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.

What is TCP/IP?

The TCP/IP model also uses the modular approach but only contains 4 while the OSI model uses 7. The layers of the TCP/IP model have much more diverse tasks than the OSI model, considering that certain layers of the TCP/IP model are the same as several of the OSI model. The layers of the TCP/IP are NETWORK ACCESS LAYER – this specifies the form in which data must be routed whichever type of network is being used, the INTERNET LAYER – that is responsible for supplying the data packet or datagram, TRANSPORT LAYER – that provides the routing data along with the mechanisms making it possible to know the status of the transmission and finally the APPLICATION LAYER – this incorporates standard network applications such as telnet, SMTP, FTP etc.

What is the OSI model?

The OSI model (open systems interconnection) was established by ISO in order to implement a communications standard between computers that are on a network (the rules that manage the communications between computers on a network). The aim of having the system, in layers is to separate the problem into different parts depending on their level of abstraction. Each of the layers of the OSI model will communicate with an adjacent level that would be above or below it. The OSI model has 7 layers while the TCP/IP model only has 4. These layers are the PHYSICAL LAYER – that defines the way in which data is physically converted into digital signals on the communication media such as electric pulses, light modulation etc., THE DATA LINK LAYER – that defines the interface with the network interface card and the sharing of the transmission media, THE NETWORK LAYER – that makes it possible to manage addressing and routing of any data (their path via the network), THE TRANSPORT LAYER – this is in charge of data transport, its division into packets and management of potential transmission errors, THE SESSION LAYER – which defines the opening and destruction of communication sessions between networked machines, THE PRESENTATION LAYER – that defines the format of data that is handled by the application layer independently of the system, and finally THE APPLICATION LAYER – that provides the interface with applications and is the closest level to the users, managed directly by the software.

What are LAN topologies?

Three common local area network topologies are star, ring and linear bus. Star network s would be one of the most common topologies. The simplest star network would consist of a central switch, hub or computer that will act as a conduit to transmit messages and any other computers in the network would be connected to that one main computer, and the transmission lines between them would form a graph with the topology of a star. A ring topology is when each of the nodes of the network are connected to two other nodes in the network and with the first and last nodes being connected to each other, that would form a ring. Any data that would be transmitted would travel from one node to the next until its destination has been reached. Data would usually flow in only one direction only. Linear bus topologies is a topology in which all of the nodes in a network are connected to a common transmission medium which has two end points( this is the bus, but is also known as the backbone) any data that is transferred between nodes in the network would be transmitted over this common transmission medium and is able to be received by all of the nodes in the network at the same time.

What are LANs and WANs and what components are needed to make one?

What are LANs and WANs?

Local area networks (LANs) are computer networks that cover a small area such as a home, school, office or small group of buildings. Local area networks have a high-transfer rate, a smaller geographic range and don’t need leased communication lines as much. Wide area networks are computer networks that cover a much larger area compared to a LAN such as metropolitan, regional or national boundaries. An example of a WAN is the internet which is the largest and most well known.

What is needed to make a Local area network or Wide area network?

File servers, workstations, network interface cards, hubs, switches, repeaters, bridges, routers and gateways would all be needed for a LANs and WANs, but in LANs, routers would not be used.

File servers will stand at the heart of most networks and is a very fast computer with a very large amount of RAM, storage space and has a fast network interface card. The network operating system software resides on this computer along with any software and data that would need to be shared. The server will control the communication of information between the nodes on a network such as when it might be asked to send a word processor programme to one of the workstations, receive a database file from another workstation and store e-mail messages during the same time. This all requires a computer that can store a lot of information and share it very quickly.

Workstations are the user computers that are connected into a network. A normal workstation is a computer that is configured with a network interface card, networking software and the proper cables. The workstations do not necessarily need floppy disk drives because files can be saved on the networks file server. Almost any computer can serve as a workstation in a network.

Network interface cards provide the physical connection between the network and the computer workstation. Most network interface cards are internal with the card having been fitted into a expansion slot inside the PC. Laptops can be bought with a NIC built in or with network cards that slot into a PCMCIA slot. The network interface card is a major factor in finding out the speed and performance of a network. It is a good idea to use the fastest network card available for the type if workstation you have. A NIC would now come with speeds from 10 Mbps to 1000 Mbps or 1 Gbps. The most common NICs are Ethernet cards and token ring cards. Ethernet would be the most popular method, although wireless access is now common.

Hubs are devices that connect twisted pair or fibre optic Ethernet devices together. Hubs would work at the physical part of a network. Hubs may be active or passive, an active hub regenerates signals and passes them along (also called a mulitport repeater) and a passive hub is simply a central connection point with no amplification or regeneration of the data being transmitted. Hybrid hubs will maximize networks efficiency by interconnecting different types of cables and topologies. Hubs today are no longer used, switches would be used instead.

A switch determine destinations of messages and sends it only to the destination port, it provides full bandwidth to each station on a network. Switches will also handle several conversations on a network at one time. Switches used to be more expensive than hubs but you can’t buy hubs anymore because switches provide for better performance and would be the device of choice for many technicians.

Since signals will lose their strength as it passes along a cable if would be necessary to boost the signal with a device called a repeater. A repeater will electrically amplify the signal it receives and will rebroadcast it. Repeaters can be separate devices or they can be incorporated into a switch or hub. They would be used when the total length of your network cable exceeds the standards that are set for the type of cable being used.

A bridge is a device that will take a large LAN a segment it into two smaller, more efficient networks. A bridge will monitor the information traffic on both sides of the network so it can pass packets of information to the correct location. Most bridges can “listen” to the network and figure out the address of each computer on both sides of the bridge automatically. The bridge can look at each message and, if it necessary, broadcast it on both sides of the network. The bridge manages the traffic to keep optimum performance on each side of the network. It works as if it is a traffic policeman at a busy crossroads, it keeps information flowing on both sides of the network but it will not allow unnecessary traffic through. Bridge can be used to connect different types of cabling or physical topologies, but bridges must be used between networks with the same protocol.

A router will connect different types of networks together and is typically used for wide area networks (WANs). Routers use IP addresses and select the best path to route a message, based on the destination’s address and where it is located. A router can direct traffic to prevent collisions and is smart enough to know when the traffic will be needed to be directed along back roads and shortcuts. While bridges know the MAC addresses of all computers on each side of a network, the routers will know the IP addresses of computers, bridges and any other routers on the network. Routers can even listen top all the traffic on a network and determine which parts are the busiest and redirect the traffic away from the busiest paths until they become cleared so that traffic can use those routes again. If you only have one single computer on a local area network that you want connected to the internet you will need a router. In this case the router will act as a translator between the information on your computer/LAN and the internet and will also determine the best route for the data to be sent over. A router can direct signal traffic efficiently, route messages between any two protocols, route messages between linear bus, star and star-wired topologies, route messages across fibre optic, coaxial and twisted-pair cabling.

A gateway in a network is a device that will translate between two dissimilar protocols. For example a gateway can link and translate between local area networks with different protocols.