3G LTE & WLAN | WIRELESS NETWORK ARCHITECTURES

3G LTE

The 3G LTE standard (3GPP Release 8, 2009) has defined a simple network architecture of E-UTRAN (evolved universal terrestrial radio access network). The E-UTRAN consists of eNBs (evolved node Bs) which are interconnected with each other by the X2 interface. As presented in Figure 1, each eNB is connected with a S-GW (serving gateway). The S-GW terminates the S1 interface between an eNB and the MME (mobility management entity). The eNB hosts the functions of RRM and dynamic resource allocation as in other BSs, while the S-GW hosts the function of mobility anchoring by assigning an IP address to an end host. This architecture is similar to the Profile C of WiMAX ASN, since most RRMfunctions are fulfilled by the eNB in a flat manner while some mobility functions are fulfilled by the S-GW in a hierarchical manner.

 

Figure 1: The overall architecture of 3G-LTE E-UTRAN

WLAN

Similarly, it is also a controversial issue how to implement the access network in the IEEE 802.11 wireless LAN systems. A subnet is composed of an AR and APs, where the hierarchical structure is also similar to cellular networks. Three types of APs are considered—Fat AP, Thin AP, and Fit AP—according to the role assigned to the AP (Sridhar, 2006). The Fat AP provides router-like functions, so there is no backhauling of traffic. This scenario is very close to the all-IP networking. In contrast, the Thin AP is close to the BS in the WiMAX Profile A. The primary role of Thin APs is to receive and transmit wireless traffic, but in this case, a group of APs are managed by a centralized access controller which acts as an ASN gateway in the WiMAX ASN. In the Fit AP architecture, MAC functions are split between the AP and the access controller, so this architecture is compromised between the Fat AP and the Thin AP models.

All-IP Cellular Networks | WIRELESS NETWORK ARCHITECTURES

In existing cellular networks, an access network consists of many entities for supporting radio resource management and mobility management. For example, in 2G GSM/GPRS networks, the base station subsystem (BSS) consists of the base transceiver system (BTS) that handles the physical layer and the base station controller (BSC) that handles radio resource management and handoff. Also, the mobile service center (MSC) fulfills upper layer functionality and acts as the visitor location register (VLR) that is required to update the location of every MS for paging. Protocols defined in each layer in GSM systems are exhibited in Figure 1, where several protocols are defined for communication between any two entities.

 

Figure 1: An example of protocol stacks in GSM systems

4G networks, in contrast, will make such a complicated protocol stack much simpler, by enabling IP packets to traverse between a base station (BS) and a mobile station (MS). Each BS may need to perform all the functionalities required in BSS, BSC, and MSC. This makes the BS play a role of an access router (AR). This architecture is shown in Figure 2. It incurs high overhead, however, especially when an MS configures a mobile IP (MIP) address for handoff. As it is known that it takes several seconds to run the MIP handoff (Yokotaet al, 2002), MIP hinders an MS from carrying out smooth handoff. In addition, the 4G network is expected to have a small cell radius due to use of high frequency band, which possibly results in short cell residence time. For this matter, reducing the latency in performing the MIP handoff is still a challenging issue. For instance, a fast handoff scheme(Koodli, 2004) proposes to decrease the address resolution delay by pre-configuration.

 

Figure 2: The pure all-IP 4G Network

Another feature of such all-IP networks is their flat architecture. All the radio resource management and mobility management will be performed at each BS independently of the other BSs. Unlike traditional cellular networks of a hierarchical architecture, the flat all-IP network can be operated flexibly but at the cost of complexity in terms of intercell RRM (e.g., coordination among cells). There are increasing demands for intercell RRM for efficient network management; for example, fractional frequency planning for OFDMA wireless networks is needed to improve cell-edge performance. The upper entity such as the BSC in hierarchical cellular networks could be a good coordinator for such a scheme.

To alleviate the difficulty in radio resource and mobility management of all-IP cellular networks, a semi (i.e., subnet-based) all-IP cellular network can be considered as shown in Figure 3, an example of a simple network where an AR manages several BSs. The functionality of an AR is separated from that of a BS in order that each undertakes L3 and L2 protocols, respectively. This relation is similar to that between BSC and BTS in GSM networks. Then, an MS moving within the subnet (i.e., changing BSs) performs L2 handoff without changing MIP attachment. The MS only needs to trigger L3 handoff, when it moves into another AR area.

 

Figure 3: The subnet-based 4G network

A main difference is that the former is decentralized while the latter is centralized. Since the pure all-IP network incurs a L3 protocol in the end access link, it requires a long handoff latency and high signaling overhead. However, the architecture is simple and cost-efficient for implementation. On the other hand, the subnet-based all-IP network implements hierarchical architecture, so it is possible to fulfill efficient resource management in spite of its inflexibility. Both network architectures are being considered in WiMAX and 3G-LTE systems, which are described in the following.