a)GSM Interfaces
Figure 2.1 GSM inter-node Interfaces
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78
b)GSM Signaling model
Sig.L1 = Signaling Layer 1 on the Abis & Um interfaces
BTSM=BTS Management
RR
=Radio Resource management
MM
=Mobility Management
CM
=Connection Management
Figure 2.2 a) GSM Signaling model 1
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79
Figure 2.2 b) GSM Signaling model 2
The A Interface
Layer 3 towards the A interface has three sublayers: RR, MM
& CM. While the CM & MM reside in the MSC, the RR layer
resides in the BSC.
CM of layer 3 is equivalent to ISUP of SS7 and is related to
circuit-switched connections towards MS. Any mapping of ISUP
messages to CM messages needed for interworking is handled by call
handling software in the MSC.
CM includes call control (CC) messages. Some CC messages such as
registration of supplementary services need to be mapped to MAP
messages in the MSC. Some other MAP messages need to be mapped to
MM messages.
Services of BSSAP are used to transfer CM & MM messages. The
BSC & BTS do not process the CM/MM messages but only relay
them.
BSSAP is also used for direct control of BSC as when an
assignment request is made.
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The Abis Interface
LAPD protocol of the ISDN DSS1 is used as the layer 2 protocol
on the Abis interface.
Layer 3 messages related to CM & MM pass through the BTS
transparently.
Most of the RR messages also pass through the BTS transparently,
while some other RR messages related to radio equipment are handled
by the BTS. Any conversion to the RR messages towards the MS is
handled by the BTS. The ciphering message is an example where the
ciphering key Kc is retained in the BTS.
The Um Interface
Layer 2 protocol used on the Um is called LAPDm, a modified
version of LAPD. The error detection & correction of the LAPD
is removed since this function is handled in the layer 1 of the
radio interface. Also, layer 3 messages are segmented and the LAPDm
frame lengths are adjusted to fit into the radio bursts. Details of
the Um interface are described below.
The Signaling layer 1
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81
Figure 2.3 Radio interface protocol structure
Signaling layer 2 - LAPDmEach logical control channel is
allocated a separate protocol entity. Some control frames such as
SABM & UA carry piggy-backed layer 3 information. Layer 3
messages are segmented and the data exchanged
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82
between the data link layer & the physical layer has either
23 octets (BCCH, CCCH, SDCCH & FACCH) or 21 octets (SACCH).
UIUnacknowledged
MFMultiple frame
Figure 2.4 Data link layer connections in the mobile
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83
Signaling layer 3Layer 3 consists of three sublayers Radio
Resource management (RR), Mobility Management (MM) & Connection
Management (CM).
Radio Resource management (RR)
RR sublayer consists of functions needed to establish, maintain
and release RR connections over dedicated channels.
Mobility Management (MM)
Messages to & from the CM layer are transferred
transparently by the MM layer. When the CM layer requests for the
establishment of an MM connection, the MM layer in turn requests
for an RR connection.
When an IMSI detach procedure is carried out, the network marks
the mobile as not reachable and either forwards or blocks an
incoming call without paging for the mobile.
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Connection Management (CM)
As indicated in figure 3.3, this sublayer consists of three
entities, CC, SS & SMS.
Call control (CC)
The procedures & functions adapted from the ISDN call
control and modified to suit the radio environment.
The mobile always uses en bloc sending to save on radio
resources.
Call re-establishment & in-call modification specially
included
Call specific supplementary services e.g., user-user
signaling
Supplementary services (SS)
Non-call related supplementary services such as call
forwarding
Short Message Service (SMS)
Higher-layer protocols for transfer of short message between the
network & a specific MS
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85
2.GSM Protocols
M
S
B
T
S
B
S
C
M
S
C
EIR
Sig.L1
HLR
VLR
VLR
MSC
Um
Abis
A
B
C
D
E
F
G
LAPDm
RR
MM
CM
RR
LAPDm
Sig.L1
BTSM
LAPD
Sig.L1
BTSM
LAPD
Sig.L1
RR
CM
MM
MTP
SCCP
BSSAP
BSSAP
SCCP
MTP
A
Abis
Um
BSSAP
I
S
U
P
A
CM
MM
MS
BTS
BSC
MSC
MTP
TCAP
SCCP
MTP
SCCP
MAP
B/C/D/E/F
MSC
The signaling layer 1 functions
Mapping of logical to physical channels
Channel coding for error detection (CRC) & error correction
(FEC)
Ciphering
Idle-mode cell selection/reselection by the MS
Interfaces to speech coder & terminal adaptor
Establishment of dedicated physical channels
Measurement of signal strength & quality of the dedicated
channel
Measurement of signal strength of surrounding cells by the
MS
Timing & power control order by BTS and setting the timing
advance & transmission power by the MS
Signaling layer 3
Connection Management CM
CC
SS
SMS
Mobility Management MM
Radio Resource Management RR
Signaling layer 2
BCCH
PCH
AGCH
RACH
SDCCH
FACCH
SACCH
Cell
Broadcast
Functions
CBCH
TCH
Signaling layer 1
R
A
C
H
Proc
Data link proc
UI
Data link proc
UI
Data link proc
MF UI
Data link proc
MF UI
Data link proc
UI
Data link proc
MF UI
SAPI=0
SAPI=3
Data link Distribution procedure
RACH
BCCH
PCH
AGCH
SDCCH
FACCH
SDCCH
SACCH
Layer 3
Layer 2
Physical
Layer
RR functions
RR connection establishment
Cipher mode setting
Change of dedicated channel e.g., SDCCH to TCH
Handover from one cell to another
Frequency re-definition (frequency hopping)
MM functions
Authentication
TMSI re-allocation
Identification of the mobile, IMSI or IMEI
Location registration
IMSI detach / attach
