This also depicts the evolution
of GSM wireless as defined by 3GPP. When looking at the GSM network, one can see
where GPRS was added for data support. GPRS provides a network where data can be
offloaded from the wireless switches, and also provides some additional bandwidth at
the radio interface.
The GSM radio reference points are identified as single-letter identifiers. The predominant
protocol used over these interfaces is the Mobile Application Part (MAP). The
E interface sits between two MSCs within the same network. The Gateway MSC in this
figure identifies a gateway function interfacing into another GSM network.
The MSC in turn interfaces with the base station controllers (BSCs) in the network
via the A interface. The A interface allows the mobile switches to communicate with
the radio resources in the network using SS7 signaling, as well as bearer path (voice
channels from the radio resources into the mobile switching center).
Architecture of the IMS 23
The BSCs interface to base transceiver stations (BTSs), which provide the GSM radio
function. The BTSs provide the radio functions interfacing to the antenna(s) at the cell
site, under the specific control of the BSC. They communicate to the BSC via the A-bis
interface.
For accessing subscriber data, the MSC must communicate with the HLR. This is
supported via the D interface. The VLR function is typically integrated with the MSC,
even though here it is depicted as a separate entity.
Pages:
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79