Transcript Signaling System #7

Signaling System #7
Contents
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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
Signaling in Telecommunications Network
Channel Associated Signaling (CAS)
Common Channel Signaling (CCS)
Signaling System Number (SS7) is a form
of Common Channel Signaling.
Channel Associated Signaling
(CAS)
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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 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.
Voice Trunks
Switch
A
Switch
B
Signaling Link
Advantage 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 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.
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
Reliability and flexibility
SS7 Protocol Stack
OSI
SS7
Protocols
Message Transfer Part (MTP Level 1) Physical
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Provides an interface to the actual physical channel over
which communication takes place
 CCITT recommends 64Kbps transmission whereas ANSI
recommends 56 Kbps
Protocols
MTP Level 2 (Data Link)
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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
Protocols
MTP Level 3 (Network)
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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
Protocols
Signaling Connection Control Part (SCCP)
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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
Protocols
Transaction Capabilities Applications Part (TCAP)
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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
Protocols
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 Networks
STP
STP
STP
STP
STP
SS7 Components
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)
–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 Levels
SSP
INTERNATIONAL
GATEWAY
NATIONAL
ITU-TS
SSP
STP
STP
STP
ANSI
SSP
SSP
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 ITUTS 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)
– An SCP is usually a computer used as a front end to a
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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 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 Types
STP
STP
STP
STP
STP
STP
STP
STP
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
Connects two signaling end points (SSPs and SCPs)
(fully associated) in the same local network
Addressing 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).
1
NETWORK
IDENTITY
1
CLUSTER
IDENTITY
1
MEMBER
IDENTITY
What goes over 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)
Signaling Units
Message signal unit (MSU)
Carries signaling associated with call setup & teardown,
database query and response and SS7 network input
1
1
FLAG BSN/
BIB
1
FSN/
FIB
1
1
8-272
1
Length
Service Signaling Check
Indicator Info. octet Info. field sum
Service Information Octet
1
2
3
4
5
6
7
8
• Bit 1 - 4  Type of Information in Signaling
Information Field
• Bit 5 - 6  Whether Message is intended for National
or International Network
• Bit 7 - 8  To identify Message Priority
Signaling 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
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– Destination Point Code – 3 Octets
– Originating Point Code – 3 Octets
– Signaling Link Selection – 1 Octet
Signaling Units
Link status signaling units (LSSU)
– Inform the far end about the changes in status
of link
– Message length can be 1 or 2 bytes
1
FLAG
1
BSN/
BIB
1
FSN/
FIB
1
1 or 2
Length Status
Indicator Field
1
Check
Sum
Signaling Units
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.
1
FLAG
1
BSN/
BIB
1
FSN/
FIB
1
1
Length
Indicator
Check
Sum
Basic Call Setup Example
15
2
1
6,10
13
9
5
ISUP Messages
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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)
Applications
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Prepaid Calling
 Local Number
Portability (LNP)
 Global Roaming
 International Callback
 Virtual Office
 Internet Call Waiting/
Caller ID
 Least Cost Routing
 Toll Bypass
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Unified Messaging
800 / Free Phone
Services
Short Message
Service (SMS)
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Tele-voting
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Location-based
Services
Caller Ring Back
Tone (CRBT)
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Implementation of SS7 in GSM
radio
subsystem
MS
network and switching
subsystem
Fixed partner networks
MS
ISDN
PSTN
MSC
Um
BTS
BSC
EIR
SS7
BTS
Abis
VLR
BTS
BTS
BSS
HLR
BSC
A
MSC
IWF
ISDN
PSTN
PDN
SS7/IP Inter-working
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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 IP
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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 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 Gateway
Media Gateway Controller
Signaling Gateway.
VoIP Network Configuration
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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 Model
The
SIGTRAN’s
protocol
specify the means by which
SS7 messages can be reliably
transported over IP network.
The architecture identifies three
component
A standard IP
A common signaling transport
protocol for SS7 protocol layer
being carried.
An adaptation module to
emulate lower layers of the
protocol.
Stream Control Transmission
Protocol(SCTP)
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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-ofline Blocking.
Transporting MTP over IP
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ITU specified following requirements
– MTP Level 3 peer to peer procedure require response
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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.
M2PA
M2PA:-
MTP2 User Peer-toPeer 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.
M2UA
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.
M3UA
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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
SUA
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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 &
Security Requirement for SS7/IP
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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
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Authentication
Integrity
Confidentiality
Availability
Network Evolution to an All-IP
Network
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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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