Wireless Networks

1. Wireless Networks Instructor: Fatima Naseem Computer Engineering Department, University of Engineering and Technology, Taxila

2. Lecture # 10 Wireless LAN

3. Overview • Overview of WLAN • Modes of operation • Requirements of WLAN • IEEE 802.11 Protocols • Architecture • MAC Protocols • DCF • PCF • Services

4. Wireless LANs • Fastest growing technology • Idea: just a LAN, but without wires • Not as easy since signals are of limited range • Uses unlicensed frequencies and low power

5. Modes of operation • 2 modes of operation: • Infrastructure: • In presence of a Control Module (CM) often called a “Base Station (BS)” or 802.11 calls them “Access Points”. The area is referred to as “hot spot”. • Ad-Hoc: • Or peer-to-peer connectivity, where there is no CM

6. Ad-Hoc Networks Infrastructure

7. Wireless LAN Applications • LAN Extension • Cross-building interconnect • Nomadic Access • Ad hoc networking

8. LAN Extension • Wireless LAN linked into a wired LAN on same premises • Wired LAN • Backbone • Support servers and stationary workstations • Wireless LAN • Stations in large open areas • Manufacturing plants, stock exchange trading floors, and warehouses

9. Multiple-cell Wireless LAN

10. Cross-Building Interconnect • Connect LANs in nearby buildings • Wired or wireless LANs • Point-to-point wireless link is used • Devices connected are typically bridges or routers

11. Nomadic Access • Wireless link between LAN hub and mobile data terminal equipped with antenna • Laptop computer or notepad computer • Uses: • Transfer data from portable computer to office server • Extended environment such as campus

12. Ad Hoc Networking • Temporary peer-to-peer network set up to meet immediate need • Example: • Group of employees with laptops convene for a meeting; employees link computers in a temporary network for duration of meeting

13. Wireless LAN Requirements • Throughput • Number of nodes • Connection to backbone LAN • Service area • Battery power consumption • Transmission robustness and security • Collocated network operation • License-free operation • Handoff/roaming • Dynamic configuration

14. Evolution of Standard IEEE 802.11 IEEE 802.11 Committee started (1995) 1997 802.11 2.4GHz ISM band FHSS, 1 or 2 MBPS DSSS, 1 or 2 Mbps 802.11 Diffused IR version 1 or 2 Mbps 2000 802.11b, 2.4GHz ISM band HR-DSSS, upto 11 Mbps (Compatible with DSSS) 802.11a 5 GHz ISM band OFDM upto 54Mbps Like ADSL – 52 freq 48 for dta,4 for synch Compatible with EU HiperLAN Late 2001 802.11g, 2.4 GHz ISM band OFDM upto 54 Mbps

15. Overview, IEEE 802.11 Committee • Committee formed in 1990 • Wide attendance • Multiple Physical Layers • Frequency Hopping Spread Spectrum • Direct Sequence Spread Spectrum • Infrared • 2.4GHz Industrial, Scientific & Medical shared unlicensed band • 2.4 to 2.4835GHz with FCC transmitted power limits • 2Mb/s & 1Mb/s data transfer • 50 to 200 feet radius wireless coverage • Draft 5.0 Letter Ballot passed and forwarded to Sponsor Ballot • Published Standard adopted in 1997

16. 802.11 LAN Architecture • BS Base Station/ Access Point/ Central Module • BSS Basic Service Set • Stations/ Hosts

17. 802.11 LAN Architecture • ESS -> Extended Service Set • Two or more BSS with APs

18. IEEE 802.11 Architecture • WLAN is based on cellular architecture • Each cell/Basic Service Set (BSS) is controlled by a base station/Access Point (AP). • APs are connected with backbone called Distribution System (DS). • The whole interconnected WLAN through DS form Extended Service Set (ESS) as a single layer in OSI model. • Mobile Station (MS) in BSS with no connection to other BSSs form Independent BSS (IBSS).

19. IBSS Basic Service Set Wireless LAN / IEEE 802.11 Fixed Host Distribution System Access Point Mobile Host Expanded Service Set

20. AP functions as a bridge and a relay point. • In BSS, MS communicate through AP • IBSS is typically an ad hoc network, where station communicate directly. • To integrate 802.11 with 802.2 (Wired LAN), a portal is used. • Portal is a device such as bridge or router attached to DS.

21. The 802.11 Protocol Stack • One of Five techniques are used to send MAC frame from one station to another

22. 802.11: Infrared • Uses T/X at 0.85 microns or 0.95 microns • Two speeds permitted -> 1 Mbps & 2 Mbps • 1 Mbps encoding scheme • Group of 4 bits are encoded as 16-bit code-word containing fifteen 0s & a single 1 • Small error in time synchronization leads to only single bit error in the output • 2 Mbps encoding scheme • Takes 2 bits & produces 4-bit codeword, with a single 1 • Signals cannot penetrate walls • Cells in different are well isolated • Not a popular option • Low bandwidth • Sunlight swamps infrared signals

23. 802.11: Frequency HoppingSpread Spectrum (FHSS) • Band • Uses 79 channels each 1 MHz wide, starting at low end of 2.4 GHz ISM band • Pseudorandom generator is used to select the frequencies for hoping • As long as all stations use the same seed as of generator will stay synchronized in time & will hop to the same frequencies simultaneously • Modulation & data rate • FSK at 1 Mbaud/s • Allows 1 or 2 bits/baud • Date rate of 1 or 2 Mbps • Secure due to spreading • Popular for building-to-building links • Disadvantage: Low BW &

24. 802.11:Direct Sequence SpreadSpectrum (DSSS) • Each bit is transmitted as 11 chips, known as Barker sequence • Band • Uses 2.4 GHz ISM band • Bit sequence uses the entire band • Modulation & data rate • Phase shift modulation at 1 Mbaud/s • 1 bits/baud has 1 Mbps while 2 bits/baud results 2 Mbps

25. 802.11a:Orthogonal FrequencyDivision Multiplexing (OFDM) • Method for signal generation in 5 GHz ISM band • Deliver up to 54 Mbps • Difference with FDM • All the subbands are used by one source at a given time • Sources contend one another at the data link layer for access • Band • Divided into 52-subbands • 48-subbabnds for data & 4-subbands for synchronization • Modulation & data rate • Uses Phase shift modulation up to 18 Mbps & QAM on above • Better immunity to multipath fading

26. 802.11a - Pros & Cons • Pros • Fast maximum speed • Regulated frequencies prevent signal interference from other devices • Cons • Highest cost • Shorter range signal that is more easily obstructed

27. 802.11b: High Rate Direct SequenceSpread Spectrum (HR-DSSS) • Uses 11 million chips/sec to achieve 11 Mbps in a 2.4 GHz ISM band • Date rate • 1, 2, 5.5 & 11 Mbps are supported • 1 & 2 Mbps run at 1Mbaud, with 1 or 2 bits/baud using phase shift modulation • 5.5 & 11 Mbps run at 1.375Mbaud with 4 & 8 bits/baud using Walsh/Hadamard codes • Slower than 802.11a but • Range is about 7 times greater than 802.11a

28. 802.11b - Pros & Cons • Pros • Lowest cost • Signal range is good and not easily obstructed • Cons • Slowest maximum speed • Home appliances may interfere on the unregulated frequency band

29. 802.11g: OFDM • First Draft approved by IEEE in • November 2001 • Final Approval - 2003 • Uses OFDM with 2.4 GHz ISM band • Maximum date rate up to 54 Mbps • Works in the 2.4 GHz band • Backwards compatible with 802.11b

30. 802.11g - Pros & Cons • Pros • Fast maximum speed • Signal range is good • Not easily obstructed • Cons • Costs more than 802.11b • Appliances may interfere on the unregulated signal frequency

31. 802.11n – Newest Standard(Not Finalized Yet) • Designed to improve on 802.11g in the amount of bandwidth supported by • Utilizing multiple wireless signals and antennas (called MIMO technology) instead of one • Data rate • Upto 600 Mbps • Available bandwidth • 83.5/580 MHz • Frequency band of operation • 2.4/5 GHz • Modulation Type • DSSS, CCK, OFDM, MIMO • Better range over earlier Wi-Fi standards

32. 802.11n - Pros & Cons • Pros • Fastest maximum speed and best signal range • More resistant to signal interference from outside sources • Cons • Standard is not yet finalized • Costs more than 802.11g • Use of multiple signals may greatly interfere with nearby 802.11b/g based networks

33. Medium Access Methods • The 802.11 standard ensures that all stations, both radio-based network interface cards (NICs) and access points, implement access methods for sharing the air medium. • When installing wireless LANs (WLAN), most people don't give much thought to these mechanisms. • A solid understanding of 802.11's medium access methods, will enable us to deal more effectively with issues such as radio frequency interference, denial of services attacks and throughput issues.

34. Medium Access Control (MAC) • Protocol required to coordinate access • i.e. transmitters must take turns • Similar to talking in a crowded room • Also similar to hub based Ethernet

35. Carrier Sense Multiple Access (CSMA) • Procedure • Listen to medium and wait until it is free (no one else is talking) • Wait a random back off time then start talking • Advantages • Fairly simple to implement • Functional scheme that works • Disadvantages • Can not recover from a collision (inefficient waste of medium time)

36. Carrier Sense Multiple Accesswith Collision Detection (CSMA-CD) • Procedure • Listen to medium and wait until it is free • Then start talking, but listen to see if someone else starts talking too • If a collision occurs, stop and then start talking after a random back off time • This scheme is used for hub based Ethernet • Advantages • More efficient than basic CSMA • Disadvantages • Requires ability to detect collisions

37. Collision Detection Problem • Transmit signal is MUCH stronger than received signal • Due to high path loss in the wireless environment • Impossible to “listen” while transmitting because you will drown out anything you hear • Also transmitter may not even have much of a signal to detect due to geometry

38. Carrier Sense Multiple Accesswith Collision Avoidance (CSMA-CA) • Procedure • Similar to CSMA but instead of sending packets control frames are exchanged • RTS = request to send • CTS = clear to send • DATA = actual packet • ACK = acknowledgement

39. Carrier Sense Multiple Accesswith Collision Avoidance (CSMA-CA) • Advantages • Small control frames lessen the cost of collisions (when data is large) • RTS + CTS provide “virtual” carrier sense protects against hidden terminal collisions (where A can’t hear B) A B

40. Carrier Sense Multiple Accesswith Collision Avoidance (CSMA-CA) • Disadvantages • Not as efficient as CSMA-CD • Doesn’t solve all the problems of MAC in wireless networks

41. Random Contention Access • Slotted contention period • Used by all carrier sense variants • Provides random access to the channel • Operation • Each node selects a random back off number • Waits that number of slots monitoring the channel • If channel stays idle and reaches zero then transmit • If channel becomes active wait until transmission is over then start counting again

42. 802.11: Modes of Operation • 802.11 supports two modes of operation • Distributed Coordination Function (DCF) • Point Coordination Function (PCF) • All implementations must support DCF but PCF is optional • MAC layers in 802.11 standard is shown below

43. Distributed Coordination Function (DCF) • The 802.11 standard makes it mandatory that all stations implement the DCF, a form of carrier sense multiple access with collision avoidance (CSMA/CA). • CSMA is a contention-based protocol making certain that all stations first sense the medium before transmitting. • The main goal is to avoid having stations transmit at the same time, which results in collisions and corresponding retransmissions.

44. Distributed Coordination Function (DCF) • If a station wanting to send a frame senses energy above a specific threshold on the medium (which could mean the transmission of another station), the station wanting access will wait until the medium is idle before transmitting the frame. • The collision avoidance aspect of the protocol pertains to the use of acknowledgements that a receiving station send to the sending station to verify error-free reception. • Think of this process of accessing the medium as a meeting where everyone is polite and each person only speaks when no one else is talking. • In addition, everyone who understands what the person is saying nods their head in agreement.

45. Distributed Coordination Function (DCF) • The DCF protocol is somewhat more complex than this, though. • For example, an 802.11 station utilizes information it gains from other frames that stations are sending over the wireless network. • In the control field of each frame, there is a duration field that a sending station places a value in, to indicate how long the station will require the medium. • As part of making a decision on whether to transmit a frame, a station must see that the time associated with the duration value of the last frame sent has expired, as well as sense that no physical transmission is taking place. • The duration field enables stations to reserve the medium for subsequent frames of some specific 802.11-defined frame exchanges (e.g. RTS/CTS)

46. B1 = 25 B1 = 5 wait data data wait B2 = 10 B2 = 20 B2 = 15 802.11 DCF Example with Backoff B1 and B2 are backoff intervals at nodes 1 and 2 cw = 31

47. 802.11 Contention Window • Random number selected from [0,cw] • Small value for cw • Less wasted idle slots time • Large number of collisions with multiple senders (two or more stations reach zero at once) • Optimal cw for known number of contenders & know packet size • Tricky to implement because number of contenders is difficult to estimate and can be VERY dynamic

48. 802.11 Adaptive Contention Window • 802.11 adaptively sets cw • Starts with cw = 31 • If no CTS or ACK then increase to 2*cw+1 (63, 127, 255) • Reset to 31 on successful transmission • 802.11 adaptive scheme is unfair • Under contention, unlucky nodes will use larger cw than lucky nodes (due to straight reset after a success) • Lucky nodes may be able to transmit several packets while unlucky nodes are counting down for access • Fair schemes should use same cw for all contending nodes (better for high congestion too)

49. 802.11 DCF (CSMA-CA) • Full exchange with “virtual” carrier sense (called the Network Allocation Vector) A B Sender Receiver RTS DATA Sender CTS ACK Receiver NAV (RTS) A NAV (CTS) B

50. 802.11 DCF (CSMA-CA) • Because of its nature, DCF supports the transmission of asynchronous signals. • A distinguishing factor of asynchronous signaling is that there are no timing requirements between data carrying frames. • For example, the DCF protocol doesn't make any attempt to deliver a series of data frames within any timeframe or at any instant in time. • As a result, there is a random amount of delay between each data frame transmission. • This form of synchronization is effective for network applications, such as e-mail, Web browsing and VPN access to corporate applications.