LAN Design / LAN design goals

LAN Design 
LAN design goals 
5.1.1 The first step in LAN design is to establish and document the goals of the design. These goals are unique to each organization or situation. This page will describe the requirements of most network designs:

• Functionality - The network must work. The network must allow users to meet their job requirements. The network must provide user-to-user and user-to-application connectivity with reasonable speed and reliability.
• Scalability - The network must be able to grow. The initial design should grow without any major changes to the overall design.
• Adaptability - The network must be designed with a vision toward future technologies. The network should not include elements that would limit implementation of new technologies as they become available.
• Manageability - The network should be designed to facilitate network monitoring and management to ensure continuous stability of operation.

The Interactive Media Activity will help students become more familiar with the four main design goals.
The next page will discuss some LAN design considerations.

LAN design considerations

5.1.2 The four figures in this TI are useful discussion points for the class. Three key components of LAN design are placement of servers, segmentation, and bandwidth versus broadcast domain. Servers and segmentation were covered in Module 4. Bandwidth domain is everything associated with one port on a bridge or switch. The term bandwidth domain emphasizes the area of a network in which bandwidth is shared. When used in the context of an Ethernet switch, a bandwidth domain is the same as a collision domain. The best practices for teaching this TI are graphical organizers. For example, print out the four figures and have students make their own notations of collision domains, bandwidth domains, broadcast domains, and network segments. The two major categories of servers to be considered in a network design are enterprise and workgroup servers. Enterprise servers support all the users on the network through, for example, e-mail and DNS. Workgroup servers support a specific category of users such as engineers. Have the students study the layout of the network in the case study and discuss the placement of servers in this scenario and in the LAN setup at their campus. Compare this to their work in Module 4 on collision and broadcast domains.

This page will describe some important factors to consider when a LAN is designed.
Many organizations have upgraded their current LANs or plan to implement new LANs. This expansion in LAN design is due to the development of high-speed technologies such as Asynchronous Transfer Mode (ATM). This expansion is also due to complex LAN architectures that use LAN switching and virtual LANs (VLANs).
To maximize available LAN bandwidth and performance, the following LAN design considerations must be addressed:

• The function and placement of servers
• Collision domain issues
• Segmentation issues
• Broadcast domain issues

Servers allow network users to communicate, and share files, printers and application services. Servers typically do not function as workstations. Servers run specialized operating systems, such as NetWare, Windows NT, UNIX, and Linux. Each server is usually dedicated to one function, such as e-mail or file sharing.
Servers can be categorized as either enterprise servers or workgroup servers. An enterprise server supports all the users on the network as it offers services, such as e-mail or Domain Name System (DNS). E-mail or DNS is a service that everyone in an organization needs because it is a centralized function. A workgroup server supports a specific set of users and offers services such as word processing and file sharing.
As seen in Figure , enterprise servers should be placed in the main distribution facility (MDF). Whenever possible, the traffic to enterprise servers should travel only to the MDF and not be transmitted across other networks. However, some networks use a routed core or may even have a server farm for the enterprise servers. In these cases, network traffic travels across other networks and usually cannot be avoided. Ideally, workgroup servers should be placed in the intermediate distribution facilities (IDFs) closest to the users who access the applications on these servers. This allows traffic to travel the network infrastructure to an IDF, and does not affect other users on that network segment. Layer 2 LAN switches located in the MDF and IDFs should have 100 Mbps or more allocated to these servers.
Ethernet nodes use CSMA/CD. Each node must contend with all other nodes to access the shared medium, or collision domain. If two nodes transmit at the same time, a collision occurs. When collisions occur, the transmitted frame is destroyed, and a jam signal is sent to all nodes on the segment. The nodes wait a random period of time, and then resend the data. Excessive collisions can reduce the available bandwidth of a network segment to thirty-five or forty percent of the available bandwidth.
Segmentation is when a single collision domain is split into smaller collision domains. Smaller collision domains reduces the number of collisions on a LAN segment, and allows for greater utilization of bandwidth. Layer 2 devices such as bridges and switches can be used to segment a LAN. Routers can achieve this at Layer 3.
A broadcast occurs when the destination media access control (MAC) address is set to FF-FF-FF-FF-FF-FF. A broadcast domain refers to the set of devices that receive a broadcast data frame that originates from any device within that set. All hosts that receive a broadcast data frame must process it. This process consumes the resources and available bandwidth of the host. Layer 2 devices such as bridges and switches reduce the size of a collision domain. These devices do not reduce the size of the broadcast domain. Routers reduce the size of the collision domain and the size of the broadcast domain at Layer 3.
The next page will explain the methodology that should be followed for a LAN design.