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LAN Design



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When we talk about the performance of a network then the design of a network affects the most.

Here are some steps that tells you about the general process of designing the networks. Generally, the network design problem consists of the three general elements given in the following table:

Element Description
Environment given Environment given includes the location of hosts, servers, terminals, and other end nodes; the projected traffic for the environment; and the projected costs for delivering the different service levels.
Performance constraints Performance constraints consist of network reliability, traffic throughput (performance measure) and host/client computer speeds (for example, network interface cards and hard drive access speeds)
Internetworking variables Internetworking variables include the network topology, line capacities, and packet flow assignments

The goal is only to minimize the cost based on these elements while delivering the service that does not compromise capacities, and packet flow assignments.

Good Network Design - The 80-20 Rule

The key to design a good network is how you place clients in relation to the servers.

Ideally, client computers should be placed on the same logical network as the servers they access most often. Whereas a physical network connection would mean that a client and server were attached to the same hub. A logical connection can be defined in your network software, so that users in one area of a building can be in the same logical network segments as a server located at the opposite end of that building.

This simple task minimizes the load on the network backbone, which carriers traffic between segments.

The 80 percent of the traffic on a given network segment is local (destined for a target in the same workgroup), and not more than 20 percent of the network traffic should need to move across a backbone (the spine that connects various segments or subnetworks) in a properly designed small to medium sized network environment.

Backbone congestion can indicate that traffic patterns are not meeting the 80-20 rule. In this case, in the place of adding switches or upgrading hubs, it is easier to improve the network performance just by doing one of the methods given below:

Now, its time to determine the optimal technology to meet your growing needs after you have ensured a proper network design and resource location.

LAN Hardware

Here are the list of hardware technologies that can be applied to LAN design:

Using Repeaters

As you already know that repeaters are the basic unit used in networks used to connect separate segments.

One basic rule of using repeaters is the 5-4-3 Rule. The maximum path between the two stations on the network should not be more than 5 segments with 4 repeaters between those segments and no more than 3 populated segments.

Using Bridges

As you already known that bridges are used to connect separate segments of a network.

Bridges differ from repeaters in that bridges are intelligent devices that operate in the data link layer of OSI model.

Bridges control the collision domains on the network.

Bridges also learn the MAC layer addresses of each node on each segment and on which interface they are located.

For any incoming frame, bridges forward the frame only if the destination MAC address is on another port or if the bridge is not aware of its location. The latter is called flooding.

Bridges filter any incoming frames with the destination MAC addresses that are on the same segment from where the frame arrives, they do not forward the frame on.

Bridges implement the Spanning-Tree Protocol to build a loop free network topology. This means that on a network, one or more bridges may be blocked if they are forming a loop.

Bridges communicate with each other, exchanging the information such as priority and bridge interface MAC addresses. They select a root bridge and then implement the Spanning-Tree Protocol.

Some interfaces are placed in a hold state, while other bridges will have interfaces in forwarding mode.

Please not that there is no loop being formed on the network as per spanning-tree protocol.

When to Use Hubs/Bridges/Switches/Routers

To establish a network between just two computers is easy, in this you just need to install NIC in both the computers and then attach them using a crossover cable.

And if you need to increase the distance between the computers, then use the Repeaters. Repeaters will themselves amplify and retime signals.

Using repeaters was fine as long as a business only needed two computers networked.

And in case, if a business wanted a third computer to attach to the network, or a fourth computer, and so on, then for this, simply use a multi-port repeater called Hub.

Problem with Hubs

Hubs share bandwidth among all the attached devices. Hubs can not filter the traffic. They simply broadcast data to all the devices.

Broadcast means data frames are sent to all the connected nodes.

The solution for this problem is to use a smarter hub called Bridge. Bridges can filter network traffic based on MAC addresses.

Now the bridges will ensure that traffic goes only to intended segment and not all the segments as with the hubs. As the spanning-tree protocol ensures that bridges in a network do not form a loop.

A Switch (or a multi-port bridge) can effectively replace the four bridges. It not only replaces the four bridges but also gives each LAN segment a dedicated bandwidths.

Problem with Switch

A switch can not stop traffic (data packets) destined for a different LAN segment from travelling to all the other LAN segments.

The solution for this problem is a Router. The Routers filter the network traffic based on the IP addresses.

The IP address tells the router which LAN segment the network traffic or data packets belongs to.

Combining Hubs, Switches, Routers

Available in Ethernet and Fast Ethernet, hubs are the best used in small networks where there are few nodes on the segment. Hubs do not control the broadcasts nor do they filter the collision domains on the network. If higher bandwidth is required, then use 100 Mbps hubs.

When the number of nodes on the network grows, then move to the switches.

With the cost of switch ports comparable to hubs, use switches as the basic network connectivity devices on the network. Switches reduce collisions and resolve media contention on the network by providing a collision domain per port

Replace hubs with switches if the utilization is over 40 percent on the Ethernet networks or above 70 percent on Token Ring and FDDI networks.

Switches can not resolve the broadcast characteristics of protocols, use routers to resolve protocol related problems.


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