q
Um: The air interface is used for exchanges between an MS
and a BSS. LAPDm, a modified version of the ISDN LAPD, is used for signalling.
q
Abis: This is the internal interface linking the BSC and a
BTS, and it has not been standardised. The Abis interface allows control of the
radio equipment and radio frequency allocation in the BTS.
q
A: The A interface is between the BSS and the MSC. The A
interface manages the allocation of suitable radio resources to the MS’s and
mobility management.
q
B: The B interface between the MSC and the VLR uses the
MAP/B protocol. Most MSC’s are associated with a VLR, making the B interface
"internal". Whenever the MSC needs access to data regarding an MS
located in its area, it interrogates the VLR using the MAP/B protocol over the
B interface
q
C: The C interface is between the HLR and a GMSC or an
SMS-G. Each call originating outside of GSM (i.e., a MS terminating call from
the PSTN) has to go through a Gateway to obtain the routing information
required to complete the call, and the MAP/C protocol over the C interface is
used for this purpose. Also, the MSC may optionally forward billing information
to the HLR after call clearing.
q
D: The D interface is between the VLR and HLR, and uses
the MAP/D protocol to exchange the data related to the location of the MS and
to the management of the subscriber.
q
E: The E interface interconnects two MSC's. The E
interface exchanges data related to handover between the anchor and relay MSC's
using the MAP/E protocol.
q
F: The F interface connects the MSC to the EIR, and uses
the MAP/F protocol to verify the status of the IMEI that the MSC has retrieved
from the MS.
q
G: The G interface interconnects two VLR’s of different
MSC's and uses the MAP/G protocol to transfer subscriber information, during
e.g. a location update procedure.
q
H: The H interface is between the MSC and the SMS-G, and
uses the MAP/H protocol to support the transfer of short messages.
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