Wireless Networking (The Morgan Kaufmann Series in Networking)
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Next, we provide ataxonomy of current wireless networks. The material in the book is organizedalong this taxonomy.
The Morgan Kaufmann Series in Networking
Then, in this chapter, we identify the common basictechnical elements that underlie any wireless network as being 1 physical wirelesscommunication; 2 neighbor discovery, association, and topology formation; and 3 transmission scheduling. Finally, we provide an overview of the contents of the remaining nine chaptersof the book. As shown in Figure 1. The information services layer comprises all the hardwareand software required to facilitate the necessary transport services, and to attachthe sources or sinks to the wireless network; for example, voice coding, packetbuffering and playout, and voice decoding, for packet telephony; or similar Wireless networking is concerned with algorithms for resource allocationbetween devices sharing a portion of the radio spectrum.
We turn now to the bottom layer in Figure 1. In wireline networks theinformation to be transported between the endpoints of applications is carriedover a static bit-carrier infrastructure. These networks typically comprise high-quality digital transmission systems over copper or optical media. The left side of the bottom layer in Figure 1. Typically, each wireless network system is constrained to operate insome portion of the RF spectrum.
For example, a cellular telephony system maybe assigned 5 MHz of spectrum in the MHz band. Information bits aretransported between devices in the wireless network by means of some physicalwireless communication technique i. It is well known, however, that unguided RF communicationbetween mobile wireless devices poses challenging problems.
Unlike wirelinecommunication, or even point-to-point, high-power microwave links betweendish antennas mounted on tall towers, digital wireless communication betweenmobile devices has to deal with a variety of time-varying channel impairments In order to combat these problems, it is imperative that in a mobile orad hoc wireless network the PHY layer should be adaptable. In fact, in somesystems multiple modulation schemes are available, and each of these may havevariable parameters such as the error control codes, and the transmitter powers.
Hence, unlike a wired communication network, where we can view networkingas being concerned with the problems of resource sharing over a static bit carrierinfrastructure, in wireless networking, the resource allocation mechanisms wouldinclude these adaptations of the PHY layer. Thus, in Figure 1.
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Thus we beginour treatment by taking a look at a taxonomy of the current practice of wirelessnetworks. Figure 1. Several commonly used terms ofthe technology will arise as we discuss this taxonomy. These will be highlightedby the italic font, and their meanings will be clear from the context.
Of course,the attendant engineering issues will be dealt with at length in the remainder ofthe book. Fixed wireless networks include line-of-sight microwave links, which untilrecently were very popular for long distance transmission. Such networks basicallycomprise point-to-point line-of-sight digital radio links.
When such links are setup, with properly aligned high gain antennas on tall masts, the links can be viewedas point-to-point bit pipes, albeit with a higher bit error rate than wired links. Currently, the most importantrole of wireless communications technology is in mobile access to wired networks. The cellular systems that have the most widespread deployment are the ones thatshare the available spectrum using frequency division multiplexed time divisionmultiple access FDM-TDMA technology.
Among such systems by far the mostcommercially successful has been the GSM system, developed by a Europeanconsortium. Each of these bands is staticallyor dynamically partitioned into reuse subbands, with each cell being allocatedsuch a subband this is the FDM aspect. The partitioning of the up-link anddown-link bands is done in a paired manner so that each cell is actually assigneda pair of subbands. Each arriving call request in a cell is then assigned a slot inone of the carriers in that cell; of course, a pair of slots is assigned in pairedup-link and down-link channels in that cell.
Thus, since each call is assigneddedicated resources, the system is said to be circuit multiplexed, just like thewireline phone network. These are narrowband systems i.
The needfor allocation of frequency bands over the network coverage area perhaps evendynamic allocation over a slow timescale , and the grant and release of individualchannels as individual calls arrive and complete, requires the control of suchsystems to be highly centralized.
Note that call admission control, that is, callblocking, is a natural requirement in an FDM-TDMA system, since the resourcesare partitioned and each connection is assigned one resource unit. Another cellular technology that has developed over the past 10 to 15 yearsis the one based on code division multiple access CDMA. In these networks, theentire available spectrum is reused in every cell. Although no frequency planning is required for CDMA systems, the performanceis interference limited as every transmitted signal is potentially an interferer for Thus at any point of time there is an allocation of powersto all the transmitters sharing the spectrum, such that their desired receivers candecode their transmissions, in the presence of all the cross interferences.
Thesedesired power levels need to be set depending on the locations of the users, and theconsequent channel conditions between the users and the base stations, and needto be dynamically controlled as users move about and channel conditions change. Hence tight control of transmitter power levels is necessary. Further, of course, theallocation of spreading codes, and management of movement between cells needsto done.
We note that, unlike the FDM-TDMA system described earlier, there is nodedicated allocation of resources frequency and time-slot to each call. Indeed,during periods when a call is inactive no radio resources are utilized, and theinterference to other calls is reduced.
If there are several calls in the system, each needing certain qualityof service QoS bit rate, maximum bit error rate , then the number of calls in thesystem needs to be controlled so that the probability of QoS violation of the calls iskept small. This requires call admission control, which is an essential mechanismin CDMA systems, in order that QoS objectives can be achieved. Evidently, theseare all centrally coordinated activities, and hence even CDMA cellular systemsdepend on central intelligence that resides in the base station controllers BSCs. Earlier, we have described twotechnologies for second generation 2G cellular wireless telephony.
Recently, withthe growing need for mobile Internet access, there have been efforts to providepacketized data access on these networks as well. A further evolution is the EDGE Enhanced Data ratesfor GSM Evolution system, where, in addition to combining TDM slots, higherorder modulation schemes, with adaptive modulation, are utilized to obtain sharedpacket switched links with speeds up to Kbps. These two systems often areviewed, respectively, as 2. These aredata evolutions of an intrinsically circuit switched system that was developed formobile telephony.
In fact, both voice and data can be carried in the packet mode, with the user bitrate, the amount of spreading, and the allocated power changing on a packet-by-packet basis. Cellular networks were developed with the primary objective of providingwireless access for mobile users. With the growth of the Internet as the de factonetwork for information dissemination, access to the Internet has become anincreasingly important requirement in most countries.
It is with such an applicationin mind that the IEEE The PHY technologies that have been utilized areorthogonal frequency division multiple access OFDMA and multiple antennasat the transmitters and the receivers. The latter are commonly referred to asMIMO multiple-input-multiple-output systems. In order to permit up-link and down-link transmissions,time is divided into frames and each frame is further partitioned into an up-link anda down-link part this is called time division duplexing TDD.
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This kindof TDD MAC structure has been used in several earlier systems; for example,satellite networks involving very small aperture satellite terminals VSATs , andeven in wireline systems such as those used for the transmission of digital dataover cable television networks. Cellular networks have emerged from centrally managed point-to-point radiolinks, but another class of wireless networks has emerged from the idea of randomaccess, whose prototypical example is the Aloha network.
Spurred by advancesin digital communication over radio channels, random access networks can nowsupport bit rates close to desktop wired Ethernet access. Hence random accesswireless networks are now rapidly proliferating as the technology of choice forwireless Internet access with limited mobility.
The most important standards forsuch applications are the ones in the IEEE Networks based onthis standard now support physical transmission speeds from a few Mbps overs of meters up to Mbps over a few meters. Nodes contend for thechannel, and possibly collide. In the event of a collision, the colliding nodes back Whena node is able to acquire the channel, it can send at the highest of the standardbit rates that can be decoded, given the channel condition between it and itsreceiver. In thelatest enhancements to the IEEE With the widespread deployment of IEEE The emerging concept of fourth-generation wireless access networks envisionsmobile devices that can support multiple technologies for physical digital radiocommunication, along with the resource management algorithms that wouldpermit a device to seamlessly move between 3G cellular networks, IEEE With reference to the taxonomy in Figure 1.
Thus, insuch networks, in the path between two user devices there is only one or at mosttwo wireless links. On the other hand a wireless ad hoc network comprises severaldevices arbitrarily located in a space e. Each device is equipped with a radio transceiver, all of which typically sharethe same radio frequency band. In this situation, the problem is to communicatebetween the various devices. Nodes need to discover neighbors in order to form atopology, good paths need to be found, and then some form of time scheduling oftransmissions needs to be employed in order to send packets between the devices.
Packets going from one node to another may need to be forwarded by othernodes. Thus, these are multihop wireless packet radio networks, and they havebeen studied as such over several years. Interest in such networks has again beenrevived in the context of multihop wireless internets and wireless sensor networks. In some situations it becomes necessary for several mobile devices suchas portable computers to organize themselves into a multihop wireless packetnetwork.
Such a situation could arise in the aftermath of a major natural disastersuch as an earthquake, when emergency management teams need to coordinatetheir activities and all the wired infrastructure has been damaged. Thus, we can call such a network a In general, such a network could attach at some pointto the wired Internet. Whereas multihop wireless internets have the service objective of supportinginstances of point-to-point communication, an ad hoc wireless sensor networkhas a global objective.
Each sensor monitors itsenvironment and the objective of the network is to deliver some global informationor an inference about the environment to an operator who could be locatedat the periphery of the network, or could be remotely connected to the sensornetwork. An example is the deployment of such a network in the border areas of acountry to monitor intrusions.
We organized our presentation around a taxonomy of wireless networksshown in Figure 1. Although the technologies that we discussed may appear tobe disparate, there are certain common technical elements that constitute thesewireless networks. The following is an enumeration and preliminary discussion of the technicalelements.
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There is, of course, no communication network unless bits can be transported between users. Digital communication over mobile wireless links has evolved rapidly over the past two decades. Several approaches are now available, with var- ious tradeoffs and areas of applicability.
Even in a given system, the digital communication mechanisms can be adaptive. First, for a given digital mod- ulation scheme the parameters can be adapted e. This adaptivity is very useful in the mobile access situation where the channels and interference levels are rapidly changing. Neighbor discovery, association and topology formation, routing. For example, in an access network each mobile device could be in the vicinity of more than one BS or access point AP. To simplify our writing, we will refer to a BS or an AP as an access device.
It is a nontrivial issue as to which access device a mobile device connects through.