Wireless Local Area Network (WLAN) - PDF

Kingdom of Saudi Arabia Ministry of Higher Education Al-Imam Muhammad Ibn Saud Islamic University College of Computer and Information Sciences Wireless Local Area Network WLAN 1 Outlines WLANs Overview. WLAN...

Kingdom of Saudi Arabia Ministry of Higher Education Al-Imam Muhammad Ibn Saud Islamic University College of Computer and Information Sciences Wireless Local Area Network WLAN 1 Outlines WLANs Overview. WLANs (design goals, benefits, known problems). IEEE 802 (standards, protocols). WLAN Standards. WPAN. WMAN. IEEE 802.11. IEEE 802 Security Methods. 2 Overview A wireless LAN or WLAN is a wireless local area network that uses radio waves as its carrier. The last link with the users is wireless, to give a network connection to all users in a building or campus. The backbone network usually uses cables 3 WLANs Goals A mature 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, increase flexibility of connection especially for portable devices and, additionally, to introduce a higher flexibility for ad hoc communication in, e.g., group meetings. 4 Design Goals Global, seamless operation of WLAN products Low power 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 5 Benefits Mobility Increases working efficiency and productivity Extends the On-line period Installation on difficult-to-wire areas Inside buildings Road crossings Increased reliability: more robust against disasters (e.g., earthquakes, fire) Reduced installation time cabling time and convenient to users and difficult-to-wire cases Long-term cost savings Cheaper that for wired networks Easy maintenance, cabling cost, working efficiency and accuracy 6 Known Problems Wireless link characteristics: media is error prone and the bit error rate (BER) is very high compared to the BER of wired networks. Carrier Sensing/collision detection is difficult in wireless networks because a station is incapable of listening to its own transmissions in order to detect a collision. The Hidden Terminal problem also decreases the performance of WLANs. Mobility (variation in link reliability, seamless connections required, battery limitations) 7 Wireless Link Characteristics Differences from wired link: Decreased signal strength: radio signal attenuates as it propagates through matter (path loss) Interference from other sources: standardized wireless network frequencies (e.g., 2.4 GHz) shared by other devices (e.g., Phone); also devices (e.g. Motors) interfere as well (noise) Multipath propagation: radio signal reflects off objects, arriving at destination at slightly different times (channel quality varies over time) Shared with other technologies and spectrum users More difficult security (shared medium) => Make communication across (even a point to point) wireless link 8 much more “difficult” IEEE 802 IEEE 802 is a family of IEEE standards dealing with local area networks (LANs). Mapping to the lower two layers (Data Link and Physical) of the ISO/OSI reference model. IEEE 802 splits the Data Link Layer into two sub-layers: Logical link control (LLC) Media access control (MAC) IEEE 802.11 is part of the IEEE 802 set of LAN protocols and it deals with wireless LAN communications. 9 IEEE 802.11 Standard 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 to the special requirements of wireless LANs, but offers the same interface as the others to higher layers to maintain interoperability. The primary goal of the standard was the specification of a simple and robust WLAN which offers time-bounded and asynchronous services. 10 IEEE 802.11 Protocols IEEE 802.11b offering 11 Mbit/s at 2.4 GHz 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 802.11a, operating at 5 GHz and offering gross data rates of 54 Mbit/s. IEEE 802.11g offering up to 54 Mbit/s at 2.4 GHz. IEEE 802.11n up and coming standard up to 300 Mbit/s (2 spatial streams; 600 Mbit/s with 4 spatial streams) All use Carrier-sense multiple access with collision avoidance (CSMA/CA) 11 for multiple access WLAN Standards Wireless LAN 2.4 GHz 5 GHz 802.11 802.11b 802.11g HiSWANa 802.11a HiperLAN2 (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) 802.11e 802.11f 802.11h 802.11i (QoS) (IAPP) (TPC-DFS) (Security) 12 Wireless Network Technology Options Network definition IEEE standard Known as Wireless personal area IEEE 802.15.1 Bluetooth network (WPAN) Low-rate WPAN (LR- IEEE 802.15.4 ZigBee WPAN) Wireless local area IEEE 802.11 WiFi network (WLAN) Wireless metroplitan IEEE 802.16 WiMAX area network (WMAN) 13 WPAN Wireless personal area network is a short-link radio technology known as Bluetooth It is short range, low power consumption and low cost. Bluetooth operates at frequencies between 2402 and 2480 MHz, or 2400 and 2483.5 MHz This band is part of industrial, scientific and medical (ISM) 2.4 GHz short- range radio frequency band – unlicensed band Bluetooth uses a radio technology called frequency-hopping spread spectrum (FHSS). It divides transmitted data into packets, and transmits each packet on one of 79 designated Bluetooth channels – each channel has a bandwidth of 1 MHz 14  LR-WPAN Low-rate WPAN is designed for low data rate but very long battery life (months or even years) and very low complexity applications. Zigbee is an IEEE 802.15.4-based specification for a suite of high-level communication protocols used to create personal area networks with small, low-power digital radios. Zigbee is similar to Bluetooth (same frequency band …etc) but more oriented towards remote control and automation. Typical application areas include:  Home automation – smart homes  Wireless sensor networks, part of IoT  Industrial control systems  Embedded sensing  Healthcare - medical data collection 15  Smoke and intruder warning … etc WMAN Metroplitan area network (MAN) is a network covers larger area than LAN but smaller than WAN. It usually covers a city. WiMAX is a wireless MAN standard which was designed to provide 30-40 Mbps (can reach up to 1 Gbps) over a radius of several kilometers (4 to 10 km). Uses of WiMAX  Providing portable mobile broadband connectivity across cities.  Providing a wireless alternative to cable broadband access.  Providing data, telecommunications (VoIP) and IPTV services.  Providing Internet connectivity as part of a business continuity plan.  Smart grids and metering. 16 Selected Wireless Link Standards 17 IEEE Standard 802.11 fixed terminal mobile terminal infrastructure network access point application application TCP TCP IP IP LLC LLC LLC 802.11 MAC 802.11 MAC 802.3 MAC 802.3 MAC 802.11 PHY 802.11 PHY 802.3 PHY 802.3 PHY 18 Architecture Aims Protocol Applications should not notice any difference apart from the lower bandwidth and perhaps higher access time from the wireless LAN. => WLAN behaves like, perhaps a ‘slower’, 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 ÞDifferent media and access control 19 IEEE 802.11 Terminology Basic Service Set (BSS): group of stations using same radio frequency. Access Point (AP): station integrated into the wireless LAN and the distribution system. Station (STA): terminal with access mechanisms to wireless medium and radio contact to access point. Portal: bridge to other (wired) networks Distribution System (DS): interconnection network to form one logical network. Extended Service Set (EES): based on several BSSs. 20 Distribution System (DS) Used to interconnect wireless cells (multiple BSS to form an ESS) Allows multiple mobile stations to access fixed resources Interconnects 802.11 technology. 21 Access Points (AP) There is a need of an access point that bridges wireless LAN traffic into the wired LAN. It can also act as a repeater for wireless nodes, effectively doubling the maximum possible distance between nodes. Allows stations to associate with it. Supports Distributed Coordination Function (DCF) and Point Coordination Function (PCF) Provides management features Join/Associatewith BSS Time synchronization (beaconing) Power management All traffic flows through APs Supports roaming. 22 BSS, ESS and DS 23 Physical layer Wireless Transmission: infrared (IR) or radio frequency (RF) Radio: typically using the license free frequency (in USA) 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) Higher transmission rates (~11 – 54 Mbit/s) Disadvantages: Very limited license free frequency bands. Shielding more difficult, interference with other senders, or electrical devices. 24 Media Access Control-MAC MAC sublayer and the logical link control (LLC) sublayer together make up the data link layer. MAC provides flow control and multiplexing for the transmission medium i.e. several terminals or network nodes to communicate in a network. Two basic access mechanisms have been defined for IEEE 802.11 CSMA/CA (mandatory) summarized as distributed coordination function (DCF). A contention-free polling method for time-bounded service called point coordination function (PCF). Within the MAC layer, (DCF) (asynchronous service) is used as a fundamental access method, while (PCF) (synchronous service) is optional. 25 DCF and PCF DCF only offers asynchronous service, while PCF offers both asynchronous and time-bounded service, but needs the access point to control medium access and to avoid contention. DCF is also known as Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) protocol:  It is an asynchronous access method based on the contention for the usage of shared channels. A contention-free access mechanism is provided through the RTS/CTS (Request to Send/Clear to Send) exchange. PCF is used for time-bounded transfer of data. An access point polls terminals according to a list. 26 Carrier Sense Multiple Access (CSMA) Carrier means a shared medium. Sense means a node can listen and detect. Multiple Access means every node in the network has a right to access (and use) the shared medium. CSMA means a node in the network can detect what is going on in the shared transmission medium (regardless what the type of this medium is). If a node detects the medium is in used, it waits (backoff) for a random amount of time before rechecking again. 27 Hidden Station Problem A B C A is talking to B. C does not know this communication and starts talking to B => Collisions. 28 Collisions in WLAN Collisions occurred when two or more nodes transmitting at the same time. Most important differences between WLAN and LAN protocol design is the impossibility to detect all collisions in WLANs.  Difficult to receive (sense collisions) when transmitting due to weak received signals (fading).  With receiving and sending antennas immediately next to each other, a station is unable to see any signal but its own.  As a result, the complete packet will be sent before the incorrect checksum reveals that a collision has happened.  Furthermore, receiver and transmitter mostly not on at the same time. Can NOT sense all collisions in any case: hidden terminal, fading 29 CSMA/CA In LAN, the protocol use collisions detection (CD) mechanism. If a node detects a collisions after sending packets, then …. In WLAN, detection is difficult => then it use collision avoidance (CA) techniques CA means try to prevent collisions happening before transmit rather than transmit then detect collisions(CD) 30 CSMA/CA - WLAN Utmost importance that number of collisions be limited to the absolute minimum. DCF’s CSMA/CA (CA-Collision Avoidance) is the MAC method used in a WLAN. (Wireless stations cannot detect collisions, i.e. the whole packet will be transmitted anyway). Basic CSMA/CA operation: If medium is free, then wait a specified time (DIFS), then start transmitting If medium is busy, then backoff DIFS stands for DCF Interframe spacing 31 CSMA/CA Protocol Basics Medium can be busy or idle (detected by the Clear Channel Assessment (CCA) signal of the physical layer). 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 medium. Optionally, the standard allows for collision free operation through small reservation packets (RTS, CTS) 32 CSMA/CA Algorithm Sender: 1 if sense channel idle for DIFS then transmit entire frame (no CD) 2 if sense channel busy then start random backoff time timer counts down while channel idle transmit when timer expires if no ACK, increase random backoff interval, repeat 2 Receiver if frame received OK then return ACK after SIFS (shortest interframe spacing) 33 CSMA/CA Scenario ACK needed due to hidden terminal problem sender receiver DIFS data SIFS ACK 34 Collision Avoidance: RTS-CTS Exchange The idea is to allow sender to “reserve” channel rather than random access of data frames: avoid collisions of long data frames. Sender first transmits small request-to-send (RTS) packets to receiver using CSMA. RTSs may still collide with each other (but they’re short) Receiver broadcasts clear-to-send CTS in response to RTS CTS heard by all nodes. sender transmits data frame. other stations defer transmissions. 35 RTS-CTS Exchange Scenario A AP B RTS(A) RTS(B) reservation RTS(A) collision CTS(A) CTS(A) DATA (A) time defer ACK(A) ACK(A) 36 Addressing 2 2 6 6 6 2 6 0 - 2312 4 frame address address address seq address duration payload CRC control 1 2 3 control 4 Address 4: used only in ad- hoc mode Address 1: MAC address of wireless host or AP Address 3: MAC address to receive this frame of router interface to which AP is attached Address 2: MAC address of wireless host or AP transmitting this frame 37 Addressing Internet H1 R1 router AP R1 MAC addr H1 MAC addr dest. address source address 802.3 frame AP MAC addr H1 MAC addr R1 MAC addr address 1 address 2 address 3 802.11 frame 38 Addressing To From Addre Addre Addre Addre DS DS ss 1 ss 2 ss 3 ss 4 0 0 DA SA BSSID N/A 0 1 DA Sendin SA N/A g AP 1 0 Receivi SA DA N/A ng AP 1 1 Receivi Sendin DA SA ng AP g AP DS: Distribution System DA: Destination Address BSSID: Basic Service Set ID SA: Source Address 39 Addressing: Case 00 11-22-33-02-02-02 11-22-33-01-01-01 A1: 11-22-33-01-01-01 DA A2: 11-22-33-02-02-02 SA A3: BSS ID A4: not used 40 Addressing: Case 01 (Wired to Wireless) wireless wired 802.11 802.3 11-22-33-01-01-01 99-88-77-09-09-09 11-22-33-02-02-02 DAA1: 11-22-33-01-01-01 DA: 11-22-33-01-01-01 Sending AP A2: 99-88-77-09-09-09 SA: 11-22-33-02- SA A3: 11-22-33-02-02-02 02-02 A4: not used 41  Addressing: Case 10 (Wireless to Wired) wireless wired 802.11 802.3 11-22-33-01-01-01 99-88-77-09-09-09 11-22-33-02-02-02 Receiving AP A1: 99-88-77-09-09-09 DA: 11-22-33-02- SA A2: 11-22-33-01-01-01 02-02 SA: 11-22-33-01- DAA3: 11-22-33-02-02-02 01-01 A4: not used 42  Addressing: Case 11 (Via Wireless) wired wireless wired wired 802.3 802.11 802.3 802.3 99-88-77-09-09-09 99-88-77-08-08-08 11-22-33-01-01-01 11-22-33-02-02-02 DA: 11-22-33-02- A1: 99-88-77-08- DA: 11-22-33-02- 02-02 08-08 A2: 99-88-77-09- 02-02 SA: 11-22-33-01- SA: 11-22-33-01- 01-01 09-09 A3: 11-22-33-02- 01-01 02-02 A4: 11-22-33-01- 01-01 43 Wireless Bridge Building A Building B Ethernet Backbone Ethernet Backbone Case 11 Wireless Wireless Bridge Bridge 44 Security Not so efficient 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 (WEP) Data encryption technique using shared keys and a pseudorandom number as an initialization vector. 64-bit key level encryption BUT several vendors support 128-bit key level encryption. Wi-Fi Protected Access (WPA2) – replaced WEP WPA2 uses encryption device which encrypts the network with a 256 bit key 45 IEEE 802 Security Methods 802.11i (Advanced Encryption Standard – AES - uses a symmetric block data encryption technique) 802.1X for port based Network Access Control. Provides an authentication mechanism to devices wishing to attach to LAN/WLAN (governs Extensible Authentication Protocol (EAP) encapsulation process that occurs between clients, wireless APs, and authentication servers (RADIUS)). EAP allows developers to pass authentication data between RADIUS servers and wireless APs. Has a number of variants, including: EAP MD5, EAP-TLS, EAP-TTLS, LEAP, and PEAP 46 WEP Wired Equivalent Privacy (WEP) – Least secure - A network that is secured with WEP has been cracked in 3 minutes by the FBI Shared key encryption Stations use the same key for encryption. RC4 encryption algorithm Key: 40 bits or 128 bits User Authentication Not specified in 802.11. 802.1X VPN 47 WPA2 WPA intermediate measure to replace WEP pending availability of full IEEE 802.11i standard. Adopts Temporal Key Integrity Protocol (TKIP) TKIP employs per-packet key; dynamically generates new 128-bit key for each packet – prevents types of attacks that compromised WEP. Includes message integrity check to prevent an attacker from capturing, altering and/or resending data packets. WPA2 replaced WPA. Provides authentication and encryption for wireless networks 48 WPA2 is more secure because it uses AES encryption method