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Departamento de Tecnología Electrónica Some of these slides are copyrighted by: Chapter 4. Wireless Local Area Networks Computer Networking: A Top Down Approach 5th edition. Jim Kurose, Keith Ross Addison-Wesley, April 2009. 1 Chapter 4: Wireless Local Area Networks 4.1 Introduction 4.2 Standard IEEE 802.11 4.2.1 Standards 4.2.2 IEEE 802.11 network components 4.2.3 Standard IEEE 802.11 4.2.4 IEEE 802.11 network topologies Wireless Local Area Networks 2 Chapter 4: Wireless Local Area Networks 4.1 Introduction 4.2 Standard IEEE 802.11 4.2.1 Standards 4.2.2 IEEE 802.11 network components 4.2.3 Standard IEEE 802.11 4.2.4 IEEE 802.11 network topologies Wireless Local Area Networks 3 Introduction Introduction No cables They use the electromagnetic spectrum Generally they are integrated within wired LANs Why? Allow mobility Allow installation in places where cables can’t be installed (or are expensive) Wireless Local Area Networks 4 Introduction Introduction Wireless Local Area Networks 5 Introduction Advantages: Allow the same features as wired LANs, but without cable limitations. Mobility Reduce installation time/cost Flexibility May work inside buildings or between buildings Disadvantages: Need a transmission medium based on radio frequency (RF) -> Electromagnetic spectrum is limited Transmission rates are slower than in wired LANs Security problems Wireless Local Area Networks 6 Electromagnetic spectrum Use of electromagnetic spectrum 0-200 MHz: Radio, television, remote controls, wireless phones, etc. 200 MHz- 1GHz: alarms, medical implants, walkie-talkies, television, mobile phones. 1- 2 GHz: GPS, medical telemetry, phone mobiles 2.4 GHz: free band… satellite radio, satellite phones, microwave ovens, weather radars, WI-FI, BLUETOOTH. 2.5- 5 GHz: satellite communications (e.g, TV) 5-50 GHz: Wi-fi, police radars 50-300 GHz: short-distance signals. Wireless Local Area Networks 7 Electromagnetic spectrum ISM frequency bands: 900 – 928 MHz 2,400 – 2,4835 GHz 5,725 – 5,850 GHz Wireless Local Area Networks 8 Chapter 5: Wireless Local Area Networks 5.1 Introduction 5.2 Standard IEEE 802.11 5.2.1 Standards 5.2.2 IEEE 802.11 network components 5.2.3 Standard IEEE 802.11 5.2.4 IEEE 802.11 network topologies Wireless Local Area Networks 9 Standards Standars Standardization of WLANs is in charge of IEEE & WIFI Alliance. IEEE in standard 802.11 is in charge of: Define specifications of high performance WLANs Ensures Interoperability Security Quality of Service. 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 10 Standards LAN/MAN Standards Wireless Local Area Networks 11 Chapter 5: Wireless Local Area Networks 5.1 Introduction 5.2 Standard IEEE 802.11 5.2.1 Standards 5.2.2 IEEE 802.11 network components 5.2.3 Standard IEEE 802.11 5.2.4 IEEE 802.11 network topologies Wireless Local Area Networks 12 802.11 Network components IEEE 802.11Network components Networks that follow standard 802.11 are formed by four basic elements Distribution system Access Point Station or client Wireless medium Wireless Local Area Networks 13 802.11 Network components Station or client: Device using a NIC that follows standard IEEE 802.11 PC, laptop, PDA,… Wireless Network Adapter Wireless Local Area Networks 14 802.11 Network components Station or client Wireless Network Adapters Radio units Several types of Wi-Fi adapters PCI Cards USB Adapters Wireless Local Area Networks 15 802.11 Network components Access Point (AP): Device in charge of “Medium Control Access” for the clients of a WLAN and allows connection to a wired network (bridge) An AP is a wireless hub. 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 16 802.11 Network components Access Point (AP): Bridge: allows interconnecting different networks, regardless of the protocol they are using. Works in physical & data link layers. A router also allows interconnecting several networks, but the network layer protocol must be common (E.g, IP) If we want to interconnect two networks that are using the same protocol, it is desirable to use a router. Wireless Local Area Networks 17 802.11 Network components Wireless Medium: Use of RF for the transport of MAC_PDUs. Frequency Standard Wavelength 2.4 Ghz 802.11b/g 12.5 cm 5.x GHz 802.11a 5-6 cm Wireless Local Area Networks 18 802.11 Network components Distribution System: LAN or WLAN technology used to make WLAN range wider. 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 19 Chapter 5: Wireless Local Area Networks 5.1 Introduction 5.2 Standard IEEE 802.11 5.2.1 Standards 5.2.2 IEEE 802.11 network components 5.2.3 Standard IEEE 802.11 5.2.4 IEEE 802.11 network topologies Wireless Local Area Networks 20 Standard IEEE 802.11 Standard IEEE 802.11 LLC (802.2) MAC PHY Wireless Local Area Networks 21 Standard IEEE 802.11. Physical Layer Physical Layer (PHY) Cell topology. Half-duplex Frequency bands: 2,4 GHz & 5 GHz A license is not necessary. Every band has a set of channels Wireless Local Area Networks 22 Standard IEEE 802.11. Physical Layer Physical Layer (PHY) Cell topology. Physical layer options Standard Band Tx Rate 802.11a 5 GHz 54 Mbps 802.11b 2,4 GHz 11 Mbps 802.11g 2,4 GHz 54 Mbps 802.11n Both 200 Mbps Wireless Local Area Networks 23 Standard IEEE 802.11. Data Link Layer MAC Sublevel Every MAC_PDU containing data is ACKed by the rcvr Encryptation and authentication algorithms are implemented. A MAC_PDU may contain up to 4 MAC addresses (source, destination, transmitter & receiver). Wireless Local Area Networks 24 Standard IEEE 802.11. Data Link Layer LLC/SNAP header (802.2) 802.11 Data frame Frame Control Duration 2 Bytes 2 Bytes Dest Address Source Address Address 3 6 Bytes 6 Bytes 6 Bytes Seq. Address 4 2 Bytes 6 Bytes Data CRC IP 0-2312 Bytes 4 Bytes Ethernet frame Dest Address Source Address 6 Bytes 6 Bytes EType Data CRC IP 2 Bytes 46-1500 Bytes 4 Bytes Wireless Local Area Networks 25 Standard IEEE 802.11. Data Link Layer MAC Sublevel Three types of MAC_PDUs: Data Control Management Wireless Local Area Networks 26 Standard IEEE 802.11. Data Link Layer MAC level Data MAC_PDUs: Superior layer info (MAC_SDU) is carried MAC_PCI is 34 bytes long. MTU is typically 2312 bytes. In case of the existence of many interferences MAC_SDU fragmentation is enabled Wireless Local Area Networks 27 Standard IEEE 802.11. Data Link Layer MAC sublevel 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 28 Standard IEEE 802.11. Data Link Layer MAC sublevel 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 29 Standard IEEE 802.11. Data Link Layer MAC Sublevel 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 30 Standard IEEE 802.11. Data Link Layer MAC Sublevel 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 31 Standard IEEE 802.11. Data Link Layer MAC Sublevel 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 32 Standard IEEE 802.11. Data Link Layer Data to tx arrival Wait MAC Sublevel Medium Access Technique: CSMA/CA IFS IFS IFS Data CW Data IFS Station A CW B Data Backoff C Data Backoff CW D Backoff Data CW E Wireless Local Area Networks 33 Standard IEEE 802.11. Data Link Layer MAC Sublevel 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 interferes in the communication. Wireless Local Area Networks 34 Standard IEEE 802.11. Data Link Layer MAC Sublevel Medium Access Technique: CSMA/CA Problems in WLAN: Hidden nodes. Exposed nodes Wireless Local Area Networks 35 Standard IEEE 802.11. Data Link Layer MAC Sublevel Medium Access Technique: MACA Contention that allows reservations to avoid collisions (CSMA/CA, CA = Collision Avoidance) More restrictive 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 36 Standard IEEE 802.11. Data Link Layer MAC Sublevel Medium Access Technique: MACA Wireless Local Area Networks 37 Chapter 5: Wireless Local Area Networks 5.1 Introduction 5.2 Standard IEEE 802.11 5.2.1 Standards 5.2.2 IEEE 802.11 network components 5.2.3 Standard IEEE 802.11 5.2.4 IEEE 802.11 network topologies Wireless Local Area Networks 38 802.11 Network topologies The basic communication set in a 802.11 network is the BSS (Basic Service Set) or cell. Every BSS has a coverage area, so that all the stations belonging to the BSS can communicate to the others A name known as SSID (Service Set Identifier) is assigned to them. According to the number of BSSs and the kind of devices within a network, there are three types of 802.11 networks: Ad hoc networks or Independent BSS (IBSS). There are only clients. Infrastructure BSS. There are clients and an AP. EBSS (Extended BSS) Multiple BSS to allow bigger coverage area. Wireless Local Area Networks 39 802.11 Network topologies Ad hoc 802.11 Network Wireless Local Area Networks 40 802.11 Network topologies Infrastructure BSS Wireless Local Area Networks 41 802.11 Network topologies EBSS 802.11 Network BSS BSS EBSS Wireless Local Area Networks 42 802.11 Network topologies Functioning in Infrastructure Mode / EBSS Every AP has a BSSID – Basic Service Set Identifier- (MAC of its wireless interface) and a SSID (configured by the network administrator). In EBSS, every cell has the same SSID, but is distinguished by its AP’s BSSID. 802.11 does not limit the number of clients per AP. To connect to a wireless network, a client must know BSSID & SSID in a cell. APs send Beacon periodically, with BSSID and, optionally with SSID The client sends Probe request with SSID, waiting a Probe response, from the AP, containing its BSSID. Wireless Local Area Networks 43 802.11 Network topologies Functioning in Infrastructure Mode / EBSS A client who know BSSID & SSID of a cell request the association (connection) with an AP by means of the Association Request If the AP accepts the client, it sends an Association Response with an Association Id. AP registers client’s MAC in its Addressing Table. An AP controls the communication in all its associated clients Clients never communicate with each other directly They only process MAC_PDUs coming from their associated AP. Wireless Local Area Networks 44 802.11 Network topologies Functioning in Infrastructure Mode / EBSS APs keep Addressing tables, like bridges do. They learn from the traffic that goes through it. They forward info based on the dest MAC addr An AP that is connected to a Distribution System acts as a bridge, but Injects traffic to the wireless interface if the dest is one of its clients or if it is broadcast/multicast Injects traffic in the Distribution System (like a bridge) AP adapts logical addressing if it’s necessary. E.g: Distribution System based on 802.3 Wireless Local Area Networks 45 802.11 Network topologies Association Wireless Local Area Networks 46 802.11 Network topologies Sending of data MAC-PDU Wireless Local Area Networks 47