interfer-Some of the important issues in designing indoor wireless environments are sion media like infrared or radio frequency; channel coding schemes like TDMA, CDMA,etc., and spreadin
Trang 1in-in digital communications, portable devices, semiconductor technology and the
availabili-ty of license-free frequency bands Another major factor that presents a real opportuniavailabili-tyfor data networking in indoor environments is the massive growth and usage of the Inter-net Examples include homes, offices, trading floors in stock exchanges, conventions andtrade shows, and so on
Applications for indoor environments typically require ad hoc connectivity, especially
in the case of a population of mobile users while they are within range of foreign agents orstations connected to the Internet These users might need to send data files to each other,run some local applications or use any of the existing internet-based applications available
on wired terminals located within their range Ad hoc networking is a name given to thecreation of dynamic and multihop networks that are created by the mobile nodes as need-
ed for their communication purposes [26] This area has also received a lot of attentionfrom the research community, and represents interesting challenges for networking appli-cations The IETF Mobile Ad Hoc Network (MANET) group [20] is attempting to estab-lish standards for creation of ad hoc networks
Several technologies have emerged to support wireless networking in indoor ments Some of the most popular ones are wireless local area networks (LANs) [4, 10,17], HomeRF [9, 24], and Bluetooth [33] Wireless LANs are especially suitable for appli-cations that involve Internet addressable devices Wireless LAN devices communicatepackets “over the air,” and their programming models are similar to those of wired LANs.HomeRF, as its name suggests, is chiefly aimed at the home environment This presents anopportunity to extend the reach of the PC and Internet throughout the home, to legacy ap-plications like telephony, audio/video entertainment, home appliances, and home controlsystems Bluetooth is a technology meant for low-cost, low-power, indoor environments,and provides ad hoc connectivity among wireless devices; this is fast gaining popularity inthe pervasive computing space as a cable-replacement solution
environ-The rest of the chapter is organized as follows We first discuss issues in the design of
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Copyright © 2002 John Wiley & Sons, Inc ISBNs: 0-471-41902-8 (Paper); 0-471-22456-1 (Electronic)
Trang 2the physical layer, followed by a detailed description of some media access control cols proposed specifically for the indoor environment We then discuss network topolo-gies, with special reference to Bluetooth Finally, we point to possible characterizations ofindoor environments, and the new emerging paradigm of nomadic computing
At the physical layer, the main objective is to detect signals between the two endpoints of
a wireless communication link Wireless media typically have vague and uncontrollableboundaries for broadcast range, low to medium bandwidth, and possibly asymmetric con-nectivity One of the main problems encountered with indoor radio wave propagation isthe multipath spread of signals due to reflection off walls and internal objects, resulting infast (or short-term) fading Slow (or long-term) fading is also a characteristic of indoorchannels; it is due to mobile objects in the range of the transmitter or receiver
Multipath propagation is the simultaneous arrival at the receiver of signals propagatedover different paths, with different path lengths When the path lengths differ by more than
a small fraction of the symbol time, multipath propagation produces intersymbol ence—the presence of energy from a previous symbol at the time of detection of the cur-rent symbol When the path lengths differ by a (a multiple of) half a wavelength, signalsarriving over different paths may partially or totally cancel at the receiver This phenome-non is called Raleigh fading Multipath effects can be mitigated by spread spectrum tech-niques and diversity in the receivers [34] Considerable work has been done on diversitytechniques, though this is not the focus of this chapter
interfer-Some of the important issues in designing indoor wireless environments are sion media like infrared or radio frequency; channel coding schemes like TDMA, CDMA,etc., and spreading techniques like direct spread, frequency hopping, etc From the point
transmis-of view transmis-of higher-level protocols, the channel encoding scheme and physical medium areorthogonal issues
28.2.1 Transmission Medium
The choice of transmission media [4], namely infrared or radio frequency, is one of thefirst issues to be resolved when designing a wireless network Infrared (IR) frequenciesrequire line-of-sight transmission and reception There is also severe attenuation by walls,people, etc., which can be used to one’s advantage since it makes it difficult to intercept.The transmitters and receivers are also less expensive since it detects the power of opticalsignals and not their frequency or phase Another advantage is that it is license-free How-ever, the need for line-of-sight signaling makes it extremely susceptible to mobility andthere is also a strong possibility of collisions going undetected IR systems share a region
of the electromagnetic spectrum dominated by natural sunlight and also used by cent lights and fluorescent lights, which limits the environments in which it is used Al-though IR signals are impaired by multipath propagation, they are not significantly affect-
incandes-ed by Raleigh fading because of their extremely short wavelength and hence extremelysmall spatial extent of a fade
Trang 3Radio frequency (RF) transmission has been around for a long time and has thereforeplaced demands on the frequency spectrum, limiting its availability This also leads to theinterest in the Industrial, Scientific, and Medical (ISM) band, which is license-free TheFederal Communications Commission (FCC) has set forth rules and regulations for use ofthis band for use in the United States The ITU-T coordinates these assignments world-wide Limited availability has also lead to the growth of spread spectrum signaling tech-niques, which require complex transmitters and receivers An advantage of RF over IR isthat it is not easily attenuated by walls, floors, etc and hence can be used for building-wide connectivity However, RF is extremely susceptible to interference from officeequipment like copiers, etc and from microwave ovens, which operate in the same band,
in addition to atmospheric and galactic noise RF signals are also impaired by multipathpropagation and Raleigh fading Since the wavelengths used may be comparable to the di-mensions of a portable computer, the probability of occurrence of a Raleigh fade is veryhigh
28.2.2 Transmission Technology
In radio systems, the choice of transmission technology determines the performance to alarge extent, in terms of cost, interference rejection, and the capability to isolate adjacentcoverage areas The most successful signaling techniques in dealing with interference in anoisy medium are the spread spectrum techniques, which spread the signal’s energy overthe full bandwidth of the channel Two of these methods have gained popularity—frequen-
cy hopping spread spectrum (FHSS) and direct sequence spread spectrum (DSSS) Spreadspectrum techniques also have built-in security since pseudorandom sequences are used
by the transmitter and receiver for detecting the signals, which mitigates multipath effects
In FHSS systems, the signal is spread over a wide frequency band, i.e., the bandwidth
of any one chip or hop interval is much smaller than the full frequency band Both mitter and receiver hop on a pseudorandom sequence of frequencies Frequency hoppingachieves interference suppression by avoidance The time spent on each channel is called achip There are two types of frequency hopping, namely, slow frequency hopping and fastfrequency hopping, based on whether the rate at which the frequency is changed (chiprate) is less than or greater than the bit rate, respectively Fast frequency hopping systemsare more robust, but are also costly and consume more power compared to slow frequencyhopping systems FHSS systems are also not scalable to very high bandwidth systems due
trans-to physical constraints on the attainable chip rate, as is evident by the choice of DSSS forthe next-generation systems
In DSSS systems, a pseudorandom sequence is used to modulate the transmitted nal This is typically accomplished by XOR-ing the user data and the sequence At the re-ceiver, the signal is demodulated and the signal is XOR-ed with the same sequence to getback the original signal The relative rate between the pseudorandom sequence and theuser data (the spreading factor) is typically between 10 and 100 for commercial systems.DSSS systems utilize the averaging method for interference suppression, unlike the FHSSsystems
sig-The IEEE 802.11 wireless LAN draft standard provides for three different types ofphysical layers: the 2.4 GHz ISM band FHSS radio, the 2.4 GHz ISM band DSSS radio,
Trang 4and the IR light The specified data rates are 1 Mbps and 2 Mbps for each of the above.However, most of the attention has been on the radio physical layers The FH systems de-fined in this standard are slow FH systems, in which the data is transmitted over a se-quence of 79 frequencies, with the transmitter “dwelling” on each frequency for a fixedlength of time Adjacent and overlapping cells use different hopping patterns, thus making
it unlikely that the same frequency will be used at the same time by two adjacent cells Inthe DS physical layer, only one predefined spreading signal is used, with a spreading fac-tor of 11 (10.4 dB), which permits some resilience to narrowband noise
The HomeRF sees the shared wireless access protocol (SWAP) as one of the options ofthe main connectivity options SWAP has native support for TCP/IP networking and inter-net access, and for voice telephony The physical layer specification for SWAP was large-
ly adapted from the IEEE 802.11 FH and OpenAir standards with modifications to reducecosts while maintaining performance Some of the key SWAP physical layer specifica-tions include hopping time of 300 microseconds In the optional low-power mode, typicalrange is expected to be 10–20 m, but higher powers could result in a range of about 50 m Bluetooth systems operate in the 2.4 GHz ISM band, which allows a maximum datarate of 1 Mbps These are frequency hopping systems with a 79 or 23 frequency pseudo-random sequence, where the hop rate is 1600 hops per second, which makes the dwelltime on each frequency is 625 microseconds The channel is slotted, each time slot corre-sponding to one frequency The basic unit of a Bluetooth network is called a piconet (seeSection 28.4 for more details) and has a star topology, with a “master” device at the center
of the star The frequency hopping sequence is determined by the clock of the master vice, to which all the “slave” devices connected to it are synchronized This also makes thehopping sequence unique for a piconet
de-A comparison of the essential features of the physical layers of the IEEE 802.11,HomeRF, and Bluetooth standards are shown in Table 28.1
The function of media access control (MAC) is to allow multiple devices to use the sameshared medium, the wireless channel in this case, with minimum interference and maxi-mum performance benefits Existing MAC protocols can be divided into two groups: con-tention-based and contention-free A contention-based protocol requires a station to com-pete for control of the transmission channel each time it sends a packet In this section, we
TABLE 28.1 Physical layer characteristics
HomeRF IEEE 802.11 BluetoothPeak data rate 1.6 Mbps 1 Mbps & 2 Mbps 1 Mbps
Transmit power Up to +24 dBm 100 mW or less 1 mW
Hopping time 300 microseconds <400 milliseconds 625 microsecondsRange in home > 50 m > 50 m < 10 m
Trang 5give a brief overview of a well-known contention-based protocol, namely, the CSMA ily We then describe the balanced media access methods, which have been proposed forcommercial wireless LANs, followed by the hybrid CDMA/ISMA protocol Among thecontention-free protocols, we discuss the GAMA-PS protocol, which is aimed at indoorenvironments We then give a brief description of the MACs of IEEE802.11, HomeRF,and Bluetooth
ty, ALOHA systems are the simplest since the stations can only be in one of two states:transmitting or idle
Carrier Sense Multiple Access Protocols
CSMA (carrier sense multiple access) protocols, which belong to the ALOHA family,have been used in several packet radio networks as well as wireline media like Ethernet.These protocols attempt to prevent a station from transmitting simultaneously with otherstations in its radio range by asking the station to listen before it transmits These are alsotermed random access techniques since there is no predictable or scheduled time for a sta-tion to transmit In a CSMA system, stations can be in three possible states: transmitting,idle, or listening These are simple to implement When the propagation delay is smallcompared to the packet transmission time, the throughput of the CSMA system is signifi-cantly better than that of ALOHA However, the CSMA systems are also unstable underheavy loads Carrier sensing may also not always be possible in a wireless medium due tothe hidden terminal problem A station that wants to transmit a packet cannot accuratelyascertain if it will arrive without collisions at the receiver, since it cannot hear the trans-missions from other senders that might arrive at the same intended receiver The perfor-mance of CSMA degrades to that of ALOHA in the presence of hidden terminals 1-per-sistent, nonpersistent, and p-persistent CSMA [16] are some of the CSMA protocols thathave been proposed; they are rightly called CSMA/CD (CSMA with collision detection).CSMA/CD is very difficult to implement in indoor environments as it may not be possiblefor sources to actually detect a collision in the presence of severe fading Another disad-vantage is that packet delays are unbounded, which makes it unsuitable for voice traffic CSMA with collision avoidance (CSMA/CA) was proposed to alleviate the hiddenstation problem CSMA/CA with a four-way handshake is used to combat the problem
of indoor fading channels In this version of CSMA, the channel is reserved by anRTS/CTS (request to send message/clear to send message) exchange, and then transmis-sion is ensured by data/ACK exchange CSMA/CA is based on multiple access collisionavoidance (MACA) protocols [11] The basic idea is for the sender to transmit an RTSthat the receiver acknowledges with a CTS If this exchange is successful, the sender isallowed to transmit data packets If not, then the source station backs off for a random
Trang 6time period before trying again The MACA and MACAW [5] protocols perform poorlysince the time periods of RTS contentions can be very long Several other protocols havebeen proposed that are based on RTS/CTS exchanges; they differ in the methods used toresolve the collisions of RTSs FAMA [6] protocols that use carrier sensing perform well
in networks with hidden terminals, but carrier sensing is not available in several spreadspectrum radios
Balanced Media Access Methods
Since the wireless medium is a critical shared resource, it is important that the MAC tocol provide fairness and robustness to the wireless network This is called the fairnessproblem There has been some recent work on balanced MACs [26], which are easy to im-plement in commercial wireless LANs These are basically p-persistent CSMA-based al-gorithms in which a fair wireless access for each user is achieved using a precalculated
pro-link access probability, p ij , that represents the link access probability from station i to tion j In classical p-persistent protocols, the probability p is constant, and a station sends
sta-packets with this probability after the back-off period, or back off again with probability 1
– p, using the same back-off window size The balanced MAC methods show how to vary
these probabilities dynamically by using a distributed approach These probabilities arecalculated at the source station in two ways: connection-based and time-based Each ac-tive user broadcasts information either on the number of logical connections or the aver-age contention time to the stations within range This exchange provides a partial under-standing of the topology of the network of stations Based on the mechanism ofinformation exchange (during the link access or periodic), the balanced MAC can be oftwo types: connection-based and time-based
In the connection-based balanced MAC, stations calculate link access probabilities fortheir logical links based on the information of the number of connections of themselvesand neighbor stations A logical link between two stations within wireless range and “visi-ble” to each other represents the physical link between them An example topology is giv-
en in Figure 28.1 Let A i be the source station and B jbe the group of stations visible to it
Let C k be the group of stations hidden from A i Each C k is connected to at least one B j The
rest of the stations are denoted by D1 Source station A iattempts to send an RTS packet to
station B j after the back-off period with probability p ij, or backs off again with probability
1 – p ij, using the same back-off window size Each station broadcasts information on thenumber of connections to all stations within its reach The computation of link access
probabilities for station A iis described below
앫 V i : The set of stations that are visible to the source station A i The members of this
set correspond to the stations labeled B j This is called the visible set
앫 S j : The number of logical connections of station B j
앫 S: The set of all S j ’s for each B j This is called the connection set
앫 S A : The number of connections of the source station A i This is called the connection
value, which has the following property: S A j 僆Vi S j
앫 S A
max
: The maximum value of the members in the connection set, known as the
max-imum connection value and defined as Smax= max僆Vi {S}
Trang 7The link access probabilities are calculated as follows:
앫 Case 1: If S Aj 僆Vi S j , then p ij= 1 j 僆 Vi This corresponds to the case where thesource station is directly connected to all stations and there is no hidden terminal
앫 Case 2: If S A< j 僆Vi S j and S j = S Amax, then p ij = min{1, (S A /S Amax)}
앫 Case 3: If S A< j 僆Vi S j and S j S A
max
, then p ij = (S j / S Amax)
The last two cases imply that either connection A ihas hidden terminals or there is at least
one connection between at least one pair of B jstations Clearly, this method gives higherpriority to the station with the maximum connection value, since this station has higherdata traffic than other stations in a fully loaded network The priorities of the other linksare in proportion to their maximum connection value Figure 28.1 shows the link accessprobabilities calculated according to the above method
In the time-based balanced MAC, the link access probabilities are calculated according
to the average contention period, which is defined as the time interval between the arrival
of the packet to the MAC layer and its actual transmission This period covers collisions,back-off periods, and listening periods, in which another station captures the channel (Alistening period is the time interval for which the intended sender is a nonparticipant sta-tion until the channel is idle again.)
In this method, each station periodically broadcasts a packet to all its logical links This
Figure 28.1 Balanced MAC (connection-based method)
B1
C3C1
C2
B1
C3C1
C2
3/5 3/4
4/5
4/5 2/5 1/5
1/4
1 1/2
2/5
1/4 1/5 1/2 3/4
2/5 2/5 3/5
3/5
2/5 2/3
2/3 2/3
1/2 2/3
1/4
1/3
Trang 8packet contains information on the average contention period of that specific link and a
traffic link descriptor, L ij Stations update link access probabilities every time they receivethis packet The link traffic descriptor is defined as:
L ij= 冦
The link access probability from station i to station j is defined as
p ij=
where T ij is an average contention period from station i to station j and is a weight factor
of the average contention period Thus, the time-based method calculates the link accessprobability of a link by dividing its average contention period by the mean value of thecontention periods of all its neighbor links If the link is blocked, the average contentionperiod of that specific link increases and, eventually, the contention period of all its neigh-bor links decreases In this way, higher priority is given to a link that is blocked and lesspriority to a link that is dominant over other links The weight factor, , controls the rate ofincrease of the probability according to the contention period In this algorithm, the linktraffic descriptor carries the information on the traffic demand in the previous period and,hence, a link with no traffic is not taken into consideration
It should be noted that the connection-based method does not have any overheads asdoes the periodic broadcast of the time-based method Moreover, in the latter method, theweight factor needs to be estimated for each scenario However, the performance of thetime-based method is found to be better than the connection-based method when the net-work load differs from link to link, as seen from the simulation results provided in [25].The results also show that the connection-based method always achieves a very reasonablefair access
Hybrid CDMA/ISMA Protocol
It is well known that code division multiple access (CDMA) improves the survival chance
of packets in the wireless channel CDMA is a DSSS method that uses noise-like carrierwaves, which makes the effective noise the sum of all other user signals Multiple userscan use each CDMA carrier frequency, as they are allocated different codes by which theirsignals are modulated However, in an indoor environment, implementing full CDMAwould mean that the number of codes used would equal the number of terminals in thenetwork, which can be quite large It would also become expensive, since a separate re-ceiver would be needed for each code
Another MAC protocol that has gained widespread acceptance is the inhibit sense tiple access (ISMA) method In this method, the current state of the medium is signaledvia a busy tone The base station signals on the downlink (to the terminals), and the termi-nals do not transmit until the busy tone stops The base stations signals collisions and suc-
Trang 9cessful transmissions via the busy tone and acknowledgements, respectively ISMA isknown to limit contention in the channel, and p-persistent ISMA is the form of ISMA in
which a station transmits with probability p at the end of a busy period
It is natural to expect that the performance of a hybrid CDMA/ISMA network will bequite good—at least better than the performance of the protocols separately The hybridprotocol proposed in [32] for indoor wireless communications combines CDMA with p-persistent ISMA This protocol has two key features: it solves the hidden terminal problem
by routing all traffic via a central base station; it also allows many users with a relativelyshort transmission code onto the same network by having the users share the same code
function using the ISMA protocol Thus, for an indoor wireless network, N tusers will be
divided into n groups with different codes, where each group will have N t /n users with the
same code
The network is modeled as a set of receivers star-connected by wire to a central basestation Around each receiver we have a cluster of terminals sharing the same transmissioncode (see Figure 28.2) The model assumes perfect power control, and does not take thenear–far effect into account It also assumes that the terminal being serviced can deter-mine within the same time slot whether the transmission was errorless or not by listening
to the broadcast packet When a data packet needs to be transmitted by the user, the nal waits for the beginning of the next time slot and transmits the data to its receiver,which forwards it to the base station The base station, in turn, broadcasts the packet to allterminals and the destination terminal receives it In order to control the flow of traffic,the base station broadcasts the busy tone to all the terminals At the beginning of eachtime slot, the busy tone is interrupted long enough to allow all terminals to start theirtransmissions If more than one terminal forwards a packet to the base station, the basestation picks a packet to service at random and broadcasts it to all terminals All otherpackets are ignored and must be retransmitted
termi-BS
Legend
BS Base StationReceiverTerminal
Figure 28.2 CDMA/ISMA network architecture
Trang 10Thus, terminals in the network have two states: idle or blocked Initially, all terminalsare idle Whenever a packet arrives at a terminal, it goes into the blocked state and ser-vices the packet according to the algorithm described in Figure 28.3 Each blocked termi-nal waits for the start of the next time slot before it attempts transmission It then transmits
the packet with probability p
The performance of this method has also been analyzed using a Markov model [31]
p-persistent ISMA
Idle
packet arrival
Trang 1128.3.2 Contention-Free Protocols
Many contention-free protocols have also been proposed, including fixed assignment (intime or frequency), polling, token passing, or dynamic reservations With completely de-terministic access method like the time division multiple access (TDMA), a central entitywould be required for assigning the channel to the various stations Such a method is alsonot feasible for indoor environments, which have strong fading characteristics However, itmakes the management and control of the network easier and more effective Portableunits using this scheme would be simpler and would use significantly less power Recentexamples of dynamic reservation protocols that build a transmission schedule include thecollision avoidance and resolution multiple access (CARMA-NTG) [7] and the group al-location multiple access (GAMA) [22] protocols
Group Allocation Multiple Access with Packet Sensing (GAMA-PS)
The concept of GAMA protocols was first introduced in [22] to provide performanceguarantees in asynchronous MAC protocols GAMA/CD provides the advantages of bothTDMA and CSMA/CD by maintaining a dynamically sized cycle that varies in length de-pending on the network load; each cycle is composed of a contention period and a grouptransmission period During the contention period, a station with one or more packets tosend competes for membership in the transmission group Once a member of the trans-mission group, a station is able to send data without collision during each cycle; as long as
a station has data to send, it maintains its position in the group This can be viewed as ther allowing stations to “share the floor” in an organized manner, or as establishingframes that are not synchronized on a slot basis and vary their length dynamically based
ei-on demand
Wireless LANs using spread spectrum radios cannot provide accurate carrier sensingand time slotting The GAMA-PS (packet sensing), proposed in [23] is aimed at such en-vironments and provides dynamic reservations of the channel It can operate in fully con-nected wireless LANs without base stations and in wireless LANs with hidden terminals
by means of base stations With packet sensing, stations are unable to detect the carrierand must operate on the basis of complete packets they receive; asynchronous access tothe channel means that no time slotting is required for the protocol to operate The com-plexity of this protocol is comparable to that of CSMA and enables efficient wirelessLANs with inexpensive radios
The GAMA-PS divides the transmission channel into a sequence of cycles; each cyclebegins with a contention period and ends with a “group transmission” period The grouptransmission period is divided into a set of zero or more individual transmission periods,which are similar to time slots, though the GAMA-PS needs no synchronization It uses aform of dynamic reservation to improve efficiency and to ensure that there are no colli-sions involving data packets To make a “reservation,” a station transmits an RTS using thepacket sensing strategy, i.e., a station backs off only if it senses an entire packet, and notjust the carrier If the RTS is received, the destination node replies with a CTS; this ex-change occurs during a contention period Once a station has successfully completed anRTS/CTS exchange, it is allocated its own transmission period and the station maintainsownership of this period as long as it has data to transmit (not necessarily to the same sta-