GSM Layer III Messages
Signaling
Protocol In GSM
In telecommunication system, signaling is required to coordinate the distributed
entities of the network.
The
signaling protocol in GSM is structured into three general layers as shown in
the following figure.
• Layer 1 (Physical layer)
It uses the data channel structures over the
air interface
• Layer 2 (Data link layer)
It provides
error-free transmission between adjacent entities, based on the ISDN’s LAPD
protocol for the Um and Abis interfaces, and on SS7’s Message Transfer Protocol
(MTP) for the other Layer interfaces
• Layer 3 (Networking or
messaging layer)
Responsible
for the communication of network resources, mobility, code format and
call-related management messages between various network entities
Layer
III is the GSM signaling protocol, which is divided into 3 sub-layers:
Radio
Resource Management to
control the setup, maintenance, and termination of radio and fixed channels,
including handovers;
Mobility
Management to
manage the location updating and registration procedures, as well as security
and authentication; and
Connection
Management to
handles general call control and manages Supplementary Services and the Short
Message Service.
System Information Type 1
When
MS need to know which frequency band to use and which frequencies within the
band it should use during frequency hopping
The ARFCN for all frequencies used in the cell.
RACH
control parameters
Information
is also provided instructing the MS how to perform access to the system. This
information is provided in the RACH control parameters information element
Cell Bar
access (CBA, CELL_BAR_ACCESS)
Cell
barred for access is denoted by using one bit only (yes/no).
Call
reestablish permitted (RE)
MAXRET
(Max retransmissions)
Tx-Interger
Access control class ( ACC) is described using a bit map with 16 bits. All MSs are spread out on ACC 0-9. Priority group MSs use ACC 11-15. A bit set to 1 indicates that the class is barred for access.
Emergency calls EC=0 means that all MSs are allowed to make emergency calls, while EC=1 means that only MSs belonging to priority classes 11-15 are allowed to make emergency calls.
System Information Type 2
The
neighbor cells description information element denotes the frequencies of the
BCCH carriers to be monitored by the MSs in the cell.
BA-IND
This
is a BCCH allocation sequence number indication. BAIND can have the value 0 or
1. When the operator changes the frequency list, the BA-IND changes from 1 to 0
or vice versa.
BA
ARFCN
These
are absolute RF channels on which the MSs should signal strength perform measurements.
System
Information Type 3
SI3 msg are showing the information of CGI, Cell selection and basic idle mode parameter.
Location
Area Identification (LAI)
MCC
Mobile Country
Code (3 digits)
MNC
Mobile Network
Code (2-3 digits)
LAC Location Area Code (2 octets:
0-65535 different LACs)
Cell
Selection Parameters
Maximum
power level of control channel (MS_TXPWR_MAX_CCH)
Minimum
receiving level of MS permitted to access (RXLEV_ACCESS_MIN)
Additional
reselection parameter indicator (ACS)
Half-rate
supported(NECI)
Power
control indication (PWRC): The power control indicator equal to 1 means set
Discontinuous
transmission (DTX)
Radio
link timeout (RLT)
ATT Attach/Detach
allowed
AGBLK
(BS_AG_BLKS_RES) This
is the number of CCCH blocks reserved for the AGCH
CCCH_CONFThis
is the number of Basic Physical Channels used for the CCCH
CCCH-CONF
: (0) 1 basic physical channel used for CCCH, not combined with SDCCHs
MFRMS
(BS_PA_MFRMS) This
is the number of 51 multiframe periods for transmission of Paging Request messages
in the same paging group (2-9).
T3212 This
is the time-out value for MS periodic location updating. T3212 has values from
0 to 255. (Time for periodic LU = 6min *
T3212 value)
System
Information Type 4
In
system information type 4, MSs are informed if the cell broadcast function is
used in this cell and on what frequency the CBCH is found.
The
LAI, the cell selection parameters, the RACH control parameters and rest octets
are also included in the type 4 message.
CBCH
Channel Description (Optional)
In
this information element, the channel used for SMS cell broadcast is described.
CHANNEL
TYPE
This
is the channel combination and sub-channel used for CBCH. For example, SDCCH/4
+ SACCH/C4 or CBCH, sub channel 2
TN : This
represents the time slot number
TSC : This
is the training sequence code. BCC part of BSIC is recommended.
H : This
is the hopping parameter, where H=0 means single RF channel and H=1 means RF
hopping channel.
type 3
rest octets. These are PI, CBQ, CELL_RESELECT_OFFSET, TEMPORARY_OFFSET, and
PENALTY_TIME
System
Information Type 5
It
describes the neighbor cell BCCH frequencies(BA2 list). It is mandatory and is
transmitted on SACCH. MS can read the frequency described in SYS INFO 5 in
dedicated mode, report the relevant information of neighbor cells in
measurement report, and use it as the basis for handover.
Neighbor
Cells Description
The
neighbor cells description information element denotes the frequencies of the
BCCH carriers to be monitored by the MSs in the cell.
System
Information Type 6
When
in active mode, the MS needs to know if the LAI changes. In case the LAI
changes the MS must perform location updating when the call is released.
If the MS changes between cells (within the
location area) where RDLINKOUT or DTX conditions differ the new cell options
must be reported to the MS.
NCC
permitted information element is also included
System
Information Type 7 & 8
System
information type 7 & 8 are optionally sent on BCCH extended if system
information type 4 does not contain all information needed for cell
re-selection.
Measurement Report
The
mobile unit always scans BCCH of up to 16 neighboring cells, and forms a list
of the six best candidates for possible handover. This information is
transmitted to current Base Station at least once per second. BSC and MSC use
this information for handover algorithm.
No comments:
Post a Comment