Signaling System #7 Signaling System #7

ContentsContents

Types of Signaling SS7 Signaling SS7 Protocol Architecture SS7 Network Architecture Basic Call Setup SS7 Applications SS7/IP Inter-working VoIP Network Configuration SIGTRAN Protocol Stack Model Network Evolution to an All-IP Network

Types of Signaling Types of Signaling

Signaling in Telecommunications NetworkChannel Associated Signaling (CAS)Common Channel Signaling (CCS)

Signaling System Number (SS7) is a form of Common Channel Signaling.

Channel Associated Signaling Channel Associated Signaling (CAS)(CAS)

Used for 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 SignalingCommon 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.

SwitchA

SwitchB

Voice Trunks

Signaling Link

Advantage of CCS over CASAdvantage of CCS over CAS

Faster call setup No interference between signaling tones by

network and frequency of human speech pattern. 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 and voice or data identification

SS7 HistorySS7 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.

SS7 PrincipleSS7 Principle

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 Reliability and flexibility

SS7 Protocol StackSS7 Protocol StackOSI SS7

Message Transfer Part (MTP Level 1) Physical

Provides an interface to the actual physical channel over which communication takes place

CCITT recommends 64Kbps transmission whereas ANSI recommends 56 Kbps

ProtocolsProtocols

ProtocolsProtocols

MTP Level 2 (Data Link)

Ensures accurate end-to-end transmission of a message across a signaling link

Variable Length Packet Messages are defined here Implements flow control, message sequence validation,

error checking and message retransmission Monitor links and reports their status Test links before allowing their use Provides sequence numbers for outgoing messages

ProtocolsProtocols

MTP Level 3 (Network)

Message routing between signaling points in the SS7 network

Signaling network management that provides traffic, links and routing management, as well as congestion (flow) control

Re-routes traffic away from failed links and signaling points, controls traffic when congestion occurs

ProtocolsProtocols

Signaling Connection Control Part (SCCP)

Provides connectionless and connection-oriented network services

Provides global title translation (GTT) capabilities above MTP level 3; translates numbers to DPCs and subsystem numbers

Provides more detailed addressing information than MTPs

Used as transport layer for TCAP (Transaction capabilities applications part) based services

ProtocolsProtocols

Transaction Capabilities Applications Part (TCAP)

Exchange of non-circuit related data– Between applications across the SS#7 network – Using the SCCP service

Queries and responses sent between Signaling Switching Point (SSPs) and Signaling Control Point (SCPs)

Sends and receives database information– Credit card validation – Routing information

ProtocolsProtocols

Telephone User Part (TUP) Basic call setup and tear down In many countries, ISUP has replaced TUP for call

management

ISDN User Part (ISUP) Necessary messaging for setup and tear down of all

circuits (voice and digital) Messages are sent from a switch, to the switch

where the next circuit connection is required Call circuits are identified using circuit

identification code (CIC)

SS7 NetworksSS7 Networks

STP

STP

STP STP

STP

SS7 ComponentsSS7 Components

Service switching point (SSP)Service switching point (SSP)– SSPs are switches that have SS7 software and

terminating signaling links– SSPs create packets (signal units) and send

those messages to other SSPs, as well as queries to remote shared databases to find out how to route calls

– SSPs communicate with the voice switch via the use of primitives and have the ability to send messages using ISUP (call setup and teardown) and TCAP (database lookup) protocols.

– The switch can originate, terminate, or switch calls

Signaling transfer point (STP)Signaling transfer point (STP)

–STPs are packet switches, and act like routers in the SS7 network.–Routes each incoming message to an outgoing signaling link, based on routing information contained in the SS#7 message and a pre-defined route table–Does not offer termination services–STPs are paired to ensure redundancy

There are three levels of STPs.

•National Signal Transfer Point

•International Signal Transfer Point

•Gateway Signal Transfer Point

STP LevelsSTP Levels

SSP

SSP SSP

SSP

STP

STP

STPNATIONAL

INTERNATIONAL

GATEWAY

ITU-TS

ANSI

National STP exists within the national network Protocol converters often interconnect a National and an

International STP by converting from ANSI to ITU-TS.

International STP functions within an international network. All nodes connecting to an International STP must use the ITU-

TS protocol standard.

Gateway STP converts signaling data from one protocol to another.

Gateway STPs are often used as an access point to the international network.

Depending on its location, the Gateway STP must be able to use both the International and National protocol standards.

Service control point (SCP)Service control point (SCP)– An SCP is usually a computer used as a front end to a

database system.– It is an interface to application-specific databases.– The address of an SCP is a point code, and the address

of the database it interfaces with is a subsystem number.

– The database is an application entity which is accessed via the TCAP protocol.

– Databases that provides information necessary for advanced call processing capabilities

– Accepts a query for information from a subsystem at another node

– Used by STP to perform a function called global title translation

Databases Accessible via SCPDatabases Accessible via SCP

HLR

Home Location Register

Used in cellular networks to store subscriber information.

LNP

Local Number Portability

Allows people to change service providers but keep their same telephone number

OSS

Operation Support System

Associated with remote maintenance center for monitoring and managing SS7 and voice networks.

VLR

Visitor Location Register

Used when a cell phone is not recognized by the mobile switching center (MSC).

SS7 Link TypesSS7 Link Types

STP STP

STP

STP

STP

STP

STP

STP

SS7 Link TypesSS7 Link TypesA 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

Addressing in SS7 NetworkAddressing in SS7 Network

Point Codes are carried in signaling messages and exchanged between signaling points to identify the source and destination of each message (24 bit address).

NETWORK IDENTITY

MEMBER IDENTITY

1 1 1

CLUSTERIDENTITY

What goes What goes overover Signaling Link Signaling Link

Signaling information is passed over the signaling link in form of messages, which are called signaling units (SUs)

3 Types of SUs are:Message signal units(MSUs)Link status signal units(LSSUs)Fill-in signal units(FISUs)

Message signal unit (MSU)

Carries signaling associated with call setup & teardown, database query and response and SS7 network input

BSN/BIB

FLAG LengthIndicator

FSN/FIB

Checksum

SignalingInfo. field

ServiceInfo. octet

1 1 11 1 8-272 1

Signaling UnitsSignaling Units

Service Information OctetService Information Octet

1 2 3 4 5 6 7 8

• Bit 1 - 4 Bit 1 - 4 Type of Information in Signaling Type of Information in Signaling Information FieldInformation Field

• Bit 5 - 6 Bit 5 - 6 Whether Message is intended for National Whether Message is intended for National or International Networkor International Network

• Bit 7 - 8 Bit 7 - 8 To identify Message Priority To identify Message Priority

Signaling Information FieldSignaling Information Field

8 – 272 Octets The first portion of this field is routing label Routing Label Identifies Message Originator,

Intended destination & Signaling Link Selected. Routing Label is of 7 Octets

– Destination Point Code – 3 Octets– Originating Point Code – 3 Octets– Signaling Link Selection – 1 Octet

Link status signaling units (LSSU)

– Inform the far end about the changes in status of link

– Message length can be 1 or 2 bytes

FLAG BSN/BIB

FSN/FIB

LengthIndicator

CheckSum

1 1 1 1 1

Status Field

1 or 2

Signaling UnitsSignaling Units

FLAG BSN/BIB

FSN/FIB

LengthIndicator

CheckSum

1 1 1 1 1

Fill-In Signal Units (FISU)– Fill the gaps between MSU and LSSU messages– Sent only when the buffer is empty, to keep the

signaling link active– Facilitate in constant monitoring of link quality.

Signaling UnitsSignaling Units

Basic Call Setup ExampleBasic Call Setup Example

1

6,10

9

2

513

15

ISUP Messages ISUP Messages Initial address message (IAM): contains all necessary

information for a switch to establish a connection Address complete message (ACM): acknowledge to

IAM; the required circuit is reserved and the “phone is ringing” (ring back tone)

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

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

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

ApplicationsApplications

Prepaid Calling Local Number

Portability (LNP) Global Roaming International Callback Virtual Office Internet Call Waiting/

Caller ID Least Cost Routing Toll Bypass

Unified Messaging 800 / Free Phone

Services Short Message

Service (SMS) Tele-voting Location-based

Services Caller Ring Back

Tone (CRBT)

Implementation of SS7 in GSMImplementation of SS7 in GSM

Um

Abis

ABSS

radiosubsystem

MS MS

BTSBSC

BTS

BTSBSC

BTS

network and switching subsystem

MSC

MSC

Fixed partner networks

IWF

ISDNPSTN

PDN

SS

7

EIR

HLR

VLR

ISDNPSTN

SS7/IP Inter-workingSS7/IP Inter-working IP offers an economical solution to challenges

created by high volume traffic at network edge. Operators off load voice calls from PSTN to VoIP

networks because it is less costly to carry voice traffic over IP network than over Switched circuit network.

In VoIP network, digitized voice data is highly compressed and carried in packets over IP network, thereby efficiently utilizing the bandwidth and increasing the number of voice calls carried.

Saving realized in using VoIP network are passed on to users in the form of lower cost.

Features of SS7 over IPFeatures of SS7 over IP

Flow Control In-Sequence Delivery of Signaling Messages

within a single control stream Identification of the originating and terminating

signaling points Identification of voice circuits Error detection, re-transmission and other error

correcting procedures. Controls to avoid congestion on the internet. Detection of status of peer entities. Support for security mechanism.

Signaling in VoIP networksSignaling in VoIP networks

VoIP Network carry SS7 over IP using protocols defined by Signaling Transport (SIGTRAN) working group of the Internet Engineering Task Force (IETF).

In IP telephony networks, signaling information is exchanged between the following functional elements.Media GatewayMedia Gateway ControllerSignaling Gateway.

VoIP Network ConfigurationVoIP Network Configuration

Media Gateway- Terminates voice calls on Inter-switch Trunks from the PSTN, compresses and packetizes the voice data and delivers compress voice packet to the IP network. For voice calls originating in an IP network, it performs these functions in reverse order.

Media Gateway Controller- It handles the registration and management of resources at Media Gateway(s), also knows as Soft Switch.

Signaling Gateway- It provides Transparent interworking of signaling between switched ckt. and IP network. It may terminate SS7 signaling or translate and relay messages over an IP network to a media gateway controller or another signaling gateway.

SIGTRAN Protocol Stack ModelSIGTRAN Protocol Stack ModelThe SIGTRAN’s protocol specify the means by which SS7 messages can be reliably transported over IP network.

The architecture identifies three componentA standard IPA common signaling transport protocol for SS7 protocol layer being carried.An adaptation module to emulate lower layers of the protocol.

Allows the reliable transfer of signaling messages between signaling end points in an IP network

Allows signaling messages to be independently ordered with in multiple streams to ensure in sequence delivery between associated end points.

SIGTRAN recommends SCTP rather than TCP/IP for transmission of signaling messages over IP networks as TCP/IP does enforce Head-of-line Blocking.

Stream Control Transmission Stream Control Transmission Protocol(SCTP)Protocol(SCTP)

Transporting MTP over IPTransporting MTP over IP

ITU specified following requirements– MTP Level 3 peer to peer procedure require response

time with in 0.5 – 1.2 sec.– No more than 1 in 10 million messages will be lost due

to transport failure.– No more than 1 in 10 billion messages will be delivered

out of sequence.– No more than 1 in 10 billion messages will contain an

error.– Availability of any signaling route set is 99.9998%.– Message length is 272 Bytes for SS7 (MAXIMUM)

IETF SIGTRAN working group recommends 3 new protocols: M2UA, M2PA and M3UA.

M2PAM2PAM2PA:- MTP2 User Peer-to-Peer Adaptation Layer

–Support the transport of SS7 MTP3 signaling messages over IP using the services of SCTP–Allows full MTP Level3 message handling and network management capabilities between any 2 SS7 nodes communicating over IP–Used b/w signaling gateway & media gateway controller, signaling gateway & IP signaling point, 2 IP signaling points.

M2UAM2UA

M2UA:- MTP2 User Adaptation Layer –Transports SS7 MTP Level2 user messages over IP using SCTP

–Provides services as MTP Level2 provides to MTP Level3

–Used between signaling gateway and media gateway controller.

M3UAM3UA M3UA:- MTP 3 User

Adaptation Layer – Transports SS7 MTP

Level3 user signaling messages over IP using SCTP

– Provides services as MTP Level3 provides to ISUP,TUP,SCCP

– Used between signaling gateway and media gateway controller or IP telephony database

SUASUA

SUA: SCCP User Adaptation Layer– Transports SS7

SCCP User Part Signaling Message over IP using SCTP

– Used between signaling gateway and IP signaling end point and between IP signaling end points.

Performance consideration & Performance consideration & Security Requirement for SS7/IPSecurity Requirement for SS7/IP

SS7 over IP network must meet ITU standard and user expectations, eg. ITU specifies end to end call setup delay not more than 20-30 sec after IAM is transmitted.

For transmission of signaling information over internet, SIGTRAN recommends use of IPSEC, which provides following security services– Authentication– Integrity– Confidentiality– Availability

Network Evolution to an All-IP Network Evolution to an All-IP NetworkNetwork

Both traditional Circuit switch and IP based services need to be supported by single network infrastructure simultaneously.

Hybrid architecture may be the best solution. Transition to All-IP network will not happen overnight.

IP Network

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