Signallingsystem7

Signaling System #7 (SS7)

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Introduction 

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There are two essential components to all telephone calls. The first, and most obvious, is the actual content—our voices, faxes, data, etc. The second is the information that instructs telephone exchanges to establish connections and route the “content” to an appropriate destination. Signaling is concerned with the creation of standards for the latter to achieve the former. These standards are known as protocols.

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SS7 or Signaling System Number 7 is a set of protocols that describe a means of communication between telephone switches in public telephone networks.

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What is Signaling? 

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Signaling refers to the exchange of information between call components required to provide and maintain service. Examples of signaling between a telephone user and the telephone network include: dialing digits, providing dial tone, accessing a voice mailbox, sending a call-waiting tone etc.,

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PSTN Signaling 

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In order to route telephone traffic through the Public Switched Telephone Network (PSTN), it is necessary to communicate with the switches that make up the PSTN. Signaling is a means for transferring network-related information between switching nodes, and also between the end office switches and their subscribers.

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Signaling is used to do the following: Request service from the central office switch (via going off-hook). Provide central office switch with the information necessary to route a telephone call (via DTMF addressing digits in a specific format). Alert destination address of incoming call (ringing). Provide status information and call supervision for billing. Manage network lines/trunks (set up and teardown calls).

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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

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Signaling is transmitted in the same frequency band as used by voice.

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Voice path is established when the call setup is complete, using the same path that the call setup signals used. 8

Common Channel Signaling  

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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 Voice calling. Trunks Switch A

Switch B Signaling Link

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Out-of-band signaling establishes a separate digital channel for the exchange of signaling information. This channel is called a signaling link. Signaling links are used to carry all the necessary signaling messages between nodes. Thus, when a call is placed, the dialed digits, trunk selected, and other pertinent information are sent between switches using their signaling links, rather than the trunks which will ultimately carry the conversation. 10

CAS vs. CCS Switch 1

Signaling and Speech

Switch 2

Channel Associated Signaling

Signaling

STP

Switch 1

STP Speech

Switch 2

Common Channel Signaling 11

Advantage of CCS over CAS 

Faster call setup

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It allows for signaling at any time in the entire duration of the call, not only at the beginning.

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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 allows for signaling between any switches (not just directly-connected )

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SS7 • Signaling System 7 (SS7) is an architecture for performing out-of-band signaling in support of the call-establishment, billing, routing, and information-exchange functions of the public switched telephone network (PSTN). • They have been 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 

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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. 14

SS7 Principle   

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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

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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

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Capable of originating, terminating or switch calls

STP’s: 

Packet switches of SS7 network

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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

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SS7 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 19

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

Connects two signaling end points (SSPs and SCPs) in the same local network

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SS7 Protocol Suite OSI Layers Applicatio n Presentatio n Session Transport

OMAP

MAP

TCAP

ISUP

SCCP

Network

MTP Level 3

Data Link

MTP Level 2

Physical

MTP Level 1 23

The Message Transfer Part 

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The function of MTP is to ensure the signaling traffic can be transferred and delivered reliably between the endusers 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. 24

MTP Level 1 contd.., 

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Physical channels may include copper wire, twisted pair, fiber, mobile radio or satellite links. CCITT recommends 64 kbps transmissions, while ANSI recommends 56 kbps.

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MTP Level 2 

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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. 26

MTP Level 3 

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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. 27

MTP Level 3 contd..,   

Signaling network management allows Determine the outgoing link Rerouting in case of link failure Allocate alternate routing facilities in case of congestion or blockage in parts of the network.  Provides a number of services to the protocol layer above it o o

The transfer of messages indicating availability of resources MTP-Transfer request, MTP-Transfer indication, MTP-Pause indication, MTP-Resume indication, and MTP-Status indication

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SCCP (Signaling Connection Control Part) 

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SCCP provides connectionless and connection-oriented network services It also provides enhanced addressing mechanism known as GTT-global title translation. A global title is an address (e.g., a dialed 800 number, calling card number) which is translated by SCCP into a destination point code.

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Telephone User Part (TUP) 

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Telephony User Part (TUP) provides conventional PSTN telephony services across the SS7 network. TUP was the first layer 4 protocol defined by the standards bodies and as such did not provision for ISDN services. Prior to the introduction of ISUP, national variants of TUP have evolved which provide varying degrees of support for ISDN. 30

ISUP (Integrated Services User Part) 

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The ISDN User Part (ISUP) provides the services required by the Integrated Services Digital Network (ISDN). ISDN supports basic telephony in a manner similar to TUP, but with a greater variety of messages and parameters in order to implement ISDN type services within the network. Many telephony networks worldwide are migrating to ISUP. 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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ISUP (Integrated Services User Part) Contd….. Telephone User Part (TUP): 

Basic call setup and tear down.

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In many countries, ISUP has replaced TUP for call management.

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ISUP Messages 

Initial address message (IAM): contains all necessary information for a switch to establish a connection.

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Address complete message (ACM): acknowledge to IAM; the required circuit is reserved and the “phone is ringing” (ring back tone).

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Answer message (ANM): occurs when the called party picks up the phone.

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Release (REL): sent by the switch sensing that the phone hung up.

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Release complete (RLC): each exchange that receives REL, sends an RLC message back (this acknowledges receipt of REL)

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TCAP (Transaction capabilities applications part) 

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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) 

The OMAP functions include monitoring, coordination and control functions to ensure that trouble free communications are possible

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Example: signaling a POTS call 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

4. STP X forwards IAM SSP B

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X A

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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 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

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Z

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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]) 9. ANM routed to A

10. SSP A receives ANM, checks caller is connected in both directions to trunk. Call is connected!

A

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Z

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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

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 39

Example: signaling a 800 ca11 M

1. A begins signaling to set up call to number associated with 800 number

W

Z

X A A

B

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Applications          

Prepaid Calling Local Number Portability (LNP) Global Roaming International Callback Internet Call Waiting/ Caller ID Least Cost Routing 800 / Free Phone Services Tele-voting Location-based Services Caller Ring Back Tone (CRBT)

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Limitations of SS7 

SS7 was not designed for Data Applications

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Voice was the only consideration

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Limited or no Intelligence

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Doesn’t provide any information about Session Capabilities

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Today's Demands       

Presence Management Media Negotiation Convergence Enhanced Telecom Service Digital whiteboard Conferencing Intelligent Call Routing Enhanced Telecom Services   

Find – Me Follow – Me Do – Not – Disturb 43

Features of VoIP 

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Packet Switched (Better utilization of Bandwidth) Voice can be compressed up to 4 to 8 kbps compared to 64kbps of Uncompressed speech

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Silence periods can be removed

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Call setup (signaling) is out-of-Band

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Billing can be made independent of Distance

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Advanced telephony features are possible with VOIP 44

Some Applications of VoIP 

Click to talk web sites for e-commerce

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Digital whiteboard conferencing

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Broadcast audio and video

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Unified Messaging

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Fax over IP 45

Session Initiation Protocol (SIP)  

IETF Standard Signaling Protocol Establishes multimedia Sessions

Features of SIP 

User Location

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User Availability

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Call Setup

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User Capabilities

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Session Management 46

Sip Components User Agents (phones, gateways)  

UserAgentClient UserAgentServer

Servers   

Proxy Server Registrar Server Redirect Server 47

SIP Requests 

INVITE

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MESSAGE

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ACK

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BYE

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CANCEL

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REGISTER

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OPTIONS

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SIP Responses Provisional Responses (Indicate Progress) 

1XX(ringing, queuing): 100 Trying 180 Ringing

Final Responses (Terminate SIP Transactions)     

2XX(success): 200 OK 3XX(redirection): 301 Moved permanently 4XX(client failure): 401 Unauthorized 5XX(server failure): 500 Server Error 6XX(global failure): 600 Busy 49

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