mid term report on ss7

8/17/2019 Mid Term Report on SS7

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DIT UNIVERSITY

DEHRADUN (UTTARAKHAND)

Mid –Term Report

ON

NLD/ILD Operations

SHIVAM CHHABRA

IT-B


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120070103099


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Project Description

Here we are talking care of national and international calls done by telenor

subscribers which include Routing, Troubleshooting Analysis, and Creation of

new connectivity with other operators PAN India. Further I have studied RealTime Implementation of SS7 protocol based on OSI model.

It also includes working on Application level protocols, Routing for different

Levels and Troubleshooting has been performed. I have created connectivity

from Layer 1 of OSI model to Layer 7. Physical connectivity of media used is

done then further protocol based configuration is taken. Then, we have created

associations based on M3UA, SCTP and S.248 protocols.


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Signaling System 7 (SS7)

DefinitionSignaling System 7 (SS7) is an architecture for performing out-of-band signaling insupport of the call-establishment, billing, routing, and information-exchangefunctions of the public switched telephone network (PST)! "t identifies functions to

be performed by a signaling-system network and a protocol to enable theirperformance!

What Is Signaling?

Signaling refers to the exchange of information between call componentsre#uired to pro$ide and maintain ser$ice!

%s users of the PST, we exchange signaling with network elements all the time!&xamples of signaling between a telephone user and the telephone network include' dialing digits, pro$iding dial tone, accessing a $oice mailbox, sending acall-waiting tone, dialing (to retry a busy number), etc!

SS7 is characteri*ed by high-speed packet data and out-of-band signaling!

What Is Out-of-Band Signaling?

+ut-of-band signaling is signaling that does not take place o$er the same path asthe con$ersation!

e are used to thinking of signaling as being in-band! e hear dial tone, dial digits,and hear ringing o$er the same channel on the same pair of wires! hen the callcompletes, we talk o$er the same path that was used for the signaling! Traditionaltelephony used to work in this way as well! The signals to set up a call between oneswitch and another always took place o$er the same trunk that would e$entually carry the call! Signaling took the form of a series of multifre#uency (.) tones,much like touch tone dialing between switches!

+ut-of-band signaling establishes a separate digital channel for the exchange of signaling information! This channel is called a signaling link! Signaling links areused to carry all the necessary signaling messages between nodes! Thus, when a

call is placed, the dialed digits, trunk selected, and other pertinent information aresent between switches using their signaling links, rather than the trunks which will


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ultimately carry the con$ersation! Today, signaling links carry information at a rateof / or 0 kbps! "t is interesting to note that while SS7 is used only for signaling

between network elements, the "S1 1 channel extends the concept of out-of-bandsignaling to the interface between the subscriber and the switch! ith "S1 ser$ice,signaling that must be con$eyed between the user station and the local switch is

carried on a separate digital channel called the 1 channel! The $oice or data whichcomprise the call is carried on one or more 2 channels!

Why Out-of-Band Signaling?

+ut-of-band signaling has se$eral ad$antages that make it more desirable thantraditional in-band signaling!

• "t allows for the transport of more data at higher speeds (/ kbps cancarry data much faster than . outpulsing)!

• "t allows for signaling at any time in the entire duration of the call, notonly at the beginning!

• "t enables signaling to network elements to which there is no directtrunk connection!

Signaling Network r!hite!ture"f signaling is to be carried on a different path from the $oice and data traffic it

supports, then what should that path look like3 The simplest design would be to

allocate one of the paths between each interconnected pair of switches as the

signaling link! Sub4ect to capacity constraints, all signaling traffic between the two

switches could tra$erse this link! This type of signaling is known as associated

signaling, and is shown below in Figure


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%ssociated signaling works well as long as a switch5s only signaling re#uirements are between itself and other switches to which it has trunks! "f call setup and

management was the only application of SS7, associated signaling would meetthat need simply and efficiently! "n fact, much of the out-of-band signalingdeployed in &urope today uses associated mode!

The orth %merican implementers of SS7, howe$er, wanted to design a signalingnetwork that would enable any node to exchange signaling with any otherSS7− capable node! 6learly, associated signaling becomes much more complicated when

it is used to exchange signaling between nodes which do not ha$e a direct connection!

.rom this need, the orth %merican SS7 architecture was born

"he North meri!an Signalingr!hite!ture

The orth %merican signaling architecture defines a completely new andseparate signaling network! The network is built out of the following threeessential components, interconnected by signaling link'

• signal switching points (SSPs)—SSPs are telephone switches (end offices

or tandems) e#uipped with SS7−

capable software and terminating signalinglinks! They generally originate, terminate, or switch calls!

• signal transfer points (STPs)STPs are the packet switches of the SS7network! They recei$e and route incoming signaling messages towards theproper destination! They also perform speciali*ed routing functions!

• signal control points (SCPs)S6Ps are databases that pro$ideinformation necessary for ad$anced call-processing capabilities!

+nce deployed, the a$ailability of SS7 network is critical to call processing! 8nless

SSPs can exchange signaling, they cannot complete any interswitch calls! .or thisreason, the SS7 network is built using a highly redundant architecture! &achindi$idual element also must meet exacting re#uirements for a$ailability! .inally,protocol has been defined between interconnected elements to facilitate the routingof signaling traffic around any difficulties that may arise in the signaling network!

To enable signaling network architectures to be easily communicated andunderstood, a standard set of symbols was adopted for depicting SS7 networks!

Figure shows the symbols that are used to depict these three key elements of any SS7 network!


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STPs and S6Ps are customarily deployed in pairs! hile elements of a pair are not

generally co-located, they work redundantly to perform the same logical function!

hen drawing complex network diagrams, these pairs may be depicted as a single

element for simplicity, as shown in Figure

Basi! Signaling r!hite!ture

Figure 4 shows a small example of how the basic elements of an SS7 network aredeployed to form two interconnected networks!

The following points should be noted'

9! STPs and : perform identical functions! They are redundant! Together,they are referred to as a mated pair of STPs! Similarly, STPs ; and < form amated pair!

=! &ach SSP has two links (or sets of links), one to each STP of a mated pair! %ll SS7 signaling to the rest of the world is sent out o$er these links!

2ecause the STPs of a mated pair are redundant, messages sent o$ereither link (to either STP) will be treated e#ui$alently!


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>!The STPs of a mated pair are 4oined by a link (or set of links)!

0! Two mated pairs of STPs are interconnected by four links (or sets of links)! These links are referred to as a #uad!

/!S6Ps are usually (though not always) deployed in pairs! %s with STPs, theS6Ps of a pair are intended to function identically! Pairs of S6Ps are alsoreferred to as mated pairs of S6Ps! ote that they are not directly 4oined by a pair of links!

! Signaling architectures such as this, which pro$ide indirect signalingpaths between network elements, are referred to as pro$iding #uasi-associated signaling!

SS7 #ink "y$esSS7 signaling links are characteri*ed according to their use in the signaling network!

?irtually all links are identical in that they are / − kbps or 0− kbps bidirectional

data links that support the same lower layers of the protocol@ what is different is their

use within a signaling network! The defined link types are shown in Figure and

defined as follows

#inks

% links interconnect an STP and either an SSP or an S6P, which are collecti$ely

referred to as signaling end points (A%A stands for access)! % links are used for thesole purpose of deli$ering signaling to or from the signaling end points (they could


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4ust as well be referred to as signaling beginning points)! &xamples of % links are =−B, >− 7, and /− 9= in Figure 5 !

Signaling that an SSP or S6P wishes to send to any other node is sent on either of its % links to its home STP, which, in turn, processes or routes the messages! Similarly,

messages intended for an SSP or S6P will be routed to one of its home STPs, which will forward them to the addressed node o$er its % links!

% #inks

6 links are links that interconnect mated STPs! %s will be seen later, they are used toenhance the reliability of the signaling network in instances where one or se$erallinks are una$ailable! A6A stands for cross (7− B, C− 9D, and 99− 9= are 6 links)! 2links, 1 links, and 2E1 links interconnecting two mated pairs of STPs are referred toas either 2 links, 1 links, or 2E1 links! Fegardless of their name, their function is tocarry signaling messages beyond their initial point of entry to the signaling network towards their destination! The A2A stands for bridge and describes the #uad of linksinterconnecting peer pairs of STPs! The A1A denotes diagonal and describes the #uadof links interconnecting mated pairs of STPs at different hierarchical le$els! 2ecausethere is no clear hierarchy associated with a connection between networks,interconnecting links are referred to as either 2, 1, or 2E1 links (7− 99 and 7− 9= areexamples of 2 links@ B− C and 7− 9D are examples of 1 links@ 9D − 9> and C− 90 areexamples of interconnecting links and can be referred to as 2, 1, or 2E1 links)!

& #inks

hile an SSP is connected to its home STP pair by a set of % links, enhancedreliability can be pro$ided by deploying an additional set of links to a second STPpair! These links, called & (extended) links pro$ide backup connecti$ity to the SS7network in the e$ent that the home STPs cannot be reached $ia the % links! hile allSS7 networks include %, 2E1, and 6 links, & links may or may not be deployed at thediscretion of the network pro$ider! The decision of whether or not to deploy & linkscan be made by comparing the cost of deployment with the impro$ement inreliability! (9− 99 and 9− 9= are & links!)

' #inks

. (fully associated) links are links which directly connect two signaling end points!

. links allow associated signaling only! 2ecause they bypass the security featurespro$ided by an STP, . links are not generally deployed between networks! Theiruse within an indi$idual network is at the discretion of the network pro$ider! (9− =is an . link!)

Basi! %all Setu$ &am$le


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2efore going into much more detail, it might be helpful to look at se$eral basic calls

and the way in which they use SS7 signaling

"n this example, a subscriber on switch % places a call to a subscriber on switch 2!

Switch % analy*es the dialed digits and determines that it needs to send the callto switch 2!

% selects an idle trunk between itself and switch 2 and formulates an initialaddress message ("%), the basic message necessary to initiate a call! The "%is addressed to switch 2! "t identifies the initiating switch (switch %), thedestination switch (switch 2), the trunk selected, the calling and called

numbers, as well as other information beyond the scope of this example!

Switch % picks one of its % links (e!g!, %) and transmits the message o$er thelink for routing to switch 2!

STP recei$es a message, inspects its routing label, and determines that it is to be

routed to switch 2! "t transmits the message on link 2!

Switch 2 recei$es the message! +n analy*ing the message, it determines that itser$es the called number and that the called number is idle!

Switch 2 formulates an address complete message (%6), which indicates thatthe "% has reached its proper destination! The message identifies the recipientswitch (%), the sending switch (2), and the selected trunk!

Switch 2 picks one of its % links (e!g!, 2:) and transmits the %6 o$er the link for routing to switch %! %t the same time, it completes the call path in the

backwards direction (towards switch %), sends a ringing tone o$er that trunk towards switch %, and rings the line of the called subscriber!

STP : recei$es the message, inspects its routing label, and determines that it is to

be routed to switch %! "t transmits the message on link %:!

+n recei$ing the %6, switch % connects the calling subscriber line to theselected trunk in the backwards direction (so that the caller can hear the


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ringing sent by switch 2)!

hen the called subscriber picks up the phone, switch 2 formulates an answermessage (%), identifying the intended recipient switch (%), the sendingswitch (2), and the selected trunk!

Switch 2 selects the same % link it used to transmit the %6 (link 2:) andsends the %! 2y this time, the trunk also must be connected to the calledline in both directions (to allow con$ersation)!

STP : recogni*es that the % is addressed to switch % and forwards it o$erlink %:!

Switch % ensures that the calling subscriber is connected to the outgoingtrunk (in both directions) and that con$ersation can take place!

"f the calling subscriber hangs up first (following the con$ersation), switch % will generate a release message (F&G) addressed to switch 2, identifying thetrunk associated with the call! "t sends the message on link %!

STP recei$es the F&G, determines that it is addressed to switch 2, andforwards it using link 2!

Switch 2 recei$es the F&G, disconnects the trunk from the subscriber line, returns

the trunk to idle status, generates a release complete message (FG6) addressed

back to switch %, and transmits it on link 2:! The FG6 identifies the trunk used to carry the call!

STP : recei$es the FG6, determines that it is addressed to switch %, andforwards it o$er link %:!

+n recei$ing the FG6, switch % idles the identified trunk!

#ayers of the SS7 roto!ol


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%s the call-flow examples show, the SS7 network is an interconnected set of network elements that is used to exchange messages in support of telecommunications functions! The SS7 protocol is designed to both facilitate thesefunctions and to maintain the network o$er which they are pro$ided! Gike mostmodern protocols, the SS7 protocol is layered!

hysi!al #ayer

This defines the physical and electrical characteristics of the signaling links of the

SS7 network! Signaling links utili*e 1SHD channels and carry raw signaling data at a

rate of / kbps or 0 kbps (/ kbps is the more common implementation)!

*essage "ransfer art+#e,el

The le$el = portion of the message transfer part (TP Ge$el =) pro$ides link-layer

functionality! "t ensures that the two end points of a signaling link can reliably exchange signaling messages! "t incorporates such capabilities as error checking,flow control, and se#uence checking!

*essage "ransfer art+#e,el .

The le$el > portion of the message transfer part (TP Ge$el >) extends thefunctionality pro$ided by TP le$el = to pro$ide network layer functionality! "tensures that messages can be deli$ered between signaling points across the SS7network regardless of whether they are directly connected! "t includes such

capabilities as node addressing, routing, alternate routing, and congestion control!

6ollecti$ely, TP le$els = and > are referred to as the message transfer part(TP)!

Signaling %onne!tion %ontrol art

The signaling connection control part (S66P) pro$ides two ma4or functions that arelacking in the TP! The first of these is the capability to address applications withina signaling point! The TP can only recei$e and deli$er messages from a node as a

whole@ it does not deal with software applications within a node!

hile TP network-management messages and basic call-setup messages areaddressed to a node as a whole, other messages are used by separate applications(referred to as subsystems) within a node! &xamples of subsystems are BDD callprocessing, calling-card processing, ad$anced intelligent network (%"), and customlocal -area signaling ser$ice (6G%SS) ser$ices (e!g!, repeat dialing and call return)!The S66P allows these subsystems to be addressed explicitly!

/lo0al "itle "ranslation

The second function pro$ided by the S66P is the ability to perform incremental


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routing using a capability called global title translation (ITT)! ITT frees originatingsignaling points from the burden of ha$ing to know e$ery potential destination to

which they might ha$e to route a message! % switch can originate a #uery, forexample, and address it to an STP along with a re#uest for ITT! The recei$ing STPcan then examine a portion of the message, make a determination as to where the

message should be routed, and then route it!

ISDN 1ser art (IS1)

The "S8P user part defines the messages and protocol used in the establishmentand tear down of $oice and data calls o$er the public switched network (PS), andto manage the trunk network on which they rely! 1espite its name, "S8P is used for

both "S1 and non− "S1 calls! "n the orth %merican $ersion of SS7, "S8Pmessages rely exclusi$ely on TP to transport messages between concerned nodes!

"ransa!tion %a$a0ilities $$li!ation art("%)

T6%P defines the messages and protocol used to communicate betweenapplications (deployed as subsystems) in nodes! "t is used for database ser$icessuch as calling card, BDD, and %" as well as switch-to-switch ser$ices includingrepeat dialing and call return! 2ecause T6%P messages must be deli$ered toindi$idual applications within the nodes they address, they use the S66P fortransport!

O$erations2 *aintenan!e2 and dministrationart (O*)

+%P defines messages and protocol designed to assist administrators of the SS7network! To date, the most fully de$eloped and deployed of these capabilities areprocedures for $alidating network routing tables and for diagnosing link troubles!+%P includes messages that use both the TP and S66P for routing!

What /oes O,er the Signaling #ink

Signaling information is passed o$er the signaling link in messages, which arecalled signal units (S8s)!

Three types of S8s are defined in the SS7 protocol!

• message signal units (S8s)

• link status signal units (GSS8s)

• fill-in signal units (."S8s)

S8s are transmitted continuously in both directions on any link that is in ser$ice! % signaling point that does not ha$e S8s or GSS8s to send will send ."S8s o$er the


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link! The ."S8s perform the function suggested by their name@ they fill up thesignaling link until there is a need to send purposeful signaling! They also facilitatelink transmission monitoring and the acknowledgment of other S8s!

%ll transmission on the signaling link is broken up into B-bit bytes, referred to as

octets! S8s on a link are delimited by a uni#ue B-bit pattern known as a flag! The flagis defined as the B-bit pattern AD999999DA! 2ecause of the possibility that data withinan S8 would contain this pattern, bit manipulation techni#ues are used to ensurethat the pattern does not occur within the message as it is transmitted o$er the link!(The S8 is reconstructed once it has been taken off the link, and any bitmanipulation is re$ersed!) Thus, any occurrence of the flag on the link indicates theend of one S8 and the beginning of another! hile in theory two flags could beplaced between S8s (one to mark the end of the current message and one to mark the start of the next message), in practice a single flag is used for both purposes!

ddressing in the SS7 Network

&$ery network must ha$e an addressing scheme, and the SS7 network is no different!etwork addresses are re#uired so that a node can exchange signaling nodes to which itdoes not ha$e a physical signaling link! "n SS7, addresses areassigned using a three-le$el hierarchy! "ndi$idual signaling points are identified as belonging to a cluster of signaling points! ithin that cluster, each signaling point is assigned a membernumber! Similarly, a cluster is defined as being part of a network! %ny node in the

%merican SS7 network can be addressed by a three-le$el number defined by itsnetwork, cluster, and member numbers! &ach of these numbers is an B-bit number

and can assume $alues from D to =//! This three-le$el address is known as the pointcode of the signaling point! % point code uni#uely identifies a signaling point withinthe %merican SS7 network and is used whene$er it is necessary to address thatsignaling point!

etwork numbers are assigned on a nationwide basis by a neutral party! Fegional2ell operating companies (F2+6s), ma4or independent telephone companies, andinterexchange carriers (":6s) already ha$e network numbers assigned! 2ecausenetwork numbers are a relati$ely scarce resource, companiesJ networks are expectedto meet certain si*e re#uirements in order to be assigned a network number! Smallernetworks can be assigned one or more cluster numbers within network numbers 9, =,

>, and 0! The smallest networks are assigned point codes within network number /!The cluster to which they are assigned is determined by the state in which they arelocated! The network number D is not a$ailable for assignment and network number=// is reser$ed for future use!

Signal 1nit Stru!ture

S8s of each type follow a format uni#ue to that type! % high-le$el $iew of thoseformats is shown in Figure


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%ll three S8 types ha$e a set of common fields that are used by TP Ge$el =!They are as follows'

'lag

.lags delimit S8s! % flag marks the end of one S8 and the start of the next!

%he!ksum

The checksum is an B-bit sum intended to $erify that the S8 has passed across thelink error-free! The checksum is calculated from the transmitted message by thetransmitting signaling point and inserted in the message! +n receipt, it isrecalculated by the recei$ing signaling point! "f the calculated result differs fromthe recei$ed checksum, the recei$ed S8 has been corrupted! % retransmission isre#uested!

#ength Indi!ator

The length indicator indicates the number of octets between itself and thechecksum! "t ser$es both as a check on the integrity of the S8 and as a means of discriminating between different types of S8s at le$el =! %s can be inferred from

Figure 8, ."S8s ha$e a length indicator of D@ GSS8s ha$e a length indicator of 9 or =(currently all GSS8s ha$e a length indicator of 9), and S8s ha$e a length-indicator greater than =! %ccording to the protocol, only of the B bits in the lengthindicator field are actually used to store this length@ thus the largest $alue that can

be accommodated in the length indicator is >! .or S8s with more than > octetsfollowing the length indicator, the $alue of > is used!


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BSN3BIB 'SN3'IB

These octets hold the backwards se#uence number (2S), the backwards indicator

bit (2"2), the forward se#uence number (.S), and the forward indicator bit (."2)!These fields are used to confirm receipt of S8s and to ensure that they are recei$edin the order in which they were transmitted! They also are used to pro$ide flow control! S8s and GSS8s, when transmitted, are assigned a se#uence number thatis placed in the forward se#uence number field of the outgoing S8! This S8 is stored

by the transmitting signaling point until it is acknowledged by the recei$ingsignaling point!

2ecause the se$en bits allocated to the forward se#uence number can store 9=Bdistinct $alues, it follows that a signaling point is restricted to sending 9=Bunacknowledged S8s before it must await an acknowledgment! 2y acknowledgingan S8, the recei$ing node frees that S8Js se#uence number at thetransmitting node, making it a$ailable for a new outgoing S8! Signaling pointsacknowledge receipt of S8s by placing the se#uence number of the last correctly recei$ed and in-se#uence S8 in the backwards se#uence number of e$ery S8 they transmit! "n that way, they acknowledge all pre$iously recei$ed S8s as well! Theforward and backwards indicator bits are used to indicate se#uencing or data-corruption errors and to re#uest retransmission!


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THANK YOU

mid term report on ss7

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