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Technical Manual – Signaling & ProtocolsTable of Contents
Table of Contents
Chapter 3 MTP and MTP3B.........................................................................................................3-1
3.1 MTP................................................................................................................................. 3-1
3.1.1 Overview...............................................................................................................3-1
3.1.2 MTP3 Functions....................................................................................................3-2
3.1.3 Message Format....................................................................................................3-4
3.1.4 Signaling Procedures..........................................................................................3-15
3.2 MTP3B........................................................................................................................... 3-17
3.2.1 Overview.............................................................................................................3-17
3.2.2 Introduction of MTP3B.........................................................................................3-18
3.2.3 MTP3B Message Structure.................................................................................3-20
3.3 SAAL.............................................................................................................................. 3-22
3.3.1 SAAL Function Structure.....................................................................................3-22
3.3.2 SSCOP................................................................................................................3-23
3.3.3 SSCF................................................................................................................... 3-28
3.3.4 LM....................................................................................................................... 3-29
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Technical Manual – Signaling & ProtocolsChapter 3 MTP and MTP3B
Chapter 3 MTP and MTP3B
3.1 MTP
3.1.1 Overview
Narrowband Message Transfer Part (MTP) is the traditional TDM based transmission
system. Its major function is to enable reliable transmission of signaling messages
over signaling network, and to take measures to avoid or minimize message loss,
duplication or mis-sequencing in case of system fault or signaling network fault. The
functions of the MTP are separated into three functional levels: signaling data link
(MTP1), signaling link functions (MTP2) and signaling network functions (MTP3). The
structure of the MTP protocol stack is illustrated in Figure 3-1.
Figure 3-1 Structure of the MTP protocol stack
The MTP in the signaling-processing module of MSC and HLR is used to convey SS7
user signaling (ISUP/SCCP). It is designed completely in compliance with the ITU-T
Recommendations Q.701 to Q.710 Series.
I. MTP1
Signaling data link is the level 1 function (MTP1) of the MTP. It defines the physical,
electrical and functional characteristics of a signaling data link and the means to
access it. It is equivalent to the physical layer of the OSI reference model and is used
to generate and receive the signals through the physical channels.
A signaling data link is a bidirectional transmission path for signaling, comprising two
data channels operating together in opposite directions at the same data rate. The
standard bit rate on a digital bearer is 64kbit/s. A transmission link at a lower bit rate
(for example, 4.8kbit/s) or at a higher bit rate (for example, 2048kbit/s) may also be
applied.
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Technical Manual – Signaling & ProtocolsChapter 3 MTP and MTP3B
II. MTP2
Signaling link functions are the level 2 functions (MTP2) of the MTP. They are used to
transfer signaling to a data link. The level 2 functions together with a level 1 signaling
data link provide a signaling link for reliable signaling transfer between two directly
associated signaling points.
The signaling link functions include signal unit delimitation, signal unit alignment, error
detection, error correction, initial alignment, processor outage, level 2 flow control and
signaling link error monitoring.
III. MTP3
Signaling network functions are the level 3 functions (MTP3) of the MTP. They
implement the functions of the network layer of the OSI reference model, and are
used to enable management message transmission between the signaling points for
the purpose of ensuring a reliable transfer of the signaling messages over the
signaling network in case that signaling links and signaling transfer points fail.
3.1.2 MTP3 Functions
The signaling network functions provided by the MTP3 must ensure a reliable transfer
of the signaling messages even in the case of the failure of signaling links and
signaling transfer points. Therefore, they include the appropriate functions and
procedures necessary both to inform the remote parts of the signaling network of the
consequences of a fault, and to appropriately reconfigure the routing of messages
through the signaling network
The signaling network functions are divided into two basic categories, namely
signaling message handling and signaling network management. See Figure 3-2.
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Technical Manual – Signaling & ProtocolsChapter 3 MTP and MTP3B
Figure 3-2 Signaling network functions
I. Signaling Message Handling
The purpose of the signaling message handling functions is to ensure that the
signaling messages originated by a particular User Part at a signaling point
(originating point) are delivered to the same User Part at the destination point
indicated by the sending User Part.
The signaling message handling functions are divided into:
the message routing function, used at each signaling point to determine the
outgoing signaling link on which a message has to be sent towards its
destination point;
the message discrimination function, used at a signaling point to determine
whether or not a received message is destined to the point itself. When the
signaling point has the transfer capability and a message is not destined to it,
that message is transferred to the message routing function;
the message distribution function, used at each signaling point to deliver the
received messages (destined to the point itself) to the appropriate User Part.
I. Signaling Network Management
The purpose of the signaling network management functions is to provide
reconfiguration of the signaling network in the case of failures and to control traffic in
case of congestion. Such a reconfiguration is effected by use of appropriate
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Technical Manual – Signaling & ProtocolsChapter 3 MTP and MTP3B
procedures to change the routing of signaling traffic in order to bypass the faulty links
or signaling points. Moreover, in some circumstances it is necessary to activate and
align new signaling links, in order to restore the required signaling traffic capacity
between two signaling points. When the faulty link or signaling point is restored, the
opposite actions and procedures take place, in order to reestablish the normal
configuration of the signaling network.
The signaling network management functions are divided into:
Signaling traffic management
Signaling link management
Signaling route management
These three signaling network management functions are activated in the appropriate
circumstances when some change occurs to the state of a signaling link, route or
signaling point. The details are described as follows:
2) Signaling traffic management function: This function is used for the diversion of
signaling traffic from one link or route to one or more alternative link or route,
used for MTP restart of signaling points, or used to temporarily slow down
signaling traffic in the case of congestion at signaling points.
3) Signaling link management function: This function is used to restore a faulty
signaling link, activate an idle (unaligned) link, and deactivate an aligned
signaling link.
4) Signaling route management function: This function is used to distribute the
information about the signaling network status with the objective of blocking or
unblocking a signaling route.
3.1.3 Message Format
For the purpose of meeting the requirements of the MTP for transmitting a variety of
signaling messages, three basic types of signal unit are defined: Message Signal Unit
(MSU), Link Status Signal Unit (LSSU), and Fill-In Signal Unit (FISU).
Message signal units are used to carry messages of the user parts, signaling
network management messages, and signaling network testing and maintenance
messages.
Link status signal units provide the information about the link status in order to
perform control actions such as connection and restoration on the signaling link.
Under normal conditions, when no message signal units or link status signal
units are to be transmitted over the signaling links, fill-in signal units are sent
continuously with the feeding objective, for the purpose of maintaining the normal
operation of the signaling links.
The structure of the signal units is illustrated in Figure 3-1.
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Technical Manual – Signaling & ProtocolsChapter 3 MTP and MTP3B
MSU F CK SIF SIO LI FIB FSN BIB BSN F
8 16 8N(N≥2) 8 2 6 1 7 1 7 8
Basic format of a message signal unit (MSU)
First bittransmitted
LSSU F CK SF LI FIB FSN BIB BSN F
8 16 8 or 16 2 6 1 7 1 7 8
Format of a link status signal unit (LSSU)
First bittransmitted
FISU F CK LI FIB FSN BIB BSN F
8 16 2 6 1 7 1 7 8
Format of a fill-in signal unit (FISU)
First bittransmitted
MSU F CK SIF SIO LI FIB FSN BIB BSN F
8 16 8N(N≥2) 8 2 6 1 7 1 7 8
Basic format of a message signal unit (MSU)
First bittransmitted
MSU F CK SIF SIO LI FIB FSN BIB BSN F
8 16 8N(N≥2) 8 2 6 1 7 1 7 8
Basic format of a message signal unit (MSU)
First bittransmitted
LSSU F CK SF LI FIB FSN BIB BSN F
8 16 8 or 16 2 6 1 7 1 7 8
Format of a link status signal unit (LSSU)
First bittransmitted
LSSU F CK SF LI FIB FSN BIB BSN F
8 16 8 or 16 2 6 1 7 1 7 8
Format of a link status signal unit (LSSU)
First bittransmitted
FISU F CK LI FIB FSN BIB BSN F
8 16 2 6 1 7 1 7 8
Format of a fill-in signal unit (FISU)
First bittransmitted
FISU F CK LI FIB FSN BIB BSN F
8 16 2 6 1 7 1 7 8
Format of a fill-in signal unit (FISU)
First bittransmitted
Figure 3-1 Format of the signal units
A signal unit is divided into two parts from the structure point of view. One is shared
by the variety of signal units and required by the MTP processing; this part comprises
8 fixed length fields. The other contains the signaling information to be handled by the
user part.
I. The Part Required by the MTP Processing
This part includes Flag (F), Forward Sequence Number (FSN), Forward Indicator Bit
(FIB), Backward Sequence Number (BSN), Backward Indicator Bit (BIB), Length
Indicator (LI), Check bits (CK), Status Field (SF), and Service Information Octet (SIO)
(SIO only exists in message signal units).
Flag (F)
There is a flag at the start and the end of every signal unit. In the transmission of
signal units, the opening flag of a signal unit is normally the closing flag of the
preceding signal unit. Therefore, a signal unit will be delimitated once the opening
and closing flags are successfully found from the information stream.
The bit pattern for the flag is 01111110.
In addition to signal unit delimitation, several flags may be inserted between signal
units, in case that the signaling links are overloaded, in order to cancel controlling and
reduce loading.
Forward sequence number (FSN)
The forward sequence number is the 7-bit sequence number of the message signal
unit in which it is carried. At the transmitting terminal, all the transmitted message
signal units are allocated with a forward sequence number which is numbered from a
cyclic sequence ranging from 0 to 127. At the receiving terminal, the forward
sequence numbers of the received message signal units are used to detect the order
of the message signal units, as a part of the acknowledgement function. If
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Technical Manual – Signaling & ProtocolsChapter 3 MTP and MTP3B
retransmission is required, the forward sequence number also serves to identify the
signal unit to be retransmitted. A fill-in signal unit and a link status signal unit share
the forward sequence number of the last transmitted message signal unit instead of
being assigned again.
Forward indicator bit (FIB)
One bit is occupied. The forward indicator bit is used in the retransmission procedure
of message signal units. It has the same status as the received backward indicator bit
during non-error operation. A change made to the value of the received backward
indicator bit indicates a request for retransmission. The signaling terminal also
changes the value of the forward indicator bit (changing 1 to 0 or 0 to 1) when
retransmitting the message signal unit, in order to keep consistent with the backward
indicator bit value, until the value of the backward indicator bit changes at receiving
another retransmission request.
Backward sequence number (BSN)
The backward sequence number is the sequence number of a message signal unit
being acknowledged. It is sent by the receiving terminal to indicate to the transmitting
terminal that the message signal unit is acknowledged (accepted successfully).
In the case of a request for a retransmission, the backward sequence number
indicates the sequence number for starting the retransmission.
In the operation of the signaling network, the transmitting terminal and the receiving
terminal of a message independent assign the forward sequence number.
Limited by the forward sequence number and the backward sequence number, not
more than 127 signal units can be transmitted while not be acknowledged.
Backward indicator bit (BIB)
The backward indicator bit provides a retransmission request for the received error
signal unit. If the received message signal unit is correct its value will be invariable
when a new signal unit is sent; otherwise this bit will be sent with a conversed value
(that is, 0 is changed to 1 or 1 is changed to 0), requesting the terminal peer to
retransmit the error message signal unit.
Length indicator (LI)
The length indicator is used to indicate the number of octets following the length
indicator octet and preceding the check bits. The length indicator differentiates
between the three types of signal units.
The 6-bit length indicator field is a number in binary code in the range 0~63 (decimal).
The length indicator values of the three types of signal units are as follows:
Length indicator = 0 Fill-in signal unit
Length indicator = 1 or 2 Link status signal unit
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Technical Manual – Signaling & ProtocolsChapter 3 MTP and MTP3B
Length indicator > 2 Message signal unit
In the national signaling network, the length indicator is invariably set to 63 if the
signaling information field of a message signal unit is more than 62 octets. In the case
that the length indicator equals 63, the maximum length indicated by it cannot be
more than 272 octets.
Note that it is necessary to calculate the number of bits and the number of octets
between two flags in the receiving process of signal units. According to the CCITT, the
number of bits between two signal unit flags must be an integral multiple of 8. The
number of octets may be equal to 0 (if only flags are sent), be equal to 5 (fill-in signal
unit), or be less than or equal to m+7 (m is 272). For a number out of such range, the
signal unit is treated as error.
Check bits (CK)
The check bits field is used for error detection of a signal unit. It is composed of 16
bits.
The seven fields described above appear in all the three types of signal units. (Eight
such fields are mentioned in the previous section, where the closing flag is included.)
They are mandatory to every signal unit.
Status field (SF)
The status field is unique to link status signal units and is used to indicate the status
of a signal link.
The length of the status field may be one octet (8 bits) or two octets (16 bits).
If the status field is one octet, the link status is indicated by the lower three bits
currently, as shown in Table 3-1:
Table 3-1 Meanings of the link status indications in the status field
CBA Bits Identifier Indication Meaning
000 SIO Status indication “O” Out of alignment
001 SIN Status indication “N” Normal alignment
010 SIE Status indication “E” Emergency alignment
011 SIOS Status indication “OS” Out of service
100 SIPO Status indication “PO” Processor outage
101 ISB Status indication “B” Busy (link congestion)
Service information octet (SIO)
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Technical Manual – Signaling & ProtocolsChapter 3 MTP and MTP3B
The service information octet field is unique to the message signal units. It contains
the service indicator (SI) and the sub-service field (SSF), as shown in Figure 3-2.
The field has 8 bits. The service indicator and the sub-service field occupy 4
respectively.
SISSF
MeaningDCBA
International networkSpare (for international use only)National networkReserved for national use
0 0 0 00 1 0 01 0 0 01 1 0 0
MeaningDCBA
Signaling network management messagesSignaling network testing and maintenance messagesSpareSCCPTelephone User PartISDN User Part
Data User Part
Spare
0 0 0 00 0 0 10 0 1 00 0 1 10 1 0 00 1 0 10 1 1 00 1 1 11 0 0 0
1 1 1 1
F CK SIF SIO
SISSF
MeaningDCBA
International networkSpare (for international use only)National networkReserved for national use
0 0 0 00 1 0 01 0 0 01 1 0 0
MeaningDCBA
Signaling network management messagesSignaling network testing and maintenance messagesSpareSCCPTelephone User PartISDN User Part
Data User Part
Spare
0 0 0 00 0 0 10 0 1 00 0 1 10 1 0 00 1 0 10 1 1 00 1 1 11 0 0 0
1 1 1 1
MeaningDCBA
Signaling network management messagesSignaling network testing and maintenance messagesSpareSCCPTelephone User PartISDN User Part
Data User Part
Spare
0 0 0 00 0 0 10 0 1 00 0 1 10 1 0 00 1 0 10 1 1 00 1 1 11 0 0 0
1 1 1 1
F CK SIF SIO
Figure 3-2 Format and codes of the service information octet
1) Service indicator (SI)
The service indicator is used to indicate the particular user part associated with the
transmitted message. In the message transfer part of the signaling network, the
message handling functions will base the service indicator to distribute the message
to the specified user part.
The code allocation for the service indicator is shown in Figure 3-2. The service
indicator capacity is enough to indicate 16 types of user part messages. Several
common types are listed in the figure.
2) Sub-service field (SSF)
It is composed of 4 bits. The higher two bits are the network indicator; the lower two
are currently spare bits, coded 00.
The network indicator is used to identify the nature of the network where the message
is transferred, that is,, it is an international or national signaling network message.
The code allocation of the sub-service field is shown in Figure 3-2.
According to the CCITT, the spare bits in the sub-service field may be used as per the
national signaling network. For example, the network indicator may be set to 10 or 11
to indicate the local signaling network or toll signaling network in case of 14-bit
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Technical Manual – Signaling & ProtocolsChapter 3 MTP and MTP3B
signaling point encoding scheme. If the unified 24-bit encoding scheme is utilized, the
network indicator is set to 10 or 11 in order to identify the unified 24-bit encoding
scheme or temporary 24-bit encoding scheme (where ten “0” at the higher bit is
added/removed).
I. The Signaling Information Part Processed by a User Part
The signaling information part processed by the user parts is the signaling information
field (SIF) in the message signal unit format. The signaling information field only
exists in a message signal unit. It consists of three parts: the label for message
addressing, the heading code of the user signaling information, and the user signaling
information.
Label
The label contains the information necessary to deliver the message to its the
destination point. The standard routing label has a length of 32 bits and is placed at
the beginning of the signaling information field. The label includes the destination
point code (DPC), the originating point code (OPC) and the signaling link selection
(SLS) field.
A signaling point code is a numeric address, uniquely identifying one signaling point in
the SS7 network. When the destination point code contained in the message
indicates the receiving signaling point, the message is distributed to the
corresponding user part (such as ISUP or SCCP) identified by the service indicator in
the service information octet.
The signaling link selection is used in the following cases:
3) In ensuring message sequencing. Any two transmitted messages with the same
signaling link selection will normally arrive at the destination in the order in which
they were first transmitted.
4) In performing average load sharing of the stream between all available links. If a
certain user part periodically transmits messages and the signaling link selection
value is assigned in the cyclic manner, all the traffic to the destination has the
same traffic level.
The label structure determines four types of label as shown in Figure 3-1:
Type A MTP management messages
Type B TUP messages
Type C ISUP messages
Type D SCCP messages
As TCAP messages have to be transferred by the SCCP, the TCAP messages are
classified as SCCP messages, that is, type D.
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Technical Manual – Signaling & ProtocolsChapter 3 MTP and MTP3B
F CK SIF FIBLI FSNSIO BIB BSN
Management message SLC OPC DPC Type A: MTP management messages
Signaling messageCIC
OPC DPC Type B: TUP messagesSLS
Signaling message OPC DPC Type C: ISUP messagesCIC SLC
F
SCCP user data SLS OPC DPC Type D: SCCP messages
F CK SIF FIBLI FSNSIO BIB BSN
Management message SLC OPC DPC Type A: MTP management messages
Signaling messageCIC
OPC DPC Type B: TUP messagesSLS
Signaling message OPC DPC Type C: ISUP messagesCIC SLC
F
SCCP user data SLS OPC DPC Type D: SCCP messages
Figure 3-1 Label structure of the four types
Heading code
The heading code is a field following the label It is composed of the 4-bit heading
code H0 and the 4-bit heading code H1, and identifies the message group and the
message type. For instance, in a TUP message, the heading code H0 coded 0001
and the heading code H1 coded 0001 indicate an Initial Address Message (IAM); the
heading code H0 coded 0001 and the heading code H1 coded 0100 indicate an
Address Complete Message (ACM). Another example is about a signaling network
management message. The H0 coded 0001 and the H1 coded 0001 indicate a
Changeover-order signal (COO); the H0 coded 0001 and the H1 coded 0100 indicate
a Transfer-prohibited signal. As both the H0 and the H1 occupy 4 bits, the maximum
capacity of a class of user messages is 256.
Signaling information
The signaling information part is also named service information part. This part is
further divided into several sub-fields. These sub-fields may be mandatory or optional
with fixed length or variable length in order to meet the requirements of various
functions and supplements, which makes it possible for message signal units to be
suitable for a variety of user messages and also makes it possible for the variety of
user message to be conveyed through common signaling channels.
For the format and encoding of the service information field, please reference the
user messages.
II. MTP Messages
In a signal unit, the flag, the backward sequence number, the backward indicator bit,
the forward sequence number, the forward indicator bit, the length indicator and the
check bits are mainly used for transmission, receiving sequence, error detection and
correction of the message signal unit. These fields are all analyzed and handled at
the second functional level of the signaling network, that is, the signaling link level.
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Technical Manual – Signaling & ProtocolsChapter 3 MTP and MTP3B
Fill-in signal units are used for “feeding” purpose on a signaling link and composed of
several fields that mainly involve transmission control. Fill-in signal units are
generated and handled by the level 2 functions.
Link status signal units are used to carry the status indication information of a
signaling link. They are also generated and handled at the functional level 2. The
functional level 2 may base both the related indication from the level 3 and the
judgment of itself to generate a corresponding status signal unit and transmit it out;
the functional level 2 may also accept the status indication of the signaling link from
the peer and process it. If necessary, the information relating to congestion and
processor outage will be reported to the level 3.
Message signal units are divided into three classes according to their role in the
signaling network: the message signal units used for signaling network management
(MSU-SNM), the message signal units used for signaling network testing and
maintenance (MSU-SNT), and the message signal units generated by user parts
(MSU-UP). The first two classes utilize the type A label structure and are transmitted
between the MTPs. They are generated at the functional level 3 of the signaling
network and also processed at the level 3. The third class includes the messages of
type B, C and D label structure. Through the MTP, these messages are delivered to a
particular user part. The level 3 functions of the signaling network are responsible for
analyzing the label contained in the message to determine where the message will be
distributed. The generating and handling of the signaling information part (service
information part) is implemented by the functional level 4, that is, the user parts.
The signaling network management messages are critical to the MTP, and described
in details in the following section.
General format for the signaling network management messages
In the signaling network, the signaling network management messages are
distinguished by the configuration 0000 of the service indicator (SI) contained in the
service information octet in the signal unit.
As a type of message signal unit, the signaling information of a signaling network
management message is carried by the service information field. It structure is
illustrated in Figure 3-2.
Figure 3-2 General format for the signaling network management messages
Label
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Technical Manual – Signaling & ProtocolsChapter 3 MTP and MTP3B
It comprises the destination point code (DPC), the originating point code (OPC) and
the signaling link code (SLC).
The destination point code and the originating point code are described the same as
the preceding section.
The signaling link code indicates the signaling link interconnecting the destination and
originating points. If the message is not related to a signaling link, or another
particular code is not specified, it is coded 0000. Currently 4 bits are used. The spare
4 bits are coded 0000.
Heading code
The heading codes include the 4-bit heading code H0 and the 4-bit heading code H1.
The heading code H0 identifies the management message group. The heading code
H1 determines the specific message from the message group. As both the H0 and the
H1 occupy 4 bits, the message capacity reaches 256 types. That is, there are 16
message groups and 16 message types in each group are available. Now not all of
them are used. See Table 3-1.
Table 3-1 Heading code allocation of signaling network management messages
Messa
ge
Group
H1
H0
000
0
000
10010
001
1
010
0
010
1
011
0
011
1
100
0
100
1
101
0
101
1
110
0
110
1
111
0
111
1
000
0
CHM000
1
CO
OCOA
CB
D
CB
A
ECM001
0
EC
OECA
FCM001
1RCT TFC
TFM010
0TFP * TFR TFA *
RSM010
1RST RSR
MIM011
0LIN LUN LIA LUA LID LFU LLT LRT
TRM011
1TRA
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Technical Manual – Signaling & ProtocolsChapter 3 MTP and MTP3B
Messa
ge
Group
H1
H0
000
0
000
10010
001
1
010
0
010
1
011
0
011
1
100
0
100
1
101
0
101
1
110
0
110
1
111
0
111
1
DLM100
0DLC CSS
CN
S
CN
P
100
1
UFC101
0UPU
101
1
110
0
110
1
111
0
111
1
The meaning of the signaling network management messages is listed in Table 3-2.
Table 3-2 Signaling network management messages
Message Full name
CHM Changeover and changeback messages
COO Changeover-order signal
COA Changeover-acknowledgement signal
CBD Changeback-declaration signal
CBA Changeback-acknowledgement signal
ECM Emergency-changeover message
ECO Emergency-changeover-order signal
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Technical Manual – Signaling & ProtocolsChapter 3 MTP and MTP3B
Message Full name
ECA Emergency-changeover-acknowledgement signal
FCM Signaling-traffic-flow-control messages
RCT Signaling-route-set-congestion-test signal
TFC Transfer-controlled signal
TFP Transfer-prohibited signal
TFR Transfer-restricted signal (national option)
TFA Transfer-allowed signal
RSM Signaling-route-set-test message
RST Signaling-route-set-test signal for prohibited destination
RSRSignaling-route-set-test signal for restricted destination
(national option)
MIM Management inhibit messages
LIN Link inhibit signal
LUN Link uninhibit signal
LIA Link inhibit acknowledgement signal
LUA Link uninhibit acknowledgement signal
LID Link inhibit denied signal
LFU Link forced uninhibit signal
LLT Link local inhibit test signal
LRT Link remote inhibit test signal
TRM Traffic-restart-allowed message
TRA Traffic-restart-allowed signal
DLM Signaling-data-link-connection-order message
DLC Signaling-data-link-connection-order signal
CSS Connection-successful signal
CNS Connection-not-successful signal
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Technical Manual – Signaling & ProtocolsChapter 3 MTP and MTP3B
Message Full name
CNP Connection-not-possible signal
UFC User part flow control messages
UPU User part unavailable signal
III. Message Examples
Transfer-allowed message (TFA)
The format of the transfer-allowed message is shown as follows:
Destination HeadingCode H1
HeadingCode H0 Label
DCBA 0100
First bittransmitted
24 4 4 56
Destination HeadingCode H1
HeadingCode H0 Label
DCBA 0100
First bittransmitted
24 4 4 56
The heading code H1 contains one signal code as follows:
D C B A
0 1 0 1 Transfer-allowed signal
3.1.4 Signaling Procedures
I. Message Routing
The message routing function is based on the information contained in the routing
label, namely on the destination point code and on the signaling link selection field.
Each signaling point has the routing information that enables it to determine the
signaling link over which a message is sent on the basis of the destination point code
and the signaling link selection field.
Typically the destination point code is associated with more than one signaling link
that may be used to carry the message; the selection of the particular signaling link is
made by means of the signaling link selection field, thus effecting load sharing.
There are two basic cases of load sharing, namely:
load sharing between the links belonging to the same link set;
load sharing between the links not belonging to the same link set.
Messages not related to a signaling link may be assigned any signaling link code
(SLC) to allow load sharing of the messages, or may be assigned a default SLC such
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Technical Manual – Signaling & ProtocolsChapter 3 MTP and MTP3B
as 0000. They are routed in accordance with the normal routing function, where the
(SLC) is used as SLS for load sharing.
II. Changeover
The purpose of the changeover procedure is to ensure the signaling traffic carried by
the unavailable signaling link is diverted to the alternative signaling link(s) as quickly
as possible while avoiding message loss, duplication or mis-sequencing.
To implement this purpose, in the normal case the changeover procedure includes
buffer updating and retrieval, which are performed before reopening the alternative
signaling link(s) to the diverted traffic. Buffer updating consists of identifying all the
messages in the retransmission buffer of the unavailable signaling link which have not
been received by the far end. Retrieval consists of transferring the concerned
messages to the transmission buffer(s) of the alternative link(s).
In the case of unavailability of a signaling link, changeover is initiated at a signaling
point. The following actions are then made:
transmission and acceptance of message signal units on the concerned
signaling link is terminated;
transmission of link status signal units or fill-in signal units takes place;
the alternative signaling link(s) are determined;
a procedure to update the content of the retransmission buffer of the unavailable
signaling link is performed;
signaling traffic is diverted to the alternative signaling link(s).
III. Changeback
The objective of the changeback procedure is to ensure that signaling traffic is
diverted from the alternative signaling link(s) to the signaling link made available as
quickly as possible, while avoiding message loss, duplication or mis-sequencing.
Changeback includes the basic procedures to be used to perform the opposite action
to changeover.
Changeback is initiated at a signaling point when a signaling link is reconnected,
restored, or unblocked, and therefore it becomes once again available. The following
actions are then made:
the alternative signaling link(s) to which traffic normally carried by the signaling
link made available was previously diverted are determined;
transmission of the concerned traffic on the alternative signaling link(s) is
stopped, and such traffic is stored in a changeback buffer;
a changeback declaration is sent to the remote signaling point of the signaling
link made available through the concerned alternative signaling link; this
message indicates that message traffic on the alternative signaling link will be
sent by the signaling link made available;
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the concerned signaling point will restart diverted traffic over the signaling link
made available when it receives a changeback acknowledgement from the far
signaling point of the link made available.
IV. Signaling Link Activation
When a decision is taken to activate an inactive signaling link, initial alignment starts:
if the initial alignment procedure is successful, the signaling link is active and a
signaling link test is started;
if the signaling link test is successful, the link becomes ready to convey signaling
traffic;
in the case when initial alignment is not possible, new initial alignment
procedures are started on the same signaling link after the timer expires;
if the signaling link test fails, link restoration starts until the signaling link is
activated or a manual intervention is made.
V. Signaling Link Restoration
After a signaling link failure is detected, signaling link initial alignment will take place.
if the initial alignment procedure is successful, a signaling link test is started;
if the signaling link test is successful, the link becomes restored and thus
available for signaling;
if the initial alignment is not possible, new initial alignment procedures may be
started on the same signaling link;
if the signaling link test fails, the restoration procedure is repeated until the link is
restored or a manual intervention made.
VI. Signaling Link Deactivation
An active signaling link may be made inactive by means of a deactivation procedure,
provided that no signaling traffic is carried on that signaling link. When a decision has
been taken to deactivate a signaling link, the signaling terminal of the signaling link is
taken out of service.
VII. Signaling Route Management Procedures
The purpose of the signaling route management function is to ensure a reliable
exchange of information between the signaling points (to ensure the availability of the
signaling routes).
The unavailability, restriction and availability of a signaling route is communicated by
means of the transfer-prohibited, transfer-restricted and transfer-allowed procedures.
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VIII. Transfer Prohibited
For the purpose of being described conveniently, it is assumed that Y is the
originating signaling point, X is the destination signaling point, and Z is a signaling
transfer point.
when the signaling point Y starts to route signaling destined to the signaling point
X through the signaling transfer point Z which is currently unavailable for the
signaling point Y, the transfer-prohibited message is sent to the signaling transfer
point Z;
when the signaling transfer point Y recognizes the inaccessibility of the signaling
point X, the transfer-prohibited message is sent to all accessible adjacent
signaling points (broadcast method);
when a message destined to the signaling point X is received at the signaling
transfer point Y and Y is unable to transfer the message, the transfer-prohibited
message is sent to the adjacent signaling point from which the concerned
message was received.
3.2 MTP3B
3.2.1 Overview
Broadband MTP provides the transfer capability of broadband signaling cross the
ATM network and consists of Message Transfer Part (broadband) (MTP3B) and
Signaling ATM Adaptation Layer (SAAL).
The major differences between the broadband SS7 and narrowband SS7 are the
relevant modifications of the MTP layer. To widen the signaling bandwidth, the MTP-1
and the MTP-2 are changed to SAAL (Service Specific Connection Oriented Protocol,
Service Specific Coordination Function) and the MTP-3 is changed to MTP3B. In the
aspect of physical connection, E1 trunk connections are changed to ATM (Permanent
Virtual Channel) connections.
In MSOFTX3000, the broadband MTP provides signaling transfer services for the
SCCP, BICC and H.248 protocols, as shown in Figure 3-3.
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ATM
AAL5
SSCOP
SSCF AT NNI
MTP3b
LM
SAAL
SCCP/BICC/H.248User Part
Broadband MTP
ATM
AAL5
SSCOP
SSCF AT NNI
MTP3b
LM
SAAL
SCCP/BICC/H.248User Part
Broadband MTP
Figure 3-3 Structure of the broadband MTP
Currently in UMTS, the broadband MTP is mainly applicable to the Iu-CS interface
and provides signaling transfer services for the RANAP/SCCP. If necessary, the
broadband MTP is also used on the Nc interface and provides services for the BICC
protocol.
3.2.2 Introduction of MTP3B
MTP3B is a protocol specification designed for ATM features on the basis of the
MTP3. The MTP3B is not only responsible for carrying signaling messages, but also
responsible for managing the signaling network and signaling links. The MTP3B uses
the services provided by the SAAL for message exchange.
I. MTP3B Structure
Similar to the MTP3, the functional structure of the MTP3B protocol is composed of
signaling message handling and signaling network management.
5) Signaling message handling
The purpose of the signaling message handling functions is to ensure that the
signaling messages originated by a particular User Part at a signaling point are
delivered to the same User Part at the destination point indicated by the related field
in the message signal unit (there are only SCCP and STC user parts at the Iu
interface). To achieve these functions, signaling message handling is further divided
into message routing, discrimination and distribution functions.
6) Signaling network management
The purpose of the signaling network management functions is to provide
reconfiguration of the signaling network in the case of failures. Activation and
alignment of a new signaling link is also included. With the enlargement of a signaling
network and increasing of the load over signaling links, congestion may appear in the
signaling network. Thus controlling congestion is one of the signaling network
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management functions. The signaling network management functions comprise
signaling traffic management, signaling link management and signaling route
management.
II. MTP3B Functions
The major functions provided by the components in the MTP3B protocol structure are
described as follows:
7) Message discrimination
The purpose of the message discrimination function is to examine the standard field
in the message header to judge whether or not a received message from the lower
layer (SAAL) is valid and, if valid, to determine where the message will be delivered.
If the message is not valid, the message will be discarded.
If the message is valid, there are the following possibilities:
a) When the received message is destined to the signaling point itself, the message
will be delivered to the message distribution module;
b) When the received message is not destined to the point itself and the signaling
point has no the transfer capability, the message will be discarded; otherwise, the
message will be delivered to the message routing module for further handling.
8) Message distribution
The purpose of the message distribution function is to direct a received message to
the appropriate upper-layer module which is the destination for processing the
message. If the message does not exist in the particular level 4 module indicating to
process it or the field is not valid, the message will be discarded.
9) Message routing
The purpose of the message routing function is to base the header information of a
received message to select an appropriate route for it, base the route to select a link
set, base the link set to select a link, and use the selected link to finally transmit the
message out. The handled message has the following possibilities:
The message is delivered from the upper-layer. The message routing module
has to determine an available route to transmit it. An exception is there is not
such a satisfactory route.
When the message is not destined to the point itself and the signaling point has
the signaling transfer function, its destination signaling point can be found from
the destination signaling point table at this signaling point, so as to direct the
message out.
When the message is not destined to the point itself and the signaling point has
the signaling transfer function but the destination signaling point of the message
cannot be found from the destination signaling point table at this signaling point,
the message will be discarded.
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10) Signaling traffic management
The purpose of the signaling traffic management function is to ensure a reliable and
in-sequence transfer of signaling messages. In the case of unreliability or
unavailability of a link, the function is used to divert the messages to one or more
alternative links with the objective of avoiding message loss or mis-sequencing.
11) Signaling route management
The purpose of the signaling route management function is to provide the basis for
message routing and, in the case of unavailability or unreliability of the currently
applied route, provides rerouting function and re-configures the network in order to
provision a reliable route to achieve signaling transfer.
12) Signaling link management
The purpose of the signaling link management function is to perform a proper
handling procedure on a signaling link in the case of unavailability or unreliability, in
order to stop using the unreliable link and repeatedly restart the link with the objective
of making it available again. The link management function also provides the link
testing function which periodically performs testing on a link so as to confirm the
availability of the link.
3.2.3 MTP3B Message Structure
The message structure of the MTP3B is basically same as that of the MTP3. Please
reference “Narrowband MTP” for more information. Here in this chapter only their
differences are covered.
I. Length of User Data
The MTP3B extends the length of the user data contained in a signal unit. The
maximum amount of the user data supported by MTP3B signaling links is 4091 octets
(that supported by narrowband MTP is 272 octets).
II. Service Indicator (SI)
The following codes of the service indicator are additionally used in the MTP3B:
SI code Meaning
1 0 0 1 Broadband ISDN User Part
1 0 1 0 Satellite ISDN User Part
In MSOFTX3000 product, the MTP3B has three users, namely SCCP, BICC and
H.248. The service indicator codes respectively corresponding to them are as follows:
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SI code Indicating user
0 0 1 1 SCCP
1 1 0 1 BICC
1 1 1 0 H.248
III. Changeover Procedure
By contrast with the narrowband MTP, the MTP3B changeover procedure applies with
the following exceptions and clarifications:
The signaling link failure indication causes by MTP2 link do not apply, here is In
Service to Out Of Service state causes by SAAL or when a request (automatic or
manual) is obtained from a management or maintenance system.
Moreover a signaling link that is available is recognized by level 3 as failed when
an extended changeover order or an emergency changeover order is received.
The changeover message of the signaling network management messages is
modified by using XCO/XCA to replace COO/COA. Heading code allocation of
MTP3B signaling network management messages is shown in the following
table:
Message
GroupH1
H000
00
00
01
00
10
00
11
01
00
01
01
01
10
01
11
10
00
10
01
10
10
10
11
11
00
11
01
11
10
1
1
1
1
0000
CH
M0001
C
O
O
C
O
A
X
C
O
X
C
A
C
B
D
C
B
A
EC
M0010
E
C
O
E
C
A
FC
M0011
R
C
T
T
F
C
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Technical Manual – Signaling & ProtocolsChapter 3 MTP and MTP3B
Message
GroupH1
TFM 0100
T
F
P
*
T
F
R
T
F
A
*
RS
M0101
R
S
T
R
S
R
MIM 0110LI
N
L
U
N
LI
A
L
U
A
LI
D
LF
U
LL
T
L
R
T
TR
M0111
T
R
A
DLM 1000
D
L
C
C
S
S
C
N
S
C
N
P
1001
UFC 1010
U
P
U
1011
1100
1101
1110
1111
3.3 SAAL
3.3.1 SAAL Function Structure
In the broadband network, signaling adaptation is required in the transmission of
signaling information across ATM network. That is to say, signaling information in a
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variety of message formats has to be converted to a format suitable for transportation
over ATM network and ATM Adaptation Layer (AAL) connections have to be set up for
signaling. What implements this function is the Signaling ATM Adaptation Layer
(SAAL).
The SAAL protocol used in MSOFTX3000 product is in full compliance with the ITU-T
Recommendations Q.2110, Q.2140 and Q.2144.
The SAAL makes use of the specification of AAL type 5 (AAL5). As shown in
Figure 3-1, The SAAL comprises the Convergence Sublayer (CS) and the
Segmentation And Reassembly (SAR). The CS is divided into the Service Specific
Convergence Sublayer (SSCS) and the Common Part Convergence Sublayer
(CPCS). Further, the SSCS includes three parts: the Service Specific Coordination
Function (SSCF) sublayer (ITU-T Q.2140), the Service Specific Connection Oriented
Protocol (SSCOP) sublayer (ITU-T Q.2110), and the Layer Management (LM) (ITU-T
Q.2144).
Figure 3-1 Structure of the SAAL protocol in MSOFTX3000
In MSOFTX3000, the CPCS and the SAR are implemented by the BSG hardware,
thus the SSCOP, the SSCF and the LM constitute the core of the SAAL protocol.
3.3.2 SSCOP
I. SSCOP Functions
The SSCOP performs the following functions:
Sequence integrity: This function preserves the order of SSCOP SD PDUs that
were submitted for transfer by SSCOP.
Error correction by selective retransmission: Through retransmission, sequence
errors are corrected when the receiving SSCOP entity detects missing SSCOP
Service Data Units (SDUs).
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Flow control: By sending the movement of the sliding window, this function
allows to adjust the information transmission rate to perform flow control.
Error reporting to Layer Management: This function indicates to layer
management errors which have occurred.
Keep alive: This function verifies that the two peer SSCOP entities participating
in a connection are remaining in a link connection established state even in the
case of a prolonged absence of data transfer.
Local data retrieval: This function allows the local SSCOP user to retrieve in-
sequence SDUs which have not yet been released by the SSCOP entity when a
link changeover procedure takes place at the higher layer.
Connection control: This function performs the establishment, release, and
resynchronization of an SSCOP connection. It also allows the transmission of
variable length user-to-user information without a guarantee of delivery.
Transfer of user data: This function is used for the conveyance of user data
between users of the SSCOP. SSCOP supports both assured and unassured
data transfer.
Protocol error detection and recovery: This function detects and recovers from
errors in the operation of the protocol.
Status reporting: This function allows the transmitter and receiver peer entities to
exchange status information.
II. SSCOP Protocol Data Units
What are conveyed between two SSCOP peer layers for the establishment or release
of a connection and for the guarantee of a reliable message transmission are protocol
data units (PDUs) of the SSCOP. Basic PDUs are listed and described as follows:
BGN PDU (Begin): The BGN PDU is used to establish an SSCOP connection
between two peer entities. The BGN PDU requests the clearing of the peer’s
transmitter and receiver buffers, and the initialization of the peer’s transmitter
and receiver state variables and counters.
BGAK PDU (Begin Acknowledge): The BGAK PDU is used to acknowledge the
acceptance of a connection request from the peer.
BGREJ PDU (Begin Reject): The BGREJ PDU is used to reject the connection
request of the peer SSCOP entity.
END PDU (End): The END PDU is used to release an SSCOP connection
between two peer entities.
ENDAK PDU (End Acknowledge): The ENDAK PDU is used to confirm the
release of an SSCOP connection.
RS PDU (Resynchronization): The RS PDU is used for the routine connection-
oriented reset in other connection-oriented protocols. The RS PDU is used to
resynchronize the buffers and the transmitter and receiver state variables
(counters).
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RSAK PDU (Resynchronization Acknowledge): The RSAK PDU is used to
acknowledge the acceptance of a resynchronization requested by the peer
SSCOP entity.
ER PDU (Error Recovery): The ER PDU is used to recover from protocol errors
in the operation of a connection.
ERAK PDU (Error Recovery Acknowledge): The ERAK PDU is used to
acknowledge the recovery from protocol error.
SD PDU (Sequenced Data): The SD PDU is used to transfer user service data to
the peer entity after an SSCOP connection is set up.
POLL PDU (Status Request): The POLL PDU is used to request, across an
SSCOP connection, status information about the peer SSCOP entity.
STAT PDU (Solicited Status Response): The STAT PDU is used to respond to a
status request (POLL PDU) received from a peer SSCOP entity. It is used to
notify the peer SSCOP entity of correct receipt of concerned SD PDUs and also
used to acknowledge which SD PDUs are successfully accepted and which fail
to be received. It is also used to update the position of the transmitting window.
In this way, the maximum transmitting sequence number of SD PDUs that can
be sent currently is controlled. The STAT PDU also contains the sequence
number [N(PS)] of the POLL PDU to which it is in response.
USTAT PDU (Unsolicited Status Response): The USTAT PDU is used to
respond to a detection of one or more new missing SD PDUs, based on the
examination of the sequence number of the SD PDU. It contains the data for
updating the transmitting window of the peer, but there is not the N(PS) field.
UD PDU (Unnumbered Data): The UD PDU is used for unassured data transfer
between two SSCOP users, without affecting connection-oriented sequencing in
progress, without changing the entities’ counters or variables, without re-
transmitting lost data.
MD PDU (Management Data): The MD PDU is used for unassured management
data transfer between two SSCOP management entities. Similar to the UD PDU,
the MD PDU does not ensure a reliable receipt by the peer.
III. SSCOP States
The states of an SSCOP protocol entity reflect general conditions of the SSCOP
entity in the sequences of signals and PDU exchanges with its user and peer,
respectively. The basic states are:
State 1 - Idle: Each SSCOP entity is conceptually initiated in the Idle state (State
1) and returns to this state upon the release of a connection.
State 2 - Outgoing Connection Pending: An SSCOP entity requesting a
connection with its peer is in the Outgoing Connection Pending state (State 2)
until it receives acknowledgement from its peer
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State 3 - Incoming Connection Pending: An SSCOP entity that has received a
connection request from its peer and is waiting for its user’s response is in the
Incoming Connection Pending state (State 3).
State 4 - Outgoing Disconnection Pending: An SSCOP entity requesting release
of the peer-to-peer connection goes to the Outgoing Disconnection Pending
state (State 4) until it receives confirmation that the peer entity has released and
transitioned to the Idle state (State 1), after which it does the same.
State 5 - Outgoing Resynchronization Pending: An SSCOP entity requesting
resynchronization of the connection with its peer is in the Outgoing
Resynchronization Pending state (State 5).
State 6 - Incoming Resynchronization Pending: An SSCOP entity that has
received a resynchronization request from its peer and is waiting for its user’s
response is in the Incoming Resynchronization Pending state (State 6).
State 7 - Outgoing Recovery Pending: An SSCOP entity requesting recovery
with its peer of an existing connection is in the Outgoing Recovery Pending state
(State 7).
State 8 - Recovery Response Pending: An SSCOP entity which has completed
recovery, notified its user, and is awaiting response is in the Recovery Response
Pending state (State 8).
State 9 - Incoming Recovery Pending: An SSCOP entity that has received a
recovery request from its peer and is waiting for its user’s response is in the
Incoming Recovery Pending state (State 9).
State 10 - Data Transfer Ready: Upon successful completion of the connection
establishment, resynchronization, or error recovery procedures, both peer
SSCOP entities will be in Data Transfer Ready state (State 10) and assured data
transfer can take place.
IV. SSCOP Operating Mechanism
Connection establishment of SSCOP
In order to establish a connection between two peer SSCOP entities, the SSCF sends
an AA-ESTABLISH.req primitive to the SSCOP. This primitive contains SSCOP-UU
and BR parameters used by SSCOP to generate a BGN message. The BGN
message is sent to the receiving SSCOP where it is decoded, processed and mapped
to an AA-ESTABLISH.ind signal which will be sent to the receiving SSCF. The SSCF
responds to the SSCOP with an AA-ESTABLISH.res primitive containing also
SSCOP-UU and BR primitives. Whereas, the SSCOP sends a BGAK message back
to the originating SSCOP and the originating SSCOP decodes and processes it and
sends it to the SSCF. These actions establish a connection between two SAAL
entities in two broadband signaling exchanges. See Figure 3-2.
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Figure 3-2 Connection establishment of SSCOP
Data transfer and error recovery of SSCOP
As shown in Figure 3-3, SSCOP A sends to SSCOP B four SD PDUs in the N(S)
sequence numbered from 1 to 4. Only the PDU1 and PDU2 succeed in arriving at
SSCOP B without error. The SSCOP delivers the PDU1 and PDU2 to the proper user.
The SSCOP A sends a POLL PDU. Contained in the message is N(S)=5 indicating
the N(S) value of the next new SD PDU (that is, the next SD PDU to be transmitted).
The POLL also contains N(PS)=1 which is the sequence number of the POLL PDU.
The SSCOP B responds to the POLL PDU with a STAT PDU, and the STAT PDU is
coded N(R)=3 to acknowledge the acceptance of the PDU1 and PDU2. In addition, it
is also indicated that it is expecting the next PDU, that is, PDU3. The N(PS) field
contained in the STAT must be the same as the value of the N(PS) field contained in
the concerned POLL PDU. The list element is set to 3 and 5. The information
indicated by it is described as follows. The odd element (valued 3) indicates the PDU
of a certain loss interval; the even element (valued 5) indicates the first PDU in the
next correctly accepted sequence. This message notifies the SSCOP A that 1) it must
re-transmit PDU3 and PDU4; 2) it can release PDU1 and PDU2 from the buffer; and
3) it must preserve PDU3 and PDU4 as there is not enough information about the
final result of PDU3 and PDU4. The SSCOP A then sends 3 SD PDUs to the SSCOP
B, and only the PDU7 is received. As the SSCOP is not allowed to exchange out-of-
sequence service with the user, the SSCOP B keeps PDU7 in the buffer. It sends to
the SSCOP A a USTAT PDU (where N(R)=3).
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Figure 3-3 Data transfer of SSCOP
Connection release of SSCOP
After an SSCOP receives a release request message AA-RELEASE.request, it sends
an END PDU to the peer SSCOP. On receipt of the END PDU, the peer sends an AA-
RELEASE.indication. After the connection is released, the peer sends an ENDAK
PDU. After receiving it, the receiving end sends an AA-RELEASE.confirm message to
the concerned SSCF, and releases the connection. See Figure 3-4.
Figure 3-4 Connection release of SSCOP
3.3.3 SSCF
The SSCF is used to coordinate the interface between the SSCOP and the upper-
layer MTP3B. It maps primitives from the MTP3B to required SSCOP signals, and
vice versa. In nature, the SSCF only transfers the signals between the SSCOP and
the MTP3B to and fro, playing an intermediate role. The SSCF does not transmit any
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PDUs to the peer entity in the receiver; instead, by relying on the SSCOP, its
information is carried in SSCOP PDUs.
I. SSCF Functions
Primitive mapping: The SSCF maps primitives received from SAAL user to
signals defined at the SSCOP upper layer boundary and maps signals received
from the SSCOP to primitives implicitly defined at the MTP-3 lower layer
boundary.
Local retrieve: In the case of a changeover procedure performed on a faulty link,
this function makes it possible to obtain back the data not yet transmitted and
divert the data to alternative link(s).
Flow control: The SSCF reports to the user the congestion level (or no
congestion) to avoid unnecessary cell loss. It also diverts its own PDU flow to the
lower layer in order to prevent from congestion happening at the other end.
Link status maintenance: This SSCF function receives primitives from the MTP-3
or signals from the SSCOP and maintains information pertaining to the status of
the link, such as In Service and Out Of Service. Based on the information, it can
provide primitives/signals to the MTP3 and the SSCOP as an aid to maintaining
the link.
Reporting to layer management: This SSCF function transmits MAAL primitives
to the layer management so that the layer management can perform statistics
and measurements. For instance, upon release of a link, the SSCF reports the
release to the layer management, and then the layer management can measure
the In-service duration. With the help of the layer management, the error
monitoring function can be implemented.
Performing link alignment.
II. SSCF Link Alignment
Alignment procedure: The procedure initiated according to user's request to detect
the status of a link before it is put into service in the case of successful establishment.
On receipt of the user’s (MTP3B) request (by sending a STAR_req primitive), the
SSCF transmits a BGN PDU to the peer entity in the receiving exchange to start the
alignment procedure, and moves a link from the Out Of Service status to the
Alignment status.
These operations require the SSCOP to establish a link between the two exchanges.
After the link is successfully established, the SSCF indicates the layer management
to start the monitoring action. Then the SSCF enters the Proving status for the link.
At this moment, proving PDUs are transported between the exchanges. A links proves
to be good by the means that n (1000 by default) proving PDUs can be successfully
transmitted. In the end, if 1000 proving PDUs are really transmitted successfully and
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errors are not found then the link is recognized as passing the alignment and can be
put into service.
The SSCF alignment procedure provides a normal or emergency proving. Whether to
begin proving can be initiated by the layer management and the MTP3B. In the
normal proving,
The proving algorithm on SAAL link is based on the alignment error rate monitoring
process used for proving a link. Transmission of testing PDUs of N1 amount (1000 by
default) at a specified rate (one PDU per millisecond by default) must be completed
within 30 seconds from the start to the proving success. If one or two (one by default)
of the transmitted N1 PDUs are re-transmitted, the proving fails. If no error occurs, the
link succeeds in being proved and moves to In Service.
3.3.4 LM
The position of the Layer Management (LM) in the SAAL is shown in Figure 3-1. The
SSCS LM is the layer management entity of the Service Specific Convergence
Sublayer. It makes a direct interaction with the sublayers to implement a number of
Operation Administration and Maintenance (OAM) functions. Therefore, the SSCS LM
is described as an entity having interactions with all SAAL layers since CPCS and
SAR (AAL Type 5) are implemented by the hardware and there are no interactions
defined at these two layers. The SSCS LM is responsible for conducting the following
tasks:
Determining whether a link should be out of service or in service. As a
component of these operations, a link has to be monitored against excessive
delays during service transmission. In order to avoid unnecessary alteration, the
layer management allows a certain number of errors occurring at the link.
Periodically conducting a number of measurements. For instance, the layer
management uses counters to count how long each link is in service, how
frequent faults take place, how frequent congestions happen, as well as other
information.
Performing alarm handling.
The layer management has the following states:
Out Of Service
Alignment
Proving
Aligned Ready
In Service
I. LM Error Monitoring Algorithms
The layer management provides three algorithms for error monitoring. These
algorithms ensure to detect an error burst keeping for more than 400ms.
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Algorithm 1 is mainly used for heavy load. If the volume of the transmitted data is
too large, the receiver has not enough time to handle the data. This causes the
fact that the data in the sending buffer cannot be released so long that the sum
of the transmission queue continues to increase to a particular value. At this
moment, the link will be released.
Algorithm 2 is mainly used for intermediate load. This algorithm monitors data
retransmissions. When data retransmissions occur so frequently within a
particular interval that the occurrence sum exceeds a threshold, it indicates a
bad quality on the link. Once the delay is beyond tolerance, the link will be
released.
Algorithm 3 is mainly used for light load. If within a particular interval the
difference between the number of transmitted POLL PDUs and the number of
accepted STAT PDUs (the difference is actually the number of lost STAT PDUs)
exceeds a threshold, it also indicates a bad quality on the link. In this case, the
link will be released.
II. SAAL Compound States
The states for coordinative operation among the three sub-layers are defined as
follows: (“m” indicates the state number of SSCF; “n” indicates the state number of
SSCOP; “r” indicates the state number of LM; and “m/n/r” indicates the compound
state of the three sub-layers.)
1/1/1 Out Of Service/Idle: In this state, the connection is idle.
1/4/1 Out Of Service/ Outgoing Disconnection Pending: In this state the MTP3B,
or alternatively the Layer Management, has issued an AAL-STOP-request, or an
AA-RELEASE-request or an MAAL_RELEASE-Request, respectively, which
caused the SSCF to issue an AA-RELEASE-request, and the SSCF is waiting for
a confirmation of the SSCOP connection release, AA-RELEASE-confirm.
2/1/2 Alignment/Idle: In this state, the SAAL user requested the SSCF to provide
an AAL connection. This request was passed to SSCOP by means of an AA-
ESTABLISH-request, but the connection establishment or proving was
unsuccessful. SSCF is waiting to reattempt this process. This process will be
repeated until a supervisory function indicates that the establishment of an AAL
connection is to be abandoned.
2/2/2 Alignment/Outgoing Connection Pending: In this state, the user has issued
an AAL-START-request, and the SSCF is waiting for a confirmation of SSCOP
connection.
2/4/2 Alignment/Outgoing Disconnection Pending: In this state the SSCF, or in
the case of unsuccessful proving, the Layer Management, requested the release
of the SSCOP connection. This request was passed to SSCOP by means of an
AA-RELEASE-request, and the SSCF is waiting for a confirmation of the SSCOP
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Technical Manual – Signaling & ProtocolsChapter 3 MTP and MTP3B
connection release, AA-RELEASE-confirm. This state transition within SSCF is
not indicated to the SAAL user.
3/10/5 In Service/Data Transfer Ready: In this state, the signaling
connection is in service and may be used by the user to transfer signaling
messages.
2/10/3 Proving/Data Transfer Ready: In this state, an SSCOP connection
has been established, and SSCS layer management is conducting alignment
error rate monitoring to verify the quality of the link.
2/10/4 Aligned Ready/Data Transfer Ready: In this state, the SSCF has
completed proving and is awaiting an indication from its peer that the signaling
link can be put into service.
Figure 3-5 is the normal start flow diagram of the SAAL protocol. The transition of the
eight states above mentioned is shown in the figure.
T1167200-94/d06
. . . . . . . . . . . .. . . . . . . . . . . .
AAL-START-req.
MAL-REPORT-ind.
(-,ALN,-)
1 1
2
2
2 2
1 1 1
2
2
3
4
5
3
1/1/1
2/2/2
2/10/3
3/10/5
2/10/4
10 10
1/1/1
2/2/2
2/10/3
2/10/4
3/10/5
5
3
4
3MAAL-PROVING-ind.
T3 expiresC1 > 0
T3 expiresC1 > 0
T3 expiresC1 = 0
MAAL-STOP_PROVING-ind.
AAL-IN_SERVICE-ind.
MAAL-REPORT-ind.
(-,INS,-)
AA-ESTABLISH-req.
AA-ESTABLISH-conf.
AA-DATA-req.(NM)
AA-DATA-ind.(NM)
AA-DATA-req.(IS)
AA-DATA-ind.(IS)
BGN BGN
BGAK BGAK
SD SD
POLL POLL
STAT STAT
POLL
AA-ESTABLISH-req.
AA-ESTABLISH-conf.
AA-DATA-req.(NM)
AA-DATA-ind.(NM)
AA-DATA-req.(IS)
AA-DATA-ind.(IS)
AAL-START-req.
MAL-REPORT-ind.(-,ALN,-)MAAL-PROVING-ind.
T3 expiresC1 > 0
T3 expiresC1 > 0
T3 expiresC1 = 0
MAAL-STOP_PROVING-ind.
AAL-IN_SERVICE-ind.
MAAL-REPORT-ind.(-,INS,-)
LM MTP3 SSCF-NNI SSCOP SSCOP SSCF-NNI MTP3 LM
FIGURE II.1/Q.2140Time flow diagram for connection estabishment Both UPS=Normal, Case 1
AA-DATA-req.(NM)
AA-DATA-ind.(NM)
SD SDAA-DATA-req.(NM)
AA-DATA-ind.(NM)
STAT STAT
SD SD
POLL
1
Figure 3-5 Normal start flow diagram of SAAL
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Technical Manual – Signaling & ProtocolsChapter 3 MTP and MTP3B
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