WIRELESS LAN (WLAN) Efthyvoulos C. Kyriacou (PhD) Assoc. Prof. Computer Science and Engineering Department

Transcription

1 ACOE 422 Wireless Computer Networks WIRELESS LAN (WLAN) Efthyvoulos C. Kyriacou (PhD) Assoc. Prof. Computer Science and Engineering Department Resources: Mobile Communications, J. Schiller book slides Prof. A. Pitsillides Univ. of Cyprus

2 WIRELESS LAN (WLAN) Selected topics Introduction WLAN Standards WLAN definition WLAN characteristics WLAN design goals Infrared vs radio transmission Infrastructure based vs ad hoc networks IEEE WLAN Roaming WLAN Security Other technologies 2

3 Introduction Several WLAN standards: IEEE b offering 11 Mbit/s at 24GHz 2.4 The same radio spectrum is used by Bluetooth A short range technology to set up wireless personal area networks with gross data rates less than 1 Mbit/s IEEE released a new WLAN standard, a, operating at 5 GHz and offering gross data rates of 54Mbit/s uses the same physical layer as HiperLAN2 does tries to give QoS guarantees IEEE g offering up to 54 Mbit/s at 2.4 GHz. 3

4 WLAN Standards d Wireless LAN 2.4 GHz 5 GHz b g HiSWANa a HiperLAN2 (54 Mbps) (2 Mbps) (11 Mbps) (22-54 Mbps) (54 Mbps) (54 Mbps) (54 Mbps) HomeRF 2.0 Bluetooth HomeRF 1.0 (10 Mbps) (1 Mbps) (2 Mbps) e f h i (QoS) (IAPP) (TPC-DFS) (Security) 4

5 WLAN definition A fast growing market introducing the flexibility of wireless access into office, home, or production environments. Typically restricted in their diameter to buildings, a campus, single rooms etc. The global goal of WLANs is to replace office cabling and, additionally, to introduce a higher flexibility for ad hoc communication in, e.g., group meetings. 5

6 WLAN characteristics Advantages: very flexible within radio coverage ad hocnetworks without previousplanningplanning possible wireless networks allow for the design of small, independent devices more robust against disasters (e.g., earthquakes, fire) Disadvantages: typically very low bandwidth compared to wired networks (~11 54 Mbit/s) due to limitations in radio transmission, higher error rates due to interference, and higher delay/delay variation due to extensive error correction and error detection mechanisms offer lower QoS many proprietary solutions offered by companies, especially for higher bit rates, standards take their time (e.g., IEEE ) slow standardization procedures standardized functionality plus many enhanced features these additional features only work in a homogeneous environment (i.e., when adapters from the same vendors are used for all wireless nodes) products have to follow many national restrictions if working wireless, it takes a very long time to establish global solutions 6

7 WLAN design goals global, seamless operation of WLAN products lowpower for battery use (special power saving modes and power management functions) no special permissions or licenses needed (license free band) robust transmission technology easy to use for everyone, simple management protection of investment in wired networks (support the same data types and services) security no one should be able to read other s data, privacy no one should be able to collect user profiles, safety low radiation 7

8 Infrared vs radio transmission Infrared light uses IR diodes, diffuse light reflected at walls, furniture etc, or directed light if a LOS exists btn sender and receiver Advantages simple, cheap, available in many mobile devices (PDAs, laptops, mobile phones) no licenses needed Disadvantages interference by sunlight, heat sources etc. many things shield or absorb IR light cannot penetrate obstacles (e.g., walls) low bandwidth (~115kbit/s 115kbit/s, 4Mbit/s) Example IrDA (Infrared Data Association) interface available everywhere Radio typically using the license free frequency band at 2.4 GHz Advantages experience from wireless WAN (microwave links) and mobile phones can be used coverage of larger areas possible (radio can penetrate (thinner) walls, furniture etc.) higher transmission rates (~11 54 Mbit/s) Disadvantages very limited license free frequency bands shielding more difficult, interference with other senders, or electrical devices Example IEEE , HIPERLAN, Bluetooth 8

9 Infrastructure based vs ad hoc wireless networks (I) Infrastructurebased wireless networks AP AP wired network AP: Access Point AP Infrastructure networks provide access to other networks. Communication typically takes place only between the wireless nodes and the access point, but not directly between the wireless nodes. The access point does not just control medium access, but also acts as a bridge to other wireless or wired networks. 9

10 Infrastructure based vs ad hoc wireless networks (II) Several wireless networks may form one logical wireless network: The access points together with the fixed network in between can connect several wireless networks to form a larger network beyond actual radio coverage. Network functionality lies within the access point (controls network flow), whereas the wireless clients can remain quite simple. Use different access schemes with or without collision. Collisions i may occur if medium access of the wireless nodes and the access point is not coordinated. If only the access point controls medium access, no collisions are possible. Useful lfor quality of service guarantees (e.g., minimum i bandwidth hfor certain nodes) The access point may poll the single wireless nodes to ensure the data rate. 10

11 Infrastructure based vs ad hoc wireless networks (III) Infrastructure based wireless networks lose some of the flexibility l wireless networks can offer in general: They cannot be used for disaster relief in cases where no infrastructure isleft left. 11

12 Infrastructure based vs ad hoc wireless networks (III) Ad hoc wireless networks No need of any infrastructure to work greatest possible flexibility Each node communicate with other nodes, so no access point controlling medium access is necessary. The complexity of each node is higher implement medium access mechanisms, forwarding data 12

13 Infrastructure based vs ad hoc wireless networks (IV) Nodes within an ad hoc network can only communicate if they can reach each other physically if they are within each other s radio range if other nodes can forward the message IEEE and HiperLAN2 are typically infrastructurebased networks, which additionally support ad hoc networking Bluetooth is a typical wireless ad hoc network 13

14 IEEE () (I) As the standards number indicates, this standard belongs to the group of 802.x LAN standards. This means that the standard specifies the physical and medium access layer adapted dto the special ilrequirements of wireless LANs, but offers the same interface as the others to higher h layers to maintain i interoperability. The primary goal of the standard was the specification of a simple and robust WLAN which offers time bounded and asynchronous services. 14

15 WLAN components Figure 2.11 Photographs of popular b WLAN equipment. Access points and a client card are shown on left, and PCMCIAClient Client card is shownon on right. (Courtesy ofcisco Systems, Inc.) 15

16 IEEE (II) System Architecture of an infrastructure network STA 1 ESS LAN 802.x LAN BSS 1 Access Portal Point Distribution System Access Point BSS 2 STA LAN STA 3 Station (STA) terminal with access mechanisms to the wireless medium and radio contact to the access point Basic Service Set (BSS) group of stations using the same radio frequency Access Point station integrated into the wireless LAN and the distribution system Portal bridge to other (wired) networks Distribution System interconnection network to form one logical network (EES: Extended Service Set) based on several BSS 16

17 IEEE (III) Stations can select an AP and associate with it. The APs support roaming (i.e., changing access points), the distribution system then handles data transfer between the different APs. Furthermore, APs provide synchronization within a BSS, support power management, and can control medium access to support time bounded service. 17

18 IEEE (IV) IEEE allows the building of ad hoc networks between stations, thus forming one or more BSSs. In this case, a BSS comprises a group of stations using the same radio frequency. Several BSSs can either be formed via the distance between the BSSs or by using different carrier frequencies. 18

19 IEEE (V) IEEE standard mobile terminal fixed terminal application TCP IP LLC access point LLC infrastructure network application TCP IP LLC MAC MAC MAC MAC PHY PHY PHY PHY 19

20 IEEE (VI) Protocol architecture Applications should not notice any difference apart from the lower bandwidth and perhaps higher access time from the wireless LAN. WLAN behaves like a slow wired LAN. Consequently, the higher layers (application, TCP, IP) look the same for the wireless node as for the wired node. The differences are in physical and link layer different media and access control 20

21 IEEE (VII) The physical layer provides a carrier sense signal, handles modulation and encoding/decoding gof signals. The basic tasks of the MAC medium access control protocol comprise medium access, fragmentation of user data, and encryption. The standard also specifies management layers. The MAC management supports the association iti and reassociation of a station to an access point and roaming between different APs. Furthermore, it controls authentication mechanisms, encryption, synchronization of a station with regard to an AP, and power management to save battery power. 21

22 Physical layer IEEE (VIII) Includes the provision of the Clear Channel Assessment CCA signal (energy detection). This signal is needed for the MAC mechanisms controlling medium access and indicates if the medium is currently idle. 22

23 IEEE (IX) MediumAccess Control The basic services provided by the MAC layer are the mandatory asynchronous data service and an optional time bounded service. IEEE offers only the asynchronous data service in ad hoc network mode Both servicetypes can be offered using an infrastructure based network together with the access point coordinating medium access. The asynchronous service supports broadcast and multicast packets, and packet exchange is based on a best effort model dl no delay bounds can be given for transmission cannot guarantee a maximum access delay or minimum transmission bandwidth Need for a time bounded service provision 23

24 IEEE (X) Medium Access Control (cnt d) Three basic access mechanisms have been defined for IEEE CSMA/CA (mandatory) Optional method avoiding the hidden terminal problem A contention free polling method for time bounded service access point polls terminals according to a list The first two methods are also summarized as distributed coordination function (DCF) The third method is called point coordination function (PCF) DCF only offers asynchronous service, while PCF offers both asynchronous and time bounded dservice, but needs an access point to control medium access and to avoid contention. 24

25 IEEE (XI) Medium Access Control o (cnt d) The medium can be busy or idle (detected by the CCA) If the medium is busy this can be due to data frames or other control frames During a contention phase several nodes try to access the medium Short inter frame spacing (SIFS) the shortest waiting time for medium access defined for short control messages (e.g., ACK of data packets) DCF inter frame spacing (DIFS) the longest waiting time used for asynchronous data service within a contention period SIFS + two slot times PCF inter frame spacing (PIFS) an access point polling other nodes only has to wait PIFS for medium access (for a time bounded service) SIFS + one slot time 25

26 IEEE (XII) Medium Access Control (cnt d) The mandatory access mechanism of IEEE is based on carrier sense multiple access with collision avoidance (CSMA/CA). a random access scheme with carrier sense (with the help of the Clear Channel Assessment CCA signal of the physical layer) and collision avoidance through random back off. The standard defines also two control frames: RTS: Request To Send CTS: Clear To Send 26

27 IEEE (XIII) Broadcast data transfer (DCF) DIFS DIFS contention window (randomized back-off mechanism) medium busy next frame direct access if medium is free DIFS slot time station ready to send starts sensing the medium (Carrier Sense based on CCA-Clear Channel Assessment) if the medium is free for the duration of a Distributed Inter-Frame Space (DIFS), the station can start sending if the medium is busy, the station has to wait for a free DIFS, then the station must additionally wait a random back-off time (collision avoidance) if another station occupies the medium during the back-off time of the station, the back-off timer stops (fairness during the next phase this node will continue its timer from where it stopped) t 27

28 IEEE (XIV) Unicast data transfer sender receiver other stations DIFS data SIFS waiting time ACK DIFS contention station has to wait for DIFS before sending data data receivers acknowledge after waiting for a duration of a Short Inter-Frame Space (SIFS), if the packet was received correctly t 28

29 IEEE (XV) Sending unicast packets with RTS/CTS control frames sender receiver DIFS RTS SIFS CTS SIFS data SIFS ACK NAV (RTS) DIFS other NAV (CTS) data stations t defer access contention station can send RTS with reservation parameter after waiting for DIFS (reservation determines amount of time the data packet needs the medium and the ACK related to it). Every node receiving this RTS now has to set its net allocation vector it specifies the earliest point at which the node can try to access the medium again acknowledgement via CTS after SIFS by receiver (if ready to receive) sender can now send data at once, acknowledgement via ACK Other stations store medium reservations distributed via RTS and CTS 29

30 Masters thesis dgoodman/fainberg.pdf 30

31 31

32 Coverage Figure 2.13 A predicted coverage plot for three access points in a modern large lecture hall. (Courtesy of Wireless Valley Communications, Inc., 2000, all rights reserved.) 32

33 IEEE enhancements () (I) IEEE b 11Mbit/s in 24GH 2.4GHz frequency band widely used IEEE a offers up to 54 Mbit/s 5 GHz band Shading is much more severe compared to 2.4 GHz Depending on the SNR, propagation conditions and the distance bt between sender and receiver, data rates may drop fast 33

34 IEEE enhancements (II) IEEE e MAC enhancements for providing some QoS No QoS in the DCF operation mode Some QoS guarantees can be given only via polling using PCF For applications such as audio, video, or media stream, distribution IEEE g service classes have to be provided For this reason, MAC layer must be enhanced offers up to 54 Mbit/s 2.4 GHz band Benefits from the better propagation characteristics at 2.4 GHz compared to 5 GHz Backward compatible to b 34

35 Reassociation Response Reassociation Request 35 WLAN Roaming (I)

36 WLAN Roaming (II) No or bad connection? Then perform: Scanning scan the environment, i.e., listen into the medium for beacon signals or send probes into the medium and wait for an answer Reassociation Request station sends a request to one or several AP(s) Reassociation Response success: AP has answered, station can now participate failure: continue scanning AP accepts Reassociation Request signal the new station to the distribution system the distribution system updates its data base (i.e., location information) typically, the distribution system now informs the old AP so it can release resources 36

37 WLAN Roaming (III) L2 handover If handover from one AP to another belonging to the same subnet, then handover is completed at L2 L3 handover If new AP is in another domain, then the handover must be completed at L3, due to the assignment of an IP belonging to the new domain hence routing to the new IP. Mobile IP deals with these issues more later 37

38 WLAN Security (I) Not so efficient i compared with Ethernet security due to the nature of the medium & the requirements of the users Security mechanisms Service Set Identifiers (SSID) Used to name the network and provide initial authentication for each client Wired Equivalent Privacy Pi (WEP) Data encryption technique using shared keys and a pseudorandom number as an initialization vector 64 bit key level encryption BUT several vendors now support 128 bit key level encryption Also a VPN could operate on top of the WLAN providing increased security IEEE developing new standards e (Enhanced Security, QoS) i (Advanced Encryption Standard AES) Requires physical replacement of Access Points and WLAN Cards 38

39 WLAN Security (II) Late-2001 Mid-2002 Q Q IEEE 802.1X ratified Mutual Authentication Key Management WEP Encryption WEP Attacks go public Rapid re-keying 802.1X authentication WEP Encryption WEP key tumbling via 802.1X key management Wi-Fi Protected Access 802.1X Authentication 802.1X key Management TKIP data protection IEEE i 802.1X authentication Enhanced 802.1X key management AES-based Encryption Enhanced support infrastructure 39

40 OTHER WIRELESS TECHNOLOGIES 40

41 HIPERLAN High Performance LAN (I) The European Telecommunications Standards Institute (ETSI) standardized HIPERLAN as a WLAN allowing for node mobility and supporting ad hoc and infrastructurebased topologies. It is a wireless LAN supporting priorities and packet life time for data transfer at 23.5 Mbit/s, including forwarding mechanisms, topology discovery, user data encryption, network identification and power conservation mechanisms. HIPERLANs operate at GHz with a range of 50m in buildings at 1 W transmit power. 41

42 HIPERLAN High Performance LAN (II) The service offered by a HIPERLAN is compatible with the standard MAC services known fromieee x LANs. The HIPERLAN Channel Access Control mechanism was specifically designed dto provide channel access with ih priorities. The CAC contains the access scheme EY NPMA, which is unique for HIPERLAN. 42

43 HIPERLAN High Performance LAN (III) Elimination yield non preemptive priority multiple access (EY NPMA) not only a complex acronym, but also the heart of the channel access providing priorities and different access schemes. divides the medium access of different competing nodes into three phases: Prioritization: i i i Determine the highest h priority i of a data packet ready to be sent on competing nodes Contention: Eliminate all but one of the contenders, if more than one sender has the highest current priority. Transmission: Finally, transmit the packet of the remaining node. 43

44 HIPERLAN High Performance LAN (IV) The contention phase is further subdivided into an elimination phase and a yield phase. The purpose of the elimination phase is to eliminate as many contending nodes as possible. The result is a more or less constant tnumber of remaining i nodes, almost tindependent d of the initial number of competing nodes. The yield phase completes the work of the elimination phase with the goal of only one remaining node. 44

45 HIPERLAN High Performance LAN (IV) The contention phase is further subdivided into an elimination phase and a yield phase. The purpose of the elimination phase is to eliminate as many contending nodes as possible. The result is a more or less constant tnumber of remaining i nodes, almost tindependent d of the initial number of competing nodes. The yield phase completes the work of the elimination phase with the goal of only one remaining node. 45

46 BLUETOOTH () (I) Bluetooth technology aims at so called ad hoc piconets, which are local area networks with a very limited coverage and without the need for an infrastructure. Needed to connect different small devices in close proximity without expensive wiring or the need for a wireless infrastructure. Represents a single chip, low cost, radio based wireless network technology. 46

47 BLUETOOTH (II) Up to now Bluetooth is not a standard like IEEE or HIPERLAN, but it soon become a de facto standard established by the industry and promoted by the Bluetooth consortium. Bluetooth uses the license free frequency band at 2.4GHz allowing for worldwide operation. 47

48 BLUETOOTH (III) Physical layer: A frequency hopping\time division duplex scheme is used for transmission with a fast hopping rate of 1,600 hops per second. The time between two hops is called a slot, which is an interval of 625μs, thus each slot uses a different frequency. On average, the frequency hopping sequence visits each hop carrier with an equal probability. All devices using the same hopping sequence with the same phase form a Bluetooth piconet. 48

49 BLUETOOTH (IV) With transmitting power of up to 100 mw, Bluetooth devices have a range of up to 10m (or even up to 100m with special transceivers). Having this power and relying on battery power, a Bluetooth device cannot be in an active transmit mode all the time. Bluetooth defines several low power states for the device. 49

50 BLUETOOTH (V) States of a possible Bluetooth device and possible transitions: Standby mode: Every device which is currently not participating in a piconet (and not switched off) In this mode, a device listens for paging messages. Connections can be initiated by any device which becomes the master. This is done by sending page messages if the device already knows the address of the receiver, or inquiry messages followed by a page message if the receiver s address is unknown. 50

51 BLUETOOTH (VI) To save battery power, a Bluetoth device can go into one of three low power states if no data is ready to be sent: PARK state: The device has the lowest duty cycle, and thus the lowest power consumption. The device releases its MAC address, but remains synchronized with the piconet. The device occasionally listens to the traffic of the master device to resynchronize and check for broadcast messages. HOLD state: The power consumption of this state is a little higher. The device does not release its MAC address and can resume sending at once after transition out of the HOLD state. SNIFF state: It has the highest power consumption of the low power states. The device listens to the piconet at a reduced rate. 51

52 BLUETOOTH (VII) STANDBY unconnected inquiry page connecting transmit connected active PARK HOLD SNIFF low power 52

53 BLUETOOTH (VIII) MAC layer: Several mechanisms control medium access in a Bluetooth system. First of all, one device within a piconet acts as a master, all other devices (up to seven) act as slaves. The master determines the hopping sequence as well as the phase of the sequence. All Bluetooth devices have the same networking capabilities, i.e., they can be master orslave slave. The unit establishing the piconet automatically becomes the master and controls medium access; all other devices will be slaves. 53

54 WLAN technologies summary () (I) The basic goals of all three LAN types (WLAN, HIPERLAN, BUETOOTH) arethe provision of a much higher flexibility for nodes within a network. All WLANs suffer from limitations of the air interface and higher complexity compared to their wired counterparts but allow for a new degree of freedom for their users within rooms, buildings etc. 54

55 WLAN technologies summary (II) The three technologies differ in some respects. HIPERLAN comprises many interesting i features, is much more powerful than IEEE , has a higher data rate (23.5 Mbit/s) but it is questionable if it will ever be a commercial success. The 5 GHz band required for HIPERLAN is not available worldwide compared to the 2.4 GHz used for IEEE

56 WLAN technologies summary (III) No standardization body has set up any specification regarding Bluetooth. The primary goal of Bluetooth is not a complex standard covering many aspects of wireless networking, but a quick ik and very cheap solution enabling ad hoc personal communication i within ihi a short range in the license free 2.4 GHz band. 56