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GB_BT05_E1_0
GSM Signaling System Introduction
Course Objectives:
Understand Protocol stack of GSM system
Understand MTP, LAPD, LAPDm protocols
State main calling process
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Contents
1 Interfaces and Protocols ...............................................................................................................................1
1.1 Interfaces ..................................................................................................................................................1
1.1.1 A Interface .................................................................................................................................1
1.1.2 Abis interface ............................................................................................................................1
1.1.3 Ater Interface ............................................................................................................................1
1.1.4 Gb Interface ..............................................................................................................................2
1.1.5 Qx Interface ..............................................................................................................................2
1.2 Protocols ...................................................................................................................................................2
1.2.1 Circuit Service Protocols ..........................................................................................................3
1.2.2 Packet Service Protocols ........................................................................................................12
1.2.3 TCP/IP ....................................................................................................................................14
1.2.4 X.25 Protocol ..........................................................................................................................15
1.2.5 HDLC Protocol .......................................................................................................................16
2 Basic Signaling Procedure ..........................................................................................................................17
2.1 MS Location Update Procedure .............................................................................................................17
2.2 IMSI Detach Procedure ..........................................................................................................................18
2.3 Mobile-Originated Call and Called Party On-hook Procedure .............................................................19
2.4 Mobile-Terminated Call and Calling Party On-hook Procedure ...........................................................21
2.5 Intra-cell Handover Procedure ...............................................................................................................23
2.6 Inter-cell Handover Procedure ...............................................................................................................23
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1 Interfaces and Protocols
1.1 Interfaces
1.1.1 A Interface
The interface between BSC and MSC is the A interface. TC on the BSC is connected to
MSC via A interface.
TC performs conversion between voice codes and 64 kbps A-law PCM cods. In
addition, it performs data rate adaptation in circuit data services. TC is located between
BSC and MSC and can be on either the BSC side or MSC side depending on the
requirements.
A interface is realized by E1 link with a 75 ohm coaxial cable or a 120 ohm twisted
pair cable for connection.
A interface uses Message Transfer Part level-2 (MTP2) at the data link layer, MTP3
and SCCP protocols at the network layer and Base Station Subsystem Management
Application Part (BSSMAP) protocol at the application layer.
1.1.2 Abis interface
The interface between BSC and BTS is the Abis interface. BSC and BTS are
interconnected through Abis interface with Base station Interface Equipment (BIE)
configured on both sides.
Abis is a self-defined internal interface realized by E1 with a 75 ohm coaxial or 120
ohm twisted pair cable for connection. Supports star, chain, and tree networkingmodes.
LAPD protocol is used at the data link layer and other application protocols such as RR
are used at the Physical layer.
1.1.3 Ater Interface
SMU is used between BSC and TC to reduce transmission line costs when TC is
located on the MSC side.
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The interface between BSC and TC is the Ater interface. Transmission content is the
same as A interface except transmission rate of voice signals. The voice signal at A
interface is 64 kbps A-law PCM coded signal while the voice signal at the Ater
interface is the same as that of Abis interface. Ater interface uses SS7 for transmission.
1.1.4 Gb Interface
The interface between BSC and SGSN is the Gb interface.
Gb interface is realized by E1 link between BSC and SGSN. Access rates are N x 64
kbps (1 N 32) or 2048 kbps. Operator specifies the timeslot and bandwidth used on
E1 link.
BSC implements RLC/MAC protocol, NS protocol, and BSSGP on the Gb interface.
1.1.5 Qx Interface
The interface between BSC and the background OMC is the Qx interface. Operation
and maintenance instructions are received at the BSC while system maintenance
information is sent to OMC via the Qx interface.
Qx interface supports the following connection modes:
Connection through X.25 leased line
Connection through Packet Switched Public Data Network (PSPDN)
Connection through Ethernet interface
Semi-permanent connection between BSC and OMC through the A interface
circuit.
1.2 Protocols
Two systems can communicate only when they agree to a certain protocol. Interfaces
are the connections between two entities and protocols are the rules for information
exchange using some interface. In application systems, several pieces of equipment and
various devices work together to accomplish a single function. Therefore, they are
interconnected via a variety of interfaces based on specified protocols.
The GSM signaling protocol is based on Open System Interconnection (OSI) reference
model. Fig 1.2-1 shows the basic structure of OSI, which is a layered topology. First
layer is the Physical or Transmission layer, second is the Link or Network layer, and
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2 Basic Signaling Procedure
third is the Application layer.
11
22
33
Application
layer
Link layer or
network layer
Physical layer o
transmission lay
Fig 1.2-1 OSI Reference Model Layered Structure
ZXG10-BSC (V2.95) implements different protocols to process circuit services and
GPRS data services. Following is a brief description of circuit service and data service
protocols used on related interfaces.
1.2.1 Circuit Service Protocols
Fig 1.2-2 shows the circuit service protocol stack structure.
CM
MM
RR
LapDm
MS
RR
LapDm
Um
LapD
BTSM
LapD
Abis
RR
BTSM SCCP
MTP3
BSSAP
BTS BSC
MTP2
SCCP
MTP3
BSSAP
MTP2
CM
MM
MSC
APhysicallayer
Fig 1.2-2 Circuit Service Protocol Stack Structure
1.2.1.1 A Interface Protocols
Fig 1.2-3 shows the circuit service protocol stack on A interface.
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BSC
MTP3
MTP2
Layer1
MSC
A interface
Layer1
MTP2
MTP3
RR
SCCP SCCP
BSSAP BSSAP
MM
CM
Fig 1.2-3 Circuit Service Protocol Stack on A Interface
Physical layer
The physical layer defines physical and electrical parameters as well as the
channel structure. This layer connects BSC and MSC physically.
It is implemented through MTP1 of SS7 and uses 2 Mbps PCM digital link for
transmission.
Data link layer
Network operation program defines the data link layer.
Data link layer uses MTP2, a derivative of High-level Data Link Control (HDLC).
Frame is composed of flag, control, information, check, and flag sequence fields.
MTP3 and SCCP implements signal routing.
Application layer
The protocol used on application layer is the BSS Application Programs (BSSAP).
It implements maintenance and management of BSS resources and connection,
and removal of services.
1.2.1.2 Abis Interface Protocols
Fig 1.2-4 shows the circuit service protocol stack on Abis interface.
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2 Basic Signaling Procedure
Abis
BTS
BTSM
LAPD
Layer1
BSC
interface
Layer1
LAPD
BTSM
RR
Fig 1.2-4 Circuit Service Protocol Stack on Abis Interface
Physical layer
2 Mbps PCM link is often used
Data link layer
LapD (Link Access Procedure of D Channel) is a data link procedure for signaling
transmission between BTS and BSC, used to implement message transmission between
the L3 entities in the D channel.
LapD is a point-to-multipoint communication protocol which employs frame structure.
LapD implements the following functions:
1. Providing one or multiple data connections in the D channel.
The data link connections are identified by the data link connection identifiers
(DLCI) contained in the respective frames. DLCI consists of terminal equipment
identifier (TEI) and service access point identifier (SAPI), indicating the
terminal entity and target service access point.
2. Delimitation, location and transparency of the frame
3. Sequence control, ensuring sequential transmission of the frames
4. Error detection
5. Error recovering
6. Notifying the management entity of the unrecoverable error
7. Flow control
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Functions 1, 2 and 4 hereof are completed automatically by the hardware, while
functions 3, 5, 6 and 7 are implemented via the software.
LapD is implemented in the LapD module of RSL. Fig 1.2-5 shows the position of the
LapD module in RSL.
LapD module
OAMM FURRM
Physical layer
BSC
Fig 1.2-5 The Position of the LapD Module
The LapD module communicates with the physical layer and L3. The L3 protocol is
processed in FURRM.
OAMM configures the parameters such as TEI and values of the timer necessary for
LapD module running.
The LapD module provides two types of information transmission modes for the
FURRM: I-frame multi-frame operation and UI frame operation.
1. I-frame multi-frame operation
The L3 message is sent in the information frame mode which requires the
confirmation from the receiver. This mode provides a whole set of control
mechanism for error recovering and flow control, the establishment mechanismand release mechanism for multi-frame operations.
As shown in Fig 1.2-6, the I frame consists of flag sequence, address field,
control field, information field and check field.
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flag Ad dress Con tro l Info rmati on FCS fl ag
SAPI TEI N(S) N(R)
1 0-260 2 1
Fig 1.2-6 The Structure of the I Frame in the LapD
The address field contains SAPI and TEI. It performs addressing for various
units via TEI in the Abis interface link. Generally, a unit has multiple functional
entities, and the logical physical links between different functional entities are
identified by the functional address of SAPI. The LapD supports three types of
information: Signaling (including short message information), O&M
information and LapD layer management information. Links of the three types
of information are distinguished by SAPI. SAPI = 0 represents the signaling
link, SAPI = 62 represents the O&M link, and SAPI = 63 represents the
management link of the LapD layer.
In the control field, N (S) represents the transmitting serial number and the serial
number of the I frame currently transmitted at the sending end; N (R) represents
the receiving serial number and the transmitting serial number of the next
expected I frame. N (R) is used to predict the instruction from the receiving end.
Frame check sequence (FCS) is used for error code detection.
Flag is the beginning and the end token of a frame, namely, eight bits containing
six consecutive 1s.
2. UI frame operation
The L3 message is transmitted in the non-SN frame mode, and the receiver is
not required to send a confirmation upon reception of a UI frame. This operation
mode does not provide flow control or error recovering mechanism.
Fig 1.2-7 shows the frame structure of a UI frame. It consists of address field,
control field and information field.
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Address Control Information
000 P 0011TEISAPI
Fig 1.2-7 The Structure of UI Frame in the LapD
The address field contains SAPI and TEI. P in the control field is a query bit. When it is
set to 1, it means that a response frame from the peer entity is required.
Application layer
This layer implements the BTS Management (BTSM) and RR protocol
transmission that includes the radio link management function and operation and
management function.
1.2.1.3 Um Interface Protocols
Fig 1.2-8 shows the circuit service protocol stack on Um interface.
CM
MM
RR
LAPDm
Layer1 Layer1
LAPDm
RR
MSBTS
Um interface
Fig 1.2-8 Circuit Service Protocol Stack on Um Interface
Physical layer
This layer provides transmission channel for wireless links to transmit data
through the radio carrier. It also provides different functional channels for higher
layers including service and logic channels.
Data link layer
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This layer provides reliable data links between MS and BTS. It uses the LAPDm
protocol, a derivative of the LAPD and dedicated for GSM.
In GSM, LapDm is a data link protocol for signaling transmission between MS and
BTS, used to implement message transmission in the Dm channel to the L3 entities via
the radio interface. LapDm is based on LapD with some simplification and
modification.
LapDm implements the following functions:
1. Providing a point-to-point data link connection in a Dm channel and multiple
services for the upper layer. The data link connections are identified by the
DLCIs in the respective frames. The DLCI in the LapDm protocol only contains
SAPI, indicating the target service access point.
2. Supporting to identify diversified frame types.
3. Supporting the transparent transmission of L3 messages between L3 entities.
4. Sequence control, to maintain the sequence of respective frames connected via
data link.
5. Checking the format and operation errors in the data link layer.
6. Notifying the L3 entities to process the unrecoverable errors.
7. Flow control
8. Supporting access of the burst solution mode after the RACH channel access is
instantly assigned.
LapDm is implemented in the LapDm module of RSL.
Fig 1.2-9 shows the position of the LapDm module in RSL.
LapDm module
OAMM FURRM
Physical layer
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Fig 1.2-9 LapDm Module
The LapDm module communicates with the physical layer and L3. The L3 protocol is
processed in FURRM. OAMM configures the value of the timer necessary for LapDm
module running.
LapDm module provides two types of message transmission modes for FURRM: I-
frame multi-frame operation and UI frame operation In terms of frame structure,
LapDm eliminates the frame delimiter flag (FLAG) and the frame check sequence
(FCS). In LapDm, frame delimitation information is transmitted by means of
synchronization scheme of the radio interface without the beginning frame and end
frame flags. FCS is not available in the LapDm because the transmission scheme in the
physical layer of the Um interface has the error check function.
1. I-frame multi-frame operation
The L3 message is sent in the information frame mode which requires the
confirmation from the receiver. This mode provides a whole set of control
mechanism for error recovering and flow control, the establishment mechanism
and release mechanism for multi-frame operations.
The structure of I frame in LapDm is shown inFig 1.2-10.
Address Control Information
SAPI N(S) N(R)
Fig 1.2-10 The Structure of I Frame in LapDm
The I frame in LapDm is made up of the address field, control field and
information field.
The address field contains the service access point identifier (SAPI). On the
radio interface, LapDm supports two types of messages: Signaling and short
message service, distinguished by the SAPI. SAPI = 0 represents the signaling
link, and SAPI = 3 represents the short message link.
The maximum length of LapDm frames on the TCH is 23 bytes, and 21 bytes on
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2 Basic Signaling Procedure
the SACCH. The reason for this difference is that there are two special-purpose
bytes in each SACCH block: timing advance and transmit power control. Since
the maximum length of a frame on the radio interface is 21 or 23 bytes which
cannot meet the need of majority signaling, segmentation and regrouping are
defined in LapDm. An additional bit is introduced to distinguish the last frame
of a frame from other frames. Thanks to this mechanism, packet length on the
radio channel is not restricted. The only restriction is that these packets must
also be transmitted on other interfaces, namely, 260 bytes mentioned in the radio
interface specification.
In the control field, N (S) represents the sending serial number and the I frames
serial number currently sent by the sending end; N (R) represents the receiving
serial number, the expected sending serial number of the next I frame. N (R) is
used to predict the instruction from the receiving end.
2. UI frame operation
The L3 message is sent in the non-SN frame mode, and the receiver is not
required to send a confirmation upon reception of a UI frame. This operation
mode does not provide flow control or error recovering mechanism.
The structure of UI frame in LapDm is shown in Fig 1.2-11.
Address Control Information
000 P 0011SAPI
Fig 1.2-11 The Structure of UI Frame in LapDm
The UI frame in LapDm is made up of address field, control field and information
field. The address field contains the service access point identifier (SAPI). P in the
control field is a query bit. When it is set to 1, it means that a response frame from the
peer entity is required.
Application layer
This layer is responsible for control and management protocols and implements
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CM, Mobility Management (MM), and RR protocols. It arranges subscriber
information and system control process into designated logical channels
according to protocol packets.
CM layer implements communication management, establishes connections, and
maintain/release calls between users. This layer includes Call Control (CC),
Supplementary Service Management (SSM), and SMS.
MM layer performs mobility and security management, for example, the
necessary processing when MS initiates location update.
RR layer performs radio resource management to establish and release a
connection between MS and MSC during the call process.
1.2.2 Packet Service Protocols
Fig 1.2-12 shows the packet service protocol stack structure.
MS
Um Gb
BSS SGSN
BSSGP
LLC
SNDCP
NetworkService
L1bis
relay
BSSGP
L1bis
RLC
MAC
GSM RF
relay
RLC
MAC
GSM RF
SNDCP
IP/X.25
application
LLC
NetworkService
Fig 1.2-12 Packet Service Protocol Stack Structure
1.2.2.1 Um Interface Protocols
Fig 1.2-13 shows the packet service protocol stack on Um interface
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MS
Um interface
BSS
RLC
MAC
relay
RLC
MAC
LLC
SNDCP
IP/X.25
application
GSM RFGSM RF
Fig 1.2-13 Packet Service Protocol Stack on Um Interface
GSM RF
GSM RF specifies carrier features, channel structure, modulation mode, and
radio frequency indices. RF part uses the same transfer mode as the GSM circuit
services.
RLC/MAC layer
RLC is the air interface protocol between BTS and MS. Its main functions
include error detection in the Um interface data block, confirmation of the error,
and re-sending selection of the data block.
MAC controls access signaling process in the radio channel. In addition, it also
maps LLC frames to the GSM physical channels.
LLC layer
This layer is a reliable encrypted logical link. It is independent of lower layer
wireless interface protocols, which ensure minimum modification to the network
when another GPRS wireless solution is introduced.
SNDCP
Sub Network Dependent Convergence Protocol (SNDCP) is a transition between
the network and data link layers and segments. It compresses IP/X.25 user data
before sending it to the LLC layer for transmission.
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Relay
LLC PDU relay between Um and Gb interfaces.
1.2.2.2 Gb Interface Protocols
L1bis
Physical Transport layer
NS
This protocol includes two sub layers: Network Service Control (NSC) layer and
Frame Relay (FR) layer. Based on the FR, the NSC layer transmits the upper-
layer BSSGP PDU.
BSSGP
This protocol provides a connectionless link between BSS and SGSN for
unconfirmed data transmission on the transmission platform.
1.2.3 TCP/IP
Transmission Control Protocol/Internet Protocol (TCP/IP) is used for foreground and
background communication between BSC and OMCR and implements TCP/IP on the
MP. Following are the TCP/IP standard functions:
Link establishment
The background server initiates link establishment procedure to establish a TCP
link and the foreground MP receives it. MP receives port number of the latest
link establishment request as the port number of current link. Original port
number is not used for data switching by MP. Background is responsible for re-
establishing the link in case it is broken.
Data transmission
Data transmission is the radical objective of TCP/IP. FAM and BAM data
transmission is performed in TCP mode.
Connection termination
If MP finds any error in the message, it terminates the connection to enable the
opposite party to resume normal status. MP never terminates an active
connection.
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2 Basic Signaling Procedure
1.2.4 X.25 Protocol
PCOM board implements the X.25 protocol and is used for BSC and CBC
communication. X.25 protocol provides an interface between the subscribers device
and the packet switching network. Fig 1.2-14 shows the X.25 protocol model.
The three layers of X.25 have a one-to-one correspondence with the lowest three layers
of OSI model. Network layer in OSI is called Packet layer in X.25 but its functions are
the same.
X.25 provides a reliable basis for upper-layer communication protocols between BSC
and remote Data Communications Equipment (DCE). In the PCOM board, CBC
protocol is used as the upper-layer protocol and CBC messages are transferred in the
user data of an X.25 data packet.
Upper-layer
protocol
...
.
.
.
Upper-layer protocolwith far-end DTE
Packetlayer
Data linklayer
Physicallayer
Packetlayer
Data linklayer
Physicallayer
Physical connection
Interface
DTE DCE
X.25
Physical levelprotocol
Frame levelprotocol
Packet levelprotocol
Fig 1.2-14 X.25 Protocol Model
BSC sends messages to CBC through X.25. After the message is assembled in BSC, it
enters the message-sending queue in X.25. The X.25 transmission processor obtains
messages from the sending queue and assembles them into one or more X.25 data
packet formats for transmission.
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CBC sends messages to BSC through X.25. The X.25 receiving processor receives the
CBC message, transfers it to the X.25 message-receiving queue, and then sends the
message to MP.
Communication control implements the control functions of X.25 communication link
including link connection/disconnection, restoring after fault, resetting communication
protocol stack, abnormal communication protection, and communication link
active/standby switching.
1.2.5 HDLC Protocol
Internal communication in the BSC uses HDLC protocol to ensure reliable and
efficient information transmission.
Following are the four main functions of HDLC protocol:
Link establishment
Link establishment employs three-channel handshake mode when both
directions of the link are normal. This mode enables one party to originate
synchronous handshake so the other party can respond or both parties can
originate a synchronous handshake simultaneously.
Link selection
Any PP or T network can interact with MP via a pair of COMM boards for data.
MP decides the communication link of COMM board to be used.
Communication control process performs link selection, monitors the link status,
and selects a link for the link-established PP on time basis.
Link holdover
COMM board regularly sends a link holdover message to PP for timely fault
identification of links over which no messages are transmitted for a long time. If
the PP cannot receive the message, service identifier of the link is eliminated and
the link is re-established.
Data transmission
Data transmission is the fundamental aim of HDLC. Reliable transmission is
expected when the data moves from MP to PP or PP to MP.
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2.1 MS Location Update Procedure
MS sends a CH REQ (Channel Request) message through RACH to BTS. Upon
receiving CH REQ message, the BTS processes it and then sends it to BSC.
After receiving CH REQ message, BSC sends a CH ACT message to the BTS to
activate SDCCH. After activating the channel, BTS returns a CH ACT ACK message.
BSC sends IMM ASS CMD to BTS. Upon receiving the message, BTS sends IMM
ASS through AGCH to MS. When receiving message, MS sends SABM to BTS. BTS
sends UA to MS.
At the same time, BTS sends a channel establishment indication (EST IND) to BSC,
containing location update request of MS. BSC forwards the location update request
(LOC UPD REQ) to MSC in CR. As receiving message, MSC returns a CC message to
BSC.
SDCCH is established between MS and BTS, and location update message is sent to
MSC through SDCCH. MSC selects the encryption mode and sends a location update
acceptance message (LOC UPD ACCEPT) to MS.
Fig 2.1-15 shows MS location update procedure.
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RLC
RLSD
DT1: CIPH MODE CMD
RF CH REL ACK
RF CH REL
REL INDUA
DISCDEACT SACCH
DR: CH RELCH REL
DT1: Clear COM
DT1: Clear CMD
DT1: CIPH MODE COMDI: CIPH MODECOM
CIPH MODE COM
CIPH MODE CMDENCRY CMD
CC
CR: LOC UPD REQEST IND
UA
SABM
IMM ASSIMM ASS CMD
CH ACT ACK
CH ACT
CH RQDCH REQ
MS BTS BSC MSC
DTAP: LOC UPD ACCEPT
Fig 2.1-15 MS Location Update Procedure
MSC sends a Clear CMD message to BSC, which returns a Clear COM message to
MSC. Meanwhile, BSC sends to BTS a CH REL message to release SDCCH and a
DEACT SACCH message to deactivate SACCH.
BTS sends a CH REL message to MS. MS requests BTS to release radio link (DISC).
BTS returns UA and reports the channel release indication to the BSC.
BSC sends a RF CHL REL message to BTS. BTS returns a RF CHL REL ACK
message. Radio channel is released.
2.2 IMSI Detach Procedure
After the SDCCH is established, an IMSI DETACH message is sent through SDCCH
to MSC. After receiving the message, MSC releases SDCCH.
Fig 2.2-16 shows the IMSI detach procedure.
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2 Basic Signaling Procedure
RF CH REL ACK
RF CH REL
REL INDUA
DISCDEACT SACCH
DRCH RELCH REL
CREF
CR IMSI DETACHEST IND
UA
SABM
IMM ASSIMM ASS CMD
CH ACT ACK
CH ACT
CH RQDCH REQ
MS BTS BSC MSC
Fig 2.2-16 IMSI Detach Procedure
2.3 Mobile-Originated Call and Called Party On-hook Procedure
When a mobile subscriber originates a call, the SDCCH is established first, and then a
request for TCH is sent through SDCCH to the MSC.
MSC sends an Assignment Request message to BSC. After receiving the message, BSC
sends an IMM ASS CMD message to MS which establishes a TCH with the BTS.
The BTS then sends a channel establishment indication, completes immediate
assignment, and releases SDCCH.
MSC sends a ring-back tone to MS over the established TCH. After the Connect and
Connect ACK messages are exchanged, the call is set up.
When the called party hangs up, MSC sends a Disconnect message to MS which
releases TCH and MSC replies with a Release Complete message and releases TCH.
Fig 2.3-17 shows the mobile-originated call and the called party on-hook procedure.
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RF CH REL ACK
RF CH REL
RLC
RLSD
CH REL
DISC
UARF CH REL
RF CH REL ACK
REL IND
DEACT SACCHDR CH REL
EST IND
ASS COMDT1 ASS COM
DT1:ASS REQ
DT1 CIPH MODE CMD
CH ACT ACK
CH ACT
PHY CONT CONF
UA
SABM
PHY CONT REQ
DR ASS CMDASS
CMD
DT1 Clear COM
DT1 Clear CMD
DT1 CIPH MODE COMDI CIPH MODE COM
CIPH MODE COM
CIPH MODE CMDENCRY CMD
CC
CR CM SERV REQEST IND
UA
SABM
IMM ASSIMM ASS CMD
CH ACT ACK
CH ACT
CH RQDCH REQ
MS BTS BSC MSC
DTAP:SETUP
DTAP:CALL PROC
DI ASS COM
DTAP A lerting
DTAP Connect
DTAP Connect ACK
Data flow
DTAP D isconnect
DTAP Release
DTAP Release COM
DTAP: CM SERV AC CP
Fig 2.3-17 Mobile-Originated Call and Called Party On-hook Procedure
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2 Basic Signaling Procedure
2.4 Mobile-Terminated Call and Calling Party On-hook Procedure
When MS is called, MSC sends the paging message to MS. After receiving paging
message, MS establish a SDCCH. Then, MSC establishes a TCH and releases SDCCH.
TCH is used to complete the call connection.
After the conversation is over, TCH is released. shows mobile-terminated call and
calling party on-hook procedure.
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GB_BT05_E1_0 GSM Signaling System Introduction
UDT PAGPAG CMDPAG REQ
RF CH REL ACK
RF CH REL
RLC
RLSD
CH REL
DISC
UARF CH REL
RF CH REL ACK
REL IND
DEACT SACCH
DR CH REL
EST IND
ASS COM
DT1 ASS COM
DT1:ASS REQ
DT1 CIPH MODE CMD
CH ACT ACK
CH ACT
PHY CONT CONF
UA
SABM
PHY CONT REQ
DR ASS CMDASS CMD
DT1 Clear COM
DT1 C lear CMD
DT1 CIPH MODE COM
DI CIPH MODE C OMCIPH MODE COM
CIPH MODE CMDENCRY CMD
CC
CR PAG RESEST IND
UA
SABM
IMM ASSIMM ASS CMD
CH ACT ACK
CH ACT
CH RQDCH REQ
DTAP:SETUP
DTAP:CALL CONF
DI AS S COM
DTAP Alertin
gDTAP Connect
DTAP Connect ACK
Data flow
DTAP Disconnec
t
DTAP
Releas
e
DTAP Release COM
BSC MSCBTSMS
Fig 2.4-18 Mobile-Terminated Call and Calling Party On-Hook Procedure
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2 Basic Signaling Procedure
2.5 Intra-cell Handover Procedure
Based on the measurement reports submitted by MS continuously, BSC judges whether
to perform handover.
If intra-cell handover is expected, BSC activates another TCH in the same cell and
assigns that TCH to MS immediately.
After MS completes the immediate assignment, BSC notifies MSC of intra-cell
handover occurred to MS and releases the original TCH.
Fig 2.5-19 shows the intra-cell handover procedure.
RF CH REL ACK
RF CH RELDI : ASS COM
ASS COM
EST IND
DR : ASS CMD
UA
DT1 : HO PERF
CH ACT ACK
ASS CMD
SABM
CH ACT
PHY CONT CON
PHY CONT REQ
MEAS RESMEAS REP
MS BTS BSC MSC
Fig 2.5-19 Intra-Cell Handover Procedure
2.6 Inter-cell Handover Procedure
Based on the measurement reports sent by MS continuously, BSC judges whether to
perform handover.
If an inter-cell handover is necessary, BSC activates a TCH in the target BTS and sends
a HO CMD message to MS. MS sets up a connection with TCH of the target BTS and
performs the handover.
BSC informs MSC of inter-cell handover occurred to MS and releases TCH in original
cell.
Fig 2.6-20 shows inter-cell handover procedure.
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GB_BT05_E1_0 GSM Signaling System Introduction
DT1HO PERF
HO CMD
CH ACT
MEAS REP
RF CH REL ACK
RF CH REL
DIHO COM
EST IND
HO DET
CH ACT ACK
MS BTS1 BTS2 BSC MSC
MEAS RES
DRHO CMD
HO ACCESS
PHY INFO
SABM
UA
HO COM
Fig 2.6-20 Inter-cell Handover Procedure
When an MS is in dedicated mode, it is assigned with an SACCH besides a TCH.
SACCH transmits measurement report, power control, timing advance control, and link
monitoring information under mobile environment.
Fig 2.6-21 and Fig 2.6-22 show the measurement report and transmit power control
procedures respectively.
MS BTS BSC
MEAS REP
MEAS REP
Fig 2.6-21 Measurement Report Procedure
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Power ControlSignaling
Procedure
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2 Basic Signaling Procedure
MS BTS BSC
MS POWER CTRL
BS POWER CTRL
MS POWER CTRL
Fig 2.6-22 Transmit Power Control Procedure
MS reports the measurement data through SACCH. BSC makes the power control
decision and sends the related control commands to BTS. BTS executes power control
commands or forwards the commands to MS.
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