Contents

1 Introduction

The PSTN consists of telephone lines, fiber optic cables, microwave transmission links, cellular networks, communications satellites, and undersea telephone cables, all inter-connected by switching centers, thus allowing any telephone in the world to communicate with any other http://en.wikipedia.org/wiki/Public_switched_telephone_network.

2 Leased lines

3 Some device terminology

• Customer Premises Equipment (CPE): The devices and inside wiring located at the premises of the subscriber and connected with a telecommunication channel of a carrier. The subscriber either owns the CPE or leases the CPE from the service provider. A subscriber, in this context, is a company that arranges for WAN services from a service provider or carrier.
• Data Communications Equipment (DCE): Also called data circuit-terminating equipment, the DCE consists of devices that put data on the local loop. These devices are controlled by the carrier (who transports data). The DCE is the end of the “local loop” (also called the “last-mile”). The DCE can be a modem if the local loop is an voiceband line, a CSU/DSU (Channel Service Unit/Data Service Unit) if the local loop is a T1 or T3 carrier lines, an access server (a device that concentrates dial-in and dial-out user communications), or a router with a WAN access interface. The CPE, which is generally a router, is the DTE. The DTE could also be a terminal, computer, printer, or fax machine if they connect directly to the service provider network. The Electronics Industry Association (EIA) and the International Telecommunication Union Telecommunications Standardization Sector (ITU-T) have been most active in the development of standards that allow DTEs to communicate with DCEs. The EIA refers to the DCE as data communication equipment, while the ITU-T refers to the DCE as data circuit-terminating equipment.
• Data Terminal Equipment (DTE): The customer devices that pass the data from a customer network or host computer for transmission over the WAN. The DTE connects to the local loop through the DCE. Device controlled by the customer (who produces data). The DTE is usually a router which connects a LAN. The Universal Asynchronous Receiver/Transmitter (UART) is the DTE agent of your PC and communicates with the modem or other serial device, which, in accordance with the RS-232C standard, has a complementary interface called the DCE interface. The CPE, which is generally a router, is the DTE. The DTE could also be a terminal, computer, printer, or fax machine if they connect directly to the service provider network.
• Demarcation Point: A point established in a building or complex to separate customer equipment from service provider equipment. Physically, the demarcation point is the cabling junction box, located on the customer premises, that connects the CPE wiring to the local loop. It is usually placed for easy access by a technician. The demarcation point is the place where the responsibility for the connection changes from the user to the service provider. This is very important because when problems arise, it is necessary to determine whether the user or the service provider is responsible for troubleshooting or repair. Prior to deregulation in North America and other countries, telephone companies owned the local loop, including the wiring and equipment on the premises of the customers. Deregulation forced telephone companies to unbundle their local loop infrastructure to allow other suppliers to provide equipment and services. This led to a need to delineate which part of the network the telephone company owned and which part the customer owned. This point of delineation is the demarcation point, or demarc. The demarcation point marks the point where your network interfaces with the network owned by another organization. In telephone terminology, this is the interface between customer-premises equipment (CPE) and network service provider equipment. The demarcation point is the point in the network where the responsibility of the service provider ends.
• Local Loop: The copper or fiber telephone cable that connects the CPE at the subscriber site to the CO of the service provider. The local loop is also sometimes called the "last-mile."
• Central Office (CO): A local service provider facility or building where local telephone cables link to long-haul, all-digital, fiber-optic communications lines through a system of switches and other equipment.
• CSU/DSU: Digital lines, such as T1 or T3 carrier lines, require a channel service unit (CSU) and a data service unit (DSU). The two are often combined into a single piece of equipment, called the CSU/DSU. The CSU provides termination for the digital signal and ensures connection integrity through error correction and line monitoring. The DSU converts the T-carrier line frames into frames that the LAN can interpret and vice versa.
• Point Of Presence (POP): the point of interconnection between the communication fecilities provided by the telephone company and the building’s main distribution facility.

4 Link protocols

The data link layer (OSI layer 2) protocols define how data is encapsulated for transmission toward a remote location and the mechanisms for transferring the resulting frames.

1. EIA/TIA-232 (RS-232): Protocol that allows a signal speeds of up to 64 kbps. 25-pin D-connector over short distances.
2. EIA/TIA-449/530: Protocol that allows a signal speeds of up to 2 Mbps. 36-pin D-connector.
3. EIA/TIA-612/613: Protocol that allows a signal speeds of up to 52 Mbps. 60-pin D-connector.
4. V.35: Protocol that allows a signal speeds of up to 2.048 Mbps. 34-pin rectangular connector.
5. X.21: 15-pin D-connector.