Case Study: Transmission Media
This case study of the fictitious company, Superior Views Industrial, was created to illustrate what a network professional may need to consider improving connectivity among geographically dispersed locations. The home office of the company is in Portland, and branch offices are located in Pittsburgh, Princeton, Nashville, and New Orleans.
Superior Views Industrial is typical of any company that may have grown through merger or acquisition and needs to improve a system of connectivity that may be outdated or non-optimal. The current system has a number of older 4800 and 9600 Kbps modems. If the company continues to grow, a properly designed system will allow new sites to be added to the network without affecting the operations of the other sites.
According to Connolly and Begg (1999), companies with a well-planned distributed environment are better able to handle expansion. It is therefore important that the network and information technology professionals at Superior Views Industrial ensure a sound connectivity infrastructure.
Although there are various costs that must be incurred in any networked system (e.g., maintenance, equipment costs, operating personnel), the most important recurring cost that management must consider is the cost of the actual transmission service. Transmission costs increase with distance (Carne, 1999). Therefore, due to the high cost of voice and data communication facilities and services, multiplexing has become a very practical procedure for reducing costs and improving transmission efficiency. Multiplexing is a procedure where two or more information sources share a larger transmission capacity (i.e., several signal streams are placed on a single bearer), and wherein each information source has its own channel.
In general, multiplexers should be logically placed in areas with a high number of current lines and input/output (I/O) ports. There are two main benefits that Superior Views Industrial will realize upon the implementation of multiplexing. The first benefit is improved efficiency.
Efficiency can be improved with statistical multiplexing, which is a method of time-division multiplexing, where time slots on a shared transmission line are assigned to devices on demand, as needed. Arrival time at the multiplexer by signals, not what input line they are on, determines the order in which they are sequenced. As the signals are placed on the multiplexed line, an identifier is added by the multiplexer so that the signals will be forwarded properly.
A second way that the efficiency can be improved is with compression of data. Compression allows fewer bits of data to represent the original amount of data. There are several compression techniques that may be used, depending on the data type. Run-length encoding is one type of compression and is used to compress any form of character sequence that repeats.
The second main benefit that the company will realize with multiplexing is a cost savings, because rather than have to pay for a number of lines with varying capacity, fewer lines will mean less in access costs. Although the proposed lines may be greater in capacity than the sum total of the current line capacity, prices become more cost effective as higher capacity lines are chosen.
Table 1 shows the capacity requirements for Superior Views Industrial prior to the adoption of multiplexers. After Superior Views Industrial adopts multiplexers, they will be able to reduce the total number of lines, thereby reducing transmission costs. The company is wise to consider the adoption of multiplexers, due to the number of lines that they currently have of varying capacities. The placement of the private branch exchange3 (PBX) in Portland, Pittsburgh, and New Orleans is an indication of the cities that are deemed most important for communications capacity. After multiplexing, these same three cities would maintain their communications importance, and would act as three main hubs, each with one multiplexer. Princeton and Nashville, already having few total numbers of lines, and needing less line capacity than the other three cities, do not need multiplexers. Princeton and Nashville can be connected through the Pittsburgh and New Orleans multiplexers, respectively, through router connection. The three multiplexers would be placed in Portland, Pittsburgh, and New Orleans. The total number lines needed would be reduced upon implementation of multiplexing.
There are two main ways that the company’s lines can be set up using multiplexing. The first option is to use private lines. The second option is to connect through a frame relay.
A T-1 line is sufficient to handle what Portland’s capacity requires, both from Pittsburgh and from New Orleans. The connection between Pittsburgh and New Orleans requires less capacity than do either of those two cities to Portland. Therefore, a 336 Kbps4 line is sufficient. One advantage of this setup is that if ever the connection from Pittsburgh to Portland, or from New Orleans to Portland, is temporarily down, data may be rerouted back through the system of Pittsburgh or New Orleans to reach its destination in Portland. The most important part of the connection that needs to be accommodated in case of a downed system is the voice connection from the PBX; the employees must be able to verbally communicate with each other. Since the PBX currently requires a 254 Kbps capacity, a capacity for the proposed system must be at least 254 Kbps or greater. A capacity of 336 Kbps was chosen because it is the next available increment above the 254 Kbps capacity of the current system, thereby providing a conservative surplus.
The second main multiplexing option to consider for Superior Views Industrial is to use a frame relay system. One advantage of using a frame relay system is that, unless the company chooses to buy frame relay equipment, another entity is responsible for owning and maintaining the frame system. The line capacity to the frame relay system must accommodate the capacity requirements of each city, and each city would connect via whatever CIR (committed information rate) that they pay for. Three main lines would leave the three main cities and would connect with a frame relay. Portland’s capacity requirements would be satisfied with a T-1 line capable of 1.544 Mbps. The line capacity of Pittsburgh and of New Orleans would be satisfied with the choice of 672 Kbps. The choice of 672 Kbps from Pittsburgh and New Orleans accommodates their need for 254 Kbps from the PBX system and the rest of the workstations.
One other consideration worth noting is the option of using modems to connect certain aspects of Princeton’s and Nashville’s systems via the normal phone lines. Nashville and Princeton are currently using 9,600 bps connection speeds for their workstations that are connected through a cluster controller. If modems were actually considered, the company should at least upgrade to modern 56 Kbps modems. However, for the purposes of this case study indicating the benefits of multiplexing, Nashville and Princeton were shown to connect through New Orleans and Pittsburgh, respectively, via routers. In other words, New Orleans operates as a hub for Nashville, and Pittsburgh operates as a hub for Princeton. If Superior Views Industrial adopts multiplexing, using either the private line option or the frame relay option, they will have improvements in the efficiency of data transmission, and will be able to recognize cost savings. Line capacities were proposed that were greater than their current requirements so that they will have room to “grow” into the new system if they decide to expand operations in the future.
Questions:
1. Give a brief summary of the case study.
2. Bring out the critical points highlighted in the case.
3. What are the main ways in which the company’s lines can be set up using multiplexing?
4. Explain the two main benefits that Superior Views Industrial will realize upon the implementation of multiplexing.
5. Discuss Table No 1 in context with the case study.