Telecommunication Switching system Signalling.pdf

Telecom… Signaling

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.

Customer Line Signalling

• Non-harmonically related

frequencies are used to

avoid ambiguity.

• * and # are used for

additional conversation.

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.

5

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.

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

7

CAS vs. CCS

Switch 1 Signaling and Speech Switch 2

Channel Associated Signaling

Switch 1 Switch 2

Common Channel Signaling

STP STP

Signaling

FDM carrier systems

• Out-band signalling

PCM signalling

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

Inter-Register Signalling

• CCITT from signalling system

no.2 provides both forward and

backward signalling.

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).

13

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.

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.

15

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

Components of SS7

SSP

STP

SCP

SSP

– Signaling Switching Points

STP

- Signaling Transfer Points

17

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

Typical SS7 Network

SSP’s STP’s

19

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

SS7 Network Architecture

STP

STP

STP STP

21

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

23

SS7 Protocol Suite

ISUP

TCAP

SCCP

MTP Level 3

MTP Level 2

MTP Level 1

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.

25

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.

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

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

29

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.

OMAP

(Operation Maintenance and Administration Part)

• OMAP functions

– Monitoring

– Coordination

– Control functions to ensure that trouble free

communications are possible.

31

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

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)

33

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!

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