Public Switched Telephone Network

1. Public Switched Telephone Network

2. Public Switched Telephone System (PSTN) • Term PSTN refers to the public communication system, as a domestic telecommunications network that provides local, extended local and long distance telephone services. • It is public because the system is available to anyone who can afford the service,through a group of common communications carriers . • PSTN is composed of telephone exchanges networked together to form a nationwide and worldwide telephone communications system. • All calls are switched, as caller's conversation is broken into pieces (packets) and these pieces are sent simultaneously over many connections to reach a receiver at the other end. • The individual pieces are 'switched' from one telephone device to another until they reach at the receiving end. • All phones in the PSTN are networked in that any phone can make a call to any other phone because all the local phone systems around the country are connected to each other.

3. PSTN - Circuit Establishment • PSTN is a standard, circuit-switched network that is optimized for real-time voice communication. • When you place a call, you close a switch by dialing and establish a circuit with the other party. • PSTN guarantees quality of service (QoS) by dedicating the circuit to your call until you disconnect.

4. PSTN network equipment & Components • Telephone network equipment can be broadly classified as • Subscriber: • User • Devices attached to network • Subscriber line: • Local Loop • Subscriber loop • Connection to network • Few km up to few tens of km • Trunks circuits: • Branches between exchanges • Multiplexed • Exchange: • Switching centers • End office - supports subscribers.

5. PSTN Network

6. Switching Offices SWITCHING OFFICE CONTROL NETWORK

7. PSTN Subscribers & Subscriber Lines • Subscriber:In a PSTN, the ultimate user, i.e., customer of a communications service. • A subscriber is anyone who purchases telephone services or access to the PSTN and pays a monthly fee for this access. • Note1: Subscribers include individuals, activities, organizations, etc. • Note 2: Subscribers use end instruments, such as telephones, modems, facsimile machines, computers, and remote terminals, that are connected to a central office. • Note 3: Subscribers are usually subject to tariff. • Subscriber Line: The subscriber's phone line is referred to as a subscriber line. • PSTN accommodates two types of subscribers: • public &private (any organization) . • Subscribers to the private sector are customers who lease equipment, transmission media (facilities), and services from telephone companies on a permanent basis. • Leased circuits are designed and configured for their use only and are often referred to as dedicated circuits or private lines..

8. PSTN - Trunk Circuits • Trunk circuit is similar to a local loop except trunk circuits are used to interconnect two telephone offices (can be between two countries), • Trunk circuit can be as simple as a pair of copper wires twisted together or as sophisticated as an optical fiber cable. • Primary difference between a local loop and a trunk is that a local loop is permanently associated with a particular station, whereas a trunk is a conditional usage connection.

9. Trunk Circuit Classification • Trunk circuits can be classified as : • Inter office Trunks • Tandem Trunks • Toll Trunks • Inter Toll Trunks • Trunks are used to interconnect the central offices through the use of interoffice trunks and tandem trunks. • In largely populated area, a tandem office is used to minimize the number of trunks that a call must be routed through to reach its destination. • Outside the local area , toll trunks are used to connect the central office to toll centers. • Toll centers may be located in adjacent cities outside the local area. • To achieve even longer distance, toll centers are connected by intertoll trunks.

10. PSTN - Trunk Circuits Interoffice Trunks: Telephone switching machines in local exchanges are interconnected to other local exchange offices on special facilities called trunks or interoffice trunks. • A subscriber in one local exchange can call a subscriber connected to another local exchange over an interoffice trunk circuit in much the same manner that they would call a subscriber connected to the same exchange. Tandem Trunks: In larger metropolitan areas, it is virtually impossible to provide interoffice trunk circuits between all the local exchange offices. • To interconnect local offices that do not have interoffice trunks directly between them, tandem offices are used. • A tandem office is an exchange without any local loops connected to it. • The only facilities connected to the switching machine in a tandem office are trunks. Tandem switches interconnect local offices only. • A tandem switch is called a switcher's switch, and trunk circuits that terminate in tandem switches are called tandem trunks or sometimes intermediate trunks. Toll-Connecting Trunks: Interstate long distance telephone calls require a special telephone office called a toll office. • When a subscriber initiates a long-distance call, the local exchange connects the caller to a toll office through a facility called a toll-connecting trunk. • Toll offices are connected to other toll offices with intertoll trunks.

11. PSTN - Trunk Circuits Switching

12. Telephone Switching Hierarchy • As the network grew, more & more switches could not be directly connected with each other, so the need for higher level central switching centers arose. • In order to organize Direct Distance Dialing (DDD) American Telephone & Telegraph (AT&T) divided the various switches in the U.S. PSTN into a hierarchy containing five levels (or classes). • Class 1(regional centers): international gateways - handing off and receiving traffic from outside the USA and Canadian networks (outside the country). • Class 2(sectional centers): tandem exchanges - which interconnected whole regions of the AT&T network (with in country). • Class 3(primary centers): tandem exchanges- connecting major population centers within particular region of the AT&T network (with in province). • Class 4 (toll centers): tandem exchanges- connecting the various areas of a city or towns in a region (outside the local area). • Class 5(End office): to which subscribers and end-users telephone lines would connect (with in local area). • In modern times only the terms Class 4 and Class 5 are much used, as any tandem office is referred to as a Class 4. • Trunks connecting different levels and locations of switches formed alternate routes for call traffic. This network provided both local and long distance calling for the country.

13. Telephone Switching Hierarchy

14. PSTN Switching Plan

15. U.S. and Canadian Telephone Switch Hierarchy AT&T PSTN Office Classification Hierarchy

16. Class 1 (Regional center) • The class 1 office is the Regional Center (RC). • The Class-1 office, appears in only a few places in the country. • Regional centers served three purposes in the North American toll network • their connections are the "last resort" for final setup of calls when routes between centers lower in the hierarchy were not available. • they were initially staffed by engineers who had the authority to block portions of the network within the region in case of emergencies or network congestion. • they provided collection points for circuits that would be passed along to one of the international overseas gateways (which operated as special centers outside the formal North American hierarchy). • The regional centers updated each other on the status of every circuit in the network.

17. Class 2 (sectional center) • The class 2 office is the Sectional Center (SC). • The sectional center typically connected major toll centers within one or two states or provinces, or a significant portion of a large state or province, to provide interstate or interprovincial connections for long-distance calls. • At various times, there were between 50 and 75 active class two offices in the network.

18. Class 3 (primary center) • The class 3 office is the Primary Center (PC). • Calls being made beyond the limits of a small geographical area where circuits are not connected directly between class 4 toll offices would be passed from the toll center to the primary center. • These locations use high usage trunks to complete connection between toll centers. • The primary center never served dial tone to the user. • The number of primary centers in the network fluctuated from time to time, ranging between 150 and 230.

19. Class 4 (toll center) • The class 4 office is the Toll Center (TC), Toll Point (TP), Intermediate Point (IP) or Tandem switch central office telephone exchange used for long distance communications in the PSTN to interconnect telephone company offices. • If call going between two end offices not directly connected together, or whose direct trunks are busy, is routed through the toll center. • Toll center also called the tandem office because calls have to pass through this location to get to another part of the network. • A tandem switch is an intermediate switch or connection between an originating telephone call or location, and the final destination of the call. • Class 4 switches do not provide dial tone - they simply route calls between other switches, so they are more concerned with efficient switching and signaling. It delivers dial tone to the customer.

20. Class 5 (local exchange) • The class 5 office is the local exchange or end office , is the closest connection to the end customer, is primarily used to connect and terminate local calls to the customer • A Class 5 switch refers to a telephone switch or exchange located at the local telephone company’s central office, directly serving subscribers . • Class 5 switch services include basic dial tone , calling features, local switching, additional digital and data services to subscribers using the local loop. • Typically a Class 5 switch will cover an area of a city, an individual town, or several villages and could serve from several hundred to 100,000 subscribers. • In modern times only the terms Class 4(end office) and Class 5 (tandem office) are much used.

21. PSTN Routing

22. 1 2 3 Signaling Terminating Switching Office Originating Switching Office Originating CPE Terminating CPE Idle Off-hook Dial Tone Dialed Digits Off-hook Off-hook (wink) On-hook (wink) Dialed Digits Ringing Audible Ring Answer Off-hook Disconnect

23. 1 2 3 4 5 6 6 Signaling Terminating Switching Office Terminating Switching Office Originating Switching Office Originating CPE Terminating CPE Idle Off-hook Dial Tone Dialed Digits Off-hook Off-hook (wink) On-hook (wink) Dialed Digits Ringing Audible Ring Answer Off-hook Disconnect

24. 1 2 3 4 5 6 6 7 8 Signaling Terminating Switching Office Terminating Switching Office Originating Switching Office Originating CPE Terminating CPE Idle Off-hook Dial Tone Dialed Digits Off-hook Off-hook (wink) On-hook (wink) Dialed Digits Ringing Audible Ring Answer Off-hook Disconnect

25. 1 2 3 4 5 6 6 7 8 9 10 10 Signaling Terminating Switching Office Originating Switching Office Originating CPE Terminating CPE Idle Off-hook Dial Tone Dialed Digits Off-hook Off-hook (wink) On-hook (wink) Dialed Digits Ringing Audible Ring Answer Off-hook Disconnect

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27. Define local loop.