Structure of the PSTN hierarchy

The PSTN hierarchy is made up of main distribution frames, wide area tandems, digital junction switching units, remote concentrator units and the digital local exchange. The subscriber lines will go into the main distribution frame or MDF and from there go to the digital local exchange or DLE. The signal is then sent on to the wide area tandem (WAT)/ digital junction switching unit (DJSU) or to the digital main switching unit (DMSU) and then sent to the international gateway.

Digital Local Exchanges, Remote Concentrator Units, Digital Main Switching Unit, Digital Derived Services Switching Centre, Gateways all of hese are major elements of the PSTN and are described below.

The digital local exchange relates to an exchange for connecting telecommunications terminals of subscribers to a telecommunications network, comprising subscriber lines in which the telecommunications terminals are linked with the exchange. The exchange is operated by a first network operator and is used by further network operators. The subscriber lines and subscribers are associated with one of said network operators each and the subscribers select the exchange through the telecommunications terminals to make a telecommunications link through the network operator.

　

The remote concentrator unit is a unit that consolidates subscriber lines. This equipment facilitates the use of lesser amounts of loop plant to serve a greater number of end users. All calls are switched by the central office switch to which the remote concentrator is connected. The voice or data path will always extend to the host switch even for calls between subscribers served by the same remote concentrator.

The Digital Local Switching Unit connects to the concentrator and routes calls to different DLSUs or DMSUs depending on the destination of the call. The main part of the DLSU is the Digital Switch which consists of time switches and a space switch. Incoming signals on the 30 channels from the concentrator units is connected to time switches. The purpose of these is to take any incoming individual time slot and connect it to an outgoing time slot and so perform a switching and routing function. Each DMSU is connected to all the other DMSUs and involves a much smaller number of links than required to link together DLEs. If used to route international calls is known as an international Gateway. The DMSUs are the original long-distance exchanges.

The DDSN intelligent network will allow a range of new features to be offered as Advanced Linkline to linkline service providers. These will include: time and day routing, call allocator, call queuing, call barring, alternative destination on busy, call prompter, courtesy response and command routing.

A default gateway is a router on a computer network that serves as an access point to another network. In homes, the gateway is usually the ISP-provided device that connects the user to the Internet. In enterprises, however, the gateway is the computer that routes the traffic from a workstation to the outside network. In such a situation, the gateway node often acts as a proxy server and a firewall. The gateway is also associated with both a router, which uses headers and forwarding tables to determine where packets are sent, and a switch, which provides the actual path for the packet in and out of the gateway.