Telecom… Signaling
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Signalling
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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. Voice Trunks Switch A
Switch B
Signaling Link 6
CAS vs. CCS Switch 1
Signaling and Speech
Switch 2
Channel Associated Signaling
Signaling
STP
Switch 1
STP Speech
Switch 2
Common Channel Signaling 7
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). 10/18/2012
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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. 13
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 16
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
SCP’s
SSP’s
STP’s 18
SS7 Network Flow signaling transfer point (STP): • packet-switches of SS7 network
• send/receive/route signaling messages
signaling control point (SCP): •“services” go here • e.g., database functions
signaling switching point (SSP):
• attach directly to end user • endpoints of SS7 network
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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 OSI Layers Application
OMAP
MAP
Presentation Session
TCAP
Transport
SCCP
ISUP
Network
MTP Level 3
Data Link
MTP Level 2
Physical
MTP Level 1 23
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. 26
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 28
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 4. STP X forwards IAM SSP B 2. SSP A formulates Initial Address Message (IAM), forwards to STP W
3. STP W forwards IAM to STP X
Y 1. caller goes offhook, dials callee. SSP A decides to route call via SSP B. Assigns idle trunk A-B
W
X A
B 31
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 6. ACM routed to Z to Y to A
7. SSP A receives ACM, connects subscriber line to allocated A-B trunk (caller hears ringing)
A
W
Z
Y
X B 32
Example: signaling a POTS call 8. Callee goes off hook, B creates, sends answer message to A (ANM[A,B,trunk])
9. ANM routed to A
10. SSP A receives ANM, checks caller is connected in both directions to trunk. Call is connected!
A
W
Z
Y
X B 33
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 3. M performs lookup, sends reply to A
2. STP W forwards request to M
1. Caller dials 800 number, A recognizes 800 number, formulates translation query, send to STP W
M
W
Y A A
B 34
