Backbone Design A back backb bone one is a means eans of con connec nectin ting two or more more LANs LANs.. It pro provide videss a transmission channel for packets being transmitted from one LAN to another. After connection to a backbone, a LAN may remain distinct or be merged with another. Indivi Individua duall LANs LANs are connec connected ted to the backbone backbone networ network k using using some some type type of device. The type of device used to connect to the backbone can vary—it may be a hub, a repeater, a bridge, a router, or a gateway—depending on the purpose of the backbone. ackbone ackbone networks networks can be applied applied to a single single building building environment environment as well as to campus environments, where the backbone is used to connect LANs in different buildings. It is possible to find both types of backbone networks in a single facility —there is a backbone network in place in each building and a separate campus backbone is used to connect connect these in!building in!building backbones. ackbo ackbone ne networ networks ks may also also be used used to implem implement ent a LAN enviro environme nment nt that that provides for centrali"ed management. In I n such an environment, all network servers are moved to one central location. The servers are connected to each other via a high!speed network backbone. The different LANs are also connected to this same backbone using bridge technology. technology. The network backbone allows the stations on the LANs to communicate with the servers.
Backbone network classification
A backbone network can be broadly classified into five categories on the basis of transmission media, topology, access control, transmission mode and speed #. Transmission $edia It is recommended that the media to be used as a transmission channel for a local %i.e., non wide!area& backbone network follow the specifications made in the AN'I(TIA()IA!*+!A standard. AN'I(TIA()IA!*+!A recogni"es four backbone transmission media, which shall be used individually or in combination #// 0 1T2 backbone cable. #*/ 0 'T2!A cable. +3.*(#3* 4m multi!mode optical fiber cable. 'ingle!mode optical fiber cable. 3. Topology As is the case in LAN architecture, the backbone network has both a physical topology and a logical topology. The physical topology describes the actual appearance of the network—the manner in which the transmission media are placed and how devices are attached to the backbone network. That is, the actual pathway for transmissions. The logical topology describes how signals are sent over the backbone network. This is largely a factor of the technology used to access the backbone. Physical design
The physical design of the backbone network differs very little from other LAN designs. 5ften, it follows a linear bus, a ring or a hierarchical star topology. a& Linear bus backbone 1sing a linear bus configuration for the backbone network re6uires a separate backbone cable to which backbone access devices are connected. These devices also connect to their respective LANs.
b& 7ierarchical star backbone The hierarchical star is the topology recommended by AN'I(TIA()IA!*+!A. In such a configuration, the devices connecting to the LANs are also connected to a central device, forming the star.
Logical design
The two choices for carrying backbone network traffic are the distributed environment and the centrali"ed environment. They differ in the manner with which multiple networks are connected. a& 8istributed backbone network A distributed backbone network is one where the backbone transmission media is shared among all of the devices accessing this backbone. An e9ample of such an environment is an :88I backbone. b& ;entrali"ed optical cabling ecause of the increased distance capabilities of optical fiber, selected users of fiber!to the! desk are taking the collapsed backbone one step further. This approach allows the telecommunications closet to be entirely passive< however, it does re6uire a two fiber path in the backbone for each station. =. Accessing the backbone Network The various devices used to link LAN segments operate at different layers of the 5'I model. These linking devices, or relay devices, can be categori"ed as belonging to one of four basic types. These are as follows The repeater. The bridge. The router. The gateway. a& >epeater A repeater is considered to be the simplest form of an interconnection device and is used to link networks at the 2hysical layer of the 5'I model. It is essentially a non! intelligent device used to move all data packets or frames received from one LAN segment to another LAN segment. A repeater makes no decision regarding the source or destination of a packet. The main function of a repeater is to e9tend the length of a network. >epeaters are add!on devices used to strengthen a transmission signal. In the case where digital signaling is used, the repeater actually regenerates the signal.
b& ridges A bridge is a device that provides a communications pathway between two LANs, typically using the same network technology. ridges are used to create a single logical network and are sometimes used to interconnect networks that have already been e9tended using repeaters. In its simplest form, the bridge is considered to be an intelligent repeater because it is capable of deciding whether to forward a frame or not. A bridge functions at the 8ata Link layer of the seven!layer 5'I reference model. c& >outers >outers operate at the Network Layer of the 5'I model. The addressing scheme of this protocol is used to move data frames through the system from a source device to a destination device. >outers can provide interconnectivity in local and(or wide area environments. They also provide traffic control and filtering functions when there are multiple pathways between the end LANs. d& ?ateways ?ateways are devices used to translate transmissions between two computer systems. They are used when there are significant differences between the network architectures which need to communicate—including differences in communications protocols, data formatting structures, and(or languages. $ost gateways operate at the Application layer of the 5'I model. A common use of gateways is to interconnect and translate two proprietary protocol stacks such as 'NA %'ystems Network Architecture& to 8NA %8igital Network Architecture& or AppleTalk to T;2(I2.
