3.2 Network Modification


3.2 Network Modification

Previously, it was noted that the bit rate on the Fast Ethernet network was projected to be approximately 20 percent of the available network bandwidth. Based on the projected network growth, the bit rate is expected to increase to approximately 22 Mbps, which would be (22/100) * 100, or approximately 22 percent of the available network bandwidth. Thus, the relatively light level of network utilization is projected to slightly worsen over time if one network is used to provide a communications capability for both existing and anticipated employees . Now let us assume another series of computations resulted in a network utilization projection of 65 percent. This would be significantly higher than the 50 percent level of utilization on Ethernet where performance problems begin to be noticed. To alleviate potential problems associated with a high level of network utilization, one of the first modifications to a network infrastructure that you should consider is the installation of dual networks interconnected by a local bridge.

The use of a bridge to subdivide a network into two or more interconnected segments essentially doubles your effective bandwidth prior to considering inter-segment communications, a topic that is discussed in the next section of this chapter. Because the cost of a two-port bridge is usually considerably less expensive than an intelligent switch or router, from a cost perspective most network modification efforts begin by examining a network with respect to its subdivision into segments via a bridge.

3.2.1 Network Subdivision Considerations

In examining the entries in the completed traffic estimation worksheet contained in Table 3.2, you will note that the vast majority of network usage associated with workstations used by design engineers involves the loading of programs and graphic images as well as the saving and printing of graphic images. Thus, it appears that the requirement of design engineers to interact with other network users through the transmission and reception of electronic mail messages is relatively limited with respect to their total projected transmission on the LAN. Therefore, it appears you could safely place all design engineers on one Ethernet network, while all other network users could be placed on a second Ethernet network.

If we assume the growth in network utilization maintains a similar bit rate ratio between design engineers and other workstation classes, then from Table 3.4 the design engineers can be expected to account for (11.77 Mbps/21.99 Mbps) * 100, or approximately 54 percent of network traffic. Thus, if network utilization increased to 65 percent, splitting the network into two segments would result in one segment with a projected utilization of 65 * .54, or 35 percent, while the projected utilization on the other segment would be 65 * 46 percent, or 30 percent. While not a perfect subdivision, as per a popular movie staring Jack Nicholson and Helen Hunt, this may be "as good as it gets!"

Figure 3.2 illustrates the resulting subdivided Ethernet network in which it is assumed that all design engineers are placed on network segment A. This method of subdivision minimizes inter-LAN communications and reduces the forwarding rate the local bridge has to support.

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Figure 3.2: Resulting Subdivided Network

As an alternative to placing all design engineers on network A, let us assume that you wish to consider network subdivision by placing half of the employees on each network. Although this would more evenly balance the utilization level of each network, what would be the effect on inter-LAN communications? Unless you could distribute the programs and graphic images used by design engineers onto servers on each network, the design engineers on network B would require a significant amount of communications with the server located on network A. Although we did not analyze the activities of other classes of network users, we can safely assume that any subdivision in which a class of workstation users is subdivided onto two or more interconnected networks will result in a high level of inter-LAN communications unless the program's users access are located on servers on each network segment. Because the cost of a program license to operate on separate servers typically exceeds the cost of a program license to operate on one server, the additional cost associated with the separation or subdivision of users within a workstation class is normally not recommended. Thus, most organizations should consider the subdivision of network users based on the class of user .

3.2.2 Inter- versus Intra-Network Communications

One area that remains to be discussed concerning LAN traffic is the relationship between inter- and intra-network communications. This is especially important if your organization previously established separate LANs and now needs to interconnect those networks.

If you previously estimated traffic carried by separate networks or have access to monitoring equipment, you can estimate or determine the actual data flow on separate networks. However, what can you expect to occur when you connect two or more networks together?

Unless you use a poor design philosophy and require workstations on one network to access many applications on a server on a different network, the majority of transmissions will be local with respect to each LAN. In fact, the interconnection of separate networks usually results in traffic following according to what is commonly referred to as the 80/20 rule. That rule states that when separate networks are interconnected, intra-LAN communications will account for 80 percent or more of communications traffic while 20 percent or less of the traffic on a network will require the services of a bridge or router to flow between networks. The one exception to this is when organizations create a tiered network structure based on the use of LAN switches, which we will shortly discuss.

The effect of the 80/20 rule should be taken into consideration when estimating the potential effect on bandwidth resulting from the segmentation of a network. To illustrate the effect of the 80/20 rule, or a variation of the percentages of that rule to better fit your organization's operational requirements, consider Figure 3.3. This example illustrates the effect on the bandwidth of a segmented Token Ring network from inter-LAN communications, and its analysis is also applicable to each version of Ethernet. In this example, it was assumed that network A's initial utilization level was projected to be 50 percent, while network B's was projected to be 55 percent. It was also assumed that 80 percent of all traffic on each network segment would be local and remain on the segment from which it was placed on the network, while 20 percent of the traffic on each segment would be destined to a device on the other segment. This means that at an expected 50 percent utilization level, 0.50 0.20, or 10 percent of the utilization of network A will flow onto network B. Similarly, if network B has an expected utilization level of 55 percent, then 0.55 0.20, or 11 percent of the utilization of network B can be expected to flow onto network A. Because both networks are considered to represent the same type of Token Ring network, the inter-LAN communications result in an increase in the utilization of network A to 61 percent and an increase in the utilization of network B to 65 percent. Thus, it is extremely important to consider the effect of inter-LAN communications because such communications not only represent a consumption of bandwidth on the initial network on which a frame is placed but, in addition, a consumption of bandwidth for each network it is placed in.

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Figure 3.3: The Effect on the Bandwidth of a Segmented Network Resulting from Inter-LAN Communications

As briefly mentioned at the beginning of this section, a key exception to the 80/20 rule concerning inter-LAN communications is when an organization uses switches to develop a tiered network topology. Figure 3.4 illustrates an example of a two-tiered network structure. In this example, note that two enterprise servers (ES) are connected to the 100-Mbps Ethernet switch at the top of the switch tier, while departmental servers (DS) are connected to each of the 10-Mbps Ethernet switches at the bottom of the tier. Under this server arrangement, traffic flowing through each lower tier is probably very similar to the 80/20 rule, with only 20 percent of network traffic flowing beyond a local switch. Now suppose that instead of departmental servers being distributed throughout a building, the organization decided to place all servers in one controlled access area. In doing so let us assume that all servers were connected to the 100-Mbps Ethernet switch at the top of the tier shown in Figure 3.4. This would result in the creation of what is referred to as a server farm.

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Figure 3.4: Data Flow on a Tiered Switch-Based Network Topology

Although the creation of a server farm facilitates administrative operations and the physical security of servers, from a network perspective it results in all traffic from lower-tier switches flowing to the backbone switch that provides the connections to the servers in the server farm. Thus, instead of an 80/20 rule concerning inter-LAN communications, you obtain a 100/100 rule because all traffic from each station on the lower-tier switch must be forwarded to an upper-tier switch. In addition, if the upper-tier switch should fail or if the connection between switches becomes inoperative, a large number of employees can lose their ability to perform productive work. For these reasons, many switch vendors include a level of redundancy in their products and the connection between lower- and higher-tier switches occurs at a higher operating rate than the operating rate of the lower-tier switches. Despite a higher operating rate to connect switches, when too many stations on the lower-layer switch become active there can be delays in accessing the higher-tier switch. Readers are referred to Chapter 11 for detailed information on the operation and utilization of LAN switches.




Enhancing LAN Performance
Enhancing LAN Performance
ISBN: 0849319420
EAN: 2147483647
Year: 2003
Pages: 111
Authors: Gilbert Held

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