In the case where the network technology involves BSCs (or RNCs) that are
separated from the MSC, then the MSC capacity generally has two limitations�"
maximum BHCA and maximum Erlang. In networks where the
BSC functionality is included within the same machine as the MSC, then
there will also be limitations in termed of RF elements supported (such as
sites, sectors, and TRXs).
The separation of BSCs or RNCs from the MSC is the most common configuration
in 3G networks. Consequently, the MSC capacity is generally not
limited by RF-specific factors. Thus, the capacity is Erlang or BHCA limited.
(There may be a total cell limit, but this is generally sufficiently large
that it is not a limiting factor.)
Although we say that the capacity of an MSC is typically Erlang or
BHCA limited, the reality is that the BHCA limit is the real bottleneck.
Although BHCA and Erlangs are closely related, Erlangs reflect the switching
capacity and port capacity of the MSC, whereas BHCA reflects the processing
power of the MSC. In general, the number of supported Erlangs can
be increased by the addition of extra MSC hardware, while the maximum
BHCA for a given release of MSC is typically fixed. Thus, it is usually possible
to add hardware and increase the supported Erlangs until such time
as the BHCA limit is reached. Adding extra hardware after this point provides
no extra capacity. Thus, when determining the number of MSCs
required to support a given market, the calculation is BHCA-based.
When we come to distributed architectures, such as the MSC Server�"
Media Gateway architecture of 3GPP Release 4, many of the same dimensioning
rules will still apply. In this case, the MSC Server is most likely to
be BHCA limited, while the Media Gateway is likely to be Erlang limited.
Today’s MSCs typically have BHCA limitations in the order of 300,000 to
500,000 BHCA. As technology advances, these numbers will increase, and
capacities of up to 1,000,000 BHCA will be common in the next few years.
For most vendors, the configuration of a given MSC in a given market is
a custom configuration. In other words, the size of the switching matrix and
the numbers and types of ports are custom designed to meet the specific
market requirements. If one is building a limited number of markets at a
given time, this is the optimum approach. On the other hand, if one is
attempting to build a large network (such as a nationwide deployment)
with many MSCs, custom design of the hardware configuration for each
MSC may be overly time consuming and may jeopardize a timely launch. In
such a situation, it is often wise to work with the MSC vendor to define a
number of network-specific standard configurations, such as small,
medium, and large configurations, depending on the types of markets to be
supported. Thus, in a large metropolitan city, one might need to deploy two
large MSCs, although is a smaller city, one might need only a single
medium-sized MSC or a single small-sized MSC. Although this approach is
not optimal from a hardware perspective, judicious determination of the different
configurations is likely to ensure that there is not a great deal of over
dimensioning. The resulting ease of cookie-cutter design and easier ordering and delivery may well result in savings in the design effort and
more rapid deployment.
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