Public Switched Telephone Network (S-PSTN)

Public Switched Telephone Network (S-PSTN)
Student Notes S-PSTN Public Switched Telephone Network (S-PSTN) Global Wireless Education Consortium  July, 2001

Student Notes S-PSTN S-PSTN Partial support for this curriculum material was provided by the National Science Foundation's Course, Curriculum, and Laboratory Improvement Program under grant DUE and Advanced Technological Education Program under grant DUE‑ GWEC EDUCATION PARTNERS: This material is subject to the legal License Agreement signed by your institution. Please refer to this License Agreement for restrictions of use. Partial support for this curriculum material was provided by the National Science Foundation's Course, Curriculum, and Laboratory Improvement Program under grant DUE and Advanced Technological Education Program under grant DUE‑ GWEC EDUCATION PARTNERS: This material is subject to the legal License Agreement signed by your institution. Please refer to this License Agreement for restrictions of use. Global Wireless Education Consortium  July, 2001

Table of Contents Overview 5 Learning Objectives 6 Network Overview 7
Student Notes Table of Contents S-PSTN Overview 5 Learning Objectives 6 Network Overview 7 Switching Systems 13 Post-Divestiture Network 19 Signaling 28 Contributors 38 Global Wireless Education Consortium  July, 2001

Overview Network setup Switching systems Post-Divestiture network
Student Notes Overview S-PSTN Network setup Switching systems Post-Divestiture network Signaling SS7 CCS Overview This module will introduce you to the Public Switched Telephone Network (PSTN). The PSTN is a publicly accessible network of interconnected elements designed to carry telecommunications services. The major components of the PSTN are: Switching Offices Transmission Facilities Customer Premise Equipment There is a defined telephony switching hierarchy in the PSTN with a standard numbering plan that allows users to initiate telephone calls. In this module you will learn about the signaling required to control the connection of a typical telephone call in the PSTN. Originally the signaling in the PSTN was inband signaling that occurred on the same transmission facility that was used for the call. Currently the majority of signaling is accomplished using an out-of-band network referred to as the Signaling System 7 (SS7) Signaling Network. It acts as an overlay network to the PSTN and carries the signaling using labeled messages between elements in the PSTN. Global Wireless Education Consortium  July, 2001

Learning Objectives After completing this module you will be able to:
Student Notes Learning Objectives S-PSTN After completing this module you will be able to: Discuss the basic operations of the Public Switched Telephone Network (PSTN) Define the telephony switching hierarchy Describe the signaling required to complete a typical telephone call through the PSTN Describe how the SS7 network is overlayed onto the PSTN After completing this module and all its activities, you will be able to: Discuss the basic operations of the Public Switched Telephone Network (PSTN) Define the telephony switching hierarchy Describe the signaling required to complete a typical telephone call through the PSTN Describe how the SS7 network is overlayed onto the PSTN Global Wireless Education Consortium  July, 2001

Network Overview Student Notes S-PSTN
Global Wireless Education Consortium  July, 2001

What Is A Network ? Switching Switching Office Office
Student Notes What Is A Network ? S-PSTN Switching Office Switching Office CPE Network A network is a system of interconnected elements. It can be represented by a group of nodes interconnected by links. A telecommunications network is a network of interconnected elements designed to carry telecommunications services. The nodes of a telecommunications network are switching offices and user station equipment called customer premise equipment (CPE). The links represent the transmission facilities. The flow of messages and information between the nodes over the links is called traffic. Traffic can be generated by simple telephone conversations or complex data, audio, and video services. A telecommunications network must be able to carry traffic between customers over large geographic areas, generated between any customers at any time, with relatively short delay. CPE Transmission Facilities Global Wireless Education Consortium  July, 2001

Public Switched Telephone Network (PSTN)
Student Notes Public Switched Telephone Network (PSTN) S-PSTN Major Components of the Public Switched Telephone Network (PSTN): Switching Offices Transmission facilities Customer Premise Equipment (CPE) Public Switched Telephone Network (PSTN) The Public Switched Telephone Network, or PSTN, is the worldwide telecommunications network accessible to everyone. The basic elements of the PSTN are: Switching Offices that route or switch traffic through the PSTN over transmission facilities. Transmission Facilities that provide the communication paths that carries traffic between any two nodes in the network. Customer Premise Equipment (CPE) is the user station equipment the customer provides to interface with the PSTN. Global Wireless Education Consortium  July, 2001

Switching Offices SWITCHING OFFICE CONTROL NETWORK Switching Office
Student Notes Switching Offices S-PSTN SWITCHING OFFICE CONTROL Switching Office Switching offices interconnect transmission facilities at various key locations and route, or switch traffic through the PSTN. The switching offices are centrally located throughout the PSTN and also referred to as Central Offices. There are two essential parts of a switching office: Switching Network that provides the connection between customer lines and the transmission facilities that interconnect the switching offices. Switching Control mechanism that responds to signals on the switching network portion and directs the switching of connections through the switching network. Both the switching network and switching control architecture have advanced over the years as the PSTN has evolved. Switching Evolution Manually operated cord boards were the original switching office with operators using a system of cords with plugs and jacks to manually switch calls. Electromechanical switches were developed that were motor driven or electromagnetically operated that responded to electrical signals or pulses. Electronic switching elements made from semiconductor devices are now used. They are computer driven under stored program control. Electronic switching offices use either an analog switching network that switches a call as an analog voice grade signal or a digital switching network that converts the call from analog to digital and switches the call as a digital signal. NETWORK Global Wireless Education Consortium  July, 2001

Transmission Facilities
Student Notes Transmission Facilities S-PSTN Switching Office Switching Office Trunks Line Line Special Service Circuit Transmission facilities Transmission facilities provide the communication paths that carry traffic between nodes in the PSTN. There are two main categories of transmission paths in the PSTN: Lines are communication paths from the customer premise equipment to the switching office. Lines are also called customer loops, a term derived from from the original pair of wires that formed a loop between the customer and the switching office. Lines are dedicated to the individual customer. Trunks are communication paths between two switching systems. A call between two customer lines may involve several switching offices with trunks interconnecting the various switching offices. Trunks are used during a call then released afterward to be used repeatedly. A third type of transmission facility is the special service circuit. This is a dedicated path through the PSTN to provide a special service to a specific customer. Special service circuits can encompass both a dedicated path to the customer’s premise and a dedicated path between switching offices. Global Wireless Education Consortium  July, 2001

Customer Premise Equipment (CPE)
Student Notes Customer Premise Equipment (CPE) S-PSTN Customer Premise Equipment (CPE) is the term used to identify any piece of equipment supplied by the customer to interface with the PSTN. Examples include: Single Line Telephone Set Modems or Data Sets Private Branch Exchange (PBX) Customer Premise Equipment (CPE) Customer Premise Equipment (CPE) is the term used to denote the station equipment the customer uses to interface with the PSTN. It is a generally used term and covers any equipment the customer uses when calling on the PSTN. There are many services available on the PSTN and many different varieties of CPE. Some examples of the most popular services are: The single line telephone set. It is the most common type of CPE. It allows the customer to convert a voice conversation into an electrical signal to be sent on a transmission facility. Conversely it will convert an electrical signal from the transmission facility, most likely a customer loop from a switching office, into an audio signal. Modems or data sets are in wide use and allow computers to communicate through the PSTN by converting data first into a series of tones then into an electrical signal to be sent over a transmission facility. Private Branch Exchange (PBX) is a type of CPE that interfaces a small to medium sized business customer with the PSTN. The PBX is a relatively small switching system located on a customer’s premise and connected to a local switching office over dedicated PBX trunks. The PBX provides many features such as station to station calling for a related group of users, attendant capabilities for a centralized answer point, and other enhanced features such as call transfer. Global Wireless Education Consortium  July, 2001

Switching Systems Switching
Student Notes S-PSTN Switching Systems Switching This section provides a broad characterization of switching systems as elements (nodes) of the facilities network. The primary function of switching systems is to interconnect circuits. Depending on the types of circuits involved, switching systems fall into two functional categories: Local Switching - Local switching systems connect customer loops directly to other customer loops or to trunks. Local switching systems, which may serve many thousands of customer loops, are also referred to as central offices. More than one central office may occupy the same building which may also contain transmission and signaling equipment. Tandem Switching - The term tandem is used generically for any switching system that connects trunks to trunks. In a more specific sense, it is often used to denote systems typically found in metropolitan networks within the PSTN. These local tandem switching systems connect local switching systems to each other or to other systems in the PSTN, or they can interconnect other metropolitan tandem systems. Tandem systems that perform the class 1 through class 4 functions described below in a toll switching hierarchy are called toll switching systems. Some switching systems perform both local and tandem switching functions. Local switching also incorporates the use of remote switching systems that serve small population centers. Customer loops are connected to these systems, which are connected to central offices in larger population centers. The remote systems are extensions of the host office. In addition to network switching systems located in telephone company buildings, another type of switching system, the Private Branch Exchange (PBX), is typically located on a customer’s premises. A PBX connects a localized community of users to each other and to a network switching system. Global Wireless Education Consortium  July, 2001

Traffic Networks The Public Switched Telephone Network:
Student Notes Traffic Networks S-PSTN The Public Switched Telephone Network: Pre-divestiture Post-divestiture Traffic Network The description of the telecommunications network is composed of physical elements, including station equipment, transmission facilities, and switching systems. In this context, the network is seen as a facilities network. It is also important to discuss how this network provides various telecommunications services. From this perspective, the network is thought of as a set of traffic networks sharing common facilities. The PSTN is the largest and best-known traffic network. Other traffic networks provide a variety of special services such as private-line voice and data, packet, and audio and video program services. Each traffic network is designed to meet a particular set of requirements related to transmission performance, reliability, maintenance, and expected traffic volume. The various types of traffic in the PSTN represent communications between any two end points in the network on demand, that is, upon request at any time during the 24-hour day. Traffic is switched through each switching office (node) it encounters and travels between nodes on trunk groups. In the pre-divestiture (prior to 1984) network, the offices and trunk groups were arranged in a hierarchical routing structure, as described below. The PSTN – Pre-Divestiture The PSTN actually consists of two interdependent networks: the local network (sometimes called the exchange area network) and the toll network. Global Wireless Education Consortium  July, 2001

Local Network Structure
Student Notes Local Network Structure S-PSTN Exchange Area 2 Exchange Area 5 Exchange Area 1 Wire Center B • Wire Center A Local Network The structure of the local network begins with customer station equipment connected by loops to local switching systems. All customers connected to a local switching system (central office) in a particular central office building are said to be located in a wire center area, and the location of the building is called the wire center. Customers located within a wire center area communicated with each other through the local switching system, or systems, at the wire center. An exchange area boundary is an area within which there is a heavy customer calling volume. There is a single uniform set of charges for telephone service in the area. An exchange area may be served by a number of local switching systems. A call between any customers within an exchange’s boundary area is a local call. • Exchange Area 4 Exchange Area 3 Global Wireless Education Consortium  July, 2001

Typical Local Network Tandem Office T Wire Center C Tandem Trunk
Student Notes Typical Local Network S-PSTN Tandem Office T Wire Center C Tandem Trunk Groups (Final) Wire Center A Wire Center B Local Network In the figure above, high traffic volumes between offices at wire centers A and B justify a direct trunk group. Traffic between A and C or B and C does not warrant direct trunk groups and are therefore carried by tandem groups through tandem office T. Using tandem trunk groups and switching systems usually involves longer transmission paths and more switching systems but proves to be more economical when the traffic volumes between office pairs are low. Note that there are alternate routes between offices, for instance, between offices A and B through office T, if traffic is allowed to overflow that way. Direct Trunk Groups (High Usage) Global Wireless Education Consortium  July, 2001

Toll Network Structure Pre-Divestiture
Student Notes Toll Network Structure Pre-Divestiture S-PSTN Regional Center D Class 1 Regional Center C Class 2 Regional Center B Class 3 Toll Center A Class 4 FINAL Regional Center E Sectional Center F Primary Center G HU6 HU5 HU4 HU3 HU2 HU1 Toll Center H Toll Connecting Local Office (End Office) Class 5 Local Tandem Office HU7 Toll Network (Pre-Divestiture) The toll network hierarchy provided an economical structure for interconnecting all switching offices in the nationwide network. The switching hierarchy used in the United States had five levels, which successively higher level offices (also called classes) concentrated traffic for increasingly larger geographical areas. Class 5 offices (also called local switching offices and end offices) were part of the local network previously discussed. The toll network consisted of the class 4 and higher switching offices, known as toll centers, primary centers, sectional centers, and regional centers, and the trunks interconnecting them. All trunks within the toll network were called intertoll trunks. Class 5 end offices were connected to the class 4 toll centers by toll connecting trunks. An office connected by a final trunk group to a higher class office is said to “home” on that office, although it should be noted that offices do not always home on the next higher class office. For example, while most class 5 offices (end offices) home on class 4 offices (toll centers), some class 5 offices (end offices) home on class 3, 2, or 1 offices. Telephone 1 Telephone 2 Global Wireless Education Consortium  July, 2001

Toll Network Structure Pre-Divestiture
Student Notes Toll Network Structure Pre-Divestiture S-PSTN Regional Center D Class 1 Regional Center C Class 2 Regional Center B Class 3 Toll Center A Class 4 FINAL Regional Center E Sectional Center F Primary Center G HU6 HU5 HU4 HU3 HU2 HU1 Toll Center H Toll Connecting Local Office (End Office) Class 5 Local Tandem Office Telephone 1 Telephone 2 HU7 Typically, offices in the same homing chain are located relatively near each other. Consequently, many final trunk groups are only a few miles long. The Final and High Usage (HU) trunk groups that interconnect two different regions (that is, two different homing chains) are the long-haul trunks of the toll network. If the volume of traffic between offices of the same class or differing in class by one is high enough, it is more economical to connect them directly by HU trunks (such as HU1 through HU6 in the Figure) than to send them by some indirect path. Sometimes traffic volume between offices in the same homing chain justifies interconnection by an HU group (such as HU7 shows). The basic rule for routing a call is to complete it using the lowest level of hierarchy, thus using the fewest trunks. In the Figure, a call from telephone 1 to telephone 2 first goes to the local office. That office recognizes (through digit analyses) the call as a toll call and sends it over a toll connecting trunk to toll center A. Toll center A searches for an idle HU trunk, first in the HU1 group, then in HU2. If all trunks in those groups are busy, the call overflows to the final trunk group connecting toll center A and primary center B. If all the trunks in this final group are busy, the call is blocked and the customer receives a fast busy tone (reorder). Otherwise, the call reaches primary center B, and the sequential search procedure continues up through the hierarchy, searching HU3 through HU6 and related final groups until either an idle path between the two offices is found or all possible routing alternatives are exhausted. The maximum number of trunks that may be used in a connection is nine, although less than that are usually needed. Global Wireless Education Consortium  July, 2001

The Post-Divestiture Network
Student Notes S-PSTN The Post-Divestiture Network Summary of the Modification of Final Judgment The 1982 Modification of Final Judgment (MFJ) required AT&T to divest itself of the twenty-two Bell Operating Companies (BOCs). The major provisions of the MFJ were extensive, but primarily required that exchange and exchange access services be provided by the BOCs, independent of AT&T, and that equal access be provided to all interexchange carriers, such as AT&T. Thus, the nationwide Bell System network, which was designed, built, and operated as a single unit prior to divestiture was divided into two components: An exchange and exchange access portion provided by the divested BOCs and an Interexchange portion provided by AT&T. This division did not correspond to the pre-divestiture distinctions between AT&T Long Lines and BOC operations, between intrastate and interstate jurisdictions, or between toll and local services. It was based instead on a definition of an exchange used in the MFJ. The MFJ concept was to group large segments of population with common social and economic interests within an exchange. The territory served by the Bell System was divided into approximately 160 of these exchanges, which were named as Local Access and Transport Areas (LATAs). Depending on population densities and other factors, most LATAs serve territories ranging from major metropolitan areas to entire states. Accordingly, LATAs generally contain a number of pre-divestiture exchange areas. The pre-divestiture BOCs performed both inter-LATA and intra-LATA functions. The MFJ specifies that BOCs offer regulated telecommunications services within LATAs, while AT&T and other interexchange carriers (ICs) offer services between LATAs. Thus, the BOCs provide basic local telephone service, public telephone and operator services. Global Wireless Education Consortium  July, 2001

The Post-1984 Network LATA x LATA y InterLATA Carriers IEC1 IEC2 IEC3
Student Notes The Post-1984 Network S-PSTN IEC1 IEC2 IEC3 LATA x InterLATA Carriers LATA y Post-Divestiture Network As mentioned, the pre-divestiture serving areas were subdivided into LATAs. The BOCs provide local and toll service within their LATAs, but calls between the LATAs were required to be handled by the customer’s designated Inter-LATA carrier, as shown above. As shown in the following figure, the BOC network and the Inter-LATA networks were interconnected at defined Points of Presence (POPs). The Telecommunications Act of 1996 brought relief from many of the regulations imposed by the MFJ. Included was a provision that allows BOCs to enter the inter-LATA long distance market and Inter-LATA carriers to enter the local market (after meeting a list of requirements specified in the Act). Global Wireless Education Consortium  July, 2001

Typical LEC Network LEC IC Central Office Tandem Office IC POP
Student Notes Typical LEC Network S-PSTN Central Office Tandem Office IC POP Switching Systems Distribution Facility/Local Loop Interoffice Facilities/Trunks Local Exchange Network (LEC) Components Switching System – A system used to select paths for connecting the calling party to the called party. Central Office – A telephone company building in which switching system and other telephone equipment are installed (also called a Wire Center). Distribution Facility – The physical connection between a Central Office and the customer location. Local Loop – A channel between a customer’s terminal and a Central Office. Also called a line. Interoffice Facility – A physical connection between two Central Offices. Trunk – A communication path between two switching systems used in the establishment of an end-to-end connection. Tandem Office – A switching system that establishes trunk-to-trunk connections. IC POP – Interexchange Carrier Point of Presence is the physical location within a LATA established by an IC for the purpose of obtaining access to the end users located in that LATA. Distribution Facility/Local Loop Central Office LEC IC Global Wireless Education Consortium  July, 2001

LATA Access Services Switched Access Special Access (Nonswitched)
Student Notes LATA Access Services S-PSTN Switched Access Special Access (Nonswitched) Access Services There are two types of services that LECs provide to ICs and other carriers to provide access to telephone subscribers within a LATA: Switched Access – Provided by most LECs as either equal access for conforming end offices (Feature Group D) or by several other forms of access for nonconforming end offices (Feature Groups A, B, and C, described on the next page). Special Access (Nonswitched) – A transmission path used to connect end user-designated premises, either directly or through a LEC hub. Global Wireless Education Consortium  July, 2001

Switched Access Service
Student Notes Switched Access Service S-PSTN Feature Group A Feature Group B Feature Group C Feature Group D Equal Access End Office Feature Groups Feature Group A (FGA) is a line-side access that includes foreign exchange service and inter-LATA Off-Network Access Line (ONAL) service from private networks. Feature Group B (FGB) is a trunk-side access arrangement. Calls to the IC or other carriers use the telephone number 950-WXXX, where W equals 0 or 1. The call may go directly from the end office or be tandemed through a second office known as an access tandem to reach the carrier. Feature Group C (FGC) contains the arrangements originally used between the BOC and AT&T for provision of switched-access services. Originally it was thought that FGC would be used only until equal access was implemented under Feature Group D However, many BOCs left FGC in place until Common Channel Signaling was available in the LATA. Feature Group D (FGD) consists of Equal Access End Office (EAEO)-to-IC, EAEO-to-access tandem, and access tandem-to-carrier interconnection arrangements. In an Equal Access End Office (EAEO), the calling party enters all the address as usual. If not presubscribed to carrier, the calling party can dial the access-code 10XXX. This is followed by 0 or 1 to indicate whether the call requires operator assistance (0) or does not need it (1). Then the 7- or 10-digit telephone number of the called party is dialed. Global Wireless Education Consortium  July, 2001

North American Numbering Plan
Student Notes North American Numbering Plan S-PSTN Numbering Plan Area Central Office Station N X X N X X X X X X North American Numbering Plan An essential element of network dialing is a numbering system wherein each station has a unique address which is convenient to use, readily understandable, and identical in its format to those of all stations connected to the network. With such a numbering system, operators or customers, wherever located, may use this address to reach the desired telephone through the network. This system is known as the North American Numbering Plan. The 10-digit numbers used for network dialing within the North American Numbering Plan consists of the following routing codes: 1. Numbering Plan Area (NPA) code, a 3-digit code (commonly called an area code). 2. Central Office (CO) code, a 3-digit number that identifies a specific switching system within each NPA. Also called “NXX”, it was formerly noted as NNX, until a code expansion was needed. A Central Office may handle more than one NXX. 3. Subscriber Station, a 4-digit code that identifies a specific customer served by the switching system. Together, these ten digits comprise the network “address” for each telephone. Global Wireless Education Consortium  July, 2001

Number Plan Area (NPA) These special purposes codes include:
Student Notes Number Plan Area (NPA) S-PSTN These special purposes codes include: NXX Reserved Codes: N11 Reserved Codes: 600 Used in Canada for TWX (teletype service) 700 Assigned to ICs 800, 888, 877, and 866 Service (INWATS) 900 Service (DIAL-IT Service) 311 Non-Emergency Police and Fire 411 Local Directory Assistance 611 Repair Service* 811 Business Office* 911 Emergency Number *Some companies use 7 digits or 800 service instead. Numbering Plan Area (NPA) Numbering Plan Areas (NPAs) have been created in accordance with principles that tend to maximize customer understanding, while minimizing both dialing effort and telephone plant cost. Boundaries are established to last for long periods of time, and their locations are based on estimates of future requirements at the time they are drawn. Reevaluation of boundaries sometimes suggests that better ones could have been selected. However, making changes after the passage of time would often cause massive customer disruption, numerous number changes, and expensive plant rearrangements. Principles considered in planning NPA boundary changes resulting from either the creation of new NPAs or the realignment of existing ones are as follows: Boundaries must not extend across state lines. Boundaries should coincide with other political subdivision boundaries where practical. When this is impractical, boundaries should follow recognizable physical geographic features or structures; i.e., rivers, large lakes, mountain ranges, and major highways. Boundaries should be drawn so as to minimize the splitting of communities of interest of recognized metropolitan areas, looking both at the present and the future. Global Wireless Education Consortium  July, 2001

Student Notes Number Plan Area (NPA) S-PSTN Interchangeable area (or NPA) codes are NPA codes of the format NXX. By changing from the former N-0/1-X format to the NXX format, the maximum number of assignable NPA codes increased by 640 codes as shown: Interchangeable NPA Interchangeable NPA Codes took effect on January 1, 1995. It should be noted that NPA overlays are becoming common and that in the future NPAs may not be geographical at all. Again, the network numbering plan used in the United States is known as the North American Numbering Plan. Several other countries also use this system for telephone numbering. The purpose of the numbering plan is to assign a specific 10-digit telephone number to identify a particular network address, with the 10-digit number consisting of 3 main parts: the area code (NPA) the central office code (NXX), which represents a maximum of 800 number combinations or CO codes the 4-digit station code (XXXX), which represents as many as 10,000 lines. Old Format New format N 0/1 X 8 x 2 x 10 = 160 N X X 8 x 10 x 10 = 800 160-8* = 152 *Excluding codes of N11 format 800-8* = 792* Global Wireless Education Consortium  July, 2001

International Numbering
Student Notes International Numbering S-PSTN If you have made international calls, you know that the familiar NANP is a subset of the ITU international plan. Until recently, international numbers were limited to 12 digits. 1, 2 or 3 digits Varies by location Country Codes Trunk Code Local Code Subscriber Number Today, the trunk code, local code and subscriber number may be as many as 17 digits. So an international number can be as many as 20 digits. Some switching centers serve as International Gateway offices. Global Wireless Education Consortium  July, 2001

Signaling Student Notes S-PSTN
Global Wireless Education Consortium  July, 2001

Student Notes Signaling S-PSTN Signaling is the generation, transmission, and reception of information needed to direct and control the setup and disconnect of a call. Signaling Signaling is the method used for the switching offices and the CPE to control the switching of calls over the transmission facilities in the PSTN. Signaling is the generation, transmission, and reception of information needed to direct and control the setup and disconnect of a call. There are many kinds of signals which need to be transmitted between offices for an interoffice call, some of which include: Addressing – Outpulsing of dialed digits Information signals – Dial tone, audible ring, busy back tone, reorder, etc. Supervision – Off-hook, on-hook, seizures Global Wireless Education Consortium  July, 2001

Originating Switching Office Terminating Switching Office
Student Notes Signaling S-PSTN Originating Switching Office Terminating Switching Office Originating CPE Terminating CPE Idle 1 Off-hook 2 Dial Tone 3 Dialed Digits Off-hook Off-hook (wink) On-hook (wink) Signaling Stages A typical telephone call requires several stages of signaling before a conversation can begin. The idle state of the CPE is a supervisory state called on-hook. In the on-hook state a customer line will be connected to the common voltage supplied by the switching office. The most common line voltage is –48v. The –48v is the historical nominal voltage supplied by the switching office. However, depending on the operating state of the power supply in the switching office the power supplied to a customer’s line may vary from –42.5v to –56v. In the on-hook state the customer’s line appears as a high resistance, essentially an open circuit. To originate a call the customer must change the supervisory state to off-hook. Off-hook occurs when the CPE closes the customer loop back to the switching office causing current flow in the loop. The off-hook state is also called line seizure. In response to the off-hook the switching office returns an information signal called dial tone giving the customer the indication to start dialing. Dialed Digits Audible Ring Ringing Answer Off-hook Disconnect Global Wireless Education Consortium  July, 2001

Signaling 1 2 3 4 5 6 Idle Off-hook Dial Tone Dialed Digits Off-hook
Student Notes Signaling S-PSTN Originating Switching Office Terminating Switching Office Terminating Switching Office Originating CPE Terminating CPE Idle 1 Off-hook 2 Dial Tone 3 Dialed Digits 4 5 Off-hook Off-hook (wink) 6 On-hook (wink) Signaling Stages (Continued) The next stage of signaling is the addressing stage when the customer sends the dialed number to the switching office. The most common method is with a series of tones referred to as Dual Tone Multi Frequency (DTMF). The DTMF tones are generated by the dialing keypad on the CPE, each dialed digit sending a combination of two tones to the switching office. Still in use today is another older method called dial pulse. With dial pulse signaling a series of opens and shorts are returned to the switching office by the CPE. Each digit dialed would send a corresponding number of opens and shorts, or pulses to the switching system. At the switching office the addressing continues as the dialed digits are collected and translated to switch the call to the serving switching office of the dialed number. The originating switching office selects an available trunk to the terminating switching office and sends a supervisory off-hook condition on the selected trunk. The terminating switching office will return a “wink” back to the originating switching office which consists of a short off-hook returning to on-hook. The wink serves as an indication to the originating switching office to send the dialed digits. Dialed Digits Audible Ring Ringing Answer Off-hook Disconnect Global Wireless Education Consortium  July, 2001

Signaling 1 2 3 4 5 6 7 8 Idle Off-hook Dial Tone Dialed Digits
Student Notes Signaling S-PSTN Originating Switching Office Terminating Switching Office Terminating Switching Office Originating CPE Terminating CPE Idle 1 Off-hook 2 Dial Tone 3 Dialed Digits 4 5 Off-hook Off-hook (wink) 6 On-hook (wink) Signaling Stages (Continued) In response to the wink the originating switching office sends the dialed digits to the terminating switching office. There are two types of signaling used for inter-office signaling, inband and out-of-band signaling. Inband signaling sends the signals on the same transmission path that will be used for the call. Multi Frequency (MF) signaling is the most common method of sending inband signals by using a combination of two frequencies for every digit sent over the transmission path. Out-of-band signaling uses a dedicated data link transmission path to send signaling information separate from the transmission path used for the call. Inband signaling will be used for the remainder of this call example. Upon receiving the dialed digits the terminating switching office will translate them to customer’s number receiving the call and check the busy/idle status of the called customer’s line. If the called customer’s line is busy the terminating switching office will return busy tone to the originating switching office. If the called line is idle the terminating switching office will return audible ringing tone back to the originating switching office and also send ringing signal over the customer loop to the called customer’s CPE. The ringing signal is a-84vrms to –104vrms 20Hz signal superimposed on the nominal –48v on the customer loop. Dialed Digits 7 Audible Ring Ringing 8 Answer Off-hook Disconnect Global Wireless Education Consortium  July, 2001

Originating Switching Office Terminating Switching Office
Student Notes Signaling S-PSTN Originating Switching Office Terminating Switching Office Originating CPE Terminating CPE Idle 1 Off-hook 2 Dial Tone 3 Dialed Digits 4 5 Off-hook Off-hook (wink) 6 On-hook (wink) Signaling Stages (Continued) When the called customer answers the call it will send an off-hook signal back to the terminating switching office. The terminating switching office will disconnect both the ringing signal to the called customer and the audible ring to the the originating switching office. The terminating switching office will then send an answer signal back to the originating switching office and connect the call through to the called customer’s CPE. When the connection is released by either customer on-hook signals will be sent from the serving switching office and the connection will be disconnected. Dialed Digits 7 Audible Ring Ringing 8 Answer Off-hook 9 Disconnect 10 Global Wireless Education Consortium  July, 2001

Common Channel Signaling (CCS)
Student Notes Common Channel Signaling (CCS) S-PSTN Common Channel Signaling (CCS) is a signaling method that uses a separate dedicated channel to send and receive signaling information for a group of trunks or facilities by means of labeled messages. Common Channel Signaling (CCS) As mentioned earlier there are two methods of inter-office signaling, inband and out-of-band signaling. Inband signaling was used in the call example as the signaling was sent on the same transmission path that was used for the call. When sending the signals, out-of-band signaling uses a dedicated data link transmission path that is separate from the transmission path used for the call. Out-of-band signaling is also referred to as common channel signaling (CCS). CCS is a signaling method that uses a separate dedicated channel to send and receive signaling information for a group of trunks or facilities by means of labeled messages. CCS is a form of data communications specialized for the transfer of information between the nodes in a telecommunications network. System Signaling 7 (SS7) is the protocol used for CCS on an overlay network of the PSTN. Global Wireless Education Consortium  July, 2001

Signaling System 7 (SS7) LINKS
Student Notes Signaling System 7 (SS7) LINKS S-PSTN F CO CO CO E A A STP STP C D B Signaling System 7 (SS7) To implement SS7 a new network is used that overlays the PSTN. The central node of the SS7 network is called the Signal Transfer Point (STP). For redundancy and reliability the STPs are installed in mated pairs. Each switching office is connected to both mated STPs with high speed data links called A-links, or access links. The mated STPs are interconnected with C-links, or cross links. B-links, bridge links, and D-links, diagonal links, are used when different mated pairs of STPs interconnect. Two additional links are also defined but are seldom used. E-links are extended access links used to connect a switching office to non-home STPs. F-links are used to directly connect two switching offices. In addition to the STP and the switching offices a third node in the SS7 network is the Service Control Point (SCP). An SCP is a database containing detailed logic and data used to provide diversified calling services and features. Routing and service related queries from switching offices are received at the SCP where replies are formulated and returned to the originating switching office. Examples of an SCP database include 800 Number Services, Local Number Portability, and Advanced Intelligent Network Services. STP STP C A A SCP SCP Global Wireless Education Consortium  July, 2001

SS7 Switching Switching Office A Office B STP Customer “B”
Student Notes SS7 S-PSTN Switching Office A Switching Office B Customer “A” STP Customer “B” IAM ACM ANM REL RLC SS7 (Continued) During call setup labeled messages are sent between switching offices to control and direct call setup and disconnect. Each switching office and STP are assigned a nine digit address called a point code. Messages between switching offices are addressed using the point codes of the switching offices then are routed through the STP. Examples of SS7 messages include: Initial Address Message (IAM) – Sent in the forward direction to indicate seizure of an outgoing circuit and to transmit called number and other information related to the routing of a call. Address Complete Message (ACM) – Sent in the backward direction to indicate that all address signals required to route a call have been received. Answer Message (ANM) – Sent in the backward direction to indicate a call has been answered. Release Message (REL) – Sent in either direction indicating a circuit has been released and is ready to be placed in an idle state pending the reception of a Release Complete Message. Release Complete Message (RLC) – Sent in either direction in response to a Release Message after a related circuit has been placed in the idle state. For more information on SS7, see the GWEC module TT-SS7. Global Wireless Education Consortium  July, 2001

Network Administration, Maintenance and Services
Student Notes Network Administration, Maintenance and Services S-PSTN Network Management Traffic Measurements Billing Maintenance Customer Services Administration and Maintenance Today’s switching systems provide separate features to ensure that the switch operates reliably and efficiently. These features monitor, test, record, and permit human control of service-affecting conditions of the switching systems. Examples include: Network Management, which enables traffic to be rerouted to avoid congested portions of the PSTN. Traffic Measurement, which provides indications of the traffic loads being carried by various components of the switching system. Billing, which allows recording of call-related information required to charge properly for service. Maintenance, which involves features that automatically detect, isolate, and often locate system and component troubles to within several plug-in circuit packs. Much of the operating data are reported by the switching systems to Operational Support Systems that collect, analyze, filter, and summarize the data for human use. Customer Services In addition to connecting communication paths, modern switching systems also provide a variety of customer services. They include operator services, now performed primarily in conjunction with computer-based operator systems and automatic call distributors, coin services, Custom Calling Services and many more. This involves routing calls for nonworking numbers to an intercept system, returning deposited coins at a coin telephone for unanswered calls, routing a call to a line other than the one dialed (Call Forwarding), identifying the calling line for billing purposes on outgoing calls, etc. Global Wireless Education Consortium  July, 2001

Public Switched Telephone Network (S-PSTN)

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Public Switched Telephone Network (S-PSTN)

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