Signaling System #7b

8/6/2019 Signaling System #7b

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Signaling System #7


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Wh at PSTN needs

Challenges facing circuit switched networksRapid convergence of voice and dataGrowth of VoD, especially VoIP

Critical needs of PSTNHigh volumeNext generation network (NGN) service support


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SS#7 signaling meets t h ese needs

E fficiencyOut-of-band links at 64 KbpsShorter information transfer time

Ability to fall back to the originating end of the call (e.g., busy)

Service enablingFree phone (0-800) services

Automatic call back & calling number delivery Automatic calling card servicesWireless services such as roaming

Network reliabilityCarries extensive network management messagesNetwork architectureSecurity


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SS#7 definition

Common channel signaling system number 7(SS#7)Out-of-band signaling systemFacilitates exchange of call control informationbetween network switching officesVoice and non-voice services


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SS#7 basics

Voice and signaling are separatedControl messages (packets) are routed throughthe network for call managementNetwork elements are connected via signalinglinksE ach element capable of SS#7 controlmessages, is called a signaling point (SP)

All SPs in an SS7 network are identified by aunique code known as point code (PC)


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SS#7 networks


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SS#7 signaling points (1)

Service switching point (SSP)Capable of controlling voice circuits via a voice switchThe switch can originate, terminate, or tandem calls An SSP sends signaling messages to other SSPs to

set up, manage, and release voice circuits required tocomplete a call

An SSP can also send a query message to acentralized database (SCP) to determine how to route

a call (e.g., toll free 1-800 number)


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Signaling transfer point (STP)Routes each incoming message to an outgoingsignaling link, based on routing information containedin the SS#7 message and a pre-defined route table

Does not offer termination servicesSTPs are paired to ensure redundancy


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Service control point (SCP)Provides access to databases

Accepts a query for information from a subsystem atanother node

Used by STP to perform a function called global titletranslationThe database may not reside in the same location asthe SCP


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C ommon c h annel signaling (1)

SwitchSP

SwitchSP

E1 31 voice channelsusing 31 time slots

2 separate signaling linksusing 2 time slots


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C ommon c h annel signaling (2)

SwitchSP

SwitchSP

E1 31 voice channelsusing 31 time slots

1 signaling link and 30 voicechannels


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Signaling network terms

Associate mode: signaling links follow the same path asthe voice trunksQuasi-associate mode: signaling links follows a differentroute than the voice trunks

SP SPSP

STP

Voice trunksSignaling Links

Associate Mode Quasi-Associate Mode


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Qu ad config u rations

STP networks employ quad configurationGeographic diversificationIncrease link capacity, especially in densely populatedareasRedundancy

STP 1

STP 2

STP 4

STP 3


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L ink types (1)

Link access (A links)Provide network access for signaling end points or SSPs

SSP MTPs


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L ink types (2)

B ridge links ( B links)Connect a local STP pair to another STP pair in the

network, in order to reach remote SSPFor redundancy reasons, the connection creates quadconfigurationThe distinction between B and D links is arbitrary

SSP


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L ink types (3)

Cross links (C links)For redundancy, STPs are paired

Cross links connect STPs performing identicalfunctionsLinks connect the pair and allow messages to crossover from one to the other

SSP


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L ink types (4)

Diagonal links (D links)

Sometimes SSPs needs to communicate with numerouslocationsSome STP pairs in the network only route messages from onelocal network to another Links from local STP pairs to STPs on a different level are calledD links


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L ink types (5)

E xtended links ( E links)In addition to the local STP pair connection, signalingend points have an additional set of redundant links to

a redundant STP pair In most of the cases, the STP pair is not so localThe links extend network access for the signaling endpoint

SSP SSP


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L ink types (6)

Fully associate links (F links)In several SS7 networks, nodes (signaling end points)

provide additional, special data processing to other nodes in the same networkDirect connection links between nodes in the samenetwork are defined as fully associate links


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SS#7 link type s u mmary (1)


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SS#7 link type s u mmary (2)

A link (access) C onnects signaling end point (S C P or SSP) to STP

B link (bridge) C onnects an STP to anot h er STP; typically, a q u adof B links interconnect peer (or primary) STPs(STPs from a network connect to STPs of anot h er network)

C link (cross) C onnects STPs performing identical f u nctions,forming a mated pair (for greater reliability)

D link (diagonal) C onnects a secondary (local or regional) STP pair to a primary (inter-network gateway) STP pair in aq u ad-link config u ration; t h e distinction between B

and D links is arbitraryE link(extended)

C onnects an SSP to an alternate STP

F link(f u llyassociated)

C onnects two signaling end points (SSPs andS C Ps) in t h e same local network


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SS#7 protocol stack


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M TP

M essage transfer part:Reliably transfers messages over links or linksetsFor correct routing, the signaling point needs thesignaling point code (SPC) of the node at theend of its linksReceives the SPC by destination point code

(DPC) in the messages it routesNeeds information about other locations in thenetwork, to select the best link set for routing themessage to its destination


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M TP level 1 (p h ysical)

Defines physical, electrical hardwareDeals with physical issues at the level of links,interface cards and multiplexorsDefined for E 1, DS1, V. 3 5 and DS0A


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M TP level 2 (date link)

E nsures accurate end-to-end transmission of amessage across a signaling linkImplements flow control, message sequencevalidation, error checking and message

retransmissionM onitor links and reports their statusPlaces links in service; restores service links thatwere taken out of service

Test links before allowing their useProvides sequence numbers for outgoingmessages


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M essage types (1)

M essage signal layer ( M SU)Contains a service information field that carries dataRelated to ISUP and TCAP applicationsIncludes SIO field

Sub service field (SSF): national and international networks Service indicator (SI): associates a message with the upper

layer, e.g., ISDNM essage priorities (0 - 3 ) used in ANSI only


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M essage types (2)

Link status signaling units (LSSU)Inform the far end of changes in statusM essage length can be 1 or 2 bytes

Fill in signal units (FISU)Fill the gaps between M SU and LSSU messagesSent only when the buffer is empty, to keep thesignaling link active


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M TP level 3 (network)

M essage routing between signaling points in theSS7 networkSignaling network management that providestraffic, links and routing management, as well ascongestion (flow) controlRe-routes traffic away from failed links andsignaling points; controls traffic when congestionoccurs


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L inksets

Groups of links that connect two adjacent nodesE nsure traffic load sharingCombined linksets between STPs ensures load sharingConsist of up to 16 links in ANSI protocol and up to 8 inITU protocolSignaling link code (SLC) is uniquely assigned to each

link


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R o u tes

Virtual path that a message takes to adestination nodeComprised of one or more linksets


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M TP level 3 management services (1)

M TP level 3 provides signaling link selection(SLS)

Rotates in each session A mechanism to assign traffic to a link in the linkset

Results in load sharing of the links in the linksetSLC rotation stops for duration of message transfer


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M TP level 3 management services (2)

M TP restartB efore returning to the network, a node can sendTRW (traffic restart wait) to an adjacent node,indicating not to send traffic

When restarting, if the node is satisfied that enoughlinks are available, it is sends a TRA (traffic restartallowed)

Optional signaling link test message (SLT M ) and

signaling test acknowledge (SLTA)E xchanged when a link is in service; ensuresagreement on signaling link code


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S CC P

Signaling connection control part:Provides connectionless and connection-oriented network servicesProvides global title translation (GTT)capabilities above M TP level 3 ; translatesnumbers to DPCs and subsystem numbersProvides more detailed addressing information

thanM

TPsUsed as transport layer for TCAP-basedservices


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TC AP

Transaction capabilities applications part:E xchange of non-circuit related data

B etween applications across the SS#7 networkUsing the SCCP connectionless service

Queries and responses sent between SSPs andSCPsSends and receives database information

Credit card validationRouting information


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TUP

Telephone user part:B asic call setup and tear down

Analog circuits onlyIn many countries, ISUP has replaced TUP for call management


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ISUP

ISDN user part:Necessary messaging for setup and tear downof all circuits (voice and digital)M essages follow the paths of voice circuitsM essages are sent from a switch, to the switchwhere the next circuit connection is requiredCall circuits are identified using circuit

identification code (CIC)M ust be compatible on both sidesFollowed by each ISUP message


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ISUP messages (1)

Initial address message (IA M ): contains allnecessary information for a switch to establish aconnection

Address complete message (AC M ):acknowledge to IA M ; the required circuit isreserved and the phone is ringing (ringbacktone)

Answer message (AN M ): occurs when the calledparty picks up the phone


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ISUP messages (2)

Release (R E L): sent by the switch sensing thatthe phone hung upRelease complete (RLC): each exchange thatreceives R E L, sends an RLC message back(this acknowledges receipt of R E L)


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ISUP normal call scenario


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SG

SIP Client

SIP Server STP ISUP/MTP

E1/T1

SIP

SS7

Voice StreamSignaling

MG

RTP

MGCP SIP

SIP

IP network

ISUP/IP

MGC

DSS 1

G eneral SS7 Vo IP conversion


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R ADCO M sSignaling System #7

call generator


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SS7 Q Pro (1)

Simulates Service Switch Point (SSP)Connects to

Service Switch Points (SSPs)Signaling Transfer Points (STPs)

Signaling gateways + M edia Gateway


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SS7 Q Pro (2)

Initiates and terminates ISUP voice callsM easures voice quality and loads SS7 networkHides SS7 complexity with intuitive GUIM andatory for signaling GW/soft switch testing


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SS7 Q Pro (3)

Supports up to 8 E 1/T1 voice ports + 2 signalingonly ports per server Provides 4 / 16 SS#7 links per server E nables the user to distribute SS#7 links in allexisting E 1/T1 trunksISUP variants: ITU, ITUWHIT E , ITU B LUE ,ITU9 7, E TSIV2, E TSIV 3 , Q767, ANSI88,

ANSI 9 2, ANSI 95 and JNTTTimer control: M TP layer 2, M TP layer 3 andISUP


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SS7 Q Pro (4)

If you need a modern and state of the art SS#7call generator, the SS#7 Qpro is the tool for you.It has the most comprehensive and advanced

features list.The Qpro is the only SS#7 call generator thatcan also test call voice quality using P E SQ,PA M S and PSQ M algorithms.

For more details: www.radcom.com

Signaling System #7b

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