POTS is generally restricted to about 52 Kbps 52, bitsper second. These standards allow different networks in different countries to interconnect seamlessly. The E. The combination of the interconnected networks and the single numbering plan allow telephones around the world to dial each other. History The first telephones had no network but were in private use, wired together in pairs.
Users who wanted to talk to different people had as many telephones as necessary for the purpose. A user who wished to speak whistled loudly into the transmitter until the other party heard. However, a bell was added soon for signaling, so an attendant no longer need wait for the whistle, and then a switch hook. Later telephones took advantage of the exchange principle already employed in telegraph networks.
Each telephone was wired to a local telephone exchange, and the exchanges were wired together with trunks. Networks were connected in a hierarchical manner until they spanned cities, countries, continents and oceans. This was the beginning of the PSTN, though the term was not used for many decades. Automation introduced pulse dialing between the phone and the exchange, and then among exchanges, followed by more sophisticated address signaling including multi-frequency, culminating in the SS7 network that connected most exchanges by the end of the 20th century.
The growth of the PSTN meant that teletraffic engineering techniques needed to be deployed to deliver quality of service QoS guarantees for the users. The work of A. Erlang established the mathematical foundations of methods required to determine the capacity requirements and configuration of equipment and the number of personnel required to deliver a specific level of service. In the s the telecommunications industry began implementing packet switched network data services using the X.
In the s the industry began planning for digital services assuming they would follow much the same pattern as voice services, and conceived a vision of end-to-end circuit switched services, known as the Broadband Integrated Services Digital Network B-ISDN. Beginning in the s, voice calls began to be digitized and manual switching was replaced by automated electronic switching.
Digital voice signals can share the same wire with many other phone calls.
Lecture 2 The Public Switched Telephone Network (PSTN | Huong Tuan - jalofyduteqe.tk
The advent of fiber-optic cables now allows thousands of calls to share the same line. But fiberoptic and other high-bandwidth cables haven't changed the basic nature of circuit switching, which still requires a connection -- or circuit -- to remain open for the length of the phone call. At the turn of the 21st century, the oldest parts of the telephone network still use analog technology for the last mile loop to the end user.
The last four digits of the phone number represent the subscriber number. Extensions from the main phone number are routed through something called a private branch exchange PBX that operates on the premises. The next three digits are the exchange. In the India. To make an international call requires further instructions. The first three digits are the area code or national destination code NDC. The phone number itself is a coded map for routing the call. The call needs to be routed through your long-distance phone carrier to another country's long-distance phone carrier.
There are also private networks run by large companies which are linked to the PSTN only through limited gateways. To signal such a switch. Within a company or larger organization. Operators The task of building the networks and selling services to customers fell to the network operators. Routing calls requires multiple switching offices. In other words. PSTN nodes are sometimes referred to by different names. This approach does not scale well when you must connect a large number of nodes.
Network Topology The topology of a network describes the various network nodes and how they interconnect. You must connect each new node to every existing node. The End Office provides network access for the subscriber. There are two primary methods of connecting switching nodes. This approach does have its merits.
Public switched telephone network
Network Topology 3. While topologies in competitive markets represent an interconnection of networks owned by different service providers. The Central Office 3. It is located at the bottom of the network hierarchy. Transit switches are generally used to aggregate traffic that is carried across long geographical distances. Connects EOs together. In some cases. The second approach is a hierarchical. The three node types we discuss in this chapter include: Also called a Local Exchange.
Network Timing 3. Access and Transmission Facilities 3. Provides an interface to another hierarchical network level. Regulatory policies play a major role in exactly how voice network topologies are defined in each country. Depending on geographical region. The first approach is a mesh topology. PSTN Hierarchy 3. PSTN Public Switched Telephone Network tree in which nodes are aggregated as the hierarchy traverses from the subscriber access points to the top of the tree.
While actual network topologies vary. The original telephone system used five numbered levels. Transit switches provide further aggregation points for connecting multiple tandems between different networks. Each Class 4 office. PSTN networks use a combination of these two methods. While Class 2 and Class 3 offices are seldom used in today's system. The Class 5 offices. It is the local workhorse for the telephone and data communications traffic in one local exchange.
When you pick up your telephone at home. There are currently about 1. If a subscriber places a call to another subscriber connected to the same Class 5 office. The Class 5 office is the only office that connects to individual or business subscribers. Offices higher in this hierarchy have only lower level COs as their subscribers. Figure 3 Original Telecommunications Figure 4 Current 5.
Figure 6 Calling Handoff However. Figure 8 End Office Facility Interfaces. Figure illustrates a number of common interfaces to the Central Office. Individual telephone lines connect subscribers to the Central Office CO by wire pairs. Trunks also provide access to corporate phone environments. The wire pair consists of a tip wire and a ring wire. They are referred to as analog lines because they use an analog signal over the local loop.
Remote line concentrators. The Local Loop The local loop consists of a pair of copper wires extending from the CO to a residence or business that connects to the phone. Remote switching centers provide local switching between subtending lines without using the resources of the CO. The analog signal carries two components that comprise the communication between the phone and the CO: While terminating the physical loop.
- Public switched telephone network - Wikipedia.
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In-band signaling is primitive when compared to the out-of-band signaling used in access methods such as ISDN. The terms tip and ring are vestiges of the manual switchboards that were used a number of years ago. The following sections describe the facilities used for lines. Analog Line Signaling Currently. The signaling that takes place between the analog phone and the CO is called in-band signaling. Lines Lines are used to connect the subscriber to the CO. The DTMF signal is a combination of two tones that are generated at different frequencies. The CO also sends an audible ring-back tone over the originating local loop to indicate that the call is proceeding and the destination phone is ringing.
The voltage levels vary between different countries. The ITU G. When the destination phone is taken off-hook. Figure 9 Voice Encoding Process. Dialing When a subscriber dials a number. An analog-to-digital converter samples the analog voice times per second and then assigns a quantization value based on decision levels. Ringing and Answer To notify the called party of an incoming call.
The incoming voltage activates the ringing circuit within the phone to generate an audible ring signal. Voice Encoding An analog voice signal must be encoded into digital information for transmission over the digital switching network. A private numbering scheme is required to enable extension to extension dialling, also special codes e. It simply needs to provide a timing interface between the digital line and the switch-block; it does not need to perform all the BORSCHT functions. Any calls arriving at that time will be lost the subscriber will have to abandon the call.
The 40, subscribers originate 1. Typical both-way calling rates are around 0. Local loop: The very-short term variation in timing. For communications outside this exchange area, trunks were installed between exchanges. Networks were designed in a hierarchical manner until they spanned cities, countries, continents and oceans. Automation introduced pulse dialing between the telephone and the exchange, so that each subscriber could directly dial another subscriber connected to the same exchange, but long distance calling across multiple exchanges required manual switching by operators.
Later, more sophisticated address signaling, including multi-frequency signaling methods, enabled direct-dialed long distance calls by subscribers, culminating in the Signalling System 7 SS7 network that controlled calls between most exchanges by the end of the 20th century. The growth of the PSTN meant that teletraffic engineering techniques needed to be deployed to deliver quality of service QoS guarantees for the users. The work of A. Erlang established the mathematical foundations of methods required to determine the capacity requirements and configuration of equipment and the number of personnel required to deliver a specific level of service.
In the s, the telecommunications industry began implementing packet-switched network data services using the X.
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In the s, the industry began planning for digital services assuming they would follow much the same pattern as voice services, and conceived end-to-end circuit-switched services, known as the Broadband Integrated Services Digital Network B-ISDN. At the turn of the 21st century, the oldest parts of the telephone network still use analog technology for the last mile loop to the end user.
Several large private telephone networks are not linked to the PSTN, usually for military purposes. There are also private networks run by large companies which are linked to the PSTN only through limited gateways , such as a large private branch exchange PBX.
The task of building the networks and selling services to customers fell to the network operators. In some countries, however, the job of providing telephone networks fell to government as the investment required was very large and the provision of telephone service was increasingly becoming an essential public utility. For example, the General Post Office in the United Kingdom brought together a number of private companies to form a single nationalized company. In more recent decades, these state monopolies were broken up or sold off through privatization.
In most countries, the central has a regulator dedicated to monitoring the provision of PSTN services in that country. Their tasks may be for example to ensure that end customers are not over-charged for services where monopolies may exist. These regulatory agencies may also regulate the prices charged between the operators to carry each other's traffic.
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The PSTN network architecture had to evolve over the years to support increasing numbers of subscribers, calls, connections to other countries, direct dialing and so on. The model developed by the United States and Canada was adopted by other nations, with adaptations for local markets. The original concept was that the telephone exchanges are arranged into hierarchies, so that if a call cannot be handled in a local cluster, it is passed to one higher up for onward routing. This reduced the number of connecting trunks required between operators over long distances and also kept local traffic separate.
However, in modern networks the cost of transmission and equipment is lower and, although hierarchies still exist, they are much flatter, with perhaps only two layers. Most automated telephone exchanges use digital switching rather than mechanical or analog switching.