Wireless Local Area Network Wireless n n n

Wireless Local Area Network

Wireless? n n n 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

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

Integration With Existing Networks n n n 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.

How are WLANs Different? n n n n 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.

Physical and Data Link Layers Physical Layer: n 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: n Uses Carriers-Sense-Multiple-Access with Collision Avoidance (CSMA/CA).

802. 11 WLAN technologies n n 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.

802. 11 WLAN technologies n n n IEEE 802. 11 standards and rates n IEEE 802. 11 (1997) 1 Mbps and 2 Mbps (2. 4 GHz band ) n IEEE 802. 11 b (1999) 11 Mbps (2. 4 GHz band) = Wi-Fi n IEEE 802. 11 a (1999) 6, 9, 12, 18, 24, 36, 48, 54 Mbps (5 GHz band) n IEEE 802. 11 g (2001. . . 2003) up to 54 Mbps (2. 4 GHz) backward compatible to 802. 11 b IEEE 802. 11 networks work on license free industrial, science, medicine (ISM) bands Wi. Fi - 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 Wi. Fi 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 (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

The IEEE 802. 11 and supporting LAN Standards IEEE 802. 2 Logical Link Control (LLC) IEEE 802. 3 Carrier Sense IEEE 802. 4 IEEE 802. 5 IEEE 802. 11 Token Wireless Ring Bus a b g bus star MAC PHY OSI Layer 2 (data link) OSI Layer 1 (physical) ring

802. 11 - Architecture of an infrastructure network n. Station (STA) 802. 11 LAN n ST A 1 802. x LAN terminal with access mechanisms to the wireless medium and radio contact to the access point n. Basic Service Set (BSS) n BSS 1 Access Point ESS n. Access Point Portal n n Access Point bridge to other (wired) networks n. Distribution System n BSS 2 802. 11 LAN station integrated into the wireless LAN and the distribution system n. Portal Distribution System STA 2 group of stations using the same radio frequency STA 3 interconnection network to form one logical network (EES: Extended Service Set) based on several BSS

802. 11 - Architecture of an infrastructure network n n n n 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 1 Mbps or 2 Mbps. 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.

802. 11 - Architecture of an infrastructure network n n n Wireless networks can exhibit two different basic system architectures. Infrastructurebased 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 – BSS 1 and BSS 2 – 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.

802. 11 - Architecture of an ad-hoc network n. Direct communication within a limited range 802. 11 LAN n. Station (STA): terminal with access mechanisms to BSS 1 STA 3 STA 2 BSS 2 STA 4 802. 11 LAN STA 5 the wireless medium n. Basic Service Set (BSS): group of stations using the same radio frequency n Computers are brought together to form a network “on the fly”. n There is no structure to the n/w. n There are no fixed points. n Usually every node is able to communicate with every other node. n Election algorithm is used to elect one machine as the base station in a n/w. n Uses the broadcast and flooding method to establish who’s who in the n/w.

Protocol architecture 802. 11 mobile terminal server fixed terminal infrastructure network access point TCP application Application IP IP TCP LLC LLC 802. 11 MAC 802. 3 MAC 802. 11 PHY 802. 3 PHY

802. 11 - Layers and functions n. MAC n n access mechanisms, fragmentation, encryption Convergence Protocol n n. MAC Management n PLCP Physical Layer synchronization, roaming, MIB, power management n clear channel assessment signal (carrier sense) PMD Physical Medium LLC MAC Management PLCP PHY Management PMD Station Management PHY DLC Dependent n n PHY Management n n modulation, coding channel selection, MIB Station Management n coordination of all management functions

Function of layers n n IEEE 802. 11 wireless LAN connected to a switched IEEE 802. 3 Ethernet via a bridge. Applications should 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.

Function of layers n n 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).

802. 11: advanced capabilities 10 -1 10 -2 10 -3 BER Rate Adaptation n base station, mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves, SNR varies 10 -4 10 -5 10 -6 10 -7 10 20 SNR(d. B) 30 40 1. SNR decreases, BER (bit QAM 256 (8 Mbps) QAM 16 (4 Mbps) BPSK (1 Mbps) operating point error rate)increase as node moves away from base station 2. When BER becomes too high, switch to lower transmission rate but with lower BER

802. 11: advanced capabilities Power Management • Node-to-AP: “I am going to sleep until next beacon frame” m. AP knows not to transmit frames to this node mnode wakes up before next beacon frame • Beacon frame: contains list of mobiles with AP-tomobile frames waiting to be sent m. Node will stay awake if AP-to-mobile frames to be sent; otherwise sleep again until next beacon frame 6: Wireless and Mobile Networks 6 -19

Roaming Users maintain a continuous connection as they roam from one physical area to another n Mobile nodes automatically register with the new access point. n Methods: DHCP, Mobile IP n 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. n

Roaming n n No or bad connection? Then perform: Scanning n n Reassociation Request n n station sends a request to one or several AP(s) Reassociation Response n n n scan the environment, i. e. , listen into the medium for beacon signals or send probes into the medium and wait for an answer success: AP has answered, station can now participate failure: continue scanning AP accepts Reassociation Request n n n signal the new station to 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

Security n n 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.