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 supporting
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 experience
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 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 standardization
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 handover,
where the handover signaling is performed through the old
base station, is similar to the current GSM handover. For the forward
handover, the mobile station initiates the handover through  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 cellularspecific
infrastructure requirements.
There are several possible scenarios for mobile operators to migrate
from GSM/GPRS to UMTS. As mentioned earlier in this chapter, the complexities
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 feature
support and to what 3GPP technical specification release the various
vendors adhere to. It is therefore to be expected that in a multivendor environment,
the operator will have to compromise the architecture and the services
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 features
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 mismatch.
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).