Wireless LANS & PANS

1. Wireless LANS & PANS Lecture # 3

2. Overview • Fundamentals of WLANS • IEEE 802.11 standards • HIPERLAN standard • Bluetooth • HomeRF

3. Reference Material • Ad hoc Wireless Networks, - Ch # 2 Architectures and Protocols By C. Siva Ram Murthy • Wireless Communication - Ch # 13, 14 and Networks By William Stallings • ANSI/IEEE Standard 802.11, 1999 Edition

4. Fundamentals of WLANS

5. Technical Issues of WLANS • Differences between wireless and wired transmission • Use of WLANs • Design Goals

6. Differences Between Wireless and Wired Transmission • Address is no equivalent to physical location • Wireless nodes are not stationary, address may not always refer to a particular geographical location • Dynamic topology • Mobiles nodes may often go out of reach of each other, connectivity is partial at times

7. Differences Between Wireless and Wired Transmission • Medium boundaries are not well defined • Exact reach of wireless signal can not be determined because of variations in signal strength, noise level, atmospheric interferences, mobility etc • Error-prone medium • Transmission of a node is effected by simultaneous transmission of the neighboring nodes located within the direct transmission range • Error rate is significantly higher, of the order of 10-4 as against 10-9 in fiber optic cables

8. Important • Wireless is inherently unreliable channel • One of the Solution • Develop reliable protocols at the MAC layer • It hides the unreliability that is present at the physical layer

9. Use of WLANs • Can replaced LANS • Areas affected by earthquake or other such disasters • Wiring is difficult • Historical buildings • User would be able to surf the Internet, check email, and receive Instant messages on the move • etc, etc, etc

10. Design Goals • Operational simplicity • Quickly setup and efficient access to mobile users • Power efficient operations • Operate with minimal power consumptions • Must incorporate • Power-saving features • Use appropriate technologies • Power efficient protocols etc • License free operation • Consider ISM band for its operation which do not require an explicit licensing

11. Design Goals • Tolerance to interference • Should take appropriate measures by way of selecting technologies and protocols to operate in the presence of interferences • Global usability • Both in terms of hardware and software • Should take into account the prevailing spectrum restrictions in countries across the world • Security • Inherent broadcast nature of wireless medium adds to the requirement of security features

12. Design Goals • Safety requirements • Interference to medical and other instrumentation devices • Increased power level of transmitters that can lead to health hazards • Should follow the power emission restrictions that are applicable in the given frequency spectrum • Quality of Service requirement • Should take into considerations the possibility of supporting a wide variety of designated levels of performance for multimedia traffic • Compatibility with other technologies and applications • Inter-operability among different LANs (wired and wireless) • Inter-operability among different protocols

13. Design Goals • Handoff/roaming • MAC protocols used in the wireless LAN should enable mobile stations to move from one cell to another • Dynamic configuration • MAC addressing and network management aspects of the LAN should permit dynamic and automated addition, deletion, and relocation of end systems with disruption to other users • Throughput • MAC protocols should make as efficient use as possible of the wireless medium to maximum capacity • Number of nodes • Wireless LANs may need to support hundred of nodes across without compromising over the performance

14. Network Architecture • Types of WLANS • Components of a typical WLAN • Services offered by a WLAN

15. Network Architecture Infrastructure based Infrastructure less (Ad hoc LANs) Does not need any fixed infrastructure Network can be established on the fly Nodes directly communicate with each other or forward messages through other nodes that are directly accessible • Contains Access Points (APs) which are connected via existing networks • AP can interact with wireless nodes as well as with existing wired network • Other wireless nodes, known as mobile stations (STAs), communicate via APs • APs also act as bridge with other networks

16. Network Architecture

17. IEEE 802 Protocol Layers Compared to OSI Model

18. IEEE 802 Protocol Layers Compared to OSI Model

19. Protocol Architecture • Functions of physical layer: • Encoding/decoding of signals • Preamble generation/removal (for synchronization) • Bit transmission/reception • Includes specification of the transmission medium

20. Protocol Architecture • Functions of medium access control (MAC) layer • On transmission, assemble data into a frame with address and error detection fields • On reception, disassemble frame and perform address recognition and error detection • Govern access to the LAN transmission medium • Functions of logical link control (LLC) Layer • Provide an interface to higher layers and perform flow and error control

21. Separation of LLC and MAC • Logic required to manage access to a shared-access medium not found in traditional layer 2 data link control • For the same LLC, several MAC options may be provided

22. MAC Frame Format

23. Logical Link Control • Characteristics of LLC not shared by other control protocols: • Must support multiaccess, shared-medium nature of the link • Relieved of some details of link access by MAC layer

24. LLC Services • Unacknowledged connectionless service • No flow- and error-control mechanisms • Data delivery not guaranteed • Connection-mode service • Logical connection set up between two users • Flow- and error-control provided • Acknowledged connectionless service • Cross between previous two • Datagrams acknowledged • No prior logical setup

25. IEEE 802.11 Architecture and Services • One of the most popular standards for wireless LANs • Comes under the IEEE 802.x LAN standards • Specifies the physical and MAC layer, adapted to the specific requirements of wireless LANs • The interfaces offered 802.11 to the higher layers are the same as those offered by in other 802.x standards

26. MAC Layers in IEEE 802.11 Standard

27. Components in a Typical IEEE 802.11 Network • Access point (AP) • An entity that has station functionality and provides access to the distribution system via the wireless medium for associated stations • Basic service set (BSS) • Set of stations that can remain in contact with an AP • Executing the same MAC protocol • Competing for access to the same shared wireless medium • Basic service area (BSA) • Coverage area of an AP within which member stations (STAs or MTs) may remain in communication

28. Components in a Typical IEEE 802.11 Network • Distribution system (DS) • Refers to an existing network infrastructure • A system used to interconnect a set of BSSs and integrated LANs to create an ESS • Implementation of the DS is not specified by the IEEE 802.11 standard, rather services of DS are specified • Gives lot of flexibility in the design of DS • APs are connected by means of DS • MAC protocol data unit (MPDU) • Unit of data exchanged between two peer MAC entities using the services of the physical layer

29. Components in a Typical IEEE 802.11 Network • Portals • Logical points through which non-IEEE 802.11 packets (wired LAN packets) enter the system • Necessary for integrating wireless networks with the existing wired networks • Implemented in bridge or routers, that is a part of the wired LAN and also attached to the DS • Extended service set (ESS) • BSSs, DS, and the portals together with the stations they connect constitute ESS • Independent basic service set (IBSSs) • Ad hoc LAN has only one BSS, and is termed as IBSS

30. Components in a Typical IEEE 802.11 Network

31. Extended Service Set Wired Network Portal

32. IEEE 802.11 Services • Defines nine services that need to be provided by the wireless LAN to provide functionality equivalent to that which is inherent to the wired LANs • These services provide functions that the LLC layer requires for sending MAC Service Data Unit (MSDUs) between two entities • MAC layer implements these services • Station services • Implemented in every 802.11 station, including AP • Authentication, deauthentication, privacy, MSDU delivery • DS services • Implemented in AP or in another special purpose device attached to the DS • Association, disassociation, distribution, integration, and reassociation

33. IEEE 802.11 Services • Three of the services are used to control 802.11 LAN access and confidentiality • Six of the services are used to support delivery of MAC service data units (MSDUs) between stations • Distribution services • Helps MAC layer in transferring MSDUs between MAC entities • Association-related services • Access and privacy services

34. Distribution Services • MSDU Delivery • Provided by station • MSDU is a block of data passed down from the MAC user to the MAC layer (LLC PDU) • If MSDU is too large to be transmitted in a single MAC frame, it may be fragmented and transmitted in a single frame

35. Distribution Services • Distribution • Provided by DS • Used by stations to exchange MAC frames when the frame must traverse the DS to get from a station from one BSS to a station on another BSS • If the two stations that are communicating are within the same BSS, then the distribution service logically goes through the single AP of that BSS

36. Distribution Services • Integration • Provided by distribution service • Enables transfer of data between a station on an IEEE 802.11 LAN and a station on an integrated IEEE 802.x LAN • Takes care of any address translation and media conversion logic required for the exchanged of data • The term integrated refers to a wired LAN that is physically connected to an IEEE 802.11 LAN via the integration service

37. Transition Types Based on Mobility • No transition • Stationary or moves only within BSS • Direct communication range of the communicating stations of a single BSS • BSS transition • Station moving from one BSS to another BSS in same ESS • Delivery of data to the station requires that the addressing capability be able to recognize the new location of the station • ESS transition • Station moving from BSS in one ESS to BSS within another ESS

38. Association Related Services • To deliver a message within a DS, the distribution service needs to know where the destination station is located • Specifically, the DS needs to know the identity of the AP to which the message should be delivered in order for that message to reach the destination station • To meet this requirement, a station must maintain an association with the AP within its current BSS

39. Association Related Services • Association • Establishes initial association between station and AP • Once the association is established the AP then communicate the identity and address of the station to other APs within the ESS to facilitate routing and delivery of address frames • Re-association • Enables an established association to be transferred from one AP to another, allowing a mobile station to move from one BSS to another

40. Association Related Services • Disassociation • A notification from either a station or an AP that an existing association is terminated • A station should give this notification before leaving an ESS or shutting down • MAC management facility protects itself against stations that disappear without notification

41. Access and Privacy Services • Authentication • Used by stations to establish their identity with stations they wish to communicate with • Standard does not mandate any particular authentication scheme • Deauthentication • Invoked whenever an existing authentication is terminated • Privacy • Prevents message contents from being read by unintended recipient • The algorithm specified in the standard in WEP

42. Physical Layer Different from Wired Media • IEEE 802.11 supports three options for the medium to be used at the physical layer • One is based on infrared • Other two are based on radio transmission • Use a medium that has neither absolute nor readily observable boundaries outside of which stations with conformant PHY transceivers are known to be unable to receive network frames • Are unprotected from outside signals

43. Physical Layer Different from Wired Media • Communicate over a medium significantly less reliable than wired PHYs • Have dynamic topologies • Lack full connectivity, and therefore the assumption normally made that every STA can hear every other STA is invalid • Have time-varying and asymmetric propagation properties

44. Protocol Architecture • PMD – Physical medium dependent sublayer • PLCP – Physical layer convergence protocol

45. Physical Layer • Physical layer convergence protocol (PLCP) • Provides a mechanism for transferring MAC sublayer protocol data units (MPDUs) between two or more STAs over the PMD sublayer • Defines a method of mapping the IEEE 802.11 MPDUs into a framing format suitable for sending and receiving user data and management information between two or more STAs using the associated PMD system

46. Physical Layer • Physical medium dependent sublayer (PMD) • Defines the characteristics of, and method of transmitting and receiving data through, a wireless medium (WM) between two or more STAs (modulation/demodulation, encoding/decoding etc)

47. Physical Layer Carrier Sensing Mechanisms • Performed either physically or virtually • Physical layer sensing is through clear channel assessment (CCA) signal provided by the PLCP • CCA signal is generated based on sensing of the air interface • Either sense the detected bits in the air • Slow, but more reliable • OR check the received signal strength (RSS) of the carrier against a threshold • Potentially create a false alarm caused by measuring the level of interference

48. Physical Layer • IEEE 802.11 supports different options for the medium to be used at the physical level • One is based in infrared • Operating at wavelength 850-950 nm range, at data rates of 1 Mbps and 2 Mbps using pulse position modulation (PPM) scheme • Other five are based on the radio transmission • IEEE 802.11 FHSS • IEEE 802.11 DSSS • IEEE 802.11a OFDM • IEEE 802.11b HR-DSSS • IEEE 802.11g OFDM

49. Basic MAC layer Mechanisms • Covers three functional areas • Reliable data delivery • Access control • Security • Also supports many auxiliary functionalities • Roaming • Authentication • Taking care of power conservations

50. Reliable Data Delivery • Physical and MAC layers is subject to considerable unreliability • Noise, interference and other propagation effects • Even with error-correction codes, a number of MAC frames may not received correctly • Can the situation be dealt with reliability at higher layer protocols (TCP)? • More efficient to deal with errors at the MAC level than higher layer • Timers are of the order of seconds