Telephone Connections Network

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From the beginning, the telephone systems were not as a network. It was simply as a telephone line connected to isolated switches and served only the customer to whom they were connected directly. It became obvious that the best productive way of extending the range over which a telephone call might be connected was to plainly connect existing switches. The necessity of switches became apparent, to provide connections from any issued switch to multiple switches for routing a call. That was the beginning of network.

The rapid evolution of telecommunications after 1945 had increased dramatically to expand the line of telecom networks. During 50 signaling system was in-band. Network signal and the call were shared the same physical channel and were provided 300 to 3400 hz voice band with in the users. When user made call with other end, the connection occupied the whole distance between the two user for several seconds expect the call being accomplished. The international telephone network was causing the same problems. International bodies made careful investigation just to get an alternative technology for providing services to the masses.

The International Telecommunications Union ITU-TS (formally known as the CCITT) studied the possibility of an all-digital network. In the mid 1960s signaling went digital with a series of standards. The new digital connection and switches were the secret to transmit a call via a dedicated data link flow a few bits of the corresponding voice channel. That was the digital signaling standard #SS6 developed by ITU. That was the revolution for telephone industry. For high-speed data communication network #SS7 has been spread over #SS6 which has become the standard for the entire world

SS7 Technology overview

SS7 is a

SS7/C7 is an out-of band system used to carry out signaling and protocols for telecommunications networks. It was preliminary designed to take the place of earlier signaling systems and to serve higher using of network trunks. By implementing various databases to the network, SS7 has ability to provide extra revenue-gaining business and residential services, like as 800 numbers, caller id, end-to-end singling. Moreover, provide service for circuit related and non-circuit related applications. It is commonly popular as SS7 in America and elsewhere over the world as C7. SS7 provides services for both Cellular and fixed line networks. This protocol practices message structure and network topology, similarity with X.25 and others protocols which are on message-based. These messages carry from one network to other by requesting services from others in a packet form.

An out-of-band or Common Channel Signaling (CCS) had introduced in the USA as Common Channel Interoffice Signaling System SS6. International telecommunications union (ITU-TS) -Telecommunications Standards Society constructed it for a distinct fluency for sending signalling information to remote in place telephone offices, by using signaling channels that were apart from the subscriber voice channel. This offers network facilities to be fast released, allocated and tested. The Common Channel Signaling network can check all the way of route of a call and allocate facilities, which are available.

In the preliminary stage of SS6 used 2.4 kbps data links in the United States. Later, it upgraded to 4.8kbps. The format of messages was the form of data packets and accustomed to entreat coherence on voice trunks in between two dominant offices. That became the form of packet switching in the Public Switched Telephone Network (PSTN). The packets were built with 12 signaling amount of 28 bits each into a data area. Similar technique to the method used in SS7 today.

Signaling System #7(SS7) was descended from the SS6 that explain the similarities. SS7 services capability much more than SS6. SS6 worked on fixed -length signal units where SS7 works on variable-length signaling unit, providing more flexibility and versatility. SS7 provide 56 kbps high-speed data links, that provide the signaling network faster then SS6. In global networks, the data links perform at 64 kbps.

In United State, As SS7 spread over during 1980s in the interoffice network and was not used in local offices few years. Beginning of SS7, it was for remote database not for call set up and teardown. In the 1980s a new service offered by telephone companies called “Wide Area Telephone Service” (WATS), which adopted a common 800 area code without considering of the destination of the call. This stuck up a problem for telephone switching appliances, which passed down the distance code to drive how to route a call over the Public Switched Telephone Network (PSTN).

To resolve this difficulty, a 2nd number was accredited to every 800 number. The switching equipment is used for that number to route the call over the voice network. A centralized database was used to hold the number where all central offices allowed accessing it. This database was turned into popular resource for all telephone companies and exists until today. When a number 800 is being dialled, to access this remote database, Telephone Company uses a data communications link a-nd search the exact routing number. Massage packet is in the form, which search the network for the number. A return message packet return from database to provide the routing telephone number and billing information for the 800 number. It might Route the call from switching equipment using usual signaling methods.

Switching equipment and telephone company database connected by data communication link that is provide from SS7. Later, SS7 network was implemented more services, with call setup and teardown. This remote database accessibility has become the key point of the success of SS7 network. SS7 is proving 800 numbers, 900 number, Billing information, routing, and customisable calling feature, caller Id many more. Once Routing information was based on the location and 800 numbers at once associated with one service provider. If user used to change service provider, changing service provider they had to handover their 800 numbers. Central database store all routing information for 800 numbers and communication based on SS7. SS7 provide services for user to transfer or keeping 800 number even the may change their service provider.

Number portability has been possible only by SS7. In 1996, a service was provided by FFC “Local Number Portability (LNP)” which mandate for telephone companies to assist the process of adapting a telephone number. If subscribers want to change their Plain Old Telephone Service (POTS) to ISDN, They might be pressured to change telephone numbers. Because switching equipment organized the rages of telephone numbers. There is no mandate to change telephone no with LNP. Database assigned the switch in the network with organized number. That is similar with the way of 800 numbers are routed. In future LNP will support subscribers with a transportable constant telephone number even they may move from one area code to others. Moreover, switching equipment is able to communicate with other switching equipment at remote sites by the SS7 protocol-accessing database. Such as, roaming which rely on SS7 protocol.

Cellular service provider use SS7 to share user information. When a user register with a service provider, from his/her Home Location Register (HLRs). User no needs to register with other operator when travelling; even he/she can roam frequently from one network to other network.

Once Cellular operator were rely on X.25 networks to take IS -41 signaling information over their network before SS7. An X.25 network was not able to interconnect over the Public Switched Telephone Network (PSTN), as because this was not compatible with SS7 signaling network.

Today service operator has been deployed their own signaling network (SS7). This signaling network offering many new feature and application's because it has ability to transfer all types of digital information to a long distance. SS7 has been the largest information sharing Data communication network of the world. This new network offering many new sophisticated services to subscriber such as made to order calling features, ATM, ISDN, and cellular. Telephone operators all over the world come into a common signaling network by the blessed of SS7 network.

SS7 Architecture Overview

The SS7 architecture consists with three signaling points: Service Switching Points (SSPs), Signal Transfer Points (STPs) and Service Control Points (SCPs). Each signaling point is called an SS7 node or SP and identified with a unique point code (PC).In ANSI Point code are 24 bits and 14 bits for ITU. Signaling point is able to read a point code and determine which message is for that node and the routing of signaling messages to other Signaling point. Signaling points are joined via signaling links, (frequently 56 or 64 kbps, bidirectional X.25 data links).

Illustration of SSP in the network(s)

SSP (Service Switching Point)

Basically, An SSP switch operates setup or terminates call; perform quires of database even it is not known. SSP provide appropriate action to the response. This is the Local Exchange of telephone system. An SSP can be consists of voice switch and SS7 switch or a subordinate computer linked to the local exchange's voice switch.

This node type is associated with a switch in a telephone network. A telephone network is hi

The Service Switching Point (SSP) must convert signaling from the voice switch into SS7 signaling messages, which can then be sent to other exchanges through the SS7 network. The exchange will typically send messages related to its voice circuits to the exchanges with a direct connection to it.

In the case of database access, the SSP will be sending database queries through the SS7 network to computer systems located centrally to the network. This was the first usage of the SS7 network, as the need for 800 number lookup became necessary.

The traffic mix found in most SS7 networks is still primarily circuit-related messages. With the implementation of applications such as Local Number Portability (LNP), the traffic mix is changing significantly, becoming predominately non-circuit-related messages. These messages originate from SSPs and are used to connect voice circuits from one exchange to another exchange. The SSP does not use circuit-related messages exclusively, however.

Before a switch can route a call, it must first be able to access information regarding the destination of the call. For most Plain Old Telephone Service (POTS) systems, the telephone number dialed is sufficient for routing. However, with 800 and 900 numbers, routing is impossible, because the dialed digits do not provide enough information about the destination.

Even POTS may now require a database query. If the switch determines that the called number has been "ported," it will generate a primitive to the SSP requesting the SSP send a query to an LNP database to determine to which exchange the ported number has been reassigned. Porting is the movement of a number from one switch to another (regardless of geography) allowing subscribers to move or change primary telephone companies without changing their telephone numbers.

For this reason, the Service Switching Point (SSP) must access a remote database to learn the routing number assigned to the 800 or 900 number or the new location of a ported number. Once this information has been retrieved, the SSP can then begin circuit connections based on the new routing number information.

The SSP function is to use the information provided by the calling party (such as dialed digits) and determine how to connect the call. A routing table will identify which trunk circuit to use to connect the call, and which exchange this trunk terminates at. An SS7 message must be sent to this adjacent exchange requesting a circuit connection on the specified trunk.

The adjacent exchange grants permission to connect this trunk by sending back an acknowledgement to the originating exchange. Using the called party information in the setup message, the adjacent exchange can determine how to connect the call to its final destination. This may require several trunk connections between several adjacent exchanges. The SSP function manages these connections until the final destination is reached.

Many SSP functions are accomplished by adding a computer adjunct to existing switches. This computer receives signals from the voice switch which are used to trigger the transmission of specific SS7 messages. The called and calling party address must be passed from the voice switch to the SSP for transfer across the network.

Using adjuncts allows telephone companies to upgrade their SS7 signaling points without replacing expensive switches, providing a modular approach to networking. Upgrades are typically limited to software loads, since these computers require very little hardware.

There are very few features required of an SSP. The ability to send messages using the ISDN User Part (ISUP) protocol and the Transaction Capabilities Application Part (TCAP) protocol is the only requirement, other than the network management defined in the Bellcore publications. Specific Bellcore requirements for an SSP can be found in Bellcore publication TR-TSY-000024, Service Switching Points (SSPs) Generic Requirements (this has since been updated, and the new document is known as GR-024-CORE).

Figure 2 shows an SSP (1) connected to two pairs of STPs via access (A) links, the primary channels for all STP-SSP communications. A links simultaneously support up to 9600 connections [12]. If the number of connections exceeds 9600, the SSP diverts tra±c to extended (E) links (shown connecting SSP (1) to an alternative STP in Figure 2). The architecture does not demand that SSPs be deployed with E links. Therefore, some SSPs, e.g., SSP (2) in Figure 2, cannot handle large volumes of tra±c. SSPs communicate by sending signaling messages over A and E links to other SSPs via STPs. They send queries to SCPs via STPs using separate links. Figure 2 shows the four primary line types connecting

SS7 Using for:

• call management and setup

• wireless services like personal communications services (PCS), wireless roaming, and mobile subscriber Authentication.

• local number portability (LNP)

• toll-free (800/888) and toll (900) wire line services

• enhanced call features such as call forwarding, calling party name/number display, and

Three-way calling

• efficient and secure worldwide telecommunications

SS7 Signaling Network

@SSP = Service Switching point

@STP = Signal Transfer Point

@SCP = Service Control Point

Service Switching Point (SSP):

  • SSP are Switches for originate and terminate calls.
  • Its send singnaling message to other SSPs for setup and manage to complete a call.
  • ISDN user part (ISUP) and Transaction capabilities Application (TCAP) protocols used SSP s for sending message.