Ch 2 802 11 and NICs Part

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Ch. 2 – 802. 11 and NICs Part 2 – 802. 11 MAC Cisco Fundamentals of Wireless LANs version 1. 1 Rick Graziani Cabrillo College Spring 2005

802. 11 Overview and MAC Layer Part 1 – 802. 11 MAC and Cisco Client Adapters • (Separate Presentation) • 2. 1 Online Curriculum – 802. 11 Standards • Overview of WLAN Topologies – IBSS – ESS – Access Points • 802. 11 Medium Access Mechanisms – DCF Operations – Hidden Node Problem – RTS/CTS – Frame Fragmentation Rick Graziani graziani@cabrillo. edu • 2. 4 – 2. 6 Online Curriculum – Client Adapters – Aironet Client Utility (ACU) – ACU Monitoring and Troubleshooting Tools Part 2 – 802. 11 MAC • 802. 11 Data Frames and Addressing • 802. 11 MAC Layer Operations – Station Connectivity – Power Save Operations – 802. 11 Frame Formats • Non-standard devices (Brief) 2

Recommended Reading and Sources for this Presentation Pejman Roshan Jonathan Leary ISBN: 1587050773 • • Matthew S. Gast ISBN: 0596001835 To understand WLANs it is important to understand the 802. 11 protocols and their operations. These two books do an excellent job in presenting this information and is used throughout this and other presentations. Rick Graziani graziani@cabrillo. edu 3

Acknowledgements • • Thanks to Pejman Roshan and Jonathan Leary at Cisco Systems, authors of 802. 11 Wireless LAN Fundamentals for allowing me to use their graphics and examples for this presentation. Also thanks to Matthew Gast for author of 802. 11 Wireless Networks, The Definitive Guide for allowing me to use their graphics and examples for this presentation. Rick Graziani graziani@cabrillo. edu 4

802. 11 Frames – This isn’t Ethernet! 802. 11 Frames • Data Frames (most are PCF) – Data – Null data – Data+CF+Ack – Data+CF+Poll – Data+CF+Ac+CF+Poll – CF-Ack – CF-Poll – CF-Cak+CF-Poll • Control Frames – RTS – CTS – ACK – CF-End+CF-Ack Rick Graziani graziani@cabrillo. edu • Management Frames – Beacon – Probe Request – Probe Response – Authentication – Deauthentication – Association Request – Association Response – Reassociation Request – Reassociation Response – Disassociation – Announcement Traffic Indication 5

802. 11 Data Frames and Addressing Helps to understand this because it is not dependent upon the 802. 11 Physical layer.

Ethernet MAC Addressing X xxx Distribution System (DS) Access Point 1 xxx yyy Access Point 2 B A Y yyy C D Pseudo MAC address of hosts xxx yyy IP Packet Rick Graziani graziani@cabrillo. edu 7

802. 11 MAC Addressing The LLC encapsulation will be explained later in this presentation. General 802. 11 Frame • • Four address fields The number and function of the address fields is dependent upon the source and destination for the 802. 11 frame. Before we look at how these addresses are used, lets look at the different source and destination options. Address 4 is optional and not commonly used, except for WDS (wireless distribution system, bridge to bridge). Rick Graziani graziani@cabrillo. edu 8

802. 11 MAC Addressing - DS X Y Distribution System (DS) Access Point 1 A • Access Point 2 B C D Distribution System (DS) – “The distribution system is the logical component of 802. 11 used to forward frames to their destination. 802. 11 does not specify any particular technology for the distribution system. ” Matthew Gast – The DS is the exiting network from the AP. (For purposes of this discussion. ) – It can be a wired network (Ethernet) or a wireless network (wireless bridge) or something else. – We will assume it is a wired network for these discussions. Rick Graziani graziani@cabrillo. edu 9

802. 11 MAC Addressing – Frame Control Field General 802. 11 Frame • • To DS: indicates if frame is destined for the DS or AP (1 bit). From DS: indicates if frame is sourced from the DS or AP (1 bit). Rick Graziani graziani@cabrillo. edu 10

802. 11 MAC Addressing – Frame Control Field General 802. 11 Frame Function IBSS (no AP) To AP From AP Wireless bridge to bridge Rick Graziani graziani@cabrillo. edu To. DS From. DS 0 0 1 1 1 Note: Some documentation is misleading stating that the To. DS is set to 1 only when the destination is on the wired side of the AP. 11

802. 11 MAC Addressing – Frame Control Field Rick Graziani graziani@cabrillo. edu 12

802. 11 MAC Addressing X xxx Y Distribution System (DS) Access Point 1 Access Point 2 111 A B aaa C D bbb aaa • • • bbb 111 Pseudo MAC address of hosts and BSSID of AP 1 Let’s look at these options: – Host A to Host B – Host A to Host X – Host X to Host A Frames to and from a BSS (Basic Service Set) must go via the access point. The access point is a layer 2 bridge (translation bridge) between the 802. 11 network and the 802. 3 network. Rick Graziani graziani@cabrillo. edu 13

802. 11 MAC Addressing The BSSID X xxx Y Distribution System (DS) Access Point 1 A General 802. 11 Frame • • aaa 111 B Access Point 2 C D bbb Each BSS is assigned a BSSID. – Not to be confused with SSID or ESSID. BSSID – 48 bit identifier which distinguishes it from other BSSs in the network, used for filtering. In a BSS, the BSSID is the MAC address of the wireless interface. Remember, normal switches (bridges) may have MAC addresses, but these addresses are only used for management purposes and not for layer 2 frame forwarding (addressing). Rick Graziani graziani@cabrillo. edu 14

802. 11 MAC Addressing The BSSID X xxx Y Distribution System (DS) Access Point 1 A General 802. 11 Frame • aaa 111 B Access Point 2 C D bbb Besides the BSSID MAC address, the access point has a MAC address for other interfaces. – Ethernet (LAN) – Ethernet (WAN) – 802. 11 a for dual mode APs Rick Graziani graziani@cabrillo. edu 15

BSSID – Cisco 1200 MAC address for AP’s IP address (ARP tables) BSSID Rick Graziani graziani@cabrillo. edu BSSID for 802. 11 a WLAN 16

Linksys WRT 54 G Router Information • IP Address: (received via DHCP) • MAC Address: 00: 0 F: 66: 09: 4 E: 10 Local Network • MAC Address: 00: 0 F: 66: 09: 4 E: 0 F • IP Address: 192. 168. 1. 1 MAC address for AP’s IP address Wireless • MAC Address: 00: 0 F: 66: 09: 4 E: 11 • SSID: Guido. Net 2 • DHCP Server: Enabled • Channel: 11 • BSSID Encryption Function: Enabled Rick Graziani graziani@cabrillo. edu 17

802. 11 MAC Addressing Host A to Host B X xxx Y Distribution System (DS) Access Point 1 A General 802. 11 Frame aaa 111 B Access Point 2 C D bbb • • • Address 1 – Receiver address Address 2 – Transmitter address Address 3 – Ethernet/wireless SA, Ethernet/wireless DA, or BSSID • Transmitter: Sends a frame on to the wireless medium, but may not be the original source (didn’t necessarily create the frame), i. e. AP Receiver: Receives a frame on the wireless medium, but may not be the final destination, i. e. AP • Rick Graziani graziani@cabrillo. edu 18

802. 11 MAC Addressing X xxx Y Distribution System (DS) Host A to Host B Access Point 1 A Access Point 2 111 aaa 1 Trans. aaa Rec. Trans. bbb 111 B DA D bbb 0 AP 1 to Host B 0 Rec. 111 Host A to AP 1 C SA aaa 1 • • • Address 1 – Receiver address Address 2 – Transmitter address Address 3 – Ethernet/wireless SA, Ethernet/wireless DA, or BSSID Rick Graziani graziani@cabrillo. edu 19

802. 11 MAC Addressing Distribution System (DS) IP Packet General 802. 11 Frame L IP Packet L C • • • Access Points are translation bridges. From 802. 11 to Ethernet, and from Ethernet to 802. 11 The “data/frame body” is re-encapsulated with the proper layer 2 frame (Ethernet or 802. 11). • Rick. Certain addresses are copied between the two types of frames. Graziani graziani@cabrillo. edu 20

802. 11 MAC Addressing X xxx Y Distribution System (DS) Host A to Host X Access Point 1 A aaa Host A to AP 1 802. 11 Frame 1 Rec. Trans. 111 aaa DA Access Point 2 111 B C D bbb xxx 0 copied Host A to AP 1 xxx • aaa The Ethernet DA and SA are the source and destination addresses just like on traditional Ethernet networks. – Destination Address – Host X – Source Address – Host A Rick Graziani graziani@cabrillo. edu 21

802. 11 MAC Addressing X xxx Y Distribution System (DS) Host A to Host X Access Point 1 A aaa Host A to AP 1 802. 11 Frame 1 Rec. Trans. 111 aaa • • • B C D bbb xxx copied 0 xxx • DA 111 Access Point 2 aaa Host A to AP 1 The AP (bridge) knows which MAC address on on its wireless interface and maintains a table with those MAC addresses. (from the Association process – later) When the AP receives an 802. 11 frame, it examines the Address 3 address. If Address 3 is not in its table of wireless MACs it knows it needs to translate the frame to an Ethernet frame. The AP copies the Address 3 address to the Ethernet Destination Address, and Rick Graziani graziani@cabrillo. edu 22 Address 2 (Transmitter address) is copied to the Ethernet Source Address.

802. 11 MAC Addressing Host X to Host A X xxx Y Distribution System (DS) Access Point 1 111 A aaa Rick Graziani graziani@cabrillo. edu B bbb Access Point 2 C D 23

802. 11 MAC Addressing X xxx Y Distribution System (DS) Host X to Host A Access Point 1 A aaa Host X to AP 1 aaa 111 B Access Point 2 C D bbb xxx Destination Address – Host X Source Address – Host A copied AP 1 to Host A 802. 11 Frame 0 Rec. aaa Trans. 111 SA xxx 1 Rick Graziani graziani@cabrillo. edu 24

802. 11 MAC Addressing X xxx Distribution System (DS) Host X to Host A Access Point 1 aaa Host X to AP 1 aaa AP 1 to Host A 802. 11 Frame 0 • • Y Rec. aaa A Trans. 111 B C D bbb xxx copied 111 Access Point 2 SA Destination Address – Host X Source Address – Host A xxx 1 The AP (bridge) knows which MAC address on on its wireless interface and maintains a table with those MAC addresses. (via Association process – later) When the AP receives an Ethernet frame, it examines the Destination address. If Destination Address is in its table of wireless MACs it knows it needs to translate the frame to an 802. 11 frame. The AP copies the Destination address to the 802. 11 Address 1, and Ethernet Source is copied to the Address 3 address (SA in this case). (Flood out all ports unless in Source Rick Graziani graziani@cabrillo. edu 25 Address Table. )

802. 11 MAC Addressing xxx 1 2 xxx aaa 111 aaa • • So how do Ethernet switches know where the wireless stations are? Just like wired stations – using the source address of frames that came from the wireless station via the access point. Here the switch learns from the incoming Ethernet frame that Source Address aaa is on port 2 and enters that in its MAC address table. Any frames coming into the switch (ex. port 1) with a Destination Address of aaa, the switch knows to forward those frames out port 2 (towards the AP). Rick Graziani graziani@cabrillo. edu 26

LLC – Logical Link Control General 802. 11 Frame L IP Packet L C • • The IP Packet is in an LLC frame which is encapsulated in a MAC frame. 802. 11 does not include a protocol type field. An 8 byte SNAP field is added to the LLC to indicate the layer 3 data being carried in the data field. The rest of the information within the LLC is not really relevant. Rick Graziani graziani@cabrillo. edu 27

LLC – Logical Link Control • • The only word of caution is that there are two types of LLC encapsulation, RFC 1042 and 802. 1 h. On a rare occasion, you might find a problem with a client associating to an AP when their LLCs do not match. Rick Graziani graziani@cabrillo. edu 28

LLC – Logical Link Control Rick Graziani graziani@cabrillo. edu 29

802. 11 MAC Layer Operations Station Connectivity Power Save Operations 802. 11 Frame Formats

Station Connectivity Rick Graziani graziani@cabrillo. edu 32

Station Connectivity • • • Earlier we stated, at a minimum a client station and the access point must be configured to be using the same SSID. How does the client find these APs? Before connecting to any network, you must find it. Ethernet, the cable does that for you, but of course there is no cable with wireless. There are various applications and utilities that will do it, but what is actually happening in the 802. 11 MAC operations? Let’s take a look… Rick Graziani graziani@cabrillo. edu 33

Station Connectivity Successful Authentication State 1 Unauthenticated Unassociated Successful Association State 2 Authenticated Unassociated Deauthentication • State 3 Authenticated Associated Disassociation Station connectivity is a explanation of how 802. 11 stations select and communicate with APs. Rick Graziani graziani@cabrillo. edu 34

Station Connectivity Probe process Authentication process Successful Authentication State 1 Unauthenticated Unassociated Successful Association State 2 Authenticated Unassociated Deauthentication • • Association process State 3 Authenticated Associated Disassociation We will look at three processes: – Probe Process (or scanning) – The Authentication Process – The Association Process Only after a station has both authenticated and associated with the access point can it use the Distribution System (DS) services and communicate with devices beyond the access point. Rick Graziani graziani@cabrillo. edu 35

Station Connectivity – Probe Process • The Probe Process (Scanning) done by the wireless station – Passive - Beacons – Active – Probe Requests • Depends on device drive of wireless adapter or the software utility you are using. • Cisco adapters do active scanning when associating, but use passive scanning for some tests. • In either case, beacons are still received and used by the wireless stations for other things besides scanning (coming). Rick Graziani graziani@cabrillo. edu 36

Station Connectivity – Passive Scanning • • • Passive Scanning – Saves battery power – Station moves to each channel and waits for Beacon frames from the AP. – Records any beacons received. Beacon frames allow a station to find out every thing it needs to begin communications with the AP including: – SSID – Supported Rates Kismet/Kis. MAC uses passive scanning Rick Graziani graziani@cabrillo. edu 37

Station Connectivity – Passive Scanning Rick Graziani graziani@cabrillo. edu 38

Station Connectivity – Passive Scanning Note: Most of these beacons are received via normal operations and not through passive scanning. Rick Graziani graziani@cabrillo. edu 39

Station Connectivity – Passive Scanning • • • Passive scans, carried out by listening to Beacons from APs, are not usually displayed by a network analyzer (Ethereal, Airopeek, etc. ) but can be. Microsecond – millionth of a second Millisecond – thousandth of a second A common beacon interval is 100 time units. Beacon interval is the number of time units between beacon transmissions. – One unit of time is 1, 024 microseconds or about 1 millisecond. – A beacon interval of 100 is equivalent to 100 milliseconds or 0. 1 seconds. – That would be 10 beacons per second. Rick Graziani graziani@cabrillo. edu 40

Setting the beacon interval on an AP (later) Rick Graziani graziani@cabrillo. edu 41

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Station Connectivity – Passive Scanning • • AP features (options) – The SSID can be “hidden” or “cloaked” in the beacon frame (can be done on Cisco APs) – Do not send AP broadcast beacons (not an option with Cisco APs) From some mailing lists: – “SSID cloaking and beacon hiding isn't necessarily a bad thing, but too many places use it as the only protection because it leads to a false sense of security. ” – “Obscurity != security. Too many companies blindly trust that no beaconing or hiding their SSID means they're automatically safe. ” Rick Graziani graziani@cabrillo. edu 43

Station Connectivity – Active Scanning • • Active Scanning: Probe Request – This process is not mandatory on with 802. 11. – A Probe Request frame is sent out on every channel (1 – 11) by the client. – APs that receive Probe Requests must reply with a Probe Response frame if: • SSID matches or • Probe Request had a broadcast SSID (0 byte SSID) Net. Stumber uses active scanning From the client Rick Graziani graziani@cabrillo. edu 44

From the client Source address is the client (host) The SSID can also be a broadcast SSID which triggers a Probe Response from all APs in the area. Rick Graziani graziani@cabrillo. edu 45

Station Connectivity – Active Scanning • • Active Scanning: Probe Response – On BSSs the AP is responsible for replying to Probe Requests with Probe Responses. – Probe Responses are unicast frames. – Probe Responses must be ACKnowledged by the receiver (client). Like a beacon, Probe Response frames allow a station to find out every thing it needs to begin communications with the AP including: – SSID – Supported Rates 1 3 2 From the AP Rick Graziani graziani@cabrillo. edu 46

From the AP Destination Address is the client who issued the Probe Request Source address is the AP (same as the BSSID) • The beacon contains certain information that lets a station know if it can continue to attempt to join this network: – SSID – Supported Rates – Privacy: – WEP – None (open) Rick Graziani graziani@cabrillo. edu 47

Capturing the Probe Response Rick Graziani graziani@cabrillo. edu 48

Station Connectivity – Multiple APs Most likely Vivian will communicate with AP 2, which matches her SSID and has the stronger signal strength. • • • How a station chooses an AP is not specified in 802. 11. It is left up to the vendor. It could be, Matching SSIDs, Signal Strength, Supported data rates. Rick Graziani graziani@cabrillo. edu 49

Station Connectivity Hey, I didn’t do anything and I am on the Internet! No SSID Probe Request Broadcast (no) SSID ACK • • • Probe Response SSID = tsunami Access Points can be configured whether or not to allow clients with broadcast SSIDs to continue the connectivity process. – If there is no authentication on the AP, then the client will most likely “associate” and be on their network! Cisco APs use a default SSID of tsunami known as the “guest mode” SSID. (coming) Unless this feature is disabled or authentication is enabled, anyone can easily associate with your AP and access your network (or the Internet). Rick Graziani graziani@cabrillo. edu 50

Station Connectivity Probe process Authentication process Successful Authentication State 1 Unauthenticated Unassociated Successful Association State 2 Authenticated Unassociated Deauthentication • • Association process State 3 Authenticated Associated Disassociation Station connectivity processes: – Probe Process (or scanning) – The Authentication Process – The Association Process Only after a station has both authenticated and associated with the access point can it use the Distribution System (DS) services and communicate with devices beyond the access point. Rick Graziani graziani@cabrillo. edu 51

Authentication Process • • • On a wired network, authentication is implicitly provided by the physical cable from the PC to the switch. Authentication is the process to ensure that stations attempting to associate with the network (AP) are allowed to do so. 802. 11 specifies two types of authentication: – Open-system – Shared-key (makes use of WEP) Rick Graziani graziani@cabrillo. edu 52

Authentication Process – Open-System • • • Open-system authentication really “no authentication”. Open-system authentication is the only method required by 802. 11 – You could buy an AP that doesn’t support Shared-key The client and the station exchange authentication frames. Rick Graziani graziani@cabrillo. edu 53

Frame Control omitted in this Authentication Response • • The client: – Sets the Authentication Algorithm Number to 0 (open-system) – Set Authentication Transaction Sequence Number to 1 The AP: – Sets the Authentication Algorithm Number to 0 (open-system) – Set Authentication Transaction Sequence Number to 2 – Status Code set to 0 (Successful) Rick Graziani graziani@cabrillo. edu 54

Authentication Process – Shared-Key • • • Shared-key authentication uses WEP (Wired Equivalent Privacy) and can only be used on products that support WEP is a Layer 2 encryption algorithm based on the RC 4 algorithm. 802. 11 requires any stations that support WEP to also support shared-key authentication. WEP and WPA will be examined more closely when we discuss security. For now both the client and the AP must have a shared-key, password. Rick Graziani graziani@cabrillo. edu 55

Authentication Process – Shared-Key • • The client: – Sets the Authentication Algorithm Number to 1 (shared-key) – Set Authentication Transaction Sequence Number to 1 The AP: – Sets the Authentication Algorithm Number to 1 (shared-key) – Set Authentication Transaction Sequence Number to 2 – Status Code set to 0 (Successful) – Challenge Text (later) The client: – Sets the Authentication Algorithm Number to 1 (shared-key) – Set Authentication Transaction Sequence Number to 3 – Challenge Text (later) The AP: – Sets the Authentication Algorithm Number to 1 (shared-key) – Set Authentication Transaction Sequence Number to 4 – Status Code set to 0 (Successful) Rick Graziani graziani@cabrillo. edu 56

Authentication Process • • • We’ll look at the configuration of the client and AP later! Example of open-system authentication. Note: On “some” systems you can configure authentication (WEP) and WEP encryption separately. On the ACU you can have open-system authentication and also have WEP encryption. However, if you have Shared-key (WEP) authentication, you must use WEP encryption. Rick Graziani graziani@cabrillo. edu 57

Authentication Process • • or Authentication – Open-System – Shared-Key (WEP) Encryption – None – WEP Rick Graziani graziani@cabrillo. edu only 58

Station Connectivity • • • Hey, I REALLY didn’t do anything and I am on the Internet! Beacon SSID = tsunami Authentication Request Authentication Response (Opensystem) If not configured specifically to look for a network, some client utilities will automatically join the network that meets their vendor’s criteria (not specified in 802. 11) such as signal strength and open-system authentication. How a station chooses an AP is not specified in 802. 11. Or just find the open-system network and join. Rick Graziani graziani@cabrillo. edu 59

Station Connectivity Probe process Authentication process Successful Authentication State 1 Unauthenticated Unassociated Successful Association State 2 Authenticated Unassociated Deauthentication • • Association process State 3 Authenticated Associated Disassociation Station connectivity processes: – Probe Process (or scanning) – The Authentication Process – The Association Process Only after a station has both authenticated and associated with the access point can it use the Distribution System (DS) services and communicate with devices beyond the access point. Rick Graziani graziani@cabrillo. edu 60

Association Process 1. Association Request 2. Association Response • • • The association process is logically equivalent to plugging into a wired network. Once this process is completed, the wireless station can use the DS and connect to the network and beyond. A wireless station can only associate with one AP (802. 11 restriction) During the 802. 11 association process the AP maps a logical port known as the Association Identifier (AID) to the wireless station. – The AID is equivalent to a port on a switch and is used later in Power Save Options. The association process allows the DS to keep track of frames destined for the wireless station, so they can be forwarded. Rick Graziani graziani@cabrillo. edu 61

Association Process • • Association Request Frame (From client) – Listen Interval – This value is used by the Power Save Operation (later). Informs AP how often it will wake-up to receive buffered frames. – Supported Rates – What data rates the client station supports. Association Response Frame (From AP) – Status Code – Indicates success or reason for failure. – AID – A value assigned to this station for the Power Save Operation (later). – Supported Rates - What data rates the AP supports. Rick Graziani graziani@cabrillo. edu 62

Association Process • Association Request Frame (From client) – At this point the AP adds the source address of the wireless client to its Source Address Table. – This is how the AP knows to forward frames destined to the client out the wireless interface (802. 11) and not the wired interface (802. 3/Ethernet). – The AP usually learns the wireless client’s Source Address sooner, either in the Probe Request or Authentication Request frames, but this is where is “officially” adds the wireless client to it MAC table. Rick Graziani graziani@cabrillo. edu 63

Station Connectivity Probe process Authentication process Successful Authentication State 1 Unauthenticated Unassociated Successful Association State 2 Authenticated Unassociated Deauthentication • • Association process State 3 Authenticated Associated Disassociation Traffic can now flow between the client and the AP. Disassociation and deauthentication can be due to: – Inactivity – The AP cannot handle all currently associated stations – Station has left BSS – etc. Rick Graziani graziani@cabrillo. edu 64

Labs and Station Connectivity Configuring AP 1 is easy! Hey, what happened to my settings on AP 2! AP 1 AP 2 • • In the lab we will need to take steps to make sure you are configuring and connected to the AP that you think you are! We will first connect via a wired interface, change the SSID and IP addressing on the AP, different from what the labs show. Rick Graziani graziani@cabrillo. edu 65

Power Save (PS) Operations • • • A key factor in wireless is mobility, which implies batteries. To preserve battery power the 802. 11 specification provides for power saving operations on the wireless clients. 802. 11 categories for power savings refer to: – Unicast frames – Broadcast/Multicast frames Rick Graziani graziani@cabrillo. edu 67

Power Save (PS) Operations • • The Cisco ACU has three options for Power Saving: – CAM (Constantly Awake Mode) – MAX PSP (Max Power Savings) – Fast PSP (Fast Power Saving Mode) More on this later. Rick Graziani graziani@cabrillo. edu 68

Power Save (PS) Operations I’m awake. Let me listen for a beacon to see if there is any traffic for me. If not, I can go back to sleep. beacon • • • A client enters low-power mode by turning off its radio. The AP buffers (holds) frames destined for that station while it is in PS mode. At a certain interval the client wakes up to listen for a beacon from the AP. The beacon contains information on whether or not there are frames for this station at the AP. If there are no frames buffered for this station it can return to PS mode. Rick Graziani graziani@cabrillo. edu 69

Power Save (PS) Operations There are frames for me! Please send them to me. Beacon (frames buffered) PS-Poll (send them to me) Frame 1 ACK The basics: • If there are frames buffered for this station it will poll the AP for those frames. • The AP will then send the frames to the station. Rick Graziani graziani@cabrillo. edu 70

Unicast Power Save Operations 1. Association Request 2. Association Response • • • When a client associates with an AP it specifies listen interval. Listen interval – The number of beacons the client waits while in sleep mode before transitioning to active (awake) mode. The number of beacons per second may vary between APs, but the beacon frame has told the client how often those beacons are sent with the beacon interval, so the client knows when it needs to wake up. Rick Graziani graziani@cabrillo. edu 71

Unicast Power Save Operations There are frames for me! Please send them to me. Beacon (frames buffered) PS-Poll (send them to me) Frame 1 ACK • For example: – If the listening interval on the client is 200 the client wakes up every 200 beacons. – If the AP beacon interval is 100 (10 beacons per second) – The client will wake up every 20 seconds. to see if there any frames buffered for it. Rick Graziani graziani@cabrillo. edu 72

Power Save (PS) Operations • • How does an AP know if a station is in PS mode? Various frames contain this information, from the Station Connectivity Process, PS-Polling and Data Frames as the user may change this status any time. • This information is contained in the Power Management sub-field of the Frame Control field which is in most 802. 11 frames. – 0 = Active mode, 1 = Power Save Mode – Frames from Rick Graziani graziani@cabrillo. edu AP always have a value of 0 (it cannot sleep) 73

Power Save (PS) Operations Rick Graziani graziani@cabrillo. edu 74

FYI – A little more detail on Unicast PS Operations The AP tells me I am AID 29. 1. Association Request 2. Association Response AID = 29 • • Remember the Association Identifier (AID) in the Association Response, equivalent to a port on a switch. Each station receives a unique AID during the association phase. The TIM (Time Indication Map) in the beacon tells the station if there any frames buffered for it in the AP. If the “flag” = 0 there are no frames buffered, “flag” = 1 there are frames being buffered. Rick Graziani graziani@cabrillo. edu 75

FYI – A little more detail on Unicast PS Operations The AP told me I am AID 29. I see in the beacon that there are frames waiting for me. Let me ask for them. During Assoc. Process 1. Association Request 2. Association Response Beacon AID = 29 PS-Poll (send them to me) Frame 1 ACK • • The station sends a PS-Poll with is AID to get the frames. Much of the detail has been left out and if you are interested, see the two books I recommended at the beginning of the presentation. Rick Graziani graziani@cabrillo. edu 76

FYI – A little more detail on Unicast PS Operations Rick Graziani graziani@cabrillo. edu 77

FYI – A little more detail on Unicast PS Operations • • You won’t find an exact match here between the protocol decode and the TIM. See the Cisco Press book 802. 11 Wireless LAN Fundamentals if you are interested in how this works. Rick Graziani graziani@cabrillo. edu 78

Broadcast/Multicast Power Save Operations • • • Broadcast and multicast traffic is buffered at the AP for all stations (including non-PS stations) when at least one associated station is in PS mode. The network administrator defines the interval for the client to wake up to receive broadcast and multicast traffic. A special TIM, known as a DTIM (Delivery Traffic Indication Map) indicates whether or not there is broadcast/multicast traffic buffered on the AP. If the TIM’s, DTIM Count field is 0, the AP has broadcast/multicast frames. DTIM information is not sent in every beacon, but on every DTIM count period (10 th beacon in this example), and “getting in sync” depends on vendor. Rest of details can be found in Matthew Gast’s book if you are interested. Rick Graziani graziani@cabrillo. edu 79

802. 11 Frame Formats

802. 11 Frame Formats (Some of them) • The following diagrams are FYI and from Cisco Press book 802. 11 Wireless LAN Fundamentals by Pejman Roshan and Jonathan Leary. 802. 11 Frames • Data Frames (most are PCF) – Data – Null data – Data+CF+Ack – Data+CF+Poll – Data+CF+Ac+CF+Poll – CF-Ack – CF-Poll – CF-Cak+CF-Poll • Control Frames – RTS – CTS – ACK – CF-End+CF-Ack Rick Graziani graziani@cabrillo. edu • Management Frames – Beacon – Probe Request – Probe Response – Authentication – Deauthentication – Association Request – Association Response – Reassociation Request – Reassociation Response – Disassociation – Announcement Traffic Indication 81

802. 11 Data Frame Rick Graziani graziani@cabrillo. edu 82

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Non-standard 802. 11 Devices

Non-standard 802. 11 devices • These devices either extend or fall outside the 802. 11 standard and will be discussed in more detail in later sections: – Repeater APs – Universal Clients (Workgroup Bridges) – Wireless Bridges Rick Graziani graziani@cabrillo. edu 87

Ch. 2 – 802. 11 and NICs Part 2 – 802. 11 MAC Cisco Fundamentals of Wireless LANs version 1. 1 Rick Graziani Cabrillo College