Tài liệu Điện thoại di động mạng lưới Radio P12 ppt

Develop-From 1992 to 1994 the RACE II programme promoted the development ofthird-generation mobile radio systems, with the objective of integrating GSM,DECT, paging, mobile satellite rad

A brief overview of the current state of development of wireless broadbandsystems, particularly in Europe, including the fundamentals of ATM in B-ISDN, is given below This is followed by important aspects of development

of wireless ATM for movable and mobile stations

The importance of wireless broadband systems is evident from the number

of projects being carried out in this field, e.g., within the European researchprogramme ACTS (Advanced Communication Technologies and Services) [2]:ACTS/MEDIAN Wireless LAN at 60 GHz transmission of ATM cells;ACTS/Cobucco Multimedia terminal;

ACTS/FRANS High-bit-rate subscriber connections;

ACTS/MagicWAND Indoor wireless ATM system at 5 GHz;

ACTS/OnTheMove Mobile multimedia value-added services;

ACTS/SAMBA Cellular ATM broadband system at 40 GHz;

ACTS/CABSINET Cellular interactive multimedia communications systemfor metropolitan areas (at 5, 17, 40 GHz);

ETSI/RES 10 HIPERLAN 1 (wireless LAN with 23 Mbit/s at 5 GHz; seeSection 13.2);

∗ With the collaboration of Andreas Hettich, Arndt Kadelka, Andreas Kr¨ ling, Dietmar Petras

am-©1999 John Wiley & Sons Ltd ISBNs: 0-471-97595-8 (Hardback); 0-470-84193-1 (Electronic)

ETSI/BRAN Broadband wireless access networks that also support ATM;ATM Forum TCP over ATM, MPEG over ATM, wireless ATM;

DAVIC/LMDS Digital And Video Council/Local Multipoint DistributionSystem;

ATMmobil Key development project of the Federal German Minister of search and Technology: development of wireless ATM systems (at 5, 19,

Re-40, 60 GHz)

A brief discussion of some of the projects follows

Until 1995, the EU research programmes RACE (Research and ment in Advanced Communications Technologies in Europe) I and RACE IIwere devoted to the development and testing of prototypes of systems withbroadband radio transmission

Develop-From 1992 to 1994 the RACE II programme promoted the development ofthird-generation mobile radio systems, with the objective of integrating GSM,DECT, paging, mobile satellite radio and trunked mobile radio systems alongwith their different applications into an Universal Mobile TelecommunicationsSystem (UMTS) with a multiplex data rate up to 2 Mbit/s at the radio in-terface This effort included the development of standardized terminals and

an expansion of services with high data rates

Along with these systems, which were designed to provide a high degree

of mobility, the RACE II project MBS (Mobile Broadband System) took the development and testing of a technology and system concept for awireless ATM system at 60 GHz that demonstrated the possibility of videotransmission with a 16 Mbit/s transmission rate (net) at a 50 km/h speed ofmovement of the terminal [11, 32]

The RACE II/MBS project undertook studies of techniques for linking mobileterminals to stationary broadband networks with data rates at the multiplexradio interface of up to 155 Mbit/s Narrowband services were also to be pro-vided The MBS system made a particularly important impact, and convincedthe professional world of the possibility of providing the services of broadbandISDN to mobile users through wireless ATM transmission [6, 22, 29, 31, 33]

In addition to providing a link-up to the broadband ISDN, the MBS conceptalso supports a cooperation with other systems such as UMTS The type ofnetwork and level of integration can vary all the way from a privately operatedMBS system with a low level of service integration and mobility up to a publicMBS system with a high level of integration, extensive mobility and coverage

of a wide area [5] Figure 12.1 shows MBS in relationship to other systemswith differing levels of mobility support for their terminals and transmissionrates It can be seen that MBS combines the wide-ranging service spectrum ofbroadband ISDN with the mobility of mobile radio networks whilst offering the

9.6 2 155 kbit / s Mbit / s

B-ISDN

MBS UMTS

ISDN GSM

Figure 12.1: MBS and other data networks

services of wideband and narrowband systems such as UMTS, W-LAN, GSM,DECT and their derivatives for Radio in the Local Loop (RLL) applications.Owing to the flexibility of MBS and the availability of the services of B-ISDN, a variety of different applications are possible These are indicated(with no claim to completeness) according to the data rates required and themobility of their users in Figure 12.2

The author and his research group were responsible for designing the radioand network protocols for MBS, which, although they were not implemented

in the demonstrator system, were developed as part of the project and wereincorporated into a number of successor projects in the ACTS programme,where they were developed further (see Section 12.1.2) For example, theMBS project as one of the first proposed an ATM-based radio interface formobile use and specified it as part of the system [29, 30, 36]

Programme

As the successor to RACE II, the ACTS research programme [3] of the ropean Union was conducting field trials and demonstrations to monitor thedeveloped systems for real applications

Eu-Along with the development of UMTS, promising attributes of MBS havebeen developed further in the following ACTS projects

MEDIAN (Wireless Broadband CPN/LAN ( Customer Premises Network) forProfessional and Residential Multimedia Applications) develops transmission

Pictorial data for travel Public transport travel advice Electronic Newspaper Traffic Advice

Emergency Services

Audio/Visual Library

HD Video Phone Surveillance

of Property Freight Management

Figure 12.2: MBS applications and services

technology at 60 GHz for wireless networks at data rates of up to 155 Mbit/sfor multimedia, voice and video applications

The goal is to develop a demonstration system for multimedia applications,including research into modulation, channel coding, channel access methods

provided B-ISDN access to mobile users through transparent transmission ofthe ATM cells of B-ISDN over the radio interface

WAND (Wireless ATM Network Demonstrator ) extended the use of ATMtechnology to mobile users, and examined realistic user environments Thefield of application covered Internet services over ATM in indoor areas with a

20 Mbit/s transmission rate at 5 GHz The project realized an indoor wirelessATM demonstration network

The emphasis was on modelling the radio channel and developing channelaccess protocols, as well as new control and signalling functions to be sub-mitted to ETSI for possible adoption in a later standard for wireless ATMsystems

12.1.2.3 SAMBA

SAMBA (System for Advanced Multimedia Broadband Applications) aimed toexpand the ATM fixed network using a cellular radio access network to pro-vide mobile users with access to broadband multimedia applications MobileATM terminals have been shown to be able to access services comparable

to those used by terminals in the ATM fixed network Therefore, besides thedevelopment of the system elements, the main priorities of SAMBA were inte-gration of ATM fixed network and mobile support A demonstration system at

40 GHz has been created and demonstrated at the EXPO’98 in Lisbon whichprovided transparent ATM links with transmission rates of up to 34 Mbit/sfor all ATM categories of service

In contrast to other ACTS broadband projects, the time-critical ATM vices CBR (Constant Bit Rat) and VBR (Variable Bit Rate) were also sup-ported and appropriate provisions made for radio protocols so that the radiochannel offered a quality of service comparable to a fibre-optic transmissionpath (within the framework of ATM quality of service requirements).The SAMBA project also developed a technology, not yet provided by theATM Forum, for call handover between different ATM fixed network accesspoints The author and his research group were responsible for the imple-mentation of the protocols of the radio interface and of the ATM networkprotocols, using their experience from MBS (see Section 12.1.1) [25, 27]

The AWACS (ATM Wireless Access Communication System) project furtherdeveloped a system based on the NTT/AWA system and developed a demon-strator to support terminals with limited mobility and provide public access

to the ATM fixed network The system operates in the 19 GHz range andprovides users with user data rates of up to 34 Mbit/s

In addition to developing the demonstrator, AWACS carried out extensiveresearch in the areas of channel and source coding, intelligent antennas, op-timization of LLC protocols, 40 GHz transmission technology and mobilitymanagement

AMUSE (Advanced Multimedia Services for Residential Users) specified anddeveloped a demonstrator for advanced multimedia services to link residentialcustomers to an ATM infrastructure The services were offered under realconditions through the use of different technologies such as HFC (Hybrid FibreCoax), ADSL (Asymmetrical Digital Subscriber Line), FTTC/FTTB (Fibre

to the Curb/Building) and WLL (Wireless Local Loop)

A possibility has been explored for setting up end-to-end links to differentaccess networks Moreover, the individual local field tests have been linkedover the European ATM network

The project was set up in two phases: services such as Video on Demand(VoD), News on Demand (NoD) and high-speed Internet access were devel-oped in the first phase; other services were offered in the second phase.

This 1996–2000 programme of the German Ministry of Research and ogy is involved in the development of concepts and the corresponding demon-strators for four forms of wireless ATM systems

Technol-The concept ATM-RLL (Radio in the Local Loop) promotes using ATMpoint-to-multipoint line-of-sight radio at 26/40 GHz to bridge the last fewmiles in a local loop area

The second concept (W-ATM LAN) is examining the wireless connection

of mobile computers to support multimedia applications at 5 and 19 GHz [8].The third concept (cellular W-ATM) links mobile terminals with an ATMradio interface over a cellular network at 5 GHz to an ATM broadband networkaccess point [4]

The fourth concept (Integrated Broadband Mobile System, IBMS) involvesthe development of wireless transmission technology for indoors and outdoors.Along with infrared as the medium for indoor purposes, millimetre waves at

5, 17, 40 and 60 GHz are being used Adaptive antennas, single-carrier mission technology, radio interface, radio resources and mobility managementare the focus of research [12]

trans-Similarly to UMTS for mobile radio systems with multiplex transmissionrates of up to 2 Mbit/s at the radio interface, the integrated individual con-cepts mentioned above are being pursued within the framework of ATMmobil.The author propoded and headed the ATMmobil project, and members of hisresearch group participated in the implementation work of the first, secondand third concept mentioned above and were also involved in the ETSI-BRANstandardization (see Section 12.1.5)

of Wireless ATM Systems

Although the ATM Forum is not an official standards body, it is playing animportant role in the quasi-standardization of certain forms of the ATM fixednetwork through its association with industry and its products In June 1996the ATM Forum became involved in WLAN standardization The WLANgroup originally wanted to focus its attention on mobility support by ATMfixed networks, a project that was supposed to run until the first quarter

be addressed Based on the experience of 1998, the anticipated market forwireless ATM systems is already so large that the standardization of the radiointerface for worldwide use will not be left up to Europe (ETSI) only

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Mobile Wireless Networks Stationary Wireless Networks

Figure 12.3: The four different types of HIPERLAN

The standardization group ETSI RES 10 (Radio Equipment and Systems,RES), now ETSI BRAN (Broadband Radio Access Networks), is currently de-veloping a family of standards referred to as HIPERLAN (High-PerformanceRadio Local Area Network) for wireless broadband communication at 5 and

17 GHz There are four different types of HIPERLAN:

HIPERLAN Type 1 is a standard for wireless communication between puter systems at 5 GHz in close proximity to one another (see Chap-ter 13)

com-HIPERLAN Type 2 refers to a wireless access system at 5 GHz to ATM fixednetworks with a multiplex bit rate of 25 Mbit/s for W-ATM LANs.HIPERLAN Type 3 is also referred to as HIPERACCESS It is an application

in HIPERLAN Type 2 technology at 5 GHz for outdoor distances of up

to 1 km (W-ATM RLL)

HIPERLAN Type 4 at 17 GHz is also referred to as HIPERLINK It willoffer rates of up to 155 Mbit/s for short distances for the connection ofW-ATM systems

The ETSI BRAN group is standardizing the radio interface [10] (see ure 12.3) The status as of September 1998 can be found in [17]

Fig-Table 12.1 presents a comparison of the key features of all four systems

Multiplex data rates of up to 155 Mbit/s on the wireless user connection arenecessary for the integration of wireless broadband applications in B-ISDN.These kinds of applications require services for continuous interactive data

Table 12.1: Parameters of the ETSI BRAN HIPERLANs

Point-to-point

[GHz]

[MBit/s]

[m]

as well as for bursty-type interactive data Along with voice transmission,applications with continuous bit streams include video conferencing in whichreal-time requirements must also be strictly maintained Interactive servicesare characterized by a wide fluctuation in the requirements for bit rates Thus

a short information request to a database can result in a very long response(counted in bit time) requiring a high transmission rate A distinction is madebetween the following:

Figure 12.4: Statistical multiplexing of cells to a medium

Asynchronous Transfer Mode (ATM) is the connection-oriented switching method used in B-ISDN ATM combines the advantages of connec-tion and packet-oriented switching—specifically the statistical multiplexing ofdata from different connections to one medium and the message switching ofpackets in the network nodes between the communicating terminals The datastreams to be transmitted are divided into short blocks of a fixed length, re-ferred to as ATM cells The cells of different connections are transmitted withtime interleaving over a physical channel Depending on their data rates, theconnections are dynamically allocated varying amounts of transmission ca-pacity, with some of them transmitting a large number of cells per time unitand others only very few The cells in each connection are transmitted in theorder of their arrival

packet-The ATM multiplexer adds empty cells to the multiplex data stream ifnone of the connections requires transmission capacity and a synchronoustransmission method is being used (see Figure 12.4)

An ATM cell comprises 53 bytes, consisting of a 5-byte long header field and

a 48-byte long information field containing data of the higher ATM stack layers and user data The switching of the cells is connection-oriented.All cells in a virtual connection take the same transmission path, which wasestablished when virtual channels were set up on different switching sections inthe network during call setup The cells are controlled through the network onthe basis of the routing information stored in the cell header (see Figure 12.5).VCI (Virtual Channel Identifier ), 2 bytes An identification of the virtualchannel differentiates between the different concurrent logical channelsand their cells The virtual channel number is always only assigned toone switching section

protocol-VPI (Virtual Path Identifier), 8 or 12 bits A channel group is identified bythe parameter VPI A differentiation can be made between a large num-

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Figure 12.5: Header of an ATM cell

ber of groups in the same direction, each containing several virtual nels The cells of channels in the same group can be processed especiallyquickly by the switch-fabric of an ATM switch and then forwarded;cross-connects are used for this purpose

chan-PT (Payload Type), 3 bits This parameter describes the type of informationfield and provides a differentiation between user and signalling infor-mation The signalling information is required, for example, to updatethe routing tables managed in the switching centres The switchingcentre carries out updates by evaluating the header field as well as theinformation field of the ATM cell User data being transmitted in theinformation field of an ATM cell is not taken into account in the switch-ing

HEC (Header Error Control), 1 byte Because the header of an ATM cell tains data that is vital to the transport of the cells, it is protected by achecksum This permits the detection of transmission errors

con-CLP (Cell Loss Priority), 1 bit This parameter identifies cells with a lowerpriority, which are discarded in the ATM switching centre when there is

an overflow in the queue The bit is also used by multiplex and switchingnodes for flow control and traffic shaping

As with other packet-switched methods, ATM cells are switched on the sis of the routing information contained in the cell header The completeoriginating and target addresses are sent only during connection setup, sothat the routing information can be kept as short as possible, thus increasingthroughput Identification of the logical channels is assigned for the differ-ent sections of a connection (VCI, VPI) During connection setup, the ATMswitching centres enter relationships between input and output identification(line + logical channel identification) into their routing tables using incomingcontrol information

VCI 4

VCI 5

VCI 6

VCI 3 VCI 4 VCI 5 VCI 6 VCI 1 VCI 2

VCI 1 VCI 2

VCI 4 VCI 3 VCI 1 VCI 2 VCI 2

VCI 1

VPI 1 VPI 4

VPI 2 VPI 3 VPI 5

Figure 12.6 shows the switching elements used in the switching centres Adistinction is made between VP switches and VC switches The VP switchonly evaluates the VPI values of the cells, so that the cells can be switchedquickly The entries of the VCI fields remain unchanged The VCI identitiesare changed only if transmission has occurred through a VC switch

According to the recommendations of the OSI reference model, an ATM erence model with four layers can also be specified (see Figure 12.7) Theselayers include the physical layer, the ATM layer , the ATM adaptation layer(AAL) and a layer that represents the functions of the higher layers Sincethe physical layer corresponds to layer 1 of the ISO/OSI reference model andthe ATM layer corresponds to layer 3 (network), the data link layer has notbeen taken into account for the ATM reference model This is because ATMnetworks typically use fibre links that guarantee a very low bit-error ratio(BER) and therefore do not need any error control in layer 2

ref-Furthermore, three different planes are defined: the user plane, the trol plane and the management plane The management plane comprises thefunctions for plane management as well as the functions for layer manage-ment Plane management is responsible for management of the entire system,whereas layer management controls the individual layers

con-Physical Layer ATM Layer

Higher Layers Higher Layers

Control Plane User Plane

Layer Management Plane Management ATM Adaptation Layer

Management Plane

Figure 12.7: The ATM reference model

The services, protocols and interfaces of the physical layer depend on thetransmission medium used, and contain all the functions required for the bit-by-bit transmission of information The tasks of the ATM layer include:

• Multiplexing and demultiplexing cells of different connections

• Control of VCI and VPI-oriented functions

• Generation or evaluation of cell header information

• Generic flow control at the user-to-network (UNI) interface

• Establishment, routing, operation and release of connections

The ATM adaptation layer adapts the higher ranking layers to the ATMlayer It carries out the necessary segmentation of data streams into cells onthe transmitting side and their reassembly into messages on the receiving side,and ensures safe transmission The AAL layer is divided into two sublayers:

1 The Segmentation-and-reassembly (SAR) sublayer for mapping protocoldata units of the higher layers to the ATM cells and the reverse

2 The Convergence sublayer (CS), which compensates for the undesirableeffects caused by the different cell transit times of different services.For example, the digital sample values used for voice transmission are con-solidated over a certain period of time to enable the complete utilization ofthe capacity of an ATM cell The receiver generates a continuous data streamagain from the arriving ATM cells

The different cell transit times through the network are offset by the ATMadaptation layer in the receiver through the addition of a constant time delaybefore the output

Table 12.2: Classes of AAL Services

sync circuitswitching (voice)

Variable bitrate (video)

Conn.-orient

data trans

Conn.lessdata trans

The number of protocols required for the ATM adaptation layer has been kept

to a minimum through the classification of services into four different groupsaccording to the following parameters: time relationship between source andsink, bit rate and type of connection (see Table 12.2) Time-continuous ser-vices with constant or variable bit rates are differentiated according to thosethat are connection-oriented and those that are connectionless

In accordance with the class of service, control data is also added to theuser information This is used to allow the restoration of user informationthat is divided among several cells The control data is generated by the SARsublayer when the data is divided into cells In the receiver the correspondingSAR sublayer must join the data together in the correct sequence according

to the control data

For identification of the sequence of the individual ATM cells, each messagecontained in a cell is assigned a sequence number to enable the receiver todetect the loss of any cells Whereas the sequence number is available withall the classes of service, additional backup data and different segment typesare being planned only for some of the classes Owing to the varying controldata part, the user data part is 44–48 bytes

In its specification Traffic Management (V 4.0) [13] the ATM Forum ferentiates between different classes of service that represent different appli-cations The specified quality of service parameters for the following classes

dif-of service are listed in Table 12.3:

Unspecified bit rate No quality-of-service parameters are specified for theUBR class of service No guarantees are provided for this class, whichmeans that it only benefits from a best-effort service

Available bit rate The ABR class of service is particularly suitable for cations that are not real-time-oriented and also have no requirements

appli-in terms of transmission rate Only the cell-loss rate is defappli-ined as aparameter for the quality of service

Constant bit rate The CBR class of service is planned for real-time-orientedservices with constant bit rates that have a stringent requirement for

Table 12.3: Classes of service and their quality of service parameters

ATM Layer Service Classes

Real-time variable bit rate Real-time-oriented applications that place astringent demand on delays and their variance as well as on cell lossratios require a real-time VBR service

Because transmission errors cannot be completely prevented even with optic technology, an end-to-end error correction procedure dependent on type

fibre-of service is provided in the AAL layer

The AAL protocols type 1 and type 2 are used for the real-time-orientedCBR and VBR services They supply their protocol data units (PDU) withsequence numbers and check sums to aid in the detection of lost or incorrectlyinserted ATM cells An FEC procedure can be used as an option to correctbit errors [19] If the bit-error ratio in the ATM layer exceeds the correctioncapabilities of the code used—something that may particularly occur fromtime to time on a radio transmission path—the quality of service requested

by the user cannot be guaranteed by this procedure

An ARQ protocol based on the functions for detection of bit errors and cellloss in the lower AAL sublayers (Common Part Convergence Sublayer, CPCSand Segmentation and Reassembly, SAR) is provided in the highest sublayer

of the AAL (Service-Specific Convergence Sublayer, SSCS) in AAL protocolstype 3/4 and type 5 [14] According to [15], these ARQ protocols can be

probability for packets 1 kbyte in length is to be maintained then the

RAS: Radio Access System WT: Wireless Terminal

ATM Radio Interface

ATM Mobility Enhanced Switch

ATM Switch ATM Network

Figure 12.8: Architecture of a cellular ATM mobile radio network

radio transmission path protected by an FEC procedure usually lies abovethat limit, and is therefore too high for an ARQ procedure to be carried outefficiently in the AAL

Figure 12.8 illustrates the schematic structure of a cellular ATM mobile radiosystem [9, 32] The access points to the ATM fixed network are located be-tween the Radio Access System (RAS) and the ATM fixed network Each ac-cess network contains one or more transmit and receive facilities (transceiver,TRX) as well as a base station controller (BSC) that carries out the protocols

of the base station The UNI interface is usually provided between the RASand the ATM network

This radio networks offer wireless ATM access from moving or mobile less terminals (WT) in selected areas, e.g., in buildings, outdoors or in thevicinity of buildings

Multiplexer

Wireless local area networks (W-LAN) are a typical application for cellularATM mobile radio networks With limited operating time (because of battery-based power supply) and reduced data rates (because of radio transmission),

it is desirable to provide wireless terminals in W-LANs access to the sameservices available to ATM terminals linked to a fixed network In particular,

it should be possible for all ATM applications to be used without any needfor modification, i.e., in wireless as well as in wired terminals based on thesame AAL services

Virtual ATM Multiplexer

of a Radio Cell

Base station Application

Application

Mobile ATM Terminal

Mobile ATM Terminal

Virtual Connection

Application

ATM Fixed Network

Fixed ATM Terminal

Figure 12.9: Connection of a cellular ATM radio network to an ATM fixed network

Figure 12.9 clarifies how AAL protocols are end-to-end transport protocolsbecause they are operated between terminals only and do not occur in thenetwork nodes Transmission over the ATM radio interface takes place withinthe ATM layer with the help of individual ATM cells, with the influence of theradio interface remaining hidden from the service users of the ATM layer (theentities of the AAL) This is referred to below as transparent transmission ofATM cells From the standpoint of the user, the terminals of a radio cell thatoperate virtual connections over the RAS behave as if they were connectedover a cable to an ATM multiplexer (see Figure 12.9)

Systems

The 5.15–5.25 GHz frequency band is currently being provided for W-ATMsystems in Europe as well as in the USA, although CEPT has allocated thisband to HIPERLAN 1 The FCC in the USA makes reference to High-SpeedMultimedia Unlicensed Spectrum for its National Information Infrastructure(NII) wireless broadband systems projects WRC is still studying the prob-lems involved There are national problems with the 5.25–5.3 GHz expandedHIPERLAN band due to the original usage and in the entire 5.25–5.35 GHzband in the USA WRC99 will probably be allocating the entire 5.15–5.87 GHzband for use by broadband local radio networks Bands already being usedwill have to be reallocated for use by broadband radio networks; this is calledrefarming a band Because of the large number of different systems (old andnew), anticipated systems must ensure that they have the ability to coexist

The assumption is that different standards will have to coexist with one other in the same band Frequency-sharing rules or etiquettes are therefore

an-being discussed, and these would apply to the use of a band at a particularlocation by systems built on the basis of the same or different standards.

In general, an etiquette is a set of rules that is agreed by the parties cerned so that a group of individuals is able to cooperate with one anotherwithout having to communicate whenever a problem arises

con-Instead of an exclusive frequency allocation for a particular system, usersmore likely expect an operation similar to what is usually found in Industrial,Scientific and Medical (ISM) bands but with additional restrictions, e.g., pa-rameters for the times when a band can be used

It was proposed that shared bands should be used in such a way that tems reserving the band (e.g., circuit or period-oriented systems) are separated

sys-in the band from systems transmittsys-ing sys-in packet-oriented mode and that there

be an overlap area that can be used by both types of system if necessary [16].For the time coordination of the transmission of different systems in the sameband, a technique is favoured that uses time containers rotating cyclicallybetween the base station transceivers (TRX) of coexisting base stations andshould be capable of enabling neighbouring (interfering) systems to transmit

on an alternating basis A periodic container used by a given base station isconsidered a TDMA channel with a capacity defined by the container size inbits, and defined by the length of the periodic frame (see Section 12.3.8) Ithas been proposed [26] to introduce a dynamic channel selection scheme asused by the DECT system (see Section 9.5) to enable base stations to decen-trally access and acquire transmit capacity in terms of a number of containersfrom a given frequency band

Since the capacity of a frequency channel must be controlled in the areadefined by the interference range of a base station (which is much larger thanthe service range) in order to be able to guarantee quality of service for ATMconnections according to the standard, but interference ranges of stations arepartly overlapping, there is still work needed to define appropriate algorithms

Since a high multiplex transmission rate of approximately 25 Mbit/s at thewireless interface is being sought, DS-CDMA (Direct Sequence Spread Spec-trum Code Division Multiple Access) procedures will not be appropriate be-cause of the bandwidth required Newer demonstration systems use Orthogo-nal Frequency-Division Multiplexing (OFDM) with symbol-by-symbol paral-lel signal transmission over 16 or 64 FDM carriers on the downlink and withsome systems also on the uplink In addition, 64 QAM and hybrid mod-ulation methods are used Capacity is usually distributed in the frequencyand time domains (FDM, TDM, FDMA, TDMA, FDD, TDD) Recent ex-periments have involved the use of adaptive antennas for the prevention ofinterference and improvement to quality of service Space-Division MultipleAccess (SDMA) can also be used for this purpose [34]

ATM AAL

PHY

W-UNI Terminal

Wireless Base Station with

ATM Switch

ATM Fixed Network

Figure 12.10: Protocol stack for the ATM radio interface (user plane)

Compared with the fixed network, the radio interface as a distributed ATMmultiplexer requires that certain radio-specific aspects also be considered:Radio propagation For example, diffraction, shadowing, reflection and mul-tipath propagation

Channel access Coordination of access to shared-use radio channels for plementing the transmission sequence of the ATM cells specified by thescheduler function of the RAS

im-Error protection Unreliable transmission conditions on radio channels sitate the use of error protection schemes to fulfil the quality of servicerequirements of the individual virtual connections according to the dif-ferent categories of service

neces-The quality of service required on a radio path is ensured whereby, alongwith AAL error protection measures, a data link control protocol (ARQ pro-tocol; see Section 2.7.4) is used directly at the radio interface in the logicallink control (LLC) sublayer to guarantee transparency in relation to the AAL

An ARQ protocol specific to the class of service is used, which permits errorprotection in the LLC layer to be adapted to the individual requirements ofeach separate virtual connection over a separate access point per ATM serviceclass

Figure 12.10 shows the resulting protocol stack at the radio interface less UNI , which consists of a physical layer that considers the characteristics

Wire-of the radio transmission (Wireless Physical Layer , W-PHY) and a Data LinkControl layer (DLC) The DLC layer consists of a sublayer for the coordi-nation of channel access (Medium-Access Control, MAC) and a sublayer thatcontrols the logical channels and contains the error protection functions (Log-ical Link Control, LLC)

The protocol stack shown in Figure 12.11 is currently being discussed byETSI BRAN Along with the W-ATM terminal, a W-ATM access point and

Mobility

Convergence

Layer ATMRADIO DLC

ATM

and Mobility Support Management Radio Resource Application

Radio Resource and Mobility Management

Application

Management Radio Resource and Mobility Support

Wireless Access Point Terminal

Wireless

RADIO PHY RADIO DLC Layer Convergence Layer

Network Network

Layer Convergence Layer RADIO DLC RADIO PHY PHY

Network Layer

Figure 12.12: HIPERLAN/2 layers and architecture

an ATM switch that has been expanded to include mobility support areshown The ITU-T signalling based on Q.2931 is located above the signallingATM adaptation layer (SAAL) (SAAL), whereas applications are based on theAAL-X (X = 1, 2, , 5) Two layers at the radio interface (R)—the datalink control layer and the physical layer—are responsible for safe transmissionwith error recovery

Figure 12.12 shows an alternative design of the ETSI/BRAN protocol stackunder discussion that is heavily supported by manufacturers aiming to providewideband and broadband Internet services to their customers Instead of aW-ATM terminal, any class of wireless broadband terminals can be supported

by this protocol stack This design favours the straightforward support of ternet applications by using the Internet Protocol (IP) in the network layerinstead of ATM signalling A convergence sublayer is provided in both pro-

Compared with the fixed network, the radio interface as a distributed ATMmultiplexer requires that certain radio- specific aspects also be considered :Radio. .. transmission conditions on radio channels sitate the use of error protection schemes to fulfil the quality of servicerequirements of the individual virtual connections according to the dif-ferent...

Layer ATMRADIO DLC

ATM

and Mobility Support Management Radio Resource Application

Radio Resource and Mobility