Wireless Local Area Networks (Part 1) - PDF

Wireless Local Area Networks (Part 1) Introduction NES441 Wireless Networks Dr. Fahed Awad Department of Network Engineering & Security...

Wireless Local Area Networks (Part 1) Introduction NES441 Wireless Networks Dr. Fahed Awad Department of Network Engineering & Security Jordan University of Science and Technology NES441: Wireless Networks Copyright © Dr. Fahed H. Awad 1 Outline Elements of Wireless Networks  Wireless hosts, wireless links, base stations Topologies of Wireless Networks  Point-to-point, Star, Tree, Switched Star, and Mesh WLAN devices  Client adapters, infrastructure, bridging, etc. IEEE 802.11 WLAN Standards  IEEE 802.11 WLAN Architecture  Discovering and Joining an IEEE 802.11 WLAN  The MAC sub-layer standards  The PHY layer standards  The Network layer standards NES441: Wireless Networks Copyright © Dr. Fahed H. Awad 2  Elements of Wireless Networks NES441: Wireless Networks Copyright © Dr. Fahed H. Awad 3 Wireless Hosts Wireless Hosts Laptop, PDA, IP phone Run applications May be stationary (non- mobile) or mobile  Wireless does not always mean mobility NES441: Wireless Networks Copyright © Dr. Fahed H. Awad 4 Base Stations Base Stations Typically connected to wired network Responsible for relaying packets between the wired network and the wireless host(s) within its coverage area  e.g.; cell towers 802.11 access points NES441: Wireless Networks Copyright © Dr. Fahed H. Awad 5 Wireless Links Wireless Links Typically used to connect wireless hosts to base stations Multiple access protocol to coordinates link access Various data rates and transmission distance NES441: Wireless Networks Copyright © Dr. Fahed H. Awad 6  Topologies of Wireless Networks NES441: Wireless Networks Copyright © Dr. Fahed H. Awad 7 Wireless Point-to-point Topology The simple point-to-point topology is more common in wireless than wired networks Wireless point-to-point topology has several forms in different technologies such as:  WLAN peer-to-peer (or ad hoc) mode  WMAN backhauling  LAN wireless bridging  WPAN main topology such as Bluetooth  Wireless optical communication such as infra-red NES441: Wireless Networks Copyright © Dr. Fahed H. Awad 8 Wireless Star Topology The center device of the wireless star topology:  Plays the role of a hub in the wired network  It can be an access point, a base station, etc.  May perform a variety of functions based on the technology used NES441: Wireless Networks Copyright © Dr. Fahed H. Awad 9 Wireless Tree Topology An access point switch or controller is used to:  Provides interconnectivity and data delivery for the access points  Moves complexity away from access points towards the switch  Simplifies management and configuration of the access points NES441: Wireless Networks Copyright © Dr. Fahed H. Awad 10 Wireless Switched Star Topology The center device can spatially divide the stations using sectored or array antenna The overall throughput is multiplied by the number of transmitters used (similar to the switch in wired networks) NES441: Wireless Networks Copyright © Dr. Fahed H. Awad 11 Wireless Mesh Topology Consists of mobile nodes, stationary wireless routers, and wired access points in order to provide reliable and self-healing network Uses multi-hop node-to-node packet routing towards the destination The topology is continuously changing due to the mobility of nodes NES441: Wireless Networks Copyright © Dr. Fahed H. Awad 12  WLAN Devices NES441: Wireless Networks Copyright © Dr. Fahed H. Awad 13 Wireless LAN devices Client devices  Network interface cards (NICs) Infrastructure devices  Access points  Outdoor wireless bridges  Wireless switches or controllers WLAN antennas NES441: Wireless Networks Copyright © Dr. Fahed H. Awad 14 WLAN Client Devices Wireless Network Interface Card (NIC):  Turns a network-capable device such as a desktop, a laptop, or a PDA into a wireless device (or station)  Enables the device to communicate with other wireless devices or with an access point Wireless NIC antennas can be:  Internallyintegrated  Externally connected (or detachable) NES441: Wireless Networks Copyright © Dr. Fahed H. Awad 15 WLAN Client Devices Wireless NICs are available in a variety of form factors:  PCMCIA card type II: used in laptops and PDAs (obsolete)  PCI card: used in a desktop computer  Mini-PCI card: integrated inside the laptop  USB dongles: used in any device with a USB port  Fully integrated within the main board as part of the chipset USB PCMCIA adapter / Desk mount PCI card with external Dongle integrated antenna USB adapter desk mount antenna PCI card with PCI card with Desk mount USB detachable antenna multiple radios and or Ethernet adapter Mini-PCI Card detachable antennas NES441: Wireless Networks Copyright © Dr. Fahed H. Awad 16 Access Point (AP) Works as a switch/gateway for wireless hosts/stations Usually connected to the backbone network to provide connectivity to wired network resources Provides a full range of processing and control functions:  Security features such as authentication and encryption support  Access control based on access lists or customized filters  Network configuration capabilities such as SNMP May provide additional optional networking features:  Internet gateway: NAT routing, DHCP, VPN, etc  LAN switch: Switching for wired devices such as Ethernet  Wireless bridge: provide point-to-point connectivity between two remote network segments  Wireless repeater: provide range extension capability of another access point NES441: Wireless Networks Copyright © Dr. Fahed H. Awad 17 Types of Access Points NES441: Wireless Networks Copyright © Dr. Fahed H. Awad 18 Outdoor Wireless Bridge Provides point-to-point link between two remote wired and/or wireless network segments Usually designed for outdoor environment:  Weather proof enclosure: water, humidity, sun, lightening, wind Uses high-gain directional antennas NES441: Wireless Networks Copyright © Dr. Fahed H. Awad 19 Wireless Switch or Controller Used in environments with large-scale WLAN deployment (i.e.; tens or hundreds of APs deployed) The aim is to simplify the deployment, configuration, and management of APs Handles the majority of the AP functionality Works with “thin” or “light” dependent APs Advantages:  Lower cost of AP hardware and deployment  Simplified network configuration and management  Better mobility management of mobile hosts between APs  Simplified updating and upgrading processes (i.e.; the wireless switch instead of every AP) NES441: Wireless Networks Copyright © Dr. Fahed H. Awad 20 Sample Wireless Switch Layout NES441: Wireless Networks Copyright © Dr. Fahed H. Awad 21 WLAN Antennas Traditional fixed-gain antennas  Different types and radiation patterns per need Omni-directional, sectored, patch, yagi, and parabolic  One node can transmit on the channel at a time (i.e.; limited throughput) Smart antennas  Use multiple antenna elements called antenna array  Can form & direct radiation beams in different directions  Allows multiple nodes to transmit simultaneously for increased throughput  Examples: Switched Beam Array Antenna Adaptive Beam Antenna Multi-input Multi-output (MIMO) Antenna NES441: Wireless Networks Copyright © Dr. Fahed H. Awad 22 WLAN Antennas Traditional WLAN antenna specifications for 2.4GHz operation Beam pattern of a 6-elment smart antenna array Parabolic NES441: Wireless Networks Copyright © Dr. Fahed H. Awad 23  IEEE 802.11 WLAN Standards NES441: Wireless Networks Copyright © Dr. Fahed H. Awad 24 History of the IEEE 802.11 Standards FCC opened the three unlicensed frequency bands (900 MHz, 2.4 GHz, and 5.8 GHz) in 1985 IEEE WLAN standardization process began in late 80’s Standard Generation Year Ratified Raw Data Rate RF Band (GHz) IEEE 802.11 Wi-Fi 0 1997 1 - 2 Mbps 2.4 IEEE 802.11b Wi-Fi 1 1999 1 - 11 Mbps 2.4 IEEE 802.11a Wi-Fi 2 1999 6 - 54 Mbps 5.8 IEEE 802.11g Wi-Fi 3 2003 6 - 54 Mbps 2.4 IEEE 802.11n Wi-Fi 4 2009 72 - 600 Mbps 2.4 / 5.8 IEEE 802.11ac Wi-Fi 5 2014 433 - 6933 Mbps 5.8 Wi-Fi 6 2019 2.4 / 5.8 IEEE 802.11ax 600 - 6933 Mbps Wi-Fi 6E 2020 2.4 / 5.8 / 6.7 IEEE 802.11be Wi-Fi 7 TBD 40 Gbps 2.4 / 5.8 / 6.7 Several other standards were developed as enhancements or extensions NES441: Wireless Networks Copyright © Dr. Fahed H. Awad 25 Other IEEE 802.11 Standards Standard Main Characteristics 802.11a 54 Mbps using OFDM in the 5 GHz ISM band 802.11b 11 Mbps using DSSS & CCK in the 2.4 GHz ISM band 802.11g 54 Mbps using OFDM in the 2.4 GHz ISM band Interoperable with 802.11b equipment 802.11e Quality of service (QoS) for all 802.11 interfaces, TDMA to enhance the performance of delay sensitive applications 802.11f Inter-Access Point Protocol to facilitate roaming between inter-vendor access points 802.11i Employs advanced encryption standard (AES) and 802.1x authentication 802.11n Higher data rates of up to 600 Mbps using MIMO technology 802.11s Extending 802.11 MAC to support mesh networking. Message delivery over self- configuring multi-hop mesh topologies. NES441: Wireless Networks Copyright © Dr. Fahed H. Awad 26 The Main Characteristics of 802.11 WLANs Each IEEE 802 (local area networking) standard covers the PHY layer and MAC sub-layer IEEE divided the Data Link Layer into two sub-layers:  Logical Link Control (LLC) sub-layer (specified by the 802.2 standard): Provides the common interface to the upper layers Provides error control Provides flow control  Media Access Control (MAC) sub-layer: Enables the station to establish or join a network Appends physical addresses and transmit the frames coming from the LLC NES441: Wireless Networks Copyright © Dr. Fahed H. Awad 27 The IEEE 802.11 WLAN Components Station (STA):  Any device that implements the PHY and MAC layer protocols Access Point (AP):  A device that provides an addressable interface between a set of stations, known as a Basic Service Set (BSS), and the distribution system Distribution System (DS):  Connects two or more APs and their associated BSSs to form an Extended Service Set (ESS)  Can be a network component (usually a wired LAN component) such as a hub, a switch, or a router NES441: Wireless Networks Copyright © Dr. Fahed H. Awad 28 The Basic Service Set IEEE 802.11 WLANs are based on a cellular structure  Each cell in known as a basic service set (BSS) BSS is a group of STA’s served by a single AP The BSS must be assigned a unique identifier called the basic service set identifier (BSSID) (or the SSID for short)  SSID serves as a “network name” for the BSS The BSS has an associated Basic Service Area (BSA) called a cell  BSA is the geographical coverage area of the BSS All STA’s, including the AP, of a BSS:  Use a common SSID  Synchronize to a common timer  TX/RX on the same RF channel used by AP BSS parameters are periodically broadcasted by AP within a beacon frame NES441: Wireless Networks Copyright © Dr. Fahed H. Awad 29 The Extended Service Set The extended service set (ESS) is made up of two or more BSS’s, of which the AP’s are connected via a common distribution system (DS) AP’s are usually located such that the BSAs (or cells) are partially overlapped in order to facilitate the roaming between cells  If the STA is located in a coverage overlap area, it may choose the AP based on several factors such as the signal strength or some required QoS metrics such as the data rate  A mobile STA may move (or roam) from one cell to another The STA has to “handoff” from one AP to the other in order to retain an ongoing session NES441: Wireless Networks Copyright © Dr. Fahed H. Awad 30 The Independent Basic Service Set The Independent Basic Service Set (IBSS) is a group of two or more STA’s that communicate directly with each other without an AP or any type of wired connection The IBSS is also called peer-to-peer or ad hoc mode The IBSS allows a wireless network to be quickly and easily established for data sharing In an IBSS, all stations:  Use a common SSID  Synchronize to a common timer  Transmit and receive on the same RF channel  Periodically broadcast beacon frames NES441: Wireless Networks Copyright © Dr. Fahed H. Awad 31  The IEEE 802.11 MAC Sublayer Standards NES441: Wireless Networks Copyright © Dr. Fahed H. Awad 32 The MAC Sublayer Standards Specify Functions and Services Frame Formats Frame Types Addressing Mechanisms Discovering a WLAN Joining a WLAN Wireless Media Access NES441: Wireless Networks Copyright © Dr. Fahed H. Awad 33 MAC Sublayer Functions and Services The MAC sublayer performs several vital functions in the WLAN, which can be divided into three functional areas:  Access control Discovering a pre-existing WLAN Gaining access to the wireless medium shared by the WLAN devices Joining the WLAN and remaining connected during mobility  Reliable data transfer of the LLC frames over the WLAN  Security Services such as authentication and privacy The MAC sublayer functions are delivered using two classes of services:  Station services: implemented in every 802.11 STA including the AP  DS services: provided between BSS’s and are implemented either in the AP or any special-purpose device attached to the DS such as the AP Controller or a stand-alone server The MAC sublayer services are implemented via the transmission of three types of frames between the MAC sublayers of the communicating stations:  Management frames  Control frames  Data frames NES441: Wireless Networks Copyright © Dr. Fahed H. Awad 34 IEEE 802.11 MAC Sublayer Services There are two categories of services:  MAC Service Data Unit (MSDU) delivery The MSDU is a block of data passed down by the LLC The MSDU is typically the LLC Protocol Data Unit (PDU) If the MSDU is very large, the MAC performs frame fragmentation Service Provider Category  WLAN access and security Association Distribution system MSDU delivery The following table lists the services and the Authentication Station WLAN access and security associated provider and category Deauthentication Station WLAN access and security Dissassociation Distribution system MSDU delivery Distribution Distribution system MSDU delivery Integration Distribution system MSDU delivery MSDU delivery Station MSDU delivery Privacy Station WLAN access and security Reassocation Distribution system MSDU delivery NES441: Wireless Networks Copyright © Dr. Fahed H. Awad 35 MSDU Delivery-Related Services Association service (by DS)  Establishes initial logical connection between a STA and an AP  Essential to provide the DS with the necessary information for data delivery Reassociation service (by DS)  Enables the transfer of the association from one AP to another, allowing the station to move (or roam) from one BSS to another  Allows the STA to change some attributes of an existing association such as the data rate Disassociation service (by DS)  Association termination notice from a STA or an AP when a STA is leaving the BSS MSDU delivery service (by STA)  Ensure the delivery of the MSDU between the LLC’s in two STA’s Distribution service (by DS)  Used to exchange MAC frames from a STA in one BSS to a STA in another BSS Integration service (by DS)  Transfer of data between a STA on the IEEE 802.11 WLAN and a station on the integrated IEEE 802.x LAN NES441: Wireless Networks Copyright © Dr. Fahed H. Awad 36 Access and Security-Related Services Authentication service (by STA)  Enables a station to authenticate another station prior to association  An AP can be configured to provide either An open system authentication: minimal security A shared key authentication: requires a common secret key to be provided by both stations (usually pre- configured in both stations) Deathentication service (by STA)  Enables a station to terminate an existing authentication when it intends to stop communicating with another station  This service is invoked prior to the disassociation service Privacy service (by STA)  Enables data frames and shared key authentication frames to be encrypted before transmission  Prevents message contents from being read by unintended recipient  Uses either the Wired Equivalent Privacy (WEP), which is the original “weak” protocol provided by the IEEE 802.11 or the “strong” Wi-Fi Protected Access (WPA) protocol, which is based on the IEEE 802.11i security standard NES441: Wireless Networks Copyright © Dr. Fahed H. Awad 37  MAC Sublayer Frames NES441: Wireless Networks Copyright © Dr. Fahed H. Awad 38 MAC Sublayer Frame Format The MAC sublayer frame consists of nine fields:  Frame Control (FC): defines the type of the frame and some control information  Duration (D): defines the duration of transmission in microseconds  Addresses: four 6-byte address fields used based on the “To DS” and “From DS” subfields of the FC field  Sequence Control (SC): defines the frame sequence number to be used by the flow control mechanism  Frame Body: 0 to 2312 bytes of information based on the type and subtype subfields of the FC field  Frame Check Sequence (FCS): 4-byte CRC-32 error detection sequence The max. size of the MAC sublayer frame is 2346 bytes NES441: Wireless Networks Copyright © Dr. Fahed H. Awad 39 The Frame Control (FC) Field Specify the addressing mode used NES441: Wireless Networks Copyright © Dr. Fahed H. Awad 40 Frame types There are three types of frames: Management Frames Control Frames Data Frames NES441: Wireless Networks Copyright © Dr. Fahed H. Awad 41 Management Frames Used for:  Initializing the communications between the STA and the AP (in the infrastructure mode) or between STA’s (in the ad hoc mode)  Maintaining the connection Types of management frames (the subtype field of FC):  Authentication  Association request  Association response  Beacon  Deauthentication  Disassociation  Probe request  Reassociation request  Reassociation response NES441: Wireless Networks Copyright © Dr. Fahed H. Awad 42 Control frames Used for:  Accessing the channel Request To Send (RTS) frame Clear To Send (CTS) frame  Acknowledging frames Acknowledgment (ACK) frame Control Frame Formats NES441: Wireless Networks Copyright © Dr. Fahed H. Awad 43 Data frames There are two types of data frames:  Data carrying frames Carry LLC data to be transmitted to the destination device  Non data carrying frames Do not carry user data For example: the Null Function  Carries a power management bit in the frame control field to the AP  It indicates that the STA is changing to a low-power state NES441: Wireless Networks Copyright © Dr. Fahed H. Awad 44 Addressing Mechanism The addressing mechanism is specified by the “To DS” and the “From DS” flags of the FC field  Address 1 is 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 STA if it is not defined by Address 1. Otherwise, it is the address of the original source  Address 4 is the address of the original source STA if it is not defined in Address 2. Otherwise, it is not used NES441: Wireless Networks Copyright © Dr. Fahed H. Awad 45 Addressing Mechanism Cases Case 1 (00): the frame is not from or to a DS  The frame is from one STA to another within an IBSS  The ACK should be sent to the original sender Case 2 (01): the frame is from a DS but not to a DS  The frame is from an AP to a STA Address 2 holds the BSSID of the AP  The ACK should be sent to the AP  Address 3 contains the address of the original sender, which can be in another BSS Case 3 (10): the frame is not from a DS but to a DS  The frame is from a STA to an AP Address 1 holds the BSSID of the AP  The ACK should be sent to the original STA  Address 3 contains the address of the final destination, which can be in another BSS Case 4 (11): the frame is from a DS to another DS  The frame is from an AP to another AP in a wireless DS Addresses 1 and 2 hold the BSSID’s of the corresponding AP’s The BSSID of the AP is the MAC address of STA part of AP NES441: Wireless Networks Copyright © Dr. Fahed H. Awad 46  Discovering and Joining a WLAN NES441: Wireless Networks Copyright © Dr. Fahed H. Awad 47 Discovering and joining a WLAN In order for a wireless device to be part of a WLAN, the following steps have to be performed:  Discovering the WLAN, which includes: Beaconing by the existing WLAN to announce its existence Scanning by the wireless device in order to discover the WLAN  Joining the WLAN, which includes: Authenticating the wireless device to validate its identity Associating the wireless device to the WLAN NES441: Wireless Networks Copyright © Dr. Fahed H. Awad 48 Beaconing At (quasi) regular intervals, the AP (in an infrastructure mode) or a station (in an ad hoc mode) transmits beacon frames in order to:  Announce the presence of a WLAN BSS  Provide the information needed by other devices to join the BSS The beacon frame is a standard management frame, of which the destination address is always set to all ones (i.e.; broadcast frame) The sending of the beacon frames is considered as the “heart beat” of the WLAN In ad hoc mode, the responsibility of sending beacon frames is rotated among the stations using random drawing NES441: Wireless Networks Copyright © Dr. Fahed H. Awad 49 The Contents of the Beacon Frame The beacon frame contains the following fields:  The beacon interval: adjustable but it is typically 100 ms What are the implications of changing the beacon interval? (network traffic versus roaming success)  The timestamp of the beacon source Contains the time of the beacon source clock All stations adjust their clocks to the timestamp  The Service Set Identifier (SSID) used by the beacon source A 2 to 32 character case-sensitive network name Included only if the “Broadcast SSID” is enabled (a security feature)  The supported data rates (e.g.; 802.11b has 11, 5.5, 2, & 1 Mbps)  Parameter sets: the type of modulation used (e.g.; FHSS or DSSS)  Capability information: the requirements needed to join the WLAN  Traffic Indication Map (TIM): identifies the stations that are in a power-save mode, of which data frames are buffered at the AP NES441: Wireless Networks Copyright © Dr. Fahed H. Awad 50 Scanning The STA that attempts to join a WLAN must scan the available channels for existing service sets There are two types of scanning used:  Passive scanning The STA simply listens for beacon frames from exiting WLAN’s Typically, listens on each available channel for a pre-determined period of time  Active scanning The STA first sends out a probe request frame on each available channel Then, it waits for a probe response frame from each available AP The key difference between passive and active scanning is which device initiates the discovery The STA can:  Scan and join a specific pre-configured SSID or a list of preferred SSID’s ordered based on a specific priority (e.g.; Office SSID, Home SSID, etc.)  Passively or actively scan all available channels for a full list of available AP’s, then the STA or the user chooses a specific SSID to join from the list found  Join “ANY” available SSID (not a secure option though, usually disallowed) based on either the strongest signal received or the first available After an SSID is selected, the STA uses the information contained in the beacon frames broadcast by the BSS in order to be able to join it NES441: Wireless Networks Copyright © Dr. Fahed H. Awad 51 Authentication In wired LAN:  The physical network connection is restricted by the physical boundaries of the premises such as walls and doors  Only authorized devices and users are assumed to connect to the LAN  Therefore, the user is usually authenticated after the device is connected to the network. In wireless LAN:  The access to the radio signal can’t be restricted by wall and doors  Unauthorized devices and users can get access to the network  Therefore, the device is authenticated before it is allowed to connects to the LAN In IEEE 802.11:  The authentication is the process of accepting or rejecting a station by the AP  There are two types of authentication in the legacy standard: Open system authentication: the default and most basic authentication method Shared key authentication: an optional authentication method that uses a challenge text Both types are considered weak and more secure methods are usually used such as the methods provided by the IEEE 802.11i standard NES441: Wireless Networks Copyright © Dr. Fahed H. Awad 52 Open system authentication After discovering the WLAN, the station:  Sends an association request frame to the AP including the necessary information such as the supported data rates and the SSID After receiving the association request, the AP:  Compares the received SSID to its SSID  If they match, then the device is authenticated. Otherwise it is rejected NES441: Wireless Networks Copyright © Dr. Fahed H. Awad 53 Shared key authentication Both the AP and the station are pre-configured with the same authentication key The station sends an authentication frame to the AP The AP sends back an authentication frame including a challenge text to the station The station encrypts the text using the shared key and sends it back to the AP in an authentication frame The AP decrypts the encrypted text and compares it to the original challenge text. If they match, then the device is authenticated. Otherwise it is rejected NES441: Wireless Networks Copyright © Dr. Fahed H. Awad 54 Association The association is the last step towards accepting the STA to join a WLAN After the authentication process, the AP responds with an association response frame, which contains the acceptance or the rejection information If the STA is accepted, the AP:  Reserves memory space in the AP for the station  Establishes an association ID for the STA  Includes the association ID and the supported data rates in the association response frame NES441: Wireless Networks Copyright © Dr. Fahed H. Awad 55 Association-Related Services In order to be able to perform its services of transferring MSDUs among different MAC sublayers within ESS, DS requires information about STAs within ESS such as their locations and capabilities This information is provided by the association-related services There are three types of STA transition based on its mobility:  No transition: STA is either stationary or it moves within the range of a single BSS  BSS transition: STA moves from a BSS to another within the same ESS This requires addressing capability to be able to recognize the new location  ESS transition: STA moves from a BSS in an ESS to a BSS in another ESS Maintenance of the upper-layer connections is not guaranteed Disruption of service is likely NES441: Wireless Networks Copyright © Dr. Fahed H. Awad 56 Station Location The DS needs to know where the destination STA is in order to be able to deliver the MSDU to it:  It needs the identity of the AP to which the MSDU should be delivered  The STA must maintain an association with an AP within the current ESS in order to get the data destined to it Three services are related to this requirement:  Association: establishes the initial association between STA and AP In order to make the identity and address of STA known STA must establish an association with an AP within a particular BSS The AP’s within the ESS exchange information about STAs  Reassociation: transfer the established association to another AP This allows STA to move from one BSS to another  Disassociation: the association can be terminated by either STA or AP by sending the disassociation frame It is performed before STA leaves the ESS or shuts down The MAC management facility protects itself against STAs that disappear without notification NES441: Wireless Networks Copyright © Dr. Fahed H. Awad 57 A Sample Scenario of a STA Mobility Among APs a) The STA discovers AP1. It authenticates and associates with it b) As the STA moves, it may pre-authenticate with AP2 c) When the association with AP1 is no longer needed, the STA reassociates with AP2 d) AP2 notifies AP1 of the new location of the STA. AP1 terminates the previous association with the STA e) AP2 may be taken out-of-service. AP2 would disassociate the associated STAs f) The STA finds AP3 and authenticate and associate with it NES441: Wireless Networks Copyright © Dr. Fahed H. Awad 58