SS7 by MasoodAlam
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Transcript of SS7 by MasoodAlam
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By Masood Alam
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Introduced by ITU by its
recommendations
Q.700-Q.716, Q.721-Q.766, Q.771-Q795 International standard forCommon
Channel Signaling System, anothername
CSS7
Call setup, communication, teardown,management ,maintenance
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Signaling method in which a single
channel conveys, by means of labeled
messagesMessage structure iswell organized, and
are Packetized
High speed 56 kbps and 64 kbps
Networktopology (e.g. Redundant paths)Number portability, databases
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Cellular providers were dependent on
X.25 networkto carry IS-41 signaling
information.This did not allow them to interconnect
through PSTN.
Now they are deploying their own SS7
networks.
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Service Switching Point (SSP).
Service Control Point (SCP).
Signal TransferP
oint (STP
).
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SSPs are switches that originate,
terminate calls.
Send messages to other SSP
sMay also send query to SCP
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Interfaces to telephone company
databases.
SSP
may use information from thesedatabases to manage calls.
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It is a Packet Switch.
Networktraffic between signaling points
is routed.Acts as network hub
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The transport, session, and presentation
layers are not used in SS7
Instead transport, session andpresentation layers are grouped together
atthe application layer, which is known
as Level 4 in SS7.
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Message TransferPartLevel 1 (MTP-1)
corresponds to Physical Layer ofOSI
model.Physical , Electrical and functional
characteristics are defined.
examples
E1 (2.048 Mbps)
DS1 (1.544 Mbps)
See Q.701 Specs ofITU.
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SS7 message is called a signal unit (SU) Fill-In Signal Units (FISUs)
Link Status Signal Units (LSSUs)
Message Signal Units (MSUs)
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Transmitted continuously on a signaling link in bothdirections unless other signal units (MSUs orLSSUs)are present
Carry basic level 2 information only (e.g.,
acknowledgment of signal unitreceipt by a remotesignaling point). Because a CRC is calculated foreach FISU, signaling link quality is checkedcontinuously by both signaling points at either end ofthe link.
Inthe ITU-T Japanvariant, signaling link quality ischecked by the continuous transmission of flag octets(8-bit bytes)ratherthan FISUs; FISUs are sent only atpredefined timer intervals (e.g., once every 150milliseconds).
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LSSUs carry one ortwo octets (8-bit
bytes) of link status information between
signaling points at either end of a link.The link status is used to control link
alignment and to indicate the status of a
signaling point (e.g., local processor
outage)to the remote signaling point
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Carry all call control, database query andresponse, network management, andnetwork maintenance data inthe signaling
information field (SIF)Have a routing labelwhich allows an
originating signaling pointto sendinformationto a destination signaling point
across the network.Have Service InformationOctet (SIO)that
specified MTP user ofthis message
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Signaling message handling functions. Message discrimination, distribution and routing
to appropriate user part according to SIO.
provides message routing betweensignaling points inthe SS7 network
Signaling network managementfunctions. Reroutes traffic away from failed links and
signaling points and controls traffic whencongestion occurs.
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Defines necessary telephone signaling
functions Initial Address Message
Address complete Message
Answer Message
Clear back Message
Clear Forward Message etc. etc.
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To establish, maintain and release trunk
circuits that carry voice & data calls over
P
STN.Used for both ISDN and non-ISDN calls.
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TUP was specified before DSS1 (ISDN user signaling) during1980s
ISUP2 specs was released after DSS1 Changeover from TUPto ISUP is well under way
internationally. Innetwork core development path is
CAS -> TUP -> ISUP Changing TUPto ISUP is easy by a software upgrade in
exchanges. Limitations ofTUP
DSS1 terminal info cannot be transported inTUP
ISDN Suspend/Resume is not supported inTUP TUP does not support all supplementary services (call waiting, call
hold etc) in Euro-ISDN.
InTUP, release is non-symmetric.
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Provides connection-less network services. Provides connection-oriented network services
by using Sequence Control.
Transfer information between exchanges andspecialized centers
Makes use of MTP services. Networkto multiple network communication. This is essentially a data network combining
the features ofIP and X.25, offering bothconnection-oriented and connectionlessoperation
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Used extensively in cellularnetworks Base Station Subsystem Mobile ApplicationPart
(BSSMAP)
DirectTransfer ApplicationP
art (DTAP
)to transferradio-related messages in GSM
In conjunction with TCAP, also usedthroughoutthe GSM Network SwitchingSubsystem (NSS)to transport MAP signalingbetweenthe core GSM components toenable subscriber mobility and textmessaging
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SCCP connection-oriented control
(SCOC)
SCCP
connectionless control (SCL
C)SCCProuting control (SCRC)
SCCP management (SCMG)
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Class 0: Basic connectionless class
Class 1: In-sequence delivery
connectionless classClass 2: Basic connection-oriented class
Class 3: Flow control connection-oriented
class
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Temporary Signaling Connections connection establishment phase
data transfer phase
connectionrelease phase
Permanent Signaling Connections The setup/release service is controlled by the
administration (e.g. OMAP
)
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Routing Label one octet
Message Type code one octet
CR,CC,CREF,RLSD,RLC,DT1,DT2,AK,UDT,UDTS,
ED, EA,RSR,RSC,ERR,IT,XUDT,XUDTS,LUDT,LUDTS
mandatory fixed parameters (MF)
Pointers to mandatory variable parameters
Pointers to optional parts
Mandatory variable parameters (MV)
Optional parts (O)
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ParameterName code
End of optional Parameters 0
estination lo
al reference
Source local reference 2
Called party address 3
Calling party address 4
Protocol class 5
Segmenting/reassembling 6
Receive sequence number 7
Sequencing/segmenting 8
Credit 9
Release cause 0
Returncause
Resetcause
2Errorcause 3
Refusal cause 4
ata 5
Segmentation 16
Hop counter 17
Importance 18
Long data 19
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Routes messages received from the MTPtoappropriate local subsystem
Routes messages from local subsystems toother local subsystems
Routes messages from local subsystems tosubsystems inremote nodes by utilizingMTP's transport services. The destination
is specified inthe called party (CdPA)
address parameter, which is supplied by thesubsystem. The address can contain acombination of point code, system number,
and global title.
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Types ofnetworkrouting Point Code (PC)routing
x MTP layer can only use point code routing
x Using MTP point code routing, MSUs pass through the
STPs until they reach the SPthat has the correct DPC
Subsystem Number (SSN)routing
Global Title (GT)routing
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A subsystem is the name givento an
applicationthat uses SCCP
SSN is used to identify the SCCP user in
much the same way as the SIO identifies
the MTP3 user.
SSN examples
SCMG (1), ISUP (3), MAP(5), HLR(6), VLR(7)MSC(8), EIR(9), AC(10)
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Point code indicator indicates thatthe address contains a signaling point code
SSN indicator indicates thatthe address contains a SSN
Global title indicator (GTI) no global title included (0) includes nature of address indicator only (1) includes translationtype only (2) includes translationtype, numbering plan and encoding
scheme (3) includes translationtype, numbering plan, encodingscheme and nature of address indicator (4)
Routing Indicator Route on SSN or Route on GT.
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Signaling Point Code it, when provided, is represented by two octets
Subsystem Number (SSN) identifies an SCCP user and, when provided, consists
of one octet coded e.g. SCMG (1), ISUP (3), MAP(5),HLR(6), VLR(7)MSC(8), EIR(9), AC(10)
Global title (GT) variable length
Nature of Address indicator (NAI), Even/Oddindicator andGlobal title Address information
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Provides functions and protocols to largevariety of applications distributed overswitches and specialized centers (e.g.Databases)
TC refers to a set of communicationcapabilities that provide interface betweenapplications and network layer services
The original implementations ofTCAP were
created to support calling card and 800-number applications inNorth America bythe North AmericanT1 standards committeein around 1986
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designed to be generic to accommodate
the needs of a wide variety of different
services
Most services can be viewed as a
dialogue of questions and answers
Non-circuitrelated services
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Used to transport INAP inIntelligent
Networks
Used to transport MAP in mobile phone
networks.
Supports remote control ability to
invoke features in anotherremote
network switch.Calling card calls are validated using
TCAP query and response messages.
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MTP provides transfer across data link.SCCP provides transfer across signaling
points.TCAP core functionality is to enable the
feature inthe remote switch/SP/exchangeand querying Toll free number application.Intoll free number application ( 800)
TCAP uses SCCP because itneeds end to
end transport protocol forretrieving therouting number from the SCP which may belocated any where inthe large network
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Above transport layer ofOSI model
depends uponthe SCCP's transport
services
Transaction sublayer
Component sublayer
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Component the means by which TC conveys a requestto perform
an operation, or a replyx operation is an actionto be performed by the remote end
x reply carries an indication of success or failure in carryingoutthe operation
Components are passed individually between a TC-user and the Component sublayer
Components in a message are delivered to theremote TC-user inthe same order as they areprovided atthe originating interface
Provides dialogue facility
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Successive components exchangedbetweentwo TC-users in ordertoperform an application
Unstructured dialogueTC-users can send components that do not expect
replies without forming an explicit association
betweenthemselves
Structured dialogueAlternatively,TC-users indicate the beginning, ortheformation of a dialogue, the continuation, and the end
of a dialogue
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provides the capability forthe exchange
of components
provides the capability to send
transaction messages between peerTR-
layer entities by means ofthe services
provided by SCCP
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the neural system interconnecting thedistributed computer infrastructure of GSM(VLRs, MSCs, HLRs and other entities).
Can be interfaced via procedure callsAProtocol first presented in 1988Features
Roaming
Call handling Non-interruptive handover
Remote switching management
Management of security functions
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MAP protocol stack TCAP
SCCP
MTP
Maximum outage allowed 15 min/yr
Tighttime budget 100 ms
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When a mobile subscriberroams into a
new mobile switching center (MSC) area,
the integrated VLR requests service
profile information from the subscriber's
HLR using mobile application part (MAP)
information carried withinTCAP
messages.
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IN Architecture to quickly create and deploy customer
SERVICES Supports many services
Is a standard - multi-vendor Distributed platform Example services supported by IN
Freephone Premium Rate Voice mail Call Diversion Call baring Local NumberPortability
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Nortel HLR features network connectivity is implemented using IP
based signaling with full support for Sigtran
17 million GSM subscribers or up to 10 millionUMTS subscribers in a single rack of equipment
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