UMTS RNC Dimensioning

Although a BSC is generally limited by the number of RF network elements
(such as sites, sectors, and TRXs) that can be supported, the capacity of an
RNC tends to be traffic or throughput limited. This is because of the fact
that an RNC can be involved in traffic handling for base stations that it
does not directly control. For example, an RNC can act as a serving RNC or
drift RNC during soft handover. In such cases the RNC may be handling
traffic to/from a base station that it does not control. Thus, the number of
controlled base stations becomes less important and the amount of traffic
handled is of greater significance to the capacity of the RNC.
The capacity of an RNC is typically limited by a combination of the following
factors:
■ Total Erlangs
■ Total BHCA
■ Total voice subscribers
■ Total data subscribers
■ Total Iub interface capacity (Mbps)
■ Total Iur interface capacity (Mbps)
■ Total Iu interface capacity (Mbps)
■ Total switching capacity (Mbps)
■ Total number of controlled base stations
■ Total number of RF carriers
The determination of the number of RNCs required in a given market
will be based on which of these limitations is the most restrictive.
Unlike the situation for BSCs, it is more common for RNCs from a given
vendor to be offered in a variety of configurations. For example, the Iu interface
might be offered using different transmission interface capacities (such
as E1, T1, or STM-1). Moreover, a given vendor’s RNC might come in several
multi-cabinet configurations, where one can start with a small configuration
and expand capacity by adding additional cabinets.
The determination of the number of required RNCs is more complex than
the equivalent determination of the number of required BSCs. In particular,
the effect of soft handover needs to be considered. Imagine for example that
there are two RNCs supporting a number of base stations. One RNC limitation
will be the total switching capacity. If there is a great deal of inter-
RNC soft handover, then switching capacity is consumed on both RNCs. In
fact, switching capacity can be consumed on both RNCs even after the soft
handover is finished if SRNS relocation has not yet taken place. Thus, the
determination of the number of RNCs needs to consider not just the RF elements
and not just the overall traffic load, it must also consider the effects
of soft handover. For this reason, the calculation of the number of required
RNCs should be done is close cooperation with the RF design effort.
In most cases, we find, however, that the most limiting factor is the Iub
interface capacity. 417