Signaling System for GSM Networks

1
Signaling System for GSM Networks

• Rabindranath Nandi
• Rahul Ghosh

2
Acknowledgement

• Mr. Suvadip Basu
• Deputy Manager,
• Switch Operations
• Hutchison Telecom East Ltd.
• Kolkata 700 017

3
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
• SS7 or CCS7 (Common Channel Signaling 7)
• CAS (Channel associated Signaling)
• DTMF (Dual Tone Multi frequency)

4
Areas of Signaling

• There are mainly three areas of signaling during
  a telephone call
• between subscribers and exchanges.
• within exchanges.
• between exchanges.

between exchanges
Exchange A
Exchange B
between Subscriber and exchange
within exchange
5
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
Signaling
Signaling
Exchange A
Exchange B
Speech
Speech
Signaling
Signaling
6
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.

Speech
Exchange A
Exchange B
Speech
Common Signaling Channel
7
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.

8
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

9
CCS7 Network Architecture

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

STP (SPC200)
SP (SPC100)
SEP (SPC500)
SRP (SPC400)
STP (SPC300)
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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.

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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
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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.

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SS7 Network Structure
SPC100
SPC700
NAT 1
SPC300
NAT 1
SPC200
SPC800
SPC900
SPC800
SPC100
NAT 0
SPC100
SPC400
SPC400
SPC100
SPC600
NAT 1
SPC300
INAT 0
SPC500
SPC200
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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.

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SS7 Protocol Structure
SS7 Users
I S U P
B S S A P
TCAP Users
M U P
Other User Parts
TCAP
SCCP
MTP Layer 3 (Network Management)
MTP Layer 2 (Signaling Link)
MTP Layer 1 (Signaling Data Link)
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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.

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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.

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The Message Transfer Part
Signal Unit Formats
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The Message Transfer Part

• 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).

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The Message Transfer Part

• 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.

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The Message Transfer Part

• 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.

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The Message Transfer Part
Routing label of a message signal unit
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The Message Transfer Part

• 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

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The Message Transfer Part

• 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.

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The Message Transfer Part
MTP Functions
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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
  

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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.

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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.

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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.

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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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ISDN User Part Call Setup Procedures
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ISDN User Part Call Release Procedures
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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

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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

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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

36
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).

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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.

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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

39
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

40
SCCP Protocol Classes
41
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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SCCP Procedures - Connectionless
43
SCCP Functional Blocks
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SCCP Subsystems

• Important Subsystems
• MAPHLR 6 MAPVLR 7 MAPMSC 8 MAPEIR9 BSSAP
  254 CAP 146 SINAP - 247 GPRS -149

0
1
254
255
Subsystems
MAP
SCCP
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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

46
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

47
Questions/ Queries

• mailto rahulghosh_at_ieee.org

48

• Thank You