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Telecom… Signaling
10/18/2012 1 Signalling TSSN JNTUHCEH
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Control Signal • Signalling system must be compatible with Transmission and the switching
systems of the network.
• Control signal may be continuous (DC off-hook signal) or Pulse signal
( dialed digit pulses).
• May or may not be acknowledged.
• Acknowledgement may be continuous or pulse signal.
• Continuous signal acknowledgements are called COMPELLED signaling.
• Control signalling Functions:
Call Request or Seize (forward)
Address Signal (forward)
Answer (backward)
Clear signals(forward and backward)
• Control signalling functions can be achieved using either Channel-Associated
signalling or Common-channel Signalling.
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Customer Line Signalling • Non-harmonically related
frequencies are used to
avoid ambiguity.
• * and # are used for
additional conversation.
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Types of Signaling
Signaling in Telecommunications Network
• Channel Associated Signaling (CAS) or
• Common Channel Signaling (CCS)
Signaling System Number (SS7) is a form of
Common Channel Signaling.
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Channel Associated Signaling (CAS)
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.
Enable a wide range of services:
Caller ID, toll-free calling.
Switch
A
Switch
B
Voice Trunks
Signaling Link
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CAS vs. CCS
Switch 1 Switch 2 Signaling and Speech
Channel Associated Signaling
Switch 1 Switch 2
Common Channel Signaling
STP STP
Signaling
Speech
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FDM carrier systems • Out-band signalling
• In-band or voice frequency (VF) Signalling
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PCM signalling
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Inter-Register Signalling • Advantage of link by link signalling:
1. Signals suffer transmission
impairments of a single link
2. Different signalling systems may be
used on different links.
• In end-to end signalling, the
originating register controls the
setting up of a connection until it
reaches its final destination
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Inter-Register Signalling • CCITT from signalling system
no.2 provides both forward and
backward signalling.
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Common channel signalling • Associated signalling—direct CCS
link between two exchanges.
• Non-associated signalling---CCS links
form a signalling network, and signal
passes through various intermediate
nodes.
• Quasi-associated signalling---CCS
messages routed through only one
intermediate node(signal Transfer
point).
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SS7 • out-of-band signaling architecture
• Functionalities- supporting
• call-establishment
• Billing
• routing, and
• information-exchange functions
• Created and controlled by various bodies around the world, but the principal
organization with responsibility for their administration is the International
Telecommunications Union or ITU-T.
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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 busty and of short duration, hence
operates in connectionless mode using packet switching
Variable length signal units with maximum size limitation
Optimum use of bandwidth
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Components of SS7
SSP STP SCP
SSP – Signaling Switching Points
STP - Signaling Transfer Points
SCP – Signaling Control Points
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Components of SS7 SSP’s:
Telephone switches equipped with SS7 software
Capable of originating, terminating or switch calls
STP’s: Packet switches of SS7 network
Route incoming signaling messages to proper destination SCP’s: Databases providing information necessary for advanced call-processing
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Typical SS7 Network
SSP’s STP’s
SCP’s
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SS7 Network Flow
signaling switching point (SSP): • attach directly to end
user • endpoints of SS7 network
signaling control point (SCP): •“services” go here • e.g., database functions
signaling transfer point (STP): • packet-switches of SS7 network
• send/receive/route signaling messages
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SS7 Network Architecture
STP
STP
STP STP
STP
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SS7 Link Types
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SS7 Link Types A link (access) Connects signaling end point (SCP or SSP) to STP
B link (bridge) Connects an STP to another STP; typically, a quad of B links
interconnect peer (or primary) STPs (STPs from a network
connect to STPs of another network)
C link (cross) Connects STPs performing identical functions, forming a
mated pair (for greater reliability)
D link (diagonal) Connects a secondary (local or regional) STP pair to a
primary (inter-network gateway) STP pair in a quad-link
configuration; the distinction between B and D links is
arbitrary
E link (extended) Connects an SSP to an alternate STP
F link
(fully associated)
Connects two signaling end points (SSPs and SCPs) in the
same local network
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SS7 Protocol Suite
ISUP TCAP
SCCP
MAP
MTP Level 3
MTP Level 2
MTP Level 1
OSI Layers
Application
Presentation
Session
Transport
Network
Data Link
Physical
OMAP
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Message Transfer Part • The function of MTP is to ensure the signaling traffic can be
transferred and delivered reliably between the end-users and the
network.
• MTP is provided at three levels
• Signaling data link functions (MTP Level 1) provide an
interface to the actual physical channel over which
communication takes place.
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MTP Level 2
• Signaling link functions (MTP Level 2) correspond to the second layer in
the OSI reference model.
• Provides a reliable link for the transfer of traffic between two directly
connected signaling points.
• Variable Length packet messages (called message signal units MSUs) are
defined in MTP Level 2.
• Implements flow control, error detection and correction.
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MTP Level 3 • Signaling network functions (MTP Level 3) provide procedures that
transfer messages between signaling nodes.
• Level 3 have two major functions as Signaling Message Handling and
Signaling Network Management.
• Signaling message handling is used to provide routing, distribution and
traffic discrimination.
• Traffic discrimination is the process by which a signaling point
determines whether or not a packet data messages in intended for its use
or not.
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CCITT signalling System no.7
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ISUP (Integrated Services User Part)
It defines the procedures used to setup, manage, and release trunk circuits that carry
voice and data calls over the public switched telephone network (PSTN).
Provides services to ISDN applications.
Most used to set up and tear down phone calls between switches.
ISUP messages
IAM, initiate a call
ANM, a call has been accepted
REL, a call disconnect
A connection-oriented protocol
Related to the establishment of connections between users
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TCAP (Transaction capabilities applications part) • TCAP in SS7 refers to the application layer which invokes the services of
SCCP and the MTP in a hierarchical format
• One application at a node is thus able to execute an application at another
node and use these results.
• For connectionless signaling
• Thus, TCAP is concerned with management of transactions and procedures
between remote applications.
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OMAP (Operation Maintenance and Administration Part)
• OMAP functions
– Monitoring
– Coordination
– Control functions to ensure that trouble free
communications are possible.
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Example: signaling a POTS call
1. caller goes offhook, dials callee. SSP A decides to route call via SSP B. Assigns idle trunk A-B
A B
W
X
Y
2. SSP A formulates Initial Address Message (IAM), forwards to STP W
3. STP W forwards IAM to STP X
4. STP X forwards IAM SSP B
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Example: signaling a POTS call
5. B determines it serves callee, creates address completion message (ACM[A,B,trunk]), rings callee phone, sends ringing sound on trunk to A
A B
W
X Y
Z 7. SSP A receives ACM,
connects subscriber line to allocated A-B trunk (caller hears ringing)
6. ACM routed to Z to Y to A
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Example: signaling a POTS call
8. Callee goes off hook, B creates, sends answer message to A (ANM[A,B,trunk])
A B
W
X Y
Z
10. SSP A receives ANM, checks caller is connected in both directions to trunk. Call is connected!
9. ANM routed to A
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Example: signaling a 800 ca11
800 number: logical phone number
• Translation to physical phone number needed, e.g., 1-800-CALL_ATT translates to 162-962-1943
A B
W M
1. Caller dials 800 number, A recognizes 800 number, formulates translation query, send to STP W
2. STP W forwards request to M
3. M performs lookup, sends reply to A
A
Y