04- Signaling in MTP

Signaling in MTP

Chapter 4

OBJECTIVES: Upon completion of this chapter the student will be able to:

• Describe the functional structure of the Message Transfer Part (MTP)

• Describe the general format of MTP level 2 and level 3 messages

• Explain some important MTP procedures

• Describe MTP hardware

Signaling No. 7 in GSM

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4 Signaling in MTP

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4 Signaling in MTP

Table of Contents

Topic Page

STRUCTURE OF MTP......................................................... 37 INTRODUCTION..............................................................................................37

FUNCTION LEVELS ......................................................................................37

SIGNALING DATA LINK (LEVEL 1) ................................... 39 GENERAL........................................................................................................39

SIGNALING DATA LINK IN AXE 10 .............................................................39

SIGNALING LINK (LEVEL 2)............................................... 41 GENERAL........................................................................................................41

SIGNAL MESSAGES.....................................................................................42

SIGNAL UNIT DELIMITATION .......................................................................42

SIGNAL UNIT ALIGNMENT ...........................................................................43

ERROR DETECTION.....................................................................................44

SIGNAL UNIT ACCEPTANCE PROCEDURE............................................44

ERROR CORRECTION..................................................................................45

SIGNALING LINK ERROR MONITORING....................................................48

ALIGNMENT ERROR RATE MONITOR.......................................................49

PROCESSOR OUTAGE CONTROL............................................................50

REMOTE CONGESTION TIME OUT (LEVEL 2 FLOW CONTROL)........51

SENDING CONGESTION INDICATION (LEVEL 3 FLOW CONTROL) ......................................................................................................52

SIGNALING NETWORK FUNCTIONS (LEVEL 3)............. 53 GENERAL........................................................................................................53

MESSAGE SIGNAL UNITS (LEVEL 3)........................................................54

SIGNALING MESSAGE HANDLING............................................................57


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SIGNALING NETWORK MANAGEMENT....................................................60

MTP FUNCTION BLOCKS IN AXE 10 ............................... 83 GENERAL........................................................................................................83

BLOCK DESCRIPTION .................................................................................85

MTP HARDWARE ................................................................. 91 INTRODUCTION..............................................................................................91

CONNECTION “RP – ST – PCDD – GSS”..................................................91

CONNECTION “RPD/RPG – GSS” ..............................................................94

MTP SET-UP IN AXE 10 ...................................................... 97 GENERAL........................................................................................................97

SET-UP IN AXE 10.........................................................................................97

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STRUCTURE OF MTP

INTRODUCTION

The fundamental structuring principle of the CCITT Signaling System (SS) No.7 is the subdivision of functions into a common Message Transfer Part (MTP) and the User Parts (UPs) for different users (applications).

In this context a “user” communicates with a corresponding “user” in an adjoining node, while the MTP helps to transport the messages by packaging, transporting, unpacking and delivering it to the correct “user” (application).

The function of MTP is to serve as a common transport-system that provides reliable transmission of signaling messages between communicating users.

MTP is in fact a specification of the entire CCITT SS No.7. The MTP function is able to define all nodes (SP and STP) and signaling links in four different signaling networks.

FUNCTION LEVELS

In accordance with CCITT recommendations Q.701 - Q.707, the functions of the MTP are divided into three levels, conforming to the OSI model:

• Level 3 Signaling Network functions

• Level 2 Signaling Link functions

• Level 1 Signaling Data link

See figure 4-1.


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Signaling

Signaling

Signalingmessagehandling

link

networkfunctions

MessageTransfer

Part(MTP)

Signalingnetwork

management

Signalinglink

functions

Signalingdatalink

Signaling message

Data User TelephonyOther Users

Control signals

User Part(TUP)

Part (DUP)Level

Level

Level

Level

4

3

2

1

Figure 4-1 Message Transfer Part (MTP) as defined in CCITT recommendations

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SIGNALING DATA LINK (LEVEL 1)

GENERAL

The signaling data link forms the lowest level (level 1) in the CCITT SS No.7 functional hierarchy. The Level 1 protocol defines the physical, electrical, and functional characteristics of a signaling data link.

A signaling data link is a bi-directional transmission path for signaling, i.e. two data channels working together in opposite directions at the same bit rate. This path can be either digital or analog.

Digital Signaling Data Link

Usually one channel of a first-order PCM system is used as a signaling data link. CCITT recommends the use of channel 16 but states that, if channel 16 is unavailable, any of the channels 1-31 may be used. The bit rate of the transmission is thus 64 kbps.

Analog Signaling Data Link

In exceptional cases an analog signaling data link may be used, e.g. when a PCM system is not available in any part of the signaling network. However, since the link consists of voice-frequency analog transmission channels and modems, the bit rate must be lower than 4.8 kbps.

SIGNALING DATA LINK IN AXE 10

The signaling data link connects a signaling terminal at a signaling point with a signaling terminal at a remote signaling point. Together the signaling data connection and the signaling terminals form a signaling link.

In an SPC exchange such as AXE, a semipermanent connection is made through the Group Switch (GS) between a signaling terminal and the chosen signaling link (a time slot in PCM), see figure 4-2.


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GSPCD

ST-7

ETC

64 kbps

2 Mbps

SignalingChannel

0 1 31

SignalingChannel

0 1 31

GSPCD

ST-7

ETC

64 kbps

2 Mbps

Signaling Link

SignalingData Link

Level 1Level 2 Level 2

Figure 4-2 Signaling data link

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SIGNALING LINK (LEVEL 2)

GENERAL

The signaling data link, the signaling terminal and the signaling link functions together to form the whole Signaling Link (SL) that is used for reliable transmission of signaling messages between two signaling points.

The block Signaling Link Control is responsible for the coordination of the signaling link functions, they include error detection and error correction mechanisms. This level implements the monitoring of the errors on the signaling link as well as the flow control in case of congestion in one of the nodes.

The purpose of the signaling link functions is to ensure that the signaling messages are delivered to the remote side correctly, in correct sequence, and without loss or duplication.

Signaling link control also performs alignment on recovery or activation of Signaling Links (SLs), involving synchronization. It monitors the errors on the signaling link in service. The important functions are shown schematically in the figure 4-3.


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Supervision

Flagdetection

(“Destuffing”)

Flaggeneration

(“Stuffing”)

Sequencenumber

generation

Retrans-missionbuffer

Sequence

checknumber

Transmissionbuffer

Receivebuffer

Messagelength check

Checksumgeneration

Checksumdecoding

Signalinglink

control

Figure 4-3 Overview of the Signaling Link functions (level 2)

SIGNAL MESSAGES

As previously mentioned there are three different kinds of signaling messages:

• Message Signal Unit (MSU)

• Link Status Signal Unit (LSSU)

• Fill In Signal Unit (FISU)

See chapter 3 “Signaling Unit Structures” for details.

SIGNAL UNIT DELIMITATION

To be able to extract the signal units from the bit stream on the signaling data link, all signal units are preceded and followed by a Flag with a unique 8-bit pattern (01111110).

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MSU Message Signal Unit

SIF SIOCK Error Correction FF LI

01111110

Figure 4-4 Signal units are delimitated by flags

To ensure that “false” flags are not generated unintentionally in a message, a zero is inserted automatically after five consecutive ones in the message.

The extra zero will be removed by the signaling link control at the remote SP. This function is known as Bit Stuffing. If a bit pattern with more than six consecutive ones is found during signal reception, an error indication is given.

data

111111111110

bit stuffed data data

11111011111100

111111111110

Figure 4-5 Bit Stuffing

SIGNAL UNIT ALIGNMENT

Alignment is partially performed by the delimitation procedure previously described.

When alignment on the signaling link is lost, an error indication is given when one of the following situations occur:

• A bit pattern is received that is rejected by the delimitation procedure (more than six consecutive ones).

• A signal unit is received that is not a multiple of eight bits and is not at least six octets long.

• The length of a signal unit exceeds the maximum length of a Signal Unit (SU), i.e. 272+7.

The Length Indicator (LI) is used to determine the type of signal unit (LSSU, MSU, or FISU) and the information on where the end of the SIF field is.


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ERROR DETECTION

Error detection is performed by means of 16 ChecK bits (CK) at the end of each Signal Unit (SU). The check bits are generated by the sending signaling terminal and contain a check sum calculated from the value of the bits in the rest of the Signal Unit (SU) excluding the flag.

At the receiving signaling terminal the same algorithm is used to calculate comparative checksum. If the received checksum and the one just calculated are equal, the received SU is considered to be fault free.

If the two check sums are different, the SU is rejected and an error indication is given.

SIGNAL UNIT ACCEPTANCE PROCEDURE

The three functions previously mentioned perform an Acceptance Procedure. This procedure is summarized in figure 4-6.

Flag detection (“Destuffing”)

Comparison of checksums

F01111110 ----- x.............xx ----- 01111110

F

check of checksum

F01111110 ----- 1111111 ----- 01111110

F

incorrect flag

Synchronization of messagesF

01111110 -------------------- 01111110F

SU is not a multiple of 8 bitsSU < 6 octetsSU > 272+7 octets

Delimitation

Alignment

Error Detection

F

Figure 4-6 SU Acceptance Procedure

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Error Indications from the Acceptance Procedure activate the Error Correction Procedure and the Signaling Link Error Monitoring.

ERROR CORRECTION

There are two error correction methods - Basic Error Correction Method and Preventive Cyclic Retransmission Error Correction Method. The latter one is referred to as the PCR method. Both methods are “non-compelled”, and use retransmission to correct an incorrectly transmitted Message Signal Unit (MSU). LSSUs and FISUs are not retransmitted because they do not carry specific signaling information but information available to all following SUs.

Retransmission is made possible by keeping transmitted, but not positively acknowledged, MSUs available in a retransmission buffer in the signaling terminal.

After positive acknowledgment the message signal unit will be deleted from the Retransmission buffer in the sending terminal.

The two methods initiate retransmission in different ways.

Retransmission methods

In the Basic method the receiving terminal sends a negative acknowledgment in response to an incorrectly received message signal unit. A negative acknowledgment initiates retransmission of the incorrect or lost MSU and all those that follow it. See figure 4-7.

Signaling Terminal Signaling Terminal

Retransmission

MSU

RetransmissionbufferNegative

acknowledgement

Figure 4-7 Basic Error Correction Method

In the PCR method, retransmission of sent but not positively acknowledged Signal Units (SUs) is initiated automatically when there are no new SUs to send from the transmission buffer. The SUs are generally retransmitted as long as no positive acknowledgment is


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received.

Fill In Signal Units (FISU) are sent only when there are no new message signal units to send and the retransmission buffer is empty.

Retransmission is also initiated if the number of MSUs in the retransmission buffer reaches a certain level. This retransmission interrupts the transmission of newly received MSUs from the transmission buffer.

So with the PCR method no negative acknowledgment to the sending side for incorrect or lost message signal units is returned. Only positive acknowledgment is used.

If the distance and/or transmit time between the signaling terminals is long, signaling would be slower with the Basic method, and the efficiency of the signaling system would drop. The PCR method is therefore used on intercontinental signaling routes, where the transit time amounts to 15 ms or more. The same applies to all routes that go by satellite. No further description of the PCR method is given here.

Basic method

The Basic method is used on all land-based signaling routes with less than 15 ms transit time delay. Only the Basic Error Correction Method is described here.

The method uses the FIB, FSN, BIB, BSN fields of the Message Signal Unit (MSU). Together the fields consist of 16 bits. Each transmitted MSU is given a sequence number (0-127), which is inserted in the MSU and constitutes the Forward Sequence Number (FSN).


BSNBIBFSNFIB


1 7 1 7

Figure 4-8 MSU-fields used for error correction

If the message is correctly received, the receiving terminal sends a

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positive acknowledgment by inserting the sequence number of the received message as a Backward Sequence Number (BSN) in an ordinary Message Signal Unit (MSU), in an FISU, or in an LSSU. The same value, as used in the message before is inserted in the Backward Indicator Bit (BIB) field. When the acknowledgment is received, the message is removed from the retransmission buffer.

BSN

FSN

Positive acknowledgement if BIB = FIB

FSN

BSN

FSN = 63

FSN = 63

BSN = 62

Negative acknowledgement if BIB = inverted value of FIB

A

A

B

B

BSN = 63

Figure 4-9 Positive and negative acknowledgement

If the message was received incorrectly, the receiving terminal sends the sequence number of the last correctly received message as a BSN. A negative acknowledgment, by inverting the value in the BIB, is also sent back. See figure 4-9. The incorrect message is then retransmitted from the retransmission buffer the with Forward Indicator Bit (FIB) inverted, to indicate that it is a retransmission.

Any Message Signal Unit (MSU) in the retransmission buffer with a sequence number higher than the recently transmitted SU is also retransmitted.

If a message signal unit has been completely lost, this is revealed by the sequence numbering of the received messages not being consecutive. In this case, the receiving terminal gives no positive acknowledgment of the last received message. Instead it requests retransmission of the lost message in the manner previously described.

The Error Correction procedure, activated after Error Detection, takes care of the retransmission of incorrectly received messages. But this is only possible as long as the error correction field in the SUs is not corrupted itself. There are two main problems to deal with:


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- Abnormal BSN or FIB - Excessive Delay of acknowledgment

When two incorrect Backward Sequence Numbers (BSNs) or Forward Indicator Bits (FIBs) occur in three consecutive Message Signal Units (MSUs) a signaling link fault is indicated to level 3 and the corresponding signaling link is taken out of service.

The same will be done if the acknowledgment of message signal units takes too long.

SIGNALING LINK ERROR MONITORING

In order to assess whether the operating quality of the signaling links is correct, all signals are monitored by:

• Signal Unit Error Rate Monitor (SUERM)

• Alignment Error Rate Monitor (AERM)

The Signal Unit Error Rate Monitor (SUERM) is used continuously on signaling links in service. It consists of a counter for receiving erroneous Signal Units (SUs).

The counter is incremented whenever an error indication is received from the acceptance procedure of the signaling link. The SUERM is decremented after a certain number (256) of correctly received SUs (“leaky bucket”-principle). See figure 4-10.

The threshold value is 64. When the counter reaches this level the signaling link is flagged faulty. It will then be taken out of service and undergo a recovery test - Initial Alignment Procedure. The signaling traffic due to be sent on the link is transferred to another link by means of a Change Over Procedure.

In addition to the previously mentioned method, SUERM is also used when Signal Unit (SU) alignment is lost on the signaling link. In this case the acceptance procedure will give an indication called “Change to octet counting mode”. In this mode SUERM is incremented for a fault indication after every 16 octets received by the Acceptance Procedure.

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0

64

-1 for 256 received SU’s

+1 for each retransmitted SU

Alarm Level

SUERM

(+1 for every 16th octetin “octet counting mode”)

Figure 4-10 Signal Unit Error Rate Monitor (SUERM)

ALIGNMENT ERROR RATE MONITOR

The Alignment Error Rate Monitor (AERM) also consists of a counter. It is active only during the Initial Alignment Procedure. This procedure takes place either at command ordered activation of a signaling link or at automatic “recovery attempts” of a signaling link after error indication from SUERM.

During the Initial Alignment Procedure the Link Status Signal Unit (LSSU) is used to exchange one of the following status indications of the signaling link between two Signaling Points (SPs):

SIO - Out of Alignment SIN - Normal Alignment SIE - Emergency Alignment SIOS – Out of Service

The procedure starts by sending SIN or SIE and is complete after reception of the corresponding message from the other side. However, alignment must first be approved for a certain trial period (8.25 s during “normal” alignment, 0.5 s during “urgent” alignment, e.g. on recovery).

During the alignment procedure the error rate monitor AERM is used. The counter is reset to zero at the beginning of an alignment period and is incremented every time an incorrect SU is received.


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An alignment period is rejected if the value of AERM reaches four. At five consecutive failed alignment periods the entire procedure is failed.

An alignment period is successful as soon as a period shows an acceptable error rate (AERM<4) and no LSSUs with “abnormal” status indication have been received.

The AERM is incremented like the SUERM by error indications from the acceptance procedure and it may as well be set into Octet Counting Mode. But while the SUERM works on the “leaky bucket”-principle, the AERM works linearly.

After a successful alignment a signaling link test will be performed before the signaling link is put into service. In the case of an unsuccessful alignment or signaling link test the link will be indicated as faulty.

0

4AERM >= 4

+1 for each SU received incorrectly

Failure level

AERM

2

Alignment period

=> Alignment unsuccessful

during the alignment period

Figure 4-11 Alignment Error Rate Monitor (AERM)

PROCESSOR OUTAGE CONTROL

Processor Outage (PO) refers to a situation in which no signaling messages can be transferred to level 3 or higher. This situation can not be influenced by level 2.

Level 2 identifies a PO situation either by receiving an indication from level 3, e.g. command-initiated “Inhibiting”, or by suspecting errors

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in level 3. After that LSSUs with Status Indication Processor Outage (SIPO) are continuously sent to the remote SP and all received MSUs are rejected. When the remote SP receives an LSSU with SIPO, level 3 is informed and a continuous sending of empty FISUs is initiated.

By the stopping of MSU transmissions the signaling link is taken out of service and is given one of the following states depending on where the PO occurred:

• Local Processor Outage (LPO)

• Adjacent PO (RPO)

• Both, Local and Remote PO (BPO)

When the Processor Outage (PO) ceases, sending of SIPO stops and normal sending of MSU and FISU is resumed. When the remote side has received these SUs correctly, level 3 is informed and the signaling link returns to normal operation.

REMOTE CONGESTION TIME OUT (LEVEL 2 FLOW CONTROL)

This procedure is initiated when congestion occurs on the receiving side of a signaling link. The congested side stops acknowledging and the Status Indication “Busy” (SIB) LSSU is sent to the remote SP every 200 ms until the congestion ceases. Sending of MSUs and FISUs to the remote side continues as usual but the BSN and the BIB contain values from the last acknowledged message.

When the remote SP receives the first SIB, a 10-second watchdog timer is started. When this timer expires, the signaling link will be indicated as faulty.

If the congestion ceases, sending of SIB is stopped and the normal sending of MSUs and FISUs is resumed, i.e. acknowledgments of previously unacknowledged messages are placed in the BSN and the BIB. The watchdog timer in the remote side is cancelled.


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SENDING CONGESTION INDICATION (LEVEL 3 FLOW CONTROL)

The previous section dealt with the congestion situation on the receiving side of a signaling connection.

Congestion in the sending direction is also monitored. When the congestion threshold in the transmission buffer of the signaling link is reached, i.e. the messages that are waiting to be sent have reached a defined limit, an indication is sent to level 3.

Flow Control Level 3 function will then deal with the congestion situation and take action.

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SIGNALING NETWORK FUNCTIONS (LEVEL 3)

GENERAL

The signaling network functions on level 3 are grouped into two major functional units: Signaling Message Handling (Signaling Traffic Handling) and Signaling Network Management (SNM).

The Signaling Message Handling is responsible for the routing of messages to the appropriate link and the distribution of received messages within the own exchange.

This traffic handling is performed by three sub functions:

• Message Routing

• Message Discrimination

• Message Distribution

The Signaling Network Management (SNM) has the purpose of diverting traffic to alternative links or routes in case of link failure, controlling the message flow, establishing and restoring signaling resources, and checking and restricting the use of the network by “external” operators.

This is done by the following functions:

• Network Control

• Network Flow Control

• Signaling Resource (Link) Management

• Policing

• Accounting

The signaling network functions are shown in figure 4-12.


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Signaling message Control signals

Signalingresource

management

Networkflow

control

Networkcontrol

Policing

Messagedistribution

Messagerouting

Messagediscrimi-

nation

Signalingmessagehandling

Signalingnetwork

management

Signaling network functions

Use

r P

arts

Level 2Level 4 Level 3

Sign

alin

g lin

k fu

ncti

ons

Figure 4-12 Overview of the Signaling Network Functions (level 3)

MESSAGE SIGNAL UNITS (LEVEL 3)

Introduction

From its basic format it is evident that the core of a Message Signal Unit (MSU) is generated on level 3 or level 4. On Level 3 there are two kinds of MSUs generated:

• Signaling Network Management (SNM) MSU

• Signaling Network Testing and Maintenance (SNT) MSU

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MSU is identified by the Service Indicator (SI) in the Service Information Octet (SIO). The MSU also includes the Signaling Information Field (SIF), which contains the actual signaling information carried by a particular message. In level 3 MSUs the SIF is subdivided as follows:

• Routing label DPC Destination Point Code OPC Originating Point Code CIC Circuit Identification Code SLS Signaling Link Selection

• Signaling information

In case of network management messages the SLS field contains the Signaling Link Code (SLC). The SLC indicates the number of the signaling link to which the level 3 message relates. If the message is not related to a signaling link, SLC is given the value 0000.



DPCOPCSLSNI

Spar

e SIRouting Labelsignaling

SIF:SIO:DPC:OPC:SLS:NI:SI:

Signaling Information FieldService Information OctetDestination Point CodeOriginating Point CodeSignaling Link SelectionNetwork IndicatorService IndicatorCircuit Id CodeCIC:

info

Figure 4-13 Signaling Information Field (SIF) and Service Information Octet (SIO) in MSU


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Signaling Network Management MSU

The Service Indicator (SI) in the SIO for SNM MSUs is given the value 0000. They are grouped into the following message groups and messages:

CHM CHange over and change back Messages:

COO Change Over Order COA Change Over Acknowledgment CBD Change Back Declaration CBA Change Back Acknowledgment

ECM Emergency Change over Messages:

ECO Emergency Change over Order ECA Emergency Change over Acknowledgment

FCM signaling traffic Flow Control Messages:

TFC TransFer Controlled

TFM TransFer prohibited/allowed/restricted Messages:

TFP TransFer Prohibited TFR TransFer Restricted TFA TransFer Allowed

RSM signaling Route Set test Messages:

RST signaling Route Set Test (for restricted destinations) RSP signaling Route Set test (for Prohibited destinations)

MIM Management Inhibit Messages:

LIN Link INhibit LUN Link UNinhibit LIA Link Inhibit Acknowledgment LUA Link Uninhibit Acknowledgment LID Link Inhibit Denied LFU Link Forced Uninhibit LLT Link Local inhibit Test LRT Link Remote inhibit Test

UFC User part Flow Control messages:

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UPU User Part Unavailable

The functions of these various messages are explained in the following subchapter.

Signaling Network Testing and Maintenance MSU

The Signal Indicator (SI) for SNT MSUs is given the value 0001. They are subdivided into the following message group and messages:

SLT Signaling Link Test messages:

SLTM Signaling Link Test Message SLTA Signaling Link Test Acknowledgment

SIGNALING MESSAGE HANDLING

Introduction

Signaling traffic in MTP can be divided into incoming, outgoing, and transfer signaling traffic.

The Signaling Message Handling must deal with these various types of MSU traffic. It is responsible for the routing of messages to the appropriate link and the distribution of received messages within the own exchange. This task is performed by the three sub functions: message routing, message discrimination, and message distribution.

Figure 4-14 is a schematic diagram of the message handling functions and their interaction with the neighboring functions:


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Signalingnetworkmanagement

- Network flow control- Network control- Signaling resource management

Signaling message Control signals

Signaling

Sign

alin

g lin

k fu

ncti

ons

Messagediscrimi-

nation

Messagedistribution

Messagerouting

SI NI DPC

SLSDPCNI

SL

SL

SL

SL

link set

TUP

ISUP

SCCP

SNM

Terminating

Transfer

SLS

...

...

User P

arts

messagehandling LabelSIO

MSU

LabelSIO

MSU

Figure 4-14 Signaling Message Handling

The signaling message handling functions are based on the network indicator in the SIO field and the routing label contained in the messages, which explicitly identifies the destination (DPC) and originating points (OPC).

Message Routing

This function is used at each Signaling Point (SP) to determine the outgoing Signaling Link (SL) for the messages to the different destinations.

This task is performed for each MSU with the help of the Network Indicator (NI), located in the Service Information Octet (SIO), Signaling Link Selection field (SLS) and Destination Point Code (DPC), which are located in the Routing Label in the Signaling Information Field (SIF).

Messages with the same NI, SLS and DPC are routed over the same signaling link in the current link set. This is not applied to

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SNM-MSUs and SNT-MSUs.

Load sharing is applied to distribute the traffic over several Signaling Links (SLs) and Link Sets (LSs). To control this load sharing the SLS field in the label is used. There are two cases:

• Load sharing within the link set is the default. It uses only the least significant part of the SLS field. Bit 0 is used when the Link Set (LS) has 2 SLs. Bit 0, 1 - when the Link Set (LS) has 3 to 4 SLs. Bit 0, 1, 2 - when the Link Set (LS) has 5 to 8 SLs.

• Load sharing between Link Sets (LSs) can be applied and uses a chosen bit in the SLS field. This Load SHaring Bit (LSHB) is indicated with a parameter in the SP exchange data.

The level 3 network control, described later on in this chapter, decides which Link Sets (LSs) are to be used in each case.

In the event of a link fault, message routing is modified by the network control in accordance with fixed rules, so that the concerned traffic is routed to other Signaling Links.

Signaling Network Management (SNM)-MSUs and Signaling Network Testing and maintenance (SNT)-MSUs are routed in three different ways:

• MSUs that do not belong to a particular Signaling Link (SL) have the Signaling Link Code (SLC) 0000. These MSUs are routed by the normal routing function as previous explained. SLC is used for load sharing in the same way as SLS.

• MSUs that are to be transmitted over a particular Signaling Link (SL) are routed with the aid of a special routing function.

• MSUs that are to be sent over a particular Signaling Link (SL) specified by the SLC code are also routed, but by a routing function different from the usual one.

Routing of messages to destinations that are not available:

• When an STP can not route a received MSU to the desired destination, the MSU is declined. If the unavailability of the SP is known, the level 3 network control sends a TransFer Prohibited (TFP) message to the SP that sent the declined MSU.

• If as a result of message routing an MSU is to be sent out on the same Signaling Link (SL) it was received on, the MSU is declined. Here too, a TFP message is sent to the sending SP.


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Message Discrimination

This function determines whether an incoming message is terminating at the local SP or is to be transferred to another Signaling Point (SP). In the latter case the local SP serves as an STP and the message must be transferred to the message routing. Otherwise it is given to message distribution for further analysis.

The Discrimination is done by analyzing the DPC and NI in the received message.

Message Distribution

Messages that are to terminate at the local SP are distributed to the relevant User Part (UP) or to the Signaling Connection Control Part (SCCP). Messages to level 3 (SNM-MSUs and SNT-MSUs) are distributed to the Signaling Network Management (SNM) function.

The Distribution is determined by the values in the NI and SI fields in the SIO. See table 3-2 "Service Information Octet (SIO)" in chapter 3.

SIGNALING NETWORK MANAGEMENT

Introduction

The Signaling Network Management (SNM) has the purpose to:

• Divert traffic to alternative links or routes in case of link failure

• Control the message flow

• Establish and restore signaling resources

• Check and restrict the use of the network by “external” operators.

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These tasks are performed by the four sub functions:

• Network flow control

• Network control

• Signaling resource management (link management)

• Policing.


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Networkcontrol

Policing

Networkflow

control

Signalingresource (link)management

Signaling Network ManagementLevel 3

Figure 4-15 Signaling Network Management (SNM)

The re-configuration of the signaling network is handled by appropriate procedures to change the routing of signaling traffic in order to bypass faulty links or Signaling Points (SP). This requires communication between Signaling Points concerning the occurrence of the failures.

Network Flow Control

The purpose of network flow control is to limit the signaling traffic at its source in the case of temporary overload of Signaling Links (SLs) in the network. This is done by controlling the message flow from the user or from application parts into the MTP in congestion situations.

Information about this kind of congestion is obtained:

• Locally, i.e. Signaling Links (SL) at the local SP become congested

• At the remote end, i.e. a TransFer Controlled (TFC) message is received

• From an adjacent Signaling Point (SP)

Link congestion

The transmission and retransmission buffers for any level 2 Signaling Link (SL) have a congestion threshold. If a buffer fills up to this limit, an indication (sending congestion) is given to level 3.

4 Signaling in MTP

EN/LZT 123 4734 R3A – 63 –

This results in the following:

• If at least one SL in a LS is congested, the entire LS gets the state “congested”.

• If at least one LS of a route to a particular destination is congested, that destination gets the state “congested” as well.

An indication together with the link congestion status is sent to each local user/application part in response to every message routed to a congested SL.

Note: The state of a certain SL can be “congested” or “uncongested”. In both cases MSUs to be routed over that link are placed in the transmission buffer for transmission. But when the buffer is full the MSUs are declined

Transfer controlled procedure

The purpose of the TFC procedure is to transfer the “congestion” indication at a certain destination SP in the signaling network back to the originating SP.

The TFC message is sent back to that SP, which sent the first MSU to the congested destination. It is passed through any STP in-between. The message is repeated after every eighth received MSU with the same destination.

On reception of TFC at the originating SP an indication is sent directly to all user/application parts (level 4), which then slow down the message stream to the congested destination.

Level 4 is not informed when the congestion ceases for a given destination. Instead it decides for itself that signaling traffic can return to normal when the TFC messages stop coming.

User part unavailability

If the MTP is unable to deliver a received message because a local User Part (UP) is unavailable, the MTP sends a User Part Unavailable (UPU) message to the MTP of the originating SP.

Network Control

Network control contains rules and procedures for routing signaling traffic both in the normal state of the signaling network and in


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abnormal states, e.g. fault situations.

The resulting information is handed over to the signaling message handling function, which performs the routing. This can be information concerning the routing/rerouting of signaling traffic and the distribution and checking of a destination status.

These topics are described in the following subchapter:

Normal routing of the signaling traffic

Network Control always has up-to-date information about the signaling routes for all destinations. This information is provided by the signaling resource (link) management, which will be dealt with later in this chapter.

Signaling traffic to a certain destination is routed either to one or two Link Sets (LSs), depending on what kind of load sharing is used.

For each MSU to be sent, network control is informed about the current values for Destination Point Code (DPC), Signaling Link Selection (SLS), Network Indicator (NI) and decides upon the correct LS (see figure 4-16). If load sharing between LSs is in operation, one of the two possible LSs is chosen with the aid of SLS and LSHB. Network Control then informs the signaling message handling about which LS is to be used for that particular MSU.

4 Signaling in MTP

EN/LZT 123 4734 R3A – 65 –

2-24182-24192-2420

2-35102-18022-2420

unavailableavailableavailable

DPC StateLS

Network controlSignaling

Signaling message handling

Signaling resource management

DPC, SLS, NI LS

Sign.link state

networkmanagement

Figure 4-16 Network Control provides the Routing Information

Distribution of destination status

When a destination status anywhere in the signaling network changes, one of the following procedures is started:

• Transfer Prohibited Procedure

Used when an STP can not communicate messages to a destination, i.e. there is no Signaling Route (SR) to the destination. The SR destination is UNAVAILABLE. STP sends TransFer Prohibited (TFP) messages to its interacting nodes. When an SP receives a TFP with information about the unavailable destination in the STP, the Signaling Route (SR) for that destination is given the status PROHIBITED in that SP. The signaling traffic to the destination is routed via alternative routes if possible (see Example 4, “Forced Rerouting Procedure” , later in this chapter).

• Transfer Restricted Procedure

Used when an STP has a Signaling Route with reduced capacity, e.g. one or some of its LSs are disabled, and it is desirable to restrict the signaling traffic to the destination in question. The STP then sends TransFer Restricted (TFR) messages to its interacting nodes. When an SP receives a TFR with information about the


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destination to which traffic is to be restricted, the LS for the sending SP is given the status RESTRICTED. If the Signaling Route (SR) for that particular destination has an alternative, a not restricted LS with the same priority or higher, the signaling traffic is rerouted to that LS. If there is no alternative LS, the signaling traffic continues via the linked group with RESTRICTED status.

• Transfer Allowed Procedure

If a STP is used when a destination has become available again after having UNAVAILABLE or RESTRICTED status, TransFer Allowed (TFA) message with information about the destination in question is sent to the interacting nodes (see Example 5, “Controlled Rerouting Procedure”, later in this chapter).

• Destination Availability Indication to a UP

When a destination is not available, i.e. there is no Signaling Route (SR), this is indicated to all UPs at the local node. It is up to each UP to stop the sending of messages to the unavailable destination. When a Signaling Route (SR) is restored to the destination, all UPs are informed about this change of status. It is then again up to each UP to reestablish signaling traffic with that destination.

Checking the destination status

In addition to the above methods to distribute the destination status there is one procedure to enable a Signaling Point (SP) to ask any STP whether it is possible to transfer traffic to a certain destination.

A Signaling Route Set (SRS) test message is sent to the STP. The procedure is started on those Signaling Routes (SRs) that have been given the status UNAVAILABLE or RESTRICTED. The STP answers the request with a TFP, TFA or TFR only if the destination status has changed since the last received test message.

When a Signaling Route (SR) with a certain LS has been given the status AVAILABLE again, i.e. TFA was received from the destination, test messages are also sent for the other SRs that use the same LS to check their status.

Two different Signaling Route (SR) test messages are used:

• RST message for “restricted destination”

• RSP message for “prohibited destination

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EN/LZT 123 4734 R3A – 67 –

Rerouting of signaling traffic

If certain Signaling Links and/or destinations become unavailable due to faults in the signaling, network control must organize rerouting of the signaling traffic.

Network control continuously provides a signaling network configuration for signaling message handling to work on. In fault situations, network control reconfigures the signaling network temporarily. When the fault ceases, the signaling network is restored to its normal configuration.

Different procedures are initiated in network control, depending on the current network status and where in the network (local or remote node) the fault occurred:

• Change over procedure (re-configuration, SL)

• Change back procedure (restoration of configuration, SL)

• Forced rerouting procedure (re-configuration, SR)

• Controlled rerouting procedure (restoration of configuration, SR)

Following are some examples of fault situations and the resulting actions automatically taken by network control:

• Fault in Local Signaling Link (SL)

Network control isolates the fault by automatically performing a change over so that the traffic normally handled by the faulty link is rerouted to an alternative link (or links) specified in the SP’s exchange data. The new signaling links can either belong to the same LS or to an alternative LS. The signaling traffic is distributed as uniformly as possible across the available links.

When the fault occurs there are messages waiting for transmission in the transmission buffer of the Signaling Terminal (ST) and unacknowledged messages in the retransmission buffer. All these messages must be rescued and transferred to the alternative SL without any message being lost or the order of sequence numbering disrupted.


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MSU 103MSU 102

MSU 101MSU 100

MSU 23MSU 22

ST-0

C7ST

Transmissionbuffer

Retransmissionbuffer

Receivingbuffer

Figure 4-17 Fault in Local Signaling Link (SL)

Example 1 - Change over procedure

One of the two Signaling Links (SLs) with SLC=0 and SLC=1 between A and B becomes faulty. Therefore it is reported to be UNAVAILABE to network control, level 3. The sending of messages to the ST of the failed link is stopped and subsequent messages are buffered in a change over buffer in the C7DR2 block.

At least one alternative LS is defined for each destination. In this case the parallel Signaling Link (SL) in LS=B can be used. Until the link is in service again after repair, its signaling traffic must be rerouted to the parallel link. The change over procedure is performed automatically by network control.

Any SL fault report comes from signaling resource management and as a result network control orders the signaling message handling to stop routing any messages with destinations that normally use the faulty link. These messages are temporarily stored in a change over buffer.

A handshaking procedure between SP A and SP B is started by the signaling resource management at SP A by sending a Change Over Order (COO) message on the alternative link to B. The purpose of the handshaking is to update the retransmission buffer in the Signaling Terminals (STs) at both SPs. The procedure determines which MSU was the last one correctly received at each SP before the link fault.

The COO message sent to B contains the Forward Sequence Number (FSN) of the last correctly received MSU in A (see figure

4 Signaling in MTP

EN/LZT 123 4734 R3A – 69 –

4-18). The Signaling Link Code (SLC) field states which SL the message relates to. In this case it is SL-0. Signaling Point (SP) B can now update its retransmission buffer, which means that messages already positively acknowledged are discarded while the others are moved to the alternative LS and retransmitted.

Transmissionbuffer

MSU 25(101)


MSU 58MSU 57

MSU 103MSU 102

MSU 101MSU 100

MSU 58MSU 57

MSU 56MSU 55

Transmissionbuffer

MSU 103MSU 102

Transmissionbuffer

MSU 27(103)MSU 26(102)

Transmissionbuffer

MSU 88(58)MSU 87(57)

MSU 101MSU 100


MSU 56MSU 55


MSU 86(56)


ST-1

ST-0ST-0

ST-1

Signaling link control Signaling link control

1001 1 800SIOSLCOPCDPCH0H1 FSN

Nationalnetwork

MSU-SNM


COO COA

SL-1SL-1A B

MSU 86MSU 25

SL-1 SL-1

Figure 4-18 Change over procedure

SP B then responds with a Change Over Acknowledgment (COA) message. It contains the FSN of the last correctly received MSU in B. Thus SP A can update its retransmission buffer in the same way.

The next SL control (level 2) is given the order to move the contents of the transmission buffer and the (updated) retransmission buffer to the Signaling Terminal (ST) of the alternative SL. The messages will be stored there on top of the already existing MSU’s in the


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corresponding buffers. The already positively acknowledged MSU 55 is not moved to the buffers in ST-1. In ST-1 the messages that have been moved are given the sequence numbering that applies in this terminal, but the original sequence of message transmissions from ST-0 is maintained.

The messages that have been moved are now transmitted. Those messages that have meanwhile accumulated in the change over buffer are also routed to ST-1 for transmission.

The change over procedure is now finished. All signaling traffic normally handled by SL-0 is rerouted to SL-1 and sent together with the normal signaling traffic of that SL.

Example 2 - Emergency Change over procedure

Due to faults in the Signaling Terminal (ST) it can sometimes be impossible to obtain the sequence number of the last correctly received MSU sent on the SL.

If SP A in the previous example can not determine the sequence number of the last correctly received signaling message, it informs SP B about this by sending an Emergency Change over Order (ECO) message instead of the COO message (see figure 4-19). This message does not contain a sequence number.

When SP B receives the ECO message, it returns an acknowledgment message. It may be either the normal COA or an Emergency Change over Acknowledgment (ECA) message.

A COA message contains the sequence number of the last correctly received MSU in SP B. This information enables SP A to update its retransmission buffer as described in the previous example. An ECA message from SP B means that its Signaling Terminal (ST) has a fault as well and is unable to determine the sequence number of the last correctly received MSU from SP A.

Whenever an ECO or ECA message is received at a SP, the receiver is not able to update its retransmission buffer. Only messages buffered in the transmission buffer will be transferred to the alternative link. Any messages in the retransmission buffer are lost.

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EN/LZT 123 4734 R3A – 71 –

Transmissionbuffer

MSU 26(102)


MSU 58MSU 57

MSU 103MSU 102

MSU 58MSU 57

Transmissionbuffer

MSU 103MSU 102

Transmissionbuffer

MSU 27(103)

Transmissionbuffer

MSU 88(58)

MSU 101MSU 100


MSU 56MSU 55


MSU 87(57)


ST-1

ST-0ST-0

ST-1

Signaling link control Signaling link control


Nationalnetwork

MSU-SNM


ECO ECA

SL-1SL-1A B

MSU 87MSU 26

SL-1 SL-1

Figure 4-19 Emergency Change over Procedure

Example 3 - Change over Procedure on No Response to COO

If no COA message is received from SP B in response to a COO message from SP A within the time-out of about 2 seconds, signaling resource management orders Signaling Link (SL) control (level 2) to reject any messages in the retransmission buffer. See figure 4-20. Only messages from the transmission buffer are transferred to the alternative link.


– 72 – EN/LZT 123 4734 R3A

Transmissionbuffer

MSU 58MSU 57

MSU 58MSU 57

Transmissionbuffer

MSU 88(58)

MSU 56MSU 55


MSU 87(57)


ST-1

ST-0Signaling link control


Nationalnetwork

MSU-SNMCOO

SL-1SL-1A B

MSU 87

SL-1

noresponse

within2

seconds

Figure 4-20 Change over Procedure on no response to COO

Example 4 - Forced Rerouting Procedure (Change over)

When the fault on the Signaling Link (SL) A-B is detected (see figure 4-21), the change over procedure starts in both SPs to save the signaling traffic on the link. The change over messages are sent via the node C.

4 Signaling in MTP

EN/LZT 123 4734 R3A – 73 –

B D

C E

A FTFP

Figure 4-21 Fault on Signaling Link between the signaling points A and B

When updating of the retransmission buffers is complete, SP A reroutes the traffic of the faulty link to the SL A-C and SP B reroutes the traffic towards A to the SL B-C.

In addition, SP B informs SP C in a TransFer Prohibited (TFP) message that SP B can not transfer any messages from C via B to A. The message includes the destination of the unavailable SP A and informs SP C that the Signaling Route (SR) C-B-A is broken and that any traffic towards A must be routed by C directly.

Network control in SP C may have defined the LS C-B for traffic with destination A. The TFP message from B now causes network control to mark this LS as UNAVAILABLE. A change back takes place and the traffic is routed to the alternative link set C-A.

Besides network control in C will send a Route Set Test for prohibited destination (RST) message to B every 30 seconds in order to monitor when the SL C-B-A returns to service. But only when B considers destination A to be AVAILABLE again a TransFer Allowed (TFA) message will be sent back to C.


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C-AC-B

AvailableAvailableA

Dest. LS State

1 5 80SIOSLCOPCDPCH0H1Dest.

RST

A B

A


RST

A


TFP

A

C

C-AC-B

AvailableUnavailableA

Dest. LS State

Every30

seconds

time

Network Control

Figure 4-22 Forced Rerouting Procedure

Example 5 - Controlled Rerouting Procedure (Change back)

When the fault on SL A-B has been repaired and the link is AVAILABLE again for signaling traffic, network control automatically initiates change back. The change back messages from A to B are sent via C, but in other respects the procedure is the same as described earlier.

B D

C E

A FFTFAA

Figure 4-23 Fault on Signaling Link between A and B repaired

B also tells C in a TransFer Allowed (TFA) message that MSUs may be routed on the signaling route C-B-A again (see figure 4-24).

4 Signaling in MTP

EN/LZT 123 4734 R3A – 75 –

C-AC-B

AvailableUn availableA

Dest. LS State


RST

A B

A


TFA

A

C

C-AC-B

AvailableAvailableA

Dest. LS Statetime

Network Control

Figure 4-24 Controlled Rerouting Procedure

When the TFA message is received at C, LS C-B is marked as AVAILABLE for traffic with destination A. The transmission of Route Set Test for prohibited destination (RST) message from C to B is stopped.

A also sends an RST test messages to B. In the format of these messages the field is provided for the destination. It states the destinations that A normally reaches through B, e.g. destination D. If destination D is flagged AVAILABLE in B, B responds with a TransFer Allowed (TFA) message.

Signaling Resource Management

This function monitors and controls the signaling resources, i.e. the exchange Signaling Links (SL), by means of the following procedures:

• Link management procedure

• Processor outage procedure

• Management inhibiting procedure

• Signaling Link (SL) test procedure

A Signaling Link (SL) handled by the signaling network functions (level 3) can have one of the states AVAILABLE, UNAVAILABLE or INHIBITED. The link can be used for UP traffic (level 4) only if


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its state is AVAILABLE. It can be used for MTP traffic (level 3) if its state is AVAILABLE or INHIBITED. The state of the link may change due to the events link error, processor outage, loss of contact with level 2, or operator activities.

Link management procedure

This procedure monitors the state of the links in the LS. The following tasks are performed:

• Signaling Link (SL) activation

Activates the SL at request by the operator.

• Signaling Link (SL) restoration

Performs an attempt to restore (activate) a faulty SL.

• Signaling Link (SL) deactivation

Deactivates the SL at the operator’s request. Can be done even if the SL is in service.

• Signaling Link (SL) emergency restart

If an entire LS fails, the resource management indicates an “emergency” situation for all links in the group. In this case a restoration of each link is started and level 2 uses the short alignment period.

Processor outage procedure

If an SL goes into the level 2 state PROCESSOR OUTAGE, e.g. when level 2 is suspecting errors in level 3, level 3 is not allowed to send or receive signaling traffic from the Signaling Terminal (ST) on level 2.

Resource management informs the network control that the link is not available for traffic, i.e. it is given the level 3 state UNAVAILABLE. Network Control then initiates a change over procedure.

MTP load protection

The purpose of the load protection is to protect the exchange from the system restarts that could occur during traffic overload.

This feature makes system availability higher. The system will be more stable. There will be no disturbances in the network, which are caused by system restarts. There will be no loss of already connected calls due to system restarts.

4 Signaling in MTP

EN/LZT 123 4734 R3A – 77 –

Management inhibiting procedure

To isolate an SL for testing, it can be made unavailable for signaling traffic generated by User Parts (UPs) (level 4). This “management inhibiting” is ordered by command and must be approved by the local and the adjacent Signaling Point (SP).

Inhibiting procedure:

• The procedure is initiated by command.

• It is checked that no destination will be made unavailable as a result of the procedure. Otherwise the procedure can not be performed.

• A request for inhibition of an SL, the Link INhibit (LIN) message, is sent to the remote SP. The adjacent SP may accept or decline such a request.

• If the Link Inhibit Acknowledge (LIA) message is not received within a certain time, the request is repeated. If no response is received after the second request, the procedure is aborted.

• If the link in question is in the state AVAILABLE, the network control must perform a change over procedure. When that procedure is complete, the state of the SL changes to INHIBITED.

Uninhibited procedure:

• Uninhibiting can only be done from the SP that did the inhibiting. The remote SP can only request uninhibiting.

• The procedure is initiated by command.

• A request for the uninhibition of the link, the Link UNinhibit (LUN) message, is sent to the remote SP. If there is no response within a certain time the request is repeated until an answer is received.

• The uninhibiting procedure can also be initiated by a destination being threatened to become unavailable unless the inhibited link is returned. For this reason a request, the Link Forced Uninhibit (LFU) message, can come either from the own SP or from the remote SP.

• If the inhibited link in question has not been inhibited from the remote SP, network control performs a change back procedure. When this procedure is complete, the state of the link changes to AVAILABLE again.


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Signaling link test procedure

The procedure is used to ensure a good SL quality in the following cases:

1. Restoration or activation of a signaling link

2. Continuously when the link is in service

The test method can indicate directly whether access to and signaling on the link is working correctly, but can not pinpoint the fault to a particular part of the SL.

Procedure on restoration or activation of a Signaling Link

• When the link has been successfully aligned, a test is run from the SP at each end of the SL.

• A Signaling Link Test Message (SLTM) is sent out on the SL. A Signaling Link Test Acknowledgment (SLTA) message should be received from the remote SP. The test message includes a bit pattern that is returned in the test acknowledgment message.

Continuous test procedure

• This test is run continuously on all SLs with the level 2 state ACTIVATED, i.e. not on links with the states CONGESTED, PROCESSOR OUTAGE or DEACTIVATED. The procedure is the same as above, except that the test is repeated every 30 seconds.

Fault Indication

A test run is unsuccessful if the following events occur:

• SLTA is not received within 10 seconds on the SL that has sent the corresponding SLTM.

• The bit pattern in the received SLTA does not agree with the bit pattern in the sent SLTM.

The SL is indicated faulty if two consecutive tests fail.

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EN/LZT 123 4734 R3A – 79 –

MTP Policing

When more than one administration operates in a CCITT No.7 signaling network, one of the administrations may want to restrict others in the use of a node as an STP.

The MTP policing function enables an operator to check and restrict the use of the own signaling network by “external” operators, e.g. to prevent them from using a certain signaling point as a Signaling Transfer Point (STP).

This MTP function ensures that there is no unauthorized use of STP. Any unauthorized attempt to use an STP can be monitored. The reception of messages violating the defined restrictions is reported in the form of an “infringement report”.

The policing function consists of three sub functions:

• STP policing

• SNM policing

• “Infringement” reporting

STP policing

STP restrictions can be used on outgoing and incoming messages to prevent an adjacent node from using the local signaling point as an STP. It imposes a prohibition on the transfer of messages from an Originating Point (OP) to a Destination Point (DP) on a certain LS.

The restrictions must be combined for the incoming Link Set (LS) with a combination of an Originating Point (OP) and a Destination Point (DP) of the message in one of the following ways:

• OP restricted, all DP allowed

• DP restricted, all OP allowed

• Both OP and DP restricted

In addition all STP transfers can be prevented to load the STP if required. An LS must always be specified. Restrictions are not imposed on the traffic from those LSs for which policing is not specified.


– 80 – EN/LZT 123 4734 R3A

If a message with a prohibited combination of OP and DP is received at a policed LS, this message is declined and a TransFer Prohibited (TFA) message is sent to the node that attempted to commit this “infringement”.

SNM policing

This is a support function for STP policing. It limits the use of certain SNM messages:

• On a policed Link Set (LS), the messages TransFer Prohibited / Restricted / Allowed (TFP/TFR/TFA) are sent to a particular node only if the node is a permitted OP and if the destination field in the TFP contains a DP allowed for that OP.

• If a signaling Route Set Test (RST) message is received at an STP on a policed LS, a response is given only when the combination of OPC in the label and DP in the destination field is allowed.

“Infringement” report

Messages that are declined as a result of STP policing are logged for each LS. For SNM policing only signaling Route Set Test (RST) messages are logged.

While logging is active for an LS, all “infringements” are logged in a list. When the list is full the logging stops and an alarm is issued. The logging will resume once the list has been cleared by command.

To get an alarm after a certain number of “infringements”, i.e. before the list is full, it is possible to define two alarm limits for each “infringement log”.

The log list can be printed out at any time by command, or automatically when the logging is deactivated.

4 Signaling in MTP

EN/LZT 123 4734 R3A – 81 –

Enhanced MTP policing

The purpose of Enhanced MTP Policing is to allow traffic to be filtered according to various message criteria and to provide notification of any discarded messages due to this policing.

There are two types of Enhanced MTP Policing:

• STP traffic policing

• SNM traffic policing

STP Traffic Policing

This allows the control of the traffic through an STP to be filtered using the following MSU fields:

• Originating Point (ORIG)

• Destination Point (DEST)

• Service Indicator (SI)

• Link Set (LS)

SNM Traffic Policing

This allows control of the transmission and reception of level 3 SNM messages using the following MSU fields as filters:

• Originating Point (ORIG)

• Destination Point (DEST)

• Header Code (HCODE)

• Concerned Destination (CDEST)

• Link Set (LS)

MTP Accounting

This function allows a telecom administration charge other telecom administrations for using nodes within their network as STPs for routing of MTP messages.

The MTP Accounting function enables the registration, by the


– 82 – EN/LZT 123 4734 R3A

receiving operator, of the number of MTP messages for which payment is to be received. Optionally the number of messages sent to an operator is counted in order to verify the payment which is to be made.

The function supports the Cascade Remuneration Accounting method. This is an accounting method based on the principle that the originating SP pays the operator of the next SP for delivering the messages towards its destination. The operator of the next SP pays the operator of the SP after that node etc.

The accounting data is collected at fixed time periods and is stored in an output file.

4 Signaling in MTP

EN/LZT 123 4734 R3A – 83 –

MTP FUNCTION BLOCKS IN AXE 10

GENERAL

The following function blocks are included in the set of parts CRT 218 002.

Basic function blocks:

• C7CAC2

• C7DR2

• C7DP2

• C7LS2

• C7NSU2

• C7RODA2

• C7SL2

• C7SLDA2

• C7SPDA2

• C7STDA2

• C7LSDA2

• C7ST2

• C7ST2C

• C7MSDA2

Optional function blocks:

• C7CVR2

• C7MG2

• C7PCDD

• C7PCD

• C7PCDDA

• C7PMA2

• C7PMP2

• C7PVC2

• C7PVDA2

• C7SC2


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• C7SNM2

• C7TMA2

• C7TMP2

The basic function blocks provide all essential functions for the MTP of SS No. 7. The optional function blocks provide:

Connection to the Group Switch (GS) in AXE 10

• C7PCDD, C7PCDDA

CCITT7 Performance measurements

• C7PMA2, C7PMP2, C7SC2

CCITT7 Traffic measurements

• C7TMA2, C7TMP2, C7SC2

CCITT7 MTP Policing and policing violation reporting

• C7PVC2, C7PVDA2

CCITT7 Signaling network monitor

• C7SNM2

CCITT7 Signaling network message generator

• C7MG2

CCITT7 MTP Compatibility violation reporting

• C7CVR2

4 Signaling in MTP

EN/LZT 123 4734 R3A – 85 –

BLOCK DESCRIPTION

C7ST2 - CCITT7 Signaling Terminal

This block is implemented in central software (C7ST2U), regional software (C7ST2R), and a microprocessor controlled hardware (C7STD or ST-7) and optionally a MODEM if an analog Signaling Link (SL) is to be used. The block contains functions for:

• Signaling link (level 2) protocol

• Reception and transmission of Message Signal Units (MSUs)

• Maintenance functions for signaling terminals and signaling links

• Operational functions for signaling terminals

• Semipermanent connection of telephony devices through the Group Switch (GS)

C7ST2C - CCITT7 ST Changeable Exchange Adaptation

This block is implemented in three function units: central software (C7ST2U), regional software (C7ST2R) and hardware (RPD HW/RPG HW). The block is adapted in order for the RPD/RPG units to be optimized for connection to the GSS. The block has the same functions as the block C7ST2.

C7LS2 - CCITT7 Link Set Management

This block is implemented in central software and contains functions for reconfiguration of the signaling network on the Link Set (LS) level in order to maintain the service capability. The main functions are:

• Changeover procedure

• Changeback procedure

• Distribution of signaling network management messages


– 86 – EN/LZT 123 4734 R3A

C7DP2 - CCITT7 Destination Point Management

This block is implemented in central software and contains functions for reconfiguration of the signaling network on the destination point and signaling route level in order to maintain the service capability. The main functions are:

• Load-sharing and priority control

• Time-controlled diversion procedure

• Forced rerouting procedure

• Controlled rerouting procedure

• Distribution of destination status

C7DR2 - CCITT7 Discrimination, Distribution and Routing

This block is implemented in central software and contains the following functions:

• Message discrimination, message distribution and message routing.

• Level 3 network flow control, on destination basis.

• Message buffering on the destination and signaling link level and queuing of retrieved messages at reconfiguration.

C7SL2 - CCITT7 Signaling Link Management

This block is implemented in central software and contains functions for the operation of signaling links and functions for signaling link tests.

C7NSU2 - CCITT7 Network Supervision

This block is implemented in central software and contains the following functions:

• Link Set (LS) redundancy supervision

• Destination accessibility supervision

4 Signaling in MTP

EN/LZT 123 4734 R3A – 87 –

C7LSDA2 - CCITT7 Link Set Data Administration

This block is implemented in central software and contains the following command functions:

• Definition of Link Set (LS)

• Definition of associated adjacent SP

• Definition of signaling links in a Link Set (LS)

C7SPDA2 - CCITT7 Signaling Point Data Administration

This block is implemented in central software and contains command functions for definition of signaling points in a network.

C7RODA2 - CCITT7 MTP Routing Data Administration

This block is implemented in central software and contains command functions for definition and activation of signaling routes and alternative signaling routes.

C7SLDA2 - CCITT7 Signaling Link Data Administration

This block is implemented in central software and contains command functions for activation/deactivation of signaling links, inhibiting/uninhibiting of signaling links and the setting of signaling link parameters.

C7STDA2 - CCITT7 Signaling Terminal Data Administration

This block is implemented in central software and contains command functions for definition of signaling terminal, definition of terminal interface type and initiation of signaling terminal tests.

C7MSDA2 - CCITT7 Measurement and Statistics Data Administration

This block is implemented in central software and contains command functions for initiating and ending of counters, activation and deactivation of counters, and printing of counter specification data.


– 88 – EN/LZT 123 4734 R3A

C7CAC2 - CCITT7 Command Access Control

This block is implemented in central software and contains functions for access control of data loading and modification commands.

C7CVR2 - CCITT7 Compatibility Violation Reporting

This block is implemented in central software and provides the recording of discarded long messages and confusion messages and issues alarms when certain limits are exceeded. It also handles commands for initiating and ending the recording of messages, printing violation reports and to reset alarms.

C7PCDD - CCITT7, Pulse Code Modulation Multiplexing Digital Device

This block is implemented in hardware (PCD-D) and central software (C7PCDDU).

• C7PCDD provides a 64 kbps inlet in the Group Switch (GS) for the C7ST2. This inlet is used for establishing a signaling data link as a semipermanent connection through the Group Switch (GS). Multiple C7PCDD blocks can exist.

C7PCD - CCITT7, Pulse Code Modulation Multiplexing Device

This block is implemented in hardware (PCD) and central software (C7PCDDU).

• C7PCD provides an analog inlet in the Group Switch (GS) for the analog C7ST2. This inlet is used for establishing a signaling data link as a semipermanent connection through the Group Switch (GS). Multiple C7PCD blocks can exist.

C7PCDDA - CCITT7, Pulse Code Modulation Multiplexing Digital Device Administration

This block is implemented in central software and contains command functions for the connection and disconnection of signaling terminals (C7ST2) to the C7PCDD or C7PCD.

C7PMA2 - CCITT7 Signaling Performance Measurement Administration

This block is implemented in central software. It contains functions for the definition of performance measurements and connecting them to time schedules.

4 Signaling in MTP

EN/LZT 123 4734 R3A – 89 –

C7PMP2 - CCITT7 Signaling Performance Measurement Printouts

This block is implemented in central software. It contains functions for printing the performance measurement results.

C7PVC2 - CCITT7 MTP Policing Violation Control

This block is implemented in central software and contains functions for restricting unauthorized use of a node as an STP. By interworking with the traffic handling blocks, it starts and stops policing. It contains functions for violation reporting and alarm handling.

C7PVDA2 - CCITT7 Policing Violations Data Administration

This block is implemented in central software and contains command functions for the specification, initiation and ending of policing. The data specified, may be printed out by command.

C7SNM2 - CCITT7 Signaling Network Monitor

This block is implemented in central software and contains functions for monitoring of the state of Link Sets (LSs) , signaling links and destinations. It also monitors the signaling network management messages related to the above.

C7MG2 - CCITT7 Message Generator

This block is implemented in central software and contains functions for generating Message Signal Units (MSUs)

C7TMA2 - CCITT7 Traffic Measurement Administration

This block is implemented in central software. It contains functions for definition of traffic measurements and connecting them to time schedules.

C7TMP2 - CCITT7 Traffic Measurement Printouts

This block is implemented in central software. It contains functions for printing the traffic measurement results.


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C7SC2 - CCITT7 Scanning of Recording Individuals

This block is implemented in central software. It contains functions for periodic scanning of recorded individuals, defined by measurement functions.

C7EPC2 – CCITT7 Enhanced MTP Policing Control

This block is implemented in central software. It contains the functions for the control and processing of the enhanced policing functionality.

C7EPDA2 – CCITT7 Enhanced MTP Policing Administration

This block is implemented in central software. It contains command functions for the administration of specification, initiation and ending of enhanced policing.

C7SNT – CCITT7 Switching Network Terminal

This block is implemented in central software and is the SNT owner for the Signaling Terminal in the RPD or the RPG. The main functions are:

• Operational functions for switching network terminal

• Connection / disconnection of SNT to / from GS

• Supervision and test of TRHB hardware in RPD or RPG

4 Signaling in MTP

EN/LZT 123 4734 R3A – 91 –

MTP HARDWARE

INTRODUCTION

The Common Channel Signaling (CCS) Subsystem contains the Signaling Terminal (ST) hardware units, which is the termination of the signaling link. The Signaling Terminal (ST) is connected to the Group Switch (GS) either via multiplexer (PCD-D) or directly if RPD/RPG is used as a ST. The following connections will be described:

• RP – ST – PCDD – GSS

• RPD/RPG – GSS

CONNECTION “RP – ST – PCDD – GSS”

The Signaling Terminals (STs) are controlled by Regional Processors (RPs). Each pair of RPs controls up to four STs.

PCD-D which is connected between the ST and the Group Switch Subsystem (GSS) as an interface to the Time Switch Module (TSM). PCD-D is the unit for rate adaptation. The 64 kbps that come from the Signaling Terminals (STs) are put together into one 2 Mbps PCM line. This PCM line is connected to the Group Switch (GS).

31- -

SignalingChannel

- -0

0

1

ETC

PCD-D

GSS

ST-7

ST-7

ST-7

...

CCITT No. 7 in AXE 10 Hardware Structure

7

Figure 4-25 Signaling Data Link in AXE10


– 92 – EN/LZT 123 4734 R3A

The PCD-D is not controlled by any RP, because the device only contains circuits for rate adaptation. However, the device is supervised by the Group Switch (GS).

ST hardware

The Signaling Terminal (ST) is housed in a magazine with three circuit boards, with each magazine being one EM. There are three different magazines for the different interface types.

Each C7STD contains a power unit board (POU), a control board (ST7C) and an interface board (ST7I).

The ST7C board (Control Unit for Signaling Terminal) provides an interface between the RP and the Link Interface cards.

The ST7I board is used to provide an interface for the signaling links.

The C7STD magazines are controlled by the Regional Processors (RPs). One RP-pair can control two magazines, C7STD.

Figure 4-26 shows the layout of the magazine.

C7STD

12 17

ST7

C

ST7

I

PO

U +

-12V

/5 V

Figure 4-26 Signaling Terminal (ST) - C7STD

Connection of the Signaling Terminals (STs) to the PCD-Ds is arranged so that STs with even numbers are connected to the even-numbered PCD-D, and similarly for odd numbers.

4 Signaling in MTP

EN/LZT 123 4734 R3A – 93 –

Figure 4-27 shows how the signaling terminals C7STD with their Regional Processors (RPs) and the PCD-Ds are placed in a magazine group.

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Figure 4-27 CSS - Cabinet with fully equipped magazine modules from A to F


– 94 – EN/LZT 123 4734 R3A

CONNECTION “RPD/RPG – GSS”

Abbreviations

RP – Regional Processor

RPD – Regional Processor Device

RPG – Regional Processor with Group Switch (GS) interface

RPG1 – RPG with parallel RP-bus

RPG2 – RPG with serial RP-bus

TRHB – TRansceiver Handler Board

EM – Extension Module

SNT – Switching Network Terminal

MTP – Message Transfer Part

DL2 – Digital Link version 2

RPG Signaling Terminal

An RPG is a Regional Processor with Group switch interface. To connect a device to a Group Switch (GS) it is associated with a Switching Network Terminal (SNT), which is a unit connected to a digital switch.

The RPG hardware concept is a complete APZ product. See figure 4-28.

4 Signaling in MTP

EN/LZT 123 4734 R3A – 95 –

R P G D L 2t o G S

A B

R P - B U S

Figure 4-28 RPG hardware concept

The RPG consists of one RP and one to four Signaling Terminals (STs). The ST in one RPG represents individual devices controlled by one Extension Module (EM). The RPG board acts as an SNT and contains a multiplexing function and software support for the MTP level 2. The number of Signaling Links (SLs) per RPG is specified in the exchange data.

Two versions of the RPG exists, the RPG1 with parallel RP-bus and the RPG2 with serial RP-bus. The RPG signaling terminal is always used with a DL2 interface at 2 Mbps.

RPD Signaling Terminal

An RPD is a Regional Processor possible to integrate with Device hardware. To connect a device to a Group Switch (GS) it is associated with a Switching Network Terminal (SNT), which is a unit connected to a digital switch. This RPD hardware concept includes an RPD and a TRansceiver Handler Board (TRHB) integrated into one hardware unit. See figure 4-29.


– 96 – EN/LZT 123 4734 R3A

R P Dt o G S

A B

R P - B U S

TRHB

Figure 4-29 RPD hardware concept

The RPD consists of one RP and one to four Signaling Terminals (STs). The ST in one RPD represent individual devices controlled by one Extension Module (EM). The TRHB board acts as an SNT and contains a multiplexing function and hardware support for the MTP level 2. The semi-permanent connection still needs to be defined. The number of Signaling Links (SLs) per RPD is specified in the exchange data.

The RPD signaling terminal is always used with a Group Switch (GS) but can co-exist with other existing standard ST configurations. The RPD signaling terminal supports the White Book MTP fully and is backwards compatible with the Red and Blue Book MTP.

4 Signaling in MTP

EN/LZT 123 4734 R3A – 97 –

MTP SET-UP IN AXE 10

GENERAL

In earlier sections different entities and parameters used in the signaling network and especially in MTP were mentioned. The way these elements are implemented in AXE 10 Exchange data is described in this section.

An effort is made to mention most of the important commands. For further or more detailed information please refer to the OPI. The section starts with the definition of the Signaling Point (SP) itself and its Identity for being the home Exchange. Neighboring nodes are defined in a second step.

Signaling Links (SLs) and PCD-D are defined and assigned to the Link Set (LS). Traffic Routes and Signaling Routes are defined, an SLS is allocated to each cooperating SP. The Signaling channel(s) are linked to a Signaling Route. The Signaling Time Slot is semi-permanently connected to a Signaling Terminal (ST).

Finally the Exchange Data of two neighboring nodes is compared.

SET-UP IN AXE 10

It is required to set-up an MTP signaling system between a home Exchange and a cooperating Exchange in the network, figure 4-30

o w n E x c h a n g e Exch . 12 - 1 02 - 3 0 0

S P “ O W N 3 0 0 ” S P “ D P C 1 0 ”

L S = 2 - 1 0

s l c o d e = 0

Figure 4-30 Signaling Links (SLs) and Signaling Points (SPs)

First of all the Signaling Points (SP) must be defined. The home Exchange is the Signaling Point (SP) 2-300. Where “2” is the network indicator for the national network (0 is used for international networks), “300” means the Origination Point Code (OPC) of the Exchange. In a cooperating node this will be the Destination Point Code (DPC).

The SP nodes are also given a symbolic name for easy reference called a Signaling Point IDentification string (SPID). The SPID for


– 98 – EN/LZT 123 4734 R3A

the home Exchange in our example is “OWN300” and that for the neighboring Exchange is ”DPC10”.

Note if there are several neighboring nodes connected to the home Exchange then each one of them should be defined in the above manner. The next step is to define an ST and a PCD-D. Note that the ST is connected to a GS through a PCD-D.

Signaling Channel

C7ST2-064 kbpsPCD-D

ETC

GSS

Channel 31 is semipermanentlyconnected to C7ST2

PCM

Dev = C7PCDD-0

0 16 31

Figure 4-31 Definition of ST and PCD-Ds

When all ST and PCD-D are defined it is required to define the signaling Link Sets (LSs).

The next step is to link a Signaling Link (SL) SLC=0 and a Signaling Terminal (ST) ST2-0 to a defined Link Set (LS).

A certain destination may possess several alternative signaling routes. It is possible to assign a special priority to a route or to assign the same priority to all of them, so the load will be shared (Load Sharing).

The MTP routing specification must be done. The traffic route and the signaling route definition is required.

4 Signaling in MTP

EN/LZT 123 4734 R3A – 99 –

Signaling

C7ST2

ETC

GSS

UPD33

Dev = C7PCDD-0

TrafficRoute Route

SEM

I0

PCD-D

PCM

0 16 31

Figure 4-30 Signaling/Traffic Routes and Semi-permanent connection Definition

The device type assigned to this traffic route is UPD33. The Signaling Point (SP) 2-10 is selected for this route. Note that the route name in the cooperating Exchange and the home Exchange may be the same, because these Exchanges are different entities.

In the case of the signaling route a both way trunk with a device type UPD33 is defined.

The next step is to allocate the devices to the defined traffic- and signaling routes.

Finally the time slot 16 is semi-permanently connected to the C7PCDD-0 device. The semipermanent connection must be defined and activated.


– 100 – EN/LZT 123 4734 R3A

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04- Signaling in MTP

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