Backbone network
A backbone or core network is a part of a computer network which interconnects networks, providing a path for the exchange of information between different LANs or subnetworks. A backbone can tie together diverse networks in the same building, in different buildings in a campus environment, or over wide areas. Normally, the backbone's capacity is greater than the networks connected to it. A large corporation that has many locations may have a backbone network that ties all of the locations together, for example, if a server cluster needs to be accessed by different departments of a company  that are located at different geographical locations. The pieces of the network connections (for example: ethernet, wireless) that bring these departments together is often mentioned as network backbone. Network congestion is often taken into consideration while designing backbones. One example of a backbone network is the Internet backbone. History. The theory, design principles, and first instantiation of the backbone network came from the telephone core network when traffic was purely voice. The "core network" was the central part of a telecommunications network that provided various services to customers who were connected by the access network. One of the main functions  was to route telephone calls across the PSTN. Typically the term referred to the high capacity communication facilities that connect primary nodes. A core network provided paths for the exchange of information between different sub-networks. In the United States, local exchange core networks were linked by several competing interexchange networks; in the rest of the world, the core network has been extended to national boundaries. Core networks usually had a mesh topology that provided any-to-any connections among devices on the network. Many main service providers would have their own core/backbone networks that are interconnected. Some large  enterprises have their own core/backbone network, which are typically connected to the public networks. Functions. Core networks typically provided the following functionality: Physically, one or more of these logical functionalities may simultaneously exist in a given core network node. Besides above-mentioned functionalities, the following also formed part of a telecommunications core network: Distributed backbone. A distributed backbone is a backbone network that consists of a number of connectivity devices connected to a series of central connectivity devices, such as hubs, switches, or routers, in a hierarchy. This kind of topology allows for simple expansion and limited capital outlay  for growth, because more layers of devices can be added to existing layers. In a distributed backbone network, all of the devices that access the backbone share the transmission media, as every device connected to this network is sent all transmissions placed on that network. Distributed backbones, in all practicality, are in use by all large-scale networks. Applications in enterprise-wide scenarios confined to a single building are also practical, as certain connectivity devices can be assigned to certain floors or departments. Each floor or department possesses a LAN and a wiring closet with that workgroup's main hub  or router connected to a bus-style network using backbone cabling. Another advantage of using a distributed backbone is the ability for network administrator to segregate workgroups for ease of management. There is the possibility of single points of failure, referring to connectivity devices high in the series hierarchy. The distributed backbone must be designed to separate network traffic circulating on each individual LAN from the backbone network traffic by using access devices such as routers and bridges. Collapsed backbone. A conventional backbone network spans distance to provide interconnectivity across multiple locations. In most cases, the backbones are the links  while the switching or routing functions are done by the equipment at each location. It is a distributed architecture. A collapsed backbone (also known as inverted backbone or backbone-in-a-box) is a type of backbone network architecture. In the case of a collapsed backbone, each location features a link back to a central location to be connected to the collapsed backbone. The collapsed backbone can be a cluster or a single switch or router. The topology and architecture of a collapsed backbone is a star or a rooted tree. The main advantages of the collapsed backbone approach are  However, the drawback of the collapsed backbone is that if the box housing the backbone is down or there are reachability problem to the central location, the entire network will crash. These problems can be minimized by having redundant backbone boxes as well as having secondary/backup backbone locations. Parallel backbone. There are a few different types of backbones that are used for an enterprise-wide network. When organizations are looking for a very strong and trustworthy backbone they should choose a parallel backbone. This backbone is a variation of a collapsed backbone in that it uses a central node  (connection point). Although, with a parallel backbone, it allows for duplicate connections when there is more than one router or switch. Each switch and router are connected by two cables. By having more than one cable connecting each device, it ensures network connectivity to any area of the enterprise-wide network. Parallel backbones are more expensive than other backbone networks because they require more cabling than the other network topologies. Although this can be a major factor when deciding which enterprise-wide topology to use, the expense of it makes up for the efficiency it creates by adding increased performance  and fault tolerance. Most organizations use parallel backbones when there are critical devices on the network. For example, if there is important data, such as payroll, that should be accessed at all times by multiple departments, then your organization should choose to implement a Parallel Backbone to make sure that the connectivity is never lost. Serial backbone. A serial backbone is the simplest kind of backbone network. Serial backbones consist of two or more internet working devices connected to each other by a single cable in a daisy-chain fashion. A daisy chain is a group of connectivity devices linked  together in a serial fashion. Hubs are often connected in this way to extend a network. However, hubs are not the only device that can be connected in a serial backbone. Gateways, routers, switches and bridges more commonly form part of the backbone. The serial backbone topology could be used for enterprise-wide networks, though it is rarely implemented for that purpose.