Wang Bing Zhang Zhijiang Li zhengmao

（China Unicom Co. Ltd.， Beijing 100032， China）

AbstractThe popular application of optical fiber technology increases the bearing capacity of backbone network greatly and makes the access network the bottleneck for services.The WiMAX is characterized by large bandwidth，wide covering range，and mobility supporting.Though WiMAX system has brought many new features to the access network，it brought new problems such as security guarantee under wireless environment，dynamic resource management，and mobility management.Especially the QoS mechanism is important in providing reliable end-to-end services under complicated communication environment.All those problems need to be solved in practice gradually.

T he specifications of IEEE 8O2.16-2OO4[1]supporting fixed access were formally published in Oct.2OO4.Intel announced its Pro/Wireless 5116—its first chip supporting IEEE 8O2.16-2OO4.In the same month，Alavrion announced its lab sample of client terminalbased on Intel's chip.WiMAXsystem has become the hot spot in the communication industry.

The IEEE 8O2.16e-2OO5[2]，published in the end of 2OO5，brings an enhanced air interface with enhanced performance and optimized to support mobility.Orthogonal Frequencey Division Multiple Access（OFDMA）and multi-antenna technology improves the performance of WiMAX，and these advanced features make WiMAXa choice for wideband access technology.Series of related information predict that WiMAXis expected to make an appearance from 2OO7.

1 Main Features of the WiMAX System

Currently，people have experienced high-speed development of communication technologies and communication networks.Therefore，they have obtained basic common understanding for the future communication network.That is，the future communication network should have good wideband feature，good interoperability，QoSmechanism for end-to-end services，and powerful service supporting capacity.Popular application of optical fiber technology has greatly increased the bearing capacity of the backbone network.Access network has become the bottleneck especially for the wireless system.In the R99 version of WCDMA standard of 3GPP，the peak rate of downlink is 384 kbps for 5 MHz wideband；in R5 version，the peak rate for downstream is increased to 14 Mbps.In the RevA version of CDMA2OOO 1x EV-DO of 3GPP2，the peak rate of reverse channel can reach 3.1 Mbps for 1.25 MHz wideband.However，considering the fast increasing data services，especially in multi-media business，the 3G system still has some limitations.

As a new communication technology，the WiMAXsystem faces the demands of the future communication network.It includes advanced wireless technology to enable and increase its capabilities to fit those demands.The IEEE 8O2.16 air interface and WiMAXsystem have the following features.

（1）Adopting advanced transmission technologies such as Orthogonal Frequency Division Multiplexing（OFDM），and OFDMA，to get a big bandwidth.Under ideal transmission environment，the transmission rate is 75 Mbps on the 2OMHz bandwidth，and the bandwidth efficiency is nearly 3 bps/Hz.

（2）Adopting wireless technologies such as adaptive modulation and multiple antenna technology，including Multiple-input Multiple-output（MIMO）and smart antenna，decrease the link budget and extend the cellcoverage.Under line of sight transmission enviroment，the biggest covering radius can reach 5OKm，while under the non-line of sight environment，the radius can reach 1 Km to 5 Km.

（3）Having QoSmechanism，enabling time sensitive services extended to access network.The IEEE 8O2.16e-2OO5air interface defines five QoSservices scheduling types and three service flow state.The business establishment adopts the apply/permit method，enabling the access control and wideband management of Base Station（BS）on wireless resources，and ensuring the service quality of the inside part of access network.

▲Figure 1. The IEEE 802.16 protocol stack.

（4）Related signalling for L2 handoff on the air interface to support SS's mobility.

（5）Convergence sublayer in air interface protocol stack to support variaty upper leveltechnologies.

（6）Adopting privacy key management protocol based on PKMv1 and PKMv2 with a powerful anti-attack property.

（7）Based on the IPnetwork architecture，in accordance with the trend of general network modle of IPcore network+access network.

Comparing to the traditionalwireless access technology，the QoSmechanism is an important aspect of WiMAX.Such property can actually realize the end-to-end services.

2 The IEEE 802.16 Protocol Stack

As the air interface specification，the IEEE 8O2.16 serial specifications set the working frequency of 2 GHz to 6OGHz.It supports two multiplexing methods as Frequency Division Duplex（FDD）and Time Division Duplex（TDD）.The physical layer technology includes single carrier，Orthogonal Frequency Division Multiplexing（OFDM）and several evolution technologies based on OFDM such as Orthogonal Frequency Division Multiple Access（OFDMA）and configurable OFDM.Different available bandwidth can be chosen to different physical layer transmission technology.

Using OFDM，there are 1.75 MHz，3.5 MHz，7 MHz，3 MHz，5.5 MHz and 1OMHz bandwidth available.Using OFDMA，there are 1.25 MHz，3.5 MHz，7 MHz，8.75 MHz，14 MHz，17.5 MHz and 28 MHz bandwidth available.In the convergence sub layer，extendable options of outer carrier for ATM，IP，and Ethernet are provided.In the security aspect，it supports authentication and key management based on PKMv1 and PKMv2.Moreover，it supports various encrypting key and arithmetic of management information and services，and two authentication methods as user verification and equipment verification.Therefore，the following group of parameter set is needed to define the air interface of the WiMAXsystem:

（1）Parameter Sets for the Medium Access Control（MAC）Layer

The MAC layer parameter set includes:convergence sub layer features；corresponding classifier；whether supporting header compression；whether supporting Automatic Repeat Request（ARQ）；corresponding relationship between the Protocol Data Unit（PDU）of MAC；and outer business flow and business scheduling type.

（2）Parameter Sets for the Physical Layer

The physical layer parameter set includes the multiplexing mode，the modulation method，available bandwidth，rate，frame length，the roll-off factor，and the frequency tolerance.

（3）Parameter Sets for Power Class

The power class parameter set includes parameters such as maximum transmission power and more.

Functions，technologies and arithmetic supported by IEEE 8O2.16 protocolstack and sub layers are shown in Figure 1.

3 WiMAX System Model

The future communication network has the property of openness，which is shown in the consistence between specifications and standards.It means that network deployment might have overlapping in function and location，and owners of network won't be only the network operators.The WiMAXsystem is facing and will face such a reality.The relationship among the networks and operators of the WiMAXsystem environment is shown in Figure 2.

Applications Service Provider（ASP）can provide services through numerous Network Service Providers（NSPs），while NAPcan obtain services through several NSP.Therefore，there are crossovers in the figure.Operators adopt open interfaces to transfer services and negotiate on parameters，optimize the utilization of network resources and users have more choices for services.Therefore，the demand for quality increases.

In the open telecom environment，a mature industry chain includes many links and each link has several participants，therefore，the open interfaceis very important.In the WiMAXsystem，according to the nature of operators，should be at least five open interfaces as shown in Figure 3，which are open interfaces among operators and air interfaces.

▲Figure 2. The WiMAX system architecture.

?Figure 3.The WiMAX system interfaces.

（1）R1

Air interface R1 is the air interface between Subscriber Station/Mobile Station（SS/MS）and BS.

（2）R2

R2 the interface between SS/MSand the Connectivity Service Network（CSN），is a logical interface to realize management plane functions such as user authentication and business authorization，and IPaddress management.

（3）R3

R3 is the interface between the Access Service Network（ASN）and CSN，realizing user's data transmission and authentication between ASN and CSN，and control signal interactivities such as mobility management.

（4）R4

R4 is the interface among ASNs，in supporting the mobile application scenario.The MSdata and signal crossing regions are realized by R4.

（5）R5

R5 is the interface between home CSN and visiting CSN，as the system supporting nomadic access and mobile access.User data and signal between home CSN and roaming CSNare realized by R5.

Presently，the interface protocol hasn't finally been decided，but the openness property is certain.As the network functions further fractionizing and network entity becoming certain，there is a tendency for the interfaces among inside network entities and the ones between network management system and network entities to gradually open.

4 The QoS Guarantee Mechanisms

The QoSguarantee mechanism is one of the important characteristics of WiMAX system，as a wireless access network supporting mobility.The QoSof WiMAX faces the following problems.

·Uncertainty of wireless channel

·Dynamic distribution of mobile users

·Diversification of services

These problems force the QoSof WiMAXto have own features in access control and bandwidth management.The IEEE 8O2.16 specifications for air interface have the following features:

（1）BSCentralized Control

The BScentralized control includes access control and bandwidth management.The MAC layer access adopts the application/permission mechanism，comparing to the Carrier Sense Multiple Access with Collision Avoidance（CSMA/CA）of IEEE 8O2.11a/b.The BScan effectively carry access management and bandwidth management.

（2）Business Scheduling Type The IEEE 8O2.16 defines five business-scheduling types and clarifies the corresponding QoSparameters to each business type，enabling BSto adopt different scheduling arithmetic to meet different business requirements.

（3）Business Flow Management

The scheduling arithmetic adopted by BSis not defined in air interface，but the air interface defines different business flow symbol and corresponding QoS parameter set to different status of business flow.The switch from one business status to another can be triggered by SS/MS，but is decided by BS.This property offers flexibility to the access control and to the scheduling arithmetic.

5 IP Network QoS End-to-end Architecture Based on WiMAX Access

The QoSmanagement of WiMAXsystem is only confined to the inside of access network，while the end-to-end business needs to span the QoSmanagement region of one or more bone networks.In fact，QoSguarantee of business is closely related to every part of business connecting route.

The QoSguarantee for business runs through the development of telecom network.In the ear of narrow band，because the business is single，little，and mostly transmission is wired，the realization of QoSis rather simple.However，the QoSguarantee mechanism faces numerous problems and becomes the essential study topic in the communication technology research.That is，because the data business（especially the multimedia business）rapidly grows，the Internet becomes popular and the wireless and mobile become the important part of communication.Whether an end-to-end connection can be established or not is more decided by the transmission layer and the network layer technologies，while the end-to-end QoSguarantee involves several aspects.In the ITU-TY.qosar draft，the QoSsystem of grouped network is proposed，dividing functions of QoSmechanism into three planes，which are the data plane，the control plane and the management plane.Those planes process business flow management，signal，and users'information.Seen from the longitudinal view，the QoSsystem runs through every layer from the application layer to the physical layer.Especially the application layer，the network layer and the transmission layer are the transmission of QoSdemands and parameters.Seen from horizontalview，the QoSmechanism should be on the whole businessconnecting route，from the customer premise network，the access network，the core network to the other side networks.

Different network has different QoS mechanism and it is very difficult to realize QoSmanagement.The management that is separated in different QoSregion can be realized.Parameter mapping is used to forward business demand from one QoSregion to the next between different layers and different QoSmechanisms.

The IPnetwork is the real global network and the foundation of the Next Generation Network（NGN）.

Presently，the IPnetwork has three QoSmechanisms[3-8]:

（1）Integrated Services/Resource Reservation Protocol

The Integrated Services（IntServ）utilizes the Resource Reservation Protocol（RSVP）to reserve resources based on business demand，and realize the real end-to-end guarantee.However，such method occupies too much network resource，not suitable for big-scale network.

（2）Differentiated Services

Network equipment transmits business flow according to the business flow classification label based on Per Hop Behavior（PHB）.The network equipment only guarantees further forwarding according to business priority.The Differentiated Services（DiffServ）is an intensively adopted technology.（3）Multi-protocol Label Switching A transmission route would be established before business flow transmission，using simple label.In business transmission only label needs to be inquired，routing tables need not.The multi-protocol Label Switching is the technology for two to three layers and not suitable for big-scale networks.

Three technologies can be used together.Except IntServ/RSVP，the QoS of other two technologies cannot really realize the end-to-end QoS.However，through measures such as network light load，access control，and flow plan，the QoSof whole IPnetwork could be realized to a certain extent.

The QoSmanagement mechanism of the inside WiMAXaccess network and air interfaces have been introduced.If an IP network adopts the DiffServ mechanism，in this end-to-end modal，in order to achieve end-to-end QoS，two mappings need to be realized.

（1）Mapping from Service Level Agreement of the Application Layer to Scheduling Services Type of the Access Network

In the Service Level Agreement（SLA）signed between users and service providers，besides the main body of service，validity period and guarantee system，the QoSparameter Service Level Specification（SLS）willbe defined in a signed or anonymous way.This parameter could be for the application layer，and forwarded to the next network layer in QoSrealization or it could be for the network and transmission layers.Services are various and changeable，therefore，it is not meaningful to directly define services.Through defining service QoSparameters and regard them as the common agreement for different QoS regions，we get a common understanding for services.In fact，in the network layer and the transmission layer the service type is not considered，and the QoScontrol and management is carried out only according to those parameters.

Before WiMAX，hardly any wireless access network was capable of providing real QoS.Therefore，the study on SLA/SLSof WiMAXaccess network services has not yet started.Considering the supportive property of WiMAXto QoS，the services type becoming richer，and diversified demands for QoS，the main parameters of WiMAXaccess network should include schedule type，minimum reservation rate，minimum rate，jitter，delay，and reliability.

（2）Mapping from Access Network Scheduling Services Type to DiffServ Digital Section

Each Connection Identification（CID）of IEEE 8O2.16 air interface is correspondent to one service scheduling type and one parameter.Data package of WiMAXaccess network is forwarded to IPnetwork after convergence and encapsulation.As for the IPnetwork adopting DiffServ mechanism，the service guarantee mechanism PHB is decided by the 6-digital definition value of Digital Section（DS）domain.Therefore，it is necessary to make agreement on the parameters between the two networks.In RFC2597，twelve types of Assured Forwarding（AF）services are defined，and by default all"O"DScorresponds to Expedited Forwarding（EF）services.In RFC3246，EFservices are defined without detailed parameters.There is only priority relationship of several types of services，and operators decide the corresponding relationship between actual service and those classified services.At the boundary between the WiMAXaccess network and the DiffServ IPnetwork，the scheduling services type inside the WiMAXaccess network should be mapped to Differentiated Services Code Point（DSCP）of the DiffServ.

The QoSmechanism is different in different networks.The mapping and interactivity of parameters are required at the boundary of networks.Most QoS mechanism can only support service priority method，for instance，DiffServ，8O2.1Q.The correspondent relationship of services between different operators'networks is decided based on SLA between the two network operators，in which the corresponding service grade relationship is essential.

6 Conclusions

The WiMAXsystem brings new features to access network，also number of new problems，for instance，security in wireless environment，dynamic resource management，mobility management，and more.Nowadays，the IEEE 8O2.16 serial specifications are far from mature，and the WiMAXforum is at its starting stage in study of network.The whole industry chain has certain scale but is not mature，and product types and functions need to be enlarged.It is expected that realWiMAXsystem supporting fixed and nomadic access willform industry scale applications in 2OO7，and the WiMAX system supporting mobile access will be later.Though WiMAXis immature，people still expect more from the WiMAXsystem and want further study on this technology.The reason is that it is the important step to break through the limitation of wireless access network.

The constant emerging of new business and new technologies makes QoSbecome the forever topic in communications networks.Furthermore，the QoSis one of the reasons why WiMAXis popular.To providing end-to-end QoSis far more complicated than establishing end-toend services.Service class mapping is only a measure of QoS，and there are more problems to be solved.For instance，the definitions of services and parameters in the application layer and the ones of service grade in QoS mechanism are inconsistent.There is no end-to-end QoSmanagement and policy in real sense to realize resource information interactivity between networks with different mechanisms and between nearby layers，as well as measuring of network resource parameters and breaking down of QoS parameters in different networks.Moreover，for networks supporting mobility as the WiMAXsystem，dynamic resource reservation and management should be considered.These problems willbe gradually solved as the study progressing.