Telecom… Signaling

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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

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.

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