Signaling System for GSM Networks

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August 11, 2006

ComSoc Lecture Meet

Signaling System for
GSM Networks

Rabindranath Nandi

&

Rahul Ghosh

August 11, 2006

ComSoc Lecture Meet

Acknowledgement



Mr. Suvadip Basu




Deputy Manager,


Switch Operations




Hutchison Telecom East Ltd.


Kolkata


700 017

August 11, 2006

ComSoc Lecture Meet

Introduction to Signaling Systems



Signaling :

Signaling is defined as a mechanism by virtue of which
network entities exchange information amongst themselves , which
are required to set up a communication path.



Signaling System :

Signaling system is defined as a set of methods
or rules followed by network entities to exchange information
required for communication set up.



Examples of Signaling Systems :


SS#7 or CCS#7 (Common Channel Signaling #7)


CAS (Channel associated Signaling)


DTMF (Dual Tone


Multi frequency)

August 11, 2006

ComSoc Lecture Meet

Areas of Signaling

There are mainly three areas of signaling during a telephone call :


between subscribers and exchanges.


within exchanges.


between exchanges.


Exchange A

Exchange B

between Subscriber and
exchange

within exchange

between exchanges

August 11, 2006

ComSoc Lecture Meet

Channel Associated Signaling
(CAS)


In CAS each and every speech channel is associated with a
signaling channel. This means for each speech channel a separate
signaling channel is required.


Speech

Speech

Speech

Signaling

Speech

Signaling

Signaling

Signaling

Exchange A

Exchange B

August 11, 2006

ComSoc Lecture Meet

Common Channel Signaling (CCS)


In CCS there is a common signaling channel which takes care of all
the signaling information to be exchanged during communication. All
other channels can be used for speech or data as required.



Exchange A

Exchange B

Speech


Speech

Common Signaling Channel

August 11, 2006

ComSoc Lecture Meet

Advantages of CCS


Higher signaling capacity.



More number of speech/data channels as there is only one signaling
channel.



Central offices can exchange information , not related to
speech/data between themselves e.g. subscriber data.



Various high end features like roaming are possible by using CCS7.


August 11, 2006

ComSoc Lecture Meet

CCS7 Network Components



The CCS7 Network consists of the following
Components :


SP
-
Signaling Point :

The SP is the source or
originating entity of the signaling message.


STP
-
Signaling Transit Point

:

These entities transfer
the signaling message to another SP or STP without
processing the signaling message.


SRP


Signaling Relay Point :

These are STPs with
ability tp process the signaling message.


SEP


Signaling End Point :


The SEP is defined to be
the terminating point of the signaling message

August 11, 2006

ComSoc Lecture Meet

CCS7 Network Architecture


A typical SS7 network is a mixture of various SPs STPs SRPs and
SEPs, As shown below :





SP (SPC=100)

SRP (SPC=400)

STP (SPC=300
)

STP (SPC=200)

SEP (SPC=500)

August 11, 2006

ComSoc Lecture Meet

CCS7 Terminologies


SPC :

SPC is defined as signaling point code which uniquely
identifies each element within the SS7 network.



Signaling Links :

The signaling links transmit signaling messages
between the communicating entities.



Signaling Link Set :


A signaling link set is a collection of signaling
links between two signaling entities.



Signaling Route :

A signaling route is defined to be the path
through which signaling messages are exchanged between two
entities. There can be multiple signaling routes between two SPCs,
wherein there will be one primary route and several alternate routes.


August 11, 2006

ComSoc Lecture Meet

SS7 Routing Modes


Associated Mode :



In the associated mode
,
the signaling link is routed together with the
circuit group belonging to the link. In other words, the signaling link
is directly connected to signaling points which are also the terminal
points of the circuit group. This mode of signaling is recommended
when the capacity of the traffic relation between the signaling points
A and B is heavily utilized.


Associated Mode of Signaling

August 11, 2006

ComSoc Lecture Meet

SS7 Routing Modes


Quasi associated mode :



In the quasi
-
associated mode of signaling,

the signaling link and the
circuit group run along different routes, the circuit group connecting
the signaling point. A is directly connected with the signaling point B.
For this mode the signaling, the circuit group is carried out via one or
more defined signaling transfer points. This signaling mode is
favorable for traffic relations with low capacity utilization, as the
various signaling links can be used for several destinations.



August 11, 2006

ComSoc Lecture Meet

SS7 Network Structure



SPC=100

SPC=200

SPC=300

SPC=100

SPC=400

SPC=500

SPC=600

SPC=400

SPC=900

SPC=800

SPC=700

SPC=800

SPC=100

SPC=100

SPC=300

SPC=200

INAT 0

NAT 0

NAT 1

NAT 1

NAT 1

August 11, 2006

ComSoc Lecture Meet

SS7 Protocol Structure



SS7 protocol stack can be broadly divided into two main
categories :



A )

MTP


Message Transfer Part :


The message transfer part
has three levels , namely MTP Layer 1, Layer 2 and Layer 3.
The message transfer part (MTP) represents a user
-
neutral
means of transport for messages between the users.



B ) User Parts (UP) :

Each user part

(UP) encompasses the
functions, protocols and coding for the signaling via SS7 for a
specific user type (e.g. data service, ISDN). In this way, the user
parts control the set
-
up and release of circuit connections, the
processing of facilities as well as administration and maintenance
functions for the circuits.



August 11, 2006

ComSoc Lecture Meet

SS7 Protocol Structure


MTP Layer 3 (Network Management)

MTP Layer 1 (Signaling Data Link)

MTP Layer 2 (Signaling Link)

SCCP

B

S

S

A

P

I

S

U

P

TCAP

TCAP Users

SS7 Users

M

U

P


Other User
Parts

August 11, 2006

ComSoc Lecture Meet

The Message Transfer Part



MTP is used in SS7 by all user parts as a transport system or
message exchange.


Messages to be transferred from one user part to another are given
to the message transfer part The message transfer part ensures that
the messages reach the addressed user part in the correct order
without information loss, duplication or sequence alteration and
without any bit errors.


August 11, 2006

ComSoc Lecture Meet

The Message Transfer Part


Signal units :



The message transfer part transports messages in signal units of
varying length. A signal unit is formed by the functions of level 2. In
addition to the message it also contains control information for the
message exchange. There are three different types of signal units


Message Signal Units (MSU)


With message signal units, the message transfer part transfers user
messages, i.e., messages from user parts (level 4) and messages from
the signaling network management (level 3).


Link Status Signal units (LSSU)


LSSU contains information for the operation of the signaling link (e.g.
for the alignment).


Fill
-
In Signal Units (FISU)


FISU are used to maintain the acknowledgement cycle and quality
control when no user messages are to be sent in one of the two
directions of the signaling.


August 11, 2006

ComSoc Lecture Meet

The Message Transfer Part


Signal Unit Formats

August 11, 2006

ComSoc Lecture Meet


Forward indicator bit (FIB)


The forward indicator bit (FIB) is needed during general error
correction. It indicates whether a signal unit is being sent for the first
time or whether it is being retransmitted.


Length indicator (LI)


The length indicator (LI) gives the number of octets (one octet = 8 bits)
between the length indicator field and the check
-
bit field, and is used to
differentiate between the three signal units. The length indicator field
contains different values according to the type of signal unit:



0 = fill
-
in signal unit


1 or 2 = link status signal unit



greater than 2 = message signal unit.


The maximum value in the length indicator field is 63 even if the signal
information field contains more than 62 octets. (The message signal
unit is invalid for longer message signal units. However, it is possible to
calculate the exact length of the message signal unit).


The Message Transfer Part


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ComSoc Lecture Meet


Service information octet (SIO):
The service information octet (SIO) only
exists in message signal units. It contains the service indicator and the
network indicator. A service indicator is assigned to each user of the
message transfer part. It informs the message transfer part which user part
has sent the message and which user part is to receive it. The network
indicator indicates whether the traffic is national or international. The
message transfer part evaluates both items of information.


Signal information field (SIF):

The signal information field (SIF) only
exists in message signal units. It contains the actual user message as well
as the addresses. The maximum length of the signaling information field is
272 octets.


Check bits (CK) :

The check bits (CK) are formed on the transmission side
from the contents of the signal unit and are added to the signal unit as
redundancy. On the receive side, the message transfer part can determine
with the check bits whether the signal unit was transferred without any
errors. The signal unit is acknowledged as either positive or faulty on the
basis of the check.


Status field (SF) :
The status field (SF) only exists in link status signal
units. It contains status indications for the signaling links for the alignment of
the transmit and receive directions.



The Message Transfer Part


August 11, 2006

ComSoc Lecture Meet


Addressing of the signal units


The routing label of a signal unit is transported in the signal information field
(SIF). It consists of the following



Destination Point Code (DPC)


Originating Point Code (OPC)


Signaling Link Selection (SLS)



A code is assigned to each signaling point in the signaling network according to
a numbering plan. The message transfer part uses the code for message
routing. The destination point code in a message signal unit identifies the
signaling point to which this message is to be transferred. The originating point
code specifies the signaling point from which the message originates.


The contents of the signaling link selection determine the signaling route along
which the message is to be transmitted. In this way, the signaling link selection
is used for load sharing on the signaling links between two signaling points.


The service information octet (SIO) contains additional address information.
Using the service indicator, the destination message transfer part identifies the
user part for which the message is intended.


The Message Transfer Part


August 11, 2006

ComSoc Lecture Meet

The Message Transfer Part


Routing label of a message signal unit


August 11, 2006

ComSoc Lecture Meet


Functions of the MTP Layers


Level 1 (signaling data link)

defines the physical, electrical and
functional characteristics of a signaling data link and the access units.
Level 1 represents the bearer for a signaling link. In a digital network,
64
-
kbit/s channels are generally used as signaling data links. In
addition, analog channels (preferably with a bit rate of 4.8 kbit/s) can
also be used via modems as a signaling data link.


Level 2 (signaling link)

defines the functions and procedures for a
correct exchange of user messages via a signaling link. The following
functions must be carried out in level 2:




delimitation of the signal units by flags.




elimination of superfluous flags.




error detection using check bits.




error correction by re transmitting signal units.




error rate monitoring on the signaling data link.




restoration of fault
-
free operation, for example, after disruption of the
signaling data link


The Message Transfer Part


August 11, 2006

ComSoc Lecture Meet


Level 3 (signaling network)

defines the internetworking of the
individual signaling links. A distinction is made between the two
following functional areas:




message handling, i.e. directing the messages to the desired
signaling link, or to the correct user part.




signaling network management, i.e. control of the message
traffic, for example, by means of changeover of signaling links if
a fault is detected and change back to normal operation after the
fault is corrected The various functions of level 3 operate with
one another, with functions of other levels and with
corresponding functions of other signaling points.


The Message Transfer Part


August 11, 2006

ComSoc Lecture Meet

The Message Transfer Part


MTP Functions

August 11, 2006

ComSoc Lecture Meet

The User Parts
-

ISUP


ISDN user part (ISUP)


The ISDN user part (ISUP) covers the signaling functions for the
control of calls, for the processing of services and features and for
the administration of circuits in ISDN. The ISUP has interfaces to the
message transfer part and the signaling connection control part
(SCCP) for the transport of message signal units. The ISUP can use
SCCP functions for end
-
to
-
end signaling.


The structure of the ISUP Message is shown Below :





August 11, 2006

ComSoc Lecture Meet

ISDN User Part


The
routing label

comprises the destination point code, the
originating point code and the signaling link selection.



The
circuit identification code

(CIC) assigns the message to a
specific circuit. A circuit identification code is permanently assigned
to each circuit.



The
message type

defines the function and the format of an ISUP
message. There are different message types for the call set
-
up, the
call release and the administration of circuits.



Message types for the call set
-
up



Initial Address Message (IAM)


The IAM is the first message sent to the next network node during
call set
-
up. It is used for seizing a circuit and contains all information
necessary for routing to the terminating network node.



August 11, 2006

ComSoc Lecture Meet

ISDN User Part


Subsequent Address Message (SAM)


The SAM transports the digits which were not yet contained in the
IAM.



Address Complete Message (ACM)


The calling network node is informed with the ACM that the
terminating network node was reached.


Answer Message (ANM)


The ANM informs the calling network node that the called party has
answered. The call charge registration normally begins with the
ANM.



Message types for call release


Release Message (REL)


The REL initiates the release of a circuit connection. Any
unsuccessful circuit connection set
-
up is likewise released with REL.
It also includes the cause of the failure of the call set
-
up.



August 11, 2006

ComSoc Lecture Meet

ISDN User Part


Release Complete Message (RLC)


With the RLC, the disconnection of the set
-
up of a circuit is indicated
and the reception of the RLC is acknowledged. After the
transmission or reception of the RLC the circuit is released and
becomes available for a new call set
-
up.



Message types for the administration of circuits



Blocking message (BLO)

The BLO is used for blocking a circuit.



Unblocking message (UBL)

The UBL is used for canceling a block
on a circuit.



August 11, 2006

ComSoc Lecture Meet

ISDN User Part


The
fixed mandatory part

of the ISUP message contains parameters
which must be present for a certain message type and which have a
fixed length. For the IAM these are, for example, parameters for
-



the type of connection (e.g. connection via a satellite link)



the requirements for the transmission link (e.g. 64 kbit/s end
-
to
-
end)



the requirements for the signaling system (e.g. ISUP end
-
to
-
end)



the type of the calling party (ISDN subscriber = normal subscriber)


The
variable mandatory part

of the ISUP message contains
parameters of variable length. An example of one such parameter for
the IAM is: the directory number or at least part of the number which is
required for routing to the terminating network node.


If a message has an
optional part
,

the parameters that can be
transmitted in the optional part are specified for the message. These
may be parameters of fixed or variable length. Examples for the IAM
are:


Directory number of the calling party


parameters for the message type (e.g. closed user group)


user information


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ComSoc Lecture Meet

ISDN User Part


Call Setup
Procedures

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ComSoc Lecture Meet

ISDN User Part


Call Release
Procedures


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ComSoc Lecture Meet

SCCP


Signaling Connection
Control Part


Introduction :

The signaling connection control part (SCCP) is used
as a supplement to the message transfer part. It provides additional
functions for the transfer of messages between network nodes and
between network nodes and other signaling points.



From the point of view of the message transfer part, the SCCP is a
user with its own service indicator. The combination of the SCCP
and the message transfer part is called the network service part
(NSP).



Two Varieties of SCCP :



Connection Oriented


Connectionless


August 11, 2006

ComSoc Lecture Meet

SCCP


Signaling Connection
Control Part



Connectionless SCCP :

Connectionless SCCP does not require
logical connection between SCCP users .

Without logical signaling
connection an SCCP user can send single messages to other SCCP
users.


Connection Oriented SCCP :
With logical signaling connection an
exchange of messages between two SCCP users is possible. A logical
signaling connection arises through the mutual network node of the
originating point codes between the SCCPs in the signaling points of
the signaling relation. The messages to the other SCCP users can thus
be directly addressed. The SCCP can send messages via the MTP
network.


The SCCP possesses its own routing function. The SCCP can use the
following as address parameters:



the destination point code (DPC)


a global title (GT Address)


a subsystem number (SSN No) which identifies the SCCP User



August 11, 2006

ComSoc Lecture Meet

SCCP


Message Structure


Structure of an SCCP message


An SCCP message consists of:


a routing label


a message type


fixed mandatory part


variable mandatory part


optional part



August 11, 2006

ComSoc Lecture Meet

SCCP


Message Structure



Connectionless SCCP :



Unidata (UDT) :

SCCP messages are sent to a destination with
the UDT message. It is used for the protocol classes 0 and 1.


Unidata service (UDTS)


A transmitting SCCP is informed with the UDTS message that a
UDT message could not be conveyed to the destination. It is
used for the protocol classes 0 and 1


Extended unidata (XUDT)

:
Signaling information is sent in a
connectionless mode, whereby optional parameters are allowed
(for segmentation).


Extended unidata service (XUDTS)


Signaling information received from an XUDT message is sent
back to its originating point if the XUDTS was not able to reach
the destination. The user must already have requested this
feature (Return option)
.


August 11, 2006

ComSoc Lecture Meet

SCCP


Message Structure



Connection Oriented SCCP :


Connection Request (CR)


The far
-
end signaling point of a signaling relation is informed with the
CR message that a logical signaling connection is to be set up. The CR
message can be sent as either a message on its own or together with
another message, depending on the protocol class used.


Connection Confirm (CC)


The set
-
up of a logical signaling connection is confirmed by the distant
side with the CC message.



Message types for the release of a logical signaling connection
:


Released (RLSD)


The RLSD message initiates the release of a logical signaling
connection. It can be sent from either end of the connection
.


Release complete (RLC)


The release of a logical signaling connection is confirmed with the RLC
message.



August 11, 2006

ComSoc Lecture Meet

SCCP


Message Structure


Connection Oriented SCCP


Message types for message transfer:


Data form 1 (DT1)

SCCP messages can be transferred in both
directions with the DT1 message after the set
-
up of a logical
signaling connection. It is only used in protocol class 2.


Data form 2 (DT2)


With the DT2 message, SCCP messages can be transferred in both
directions after the set
-
up of a logical signaling connection, and the
reception of SCCP messages can be confirmed by the opposite
side. It is only used in protocol class 3.


The
fixed mandatory part

of the SCCP message contains
parameters which must be present for a certain message type and
which have a fixed length. For the CR message



these are, for example:




the local reference




the protocol class used for the message transfer


August 11, 2006

ComSoc Lecture Meet

SCCP


Message Structure


Connection Oriented SCCP



The
variable mandatory part

of the SCCP message contains
parameters of variable length. For the CR message these are, for
example:





the directory number of the called party




the identifier of the SCCP user (e.g. ISUP, TCAP)



The
optional part

of the SCCP message contains parameters
which can occur in every message type. The parameters in question
can be of either fixed or variable length. For the CR message these
are, for example:





the directory number of the calling party




user messages to be transferred


August 11, 2006

ComSoc Lecture Meet

SCCP


Protocol Classes


August 11, 2006

ComSoc Lecture Meet

SCCP


Protocol Classes


For the transfer of
connectionless
messages, the SCCP provides
the protocol classes 0 and 1:


Protocol class 0


For the protocol class 0 the SCCP messages are sent singly and
independently of one another by the message transfer part.


Protocol class 1


For the protocol class 1 the SCCP messages are sent in the order
defined by the user.


Protocol class 2


For the setting up of a logical signaling connection (Connection
Oriented), the SCCP s of the signaling points of the signaling
relation concerned send their own originating point codes to one
another. In addition, they assign local references to the process for
which they set up a logical signaling connection (e.g. for using a
feature during an existing connection), and likewise inform one
another. Messages can then be exchanged. Each SCCP can assign
incoming messages to the process concerned by means of the local
reference. This protocol class guarantees for a correct message
order.


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ComSoc Lecture Meet

SCCP Procedures
-

Connectionless

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ComSoc Lecture Meet

SCCP Functional Blocks

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ComSoc Lecture Meet

SCCP Subsystems









Important Subsystems


MAPHLR


6 MAPVLR


7 MAPMSC


8 MAPEIR

9
BSSAP


254 CAP


146 SINAP
-

247 GPRS
-
149

SCCP

0

1

254

255

Subsystems

MAP

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ComSoc Lecture Meet

SCCP Examples


Connectionless SCCP :


Connectionless SCCP is used
for a number of GSM features like Roaming (GTT), SMS.
Other user parts like TCAP use connectionless SCCP
for transactions.



Connection Oriented SCCP :

Connection Oriented
SCCP is used for DATA/FAX feature in GSM. Other
User parts like BSSAP use Connection Oriented SCCP
for signaling messages exchanged between BSC and
MSC, and also for messages exchanged between MS
(Mobile Station) and MSC



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ComSoc Lecture Meet

Few Notes


There are other user parts like :
BSSSAP
,
TCAP

etc.


The
TCAP

is an application signaling protocol (OSI layer 7,
application layer)


The A


Interface is defined to be the interface between Base
Station Subsystem (BSS) and the Switching Subsystem (SSS). This
interface connects the BSC (Base station Controller) with The MSC
(Mobile Switching Center). At the A


Interface the SCCP supports
the subsystem known as

BSSAP

or
Base Station Subsystem
Application Part.



How to check the status of different bits in a signaling
message?



Use Network Analyzer with Protocol Tester

August 11, 2006

ComSoc Lecture Meet

Questions/ Queries





mailto : rahulghosh@ieee.org

August 11, 2006

ComSoc Lecture Meet




Thank You