When comparing the logical and physical design of a network I visualize a carefully thought out plan created from start to finish by the individuals that would ultimately be using and maintaining it. Like many areas in Information Technology, Network Design cannot be taken likely. All of the necessary steps like those of the SDLC process must be done step-by-step in order to minimize unnecessary costs, downtime and loss of business opportunities. Like a good database, a good network must have a Logical and Physical design that will serve any company for many years. At my current position, I am generally the end user of such networks. My concern is that the Network operates effectively and efficiently. A good network should be easy to support. In my paper I will give a brief difference of the Logical and Physical Design of a Network. The Logical Design of a network for the purpose of my paper will begin with asking a few questions. How many computers will you need? Where will the locations be? Will sub netting be used? This entire information gathering is the beginning of the Logical design of the network. It will determine many different aspects of the network and form a basic starting point to make other major decisions. From this information the IP structure of the network will be decided. The IP structure is decided by the size of the network needed and is assigned a Class address scheme. From the size of this network a Class address scheme will be assigned and will be similar to 188.8.131.52. The three groups of numbers will be the network number and the last group will be the number of host the company assigns. When determining how many computers will be needed and where they will be located is one of the least complicated details. Once the locations of all the stations are finalized they need to decide on what specific type of system they will be using. This will involve deciding on the actual system specifics such as what type of physical cabling will be used and the topology of the system. The only problem would be if some employees where collocated as this all leads back to the importance of the planning process. A sub network (subnet) is a separate part of an organization's network. A subnet may represent all the machines at one location, in one building, or on the same local area network. Having an organization's network divided into subnets allows it to be connected to the Internet with a single shared network address. Without subnets, an organization could get multiple connections to the Internet, one for each of its physically separate sub networks, but this would require an unnecessary use of the limited number of network numbers the Internet has to assign. It would also require that Internet routing tables on gateways outside the organization would need to know about and have to manage routing that could and should be handled within an organization.
Example of Logical Design
The Physical Design of a network deals with what type of topology you will use. Included with the topology is deciding what type of cabling, switches and routers will be used. A physical layout/map usually involves a diagram of the actual floor the way it would be seen if you were on the ceiling, looking down towards to the floor. The typical physical design of most networks involve a central rack located in a computer room or a restricted access room where all wiring from the walls terminate and connect at the back of a patch panel The following topologies exist today are: Mesh, Star, Bus, Ring and Tree. The Mesh Topology is where each device has a point-to-point connection to every other device on the network. The Star Topology connects the peripheral devices through point-to-point links to a central hub. The Bus Topology is a line transmission medium that is terminated at both ends. The Ring Topology uses a closed loop configuration. The last topology, Tree is primarily used in the wireless technologies and has a hub that services the cell's area. The hubs are interconnected in order to provide end-to-end communication with the network.
Mesh Topology Star Topology Bus Topology
Ring Topology Tree Topology Token Ring
The choices of the wiring or cabling are as follows; twisted pair wire, coaxial cable and fiber-optic cable. Each of these cables has its own advantages and disadvantages and depending on the size of your business network along with the budget, will definitely be a determining factor in which to use. The Physical Design is self explanatory, the "Physical" equipment needed to connect the computers together laid out in a design. Another way of looking at this would be blueprint to the network and can be made to contain great detail. The topology of the system will also be included in the Physical Design. As we all know budget especially on the monetary funds are not substantial can be a limiting factor in the Physical Design and can influence the system greatly. If the budget does not account for the proper equipment that is needed for the project, could create problems down the road as far as upgrading and system maintenance. When deciding to upgrade the equipment that was procured for the project may not be upgradeable to the company's need and will end up costing more as new equipment will be necessary.
After the cabling and deciding the type of topology is considered the devices that tie them together would be the routers, bridges, repeaters and gateways as these four items assist in connecting and interconnecting two or more LANs together. Which one used will be determined during the Logical Design. The Logical design sets the pace and foundation of exactly what your Physical design will consist of. The Physical design cannot exist properly without the Logical design. References
Northrup, Anthony. NT Network Plumbing. Forest City CA, IDG Books Worldwide, 1998 http://www.webopedia.com/quick_ref/topologies.asp