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Introduction to Wireless LANs: Characteristics and Architectures

This text provides an introduction to wireless LANs, including their main characteristics, specific characteristics of the wireless environment, and different architectures. It also covers the IEEE 802.11 standard and its design requirements.

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Introduction to Wireless LANs: Characteristics and Architectures

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  1. Modulo 11 Wireless sensors and actuators networks Igor Bisio - Franco Davoli – Fabio Lavagetto franco.davoli@unige.it Master Universitario di II Livello "Internet of Things and Big Data" A.A.2018-2019 Università degli Studi di Genova - www.master-iot.it

  2. Obiettivi e Risultati attesi Obiettivi (per la parte di F. Davoli) • Protocolli di accesso multiplo • Reti WLAN e WPAN • WiFi e Bluetooth Risultati attesi • Conoscenza di base di architettura e protocolli per WiFi e Bluetooth Prerequisiti: Conoscenze di base di reti di TLC (architetture e protocolli) Master Universitario di II Livello "Internet of Things and Big Data" A.A.2018-2019 Università degli Studi di Genova - www.master-iot.it

  3. TESTI CONSIGLIATI PER CHI VOLESSE APPROFONDIRE J. Kurose, K. Ross, COMPUTER NETWORKING: A Top-Down Approach, 6th Ed., PearsonEducation, UpperSaddle River, NJ, USA, 2013; ISBN-13: 978-0-13-285620-1. Master Universitario di II Livello "Internet of Things and Big Data" A.A.2018-2019 Università degli Studi di Genova - www.master-iot.it

  4. IntroductionWireless LAN • Wireless LANs (WLANs) are wireless networks that support services and extensions typically offered by wired LANs; their main characteristics are: • Terminal stations (and sometimes also intermediate nodes) use wireless connections; • They are generally designed as mobile networks, but typically the mobility is considered as “slow mobility”; • The main goal of these networks is making the cabling easier and freeing the user from fixed work locations; • They are also used as access networks.

  5. IntroductionSpecific characteristics of the wireless environment • “Difficult” medium type • Noise and interference • The quality is variable in space and time • Shared with other unsolicited WLAN elements • Shared with non-WLAN elements • One cannot assume full connectivity (hidden stations) • Several different international regulations

  6. IntroductionSpecific characteristics of the wireless environment • Mobility support - which implies: • change of the connection characteristics and availability; • battery power supply: power management; • mobility management. • Security • No physical boundaries; • Overlapping LANs.

  7. Wireless-LAN • Among the different proposed standards, the following should be listed: • IEEE 802.11 • HIPERLAN (European HIghPERformance LAN) • (Bluetooth) • HomeRF

  8. WirelessLAN – IEEE 802.11 • The IEEE 802.11 standard has been published in 1997 • initially it envisaged the use of the 2.4 GHz ISM (Industrial, Scientific and Medical) band and the transmission speed of 1-2 Mbps. • Updated in 1999 (IEEE 802.11:1999) • introduction of new modulations and higher speeds; • definition of two new versions: 802.11a and 802.11b. • Also adopted in 1999 by ISO/ IEC as 8802-11:1999. • In 2003 a further evolution led to the definition of the 802.11g specification. • In 2009 an additional evolution has been released with the 802.11n specifications (which were already available in a first draft version in 2007). • This standard is also called Wireless Fidelity (Wi-Fi) from the name of an association of manufacturers that promotes and verifies the interoperability of products.

  9. IEEE 802.11Design requirements • A single MAC that supports different physical layers • Single and multiple channels • Different characteristics of "Medium sense" • Allowing the superposition of multiple networks in the same geographical area • Providing robustness to interference • Solving the problem of hidden nodes

  10. OSI Model Application Presentation Session Trasport Network Link Physical IEEE 802.11 • The specification of the protocol includes the two lower layers of the ISO/OSI model and it is part of the group of the IEEE 802 standards defined for the LAN IEEE 802 Model HigherLevels 802.11 Interface to the higher layers, flow and error control • Generation and removal of TX frames • Error control • Control of the the medium access LLC 802.2 MAC 802 Standards Physical • Encoding and decoding of signals • Generation / removal of the preambles • Transmission / reception of the bits 802.11 Medium Tx TX medium and topology

  11. Network architecture • The standard defines two different types of architectures: • Independent Basic Service Set (IBSS); • Extended Service Set (ESS). • The basic element is represented by the Basic Service Set (BSS), the area within which all stations can communicate • a station may move within the BSS, but it can no longer communicate directly with the others if it leaves it.

  12. Independent Basic Service Set • An IBSS consists of an Independent BSS: • without any backbone infrastructure; • with at least two stations inside. • An architecture of this type is defined as “ad-hoc network” • it can be deployed very quickly. • The ad-hoc architecture meets the communication needs of few users located in small areas • the coverage area is generally very limited. IBSS

  13. IEEE 802.x LANs Extended Service Set Extended Service Set Portal Distribution System AP AP Basic Service Set Basic Service Set

  14. Extended Service Set • The Basic Service Set (BSS) consists of a set of stations that compete for access to the shared transmission medium. • The AccessPoint(AP) acts as a bridge and allows the connection of a BSS to a DS. • The DistributionSystem (DS) is a backbone used for connecting several BSSs and may consist of a wired (e.g., switched) or wireless LAN. • The ExtendedServiceSet(ESS) consists of more BSSs connected to each other through a DS; the ESS appears as a single LAN to the LLC layer. • The Portal connects the WLAN (ESS) with other wired LANs.

  15. Extended Service Set • Within an ESS, the different BSS’s can be physically distinguished according to different criteria: • overlapping BSSs • to provide continuous coverage; • physically disjoint BSSs • co-located BSSs (different BSSs in the same area) • can provide redundancy to the network or allow higher performance.

  16. Mobility • The 802 .11 manages the mobility of stations by distinguishing three types of transitions: • Static: the station is immobile or moves only within the area of a single BSS; • Transition between BSSs: in this case, the station moves between two different partially overlapping BSSs belonging to the same ESS • the MAC is able to handle this situation in a transparent manner with respect to the higher layers; • Transition between ESSs: the station moves between BSSs belonging to two different ESSs • the station may move, but the MAC is not able to maintain connectivity.

  17. MAC SAP MAC Sublayer MAC Sublayer Management Entity (MLME) Station Management Entity MLME SAP PHY SAP MLME PLME SAP PLCP Sublayer PHY Sublayer Management Entity (PLME) PMD SAP PLME SAP PMD Sublayer Protocol architecture User Plane Management Plane Link Layer Physical Layer

  18. Physical Layer • PMD (Physical Medium Dependent) sublayer • defines the different transmission media; • deals with the transmission/reception of packets; • realizes the carrier sense on the basis of the medium. • PLCP (Physical Medium Convergence Protocol) sublayer • provides a common interface to the various transmission media; • defines a methodology with which to transform the MPDU in a frame suitable for Tx / Rx of user and control information through the PMD.

  19. Link layer • The MAC Sublayer has the following functions: • it handles multiple access and contention for the medium for transmission (CSMA/CA) • which is the same for all the transmission media; • it provides services with and without delay constraints • DCF and PCF; • it handles fragmentation; • it realizes encryption.

  20. Management plane • Station Management Entity (SME) • is an inter-entity layer • resides on a separate plane; • its functions are not specified in the standard; • usually has to deal with • collecting information from different layers; • setting the values of specific parameters for each layer. • Management entities of the individual layers • represent the interfaces through which management features are called: • MAC sublayer management; • PHY layer management.

  21. Standard structure Logical Link Control Service without contention Contention service HCF Contention Access (EDCA) Point Coordination Function (PCF) HCF Controlled Access (HCCA) MAC Distributed Coordination Function (DCF) DSSS (802.11b) OFDM (802.11g) MIMO+ (802.11n) MIMO+++ (802.11ac first phase) FHSS DSSS OFDM (802.11a) Infrared PHY 2.4 GHz 1-2 Mbps 2.4 GHz 5.5-11 Mbps 1-2 Mbps 2.4 GHz6-54 Mbps 2.4/5.5 GHz6-600 Mbps 5.5 GHz290-1300 Mbps 2.4 GHz 1-2 Mbps 5.5 GHz6-54 Mbps

  22. MAC services • The 802.11 standard includes a set of services that the LLC layer requires for transferring MAC Service Data Units (MSDU) between two LLC entities in the network. • The 802.11 MAC realizes and offers (to the LLC) these services. • They can be classified in two main categories • Station Services • Authentication, De-authentication, MSDU Delivery, Confidentiality, DFS, TPC, Higher Layer Timer Synchronizationand QoS Traffic Scheduling; • These services are available for both IBSS and ESS networks; • Distribution System Services • Association, Disassociation, Distribution, Integration and Re-association; • These services are available only for ESS networks.

  23. ServicesAuthentication • It is the mechanism used to establish the identity of the station that needs to communicate. • Must provide a level of safety equal to that of wired LANs. • Each 802.11 station must authenticate before being enabled to exchange data ("association") with another station. • 802.11 provides different authentication mechanisms.

  24. ServicesDe-authentication • Used to end an existing authentication to another station. • The station that wants to end this relationship sends a notification frame. • The service cannot be refused by the station receiving the notification.

  25. ServicesConfidentiality • In wireless networks the traffic can be observed by anyone in the coverage area. • The standard provides for the optional use of encryption to ensure the confidentiality of communications. • There are several mechanisms, with different levels of security: • WEP (RC4+CRC32), • TKIP (RC4+Michael) • CCMP (AES+CBC-MAC). • The default configuration of the interface is “sent without encryption”. If one recalls the Privacy services, the station is configured for encryption and accepts no more “plaintext” frames.

  26. ServicesDFS/TPC • Dynamic Frequency Selection (DFS) • WLANs operating in the 5 GHz band must implement a mechanism to avoid interference with radar systems and to ensure the uniform use of available channels; • detection of radar transmissions and stopping the use of the channel, request and publication of measures on available channels, assistance in migrating from BSS to IBSS. • Transmit Power Control (TPC) • required for WLANs operating in the 5 GHz band to reduce interference with satellite services; • association based on the ability to manage the power control, specification of the maximum power level for each channel, adaptation of transmission power on the basis of regulations and the attenuation of the channel.

  27. ServicesServices for QoS support • Higher Layer Timer Synchronization • Some applications require a very fine synchronization of their timers • transport and representation of audio/video flows • the standard provides a MAC service that allows an accurate synchronization • QoS traffic scheduling • allows provision of end-to-end QoS; • provides different ways to access the medium • distributed bandwidth allocation (with contention), • centralized bandwidth allocation managed by the AP; • provides the ability to perform access control.

  28. ServicesAssociation • In order to deliver a packet within the ESS, the Distribution Service needs to know the location of the destination station. • In particular, it is necessary to know the identity of the AP to which to deliver the message. • For this reason it is necessary that each station carries out a procedure for associating with the AP of the BSS in which it is located.

  29. ServicesReassociation • The Reassociation service allows a station to change its association from one AP to another, supporting the transition between different BSSs within the same ESS. • It corresponds to the handoff procedure in cellular networks. • The stations measure the power with which they receive the control messages (beacon) from the AP to decide which BSS to join.

  30. ServicesDisassociation • It consists of the notification of the end of the association. • A station performs Disassociation before shutting down or going out from the ESS. • An AP can disassociate all the stations before being switched off for maintenance operations. • The stations should always disassociate themself before shutting down • disassociation protects the MAC layer from the sudden "disappearance" of stations previously registered.

  31. ServicesDistribution • The Distribution service is used by stations to exchange packets that must traverse the DS. • The APs know the locations of the different stations thanks to the service of Association and they are able to exchange packets through the DS. • The operating mechanism of the DS is not defined in the standard. • If the stations belong to the same BSS, the service of Distribution logically involves only the AP of that BSS.

  32. IEEE 802.x LAN ServicesDistribution Extended Service Set Portal Distribution System AP AP D Basic Service Set Basic Service Set S

  33. ServicesIntegration • The Integration service allows the transfer of data between the stations on the 802.11 LAN and those of other IEEE 802.x LANs. • The wired LAN is physically connected to the DS and its stations can be logically connected by using the service of Integration. • The Integration service decides the possible address translation and adaptation to potentially different media.

  34. Physical Layer Logical Link Control Service without contention Contention service Point Coordination Function (PCF) HCF Controlled Access (HCCA) HCF Contention Access (EDCA) MAC Distributed Coordination Function (DCF) DSSS (802.11b) OFDM (802.11g) MIMO+ (802.11n) MIMO+++ (802.11ac) FHSS DSSS OFDM (802.11a) Infrared PHY 2.4 GHz 1-2 Mbps 2.4 GHz 5.5-11 Mbps 1-2 Mbps 2.4 GHz6-54 Mbps 2.4/5.5 GHz6-600 Mbps 5.5 GHz270-6000 Mbps 2.4 GHz 1-2 Mbps 5.5 GHz6-54 Mbps

  35. Transmission media and terminals • The standard provides for transmission through the use of electromagnetic waves in the ether: • radio • infrared • Supported Terminals: • Fixed, movable, mobile at pedestrian and (possibly) vehicular speed.

  36. Physical Layer • Transmission speed • 802.11 original specifications provided for transmission at 1 and 2 Mbps • in the 2.4 GHz ISM band for radio systems; • at wavelengths between 850 and 950 nm for infrared systems; • 802.11b brings speed to 5.5 and 11 Mbps for radio systems • still uses the 2.4 GHz ISM band; • with the introduction of the 802 .11a the speeds allowed are 6, 9, 12, 18, 24, 36, 48 and 54 Mbps • 6, 12 and 24 are mandatory; • the bandwidth used is around 5.5 GHz • 802.11g allows the same 11a speeds but in the 11b band (2.4 GHz)

  37. Physical Layer • The 802.11n standard applies to both frequencies of 5.5 GHz and 2.4 GHz. • It is able to manage functions compatible with previous standards and to adapt its rate (starting from 6 Mbps) as a function of both the channel conditions and the bandwidth availability. • The maximum speed achieved depends on various factors, but it starts from a "minimum" of 64 Mbps to achieve the most extreme version of 600 Mbps • Currently, the maximum speeds found in commercial equipment rarely attain the theoretical maximum also in optimal conditions.

  38. PhysicalLayer • The currentversion of the 802.11ac standard is IEEE Std P802.11ac-2013 (and followingupdates). Itapplies to the 5.5 GHz frequencyrange, whichislesscrowded and notshared with Bluetooth. • Itbrings the new techniquesintroduced in 11n to the extreme , in order to compete with currentcabled networks. • The maximum speedstarts from a “minimum” 290 Mbps, to reach 6 Gbps in the mostadvancedversion.

  39. PhysicalLayer

  40. Physical Layer • Coverage Areas • with omnidirectional antennas: • 50-100 m for 802.11b; • 15-30 m for 802.11a/g/n; • with directional and high gain antennas (point-to-point), one can get up to about 40 km. • Transmission bands used: • ISM 2.4 GHz, 2.4 - 2.4835 GHz; • 5.5 GHz, 5.15 - 5.825 GHz. • Transmission techniques: • Spread Spectrum: FHSS, DSSS; • OFDM.

  41. Physical LayerFrequency Hopping Spread Spectrum • The technique is to change the transmission frequency using pseudorandom sequences common to all stations. • The overall spectrum is divided into 79 channels of 1 MHz each • in Japan only 23 channels are available . • A designated station generates a list with the 79 frequencies in a specific order • the hop rate should be a minimum of 2.5 jumps / second (U.S.); • each "jump" (hop) must be at least 6 channels (5 in Japan); • the different possible sequences (78) are obtained by moving the start of the sequence of an offset and by re-computing it modulo 79. • The 78 sequences are organized into 3 sets of 26 elements • there may be a maximum of 26 co-located networks. • The throughput increases up to 15 co-located networks, in high traffic conditions.

  42. Physical LayerFrequency Hopping Spread Spectrum • It allows a good robustness to fading due to multipath (quite common in the indoor environment). • Multiple propagation paths, by interfering with one another, create frequency selective fading. • The fluctuations are related to adjacent frequencies, but they become uncorrelated after a few MHz in the indoor environments.

  43. Physical LayerDirect Sequence Spread Spectrum • The signal related to a symbol is "spread" in a sequence: • Wider band • Power less "dense”

  44. Physical LayerDirect Sequence Spread Spectrum • Symbol rate 1 MHz / 1.375 MHz • Chipping rate 11 MHz • 802 .11 (initial version) prescribed a Barker sequence of 11-bits; • 802 .11b uses a CCK (Complementary Code Keying) encoding. • A total of 14 20MHz-channels, gathered in pairs • in Europe, one of the channels of the first pair cannot be used • only 13 channels are used; • in Japan only one channel is usable ; • the channels of each pair can operate simultaneously without interference.

  45. Physical LayerOrtogonal Frequency Division Multiplexing • The signal is distributed to 48 subcarriers (in 11g, in 11n they are 52). • Each subcarrier is orthogonal with respect to the other • the various signals do not overlap. • The modulation used in each subcarrier determines the transmission rate. • Advantages: • high spectral efficiency; • resistance to radio interference and multi-path distortion.

  46. PhysicalLayer802.11n • 802.11n was a major advance over 802.11a; it introduced several major MAC sublayer and PHY layer advances, namely: • Multiple input, multiple output (MIMO), which increases speed by means of Spatial Multiplexing (though only directed to a single address – Single User MIMO – SU-MIMO) and increases reliability at the AP receiver (owing to diversity created by multi-path). • Channel bonding, doubling the channel bandwidth from 20 to 40 MHz. • Aggregation of multiple MAC PDUs or SDUs (A-MPDU, A-MSDU). Source: “802.11ac: The Fifth Generation of Wi-Fi”, Cisco Technical White Paper, Aug. 2012.

  47. PhysicalLayer802.11n (cont’d) • 802.11ac achieves its raw speed increase by pushing on three different dimensions: • More channel bonding, increased from the maximum of 40 MHz in 802.11n, and now up to 80 or even 160 MHz (for 117% or 333% speed-ups, respectively) • Denser modulation, now using 256 quadrature amplitude modulation (QAM), up from 802.11n’s 64QAM (for a 33% speed burst at shorter, yet still usable, ranges) • More multiple input, multiple output (MIMO). Whereas 802.11n stopped at four spatial streams, 802.11ac goes all the way to eight (for another 100% speed-up), and can adopt Multi-User MIMO (MU-MIMO). Source: “802.11ac: The Fifth Generation of Wi-Fi”, Cisco Technical White Paper, Aug. 2012.

  48. PhysicalLayer802.11n (cont’d) Multi-User MIMO Source: “802.11ac: The Fifth Generation of Wi-Fi”, Cisco Technical White Paper, Aug. 2012.

  49. Physical LayerPLCP Sublayer • The PLCP sublayer receives the 802.11 packets and creates the frame for transmission (PPDU, PLCP Protocol Data Unit) • The 802.11b standard provides for the possibility to use two different headers: • Long, mandatory • Short, optional. • Long Preamble and Header: • sync (128 bits), an alternating sequence of 0 and 1 • the receiver "locks" in this cycle; • start frame delimiter (16-bits), 1111001110100000 marks the actual start of the frame;

  50. Physical LayerPLCP sublayer • signal (8 bits) indicates the transmission speed of the frame • the binary value is equal to rate/100 kbps; • 0x0A 1 Mbps, 2 Mbps 0x14, 0x6E 11 Mbit, etc..; • for compatibility, the fields introduced by the PLCP are always transmitted at 1 Mbps; • service (8 bits) • bit 7 to support the speed of 11 Mbps; • bit 3 indicates the modulation (CCK or PBCC); • length (16 bits) indicates the number of ms needed to transmit the content of the PPDU • the receiver uses this value to determine the end of the frame; • service field indicates how this value was calculated; • frame check sequence (16-bit), CRC to protect the header of the PPDU • PSDU, which coincides with the MAC packet.

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