Logical design is, “The part of the design phase of the SDLC in which all functional features of the system chosen for development in analysis are described independently of any computer platform.” [1] A logical design for a network is an abstract functional specification for a telecommunications solution. A logical design lacks specific details such as technologies and standards and focuses on the needs at a general level. A logical network design can be a view of any part of a network. An entire enterprise educational network can be a composition of many logical designs. The lower level designs can be a university campus network that connects each building to the Internet, or it could be a view of the standard office telecommunications setup. The important quality of a logical design is that is communicates all needs in general terms.
Logical designs communicate with abstract concepts, such as a network, router or workstation, without specifying concrete details. A definition of abstraction that I like is, “the process of formulating general concepts by abstracting common properties of instances.” [3] Another is a “general concept formed by extracting common features from specific examples.” [3] Abstractions for complex systems, such as network designs are important because they simplify the problem space so humans can manage it. An example of a network abstraction is a WAN. A wide-area-network carries data between remote locations. To understand a WAN, you do not need to understand the physics behind fiber optic data communication, although WAN traffic may be carried over optical fiber, satellite, or copper wire. Someone specifying the need for a WAN connection on a logical network diagram can understand the concept of a WAN connection without understanding the detailed technical specifics behind it.
Logical designs are often described using terms from the customer’s business vocabulary. Locations, processes, roles from the business domain can show up in the logical design. An important aspect of a logical network design is that it is part of the requirements set for a solution to a customer problem.
The basic idea of physical design is that it communicates “decisions about the hardware used to deliver a system.” [2] A physical network design is created from a logical network design. A physical design will often expand elements found in a logical design. For instance, a WAN connection on a logical design diagram can be shown as a line between two buildings. When transformed into a physical design, that single line could expand into the connection, routers and other equipment at each end of the connection. The actual connection media might be shown on a physical design as well as manufacturers and other qualities of the network implementation.
The primary difference between logical network design and physical network design is that of iterative production of a solution from the identification of a problem. For example, when a business needs to share information in real time with remote offices, they are thinking in terms of business first and technology second. This is where identification of a problem begins, and as the problem is documented, it can be iteratively evolved from a logical solution into many possible physical designs. The logical design of a network can be re-implemented with new technology, and yet the logical design remains the same. Logical designs can span generations of technology, while a physical design is one realization of a logical design.
References
[1] http://myphliputil.pearsoncmg.com/student/bp_hoffer_modernsad_3/glossary.html
[2] http://lms.thomsonelearning.com/hbcp/glossary/glossary.taf?gid=21&start=p
[3] http://www.cogsci.princeton.edu/cgi-bin/webwn2.0?stage=1&word=abstraction