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Signaling System No. 7 (SS7/C7) - Protocol, Architecture and Services (Full Book) < Top Index >

SCCP Routing Control (SCRC) SC R C pe rform s the following thre e functions: R oute s m e ssage s re ce ive d from the MTP to appropriate local subsyste m . R oute s m e ssage s from local subsyste m s to othe r local subsyste m s. R oute s m e ssage s from local subsyste m s to subsyste m s in re m ote node s by utilizing MTP's transport se rvice s. The de stination is spe cifie d in the calle d party (C dPA) addre ss param e te r, which is supplie d by the subsyste m . The addre ss can contain a com bination of point code , syste m num be r, and global title . SC C P addre ssing capabilitie s are fle x ible in contrast to those of MTP 3. As a re sult, the addre ssing capabilitie s are som e what com ple x , the re by allowing se ve ral diffe re nt com binations of routing param e te rs. SC C P provide s a routing function that allows signaling m e ssage s to be route d to a signaling point base d on diale d digits, for e x am ple . This capability is k nown as Global Title Translation (GTT), which translate s what is k nown as a global title (for e x am ple , diale d digits for a toll fre e num be r) into a signaling point code and a subsyste m num be r so that it can be proce sse d at the corre ct application. The se ction on "Global Title Translation" e x plains global title s and GTT. The following are diffe re nt type s of ne twork addre ssing that SC C P supports: Point C ode (PC ) routing Subsyste m Num be r (SSN) routing Global Title (GT) routing The MTP laye r can only use point code routing, which is de scribe d in C hapte r 7. Figure 9-9 shows a sum m ary of MTP point code routing. Using MTP point code routing, MSUs pass through the STPs until the y re ach the SP that has the corre ct DPC . The following se ctions de scribe the SSN and GT routing.

Figure 9-9. Showing MTP Point Code Routing [View full size image]

Subsystem Number (SSN) Routing As pre viously m e ntione d, a subsyste m is the nam e give n to an application that use s SC C P; applications are pre dom inantly database drive n, e x ce pt whe re ISUP is the subsyste m (for a lim ite d num be r of supple m e ntary www.ss7-training.net/sigtran-training/ch09lev1sec4.html

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se rvice s), or whe re BSSAP use s SC C P (for radio-re late d signaling in GSM). As illustrate d in Figure 9-10, a SSN is use d to ide ntify the SC C P use r in m uch the sam e way as the se rvice indicator ide ntifie s the MTP3 use r (se e C hapte r 7).

Figure 9-10. An SSN and DPC Are Required for the Final Delivery of an SCCP Message

Figure 9-10 shows that a DPC and SSN are re quire d in orde r to de live r a m e ssage to the corre ct application at the de stination node . It should be cle ar that noncircuit-re late d signaling (for e x am ple , database transactions to support IN/ce llular, and so on) involve two distant applications (subsyste m s) e x changing inform ation. The SSN is use d to ide ntify the application. Appe ndix L contains a trace that shows the de coding of a VLR calling an HLR (to pe rform a location update ).

NOTE Applications using TC AP re ly on SC C P for m e ssage routing since TC AP itse lf has no routing capabilitie s. The re fore , e ach application is e x plicitly ide ntifie d by an SSN at the SC C P le ve l.

If SSN routing is use d, the SSN is place d inside the C dPA param e te r. The SC C P use s the SSN to se nd an SC C P m e ssage to a particular subsyste m (application) at an SP. The SSN of the originating subsyste m is also include d in the C alling Party Addre ss (C gPA) param e te r to ide ntify the subsyste m that se nt the SC C P m e ssage .

NOTE SC C P's C gPA and C dPA param e te rs should not be confuse d with the C alling Party Num be r and C alle d Party Num be r param e te rs found in TUP/ISUP.

The SSN fie ld is one octe t in le ngth and, the re fore , has a capacity of 255 possible com binations. Table 9-12 shows the SSN value s that are spe cifie d by the ITU-T.

Table 9-12. ITU-T Specified Subsystem Numbers [60] Bits 87654321

Subsystem

0 0 0 0 0 0 0 0

SSN not k nown/not use d

0 0 0 0 0 0 0 1

SC C P m anage m e nt

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0 0 0 0 0 0 1 0

R e se rve d for ITU-T allocation

0 0 0 0 0 0 1 1

ISUP (ISDN use r part)

0 0 0 0 0 1 0 0

O MAP (O pe ration, Mainte nance , and Adm inistration Part)

0 0 0 0 0 1 0 1

MAP (Mobile Application Part)

0 0 0 0 0 1 1 0

HLR (Hom e Location R e giste r)

0 0 0 0 0 1 1 1

VLR (Visitor Location R e giste r)

0 0 0 0 1 0 0 0

MSC (Mobile Switching C e ntre )

0 0 0 0 1 0 0 1

EIC (Equipm e nt Ide ntifie r C e ntre )

0 0 0 0 1 0 1 0

AUC (Authe ntication C e ntre )

0 0 0 0 1 0 1 1

ISUP supple m e ntary se rvice s [1 ]

0 0 0 0 1 1 0 0

R e se rve d for inte rnational use

0 0 0 0 1 1 0 1

Broadband ISDN e dge -to-e dge applications

0 0 0 0 1 1 1 0

TC te st re sponde r[1]

0 0 0 0 1 1 1 1

R e se rve d for inte rnational use

to 0 0 0 1 1 1 1 1 0 0 1 0 0 0 0 0

R e se rve d for national ne twork s

to 1 1 1 1 1 1 1 0 1 1 1 1 1 1 1 1

[1]

R e se rve d for e x pansion of national and inte rnational SSN

A N SI [2 ] s imply s tates this field value as res erved.

ITU-T ne twork spe cific subsyste m num be rs should be assigne d in de sce nding orde r, starting with 11111110 (for e x am ple , BSSAP is allocate d 11111110 within GSM). In GSM, subsyste m num be rs can be use d be twe e n Public Land Mobile Ne twork s (PLMNs), in which case the y are tak e n from the globally standardize d range (1–31) or the part of the national ne twork range (129–150) that is re se rve d for GSM use be twe e n PLMNs, or within a PLMN, in which case the y are tak e n from the part of the national ne twork range (32–128 and 151–254) that is not re se rve d for GSM use be twe e n PLMNs. Table 9-13 lists the globally standardize d subsyste m num be rs that have be e n allocate d by 3GPP for use by GSM/GPR S/UMTS ce llular ne twork s [106].

Table 9-13. 3GPP Specified Subsystem Numbers [60] Bits

Subsystem

0000 0110

HLR

0000 0111

VLR

0000 1000

MSC

0000 1001

EIR

0000 1010

AuC

1111 1010

BSC

1111 1011

MSC

1111 1100

SMLC

1111 1101

BSS O &M

1111 1110

BSSAP

1000 1110

R ANAP

1000 1111

R NSAP

1001 0001

GMLC

1001 0010

C AP

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1001 0011

gsm SC F

1001 0100

SIW F

1001 0101

SGSN

1001 0110

GGSN

Additionally INAP is spe cifie d as 0000 1111 [106]. Table 9-14 shows som e com m on subsyste m s that are use d within North Am e rica.

Table 9-14. Common North American Subsystem Numbers Bits

Subsystem

1111 1011

C ustom Local Are a Signaling Se rvice (C LASS)

1111 1100

PVN (Private Virtual Ne twork )

1111 1101 AC C S

Autom atic C alling C ard Se rvice (AC C S)

1111 1110

E800 (Enhance d 800)

Global Title Routing "A global title is an addre ss, such as diale d-digits, which doe s not e x plicitly contain inform ation that would allow routing in the SS7 ne twork ." Source : ITU-T-T Q .714 Subclause 2.1 [61] The re are m any e x am ple s of digit strings that are global title s: in fix e d-line ne twork s, toll fre e , pre m ium rate , num be rs porte d unde r LNP, or in the case of GSM ce llular ne twork s, the Mobile Subscribe r ISDN Num be r (MSISDN) and Inte rnational Mobile Subscribe r Ide ntity (IMSI) of the ce llular subscribe r and e ach HLR and VLR . A GT is a te le phony addre ss. As such, the GT addre ss m ust be translate d into an SS7 ne twork addre ss (DPC +SSN) be fore it can be finally de live re d. The GT is place d in the global title addre ss inform ation (GTAI) param e te r within the C gPA and C dPA fie lds. Global title routing is ofte n use d in fix e d-line ne twork s for calling-card validation and such se rvice s as telemarketing num be rs (lik e a toll-fre e or pre m ium rate ). It is use d in ce llular ne twork s for e x changing m e ssage s whe n an HLR and VLR be long to diffe re nt ne twork s or whe n se ve ral signaling points se parate the m .

Global Title Translation GTT is an incre m e ntal indire ct routing m e thod that is use d to fre e originating signaling points from the burde n of having to k now e ve ry pote ntial application de stination (that is, PC +SSN). This se ction de scribe s the GTT proce ss and the param e te rs associate d with GTT. For e x am ple , calling-card que rie s (which are use d to ve rify that a call can be prope rly bille d to a calling card) m ust be route d to an SC P that is de signate d by the com pany that issue d the calling card. R athe r than m aintaining a nationwide database of whe re such que rie s should be route d (base d on the calling-card num be r), SSPs ge ne rate que rie s that are addre sse d to the ir local STPs, which use GTT, to se le ct the corre ct de stination to which the m e ssage should be route d. STPs m ust m aintain a database that e nable s the m to de te rm ine whe re a que ry should be route d. GTT ce ntralize s SC C P routing inform ation at de signate d node s, ge ne rally an STP, although SSP or SC P node s are norm ally capable of pe rform ing GTT. Eve n the SP that has be e n re que ste d by anothe r SP to pe rform GTT doe s not have to k now the e x act final de stination of a m e ssage . Inste ad, it can pe rform inte rm e diate GTT, in which it use s its table s to locate anothe r SP that m ight have the final addre ss re quire d in its routing table s. An SP that pe rform s a final GTT provide s both the PC and SSN ne e de d to route the m e ssage to its final de stination. Inte rm e diate GTT furthe r m inim ize s the ne e d for STPs to m aintain e x te nsive inform ation about node s that are far re m ove d from the m . GTT also is use d at the STP to share a load am ong m ate d SC Ps in both norm al and failure sce narios. In the se instance s, whe n m e ssage s arrive at an SP for final GTT and routing to de stination SP, the STP that route s the m e ssage can se le ct from am ong available re dundant SPs (for e x am ple , two m ate d SC Ps). It can se le ct an SP on e ithe r a priority basis or to e qualize the load across the available SPs (this is re fe rre d to as loadsharing). As an e x am ple , GTT is pe rform e d to de te rm ine the SC P location to which que rie s should be se nt for num be r translation se rvice s such as tollfre e and LNP. If you dial 1-800-BUY-MO R E in the U.S. (toll-fre e be gins with 0800 in m any countrie s, including Gre at Britain), a que ry is se nt to an SC P to translate the toll-fre e num be r to a routing num be r. Se e C hapte r 11 for a de taile d e x planation of how num be r translation se rvice s work . W he n the SSP re ce ive s the tollfre e or LNP num be r from the subscribe r, it m ust de te rm ine the ne x t hop de stination to www.ss7-training.net/sigtran-training/ch09lev1sec4.html

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re ach the SC P that provide s the num be r translation se rvice . In Figure 9-11, the SSP pe rform s a GTT to de te rm ine that the ne x t hop de stination is the STP. The STP the n pe rform s the final GTT to route the m e ssage to the corre ct SC P. It is worth noting that whe n pe ople in the SS7 fie ld re fe r to "whe re the GTT is done ", the y are usually re fe rring to the STP that provide s the addre ss of the final de stination. In the pre vious e x am ple , GTT is actually done at the originating SSP in orde r to de te rm ine the ne x t hop de sination (the STP) towards the SC P and also at the STP to de te rm ine the final de stination.

Figure 9-11. Example of GTT [View full size image]

The SSP could always ge t the inform ation from such a database (SC P) without using GTT if the DPC and SSN of the re quire d toll fre e (or LNP) application we re pre se nt in its routing table s. Howe ve r, this would re quire m aintaining a large num be r of routing e ntrie s at the SSP. Ne w se rvice s (and applications) are fre que ntly de ploye d into the SS7 ne twork around the world. Som e of the se rvice s m ight be proprie tary and are , the re fore , only acce ssible to the SSPs in the sam e proprie tary ne twork . O the rs are inte nde d to be offe re d to othe r ne twork s for a fe e . If a se rvice be com e s unive rsally available , it should not m e an that e ve ry switch worldwide should be re quire d to add the location (DPC ) and application ide ntifie r (SSN) to its routing table s. The re fore , the GTT is use d to ce ntralize the se routing functions. SC C P routing (utilizing GTT) is an e ffe ctive solution. The GTT inform ation is place d at a lim ite d num be r of ne twork locations (such as STPs), and SSPs que ry the se ce ntralize d locations without ide ntifying from whe re the inform ation is re trie ve d. W he n a switch re quire s a GT translation (that is, to addre ss an application), it m ust only ide ntify the nature of the translation it ne e ds (for e x am ple , a toll-fre e num be r to E.164 "re al" num be r), and se nd the re que st to a location that has GT routing table s to pe rform the translation. GTT is only pe rform e d on the num be r of digits re quire d to ide ntify whe re the SC C P m e ssage should be se nt afte r translation. For e x am ple , in our toll-fre e illustration, GTT m ay only be pe rform e d on the thre e m ost significant digits (800) at the SSP to de te rm ine that all 800 num be rs should be se nt to a de signate d STP. At the STP, GTT could re quire translation of six digits (800-289) in orde r to de te rm ine the ne x t STP for inte rm e diate GTT or the final SC P de stination. The se de cisions are m ade base d on the adm inistration of the ne twork and agre e m e nts be twe e n ne twork ope rators.

NOTE It is im portant not to confuse dire ctory num be r translation with GTT. W he n a que ry involving a num be r translation se rvice is se nt, GTT de te rm ine s the SS7 addre ss of the se rvice (DPC + SSN) in orde r to de live r the m e ssage to the corre ct SP and subsyste m . The se rvice (such as toll-fre e ) translate s the num be r containe d in the TC AP portion of the m e ssage , not the GT num be r in the SC C P portion of the m e ssage .

This allows a single e ntry in the SSP's routing table (such as the location of an STP) to provide 800 num be r translations. As state d pre viously in this se ction, with inte rm e diate GTT, e ve n the first location that re ce ive s the que ry (for DPC and/or SSN of de stination application) doe s not have to m aintain a routing table of all locations on the globe . Inste ad, it m ight have a table that indicate s that all re que sts in se ve ral sim ilar cate gorie s should be se nt to one location, while re que sts in othe r cate gorie s can be se nt som e whe re e lse . The se locations e ithe r dire ctly ide ntify the corre ct de stination application (subsyste m ) or again, in the case of inte rm e diate GTT, se nd it to anothe r node for www.ss7-training.net/sigtran-training/ch09lev1sec4.html

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furthe r GT routing analysis. Figure 9-12 shows a furthe r e x am ple using the GSM ce llular ne twork .

Figure 9-12. GTT on a GSM Cellular Network [View full size image]

In Figure 9-12, a VLR in C ountry A originate s a MAP Update Location m e ssage . The m e ssage contains the DPC of a C ountry A's Inte rnational Switching C e ntre (ISC ). The MSC /VLR contains the PC of the ISC that is provisione d in its routing table s. The m e ssage also contains the GT of the HLR (an E.164 num be r). The ISC at C ountry A change s the DPC to be an ISC of C ountry B. Again, this PC is alre ady provisione d in its routing table s, and again, the GT of the HLR is pre se nt in the m e ssage . The ISC in C ountry B happe ns to have the data fill to translate the GT into a PC +SSN; the re fore , it pe rform s the GTT. Thus, the m e ssage is route d to the HLR via the GMSC using only the PC +SSN. GT translations are usually ce ntralise d at STPs to allow routing change s to be m ade e asily.

Calling Party Address (CgPA) and Called Party Address (CdPA) The C gPA contains inform ation for ide ntifying the originator of the SC C P m e ssage . The C dPA contains inform ation to ide ntify the SC C P m e ssage 's inte nde d de stination. Figure 9-13 shows the place m e nt of the C gPA/C dPA in the conte x t of an MSU. Figure 9-14 shows the fie lds that are found within the C gPA/C dPA.

Figure 9-13. Positioning of the CgPA and CdPA Fields in the Context of an MSU [View full size image]

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Figure 9-14. The Subfields that Belong to Both the CgPA and CdPA Fields

Address Indicator (AI) The AI is the first fie ld within C gPA/C dPA and is one octe t in le ngth. Its function is to indicate which inform ation e le m e nts are pre se nt so that the addre ss can be inte rpre te d—in othe r words, it indicate s the type of addre ssing inform ation that is to be found in the addre ss fie ld so the re ce iving node k nows how to inte rpre t that data. The R outing Indicator (R I) spe cifie s whe the r GTT is re quire d; it de te rm ine s whe the r routing base d on PC +SSN or GT. If routing is base d on the GT, the GT in the addre ss is use d for routing. If routing is base d on PC +SSN, the PC and SSN in the C dPA are use d. The PC from the C dPA is the n place d into the MTP3 routing labe l DPC be fore MTP routing tak e s place . The GT Indicator (GTI) spe cifie s the GT form at. In addition to those code s shown pre viously, 0101 to 0111 re pre se nt spare international use , and 1000 to 1110 re pre se nts spare national use . The subsyste m num be r is e ncode d "00000000" whe n the Subsyste m Num be r is unk nown (such as be fore GTT). Figure 9-15 shows an e x am ple of SC C P routing using a GT.

Figure 9-15. Example Routing Parameters and Values [View full size image]

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The re are four possible GT form ats (bits C -F). '0100' is a com m on form at that is use d for inte rnational ne twork applications, including INAP, which is discusse d in C hapte r 11, "Inte llige nt Ne twork s (IN)," and MAP, which is discusse d in C hapte r 13, "GSM and ANSI-41 Mobile Application Part (MAP)." Figure 9-16 shows this com m on form at.

Figure 9-16. GT Format 0100 [View full size image]

W e now e x am ine the fie lds with the form at '0100' that are found within a GT.

Translation Type (TT) The Translation Type (TT) fie ld indicate s the type of translation. W he n it is not use d, the TT is code d 00000000. A GTI of 0010 is for national use only; the translation type s for GTI 0010 is a national de cision; it can im ply the e ncoding sche m e and the num be ring plan. The ITU-T has not spe cifie d the translation type s for GTI 0011. Figure 917 shows the TT value s [60].

Figure 9-17. Translation Type Values [60] www.ss7-training.net/sigtran-training/ch09lev1sec4.html

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[View full size image]

Encoding Scheme (ES) The Encoding Sche m e (ES) te lls the re ce iving node how to translate the digits from binary code . Figure 9-18 shows the ES value s [60].

Figure 9-18. Encoding Scheme Values [60]

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Numbering Plan (NP) The Num be r Plan (NP) fie ld spe cifie s the num be ring plan that the addre ss inform ation follows. The E.164 standard for te le phony has the form at C ountry C ode , National De stination C ode , and Subscribe r Num be r. The E.212 standard for the m obile station num be ring plan has the form at Mobile C ountry C ode , Mobile Ne twork C ode , and Mobile Subscribe r Ide ntity Num be r (MSIN). The E.214 standard is a hybrid num be r with the C ountry C ode and National De stination C ode from E.164 and the MSIN from E.212. The E.214 form at e x ists be cause inte rnational signaling ne twork s re quire E.164 form at. By re placing the le ading digits of an E.212 num be r with the le ading digits of an E.164 num be r, the e x isting translations can be use d to route GTs. Figure 9-19 shows the NP value s [60].

Figure 9-19. Numbering Plan (NP) Values [60] [View full size image]

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Nature of Address Indicator (NAI) The Nature of Addre ss Indicator (NAI) fie ld de fine s the addre ss range for a spe cific num be ring plan. The e x act m e aning de pe nds on the num be ring plan, not on GTI value s. Figure 9-20 shows the NAI value s [60].

Figure 9-20. The Nature of Address (NAI) Values [60] [View full size image]

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Address Information (AI) The AI contains the actual Global Title digits. The se include e nough of the m ost significant portion of the actual addre ss digits to ide ntify the de stination node . For e x am ple , if a toll-fre e num be r of 800-123-4567 is diale d, the AI fie ld m ight contain the digits 800 to ide ntify an SC P to which the tollfre e que ry should be se nt. As shown in Figure 921, the addre ss inform ation is pre dom inantly code d in Binary C ode d De cim al (BC D) using four bits to code e ach digit.

Figure 9-21. BCD Encoding of Address Digits [View full size image]

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As an e x am ple of the param e te rs found for GT form at "0100,"we conside r SC C P addre ssing for GSM-MAP m e ssage s (which is discusse d in C hapte r 13, "GSM and ANSI-41 Mobile Application Part [MAP]). For Inte r-PLMN addre ssing, the C dPA/C gPA contains the following value s: SSN Indicator = 1 (SSN include d); GT Indicator = 0100, GT include s Translation Type , Num be ring Plan, Encoding Sche m e and Nature of Addre ss Indicator; TT = 00000000 (not use d), and R outing Indicator = 0 (routing on GT). < Top Index >

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