Impact on the Existing Network

In this section, the 3G impact on the existing network architectures will be discussed. It is important to realize that even if most legacy networks are sup-porting the same access technology, their architectures often are very

different. Thus, it can be noted that existing mobile networks architectures are very different in nature. They can be characterized by the human experi-ence pool responsible for the architecture, network age and size, vendor or vendors, and whether they interface with fixed networks (either PSTN or IP). The understanding of these facets of the legacy networks (see Figure 2.9) will, together with the need for cost-efficient solutions, be a major driver for 3G deployment and the impact of 3G network rollout on the existing legacy networks. For example, if a mobile network operator already has an ATM backbone network and considerable experience with ATM engineering, it should use this synergy to reduce deployment cost and reduce investments in new transmission and switching equipment. A legacy network that does not have an existing ATM infrastructure could benefit from architecting an IP backbone, although ATM would still need to be supported for uplink to the Iu,CS and Iu,PS. It should also be noted that it is often easier to find good IP engineering skills in comparison with ATM experience. For a single-vendor legacy network, considerable synergy could be found in operational and maintenance experience as well as network monitoring systems by choosing the existing vendor for a 3G network. The initial ease of integration and few

Figure 2.8 A possible network deployment scenario for 2G mobile operators.

interoperability issues would also be expected. The big question in this single-vendor scenario is whether the equipment and service pricing will be attractive enough when compared to the addition of a new vendor for the 3G network. The legacy network will be a significant boundary condition for architecting the 3G network. Only Greenfield operations, where no legacy network is present, or an operation with very recent legacy operations might deploy the theoretical architectures (i.e., such as all-IP or near-IP networks with the state-of-the-art IP QoS implementations) presented in standardiza-tion bodies.

With all the changes between the GSM network and the 3G network, there are two major impacts on the existing network:

1. Handover:It is assumed that the handover decision is always made inside GERAN. For inter-GERAN handover, functions that set up a path within the CN are required. Depending on the handover type, different changes in GSM are required. The backward hando-ver, where the handover signaling is performed through the old base station, is similar to the current GSM handover. For the for-ward handover, the mobile station initiates the handover through

Figure 2.9 2G legacy network.

the new base station. When trying to avoid corner effect, in which the connection to the old station is lost, a very fast handover is required to prevent the blocking of the existing users in another cell. Forward handover requires a large number of changes in GSM.

2. Transmission infrastructure: The transmission infrastructure has to meet new requirements imposed by wideband services. Because the data services are bursty and often asymmetric, the transmission solution has to be able to efficiently multiplex different types of information. ATM will provide efficient support for transmission of bursty wideband services. However, because ATM was originally designed for very high-speed transmission in the fixed network, some modifications may be needed to accommodate cellular-specific infrastructure requirements.

There are several possible scenarios for mobile operators to migrate from GSM/GPRS to UMTS. As mentioned earlier in this chapter, the com-plexities of the 3G migration depends to a high degree on the legacy mobile network and to what extend a single-vendor environment is in place and will remain after the 3G migration. In a multivendor environment, the migration could be considerably more complicated due to mismatches in software fea-ture support and to what 3GPP technical specification release the various vendors adhere to. It is therefore to be expected that in a multivendor envi-ronment, the operator will have to compromise the architecture and the serv-ices that initially will be launched. Furthermore, where the 2G and 3G vendor differs, interfaces might have to be reconsidered with the possible result of service touch-and-feel changes. A typical example is the interfaces between the HLR and the SGSN (Gr)/GGSN (Gc), and MSC/VLR and SGSN (Gs). Moreover, one vendor’s software release package might differ substantially from another vendor’s release (after all, with open interfaces fea-tures will be what differentiates the various vendors) and could allow for only basic services to be launched or result in significant development work to allow for feature match. A good example of this particular issue is in legacy networks with the open MAP interface between the MSC/VLR and the HLR (i.e., C & D interfaces).

Theoretically, it is possible to interface vendor X HLR with a vendor Y MSC/VLR, but only the basic features could be explored due to feature mis-match. In practice, an operator would always vendor match the MSC/VLR and HLR in order to get the maximum out of the architecture and network infrastructure.

1. To upgrade its existing GSM/GPRS CN for UMTS use, in this case 2G and 3G network share the same core infrastructure, as shown in Figure 2.10. The possible impact on the existing network includes:

• Redimension of the existing CN to be able to support 3G broadband services;

• Optimization of the transmission network for a suitable traffic mix;

• Network management system.

2. To deploy an independent 3G CN from the 2G CN, as shown in Figure 2.11, in this case 2G and 3G network will have minimum impact on each other. In a multivendor scenario, interoperability tests (IOTs) will be needed depending on the architecture—for example, Gc (between HLR and GGSN), Iu,PS (RAN and SGSN), and Gr (HLR and SGSN).