Departamento de Tecnología Electrónica Some of these slides are copyrighted by: Computer Networking: A Top Down Approach 5th edition. Jim Kurose, Keith Ross Addison-Wesley, April 2009. Chapter 5. Wireless Local Area Network 1

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 2

Wireless Local Area Networks 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 3

Wireless Local Area Networks 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

Wireless Local Area Networks Introduction Introduction Wireless Local Area Networks 5

Wireless Local Area Networks 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 trasnsmission medium based on radio frequency (RF) -> Electromagnetic spectrum is limited Transmission rates are slower than in wired LANs Security problems Wireless Local Area Networks

Wireless Local Area Networks 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

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

Wireless Local Area Networks 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

Wireless Local Area Networks 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

Wireless Local Area Networks Standards LAN/MAN Standards Wireless Local Area Networks 11

Wireless Local Area Networks 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

Wireless Local Area Networks 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

802.11 Network components Wireless Network Adapter Station or client: Device using a NIC that follows standard IEEE 802.11 PC, laptop, PDA,… Wireless Network Adapter Wireless Local Area Networks

Wireless Local Area Networks 802.11 Network components Station or client Wireless Network Adapters Radio units Several types of Wi-Fi adapters PCI Cards With internal antenna With external antenna USB Adapters PCMCIA Adapters Wireless Local Area Networks

Wireless Local Area Networks 802.11 Network components Station or client Wireless Network Adapters PCI Cards With internal antenna More usual Disadvantage: very sensitive to the host location With external antenna Allows locating the antenna in the best place for receiving a more powerful signal. PCI cards in 802.11n have three antennas. Wireless Local Area Networks

Wireless Local Area Networks 802.11 Network components Station or client Wireless Network Adapters PCI Cards Advantages Reliable: once they are installed, there are usually no problems Disadvantages Need HW installation Allow their use in an only host Wireless Local Area Networks

Wireless Local Area Networks 802.11 Network components Station or client Wireless Network Adapters USB Adapters With internal antenna More usual. Less range, cheaper. With external antenna More gain: more quality of signal Also in 802.11n there are three antennas. Wireless Local Area Networks

Wireless Local Area Networks 802.11 Network components Station or client Wireless Network Adapters USB Adapters Advantages Mobility: Allows locating them in the best place for receiving a more powerful signal. May be used in any host. Only a USB port is needed. If it is neccessary, they may pass from a host to another (only driver installation is needed) Disadvantages Much more unstable than Wi-Fi PCI Cards. Units with interior antenna do not usually have high gain, so that they do not work properly in places where the quality of the signal is bad. Wireless Local Area Networks

Wireless Local Area Networks 802.11 Network components Station or client Wireless Network Adapters PCMCIA Cards With internal antenna More useful for laptops but less range than units with external antenna. With external antenna Higher range. Antenna is not usually very large, and it is generally folding Also in 802.11n there are three antennas (internal, for practical reasons). Wireless Local Area Networks

Wireless Local Area Networks 802.11 Network components Station or client Wireless Network Adapters PCMCIA Cards Adavantages More quality of signal than USB Adapters and simillar to Wi-Fi PCI cards. Disadvantages May be used only in hosts with PCMCIA port. They all need driver installation Wireless Local Area Networks

Wireless Local Area Networks 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

Wireless Local Area Networks 802.11 Network components Access Point (AP): Bridge: allows interconnecting different networks, regardless of the protocol they are using. Works in 1 & 2 OSI 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

Wireless Local Area Networks 802.11 Network components Station & AP Antennas Directional antennas or beam antennas Radiate signal in a very determined direction. Narrow beam with high range. Range is determined by the combination of the gain of the antenna, emitting power in the transmitter AP & sensitivity in the receiver AP. No signal is captured out of range. Very useful for long-distance point to point communication Wireless Local Area Networks

Wireless Local Area Networks 802.11 Network components Station & AP Antennas Onmidirectional antennas Signal is radiated in all directions, but the signal’s range is short Useful for radiating a signal in an extensive area. Sectorial antennas A combination of directional & omnidirectional antennas. Expensive When long distance and extensive area range are neccesary. Wireless Local Area Networks

Wireless Local Area Networks 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

Wireless Local Area Networks 802.11 Network components Distribution System: LAN or WLAN technology used to make WLAN range wider. Wireless case: Several AP. WDS AP: Master AP. 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

Wireless Local Area Networks 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 28

Wireless Local Area Networks Standard IEEE 802.11 Standard IEEE 802.11 LLC (802.2) MAC PHY Wireless Local Area Networks

Standard IEEE 802.11. Physical Layer Physical Layer (PHY) 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 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 802.11n Both 200 Mbps Wireless Local Area Networks

Standard IEEE 802.11. Data Link Layer MAC Sublevel Medium Access Technique: CSMA/CA Before transmitting info, a station must determine the medium state (free or busy) If the channel is not busy, there is an additional wait: InterFrame Space (IFS) 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 802.11. Data Link Layer MAC Sublevel Medium Access Technique: CSMA/CA 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) 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 802.11. Data Link Layer Data to tx arrival Wait MAC Sublevel Medium Access Technique: CSMA/CA IFS IFS IFS IFS Data CW Station A Data CW B Data Backoff C Data Backoff CW D Data Backoff CW E Wireless Local Area Networks

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 intereferes in the communication. Wireless Local Area Networks

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) RTS (Request to Send) / CTS (Clear to Send) 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 802.11. Data Link Layer MAC Sublevel Medium Access Technique: MACA Wireless Local Area Networks

Standard IEEE 802.11. Data Link Layer MAC Sublevel Every MAC_PDU containing data is ACKed by the rcvr Encryptation and authenticaction algorithms are implemented. A MAC_PDU may contain up to 4 MAC addresses (source, destination, transmitter & receiver). Wireless Local Area Networks

Standard IEEE 802.11. Data Link Layer MAC Sublevel Three types of MAC_PDUs: Data Control Management Wireless Local Area Networks

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 2312. In case of the existance of many interferences MAC_SDU fragmentation is enabled Wireless Local Area Networks

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

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

Wireless Local Area Networks 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 43

Wireless Local Area Networks 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

Wireless Local Area Networks 802.11 Network topologies Ad hoc 802.11 Network Wireless Local Area Networks

Wireless Local Area Networks 802.11 Network topologies Infrastructure BSS Wireless Local Area Networks

Wireless Local Area Networks 802.11 Network topologies EBSS 802.11 Network BSS BSS EBSS Wireless Local Area Networks

Wireless Local Area Networks 802.11 Network topologies Functioning in Infrastructure Mode / EBSS Every AP has a BSSID (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

Wireless Local Area Networks 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 proccess MAC_PDUs coming from their associated AP. Wireless Local Area Networks

Wireless Local Area Networks 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 neccessary. E.g: Distribution System based on 802.3 Wireless Local Area Networks

Wireless Local Area Networks 802.11 Network topologies Association Wireless Local Area Networks

Wireless Local Area Networks 802.11 Network topologies Sending of data MAC-PDU Wireless Local Area Networks