Issues, Controversies, Problems | WIRELESS ROUTER ARCHITECTURE

The legacy mobile wireless architecture shown on the right half of Figure 1 with a hierarchy of BS (Base Station)s, BSC (Base Station Controller)s, and MSC (Mobile Switching Center)s has a number of shortcomings with respect to handling of large volumes of data in B3G networks: i) wireless frame selection for handoff management is done at BSCs resulting in the duplicate traffic flow on the backhaul, ii) even during the ideal periods of a call, transmission resources are reserved resulting resources wastage in contrast to the IP-networks equipped with the statistical multiplexing scheme, iii) only 15% of the BS-BSC traffic is payload, and the rest is overhead, iv) BSs forward erroneous frames also BSC and this results in dead payload on the backhaul, v) uneven utilization of links makes the system inefficient, cost-ineffective, and unsuitable for deployment of new data intensive services, vi) transmission delays in long BS-BSC links could cause soft-handoff failures and call drops and thereby contribute to performance degradation, and vii) single-point failures of the legacy system entities or the links between them results in low system availability.

Figure 1: A novel distributed wireless router based RAN communicating with a legacy network (each pink dot represents a wireless router)

The demand for high quality services despite large volumes of call traffic necessitates a drastic reduction in the long backhaul (BS-MSC) control and data paths used in the legacy systems. An elegant approach to address the problem is to build the next generation RANs as distributed configurations of simple but functionally comprehensive wireless router (WR)s that could replace and at the same time inter-work, as shown in Figure 1, with the hierarchy of legacy wireless system entities i.e. the BSs, BSCs, and MSCs. Thus these new versatile WRs, need to have integrated into them several overlaying features of an all-IP 3G wireless network depicted in Figure 2. Even though the functional modules of the service layer are not detailed out in the figure in view of the ever growing number of wireless/wire-line services, typical entities that provide network services are call servers, bandwidth brokers, SLA (Service Level Agreement) managers, billing servers, HLR (Home Location Register)s, HSS (Home Subscriber Server)s, MGW (Media Gateway)s, SGW (Signaling Gateway)s, legacy servers, DNS (Domain Name Servers)s, and so on. The control layer supports the services with entities such as QOS (Quality of Service)/Mobility/Location/Power managers, Call Agents, and AAA (Authentication, Authorization, and Accounting) mangers. In the legacy systems, these two functional layers roughly correspond to the MSC and BSC functions, respectively. For execution of these functions, the legacy system entities (BSs, BSCs, and MSCs) need to communicate with one another. This is supported by backhaul IP networks with wire-line topologies. A part of the inter-BS communication is supported by wireless routing. Finally, the communication between the BSs at different cell sites and the mobile device constitutes the physical layer functionality of the wireless system.

Figure 2: Several overlaying features that need to be integrated into a WR

Since the WR replaces the mobile network entities in the proposed RAN architecture, it needs to incorporate the control and routing functionalities of the legacy systems. In particular, it should have the following features: i) it should support all the data and signaling protocols for inter-router communication as well as communication with various service and control entities, ii) it should facilitate effective hand-off management to achieve nearly zero call drop rate, iii) it should provide high QOS by effective bandwidth management through traffic shaping, and iv) it should be capable of dynamically configuring its operational parameters in collaboration with its neighbors, and adapt itself to RF topology and other changes. Overall, it should render the RAN both high performing and highly available at a low cost.

IP-TRIGGERED RESOURCE ALLOCATION STRATEGY (ITRAS)

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.