Wireless Local Area Network

1. Wireless Local Area Network

2. Wireless? • 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. Common Topologies The wireless LAN connects to a wired LAN • There is a need of an access point that bridges wireless LAN traffic into the wired LAN. • The access point (AP) can also act as a repeater for wireless nodes, effectively doubling the maximum possible distance between nodes.

4. Integration With Existing Networks • Definition - What does Wireless Access Point (WAP) mean? • A wireless access point (WAP) is a hardware device or configured node on a local area network (LAN) that allows wireless capable devices and wired networks to connect through a wireless standard, including Wi-Fi or Bluetooth. WAPs feature radio transmitters and antennae, which facilitate connectivity between devices and the Internet or a network. • A WAP is also known as a hotspot. • Wireless Access Points (APs) - A small device that bridges wireless traffic to your network. • Most access points bridge wireless LANs into Ethernet networks, but Token-Ring options are available as well.

5. How are WLANs Different? • They use specialized physical and data link protocols • They integrate into existing networks through access points which provide a bridging function • They let you stay connected as you roam from one coverage area to another • They have unique security considerations • They have specific interoperability requirements • They require different hardware • They offer performance that differs from wired LANs.

6. Physical and Data Link Layers Physical Layer: • The wireless NIC takes frames of data from the link layer, scrambles the data in a predetermined way, then uses the modified data stream to modulate a radio carrier signal. Data Link Layer: • Uses Carriers-Sense-Multiple-Access with Collision Avoidance (CSMA/CA).

7. 802.11 WLAN technologies • The IEEE standard 802.11 (IEEE, 1999) specifies the most famous family of WLANs in which many products are available. As the standard’s number indicates, this standard belongs to the group of 802.x LAN standards, e.g., 802.3 Ethernet or 802.5 Token Ring. 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. The MAC layer should be able to operate with multiple physical layers, each of which exhibits a different medium sense and transmission characteristic. Candidates for physical layers were infra red and spread spectrum radio transmission techniques.

8. 802.11 WLAN technologies • IEEE 802.11 standards and rates • IEEE 802.11 (1997) 1 Mbps and 2 Mbps (2.4 GHz band ) • IEEE 802.11b (1999) 11 Mbps (2.4 GHz band) = Wi-Fi • IEEE 802.11a (1999) 6, 9, 12, 18, 24, 36, 48, 54 Mbps (5 GHz band) • IEEE 802.11g (2001 ... 2003) up to 54 Mbps (2.4 GHz) backward compatible to 802.11b • IEEE 802.11 networks work on license free industrial, science, medicine (ISM) bands • WiFi - Wireless Fidelity • Wireless Fidelity is a brand, originally licensed by the Wi-Fi Alliance to describe the underlying technology of wireless local area networks (WLAN). A person with a WiFi device can connect to the internet or Local area network using an access point, provided that the device should be in the range of that Wireless access point. Wireless access point (WAP) connects a group of wireless devices to an adjacent wired LAN. Wi-Fi can be less secure than wired connections • Wi MAX (Worldwide Interoperability for Microwave Access) is a wireless communications standard designed to provide 30 to 40 megabit-per-second data rates with the 2011 update providing up to 1 Gbit/s fixed stations. The name "Wi MAX" was created by the Wi MAX Forum, which was formed in June 2001 to promote conformity and interoperability of the standard

9. The IEEE 802.11 and supporting LAN Standards IEEE 802.2 Logical Link Control (LLC) IEEE 802.11 Wireless IEEE 802.5 Token Ring IEEE 802.3 Carrier Sense IEEE 802.4 Token Bus MAC OSI Layer 2 (data link) OSI Layer 1 (physical) PHY g b a star bus ring

10. Portal Distribution System 802.11 - Architecture of an infrastructure network • 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 802.11 LAN STA1 802.x LAN BSS1 Access Point ESS Access Point BSS2 802.11 LAN STA2 STA3

11. 802.11 - Architecture of an infrastructure network • This architecture uses fixed network access points with which mobile nodes can communicate. • Similar to cellular networks. • Network access points may be connected to landlines to enhance the LAN’s capability by bridging wireless nodes to other wired nodes. • Transmission can be direct sequence spread spectrum ,frequency-hopping spread spectrum or infrared pulse position modulation. • Data rates of 1Mbps or 2Mbps. • 802.11 standard specifies a CSMA/CA protocol. • When a node receives a packet to be transmitted ,it first listens to ensure no other node is transmitting. if the channel is clear , it then transmits the packet.

12. 802.11 - Architecture of an infrastructure network • Wireless networks can exhibit two different basic system architectures. Infrastructure-based and ad-hoc. • Several nodes, called stations (STAi), are connected to access points (AP). • Stations are terminals with access mechanisms to the wireless medium and radio contact to the AP. • The stations and the AP which are within the same radio coverage form a basic service set (BSSi). • The example shows two BSSs – BSS1 and BSS2 – which are connected via a distribution system. • A distribution system connects several BSSs via the AP to form a single network and thereby extends the wireless coverage area. This network is now called an extended service set (ESS) and has its own identifier, the ESSID. The ESSID is the ‘name’ of a network and is used to separate different networks. Without knowing the ESSID (and assuming no hacking) it should not be possible to participate in the WLAN. The distribution system connects the wireless networks via the APs with a portal, which forms the interworking unit to other LANs.

13. 802.11 - Architecture of an ad-hoc network • Direct communication within a limited range • Station (STA): terminal with access mechanisms to the wireless medium • Basic Service Set (BSS): group of stations using the same radio frequency • Computers are brought together to form a network “on the fly”. • There is no structure to the n/w. • There are no fixed points. • Usually every node is able to communicate with every other node . • Election algorithm is used to elect one machine as the base station in a n/w . • Uses the broadcast and flooding method to establish who’s who in the n/w. 802.11 LAN BSS1 STA1 STA3 STA2 BSS2 STA5 STA4 802.11 LAN

14. Protocol architecture 802.11 server mobile terminal fixed terminal infrastructure network access point application TCP IP Application TCP IP LLC LLC LLC 802.11 MAC 802.3 MAC 802.3 MAC 802.11 MAC 802.11 PHY 802.11 PHY 802.3 PHY 802.3 PHY

15. PLCP Physical Layer Convergence Protocol clear channel assessment signal (carrier sense) PMD Physical Medium Dependent modulation, coding PHY Management channel selection, MIB Station Management coordination of all management functions MAC access mechanisms, fragmentation, encryption MAC Management synchronization, roaming, MIB, power management 802.11 - Layers and functions Station Management LLC DLC MAC MAC Management PLCP PHY Management PHY PMD

16. Function of layers • IEEE 802.11 wireless LAN connected to a switched IEEE 802.3 Ethernet via a bridge. Applications should not notice any difference apart from the lower bandwidth and perhaps higher access time from the wireless LAN. The WLAN behaves like a slow wired LAN. The upper part of the data link control layer, the logical link control (LLC), covers the differences of the medium access control layers needed for the different media. In many of today’s networks, no explicit LLC layer is visible. •  The IEEE 802.11 standard only covers the physical layer PHY and medium access layer MAC like the other 802.x LANs do. The physical layer is subdivided into the physical layer convergence protocol (PLCP) and the physical medium dependent sub layer PMD. The basic tasks of the MAC layer comprise medium access, fragmentation of user data, and encryption. The PLCP sub layer provides a carrier sense signal, called clear channel assessment (CCA), and provides a common PHY service access point (SAP) independent of the transmission technology. Finally, the PMD sub layer handles modulation and encoding/decoding of signals.

17. Function of layers • Apart from the protocol sub layers, the standard specifies management layers and the station management. The MAC management supports the association and re-association of a station to an access point and roaming between different access points. It also controls authentication mechanisms, encryption, synchronization of a station with regard to an access point, and power management to save battery power. MAC management also maintains the MAC management information base (MIB). •  The main tasks of the PHY management include channel tuning and PHY MIB maintenance. Finally, station management interacts with both management layers and is responsible for additional higher layer functions (e.g., control of bridging and interaction with the distribution system in the case of an access point).

18. Rate Adaptation base station, mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves, SNR varies 802.11: advanced capabilities 10-1 10-2 10-3 BER 10-4 10-5 10-6 10-7 10 20 30 40 SNR(dB) 1. SNR decreases, BER (bit error rate)increase as node moves away from base station QAM256 (8 Mbps) QAM16 (4 Mbps) 2. When BER becomes too high, switch to lower transmission rate but with lower BER BPSK (1 Mbps) operating point

19. 802.11: advanced capabilities Power Management • Node-to-AP: “I am going to sleep until next beacon frame” • AP knows not to transmit frames to this node • node wakes up before next beacon frame • Beacon frame: contains list of mobiles with AP-to-mobile frames waiting to be sent • Node will stay awake if AP-to-mobile frames to be sent; otherwise sleep again until next beacon frame 6: Wireless and Mobile Networks

20. Roaming • Users maintain a continuous connection as they roam from one physical area to another • Mobile nodes automatically register with the new access point. • Methods: DHCP, Mobile IP • IEEE 802.11 standard does not address roaming, you may need to purchase equipment from one vendor if your users need to roam from one access point to another.

21. Roaming • 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

22. Security • In theory, spread spectrum radio signals are inherently difficult to decipher without knowing the exact hopping sequences or direct sequence codes used • The IEEE 802.11 standard specifies optional security called "Wired Equivalent Privacy" whose goal is that a wireless LAN offer privacy equivalent to that offered by a wired LAN. The standard also specifies optional authentication measures.