QoS Issues in Converged Networks

Song Jun

(Central Academy of ZTE Corporation, Nanjing 210012, China）

Abstrac t:The convergence of telecom networks requires an improved Quality of Service(QoS)solution.The introduction of the QoS resource control architecture based on the IP Multimedia Subsystem(IMS)framework is a way to solve QoS problems in the converged networks.Services,QoS guarantee,resource control and accounting functions are integrated into this IMS-based QoS control architecture.It is a solution to the problems currently facing the fixed network operators,helping them offer more QoS-guaranteed multimedia services such as 3D games,and make a transition from bandwidth providers to genuine telecom services providers.

1 QoS Issues in Converged Networks and Service Transformation of Operators

Q uality of Service(QoS)plays an important role in the converged networks.Different services require different QoS,which indicates QoSis correlative to services.

Many operators,especially the fixed network operators,are currently facing an awkward situation of communication traffic increasing but business income falling.The operators desire to transform from extensive bandwidth operation to intensive QoS-guaranteed multi-services operation.In this way,the operators can not only charge communication(bandwidth)fee,but also offer the QoS-guaranteed services,including the real-time services such as IPphone,3Dgames and IPTVservices,with special charges.Moreover,the QoS-guaranteed value-added communication services are the new service growth points that bring new bandwidth revenue.

There is no complete set of QoS transport resource control mechanism that is controlled and triggered by services on the existing networks.Both the International Telecommunication Union(ITU)and the Telecoms&Internet Converged Services&Protocols for Advanced Networks(TISPAN)are involved in working out solutions and standards for service-related QoS strategies,resource control,service assurance and accounting［1-2］.

2 Architecture of QoS Controlling on Converged Network

The architecture of QoScontrol is shown in Figure 1,where the key component is the Resource and Admission Control Function(RACF),located between the service layer and transport network.It ensures the QoSrequirements of the Service Control Function(SCF)at the service layer by controlling the system resources of the transport network.

The RACFis aimed at providing real-time application-driven and policy-based transport resource management for all kinds of services in different transport networks.The RACF provides the SCFwith an abstract transport network architecture,and it makes the service layer unnecessary to know network topology,network links,resource utilization and QoSmechanism of the transport network.It is not,however,limited to only support IMS application services.

The RACFexecutes policy-based transport resource control according to the request of the SCF,determines available transport resources,makes admission decisions,and applies the control policy to the transport functional entity for execution.It interacts with the transport function for the purpose of fulfilling the following functions:bandwidth reservation and allocation,packet filtering,traffic classification,QoS marking,traffic shaping,priority handling,Network Address Translation(NAT),and firewall control.Moreover,it interacts with the Network Attachment Control Function(NACF)to get the subscriber profile,takes the current situation of transport network resources into account,and finally makes the policies for transport resource control.

The RACFimplements the following QoScontrol functions:

·To control the QoS-related transport resources within packet networks and at the network boundaries

·To support different access and core transport technologies,while hiding network technological and administrative details from the SCF

?Figure 1.RACF-based QoS control architecture.

·To support QoS-capable user equipment

·To support resource and admission control within an administrative domain and between administrative domains

·To act as the arbitrator for QoS-related transport resource negotiation between the SCFand transport functions

·To support both relative QoS control,such as Differentiated Services(DiffServ),and absolute QoScontrol,such as Integrated Services InterServ(InterServ)

·To verify the availability of end-to-end transport resources

·To support the QoSpolicies for various media stream and users

·To support QoSsignalling

·To authorize requests for QoS,and to operate only on the authorized requests for QoS

·To support Network Address Port Translation(NAPT)controland various firewall modes

·To support NATtraversal

·To export information for supporting accounting based on resources usage and QoShandling

·To support different methods for resource-based admission control

·To support service priority handling

3.1 QoS Capability of User Equipment

The user equipment can be categorized into three types:

(1)Type 1:User Equipment without QoSNegotiation Capability

This type of user equipment(such as game terminals)has no QoSnegotiation capabilities at either the transport or service layers.It can communicate with the SCFfor service initiation,but cannot directly request QoSresources.

(2)Type 2:User Equipment with QoS Negotiation Capability at Service Layer

This type of user equipment(such as SIPphone)can perform QoSnegotiation through service signaling,but is unaware of QoSattributes specific to the transport layer.The service QoSonly concerns applications-related QoS.

(3)Type 3:User Equipment with QoS Negotiation Capability at Transport layer

This type(such as UMTSuser equipment)supports transport signaling similar to the Resource Reservation Protocol(RSVP).Hence,it is able to directly negotiate QoSat the transport layer with transport equipment.

3 QoS Service Modes and Resource Control in Converged Network

3.2 Resource Control Modes

The RACFis required to support the following QoSresource controlmodes:

(1)Push Mode

The RACFmakes the authorization and resource control decision according to the policy,and instructs the transport functional entity to execute the decision.

(2)Pull Mode

The RACFmakes the authorization according to the policy,and upon the request of the transport functional entity,re-authorizes the resource request and responds with the final resource control decision for execution.

3.3 QoS Control Processes

3.3.1 QoSResource Control Process in Push Mode

The push mode is suitable for all types of user equipment；Types 1 and 2 can only adopt the push mode.

User equipment Type 1 has no QoS negotiation capability.The SCFis responsible for deriving the QoSneeds of the requested service and sending a request to the RACFfor QoSresource authorization and reservation.

Type 2 supports QoSnegotiation at the service layer.The SCFis responsible for extracting the service QoSrequest from the service signaling and sending it to the RACFfor QoSauthorization and resource reservation.

Type 3 adopts the same QoS resource control process as Type 2.However,it requires the RACFto send authorization and QoSresource reservation information to the transport functional entity in advance.The transport-layer QoSsignaling sent by user equipment can be used to invoke QoSresources in the transport functional entity and execute the QoSpolicy.

The detailed process is depicted in Figure 2 with the illustrations below.

(1)The user equipment sends a service request(such as a SIPrequest)to the SCF.The service request may include QoSrequirement parameters.

(2)The SCFderives or extracts the QoSrequirement parameters from the request,and then sends a resource reservation request that contains the explicit QoSrequirement parameters to the RACFto ask for QoSauthorization and resource reservation.

(3)The RACFgives authorization and implements admission control according to the policy,resource situation and subscriber profiles stored in the NACF.If the resource reservation request is granted,the RACFpushes gate control,packet QoSmarking and bandwidth allocation decisions to the transport functional entity.

3.3.2 QoSResource Control Process in Pull Mode The pull mode is only applied to the user equipment Type 3.This equipment can initiate an explicit QoSresource reservation request directly to the transport function through the QoS signaling at the transport layer,following a dedicated transport path.But the QoS resource reservation needs pre-authorization of the SCF.

Detailed control process is depicted in Figure 3 with the illustrations given below.

(1)The user equipment sends a service request(such as a SIPrequest)to the SCF.The request may include QoS requirement parameters.

(2)The SCFderives or extracts the QoSrequirement parameters from the service request,and then sends a resource reservation request that contains the QoSrequirement parameters to the RACF,with a purpose to ask for QoSauthorization and resource reservation.

(3)The RACFgives authorization according to the policy.If the resources are authorized,an authorization token will be assigned to this service and the user equipment will be informed.However,the use of an authorization token is optional.(4)The user equipment sends an explicit QoSrequest for resource reservation directly to the transport function through the QoSsignaling at the transport layer.This request includes the explicit QoSparameters for the transport layer and the authorization token that was assigned before.

(5)Upon receiving the QoSrequest,the transport function sends a request to the RACFfor resource reservation and admission control,in which the authorization token may be contained.

(6)The RACFgives authorization and implements admission control according to the policy,resource availability and subscriber profiles stored in the NACF.If the resource reservation request is granted,the RACFpushes gate control,packet QoSmarking and bandwidth allocation decisions to the transport functional entity.

4 TISPAN's QoS Implementation Architecture and Research Plans

The European Telecommunications Standards Institute(ETSI)merged the Services and Protocol for Advanced Network(SPAN)organization,focusing on fixed-network standards,and the Telecommunications and Internet Protocol Harmonization over Networks(TIPHON)organization,focusing on Voice over Internet Protocol(VoIP)research,into a new committee called TISPANin September 2003.The TISPAN is responsible for all aspects of standardization for present and future converged networks including VoIP networks and NGN.It emphasizes network convergence so that fixed network and wireless users can communicate seamlessly.As a result,operators can provide fixed and mobile services through a public network.Based on the IMSarchitecture,TISPAN proposes the NGN system architecture and logical functional structure,and reuses the IMSR6 specifications as much as possible.However,the TISPAN NGN should support many access technologies,including all kinds of Digital Subscriber Lines(xDSL),Wireless Local Area Network(WLAN)and Local Area Network.The purpose is to make IMS become a common platform based on SIPand support multiple access technologies covering both fixed and mobile networks.

TISPAN is mapping out relevant NGN specifications,and has finished TISPAN-NGN R1［3］.

TISPAN proposes the Resource and Admission Control Subsystem(RACS)architecture to implement QoS control with details shown in Figure 4.

The functions of the RACSare mostly equivalent to those of the RACFas mentioned above.The RACShas two functional entities:Service-based Policy Decision Function(SPDF)and Access-Resource and Admission Control Function(A-RACF).The Application Function(AF)is an abstract of the service layer.The Network Attachment Subsystem(NASS)［4］is equivalent to the NACF.The transport functional entities include the Border Gateway Function(BGF),IPedge device(such as BRAS)and access node(such as DSLAM).

The SPDFare mainly responsible for:

·Checking if the request received from the AFis consistent with the local policy；

·Authorizing the requested resources for the AF；

·Determining the location of the BGFand A-RACF；

·Asking the A-RACFfor the requested resources and admission control；

·Asking the BGFfor the requested resources and NATcontrol enforcement；

·Hiding details of the transport layer from the AF.

The A-RACFmainly implements admission control and policy assembly.

(1)Admission Control

When the A-RACFreceives a resource reservation request from the SPDF,it obtains subscription information of the subscriber from the NASSfirst.Then,it will decide whether to allow the requested resource reservation or not,taking the local policy and access network situation into account.

(2)Policy Assembly

The A-RACFprovides the Resource Control Enforcement Function(RCEF)with an explicit policy description,or offers a pre-defined policy ID,which will be mapped into the specific L2/L3 transport policies.For practical networking,one SPDFmay control multiple A-RACFentities and is responsible for QoSpolicy decisions of the entire network.Each A-RACFis capable of managing a specific access network.It adopts corresponding transport QoSpolicies to perform management and control functions,and to hide network structure and QoS attributes of the access network.

▲Figure 2. QoS resource control process in the push mode.

▲Figure 3. QoS resource control process in the pull mode.

▲Figure 4. RACS architecture.

The BGFis a user-plane gateway,performing both QoSpolicy enforcement and NATfunctions.It fulfills gateway control of media stream,packet marking,resource allocation,NATtraversal and traffic statistics.The RCEFhas similar QoSmanagement functions to the BGF except for not having the NATtraversal function.The Layer 2 Termination Function(L2TF)is the control point for L2 communication between the user and terminating equipment.

The TISPAN has released RACSR1,while R2 is in the developing process.The following interfaces already have been standardized:Gq'between the AF and SPDF［5］,Rq between the SPDFand A-RACF［6］,and e4 between the NASS and A-RACF［7］.Moreover,these three interfaces adopt and expand the Diameter protocol.In addition to them,the interface Ia between the SPDFand BGFadopts and expands H.248［8］.However,there are no standard for Re and Ra between the A-RACFand the access network by now.Besides,standards for transport QoSenforcement entities such as the RCEFand L2TPhave not been made.As for the fixed access network,there are no complete QoS control solutions by now.TISPAN plans to cooperate with the DSL forum to finish the RACScontrolspecifications for the DSL access network first,and then make those for the Ethernet,Passive Optical Networks(xPON),WLAN and Worldwide Interoperability for Microwave Access(WiMAX)access networks,and eventually build a unified QoScontrol architecture for the converged network.

5 Conclusions

The IMS-based QoSresource control architecture is an important solution to QoSissues in the converged network.Based on the architecture,some operation problems currently encountered by fixed operators can be solved.For instance,it is possible for operators to develop various multimedia services with ensured QoS(such as 3D games).Therefore,it is beneficialfor the operator to transform from a bandwidth service provider to a true communication service operator.Moreover,the solution helps operators provide diversified value-added communication services.This article has discussed the QoS control architecture for the converged network,QoSservice modes and resource control.It has also introduced TISPAN's efforts on standardization.The IMS-based QoScontrol architecture is urgently needed by the converged network,especially by the fixed network.However,the architecture and relevant standards require further research in the future.