11 Chapter 4. Wireless Local Area Networks Departamento de Tecnología Electrónica Some of these slides are copyrighted by: Computer Networking: A Top Down Approach 5 th edition. Jim Kurose, Keith Ross Addison-Wesley, April 2009.

22 Chapter 4: Wireless Local Area Networks r 4.1 Introduction r 4.2 Standard IEEE m Standards m IEEE network components m Standard IEEE m IEEE network topologies Wireless Local Area Networks

33 Chapter 4: Wireless Local Area Networks r 4.1 Introduction r 4.2 Standard IEEE m Standards m IEEE network components m Standard IEEE m IEEE network topologies

44 Introduction r No cables r They use the electromagnetic spectrum r Generally they are integrated within wired LANs Why? r Allow mobility r Allow installation in places where cables can’t be installed (or are expensive) Wireless Local Area Networks

55 Introduction Wireless Local Area Networks Introduction

6 r Advantages: m Allow the same features as wired LANs, but without cable limitations.  Mobility m Reduce installation time/cost m Flexibility m May work inside buildings or between buildings r Disadvantages: m Need a trasnsmission medium based on radio frequency (RF) -> Electromagnetic spectrum is limited m Transmission rates are slower than in wired LANs m Security problems Wireless Local Area Networks Introduction

7 r Use of electromagnetic spectrum m MHz: Radio, television, remote controls, wireless phones, etc. m 200 MHz- 1GHz: alarms, medical implants, walkie-talkies, television, mobile phones. m 1- 2 GHz: GPS, medical telemetry, phone mobiles m 2.4 GHz: free band… satellite radio, satellite phones, microwave ovens, weather radars, WI-FI, BLUETOOTH. m GHz: satellite communications (e.g, TV) m 5-50 GHz: Wi-fi, police radars m GHz: short-distance signals. Wireless Local Area Networks Electromagnetic spectrum

8 r ISM frequency bands: m 900 – 928 MHz m 2,400 – 2,4835 GHz m 5,725 – 5,850 GHz Wireless Local Area Networks Electromagnetic spectrum

99Wireless Local Area Networks Chapter 5: Wireless Local Area Networks r 5.1 Introduction r 5.2 Standard IEEE m Standards m IEEE network components m Standard IEEE m IEEE network topologies

10 Standards Standars r Standardization of WLANs is in charge of IEEE & WIFI Alliance. m IEEE in standard is in charge of:  Define specifications of high performance WLANs  Ensures Interoperability  Security  Quality of Service. m WIFI Alliance is in charge of:  Certificates that a manufacturer’s product may interoperate with another manufacturer’s one.  Promote the use of WLANs Wireless Local Area Networks

11 Standards LAN/MAN Standards Wireless Local Area Networks

12 Wireless Local Area Networks Chapter 5: Wireless Local Area Networks r 5.1 Introduction r 5.2 Standard IEEE m Standards m IEEE network components m Standard IEEE m IEEE network topologies

Network components IEEE Network components r Networks that follow standard are formed by four basic elements Distribution system Access Point Station or client Wireless medium Wireless Local Area Networks

14 r Station or client: Device using a NIC that follows standard IEEE  PC, laptop, PDA,… Wireless Network Adapter Wireless Local Area Networks Network components

15 r Station or client m Wireless Network Adapters  Radio units  Several types of Wi-Fi adapters  PCI Cards  USB Adapters Wireless Local Area Networks Network components

16 Access Point (AP): m Device in charge of “Medium Control Access” for the clients of a WLAN and allows connection to a wired network (bridge) m An AP is a wireless hub. m Different from a wireless router (very common nowadays). A wireless router is a combination of an AP and a router and may execute more complex functions than a simple AP. Wireless Local Area Networks Network components

17 r Access Point (AP): m Bridge: allows interconnecting different networks, regardless of the protocol they are using. Works in 1 & 2 OSI layers. m A router also allows interconnecting several networks, but the network layer protocol must be common (E.g, IP) m If we want to interconnect two networks that are using the same protocol, it is desirable to use a router. Wireless Local Area Networks Network components

Network components r Wireless Medium : Use of RF for the transport of MAC_PDUs. Wireless Local Area Networks FrequencyStandardWavelength 2.4 Ghz802.11b/g12.5 cm 5.x GHz802.11a5-6 cm

19 r Distribution System: LAN or WLAN technology used to make WLAN range wider. m Wireless case:  Several AP.  WDS AP: Master AP (WDS = Wireless Distribution System).  The other APs are slaves and act as relay stations: WDS Stations.  All in the same channel  SSID may be the same one or different  Not standard. Not supported by all hosts and there may be incompatibilities.  Incompatible with some security mechanisms Wireless Local Area Networks Network components

20 Wireless Local Area Networks Chapter 5: Wireless Local Area Networks r 5.1 Introduction r 5.2 Standard IEEE m Standards m IEEE network components m Standard IEEE m IEEE network topologies

21 Standard IEEE PHY MAC LLC (802.2) Wireless Local Area Networks

22 r Physical Layer (PHY) m Cell topology.  Half-duplex  Frequency bands: 2,4 GHz & 5 GHz  A license is not neccessary.  Every band has a set of channels Wireless Local Area Networks Standard IEEE Physical Layer

23 r Physical Layer (PHY) m Cell topology.  Physical layer options StandardBandTx Rate a5 GHz54 Mbps b2,4 GHz11 Mbps g2,4 GHz54 Mbps n Both200 Mbps Wireless Local Area Networks Standard IEEE Physical Layer

24 r MAC Sublevel m Every MAC_PDU containing data is ACKed by the rcvr m Encryptation and authenticaction algorithms are implemented. m A MAC_PDU may contain up to 4 MAC addresses (source, destination, transmitter & receiver). Wireless Local Area Networks Standard IEEE Data Link Layer

25Wireless Local Area Networks Standard IEEE Data Link Layer Data frame Ethernet frame Frame Control DurationDest Address Source Address Address 3 Seq.Address 4 DataCRC 2 Bytes 2 Bytes 6 Bytes 6 Bytes 6 Bytes 2 Bytes 6 Bytes Bytes 4 Bytes Dest Address Source Address ETypeDataCRC 6 Bytes 6 Bytes 2 Bytes Bytes 4 Bytes IP LLC/SNAP header (802.2)

26 r MAC Sublevel m Three types of MAC_PDUs:  Data  Control  Management Wireless Local Area Networks Standard IEEE Data Link Layer

27 r MAC level m Data MAC_PDUs:  Superior layer info (MAC_SDU) is carried  MAC_PCI is 34 bytes long.  MTU is  In case of the existance of many interferences MAC_SDU fragmentation is enabled Wireless Local Area Networks Standard IEEE Data Link Layer

28 r MAC sublevel m Control MAC_PDUs:  Used for the “reservation” of the medium and for ACKs  ACK: sent by MAC sublevel to ACK a Data MAC_PDU. Does not indicate that MAC_PDU destination has received it.  RTS (Request to Send) / CTS (Clear to Send) for the reservation of the medium. RTS is sent by MAC sublevel to request the use of the medium and indicate the duration of the reservation (time needed). CTS is sent by MAC sublevel as a response to RTS. Indicates that the sender is able to transmit and the duration of the reservation (time left) Wireless Local Area Networks Standard IEEE Data Link Layer

29 r MAC sublevel m Management MAC_PDUs:  To manage wireless links.  Beacon. Sent periodically by MAC sublevel to inform about the existance of a wireless network Interval: configurable parameter.  Probe request. Enable MAC sublevel to search for wireless networks within an area. Informs about transmission rates.  Probe response. Response to Probe Request.  Association request. MAC sublevel requests the connection to a wireless network.  Association response. Connection confirmation.  Others. Wireless Local Area Networks Standard IEEE Data Link Layer

30 r MAC Sublevel m Medium Access Control (MAC), changes for adapting to wireless medium. In Ethernet, CSMA/CD (Carrier Sense Multiple Access / Collision Detection). In Wireless, this mechanism is hard to implement, as transmission errors are more frequent. New mechanisms for MAC are added: CSMA/CA y MACA. Wireless Local Area Networks Standard IEEE Data Link Layer

31 r MAC Sublevel m Medium Access Technique: CSMA/CA 1)Before transmitting info, a station must determine the medium state (free or busy) 2)If the channel is not busy, there is an additional wait: InterFrame Space (IFS) 3)If the channel is busy or it gets busy while IFS, the tx must wait until current transaction ends. Wireless Local Area Networks Standard IEEE Data Link Layer

32 r MAC Sublevel m Medium Access Technique: CSMA/CA 4)After current transaction ends: Backoff algorithm is executed  Additional and random wait, chosen in an interval called contention window (CW)  Measured in units of slot time (slots) 5)If during this wait, medium is not free for a time equal or higher than IFS, wait is suspended until the condition is fulfilled. Wireless Local Area Networks Standard IEEE Data Link Layer

33 r MAC Sublevel m Medium Access Technique: CSMA/CA Station A B C D E IFS CW Data Backoff Data to tx arrival Wait Wireless Local Area Networks Standard IEEE Data Link Layer

34 r MAC Sublevel m Medium Access Technique: CSMA/CA  Problems in WLAN:  Hidden nodes. Channel is busy by a station that is not heard by the correspondent node  Exposed nodes. A station thinks that the channel is busy though it is, in fact free, as other nodes does not intereferes in the communication. Wireless Local Area Networks Standard IEEE Data Link Layer

35 Standard IEEE Data Link Layer r MAC Sublevel m Medium Access Technique: CSMA/CA  Problems in WLAN: Hidden nodes. Exposed nodes Wireless Local Area Networks

36 r MAC Sublevel m Medium Access Technique: MACA  Contention that allows reservations to avoid collisions (CSMA/CA, CA = Collision Avoidance)  More resstrictive than standard CSMA/CA:  RTS (Request to Send) / CTS (Clear to Send)  DIFS (Distributed IFS)/ SIFS (Short IFS)  NAV (similar to backoff algorithm time)  Not employed (overload) if:  There are a few stations  Very dense network: all the stations are in range of every station.  Small frames. Wireless Local Area Networks Standard IEEE Data Link Layer

37 Standard IEEE Data Link Layer r MAC Sublevel m Medium Access Technique: MACA Wireless Local Area Networks

38Wireless Local Area Networks38 Chapter 5: Wireless Local Area Networks r 5.1 Introduction r 5.2 Standard IEEE m Standards m IEEE network components m Standard IEEE m IEEE network topologies

Network topologies r The basic communication set in a network is the BSS (Basic Service Set) or cell. m Every BSS has a coverage area, so that all the stations belonging to the BSS can communicate to the others m A name known as SSID (Service Set Identifier) is assigned to them. r According to the number of BSSs and the kind of devices within a network, there are three types of networks: m Ad hoc networks or Independent BSS (IBSS).  There are only clients. m Infrastructure BSS.  There are clients and an AP. m EBSS (Extended BSS)  Multiple BSS to allow bigger coverage area. Wireless Local Area Networks

40 r Ad hoc Network Wireless Local Area Networks Network topologies

41 r Infrastructure BSS Wireless Local Area Networks Network topologies

42 r EBSS Network BSS EBSS Wireless Local Area Networks Network topologies

43 Functioning in Infrastructure Mode / EBSS r Every AP has a BSSID – Basic Service Set Identifier- (MAC of its wireless interface) and a SSID (configured by the network administrator). m In EBSS, every cell has the same SSID, but is distinguished by its AP’s BSSID. r does not limit the number of clients per AP. r To connect to a wireless network, a client must know BSSID & SSID in a cell. m APs send Beacon periodically, with BSSID and, optionally with SSID m The client sends Probe request with SSID, waiting a Probe response, from the AP, containing its BSSID. Wireless Local Area Networks Network topologies

44 Functioning in Infrastructure Mode / EBSS r A client who know BSSID & SSID of a cell request the association (connection) with an AP by means of the Association Request m If the AP accepts the client, it sends an Association Response with an Association Id.  AP registers client’s MAC in its Addressing Table. r An AP controls the communication in all its associated clients m Clients never communicate with each other directly  They only proccess MAC_PDUs coming from their associated AP. Wireless Local Area Networks Network topologies

45 Functioning in Infrastructure Mode / EBSS r APs keep Addressing tables, like bridges do. m They learn from the traffic that goes through it. m They forward info based on the dest MAC addr r An AP that is connected to a Distribution System acts as a bridge, but m Injects traffic to the wireless interface if the dest is one of its clients or if it is broadcast/multicast m Injects traffic in the Distribution System (like a bridge) r AP adapts logical addressing if it’s neccessary. m E.g: Distribution System based on Wireless Local Area Networks Network topologies

Network topologies Association Wireless Local Area Networks

Network topologies Sending of data MAC-PDU Wireless Local Area Networks