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Public Switched Telephone Network Technologies

by: Lawrence Harte and Robert Flood

Some of the key technologies behind the operation of the public telephone network include interconnection lines, network common control signaling, and intelligent call processing. Several types of interconnection systems are used to provide access to different services and systems available through the PSTN. To coordinate the overall operation of the PSTN, a standard common control signaling (CCS) system is typically used. The use of intelligent call processing can combine the use of efficient high-speed interconnection lines with common control signaling to provide for advanced services such as call forwarding, telephone number portability, and prepaid services. 

Public Telephone System Interconnection

There are many types of interconnection options available to connect public telephone systems to other public telephone networks or private telephone networks. The type of connection selected depends on the type of private system, telecommunications regulations, and the needs of the company that uses the private telephone system (e.g., advanced calling features). In addition to standard telephone system connection types, there are also private-line connections that may be used to link private branch exchange PBX systems together. 


There are two types of connections that are used between switching systems: line side and trunk side. Line side connections are an interconnection line between the customer’s equipment and the last switch EO in the telephone network. The line side connection isolates the customer’s equipment from network signaling requirements. Line side connections are usually low capacity (one channel) lines. Trunk side connections are used to interconnect telephone network switching systems to each other. Trunk side connections are usually high capacity lines. Primary rate interfaces use out-of-band signaling in a dedicated signaling channel.

 
This article is Part 1 of a 5 Part Series
Series Article List

Introduction to PSTN
PSTN Technologies
PSTN Systems
PSTN Services

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POTS (dial) Line Connections


POTS dial lines are 2-wire, basic line-side connections from an End Office (EO)  with limited signaling capability. Because dial lines are line-side connections, call setup time may be longer than those connections that employ trunk-side supervision. 


Direct Inward Dialing (DID) Connections

Direct inward dialing (DID) connections are trunk-side (network side) EO connections. The network signaling on these 2-wire circuits is primarily limited to one-way, incoming service. DID connections employ different supervision and address pulsing signals than dial lines. Typically, DID connections use a form of loop supervision called reverse battery, which is common for one-way, trunk-side connections. Until recently, most DID trunks were equipped with either dial pulse (DP) or dual tone multifrequency (DTMF) address pulsing. While many wireless carriers would have preferred to use multifrequency (MF) address pulsing, a number of LEC’s prohibited the use of MF on DID trunks. 

Foreign Exchange Office (FXO)

Foreign exchange office (FXO) is an interface or channel unit that allows an analog connection (foreign exchange circuit) to be directed at the PSTN’s central office or to a station interface on a PBX. The FXO sits on the switch end of the connection. It plugs directly into the line side of the switch so the switch thinks the FXO interface is a telephone. (See also: foreign exchange station.)

 
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Foreign Exchange Station (FXS)

Foreign exchange station is a type of channel unit used at the subscriber station end of a foreign exchange circuit. A foreign exchange station (FXS) interface connects directly to a standard telephone, fax machine, or similar device and supplies ring, voltage, and dial tone. (See also: foreign exchange office.)

Type 1 Connections (TI)

Type 1 (T1) connections are trunk-side connections to an EO. The EO uses a trunk-side signaling protocol in conjunction with a feature known as Trunk With Line Treatment (TWLT). This connection was originally described in technical advisory 76 published by AT&T in 1981. This interconnection was developed because dial line and DID connections did not provide enough signaling information to allow the connection of public telephone networks to other types of networks (such as wireless and PBX networks). The switch must be equipped to provide TWLT, or its equivalent to offer Type 1 service. As a result, T1 is not universally available. The TWLT feature allows the EO to combine some line-side and trunk-side features. For example, while trunk-side signaling protocols are used, the calls are recorded for billing purposes as if they were made by a line-side connection. 
     T1 connections are usually used as 2-way trunks. Two-way trunks are 4-wire circuits, meaning they have separate transmit and receive paths, and almost always use MF address pulsing and supervision. The address pulsing normally uses wink-start control. One-way Type 1 connections can be provided on a 2-wire basis using E&M supervision or reverse battery like the DID connection. T1 connections in a digital context are also provided and these are labeled as T1 services. These T1 services include in-band signaling as well as out-of-band signaling in the later described services of primary interface.

Integrated Services Digital Network - Basic Rate Interface Connections (ISDN-BRI)

ISDN-BRI connection provides two bearer channels, each using a 64 kbps digital channel, as well as a 16 kbps data link for signaling messages. This 144 kbps combination is referred to as 2B+D, which signifies two bearer channels and one data channel. The bearer channels provide the voice portion while the data channel is used to transfer SS7 signaling messages. EO switches must have an ISDN-BRI interface and software installed to supply this connection. 

Integrated Services Digital Network - Primary Rate Interface Connections

Another variation of Type 1 is the Integrated Services Digital Network - Primary Rate Interface (ISDN-PRI). It has capabilities similar to the ISDN-BRI but employs 23 bearer channels and one signaling channel, or a 23B+D configuration. The ISDN-PRI interconnection is provided using a standard DS1-level interface that would normally provide the equivalent of 24 voice channels. It offers the same calling capabilities as noted for the Type 1 and ISDN-BRI connections. Primary rate interfaces use out-of-band signaling in a dedicated signaling channel.

Type 2A Connections

Type 2A connections are true trunk-side connections that employ trunk-side signaling protocols. Typically, they are two-way connections that are 4-wire circuits using E&M supervision with multifrequency (MF) address pulsing. The address pulsing is almost always under wink-start control. Type 2A connections allow the other public 

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or private telephone network switching systems to connect to the PSTN and operate like any other EO.

   Type 2A connections may restrict calls to specific NXX (exchange) codes and access to operator services (phone number directories, emergency calls, freephone/toll free) may not be permitted. For some interconnections, additional connections (such as a type 1) may be used to supplement the type 2A connection to allow access to other operator or network services.

Type 2B Connections

Type 2B connections are high usage trunk groups that are used between EOs within the same system. The Type 2B connection can be used in conjunction with the Type 2A. When a type 2B is used, the first choice of routing is through a Type 2B with overflow through the type 2A. Because the Type 2B connection is used for high usage connections, it can access only valid NXX codes of the EO providing that it is connected to. Type 2B connections are almost always 4-wire, two-way connections that use E&M supervision and multifrequency (MF) address pulsing.
 

Type 2C Connections

Type 2C connections were developed to allow direct connection to public safety centers (E911) via a tandem or local tandem switch. This

interconnection type must provide additional information such as the return phone number (complicated on mobile telephone systems) and the location of the caller. This information is passed on to a public safety answering point (PSAP). In recent times primary rate interface has been a more popular connection for the purposes of enhanced 911 services and the appropriate public safety answering points. Because of the outer band signaling and the dedicated channel for signaling and the PRI connection has become more flexible and versatile to meet the needs of an enhanced 911 service offering.

Type 2D Connections

Type 2D interconnection lines allow direct connection from an operator services system (OSS) switch. The OSS switch is a special tandem that contains additional call processing capabilities that enables operator services special directory assistance services. The type 2D connection also forwards the automatic number identification information to allow proper billing records to be created. Type 2D connection will normally use trunks employing E&M signaling with wink start, and multifrequency (MF) address pulsing.
 

Type S Connections
 

Type S connections transfer signaling messages that are associated with other interconnection types (out-of-band signaling). The Type S

Figure 1.1, Private to Public Telephone System Interconnection
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is a data link (e.g., 56 kbps) that is used to connect the signaling interfaces between switches. Type S connections permit additional features to be supported by the network such as finding and using call forwarding telephone numbers. Because Type S connections cost money, some smaller public telephone networks do not offer or use Type S connections.

Figure 1.1 illustrates some of the different types of private to public telephone system interconnection. This diagram shows some groups of phone lines (e.g., dial line, Type 1) that provide limited signaling information (line-side) that primarily interconnect the PSTN with private telephone systems. Another group of lines (Type 2 series) are used to interconnect switching systems or to connect to advanced services (such as operator services or public safety services). The interconnection lines (trunk-side) provide more signaling information. Also shown is the type S connection that is used exclusively for sending control signaling messages between switching system and the signaling system 7 (SS7) telephone control network.

Common Channel Signaling (CCS)

The signaling system 7 (SS7) is an international standard network signaling protocol that allows common channel (independent) signaling between telephone network elements. SS7 system protocols are optimized for telephone system control connections and they are only directly accessible to telephone network operators. ]

 Common channel signaling (CCS) is a separate Signaling System that separates content of telephone calls from the information used to set up the call (signaling information). When call-processing information is separated from the communication channel, it is called “out-of-band” signaling. This signaling method uses one of the channels on a multi-channel network for the control, accounting, and management of traffic on all of the channels of the network.
   An SS7 network is composed of service switching points (SSPs), signaling transfer points (STPs), and service control points (SCPs). The SSP gathers the analog signaling information from the local line in the network and converts the information into a digital SS7 signaling message. These messages are transferred into the SS7 network to STPs that transfer the packet closer to its destination. When special processing of the message is required such as routing a call to a call forwarding number, the STP routes a query to a SEP. The SCP is a database that can use the incoming message to determine other numbers and features that are associated with this particular call.
   In the SS7 protocol, an address, such as customer-dialed digits, does not contain explicit information to enable routing in a signaling network. It then will require the signaling connection control part (SCCP) translation function. This is a process in the SS7 system that uses a routing table to convert an address (usually a telephone number) into the actual destination address (forwarding telephone number) or into the address of a service control point (database) that contains the customer data needed to process a call.

 
 Figure 1.2,Signaling System 7 (SS7) Network

      

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Intelligence in the network can be distributed to databases and information processing points throughout the network because the network uses common channel signaling. A set of service development tools has been developed to allow companies to offer advanced intelligent network (AIN) services.

   Figure 1.2 shows the basic structure of the SS7 control signaling system. This diagram shows that a customer’s telephone is connected to a local switch. The local switch converts the dialed digits to a SS7 signaling message. The SS7 network routes the control packet to its destination using its own STP data packet switches and separate interconnection lines. In some cases, when additional services are provided, SCPs are used to process requests for advanced telephone services. This diagram also shows that the connections used for signaling are different than the voice connections. There are multiple redundant links between switches, switching points, and network databases. 

SS7 and Internet Protocol (IP) Signaling Systems

SS7 messages can be directly transported over IP networks or the functional equivalent of SS7 control message can be sent as control messages (e.g. text based messages) directly between elements connected

to a data network (e.g. the Internet). 
   Figure 1.3 shows that SS7 signaling systems can be interconnected with voice over data networks and that SS7 messages can be transported over the Internet protocol. This diagram shows that analog and digital telephones are connected to the PSTN. To interconnect these telephones to voice over data network telephones, the media portion of each communication session is routed through a media gateway where it is converted from the PSTN circuit switched form to a IP packet data media format (packetized voice.) This diagram shows that the packet media can be routed through a data network (e.g. Internet) to an endpoint communication terminal such as a multimedia computer or an IP telephone. This diagram also shows that the SS7 network can control the PSTN through SS7 signaling messages and it can communicate to the media gateway through IP signaling messages.

Advanced Intelligent Networks (AIN)

Advanced intelligent networks (AIN’s) are telecommunications networks that are capable of providing advanced services through the use of distributed databases that provide additional information to call processing 

Figure 1.3, Hybrid SS7 and Internet Protocol Network 
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Figure 1.4, Advanced Intelligent Network (AIN) 

and routing requests. 
   In the mid 1980’s, Bellcore (now Telcordia) developed a set of software development tools to allow companies to develop advanced services for the telephone network [5]. The advanced intelligent network (AIN) is a combination of the SS7 signaling network, interactive database nodes, and development tools that allow for the processing of signaling messages to provided for advanced telecommunications services. 
   The AIN system uses a service creation environment (SCE)  to create advanced applications. The SCE is a development tool kit that allows the creation of services for an AIN that is used as part of the SS7 network. Using AIN, SS7 control messages can interact with signaling end point (SEP) databases that are connected to SSPs.
   A service management system (SMS) is the interface between applications and the SS7 telephone network. The SMS is a computer system that administers service between service developers and signal control point databases in the SS7 network. The SMS system supports the development of intelligent database services. The system contains 

routing instructions and other call processing information. 
   To enable SCPs to become more interactive, intelligent peripherals (IPs) may be connected to them. IPs are a type of hardware device that can be programmed to perform an intelligent network processing for the SCP database. IPs perform processing services such as interactive voice response (IVR), selected digit capture, feature selection, and account management for prepaid services.
   To help reduce the processing requirements of SCP databases in the SS7 network, adjunct processors (APs) may be used. APs provide some of the database processing services to local Service Switching Points (SSPs).


Series Source:
Introduction to PSTN
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