01-1 Signaling Fundamentals.pdf

M900/M1800 Base Station Subsystem Signaling Analysis Manual Contents

Issue 06 (2006-08-20) Huawei Technologies Proprietary i

Contents

1 Signaling Fundamentals...........................................................................................................1-1 1.1 Interface Overview........................................................................................................................................1-2 1.2 A Interface .....................................................................................................................................................1-3

1.2.1 Overview..............................................................................................................................................1-3 1.2.2 Protocols on the A-Interface ................................................................................................................1-3

1.3 Abis interface ..............................................................................................................................................1-10 1.3.1 Overview............................................................................................................................................1-10 1.3.2 Protocols on the Abis Interface ..........................................................................................................1-13

1.4 Um Interface................................................................................................................................................1-19 1.4.1 Overview............................................................................................................................................1-19 1.4.2 Layer 1 - Physical Layer ....................................................................................................................1-20 1.4.3 Layer 2 - Data Link Layer..................................................................................................................1-20 1.4.4 L3.......................................................................................................................................................1-22

Figures M900/M1800 Base Station Subsystem

Signaling Analysis Manual

ii Huawei Technologies Proprietary Issue 06 (2006-08-20)

Figures

Figure 1-1 GSM protocol stack ..........................................................................................................................1-2

Figure 1-2 A-interface signaling protocol reference model................................................................................1-3

Figure 1-3 Signaling message processing procedure..........................................................................................1-5

Figure 1-4 Protocol model of Abis interface ....................................................................................................1-10

Figure 1-5 Structure of Abis interface ..............................................................................................................1-11

Figure 1-6 Abis interface layer 2 logical links..................................................................................................1-12

Figure 1-7 Basic structure of management object ............................................................................................1-17

Figure 1-8 Layered structure of Um interface ..................................................................................................1-19

Figure 1-9 Physical layer interface...................................................................................................................1-20

Figure 1-10 Um interface L3 protocol model...................................................................................................1-24

Figure 1-11 Communication at RR...................................................................................................................1-25

Figure 1-12 Communication at MM.................................................................................................................1-26

M900/M1800 Base Station Subsystem Signaling Analysis Manual Tables

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Tables

Table 1-1 Major functions of BSSAP.................................................................................................................1-8

Table 1-2 Management status ...........................................................................................................................1-17

Table 1-3 Operation status ................................................................................................................................1-17

Table 1-4 Availability status .............................................................................................................................1-18

M900/M1800 Base Station Subsystem Signaling Analysis Manual 1 Signaling Fundamentals

Issue 06 (2006-08-20) Huawei Technologies Proprietary 1-1

1 Signaling Fundamentals About This Chapter

The following table lists the contents of this chapter.

Title Description

1.1 Interface Overview Introduces three external interfaces of the BSS.

1.2 A Interface Describes the functions of the A interface layers and the signaling procedures on the A interface.

1.3 Abis interface Describes the functions of the Abis interface layers and the signaling procedures on the Abis interface.

1.4 Um Interface Describes the functions of the Um interface layers and the signaling procedures on the Um interface.

1 Signaling Fundamentals M900/M1800 Base Station Subsystem


1-2 Huawei Technologies Proprietary Issue 06 (2006-08-20)

1.1 Interface Overview M900/M1800 BSS offers standard external interfaces: Um interface between an MS and the BSS, A interface between the BSS and the MSC. The interface protocols and interface procedures strictly follow the ETSI specifications.

The interfaces between each BTS and BSC and those between the BSS and the OMC are internal interfaces, and are related to specific equipment from different manufacturers. There are many regulations drafted by ETSI for the Abis interface between BTS and BSC, but the regulations are still incomplete.

Figure 1-1 shows the GSM protocol stack. The following is an overall introduction to each interface.

Figure 1-1 GSM protocol stack

MS: Mobile Station CM: Connection Management BSC: Base Station Controller MM: Mobility Management BTS: Base Transceiver Station MSC: Mobile services Switching Centre, Mobile Switching

Centre MTP: Message Transfer Part (MTP) BTSM: Base Transceiver Station Site Management RR: Radio Resource Management BSSMAP: Base Station Subsystem Management Application Part SCCP: Signaling Connection Control Part

LAPD: Link Access Procedure on the D channel

LAPDm: Link Access Procedure on the Dm channel

A interface A interface is a standard interface between the BSS and the MSC. The information transferred on this interface includes MS management, BTS management, mobility management, connection management, etc.

Abis interface Abis interface defines the standard of communications between the BSC and the BTS in BSS. It is used in remote interconnection mode. This interface supports all MS-oriented services, and supports the control of BTS radio equipment and the allocation of radio frequencies.



Um interface Um interface is defined as the communication interface between MS and BTS, and is used for the interworking between MS and the fixed part of the GSM system. The physical link is a radio link. The information transferred on this interface includes the information of radio resource management, mobility management, connection management, etc.

1.2 A Interface 1.2.1 Overview

The A interface lies between the BSC and the MSC. It is a standard interface in GSM specifications, as it may involve the interworking between the equipment from different manufactures. In the GSM system, SS7 is adopted on the A interface.

Physically, the A interface is the trunk circuit and trunk interface between the BSC and the MSC. See Figure 1-2 for the A interface signaling protocol reference model.

Figure 1-2 A-interface signaling protocol reference model

DTAP: Direct Transfer Application Part MTP: Message Transfer Part (MTP) SCCP: Signaling Connection Control Part BSSAP: Base Station Subsystem Application Part BSSMAP: Base Station Subsystem Management Application Part

1.2.2 Protocols on the A-Interface

Physical layer The physical layer of the A-interface is 120-ohm symmetrical twisted pair or 75-ohm coaxial cable whose rate is 2 Mbit/s. The physical layer of A-interface has the following features:

z The 2 Mbit/s transfer rate complies with G.703. z Frame structure, synchronization and timing comply with G.705. z Fault management complies with G.732. z CRC4 complies with G.704.




Message Transfer Part (MTP) The main function of MTP is to ensure reliable signaling message transfer in the signaling network. In case of system and signaling network faults, it takes measures to avoid or reduce the loss of messages, repeated messages and out-of-sequence packets.

MTP protocols are defined in ITU-T Q.701~710 Recommendations.

MTP comprises three functional levels: signaling data link function, signaling link function and signaling network function.

Step 1 Signaling data link function

Signaling data link (layer 1) is the channel used for signaling transmission. It comprises two data channels of the same data rate but two opposite working directions. The data rate is 64kbit/s. Generally, the signaling data link occupies timeslot 16 of a trunk cable. The specific timeslot is to be determined by negotiation between BSC and MSC. By data configuration, the timeslot can be used to establish a semi-permanent connection.

The signaling data link is the information bearer of SS7. One of its important features is that the signaling link is transparent, i.e. the data transferred on it cannot be changed. Therefore, equipment such as echo canceller, digital attenuator, A/u rate converter, cannot be connected to this link.

Step 2 Signaling link function

Signaling link function (layer 2) regulates the functions and procedures to send the signaling to the data link, and together with layer 1, it implements reliable signaling message transfer between two directly-connected signaling points. Due to long-distance transmission, a certain rate of bit errors may be caused on the data link between two adjacent signaling points. However, no error is allowed in CCS7 signaling message codes. The purpose of layer 2 is to guarantee error-free transmission of message codes in the case that there exist bit errors on layer 1. Functions of layer 2 include: signaling unit delimitation, signaling unit alignment, error detection, error correction, initial alignment, processor fault, level-2 flow control, and signaling link error rate monitoring.

Step 3 Signaling network function

By controlling the route and performance of the signaling network, signaling network function (level 3) guarantees that signaling information can be reliably transferred to the user part, whether the signaling network is in normal state or not.. Signaling network functions include signaling message processing and signaling network management.

Signaling message processing

Signaling message processing function sends signaling messages to the corresponding signaling links or user parts. The user part in BSS only contains SCCP. Signaling message processing functions comprise three parts: message routing (MRT), message discrimination (MDC) and message distribution (MDT), as shown in Figure 1-3.



Figure 1-3 Signaling message processing procedure

z Message Routing MRT (Message Routing) function is used at each signaling point to determine the signaling link group and the signaling link to destination signaling point. The MRT part implements the selection of message routes. In other words, by using the information (DPC and SLS) contained in the route mark, it selects a signaling link for signaling messages, so that the messages can be transferred to the DPC.

z Message Discrimination (MDC) Message Discrimination (MDC) part is designed to receive the messages from Layer 2 to ascertain whether the destination of the messages is the local signaling point. If the destination is the local signaling point, the MDC part will send the messages to the Message Distribution (MDT) part. If the destination is not the local signaling point, the MDC part will send the messages to the Message Routing (MRT) part.

z Message Distribution (MDT) Message Distribution (MDT) part is designed to allocate the messages from the MDC part to the user part and the signaling network management and test & maintenance part accordingly.

Signaling network management

Signaling network management is to re-construct the signaling network and to keep and recover the normal transfer ability of the signaling unit when the signaling network fails. Signaling network management includes three parts: signaling traffic management, signaling link management and signaling route management.

z Signaling Traffic Management (STM) Signaling Traffic Management (STM) is to transfer the signaling data from one link/route to another or multiple available links/routes when the signaling network fails. It is also used to temporarily reduce signaling traffic in case of congestion at the signaling point.

z Signaling link management Signaling link management (SLM) is to recover or enable the signaling link in the signaling network or to disconnect the signaling link. It ensures the provision of certain




pre-determined link groups. The connection between the signaling data link and the signaling terminal is normally established by the man-machine commands. Operations in the signaling system can not automatically change the above connection relationship.

z Signaling route management Signaling route management (SRM) is used to ensure the reliable exchange of signaling route availability information between signaling points so as to block or unblock signaling routes when necessary. It mainly comprises such procedures as transfer prohibited, transfer allowed, controlled transfer and restricted transfer, signaling route group test, and signaling route group congestion test.

----End

Signaling Connection & Control Part (SCCP) The purpose of SCCP is to provide complete network layer functions with the help of MTP. Network layer provides connectionless services and connection-oriented services.

The network layer services provided by SCCP can be classified into connectionless services and connection-oriented services. The connectionless service means that MS does not establish a signaling connection in advance, and uses the routing functions of SCCP and MTP to directly transfer data information in the signaling network. It is applicable to the transfer of a small quantity of data.

The connection-oriented service means that a signaling connection is established in advance, and data are directly transferred on the signaling link, instead of using the route selection function of SCCP. It is applicable to the transfer of large quantities of data, and effectively shortens the transmission delay of batch data.

SCCP has routing and network management functions. The routing function of SCCP is to perform addressing as per the address information such as DPC, SSN, GT, etc. DPC refers to the destination signaling point code adopted by MTP, and SSN refers to the subsystem No., which is used to identify the different users (such as ISUP, MAP, TCAP and BSSAP) of SCCP in the same node, so as to compensate the insufficiency of users of MTP and to enlarge the addressing scope. GT addressing mode is not introduced as BSS does not adopt this addressing mode.

The network management function of SCCP is to implement management of signaling point state and subsystem state, switchover of active/standby subsystem, broadcasting of status messages and testing of subsystem state. SCMG (SCCP management) is to maintain the network functions by reselecting a route or adjusting the traffic volume when network fault or congestion occurs. MTP protocols are defined in ITUT Q.711~716 Recommendations

BSSAP Step 1 Protocol overview

The BSSAP protocol, which serves as A-interface specification, describes two kinds of messages, BSSMAP and DTAP message. BSSMAP messages are used for traffic flow control, and are to be processed by the internal functional module of the A interface. For DTAP messages, the A interface is merely equivalent to a transport channel, On BSS side, DTAP messages are directly transferred to radio channels. On MSC subsystem side, DTAP messages are transferred to the corresponding functional processing unit.

BSSAP protocols are defined in ETSI GSM 08.08 and ETSI GSM 04.08 specifications.

Step 2 Typical message contents



DTAP messages

The DTAP messages can be divided into Mobile Management (MM) messages and Call Control (CC) messages.

The MM messages consist of messages related to authentication, CM service request, identification request, IMSI detach, location update, MM state, TMSI re-allocation, etc.

The call control messages consist of alerting, call proceeding, connection, setup, modification, release, disconnection, notification, state query, DTMF startup messages, etc.

BSSMAP messages

The BSSMAP messages can be divided into connectionless and connection-oriented messages.

z The connectionless messages consist of Block/Unblock, Handover, Resource, Reset, Paging messages, etc. The Block/Unblock messages include Block & Block ACK messages and Unblock & Unblock ACK messages. The Circuit Group Block/Unblock messages include Circuit Group Block message, Circuit Group Block ACK message, Circuit Group Unblock and Circuit Group Unblock messages. Handover messages include Handover Candidate Enquire and Handover Candidate Enquire Response. The resources messages include Resources Request and Resource Indication messages. The Reset messages include Reset and Reset ACK messages.

z The connection-oriented messages include Assignment, Handover, Clear and Cipher messages. The Assignment messages include Assignment Request, Assignment Complete and Assignment Error messages. The Handover messages include Handover Request, Handover Request ACK, Handover Command, Handover Complete and Handover Error messages. The Clear messages include Clear Request and Clear Complete messages. The Cipher messages include Cipher Mode Command and Cipher Mode Complete messages.

BSSAP protocol functionality

The BSSAP protocol can deliver its own functions in connection-oriented mode and connectionless mode of SCCP. When MS needs to exchange service-related messages over radio resources with the network side while there is no MS-related SCCP connection between MSC and BSS, a new connection will be established. A new connection shall also be set up for external handover. There are two kinds of connection setup:

z While MS sends the Access Request message on the RACH, BSS allocates a dedicated radio resource (DCCH or TCH) to MS. After the L2 connection is set up on the SDCCH (or FACCH) where resources are allocated, BSS starts the connection setup.

z When MSC decides to execute an external handover (the target BSS might be the original BSS), it must reserve a new DCCH or TCH from the target BSS. In this scenario, MSC starts the connection setup.




The BSSAP protocol implements the functional flow as shown in Table 1-1 using the connection and connectionless messages.

Table 1-1 Major functions of BSSAP

Serial number

Function Description

1 Assign Assign is to ensure the dedicated radio resources are allocated or re-allocated properly to the MS. The initial MS random access and immediate assignment to a DCCH is processed automatically by BSS but not controlled by MSC.

2 Block/Unblock

During circuit assignment, MSC selects an available terrestrial channel. If this channel is no longer available then BSS notifies it to MSC. The Block/Unblock procedure can carry out this function.

3 Resource Indication

Resource indication serves to notify MSC: Amount of the radio resource available for TCH in BSS, Amount of all available radio resource (i.e. able to provide service or have been specified) It is not easy to get this information from the MSC-controlled services. These must be considered when MSC decides an external handover.

4 Reset Reset is to initialize the BSS or MSC. For instance, if BSS goes faulty or loses all the reference messages about processing, BSS sends a Reset message to MSC, which releases the affected calls, deletes the affected reference messages and sets all the circuits related to the BSS to idle. If MSC or BSS is only locally faulty, the affected parts can be cleared using the Clear procedure.

5 Handover Request

BSS may send a handover request to MSC requesting to perform handover of the MS, to which dedicated resources have been allocated, for the reasons as listed below: a) BSS detects a radio cause for handover. b) MSC starts the Handover Candidate Enquirer procedure. The MS is waiting for the handover. Due to congestion, the serving cell needs to be changed during the call setup such as directed retry. The Handover Request messages should be re-sent once in a while till one of the following situations occurs: Receive the "Handover Command" message from MSC; Reset message is received; All communications with MS will be interrupted and the processing is aborted; Processing is over, such as call clearing.



Serial number


6 Handover Resource Allocation

Handover Resources Allocation enables MSC to request for resources from BSS based on the handover request. The target BSS will reserve resources and wait for an MS to access this channel.

7 Handover Procedure

This is the procedure in which MSC instructs MS to access the radio resources of another cell. When handover is carried out, the original dedicated radio resources and terrestrial resources are maintained all the time until MSC sends a Clear Command message or Reset occurs.

8 Release of Radio Resources and Terrestrial Resources

When processing is done, MSC sends a Clear Command to BSS to release radio resources. On receiving the command, BSS starts the Clear procedure at the radio interface, then sets the configured terrestrial circuit to idle and returns a Clearing Complete message to MSC, which in turn releases the terrestrial resources of the local end. If resources need to be released by BSS, BSS will send a Clear Request to notify MSC to start the release procedure to release the terrestrial and radio resources concerning MSC and BSS.

9 Paging The paging to MS is transported with the SCCP connectionless service via BSSMAP. If BSS receives the Paging Response message at the radio channel interface, it will establish an SCCP connection to MSC. The paging response message, which is loaded in the BSSMAP Full L3 Message, is transported on the signaling connection to MSC.

10 Flow Control Flow control can prevent the entities from receiving too much traffic. Flow control on the A-interface is implemented by controlling the traffic at the traffic source. Two levels of flow control are available. Flow control can be implemented based on subscriber classes.

11 Classmark Update

Classmark Update serves to notify the class messages received from MS to the receiving entities. Generally, BSS notifies MSC after receiving the class messages from MS. It is also likely that when handover is complete, MSC sends the corresponding MS Classmark messages to the new BSS via the A-interface.

12 Cipher Mode Control

The Cipher Mode Control procedure allows MSC to transport the cipher mode control messages to BSS and start the subscriber equipment and signaling cipher equipment with a correct Kc.

13 Queuing Indication

This procedure is designed to notify MSC that BSS wants to delay the allocation of necessary radio resources. This procedure is valid only when the queuing function is introduced for traffic channel assignment and traffic channel handover in the BSS.




Serial number


14 Load Indication

Load indication serves to notify the traffic state of a cell to all the adjacent BSSs so that an overall control over the handover services in an MSC can be exercised. In a certain valid period, the traffic state of the adjacent cells will be taken into account by the adjacent BSS during handover.

1.3 Abis interface 1.3.1 Overview

The Abis interface is the interface between Base Station Controller (BSC) and Base Transceiver Station (BTS), and complies with the requirements of 08.5X series of the GSM specifications. It is merely an internal interface of BSS. The interworking between the BSC and BTS equipment of different manufactures has not been realized.

The terrestrial traffic channels on the Abis interface and the radio traffic channels on the Um interface are in one-to-one correspondence with one another.

Protocol Model The protocol model of the Abis interface is shown in Figure 1-4.

Figure 1-4 Protocol model of Abis interface

BTSM: Base Transceiver Station Management LAPD: Link Access Procedure on the D ChannelLAPDm: Link Access Procedure on the Dm Channel SCCP: Signaling Connection Control Part MTP: Message Transfer Part BSSAP: Base Station Subsystem Application Part RR: Radio Resource

z Layer 1 of the Abis interface is a physical link which receives data from and transmits data to the transport layer based on the bottom layer driver of the hardware.



z The layer 2 protocol of the Abis interface is based on the LAPD. LAPD addresses TRX (or BCF) through TEI, and uses different logical links for message transfer. RSL is to transfer traffic management messages. OML is to transfer network management messages. L2ML is to transfer L2 management messages.

z RR (Radio Resource Management) messages are mapped onto the BSSAP (BSS Application Part) in BSC. In BTS, most of RR messages are handled as transparent messages. However, some of them have to be interpreted and executed by BTS (for example, cipher, random access, paging and assignment), these messages are processed by the BTSM (BTS Management) entities in BSC and BTS.

z BSC and BTS do not interpret CM (Connection Management) and MM (Mobility Management) messages. These messages are transferred over the A-interface by DTAP (Direct Transfer Application Part). At the Abis interface, DTAP messages are transferred as transparent messages.

Structure of Abis interface The Abis interface can support three different internal BTS configurations (as illustrated in Figure 1-5.

z Single TRX. z Multiple TRXs are connected with the BSC via a common physical connection. z Multiple TRXs are connected with the BSC via different physical connections.

Figure 1-5 Structure of Abis interface

As shown in Figure 1-5,




z TRX (Transceiver) is the functional entity that supports 8 physical channels that belong to the same TDMA frame, which is defined in the PLMN.

z The BCF (Base Control Function) is the functional entity that performs common control functions including BTS initialization, software loading, channel configuration, operation and maintenance.

There are two types of channels at the Abis interface, which are:

z Traffic channels with the rates of 8kbit/s, 16kbit/s and 64kbit/s respectively, carrying speech or data from radio channels.

z Signaling channels with rates of 16kbit/s, 32kbit/s or 64kbit/s respectively, carrying signaling between BSC and MS, and between BSC and BTS.

Different Terminal Equipment Identifiers (TEI) are assigned to get unique addresses of TRXs. Three separate logical links are defined with each TEI (as shown in Figure 1-6).

z RSL: Radio Signaling Link used to support traffic management procedures, one for each TRX.

z OML: Operation & Maintenance Link used to support network management procedures, one for each SITE.

z L2ML, L2 management link, for transferring the management messages at L2.

Figure 1-6 Abis interface layer 2 logical links



1.3.2 Protocols on the Abis Interface

Physical layer Abis interface physical layer adopts the PCM link with the working rate at 2048 kbit/s to provide 32 channels at 64kbit/s. The electro-technical parameter at the physical layer conforms to the CCITT G.703 recommendations.

BSS is the connection point of the radio channel and terrestrial channel. Both kinds of channels have different transfer patterns and coding rates. In the radio channel of BSS, the transfer rate is 16kbit/s while it is 64kbit/s in the terrestrial channel. Therefore transcoding and rate adaptation is needed. To save cost more efficiently, different multiplexing ratio modes are adopted on Abis interface, e.g. 10:1/12:1/15:1 multiplexing mode.

Data link layer Step 1 Overview

The data link layer of Abis uses LAPD protocol. It utilizes the service on the physical layer, and provides connection-oriented or connectionless services for layer 3. The data link Service Access Point (SAP) is the point that provides services for layer 3. SAP is identified by Service Access Point Identifier (SAPI). A data link connection endpoint is identified by a data link connection endpoint identifier as seen from layer 3 and by a data link connection identifier (DLCI) as seen from the data link layer.

For information exchange between two or more layer 3 entities, an association must be established between the layer 3 entities in the data link layer using a data link layer protocol.

The communication between data link layer entities is governed by a peer-to-peer protocol specific to the layer. Messages at the data link layer are transferred between entities at layer 2 through physical layer. Inter-layer service request is implemented with service primitive.

Step 2 Function

The purpose of LAPD is to realize reliable end-to-end information transfer between layer 3 entities through the user-network interface by using the D-channel. To be specific, LAPD supports:

z Multiple terminal equipment between subscriber and interface . z Multiple L3 entities.

Functions of LAPD include:

z Establishes one or several data links on the D channel. z Delimits, locates and transmits transparently frames so that a string of bits transmitted on

the D channel in the form of frames can be identified. z Implements sequence control to keep the order of the frames that pass the data link

connections. z Checks the transmission errors, format errors and operation errors in the data link

connections. z Makes recovery based on the detected transmission errors, format errors and operation

errors. z Notifies the management layer entities of the unrecoverable errors. z Flow control.




Data link layer provides the means for information transfer between multiple combinations of data link connection points. The information may be transferred through point-to-point data link connections or via broadcast data link connections.

----End

Traffic management of Layer 3 The traffic management part of the Abis interface layer 3 is mainly described in GSM 08.58 specifications. The procedures defined in these specifications have two major functions:

z Realizing the interworking of the MS and BSS/NSS on the Um interface. z Implementing part of the radio resource management functions under the control of

BSC.

The traffic management message is divided into the transparent and non-transparent messages,

z The transparent message refers to the messages forwarded without interpretation or being processed by the BTS.

z The non-transparent message refers to the messages processed and structured by the BTS.

The traffic management messages can also be divided into four groups in terms of functions, which are:

z Radio link layer management message, used for the management of the data link layer on the radio channel.

z Dedicated channel management message used for the management of dedicated channels (SDCCH and TCH).

z Common control channel management message used for the management of common control channels.

z TRX management message used for TRX management.

Transparency and group of the message is determined by the message discriminator at the header of the message.

Step 1 Radio link layer management procedures

Radio link layer management procedures include:

z Link establishment indication procedure: BTS uses this procedure to indicate to BSC the success of setting up multi-frame link originated by the subscriber. BSC establishes a link from MSC to SCCP through the indication.

z Link establishment request procedure: This procedure is used by BSC to request the establishment of a link layer connection in multi-frame mode on the radio channel.

z Link release indication procedure: This procedure is used by BTS to indicate to BSC that a link layer connection on the radio channel has been released at the initiative of an MS.

z Link release request procedure: This procedure is used by BSC to request the release of a link layer connection on the radio channel.

z Transmission of a transparent L3-message on the Um interface in acknowledged mode: This procedure is used by BSC to request the sending of a transparent L3 message to MS on the Um interface in acknowledged mode.



z Reception of a transparent L3-message on the Um interface in acknowledged mode: This procedure is used by BTS to indicate the reception of a transparent L3 message on the Um interface in acknowledged mode.

z Transmission of a transparent L3-message on the Um interface in unacknowledged mode: This procedure is used by BSC to request the sending of a transparent L3 message to MS on the Um interface in unacknowledged mode.

z Reception of a transparent L3-message on the Um interface in unacknowledged mode: This procedure is used by BTS to indicate the reception of a transparent L3 message in unacknowledged mode.

z Link error indication procedure: Through this procedure BTS indicates BSC incase of any abnormality in the radio link layer.

Step 2 Dedicated channel management procedures

The dedicated channel management principles include:

z Channel activation procedure: This procedure is used to activate a channel at BTS for an MS which later will be commanded to this channel by an Immediate Assignment, an Assignment Command, an Additional Assignment or a Handover Command message.

z Channel mode modification procedure: This procedure is used by BSC to request a change of the channel mode of an active channel.

z Handover detection procedure: This procedure is used between the target BTS and BSC to detect the accessing of the MS being handed over.

z Start of encryption procedure: This procedure is used to start encryption according to the procedure defined in Technical Specification GSM 04.08.

z Measurement report procedure: It includes the necessary basic measurement report procedure and measurement report preprocessing procedure. BTS reports all parameters related to handover decision to the BSC through this procedure.

z Deactivate SACCH procedure: This procedure is used by BSC to deactivate the SACCH at BTS according to the Channel Release procedure defined in Technical Specification GSM 04.08.

z Radio channel release procedure: This procedure is used by BSC to release a radio channel that is no longer needed.

z MS power control procedure: This procedure is used by BSS to set the MS power level or the parameters required by TRX. MS power control decision must be implemented in BSC, and as an optional procedure in BTS.

z BTS Transmission power control procedure: This procedure used between BSC and BTS to set the TRX transmission power level or the parameters required by TRX. The BTS transmission power control decision should be implemented in BSC, or in BTS.

z Connection failure procedure: This procedure is used by BTS to indicate to BSC that an active connection has been broken.

z Physical context request procedure: This is an optional procedure which allows the BSC to obtain information on the "physical context" of a radio channel just prior to a channel change.

z SACCH information modification procedure: BSC uses this procedure to instruct BTS to change the information (system information) filled in a specific SACCH channel.

Step 3 Common channel management procedures

Common channel management regulations include:

z Channel request by MS procedure: The procedure is initiated by TRX upon detection of a random access from an MS (Channel Request message from MS).




z Paging principle procedure: It is used to page an MS on the specified paging sub-channel. The paging of an MS is initiated by BSC sending a Paging Command message to BTS. BSC determines the paging group to be used according to the IMSI of the called MS. The value of this paging group together with the identity of the mobile station is sent to BTS.

z Immediate assignment procedure: When a mobile station accesses BTS, BSC uses this procedure to assign a dedicated channel for the mobile station immediately.

z Delete indication procedure: This procedure is used by BTS to indicate that due to overload on the AGCH, an Immediate Assign Command has been deleted.

z CCCH load indication procedure: This procedure is used by BTS to inform BSC the load on a designate CCCH. Indication period is also set by OM.

z Broadcast information modification procedure: This procedure is used by BSC to indicate to BTS the new information to be broadcast on BCCH.

z Short message cell broadcast procedure: Short Message Service Cell Broadcast messages are sent to BTS as SMS Broadcast Request messages.

Step 4 TRX management procedures

This type of procedure is used for TRX management. There are:

z SACCH filling information modify procedure: This procedure is used by BSC to indicate to BTS the new information to be used as filling information on SACCHs.

z Radio resources indication procedure: This procedure is used to inform BSC on the interference levels on idle channels of a TRX.

z Flow control procedure: This procedure is defined to give some degree of flow control. It can be used for TRX processor overload, downlink CCCH overload and AGCH overload.

z Error reporting procedure: This procedure is used by BTS to report detected downlink message errors, which cannot be reported by any other procedure.

----End

Operation and maintenance part of Layer 3 Step 1 Operation and maintenance information model

z Managed objects



There are four types of management objects: site, cell, carrier and channel. The basic structure is illustrated in Figure 1-7.

Figure 1-7 Basic structure of management object

z Object addressing Addressing of network management messages is realized by means of managed object types and cases. For each object case in BTS there is a complete L2 connection description. The setup of the first connection uses one (semi-) permanent default TEI. Subsequent connections use the TEIs provided when setting up TEI procedures. Object cases can also use layer 3 addresses. The mixed use of layer 2 and layer 3 addressing enables one BTS site to have one or multiple physical links.

z Managed object state Management status includes management status, operation status and availability status. See Table 1-2, Table 1-3 and Table 1-4. The management state of managed objects is only controlled by BSC, and available state is the specific explanation of operative state.

Table 1-2 Management status

State Description

Locked BSC has disconnected all calls through this managed object, and no new calls can be connected to this object.

Shut down New services can not be connected to this managed object, but those existing calls will be maintained.

Unlocked New calls can be connected to this managed object.

Table 1-3 Operation status

State Description

Disabled Resources are completely unavailable, and can no longer provide services to the users.

Enabled All or part of resources are available and can be used.




Table 1-4 Availability status

State Description

ln test The resource is being tested. Its operational state is disabled.

Failed The source/object is not working due to some internal error. Its operational state is disabled.

Power off The resource needs power supply. Its operational state is disabled.

Off line The resource needs manual or automatic operations. Its operational state is disabled.

Dependency Services provided by this resource are degraded in a certain sense, such as rate or operational capacity. Its operational state is disabled.

Degraded Services provided by this resource are degraded in a certain sense, such as rate or operational capacity. Its operational state is enabled.

Not Installed Hardware or software of the managed objects is not installed. Its operational state is disabled.

Step 2 Basic procedures

All procedures are based on formatted O&M messages. Most formatted O&M messages initiated by BSC or BTS require the peer layer 3 endpoint to give response or acknowledgment in the form of formatted O&M messages. This pair of formatted O&M messages or a single formatted O&M message that need not be responded to is called a basic procedure. All formatted O&M messages are sent on layer 2 in the form of I frames. A group of procedures, called as structured procedures, are based on the combination of some basic procedures.

For a specific object, if a certain basic procedure is not completed, the system will not start its subsequent basic procedures. When there is no response to the formatted operation and maintenance message from the peer layer 3 before L3 timeout, the basic procedure is regarded as not completed. When the previous basic procedure has not received any response (ACK or NACK) before layer 3 timeout, then no subsequent basic procedure is sent to this object case. The default timeout for layer 3 is 10s. If part of an original message is not understood or supported, the whole message is discarded. A ACK message returned by the object indicates affirm response, it is used to notify the message sender that the command has been executed or will be executed. A NACK message returned by the object indicates disaffirm response, it is used to notify the message sender that the command executed unsuccessfully and the corresponding failure cause.

There are mainly the following types of basic procedures:

z Software loading management procedure z Abis interface management procedure z Transmission management procedure z Air interface management procedure z Test management procedure z State management and event reporting procedure z Equipment state management procedure



z Other procedure

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1.4 Um Interface Um interface (air interface or radio interface) is defined as the communication interface between MS and BSS. It is for the communication between MS and the fixed part of GSM. Its physical link is the radio link. The information transmitted via this interface include radio resource management, mobility management and connection management.

1.4.1 Overview In a GSM network, MS is connected through radio channels to the fixed network so that communication services can be routed to the specific destination. To realize the inter-working between MS and BSS, it is necessary to standardize the transmission of the signals on the radio channel. The norm concerning the signal transmission on the radio channel is the radio interface, or Um interface.

The Um interface is specified by the following features:

z Channel structure and access capability z MS-BSS protocols z Maintenance and operation characteristics z Performance characteristics z Service characteristics.

The Um interface can be divided into 3 layers, as shown in Figure 1-8.

Figure 1-8 Layered structure of Um interface

The first layer is the physical layer at the bottom. It defines the radio access capabilities of GSM, and provides basic radio channels for information transfer on higher layer.

The layer 2 is the data link layer using the LAPDm protocol. It defines various data transmission structures, and controls data transmission.

The layer 3 is the highest layer. It includes various messages and programs, and controls services. It includes three sub-layers, which are Radio Resources management (RR), Mobility Management (MM), and Connection Management (CM).




1.4.2 Layer 1 - Physical Layer The physical layer (L1) is the lowest part of the Um interface. It provides the physical links needed for transferring bit streams, and provides various logical channels for the higher layers, including traffic channels and signaling channels. Each logical channel has its own logical access point.

Physical layer interface and services:

The interfaces between the physical layer (L1) and data link layer (L2), the physical layer (L1) and radio resources management sublayer (RR) of L3, the physical layer (L1) and other functional unit, are shown in Figure 1-9.

Figure 1-9 Physical layer interface

The physical layer provides the following services:

z Access capability: the physical layer provides a series of limited logical channels for transmission service. The logical channel is multiplexed on the physical channel. There exist 8 physical channels on each TRX. Through data configuration, logical channels are mapped to physical channels.

z Error code detection: Physical layer provides error protection transmission, including error detection and correction.

z Ciphering: Use the selected encrypt algorithm to transmit bit sequence encrypted.

1.4.3 Layer 2 - Data Link Layer The purpose of the data link layer is to establish reliable dedicated data links between MS and BTS. The link layer protocol used by the GSM system at the radio interface is the LAPDm protocol which has evolved from the LAPD protocol. It receives the service from the physical layer and provides service to L3. The data link Service Access Point (SAP) is the node that provides services for layer 3. SAP is identified through SAPI. Each SAP is associated with one or multiple Data Link Connection End Points (DLCEP). Currently, two SAPI values are defined in the LAPDm protocol, 0 (main signaling) and 3 (short messages).

LAPDm

Step 1 Function



LAPDm transfers information between layer 3 entities through the radio interface on the Dm channel. LAPDm supports multiple layer 3 entities and physical layer entities, and signaling of BCCH, PCH, AGCH and DCCH.

The Dm channel is a generic term for all the signaling channels at the Um interface in the GSM system. For instance, the Dm channel can either be PCH or BCCH.

LAPDm includes functions for:

z The provision of one or more data link connections on a Dm channel. Discrimination between the data link connections is by means of a data link connection identifier (DLCI).

z Allows for frame type identification. z Allows L3 message units to be transmitted transparently between L3s. z Exercises sequence control to maintain the order of frames that pass DLC. z Check on the format and operation errors on the data links. z Flow control. z Contention resolution when establishing a data link after an access request has been

made on the RACH.

Step 2 Operation types

Two types of operation of the data link layer are defined for layer 3 information transfer: unacknowledged operation and acknowledged (multiple frame) operation. They may co-exist on a Dm channel.

z Unacknowledged mode: In unacknowledged mode, layer 3 information is transmitted in Unnumbered Information (UI) frames. At the data link layer, the UI frames are not acknowledged. Flow control mechanisms and error recovery mechanisms are not defined. Unacknowledged operation is applicable to different types of control channels except for RACH.

z Acknowledged mode: In acknowledged mode, layer 3 information is transmitted in Unnumbered Information (UI) frames. The data link layer acknowledges the transmitted I frame. Error recovery procedures based on retransmission of unacknowledged frames are specified. In case of errors, which cannot be corrected by the data link layer, a report is issued to the layer 3 entity. Flow control procedures are also defined. Acknowledged operation is applicable to DCCH.

Step 3 Information transfer mode:

On different channels, information transfer modes are different.

z Information transfer on the BCCH: The BCCH exists only in the network to MS direction and is used for broadcasting radio sub-system information to MSs. Only the acknowledged mode can be adopted on the BCCH.

z Information transfer on the PCH + AGCH: These channels exist only in the network to MS direction. On the PCH + AGCH only unacknowledged operation is possible.

z Information transfer on the DCCHs: On the DCCHs, either acknowledged or unacknowledged mode may be adopted. The mode required at any time is determined by layer 3.

Step 4 Release of data links:




Multiple frame operation may be released in the following ways:

z Normal release by exchange of commands/responses. z Local end release, i.e. without exchange of commands/responses, initiated and controlled

by layer 3.

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1.4.4 L3

Introduction The signaling layer 3 of the Um interface provides the functions to establish, maintain and terminate circuit-switched connections across a GSM PLMN and other networks to which the GSM PLMN is connected. It provides the necessary supporting functions related to supplementary services control and short messages service control. Furthermore it includes the functions necessary for mobility management and radio resource management.

The layer 3 entity consists of many functional program blocks. These program blocks transfer message units carrying various kinds of information among all layer 3 entities and between layer 3 and neighboring layers. The objectives of the layer 3 are to provide the means for:

z The establishment, operation and release of a dedicated radio channel connection (RR). z For location updating, authentication and TMSI reallocation (MM). z For establishment, maintaining and termination of circuit-switched calls (CC). z Supplementary services support (SS). z Short messages service support (SMS).

Layer 3 consists of three sub-layers including Connection Management (CM), Mobility Management (MM) and Radio Resource management (RR). The CM sub-layer contains multiple call control (CC) units, which are to implement concurrent call handling. It also contains SS units and SMS units, which are respectively used to support supplementary services and short message services.

The functions of the signaling layer 3 are performed by means of the signaling layer 3 protocols between two systems which represent the Mobile Station side and the Network side of the radio interface as viewed by the Mobile Station. GSM 04.07 does not consider the distribution of signaling functions among the different network equipment. The functions of layer 3 and its supporting lower layers, therefore, provide the Mobile Network Signaling (MNS) Service to the upper layers.

Interaction between layer 3 and higher layers and between services interfaces of layer 2 as well as that between neighboring sub-layers in layer 3 can be described in primitives and parameters. Exchange of information between two peers of the signaling layer 3 is performed by means of the three sublayer protocols.

L3 Structure As have already introduced, the three sub-layers of layer 3 are further discussed here: Among them, The CM sub-layer (the highest sub-layer) is composed of three functional entities: Call Control (CC), Short Message Service support (SMS) and Supplementary Service support (SS). In total, there are five functional entities consisted in the layer 3 radio interface. Below is the brief introduction to these entities:



z Radio Resources (RR) management handles the establishment, maintenance, and release of physical channels and logical channels, as well as cross-cell transfer on the request of CM sub-layer.

z Mobility Management (MM) deals with the all necessary functions of mobile features to support mobile subscribers. It notifies the network when the mobile station is activated and deactivated, or the location area is changed. It is also responsible for the security of activated radio channels.

z CC deals with all necessary functions to establish or release the circuit-switched connections.

z SS deals with all necessary functions to support GSM supplementary services. z SMS performs all necessary functions to support point-to-point short message services.

In addition, other functions are contained in layer 3 which are related to the transport of messages, e.g. multiplexing and splitting. Those functions are defined in the Radio Resource Management and Mobility Management. They have the task to route the messages according to the protocol discriminator (PD) and transaction identifier (TI) which are part of the message header.

The MM routing function route the messages of the CM entities and the messages of the MM entity of its own sublayer towards the service access point of RR, and multiplex them in case of parallel transactions. The routing function of Radio Resource Management shall distribute the messages to be sent according to their protocol discriminator (PD) and the actual channel configuration.

The messages provided at the different service access points of layer 2 are split by the RR routing function according to the protocol discriminator (PD). If PD equals to RR, this message will be transferred to RR at the local sub-layer. Other messages are provided to MM via the access point RR-SAP. The routing function of MM passes the messages according to the protocol discriminator (PD) and the transaction identifier (TI) towards the MM entity or towards the CM entities via the various MM-SAP's.

Figure 1-10 illustrates the protocol model of L3 signaling.

The RR sub-layer at the bottom receives services provided by layer 2 through various service access points (i.e., various types of channels) of layer 2, and provides services via RR-SAP to the MM sub-layer. The MM sub-layer provides services to the three entities (CC, SS and SMS) on the CM sub-layer through different service access points MMCC-SAP, MMSS-SAP and MMSMS-SAP respectively, provides register services to the higher layer through MMREG-SAP service access points. The 3 independent entities on the CM sub-layer provide services to higher layers through MNCC-SAP, MNSS-SAP and MNSMS-SAP respectively.




Figure 1-10 Um interface L3 protocol model

Service Characteristics Step 1 Services provided by layer 3 on the MS side

z Registration services, i.e., IMSI attach and detach operations. z Call Control services, including MS originating normal call establishment, MS

originating emergency call establishment, call hold, call termination, and call related Supplementary Services Support.

z Call independent Supplementary Services Support.



z Short Message Services Support.

Step 2 Services provided by layer 3

z on the network side Call Control Services, including call establishment, call maintaining, call termination and call related supplementary service support.

z Call independent Supplementary Services Support. z Short Message Services Support.

Step 3 Inter-layer services between the mobile station and network side

z Services provided by Radio Resource Management entity (Refer to Figure 1-11). These services are provided to MM via RR-SAP. They are used for establishing control channel connections, establishing traffic channel connections, ciphering mode indication, releasing control channel connections, and control-data transfer.

z Services provided by mobility management entities (MM) (Refer to Figure 1-12). These services support call control, supplementary services and short messages services of connection management entities.

Figure 1-11 Communication at RR




Figure 1-12 Communication at MM

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ContentsFiguresTables1 Signaling FundamentalsAbout This Chapter1.1 Interface Overview1.2 A Interface1.2.1 Overview1.2.2 Protocols on the A-Interface

1.3 Abis interface1.3.1 Overview1.3.2 Protocols on the Abis Interface

1.4 Um Interface1.4.1 Overview1.4.2 Layer 1 - Physical Layer1.4.3 Layer 2 - Data Link Layer1.4.4 L3

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01-1 Signaling Fundamentals.pdf