Lecture 7 - 802.11 WLAN Architecture
802.11 Network Architecture (1 September, 2006) February 2005 Copyright 2005 All Righs Reserved Objectives Upon completion of this lecture you will be able to: q Define the different types of service sets that can be configured q Explain what beacons are and what information they provide q Describe authentication and association and how they allow users to gain access to the wireless LAN q Define the power management features available with wireless LANs q Explain Dynamic Rate Shifting February 2005 Copyright 2005 All Righs Reserved 802.11 WLAN Topology and Architecture February 2005 Copyright 2005 All Righs Reserved Major WLAN Components q 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 q Access Points (AP) APs form a bridge between wired and wireless medium Coordinate the connection of wireless stations to the DS q Wireless Medium The Radio Frequency spectrum used to transfer frames between the wireless station and the AP or between wireless stations q Wireless Stations Computing devices with wireless network interfaces Typically battery operated laptops or handheld computers February 2005 Copyright 2005 All Righs Reserved WLAN Types q 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 February 2005 Copyright 2005 All Righs Reserved WLAN Types Contd q 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 AP rk two ine Ne Wirel Network Access Point February 2005 Copyright 2005 All Righs Reserved WLAN Types Contd q 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 February 2005 Wireline Network Access Point Copyright 2005 All Righs Reserved WLAN Types Contd q 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 February 2005 Wireline Network Access Point Copyright 2005 All Righs Reserved 802.11 Network Services q 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 February 2005 Copyright 2005 All Righs Reserved 802.11 Architecture Contd q 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 with only one AP APs but Associated 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 February 2005 Copyright 2005 All Righs Reserved 10 Probe Response Contd Frame Control Duration ID 6 DA SA 2 Sequence BSSID Control Var 0-2,312 Frame Body Beacon Capability FH DS CF SSID Timestamp Interval Param Set Param Set Param Set Info FCS IBSS Param Set q Timestamp - Synchronization between stations q Beacon Interval - Announce existence of 802.11 network q Capability Info - Advertisement of network's capabilities q Service Set Identity (SSID) - The identity of the service set q FH Parameter Set - Specifies such things a Dwell time, Hop Set and Hop Pattern q DS Parameter Set - Specifies the DSSS channel number q 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 q IBSS Parameter Set - Specifies the number of Time Units between Announcement Traffic Indication Map (ATIM) windows February 2005 Copyright 2005 All Righs Reserved 24 Probe Response Contd Indicates a Probe Response February 2005 Copyright 2005 All Righs Reserved 25 Probe Response Contd February 2005 Copyright 2005 All Righs Reserved 26 BEACON February 2005 Copyright 2005 All Righs Reserved 27 BEACON q The Beacon Management frame serves the following major purposes Time Synchronization The Beacon time stamp synchronize the client to the transmitter This ensures all time sensitive functions, (FHSS, etc) are properly timed FH/DS (FHSS/DSSS) Parameter Sets FHSS hop sequence, hop time and dwell time DSSS channel information SSID Information The beacon contains the SSID the client wishes to join and sends an authentication request to the MAC address contained in the Beacon Conversely, the client can be set to join ANY network that: I (1) sends a beacon or I (2) the strongest signal if there are multiple APs February 2005 Copyright 2005 All Righs Reserved 28 BEACON Contd Traffic Indication Map (TIM) The TIM indicates which sleeping stations have traffic queued by the Access point The time synchronized stations power up, listen to the Beacon, and check the TIM to see if they have traffic queued Supported Rates Informs the clients the speed capability of the Access Point February 2005 Copyright 2005 All Righs Reserved 29 Beacon Frame Frame Control Duration ID 6 DA SA Var Sequence BSSID Control 0-2,312 Frame Body FH Beacon Capability DS IBSS CF SSID Timestamp Interval Param Set Param Set Param Set Param Set Info FCS Var TIM 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 of the service set FH Parameter 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 30 Beacon Frame Contd Frame Control Duration ID 6 DA SA Var Sequence BSSID Control 0-2,312 Frame Body FH Beacon Capability DS IBSS CF SSID Timestamp Interval Param Set Param Set Param Set Param Set Info Mandatory Optional FCS Var TIM CF Parameter Set – Specifies 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 of Time Units between Announcement Traffic Indication Map (ATIM) windows Traffic Indication Map (TIM) - Traffic Indication map to indicate which stations have buffered traffic February 2005 Copyright 2005 All Righs Reserved 31 Beacon Frame This is the MAC Header Indicates the Beacon is contained within the MAC header The Basic Service Set identifier (BSSID) is a byte hex number identifying the Access Point It is not the same as the SSID February 2005 Copyright 2005 All Righs Reserved 32 Beacon Frame Contd This is the Beacon frame February 2005 Copyright 2005 All Righs Reserved 33 Beacon Frame Contd This is the SSID Supported Rates DSS Element TIM Element February 2005 Copyright 2005 All Righs Reserved 34 802.11 Authentication and Association - Joining a Wireless LAN- February 2005 Copyright 2005 All Righs Reserved 35 Authentication & Association q There are two separate sub-processes for connecting a client to a WLAN These are: Authentication: The process by which a node's identity is verified Authentication must occur before Association Association: A state in which a node is allowed to pass traffic through an access point q There are three distinct states Unauthenticated and Unassociated Authenticated and Unassociated Authenticated and Associated February 2005 Copyright 2005 All Righs Reserved 36 States of Authentication & Association q Unauthenticated and Unassociated The initial state The client is not connected to the WLAN and is unable to pass data q Authenticated and Unassociated The Client has passed the Authentication state but is not yet associated with an Access Point The Client cannot send or receive packets q Authenticated and Associated The client is completely connected and is able to send and receive packets A client can be authenticated with multiple Access Points but only associated with one Pre-authentication makes for faster roaming February 2005 Copyright 2005 All Righs Reserved 37 End of Presentation February 2005 Copyright 2005 All Righs Reserved 38 ... Interval - Announce existence of 802.11 network Capability Info - Advertisement of network''s capabilities Service Set Identity (SSID) - The identity of the service set FH Parameter Set - Specifies... February 2005 Copyright 2005 All Righs Reserved WLAN Types q Independent Basic Service Set (IBSS) No Access Point An ad-hoc group of wireless nodes Peer-to-peer transmission One node is elected to... De-authentication - Destroys a previously known station identity- terminates the current Association The device shuts down Out of AP range February 2005 Copyright 2005 All Righs Reserved 802.11