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SS7 - An Overview
Page 1
Contents
Signaling types
Out of Band Signaling
Common Channel Signaling
SS7 Layers
A IN Call flow using the SS7 Backbone
Page 2
The Network
Page 3
Signaling Types
Topologically the Signaling systems can be divided to :
User to Network Interface (UNI)
Analog Subscriber Signaling System (ASSS)
Digital Subscriber Signaling System #1(DSS1)
Digital Subscriber Signaling System #2 (DSS2)
Network to Network Interface (NNI)
Channel Associated Signaling (CAS)
Common Channel Signaling (CCS#7)
Page 4
Signaling Types
Functional classification of Signaling could be :
Line Signaling (Seizure and release of a local line)
Register Signaling (One exchange to another)
Page 5
Signaling Types
Based on Transmission signaling can be divided into:
Inband Signaling : Signaling is sent on the same
Speech Channels
Out of Band Signaling : A separate Digital Channel
for signaling purpose, also called a signaling link.
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CAS Signaling
Channel Associated Signaling (CAS)
Transported on 2Mbps PCM Link
Each frame contain 32 bytes (0 to 31)
A sequence of 16 Frames are used called Multi-frame
Line signal for a channel is carried on Channel 16 in an associated frame
Register Signaling is carried on the user channel itself
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CAS Frames
Page 8
CAS
Exchange 1 Exchange 2
Signaling+Speech
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CCS7
Common Channel Signaling System #7 (CCS7)
Signaling for all the Speech Channel is done on
one Common channel
Signaling is done by messages , the messages
have identifier for the speech channels
Typically one signaling channel can carry
signaling information for about 1000 speech
channel
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CCS7
Page 11
CCS7
Exchange 1 Exchange 2
Signaling
Speech
Page 12
Signaling Network view
SP SPSTP
LE LE
Signaling Network
Telephone Network
Page 13
SS7 Network Structure
SS7 signaling network is logically separated from the
telephone network and is composed of :
Signaling Points (SP) : Origin or Sink of a SS7
message
Signaling Transfer Point (STP) : Transit Nodes
Signaling Link (SL) : Links joining the SP,used to
convey the SS7 messages
Link Set : A number of signaling Links that directly
interconnect the two SP
Signaling Route (SR) : Path taken in the SS7 network
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SS7 Network Structure
Page 15
SS7 Suite
Page 16
SS7 vs OSI
TCAPI S U P
SCCP
MTP - 3
MTP - 2
MTP - 1
L7 : Application
L6 : Presentation
L5 : Session
L4 : Transport
L3 : Network
L2 : Data Link
L1 : PhysicalPage 17
SS7 Layers
Broadly the SS7 can be divided into 3 main Layers:
Message Transfer Part (MTP)
Signaling Connection Control Part (SCCP)
User Part / Application Part
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SS7 Layers - MTP
MTP provides connectionless service to SCCP and
other User and Application parts ensuring reliable
transport and delivery of signaling messages.
MTP can be further subdivided into 3 Layers:
MTP1
MTP2
MTP3
Page 19
SS7 Layers - MTP1
MTP1 : Signaling Data Link Function
Handles the Physical/Electrical layer functions for
full duplex transmission of bits on the data
link.There can be two types of data links.
Analog : Using modems at 4.8Kbps
Digital : 2Mbps PCM link (E1 or T1)
Identical to the OSI Layer1
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SS7 Layers - MTP2
MTP2 : Signaling Link Control Function
Ensures the error free/sequenced transmission of
data sent by the upper layers over the signaling
link using Signaling Units (SU)
Monitors the data link
Initializes the data link during startup
Identical to the Layer 2 of OSI
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Signaling Units
There are three types of Signaling Units (SU)
Message Signaling Unit (MSU) : Signaling Unit used to
carry information passed by the User parts/Applications
of one SP to the User parts/Applications of the other SP
Link Status Signaling Unit (LSSU) : Used to indicate the
Link status (Flow control, during startup of link)
Fill in Signaling Unit (FISU) : Fillers when no payload and
SS7 network is idle. By them the integrity of the link is
monitored.
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Signaling Units
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Signaling Units
Key fields of a SU are :
Flag (01111110) used as a message separator
Backward/Forward Sequence Number(BSN/FSN)
used to ensure transmission error correction and
sequencing.
Forward/backward Indicator bits(FIB/BIB) :Used
along with BSN/FSN for error control procedures
Page 24
Signaling Units
Length Indicator (LI) : Number of Octets following the LI
and preceding the Check bits.
LI = 0 FISU
LI =1 or 2 LSSU
LI > or = 3 MSU
Service Information Field (SIO) : Present only in
MSU,used
To identify the type of protocol (TUP,ISUP,TCAP) :
Field Service indicator (SI)
To identify the Type of Network
(International,National): Field Network indicator (NI)Page 25
Signaling Units
Signaling Information Field (SIF) : Present only in
MSU , contains :
Actual information passed by the higher layers
(User Parts)
Routing Label
Status Field (SF) : Present only in LSSU and carries
Link status information .
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Signaling Units
The parameters used for message handling functions
are :
Field SIO
SI (First 4 bits)
NI (Last two bits)
Field SFS (Routing Label part, First 4 Octets)
Destination Point Code (First 14 bits)
Origin Point Code (next 14 bits)
Signaling Link Selection (last 4 bits)
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SS7 Layers - MTP3
MTP3 : Signaling Network Function
Signaling Message Handling
Signaling Network Management
Page 28
SS7 Layers - MTP3
Signaling Message Handling Function
Message discrimination : determines for each
incoming message whether the SP is the
Destination for the message
Message distribution : Ensures the delivery of the
message to the correct User part / Application
Message Routing : To route the message correctly
to the other SP.
Page 29
SS7 Layers - MTP3
Signaling Network Management Function
Signaling Traffic management : To divert traffic
from the failed link. Traffic management is only
between point to point
Signaling Route management : From the point of
view of SP. Used to divert traffic from a specific SP.
Signaling Link management : For
activation/deactivation of link. They trigger MTP2
alignment procedure.
Page 30
SS7 Layers - MTP Shortcomings
MTP was originally designed for Classical Telephone
Networks and so had the following shortcomings:
Limited addressing capabilities
Supports only connectionless services
Not completely OSI compatible
Page 31
SS7 Layers - SCCP
Signaling Connection Control Part (SCCP) takes care
of the MTP shortcomings.
SCCP provides :
More flexible routing schemes : Concept of
Subsystem Number (SSN) /Global Titles (GT)
Connection oriented and connection les services
provided
Routing of Multiple data messages to the same
destination efficiently using Virtual Circuits
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SS7 Layers - SCCP
Flexible routing schemes (SSN)
SCCP uses the SSN to identify the SCCP user at
each SP (MAP,INAP, CAMEL)
Hence OPC/DPC + SSN make a more complete
addressing
Page 33
SS7 Layers - SCCP
Flexible routing schemes (GT)
GT is a network address in form of a digit
sequence.
SCCP translates GT into a DPC+SSN
At the destination SP the message is passed to
the local SCCP which delivers it to the correct user
Page 34
SS7 Layers : User/Application Parts
Correspond to the highest end to end Layer of the
OSI
Example of User Part are :
ISDN User Part (ISUP)
Telephone User Part (TUP)
Example for an Application part are :
Transaction Capabilities Application Part (TCAP)
Page 35
ISUP
ISDN User Part defines the messages and protocols
used in the establishment of Speech and data calls
for Public Switched telephone Network (PSTN)
Though conceived to be a NNI for ISDN originated
calls, it can be used for both ISDN/non ISDN calls.
The ISUP messages are contained in the SIF of the
MTP frame.
ISUP using the services of MTP and SCCP provides a
logical connection between two Exchanges.This
connection is identified by Circuit identification Code
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Structure of an ISUP message
Routing Label (OPC+DPC)
Circuit Identification Code
Message Type Code(H0H1) : 8 bit code
User Part data
Mandatory Fixed Part
Mandatory Variable Part
Optional Part
Page 37
TCAP
The Transaction Capabilities Application Part (TCAP)
provides a remote operation function of SS7
Provides capability for an application at one node to
invoke the execution of an operation at another
node and to receive the results from that remote
process.
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