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Transcript
Supporting Packet-Data QoS in Next-Generation Cellular Networks R. Koodli and Mikko Puuskari Nokia Research Center IEEE Communication Magazine Feb, 2001 Introduction • Traditional circuit-switched networks that support basic voice are now to support packetswitched data services • 3G: evolution of cellular network architectures are to be multi-service platforms supporting voice, video and data services • QoS is crucial issue for packet data services, especially in bandwidth-constrained and errorprone environment Introduction • UMTS (Universal Mobile Telecommunication Systems) are defined by 3GPP (Third Generation Partnership Project) Release 1999 • Based on GPRS (General Packet Radio Service) Background • UMTS phase one encompasses both circuitswitched networks (GSM) and packet-switched networks (GPRS) evolution • SGSN (Serving GPRS support node) – handles terminal mobility and authentication functions – is connected to BSS (base station subsystem) and to GGSN over an IP backbone network • GGSN (Gateway GPRS support node) – handles accounting of resource usage – edge IP router Network view of regular GPRS GPRS Operation and PDP Contexts • MS (mobile station) initiates a GPRS attach procedure, known to the SGSN • Once attached, activate a Packet Data Protocol (PDP) context to send or receive packet data • PDP context: – network layer protocol, is a virtual connection between the MS and GGSN – includes an identifier (eg. IP), QoS parameters etc. – establish a GPRS “tunnel” between GGSN and SGSN using GPRS Tunneling Protocol (GTP) QoS Approach in current GPRS • QoS profile (to each PDP context) consists of: – delay: acceptable transfer time from one edge of GPRS system to the other edge – service precedence: drop preference during network abnormalities – reliability: tolerance for error rates and need for re-transmission – mean throughput, peak throughput: specify average rate and maximum rate Current GPRS QoS • GPRS performs admission control based on QoS profile requested in PDP Context Activate message and availability of resources • actual algorithms used for admission control are not specified (can be vendor- or operator-specific) Current GPRS QoS • When PDP Context Activate succeeds: – SGSN maps QoS profile into appropriate Radio Link Control (RLC)/Medium Access Control (MAC) priority level to indicates the use in uplink access – SGSN also maps accepted QoS profile into an appropriate IP QoS procedure (e.g. marking in Differentiated Services for QoS provisioning over core networks Limitations of Current GPRS QoS • Limitations make current GPRS infeasible for supporting real-time tranffic – For a given PDP address, only one QoS profile can be used: all application flows share same PDP context, and no per-flow prioritization is possible – do not allow QoS re-negotiation – QoS parameters are too vague and ambiguous in interpreting implementations, thus raising inter-operability concerns Limitations of Current GPRS QoS – GPRS is designed for best-effort traffic only – In GPRS phase 1, BSS does not perform clever resource management or simply reserving resources for higher priority flows UMTS Packet QoS Architecture • UMTS packet data system includes: – – – – – – – MS UTRAN (UMTS Terrestrial Radio Access Network) 3G-SGSN GGSN HLR (home location register) SCp (service control point) BG (border gateway) UMTS architecture UMTS vs. GPRS • UMTS is evolved from GPRS • But, some differences in QoS approach • 2 main QoS-related enhancements: – PDP context mechanism can support multiple application flows and provide a more flexible QoS negotiation and setup – BSS (known as UTRAN) can support QoS for application flows with extension of GTP tunnels to RNC UMTS vs. GPRS • Table 1 Overview of Different Levels of QoS • Bearer service defines characteristics and functionality established between communicating end-points for end-to-end services – UMTS control plane signaling is used to set up an appropriate bearer that complies with end-toend QoS of applications within UMTS – once bearer is established, user plane transport and QoS management functions provide actual bearer service support UMTS bearer support Layered bearer model • TE (Terminal Equipment): – laptop, PDA, or mobile phone • UMTS bearer – provides QoS inside UMTS network and perform QoS functions with interworking with external networks • External bearer service – QoS support available outside UMTS, including Differentiated Services, RSVP-based services, or simply best-effort service UMTS bearer service • Realizes QoS in UMTS network, and consists of: – radio access bearer: • RLC-U (Radio Link Control’s User-plane) layer between RNS and MS support radio bearer service • Iu-bearer service provides transport services between RNS can SGSN – core network bearer • provides transport services within UMTS core network, e.g. between a SGSN and a GGSN • based on UDP/IP datagram delivery UMTS QoS Management Functions for Bearer Support • Provide end-to-end QoS for each PDP context • Control-plane and data-plane components of this architecture – – – – – admission control bearer service manager resource manager traffic conditioner packet classifier QoS components in reference architecture UMTS QoS Management • Admission Control – admission control module in SGSN to accept or reject the PDP context activation and requested QoS – GGSN and UTRAN verify whether they can support the bearers associated with QoS profile • Bearer Service Manager – coordinates control plane signaling to establish, modify, and maintain the bearer service UMTS QoS Management • Resource Manager – manages access to resources – provides support for QoS required for a bearer service – may achieve QoS by scheduling, bandwidth management, and power control • Traffic Conditioner – provide conformance of input traffic to specification agreed in the bearer service – may achieve this by traffic shaping or traffic policing UMTS QoS Management • Packet Classifier – In MS, assigns packets received from local bearer service manager to correct UMTS bearer based on DSCp, transport layer port numbers, security parameter, etc. – In GGSN, assigns packets received from external bearer service manager to appropriate UMTS bearer QoS Traffic Classes and Parameters • Conversation class – conversational real-time applications: video telephony – supported by fixed resource allocation – constant bit rate services • Streaming class – streaming media applications: video downloading – certain amount of delay variation is tolerable – variant of constant bit rate and real-time variable bit rate services QoS Traffic Classes and Parameters • Interactive class – for services requiring assured throughput: ecommerce, interactive Web – supported by traffic flow prioritization • Background class – traditional best-effort services: background download of emails and files, etc – lowest priority Traffic classes and QoS parameters • Table 2 QoS Negotiation and Setup • QoS profile for a PDP context may consist of values for: – – – – traffic class transfer delay traffic handling priority etc • per-PDP QoS provisioning • Both MS and GGSN maintain separate filters for packet classification