Future mobile communication networks are evolving from traditional circuit-switched architecctures to an all-IP based structure. It is suggested that the mobile networks should be integrated by a high-bandwidth IP-based core network and a variety of wireless access technologies such as UMTS or WiMAX. Mobile terminals will be able to access different multimedia applications and advanced services while roaming across zones covered by different access technologies. Currently, 3GPP is developing a feasibility study on providing seamless service continuity between UMTS and WLAN.
The levels of convergence may be classified into convergence of service, network and technique. The goal is to share a service system based on interworking. Providing a uniform service experience for users, through a uniform service system, would enable customers to use different terminal devices to access heterogeneous networks, to access the same service, and to achieve common billing and session management. Service convergence is the first step of the convergence. Seamless roaming and handoff between different networks is the main problem. There are significant differences between the PHY technique of 3GPP and WiMAX.
- IMS Architecture
Within the UMTS core network, IMS is defined by the 3GPP as the component that provides support for multimedia services (e.g. voice and video) based on packet switching with QoS and the provision of AAA. The above figure shows a general view of IMS architecture. From this we can appreciate how the core network is organized in two networks: a signaling or control network and a data or transport network. The signaling network is composed of a set of call session control function nodes (CSCFs). They are signaling proxies whose task is to establish, modify and release media sessions with guaranteed QoS and AAA and charging support.
Note that user equipment (UE) gains access to the IMS via UMTS terrestrial radio access network (UTRAN), which is responsible for providing access for mobile stations and managing terminal mobility. SIP, COPS, and Diameter are the major protocols involved in this architecture.
- Interworking arthitecture and interworking level
1. Two interworking modes
There are two methods for WiMAX networks to interwork with other wireless networks: loose couple and tight couple. There is little difference between loose couple and existing networks; WiMAX utilizes the AAA server of 3GPP network, and data streams are not passed through the core network of 3GPP. This methods guarantees the independence of WiMAX network, however it results in high handover latency between two networks. Therefore, it is not suitable for real-time services.
In tight couple mode, the data streams of WiMAX must pass through the RNC and the core network of 3GPP, so each of the existing networks must modify their protocols, interfaces and services to meet the requirements of interworking. The BS of WiMAX connects with RNC of WCDMA or SGSN directly. The advantage of this mode is that it reduces the handoff latency and guarantees seamless handoff. If different operators own both 3G and WiMAX networks, the integration would be troublesome for the open of network interface.
2. Interworking levels
WiMAX is commonly used to transport IP packets. Thus 3GPP-WiMAX interworking should be built on the top of the IP protocol and not be limited to a specific WiMAX technology.
Different interconnection levels must be defined to represent different operational capabilities. These levels are suitable for either interworking mode.
Six interconnection levels between WLAN and 3GPP were taken into consideration., as well as the operational capabilities of each of them, based on the interconnection levels. The interworking is not limited to 3GPP and WLAN, but also includes the internetworking between 3GPP and other wireless access technologies based on IP. To maintain consistency, interworking with WiMAX networks must be based on the same model as shown in the following table.
3GPP has included the first three level s in Release 6, and the last two will be developed in future releases. The first level is the simplest and includes common billing (the customer receives just one bill for usage of both 3GPP and WiMAX services) and common customer care. It does not have any impact on either 3GPP or WiMAX architecture. The subscriber is charged on the same bill for usage of both 3GPP and WiMAX services. Customer care will be ensured independently of the connecting platform.
The second level (3GPP system-based access control and charging) includes the usage of the 3GPP access procedures (including authentication and authorization) for WiMAX users within the 3GPP domain. In addition, Wimax nodes use UMTS charging systems for charging data records generation. A subscriber may use the WiMAX Access network to access the Internet, for example, but AAA operations are handled by the 3GPP platform.
The third level extends the IMS services to the WiMAX. However, it is a matter of implementation as to whether all services are provided or just a subset of the services. This scenario lacks service continuity, so the user must re-establish the session in the new access network. Continuity is considered in this context as the ability to maintain an active service session when moving from one access network to another (e.g. between WiMAX and UTRAN) at the signaling level, without considering a transport level-related continuity issue like bandwidth or packet loss. Level 3 allows the operator to extend 3GPP system PS based services to the WiMAX network. In this scenario, an authenticated 3GPP subscriber can access 3GPP PS services through a WiMAX access network by interworking with its 3GPP PLMN (non roaming case) or with a visited 3GPP PLMN (roaming case).
The last three levels are not considered by the 3GPP in Release 6 and may be developed in future releases. The fourth level introduces service continuity, although the handover process may be perceptibel to the user (due to data losses or delays). The fifth scenario provides seamless continuity, with no noticeable service interruption greater than that perceived in intra-3GPP handovers.
3. QoS guarantee
Due to the differences in the network bandwidth, providing users with a constant level of service is not feasible. The goal of QoS guarantee is to offer suitabel quality of service in the given network, in accordance with user's QoS profiles and application require,ents. The QoS guarantee involves the task of mapping the QoS parameters from P-CSCF, GGSN, PDF, QoS negotiation, and the resource reservation methanism.
UMTS defines four classes of QoS services based on different application requirements: conversational, streaming, interactive, and background. WiMAX also defines four classes of QoS: UGS (unsolicited grant service), real-time polling service, non-real-time polling service and BE (best effort). According to the application scenario, QoS class mapping can be implemented according to the mapping relation mentioned according to the mapping relation. The conversational and streaming services of UMTS correspond to the UGS and rt-PS services in WiMAX. The interactive service can be mapped to nrt-PS and BE services in WiMAX in different application scenarios. However, the background service in UMTS has the same requirement and application scenario as the BE service in WiMAX.
QoS negotiation between session peers is performed using the SIP offer/answer model, in which each session peer offers its QoS capabilities using Session Description Protocol (SDP) descriptions in the message body.
The following figure shows the architecture of QoS-enabled interworking based on COPs.
The PCF communicates with the GGSN via the Go interface. It enables two modes of operation. In the push mode, the PCF initiates communication with the PEP and sends the decision to GGSN. In the pull mode, the GGSN initiates communication with the PCF to request a decision for a particular IP flow.
In summary, SIP is the key signaling protocol of IMS. Interworking between SIP elements of the WiMAX and CSCFs of the IMS is a key issue in reaching a high level of interworking between WiMAX and 3GPP networks. Here the overall architecture of the interworking based on IMS is represented, as well as special issues such as QoS guarantees are discussed.
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