Lecture 7 - 802.11 WLAN Architecture

Lecture 7 - 802.11 WLAN Architecture

802.11 Network Architecture

(1 September, 2006)

 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 Shifting

Objectives

Upon completion of this lecture you will be able to:

 802.11

 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

 APs form a bridge between wired and wireless medium.

 Coordinate the connection of wireless stations to the DS

 The Radio Frequency spectrum used to transfer frames between the wireless station and the AP or between wireless stations.

 Computing devices with wireless network interfaces.

 Typically battery operated laptops or handheld computers.

 Major WLAN Components

 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

 WLAN Types Contd

 Access Point

 Networ k

 Wireline Net

work

 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

 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

 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

 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

 Distribution Services.

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.

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

 Distribution Services contd.

and the wireless device.

 The device leaves the AP area.

 The AP is shutting down.

should be forwarded by the AP - An AP uses the DS to deliver frames.

 Another AP.

 A Wireless client.

 The Wired Network.

 Integration - Provides a MAC framing service to the AP.

 Translates the 802.11 format to the Wired LAN format.

 Translates the Wired LAN format to the 802.11 format

 802.11 Architecture Contd

 A 20 byte Power Save Poll (PS-Poll) is sent to the AP.

 The AP buffers all packets destined for he device until it comes back on line

 Multicell Roaming.

 Roaming is based upon determining the S/N ratio.

 AP sends out beacon messages containing link measurement data

 The client listens and determines which AP has the stronger signal

 802.11 - Other Operations

Locating a WLAN

Locating a WLAN

 The wireless client locates the Access

Point by scanning the airways for its RF

Service Set Identifier (SSID)

 The SSID is used by WLAN as a network name

Passive Scanning

 Passive Scanning is the process of listening for Beacons on each channel for a specific period

after the station has initialized The beacons are sent by:

 Access Points in the Independent Basic Service Set (BSS) or Extended Basic Service Set (EBSS) mode.

 Client stations in the Ad Hoc mode.

 If there are multiple APs in the area then the client attempts to join the strongest signal strength

and lowest bit rate.

 Passive Scanning continues after association to allow the Client to create a list of available

access points should the current connection be broken.

Active Scanning

 Active Scanning is the process of sending a Probe Request frame from a wireless client.

 The station is actively seeking a network to join.

 The probe contains either the network Service Set Identifier (SSID) or a broadcast SSID

 Only Access Points servicing that SSID will respond with a Probe Response frame.

 This frame is identical to the Beacon frame with the exception that it does not contain a

Traffic Indication Map (TIM)

 All APs respond to the Broadcast SSID.

 The wireless station then initiates the Authentication and Association steps.

 Probe Request

 Probe Request

 Frame

Control

 Duratio n

 Probe Requests are used by stations to scan an area for 802.11

networks.

 The station sends our Probe Requests sequentially on all channels with its SSID and listens for Probe Responses

All Access Points must respond with a Probe Response.

 Some APs are configured to ONLY send a Probe Request to a matching SSID

 The Probe Response contains:

 Service Set Identity (SSID) - The identity of the service set.

 Supported Rates - Data rates supported by the 802.11 network.

 Probe Request Contd

Indicates a Probe Request is contained within the MAC header

 Probe Request Contd

 Probe Response

 Probe Response

 Frame

Control

 Duratio n

 The Probe Response is sent by the AP in response to the Probe Request.

 The Probe Response format is the same as the Beacon with the

exception that it does not include the TIM Element

 The station is not yet associated with the AP therefore it does not need to now the TIM parameter

 Probe Response Contd

 Frame

Control

 Duratio n

 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 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 of Time Units between Announcement

 Probe Response Contd

Indicates a Probe Response

 Probe Response Contd

BEACON

 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

 FHSS hop sequence, hop time and dwell time.

 DSSS channel 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:

 (1) sends a beacon or

 (2) the strongest signal if there are multiple APs.

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

Beacon Frame

 Frame

Control

 Duratio n

 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.

Beacon Frame Contd

 Frame

Control

 Duratio n

 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.

It is not the same as the SSID

This is the MAC Header

Indicates the Beacon is contained within the MAC header

Beacon Frame Contd

This is the Beacon frame

Beacon Frame Contd

This is the SSID

Supported Rates

DSS Element

TIM Element

 802.11

LAN-Authentication & Association

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

 Unauthenticated and Unassociated

 Authenticated and Unassociated.

 Authenticated and Associated.

States of Authentication & Association

with an Access Point.

packets.

associated with one

End of Presentation