To support IP QoS, the Internet Engineering Task Force (IETF) recommends integrated services (IntServ) and differentiated services (DiffServ). They are expected to be effective also in all-IP-based 4G networks. Since 4G networks will support multimedia traffic, we need to visit the issue of providing IP QoS in IP based wireless access networks, and propose ITRAS for QoS support in 4G networks, where the decision of radio resource allocation follows IntServ or DiffServ policy.
IP QoS
IntServ uses Resource Reservation Protocol (RSVP) to reserve bandwidth during the session setup. As a first step of RSVP, the source sends a QoS request message of PATH to the receiver through intermediate routers which run an admission and a policy control. If the sender receives RESV returned from the receiver through the reverse route as an indication of QoS guarantee, it initiates the session. If each router along the path receives packets, it classifies and schedules them. IntServ ensures strict QoS, but each router has to implement RSVP and maintain per-flow state, which brings difficulty in a large-scale network.
DiffServ, on the other hand, does not need any signaling protocol and cooperation among nodes. As the QoS level of a packet is indicated by the DS field of IP header (TOS field in IPv4, Traffic Class field in IPv6), each domain can deal with it independently. Once the packet is classified, each router can mark, shape or drop it according to network status. Since DiffServ is not so rigorous as IntServ, it is scalable in supporting QoS statistically.
QoS of Wireless Access Networks
In general, a wireless access network has the capability of managing QoS independently of the IP network because it becomes a bottleneck for providing end-to-end QoS. QoS control can be made possible by using some access and scheduling methods. Recently the QoS of IEEE 802.11 WLAN system is supplemented by IEEE 802.11e standard (IEEE 802.11e, 2005). It defines Extended Distributed Contention Access (EDCA) that assigns a small backoff number to high priority traffic, and Hybrid Coordination Function (HCF) that improves the conventional polling scheme of Point Coordination Function (PCF). Also, cdma2000 lx EV-DO and WCDMA-HSDPA (High Speed Downlink Packet Access) adopted opportunistic scheduling to exploit channel fluctuation. The opportunistic scheduling has brought an implementation issue in designing various scheduling algorithms for QoS.
The Third Generation Partnership Projects (3GPP and 3GPP2) define four traffic classes and their related parameters for QoS provisioning. There exist gateways between IP backbone and access networks that perform protocol conversion and QoS mapping between IP and access networks. However, direct translation is difficult since access networks have their own QoS attributes that require strict QoS provisioning within them.
Meanwhile, the importance of unified QoS management grows in 4G networks as QoS management for both access network and IP network becomes cumbersome in all-IP networks. If each network has an individual QoS model, it needs a rule that integrates their QoS models to ensure end-to-end QoS. For the unified QoS management, we propose ITRAS that considers L1, L2 and L3 together. In ITRAS, L1 and L2 allocate radio resources and logical channels, respectively, according to the QoS indication of L3.
C ITRAS
ITRAS concerns the information about IntServ and DiffServ for the resource management of L1 and L2. When IntServ sets up a real-time session, MAC reserves a dedicated channel. On the contrary, when DiffServ is used for low mobility users, MAC can exploit either a dedicated or shared channel. If the shared channel is allocated for DiffServ, the wireless scheduler runs a scheduling algorithm for QoS provisioning. In contrast, the dedicated channel allocation needs admission control that allows a limited number of users into the network for QoS support. Therefore IP QoS information helps MAC and PHY manage resources of the following in a flexible manner.
- Cell type - microcell or macrocell
- Multiple access - OFDMA or FHOFDMA
- MAC channel - dedicated or shared
- PHY scheduling - priority or fairness
IntServ is easy to be involved in radio resource management because wireless access is usually accompanied by signaling. When an MS requests a real-time service in a 4G network, the corresponding AR can initiate IntServ and allocate a dedicated channel. For a downlink call, the AR can adjust the bandwidth of a dedicated channel with the aid of RSVP As real-time traffic usually requires a constant data rate, the dedicated channel is recommended to use power control rather than AMC. In this aspect, FH-OFDMA and CDMA may have more suitability than OFDMA for real-time services.
Regarding DiffServ in 4G networks, it is enough for an MS to set the DS field properly for uplink packets because the BS controls radio resources before transferring them to the AR. For downlink traffic, the AR classifies packets according to the DS field and chooses a multiple access method, and accordingly the BS allocates a dedicated or shared channel. The dedicated channel has the advantage of simple management, while the shared channel goes well with the DiffServ because both require scheduling. Contrary to scheduling in routers which need to handle a lot of flows, wireless scheduling takes care of not many connections, which allows to use per-user buffer. So the wireless scheduler can exploit an algorithm with high granularity of radio resources. Figure 1 summarize tightly coupled resource management among three layers through a unified QoS strategy.
Figure 1: The coupled layering for resource management
Further Issues
Implementing ITRAS needs further study. Specifically, when the subnet-based all-IP network is deployed, an AR should cooperate with its subordinate BSs in performing ITRAS functions. While resource management functions are primarily handled by BSCs in the GSM networks, more functions will be imposed on BSs in 4G networks. Basically ARs will be responsible for IP QoS and BSs will play the primary role of resource management. Another challenge is the application of ITRAS to the macro/micro cellular network. In this case, a coordinator is needed in deciding whether an incoming session is served by a macrocell or a microcell. It will also have the capability of load balancing by triggering vertical handoff.
Along with the architectural evolution towards all-IP network, one of the most salient trends for future network design is emerging in the form of Fixed Mobile Convergence (FMC). The integration of wireline and wireless technologies and services realized by FMC is expected to offer benefits to both operators and consumers by delivering enhanced user experience over a unified framework. IP Multimedia Subsystem (IMS) (Poikselka, 2004) lies at the heart of this network convergence. It is a framework that provides a variety of IP based services. This framework enables wireline, wireless and cable operators to offer rich multimedia services across both legacy circuit switched and new packet switched network infrastructures. Also, together with QoS provisioning, security should be guaranteed in 4G mobile networks.
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