Wi-Fi Wireless Communications Sheldon Lou What is Wi-Fi? The standard for wireless local area networks (WLANs). It’s like a common language that all the devices use to communicate to each other. If you have a standard, people can make all sorts of devices that can work with each other. It’s actually IEEE 802.11, a family of standards. The IEEE (Eye-triple-E, Institute of Electrical and Electronics Engineers Inc.) is a non-profit, technical professional association of more than 360,000 individual members in approximately 175 countries. The Wireless Ethernet Compatibility Alliance started the Wi-Fi--wireless fidelity--certification program to ensure that equipment claiming 802.11 compliance was genuinely interoperable. US Frequency Bands Band Frequency range UHF ISM 902-928 MHz S-Band 2-4 GHz S-Band ISM 2.4-2.5 GHz C-Band 4-8 GHz C-Band satellite downlink 3.7-4.2 GHz C-Band Radar (weather) 5.25-5.925 GHz C-Band ISM 5.725-5.875 GHz C-Band satellite uplink 5.925-6.425 GHz X-Band 8-12 GHz X-Band Radar (police/weather) 8.5-10.55 GHz Wi-Fi Standards Standard Speed Freq band Notes 802.11 2 Mbps 2.4 GHz (1997) 802.11a 54 Mbps 5 GHz (1999) 802.11b 11 Mbps 2.4 GHz 802.11g 54 Mbps 2.4 GHz ISM Band ISM stands for industrial, scientific, and medical. ISM bands are set aside for equipment that is related to industrial or scientific processes or is used by medical equipment. Perhaps the most familiar ISM-band device is the microwave oven, which operates in the 2.4-GHz ISM band. The ISM bands are license-free, provided that devices are low-power. You don't need a license to set up and operate a wireless network. Wireless LAN Networks WLAN Architecture—Ad Hoc Mode Ad-Hoc mode: Peer-to-peer setup where clients can connect to each other directly. Generally not used for business networks. Ad Hoc Structure Mobile stations communicate to each other directly. It’s set up for a special purpose and for a short period of time. For example, the participants of a meeting in a conference room may create an ad hoc network at the beginning of the meeting and dissolve it when the meeting ends. WLAN Architecture--Mesh Mesh: Every client in the network also acts as an access or relay point, creating a “self- healing” and (in theory) infinitely extensible network. Not yet in widespread use, unlikely to be in homes. WLAN Architecture—Infrastructure Mode To Wired Network Infrastructure network There is an Access Point (AP), which becomes the hub of a “star topology.” Any communication has to go through AP. If a Mobile Station (MS), like a computer, a PDA, or a phone, wants to communicate with another MS, it needs to send the information to AP first, then AP sends it to the destination MS Multiple APs can be connected together and handle a large number of clients. Used by the majority of WLANs in homes and businesses. Comparison of Two Structures Infrastructure Ad hoc Expansion X Flexibility X Control X Routing X Coverage X Reliability X Extended Service Area Roaming In an extended service area, a mobile station (MS) can roam from one BSS (Basic Service Set) to another. Roughly speaking, the MS keeps checking the beacon signal sent by each AP and select the strongest one and connect to that AP. If the BSSs overlap, the connection will not be interrupted when an MS moves from one set to another. If not, the service will be interrupted. Two BSSs coverage areas can largely overlap to increase the capacity for a particular area. If so, the two access points will use different channels, as we will explain later. Antennas All WLAN equipment comes with a built-in omni-directional antenna, but some select products will let you attach secondary antennas that will significantly boost range. Antennas, continued Antennas come in all shapes and styles: Omni-directional: Vertical Whip Ceiling mount Directional: Yagi (“Pringles can”) Wall mounted panel Parabolic dish How Can Several Users Communicate Simultaneously? As we have discussed, there is a difference between a network designed for voice conversation and one for data exchange. For voice conversations, like telephone and cell phone calls, each person has a dedicated channel during the entire conversation. (3G and 4G cell phones are somewhat different, as we will explain later.) For data exchange, many users can share one channel. A user sends information when no one else is sending. New technologies try to accommodate both voice and data transmissions, as we will discuss in this course. Share one channel in data communication In data communication, data are grouped into packets/frames. Each packet/frame contains a number of bits of information. Devices (phones, computers, etc.) don’t communicate simultaneously. It’s like they are sharing one single cable (the air in this case), only one person can use it at one time. Before an MS (mobile station) sends its packets, it checks to see if someone else is sending information. Only when the medium is free can an MS sends packets. If some station is sending or receiving signal, the MS that intends to send will generate a random waiting time and wait for its turn. If several MSs are all waiting for their turns, since their waiting times are randomly generated and thus not equal, they will not start sending simultaneously. Thus collision (two or more MSs sending signals simultaneously) is avoided. It’s called Carrier Sensing Multiple Access with Collision Avoidance (CSMA/CA). How does CSMA/CA (Carrier Sensing Multiple Access with Collision Avoidance) Work? (p. 189, Example 4.18) RTS/CTS (Request-to-send/clear-to-send) Use Request-to-send/clear-to-send (RTS/CTS) mechanism (p. 191-192, Fig. 4.17 & p. 462, Fig. 11.14) to avoid collision when two MSs cannot hear each other (blocked by a wall …). A terminal ready for transmission sends an RTS packet identifying the source address, destination address, and the length of the data to be sent. The destination station responds with CTS packet. The source terminal receives the CTS and sends the data. Other terminals go to the virtual carrier-sensing mode (NAV signal on), therefore the source terminal sends its packet with no contention. After completion of the transmission, the destination station sends an ACK, opening contention for other users. Spread spectrum in 802.11 It is a requirement imposed by the regulatory authorities for devices in ISM band in order to reduce interference. There is also limitations on transmitted power. We discuss two methods specified in 802.11, FHSS and DSSS. DSSS in 802.11 Used by 802.11b Symbol transmission rate = 1Mbps Multipath spread of up to 1/1 Mbps = 1 µs does not cause ISI. For indoor applications this ensures that the system does not suffer from ISI. Chip rate = 11 Mcps Resolution is on the order of 1/11 Mcps = 90 ns. Use Barker code (Example 3.16, p. 116). Complementary code keying (CCK) Used to increase the data rate to 11 Mbps Example 17, p. 119 Sec. 11.3.4, p. 457 Frequency Hopping in 802.11 The frequency can hop over 78 hopping channels each separated by 1 MHz. The first channel, Channel 0, starts at 2.402 GHz. Channel 1 is at 2.403 GHz, Channel 2, 2.404 GHz, and so on up to Channel 77 at 2.479 GHz (US, Canada, and Europe standards). These frequencies are divided into three patterns of 26 hops each corresponding channel numbers (0, 3, 6, 9, …, 75), (1, 4, 7, 10, …, 76), (2, 5, 8, 11, …, 77), see p. 454, Fig. 11.5. Three APs can coexist without any hop collision, that results in a threefold increase in the capacity of the cell. Hop rate = 2.5 hops per second. Frequency bands for DSSS FHSS uses 1 MHz bandwidth (narrowband), but the center frequency hops over 76 MHz. DSSS uses a chip rate of 11 Mcps which occupies around 26 MHz of bandwidth (wideband). The ISM band at 2.4 GHz is divided into 11 overlapping channels spaced by 5 MHz (see Fig. 11.6, P. 455). APs located close to each other can choose different channels to mitigate interference. The coverage areas of two access points (Basic Service Sets, BSS) may overlap to increase capacity. For example, up to 8 users can use VoIP simultaneously through one access point. With two overlapping APs, 16 users can talk simultaneously. But the two APs have to use non-overlapping channels. Modulation Gaussian frequency shift keying (GFSK) is used (Sec. 3.6.1, p. 97). Wi-Fi network services Distribution and integration Association, re-association, and disassociation Authentication and deauthentication Providing privacy Distribution This service is used by mobile stations in an infrastructure network every time they send data. Once a frame has been accepted by an access point, it uses the distribution service to deliver the frame to its destination. Any communication that uses an access point travels through the distribution service, including communications between two mobile stations associated with the same access point. Integration Integration is a service provided by the distribution system; it allows the connection of the distribution system to a non-IEEE 802.11 network. The integration function is specific to the distribution system used and therefore is not specified by 802.11, except in terms of the services it must offer. Association Delivery of frames to mobile stations is made possible because mobile stations register, or associate, with access points. The distribution system can then use the registration information to determine which access point to use for any mobile station. Reassociation When a mobile station moves between basic service areas within a single extended service area, it must evaluate signal strength and perhaps switch the access point with which it is associated. Reassociations are initiated by mobile stations when signal conditions indicate that a different association would be beneficial; they are never initiated by the access point. After the reassociation is complete, the distribution system updates its location records to reflect the reachability of the mobile station through a different access point. Disassociation To terminate an existing association, stations may use the disassociation service. When stations invoke the disassociation service, any mobility data stored in the distribution system is removed. Once disassociation is complete, it is as if the station is no longer attached to the network. Disassociation is a polite task to do during the station shutdown process. The MAC is, however, designed to accommodate stations that leave the network without formally disassociating. Authetication/deauthentication Physical security is a major component of a wired LAN security solution. Wired network’s equipment can be locked inside offices. Wireless networks cannot offer the same level of physical security, however, and therefore must depend on additional authentication routines to ensure that users accessing the network are authorized to do so. Authentication is a necessary prerequisite to association because only authenticated users are authorized to use the network. (In practice, though, many access points are configured for "open-system" mode and will authenticate any station.) Deauthentication terminates an authenticated relationship. Because authentication is needed before network use is authorized, a side effect of deauthentication is termination of any current association.
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