Righs Reserved 15Service Set Identifier (SSID)●The SSID is used by WLAN as a network name.✔ Unique✔ Case sensitive✔ Alphanumeric value✔ 2-3 2 characters● The SSID is used for ✔ Segmenting networks✔... Righs Reserved 9● Station Services✔ Authentication - The client identifies itself to the AP in order to form an Association. This can be done by:☛ Service Set Identifier (SSID)☛ MAC Filtering.✔ De-authentication - Destroys a previously known station identity- terminates the current Association.☛ The device shuts down.☛ Out of AP range 802.11 Network Services February 2005 Copyright 2005... Access Points but only associated with one .✔Pre-authentication makes for faster roaming February 2005 Copyright 2005 All Righs Reserved 5● Independent Basic Service Set (IBSS).✔ No Access Point ✔An ad-hoc group of wireless nodes.✔ Peer-to-peer transmission✔ One node is elected to act as a proxy to perform the functions of the AP. WLAN Types February 2005 Copyright 2005 All Righs... link measurement data✔ The client listens and determines which AP has the stronger signal 802.11 - Other Operations February 2005 Copyright 2005 All Righs Reserved 37 States of Authentication & Association● Unauthenticated and Unassociated✔ The initial state.✔ The client is not connected to the WLAN and is unable to pass data.● Authenticated and Unassociated.✔The Client has passed... set.● FH Parameter Set - Specifies such things a Dwell time, Hop Set and Hop Pattern.● DS Parameter Set - Specifies the DSSS channel number.● CF Parameter Set – Specifies the parameters relating to the duration of the Contention Free period. It is only sent by APs supporting the Point Coordination Function (PCF). Very few APs support this function.● IBSS Parameter Set - Specifies the number... information tells the new AP the identify of the last AP. ☛ This allows the old AP to forward any remaining frames to the new AP for delivery via the DS. 802.11 Architecture Contd February 2005 Copyright 2005 All Righs Reserved 14Locating a WLAN ●The wireless client locates the Access Point by scanning the airways for its RF signal.● It locates the AP by identifying its Service Set Identifier... identifying the Access Point.It is not the same as the SSIDThis is the MAC HeaderIndicates the Beacon is contained within the MAC header February 2005 Copyright 2005 All Righs Reserved 1 802.11 Network Architecture (1 September, 2006) February 2005 Copyright 2005 All Righs Reserved 29BEACON Contd✔ Traffic Indication Map (TIM)☛ The TIM indicates which sleeping stations have traffic queued . ShiftingObjectivesUpon completion of this lecture you will be able to:February 2005 Copyright 2005 All Righs Reserved 3 802. 11 WLAN Topology and Architecture February 2005. Filtering.✔ De-authentication - Destroys a previously known station identity- terminates the current Association.☛ The device shuts down.☛ Out of AP range 802. 11
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