Ch. 2 – 802.11 and NICs Part 1 – 802.11 MAC and Cisco Client Adapters

1. Ch. 2 – 802.11 and NICsPart 1 – 802.11 MAC and Cisco Client Adapters Cisco Fundamentals of Wireless LANs version 1.2

2. Overview • Sections 2.2 and 2.3 • We will not use most of the online curriculum in these sections. • This presentation will add additional material. • However, still please read the online curriculum. Will not use curriculum. Additional information provided. MAC – Two presentations. This is Part I PHY – Separate presentation. Rick Graziani graziani@cabrillo.edu

3. 802.11 Overview and MAC Layer Part 1 – 802.11 MAC and Cisco Client Adapters • 2.1 Online Curriculum • 802.11 Standards • Overview of WLAN Topologies • IBSS • BSS • ESS • Access Points • 802.11 Medium Access Mechanisms • DCF Operations • Hidden Node Problem • RTS/CTS • Frame Fragmentation • 2.4 – 2.6 Online Curriculum • Client Adapters • Aironet Client Utility (ACU) • ACU Monitoring and Troubleshooting Tools Part 2 – 802.11 MAC • (Separate Presentation) • 802.11 Data Frames and Addressing • 802.11 MAC Layer Operations • Station Connectivity • Power Save Operations • 802.11 Frame Formats • Non-standard devices Rick Graziani graziani@cabrillo.edu

4. Recommended Reading and Sources for this Presentation • 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. Pejman Roshan Jonathan Leary ISBN: 1587050773 Matthew S. Gast ISBN: 0596001835 Rick Graziani graziani@cabrillo.edu

5. 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

6. 802.11 Standards

7. Overview of Standardization • Standardization of networking functions has done much to further the development of affordable, interoperable networking products. • This is true for wireless products as well. • Prior to the development of standards, wireless systems were plagued with low data rates, incompatibility, and high costs. • Standardization provides all of the following benefits: • Interoperability among the products of multiple vendors • Faster product development • Stability • Ability to upgrade • Cost reductions Rick Graziani graziani@cabrillo.edu

8. IEEE and 802.11 • IEEE, founded in 1884, is a nonprofit professional organization • Plays a critical role in developing standards, publishing technical works, sponsoring conferences, and providing accreditation in the area of electrical and electronics technology. • In the area of networking, the IEEE has produced many widely used standards such as the 802.x group of local area network (LAN) and metropolitan area network (MAN) standards, Rick Graziani graziani@cabrillo.edu

9. IEEE 802 Architecture Some you may recognize: • 802.3 – CSMA/CD (Carrier Sense Multiple Access with Collision Detection), often mistakenly called Ethernet • 802.1d – Spanning Tree • 802.1Q – VLANs • 802.5 – Token Ring Rick Graziani graziani@cabrillo.edu

10. IEEE 802.11 Architecture • 802.11 is a family of protocols, including the original specification, 802.11, 802.11b, 802.11a, 802.11g and others. • Officially called the IEEE Standard for WLAN MAC and PHY specifications. • 802.11 “is just another link layer for 802.2” • 802.11 is sometimes called wireless Ethernet, because of its shared lineage with Ethernet, 802.3. • The wired network side of the network could be Ethernet, Token Ring, etc.(we will always use Ethernet in our examples) • Access Points and Bridges act as “translation bridges” between 802.11 and 802.3 (or other other protocol) Rick Graziani graziani@cabrillo.edu

11. Overview of WLAN Topologies IBSS BSS ESS Access Points Quick Preview: Station/AP Connectivity

12. Overview of WLAN Topologies • Three types of WLAN Topologies: • Independent Basic Service Sets (IBSS) • Basic Service Set (BSS) • Extended Service Set (ESS) • Service Set – A logical grouping of devices. • WLANs provide network access by broadcasting a signal across a wireless radio frequency. • Transmitter prefaces its transmissions with a Service Set Identifier (SSID) • A station may receive transmissions from transmitters with the same or different SSIDs. Rick Graziani graziani@cabrillo.edu

13. Independent Basic Service Sets (IBSS) • IBSS consists of a group of 802.11 stations directly communicating with each other. • No Access Point used • Also known as an ad-hoc network. • Usage: Few stations setup up for a specific purpose for a short period of time. (ex. file transfers.) • We will have a an IBSS lab, but our main focus will be BSSs and ESSs. Rick Graziani graziani@cabrillo.edu

14. Basic Service Set (BSS) • BSS, also known as an Infrastructure BSS (never called IBSS) • Requires an Access Point (AP) • Converts 802.11 frames to Ethernet and visa versa • Known as a translation bridge • Stations do not communicate directly, but via the AP • APs typically have an uplink port that connects the BSS to a wired network (usually Ethernet), known as the Distribution System (DS). Rick Graziani graziani@cabrillo.edu

15. Extended Service Set (ESS) • Multiple BSSs can be connected together with a layer 2 “backbone network” to form an Extended Service Set (ESS). • 802.11 does not specify the backbone network • The backbone network is also known as the Distribution System (DS) and could be wired or wireless. • Stations are “associated” with only one AP at a time. • The SSID is the same for all BSS areas in the ESS (unless creating multiple BSSs, i.e. one for Marketing and another for Sales). Rick Graziani graziani@cabrillo.edu

16. Extended Service Set (ESS) • What if you want to be able to move between access points without the latency of re-association and re-authentication (these will be explained)? • Roaming gives stations true mobility allowing them to move seamlessly between BSSs. (More later) • APs need to be able to communicate between themselves since stations can only associate with one AP at a time. • IEEE 802.11 working group (Task Group F) is working on standardizing IAPP (Inter-Access Point Protocol) Rick Graziani graziani@cabrillo.edu

17. Access Points • Access Point (AP) • Translates (converts) 802.11 frames to Ethernet and visa versa • Typically provides wireless-to-wired bridging function • All BSS communications must go through the AP, even between two wireless stations Rick Graziani graziani@cabrillo.edu

18. Quick Preview: Station/AP Connectivity • This is just a preview. • Later in this module, we will take a closer look at the following: • The hardware/software: • Wireless NICs • Client Utilities (Aironet) • Using Windows to set the IP Address • The 802.11 MAC Layer Operations: • Station Connectivity Rick Graziani graziani@cabrillo.edu

19. Quick Preview: Station/AP Connectivity SSID (Service Set Identifier) • At a minimum a client station and the access point must be configured to be using the same SSID. • An SSID is: • Between 2 and 32 alphanumeric characters • Spaces okay • Must match EXACTLY, including upper and lower case • Sometimes called the ESSID • Not the same as BSSID (MAC address of the AP, later) Rick Graziani graziani@cabrillo.edu

20. Quick Preview: Station/AP Connectivity Can use windows to configure wireless NIC, but we will use the Cisco client utility, Aironet • SSIDs are sent by the APs in beacons (and other frames) • Applications such as NetStumbler or even Windows can see these beacons and interpret the information in them. SSID 2 and 3 are used for roaming where different SSIDs are used (later) Rick Graziani graziani@cabrillo.edu

21. Quick Preview: Station/AP Connectivity • The Cisco APs have the default SSID tsunami. SSID Rick Graziani graziani@cabrillo.edu

22. Quick Preview: Station/AP Connectivity Using Windows Looking for an AP? Right click Using NetStumbler Rick Graziani graziani@cabrillo.edu

23. Quick Preview: Station/AP Connectivity • Your operating system (Windows) or wireless NIC client (Aironet) will tell you whether or not you have successfully connected (associated). Windows Toolbar Icon Windows Network Properties Aironet Toolbar Icon Rick Graziani graziani@cabrillo.edu

24. 802.11 Medium Access Mechanisms DCF Operations Hidden Node Problem RTS/CTS Frame Fragmentation

25. 802.11 Frames – This isn’t Ethernet (802.3) • 802.11 has some similarities with Ethernet but it is a different protocol. • 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. • Certain addresses are copied between the two types of frames. Distribution System (DS) IP Packet General 802.11 Frame IP Packet LLC Rick Graziani graziani@cabrillo.edu

26. 802.11 Frames 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-End+CF-Ack • Management Frames • Beacon • Probe Request • Probe Response • Authentication • Deauthentication • Association Request • Association Response • Reassociation Request • Reassociation Response • Disassociation • Traffic Indication Map (TIM) Rick Graziani graziani@cabrillo.edu

27. Medium Access – CSMA/CA All stations detect the collision • Both CSMA/CD and CSMA/CA are half-duplex architectures • Ethernet uses CSMA/CD – Collision Detection • Ethernet devices detect a collision as when the data is transmitted • 802.11 uses CSMA/CA – Collision Avoidance • 802.11 devices only detect (assume) a collision when the transmitter has not received an Acknowledgement. • Stations also use a virtual carrier-sense function, NAV ACK CSMA/CA CSMA/CD Rick Graziani graziani@cabrillo.edu

28. Medium Access – CSMA/CA All stations detect the collision • The 802.11 standard makes it mandatory that all stations implement the DCF (Distributed Coordination Function), a form of carrier sense multiple access with collision avoidance (CSMA/CA). More Coming! • CSMA is a contention-based protocol making sure that all stations first sense the medium before transmitting (physically and virtually). More Coming! • The main goal of CSMA/CA is to avoid having stations transmit at the same time, which will then result in collisions and eventual retransmissions. However, collisions may still occur and when they do stations may or may not be able to detect them (hidden node problem). More Coming! ACK CSMA/CA CSMA/CD Rick Graziani graziani@cabrillo.edu

29. DCF • IEEE mandated access mechanism for 802.11 is DCF(Distributed Coordination Function) • Basis for CSMA/CA • Discussed in detail next Rick Graziani graziani@cabrillo.edu

30. DCF Operation • In DCF operation, a station wanting to transmit : • Checks to see if radio link is clear, CS/CCA – Carrier Sense, Clear Channel Assessment (Later in PHY presentation) • Checks its NAV timer (coming) to see if someone else is using the medium. • If medium is available DCF uses a random backoff timer to avoid collisions and sends the frame. • Transmitting station only knows the 802.11 frame got there if it receives an ACK. • May also use RTS/CTS to reduce collisions (coming) An example will be coming! Rick Graziani graziani@cabrillo.edu

31. Duration Field • Duration/ID field – The number of microseconds (millionth of a second) that the medium is expected to remain busy for transmission currently in progress. • Transmitting device sets the Duration time in microseconds. • Includes time to: • Transmit this frame to the AP (or to the client if an AP) • The returning ACK • The time in-between frames, IFS (Interframe Spacing) • All stations monitor this field! • All stations update their NAV (Network Allocation Vector) timer. An example will be coming! General 802.11 Frame (more on this later) Rick Graziani graziani@cabrillo.edu

32. NAV Timer • All stations have a NAV(Network Allocation Vector) timer. • Virtual carrier-sensing function • Protects the sequence of frames from interruption. • Martha sends a frame to George. • Since wireless medium is a “broadcast-based” (not broadcast frame) shared medium, all stations including Vivian receive the frame. • Vivian updates her NAV timer with the duration value. • Vivian will not attempt to transmit until her NAV is decremented to 0. • Stations will only update their NAV when the duration field value received is greater than their current NAV. An example will be coming! General 802.11 Frame (more on this later) Rick Graziani graziani@cabrillo.edu

33. Broadcast-based shared medium • Host A is sending 802.11 frames to another host via the AP. • All other 802.11 devices in BSS (on this channel) and within range of the signal will see the frame. • 802.11 framing provides addressing, so only the AP knows it is the next-hop receiver. • Other 802.11 devices within this BSS can sense that the medium is in use and will update their NAV values. What if a station is in range of the AP but not the Host A? (Hidden node problem – later) Rick Graziani graziani@cabrillo.edu

34. Interframe Spacing (IFS) • 802.11 uses four different interframe spaces used to determine medium access (note: microsecond = millionth of a second): • DIFS – DCF Interframe Space • Minimum amount of medium idle time until contention-based services begin. • PIFS – PCF Interframe Space • Used by PCF • SIFS – Short Interframe Space • Used for highest priority transmission, ACKs, RTS, CTS • EIFS – Extended Interframe Space • Not a fixed interval and used only when there is an error in frame transmission. An example will be coming! Rick Graziani graziani@cabrillo.edu

35. Wanting to transmit (1/3) • Station wanting to transmit. • Carrier Sensing: • Physical: Physically senses medium is idle (CS/CCA – coming). • Virtual: NAV timer is 0 • Waits DIFS (DCF Interframe Space) • Minimum amount of medium idle time until contention-based services begin. • Once DCF is over, stations can contend for access. • Contention window begins. • Uses random backoff algorithm to determine when it can attempt to access the medium. (next) Random backoff slots Rick Graziani graziani@cabrillo.edu

36. Wanting to transmit (2/3) • (Detail of random backoff algorthim has been left out, but this will be sufficient.) • The random backoff algorithm randomly selects a value from 0 to 255 (maximum value varies by vendor and stored in the NIC). • The random value is the number of 802.11 slot times the station must wait after the DIFS, during the contention window before it may transmit. • Stations pick a random slot and wait for that slot before attempting to access the medium. • With several stations attempting to transmit, the station that picks the lowest slot, lowest random number, wins. Contention Window Begins Rick Graziani graziani@cabrillo.edu

37. Wanting to transmit (3/3) Others update NAV • Station transmits, setting the Duration ID to the time needed to transmit data, ACK and IFSs. • Other stations with higher slots will see the new transmission and wait to transmit. • If frame arrives at AP (assuming the transmitter is a station), then an ACK will be returned (stations have updated their NAVs from original frame). • If there is not an ACK received, the sending station assumes there has been a collision (stations have not updated their NAVs because of collision). • If two stations have the same lowest slot time and both transmit, then a collision occurs. • Stations will update its retry counter (double) to determine a new randomly selected slot time and process starts all over again. General 802.11 Frame (more on this later) Rick Graziani graziani@cabrillo.edu

38. Example Scenario: • Both Vivian and George want to transmit frames. • Both stations have same NAV values and physically sense when the medium is idle. • Both are waiting for Martha’s transmission to end and the medium to become available. • The medium now becomes available. I’m waiting I’m waiting Rick Graziani graziani@cabrillo.edu

39. Example • George and Vivian are both wanting to transmit. • Both perform the following: • Both sense that medium is available using Physical and Virtual Carriers Sensing: • Physical: Physically senses medium is idle (CS/CCA – coming). • Virtual: NAV timer is 0 • Both waits DIFS (DCF Interface Space) • Contention window begins. • Uses random backoff algorithm to determine when it can attempt to access the medium. (next) Random backoff slots Rick Graziani graziani@cabrillo.edu

40. Example • Both Vivian and George calculate their random backoff algorithm to randomly selects a value from 0 to 255. • Vivian has a slot time of 7, George a slot time of 31. • Vivian wins. • The destination of her frame is George (could have been a station on the wired network.) Vivian (7), George (31) Rick Graziani graziani@cabrillo.edu

41. Martha and George receive “broadcast-based” 802.11 frame. Example ( ( ( ) ) ) Others update NAV • Vivian transmits, setting the Duration ID to the time needed to transmit data, ACK and IFSs. • George with a higher slot will see the 802.11 frame from Vivian and wait to transmit. • Assuming their was not a collision from another station, Martha and George update their NAVs. General 802.11 Frame (more on this later) Rick Graziani graziani@cabrillo.edu

42. Example • The frame arrives at the AP. • After the SIFS, the AP sends an ACK back to Vivian, which is how Vivian knows the frame was received by the AP. • The AP now has the frame and must contend for access to the medium like all other stations. • Once it sends the frame to George, George will send an ACK back to the AP. • Remember, 802.11 uses a half-duplex, shared medium and the AP has to contend for access just like all other devices! Rick Graziani graziani@cabrillo.edu

43. Your turn! • Get into teams of 2. • Each person is a wireless client contending for access to shared wireless medium. • Using the example as an example contend for the wireless medium. • Station 1: Backoff slot of 3 • Station 2: Backoff slot of 9 • Go through the steps of sending the 802.11 frame to the AP • Use a total NAV value of 1054 microseconds for both stations. • Answer is on the next slide! Don’t look until you need to check! Rick Graziani graziani@cabrillo.edu

44. Your turn! (Solution) • Both stations count down their NAV timers to 0 • Both stations physically sense the wireless medium is available • Both stations wait the DIFS • Station 1 and Station 2 prepare to send their 802.11 by updating the NAV value in the Duration/ID field to reflect the time needed to transmit to the AP, the ACK from the AP and IFS for both the originally transmitted frame and the ACK from the AP. • Station 1 sends its 802.11 frame first because it had to wait less time with a Backoff slot of 3. • Station 2 “sees” that Station 1 has accessed the shared medium, the 802.11 frame from Station 1 to the AP, and updates its NAV timer. • Station 2 waits for the NAV timer to count down to 0. (Step 1) Rick Graziani graziani@cabrillo.edu

45. 802.11 Medium Access Mechanisms DCF Operations Hidden Node Problem RTS/CTS Frame Fragmentation

46. Hidden Node Problem • What if a station is in range of the AP but not other hosts, like the transmitting host? • Wireless networks have fuzzy boundaries, sometimes where may not be able to communicate/see every other node. • Hidden nodes can be caused by: • Hosts are in range of the AP but not each other. • An obstacle is blocking the signal between the hosts. Rick Graziani graziani@cabrillo.edu

47. Hidden Node Problem • The problem is collisions. • Collisions occur at the AP (or another station in an IBSS). • Both stations assume the medium is clear and transmit near the same time, resulting in a collision. • The AP cannot properly receive either signal and will not ACK either one. • Both stations retransmit, resulting in more collisions. • Throughput is significantly reduced Rick Graziani graziani@cabrillo.edu

48. Hidden Node Problem • Solutions: • Move the node • Remove the obstacle • Use RTS/CTS (Request to Send / Clear to Send) Rick Graziani graziani@cabrillo.edu

49. 802.11 Medium Access Mechanisms DCF Operations Hidden Node Problem RTS/CTS Frame Fragmentation

50. RTS/CTS Solution • The hidden node stations cannot see the RTS. • The AP replies to Vivian with a CTS, which all nodes, including the hidden node can see. • Vivian transmits the frame. • The AP returns an ACK to Vivian. • The AP sends the message to George who returns an ACK to the AP. • Vivian attempts to reserve the medium using an RTS control frame to the AP. • The RTS frame indicates to the AP and all stations within range, that Vivian wants to reserve the medium for a certain duration of time, message, ACK, and SIFS. Rick Graziani graziani@cabrillo.edu