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Signaling System 7
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Contents

• Types of Signaling
• SS7 Signaling
• SS7 Protocol Architecture
• SS7 Network Architecture
• Basic Call Setup
• SS7 Applications
• SS7/IP Inter-working
• VoIP Network Configuration
• SIGTRAN Protocol Stack Model
• Network Evolution to an All-IP Network

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Types of Signaling

• Signaling in Telecommunications Network
• Channel Associated Signaling (CAS)
• Common Channel Signaling (CCS)
• Signaling System Number (SS7) is a form of Common
  Channel Signaling.

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Channel Associated Signaling (CAS)

• Used for In-Band Signaling
• Signaling is transmitted in the same frequency
  band as used by voice.
• Voice path is established when the call setup is
  complete, using the same path that the call setup
  signals used.

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Common Channel Signaling

• Out of Band signaling
• Employs separate, dedicated path for signaling.
• Voice trunks are used only when a connection is
  established, not before.
• Faster Call Setup.

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Advantage of CCS over CAS

• Faster call setup
• No interference between signaling tones by
  network and frequency of human speech pattern.
• Greater Trunking Efficiency- CCS has shorter
  call set up and tear down times that result in
  less call holding time, thereby reducing the
  traffic on the network.
• Information Transfer- CCS allows the transfer of
  additional information along with the signaling
  traffic providing facilities such as caller
  identification and voice or data identification

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SS7 History

• CCITT developed a digital signaling standard
  called Signaling System 6
• SS6 was based on Packet-Switched, proprietary
  data network.
• Uses 2.4 Kbps data links to send packets of data
  to distant switches to request service.
• SS7 began deployment in 1983, was initially used
  for inter office network, but now it is deployed
  in local central offices.
• Provide a global standard for call setup,
  routing, control and database access.

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SS7 Principle

• Out of band Signaling
• Higher Signaling data rates (56Kbps 64 Kbps)
• Signaling traffic is bursty and of short
  duration, hence operates in connectionless mode
  using packet switching
• Variable length signal units with maximum size
  limitation
• Optimum use of bandwidth
• Reliability and flexibility

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SS7 Protocol Stack
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Protocols

• Message Transfer Part (MTP Level 1) Physical
• Provides an interface to the actual physical
  channel over which communication takes place
• CCITT recommends 64Kbps transmission whereas ANSI
  recommends 56 Kbps

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Protocols

• MTP Level 2 (Data Link)
• Ensures accurate end-to-end transmission of a
  message across a signaling link
• Variable Length Packet Messages are defined here
• Implements flow control, message sequence
  validation, error checking and message
  retransmission
• Monitor links and reports their status
• Test links before allowing their use
• Provides sequence numbers for outgoing messages

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Protocols

• MTP Level 3 (Network)
• Message routing between signaling points in the
  SS7 network
• Signaling network management that provides
  traffic, links and routing management, as well as
  congestion (flow) control
• Re-routes traffic away from failed links and
  signaling points, controls traffic when
  congestion occurs

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Protocols

• Signaling Connection Control Part (SCCP)
• Provides connectionless and connection-oriented
  network services
• Provides global title translation (GTT)
  capabilities above MTP level 3 translates
  numbers to DPCs and subsystem numbers
• Provides more detailed addressing information
  than MTPs
• Used as transport layer for TCAP (Transaction
  capabilities applications part) based services

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Protocols

• Transaction Capabilities Applications Part (TCAP)
• Exchange of non-circuit related data
• Between applications across the SS7 network
• Using the SCCP service
• Queries and responses sent between Signaling
  Switching Point (SSPs) and Signaling Control
  Point (SCPs)
• Sends and receives database information
• Credit card validation
• Routing information

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Protocols

• Telephone User Part (TUP)
• Basic call setup and tear down
• In many countries, ISUP has replaced TUP for call
  management
• ISDN User Part (ISUP)
• Necessary messaging for setup and tear down of
  all circuits (voice and digital)
• Messages are sent from a switch, to the switch
  where the next circuit connection is required
• Call circuits are identified using circuit
  identification code (CIC)

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SS7 Networks
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SS7 Components
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Service switching point (SSP)

• SSPs are switches that have SS7 software and
  terminating signaling links
• SSPs create packets (signal units) and send those
  messages to other SSPs, as well as queries to
  remote shared databases to find out how to route
  calls
• SSPs communicate with the voice switch via the
  use of primitives and have the ability to send
  messages using ISUP (call setup and teardown) and
  TCAP (database lookup) protocols.
• The switch can originate, terminate, or switch
  calls

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Signaling transfer point (STP)

• STPs are packet switches, and act like routers in
  the SS7 network.
• Routes each incoming message to an outgoing
  signaling link, based on routing information
  contained in the SS7 message and a pre-defined
  route table
• Does not offer termination services
• STPs are paired to ensure redundancy
• There are three levels of STPs.
• National Signal Transfer Point
• International Signal Transfer Point
• Gateway Signal Transfer Point

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STP Levels
SSP
SSP
ITU-TS
INTERNATIONAL
STP
GATEWAY
STP
NATIONAL
STP
ANSI
SSP
SSP
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• National STP exists within the national network
• Protocol converters often interconnect a National
  and an International STP by converting from ANSI
  to ITU-TS.
• International STP functions within an
  international network.
• All nodes connecting to an International STP must
  use the ITU-TS protocol standard.
• Gateway STP converts signaling data from one
  protocol to another.
• Gateway STPs are often used as an access point to
  the international network.
• Depending on its location, the Gateway STP must
  be able to use both the International and
  National protocol standards.

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Service control point (SCP)

• An SCP is usually a computer used as a front end
  to a database system.
• It is an interface to application-specific
  databases.
• The address of an SCP is a point code, and the
  address of the database it interfaces with is a
  subsystem number.
• The database is an application entity which is
  accessed via the TCAP protocol.
• Databases that provides information necessary for
  advanced call processing capabilities
• Accepts a query for information from a subsystem
  at another node
• Used by STP to perform a function called global
  title translation

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Databases Accessible via SCP
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SS7 Link Types
STP
STP
STP
STP
STP
STP
STP
STP
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SS7 Link Types
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Addressing in SS7 Network

• Point Codes are carried in signaling messages and
  exchanged between signaling points to identify
  the source and destination of each message (24
  bit address).

1
1
1
NETWORK IDENTITY
MEMBER IDENTITY
CLUSTER IDENTITY
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What goes over Signaling Link

• Signaling information is passed over the
  signaling link in form of messages, which are
  called signaling units (SUs)
• 3 Types of SUs are
• Message signal units(MSUs)
• Link status signal units(LSSUs)
• Fill-in signal units(FISUs)

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Signaling Units

• Message signal unit (MSU)
• Carries signaling associated with call setup
  teardown, database query and response and SS7
  network input

1
1
1
1
1
8-272
1
FLAG
Length Indicator
FSN/ FIB
Check sum
Service Info. octet
BSN/ BIB
Signaling Info. field
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Service Information Octet

• Bit 1 - 4 ? Type of Information in Signaling
  Information Field
• Bit 5 - 6 ? Whether Message is intended for
  National or International Network
• Bit 7 - 8 ? To identify Message Priority

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Signaling Information Field

• 8 272 Octets
• The first portion of this field is routing label
• Routing Label Identifies Message Originator,
  Intended destination Signaling Link Selected.
• Routing Label is of 7 Octets
• Destination Point Code 3 Octets
• Originating Point Code 3 Octets
• Signaling Link Selection 1 Octet

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Signaling Units

• Link status signaling units (LSSU)
• Inform the far end about the changes in status of
  link
• Message length can be 1 or 2 bytes

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Signaling Units

• Fill-In Signal Units (FISU)
• Fill the gaps between MSU and LSSU messages
• Sent only when the buffer is empty, to keep the
  signaling link active
• Facilitate in constant monitoring of link quality.

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Basic Call Setup Example
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ISUP Messages

• Initial address message (IAM) contains all
  necessary information for a switch to establish a
  connection
• Address complete message (ACM) acknowledge to
  IAM the required circuit is reserved and the
  phone is ringing (ring back tone)
• Answer message (ANM) occurs when the called
  party picks up the phone
• Release (REL) sent by the switch sensing that
  the phone hung up
• Release complete (RLC) each exchange that
  receives REL, sends an RLC message back (this
  acknowledges receipt of REL)

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Applications

• Prepaid Calling
• Local Number Portability (LNP)
• Global Roaming
• International Callback
• Virtual Office
• Internet Call Waiting/ Caller ID
• Least Cost Routing
• Toll Bypass

• Unified Messaging
• 800 / Free Phone Services
• Short Message Service (SMS)
• Tele-voting
• Location-based Services
• Caller Ring Back Tone (CRBT)

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Implementation of SS7 in GSM
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SS7/IP Inter-working

• IP offers an economical solution to challenges
  created by high volume traffic at network edge.
• Operators off load voice calls from PSTN to VoIP
  networks because it is less costly to carry voice
  traffic over IP network than over Switched
  circuit network.
• In VoIP network, digitized voice data is highly
  compressed and carried in packets over IP
  network, thereby efficiently utilizing the
  bandwidth and increasing the number of voice
  calls carried.
• Saving realized in using VoIP network are passed
  on to users in the form of lower cost.

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Features of SS7 over IP

• Flow Control
• In-Sequence Delivery of Signaling Messages
  within a single control stream
• Identification of the originating and terminating
  signaling points
• Identification of voice circuits
• Error detection, re-transmission and other error
  correcting procedures.
• Controls to avoid congestion on the internet.
• Detection of status of peer entities.
• Support for security mechanism.

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Signaling in VoIP networks

• VoIP Network carry SS7 over IP using protocols
  defined by Signaling Transport (SIGTRAN) working
  group of the Internet Engineering Task Force
  (IETF).
• In IP telephony networks, signaling information
  is exchanged between the following functional
  elements.
• Media Gateway
• Media Gateway Controller
• Signaling Gateway.

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VoIP Network Configuration
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• Media Gateway- Terminates voice calls on
  Inter-switch Trunks from the PSTN, compresses and
  packetizes the voice data and delivers compress
  voice packet to the IP network. For voice calls
  originating in an IP network, it performs these
  functions in reverse order.
• Media Gateway Controller- It handles the
  registration and management of resources at Media
  Gateway(s), also knows as Soft Switch.
• Signaling Gateway- It provides Transparent
  interworking of signaling between switched ckt.
  and IP network. It may terminate SS7 signaling or
  translate and relay messages over an IP network
  to a media gateway controller or another
  signaling gateway.

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SIGTRAN Protocol Stack Model

• The SIGTRANs protocol specify the means by which
  SS7 messages can be reliably transported over IP
  network.
• The architecture identifies three component
• A standard IP
• A common signaling transport protocol for SS7
  protocol layer being carried.
• An adaptation module to emulate lower layers of
  the protocol.

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Stream Control Transmission Protocol(SCTP)

• Allows the reliable transfer of signaling
  messages between signaling end points in an IP
  network
• Allows signaling messages to be independently
  ordered with in multiple streams to ensure in
  sequence delivery between associated end points.
• SIGTRAN recommends SCTP rather than TCP/IP for
  transmission of signaling messages over IP
  networks as TCP/IP does enforce Head-of-line
  Blocking.

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Transporting MTP over IP

• ITU specified following requirements
• MTP Level 3 peer to peer procedure require
  response time with in 0.5 1.2 sec.
• No more than 1 in 10 million messages will be
  lost due to transport failure.
• No more than 1 in 10 billion messages will be
  delivered out of sequence.
• No more than 1 in 10 billion messages will
  contain an error.
• Availability of any signaling route set is
  99.9998.
• Message length is 272 Bytes for SS7 (MAXIMUM)
• IETF SIGTRAN working group recommends 3 new
  protocols M2UA, M2PA and M3UA.

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M2PA

• M2PA- MTP2 User Peer-to-Peer Adaptation Layer
• Support the transport of SS7 MTP3 signaling
  messages over IP using the services of SCTP
• Allows full MTP Level3 message handling and
  network management capabilities between any 2
  SS7 nodes communicating over IP
• Used b/w signaling gateway media gateway
  controller, signaling gateway IP signaling
  point, 2 IP signaling points.

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M2UA

• M2UA- MTP2 User Adaptation Layer
• Transports SS7 MTP Level2 user messages over IP
  using SCTP
• Provides services as MTP Level2 provides to MTP
  Level3
• Used between signaling gateway and media gateway
  controller.

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M3UA

• M3UA- MTP 3 User Adaptation Layer
• Transports SS7 MTP Level3 user signaling messages
  over IP using SCTP
• Provides services as MTP Level3 provides to
  ISUP,TUP,SCCP
• Used between signaling gateway and media gateway
  controller or IP telephony database

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SUA

• SUA SCCP User Adaptation Layer
• Transports SS7 SCCP User Part Signaling Message
  over IP using SCTP
• Used between signaling gateway and IP signaling
  end point and between IP signaling end points.

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Performance consideration Security Requirement
for SS7/IP

• SS7 over IP network must meet ITU standard and
  user expectations, eg. ITU specifies end to end
  call setup delay not more than 20-30 sec after
  IAM is transmitted.
• For transmission of signaling information over
  internet, SIGTRAN recommends use of IPSEC, which
  provides following security services
• Authentication
• Integrity
• Confidentiality
• Availability

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Network Evolution to an All-IP Network

• Both traditional Circuit switch and IP based
  services need to be supported by single network
  infrastructure simultaneously.
• Hybrid architecture may be the best solution.
• Transition to All-IP network will not happen
  overnight.

IP Network
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