Logical Vs. Physical Network Design

Logical vs. Physical Network Design

The typical Top-Down approach to network design uses a systematic method to plan, design, and implement a new network. Generally, the Top-Down methodology involves analysis of the business requirements and goals, development of the logical design based on such goals, development of the physical design, and a phase for testing, optimizing, and documenting the network design. This paper focuses on the activities performed to create a logical design of a network and then goes on to explain how the physical design is created.

Logical Network Design

It is difficult to discuss the principles of the logical design without first discussing the importance of planning and analysis. After all, the goal of initial network planning is to ascertain enough information to create the logical network design proposal. Logical design begins by determining the need of the users. How does the business use a network to share information? What services does the network need to provide? What resources are needed? What are the requirements for network protocols, applications, performance, and security? These are the most important questions to consider in the beginning stages of design.

A network engineer must gather information such as projected traffic patterns and determine if there is a potential for bottlenecks. If this is the case, the design might require multiple communication paths to resources or the implementation of clustered servers with replicated data for load balancing. Also, the job functions of the clients must be studied. What are their jobs and work patterns and how does this affect the placement of servers, high-bandwidth links, and other physical components? Furthermore, a network engineer must determine the scope of the applications requiring network communication. For example, most networks consist of at least some use of applications such as FTP, telnet, and web browsing. He/she must perform a risk assessment of common applications to identify potential security problems. The preliminary analysis will also answer questions such as who requires internet access, email, or VPN which might implicate the need for firewalls on the network to protect against unauthorized access to and from the outside of the network. Finally, the impact of any downtime whether planned or unexpected needs to be addressed. How critical is it to the business to have near 100% uptime? This determination effects how protective measures such as redundancy, clustered servers, disk arrays with RAID techniques, or multiple links between sites are built into the network.

Once the aforementioned information is gathered, the logical design process can commence. The first step in logical design is selecting a network topology. Here, the overall structure of the network is depicted by connecting lines between each node. Furthermore, the network segments along with their scope, connection points, and placement of general inter-networking devices are defined.

Logical design also includes the creation of a layered hierarchal model which includes a core layer, a distribution layer, and an access layer. The core layer is the backbone of the network and consists of high-end equipment optimized for efficient packet flow. The distribution layer defines a boundary between the core and access layer. Some functions performed at the distribution layer include departmental or workgroup access, addressing to distinguish between areas and/or groups, and security policies. Routing techniques and protocols are defined in the distribution layer. The access layer defines the point at which the local users are connected directly to the network and covers such issues as shared or switched bandwidth and MAC layer filtering. Figure 1 depicts a typical three-layered hierarchal design model.

The next stage in logical network design involves the placement of switching technologies. Using switches between shared-media LAN segments allows fewer users to compete for available bandwidth. The placement of switches in the distribution layer also provides multiple paths for VLAN segments and allows for logical workgroups to be setup regardless of their physical position on the network. This also covers a portion of the logical design process of addressing the scalability requirements.

Figure 1. A three-layered hierarchal design model

Physical Network Design

Whereas the logical design of the network most accurately reflects the needs of the business by depicting network structure with regards to the organizational units and requirements, the physical design is more concerned with distance, cabling, and connectivity issues. Generally, a physical network is tied very closely with the