Since the services were limited to voice,
this was fairly easy to accomplish; however, without IN and SS7, mobility would have
never been possible. The IN and SS7 provide the core call control functions throughout
the wireless domain. Remember it is SS7 and the MAP protocols that allow the wireless
switches and base stations to communicate with each other, providing registration and
location updates for each subscriber device.
When data services were added to this network, there came a need to offload the data
from the MSC onto a data network. Referred to as 2.5G, this consists of the General
Packet Radio Service (GPRS) network. The purpose of the GPRS network is to provide
the functions necessary to support packet services.
In other words, instead of sending the data packets through the base stations, to the
MSC, and then offloading the packet data to another network, the data is sent straight
from the base stations to the packet network (GPRS).
Figure 1.3 Wireless evolution from 2G to IMS
GMSC
MSC
BSC
BTS BTS
VLR
HLR
SMS
GMSC
GGSN
SGSN
P-CSCF I-CSCF S-CSCF MRFC
MRFP
MGCF
SGW
IM-MGW
BGCF
SGSN
RNC RNC
Node B Node A
AS HSS SLF
IMS
UMTS
GPRS GSM
Architecture of the IMS 9
The GPRS then interconnects the wireless domain with the Internet and other
packet networks, eliminating the need entirely for sending the packet data through
the MSC.
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