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Propagation delay refers to the length of time required for a signal to travel from transmitter to receiver across a transmission system. While electromagnetic energy travels at roughly the speed of light (186,000 miles per second) the nature of the transmission system impact the level of propagation delay to a considerable extent. In other words, the total length of the circuit directly impacts the length of time it takes for the signal to reach the receiver. That circuit length can vary considerably in a switched network, as the specific circuit route will vary in length from call to call, depending on availability of individual links and switches. Dedicated networks offer the advantage of a reliable and consistent level of propagation delay. In either case, the level of delay is affected by the number of network elements (devices) in the network, as each device (e.g., amplifier, repeater, and switch) acts on the signal to perform certain processes, each of which takes at least a small amount of time. Clearly, the fewer devices involved in a network, the less delay imposed on the signal.
Perhaps propagation delay is best illustrated by satellite systems. Given the fact that the radio signals must travel approximately 22,300 miles up to the satellite and the same distance on the return leg, the resulting delay is approximately .25 seconds. Considering the amount of time required for processing on the satellite, as well as at the earth stations, the total delay for a one-way transmission is about .32 seconds. Therefore, the delay between signal origination and receipt of response is about .64 seconds, assuming that the response is immediate. Hence, highly interactive voice, data, and video applications are not effectively supported via two-way satellite communications.
Security, in the context of transmission systems, addresses the protection of data from interception as it transverses the network. Clearly, increasing amounts of sensitive data are being transmitted across wide and metropolitan area networks, outside the protection of ones own premises. Therefore, security is of greater concern than ever before and will heighten as nations and commercial enterprises seek to gain competitive advantage and as they apply ever more sophisticated means to do so. In hearings (May 1996) before the United States Senate, it was stated that 120 nations either have or are in the process of developing sophisticated computer espionage capabilities.
Further, it should be noted that airwave systems (e.g., microwave and satellite) are inherently not secure, as unauthorized entities can gain access to that data through the use of a properly tuned and placed antenna, without the necessity of tapping a physical circuit. Finally, and as we discussed in a previous chapter, digital systems are inherently more secure than are their analog counterparts by virtue of the fact that the data can effectively be encrypted, or encoded, in order to conceal its true meaning. Particularly in the case of data networking, it also is important that access to a remote system and the data resident on it be limited to authorized users; therefore, some method of authentication, must be employed in order to verify that the access request is legitimate and authentic.
Mechanical strength applies most especially to wired systems. Twisted pair, coaxial, and fiber optic cables are manipulated physically as they are deployed and reconfigured. Clearly, each has certain physical limits to the amount of bending and twisting (flex strength) they can tolerate, as well as the amount of weight or longitudinal stress they can support (tensile strength), without breaking (break strength). Fiber optic cables are notoriously susceptible in this regard. Cables hung from poles expand and contract with changes in ambient temperature; while glass fiber optic cables expand and contract relatively little, twisted pair copper wire is more expansive [3-5].
The issue of mechanical strength also applies to airwave systems, as reflective dishes, antennae, and other devices used in microwave, satellite, and infrared technologies must be mounted securely to deal with wind and other forces of nature. Additionally, the towers, walls and roofs on which they are mounted must be constructed and braced properly in order to withstand such forces, and must flex as appropriate.
The physical dimensions of a transmission system must be considered as well. This is especially true, once again, in the case of wired systems. Certainly, the sheer weight of a cable system must be considered as one attempts to deploy it effectively. Additionally, the bulk (diameter) of the cable is of importance, as conduit and raceway space often is at a premium. The physical dimensions of airwave systems also must be considered, as the size and weight of the reflective dish and mounting system (e.g., bracket and tower) may require support.
Cost issues abound in the selection of an appropriate transmission medium. Such issues include the cost of acquisition, deployment, operation, and maintenance (O&M), and upgrade or replacement. Without a lengthy discussion of each cost issue, it is particularly noteworthy to compare the costs of deployment of wired versus wireless media.
Wired transmission systems require that a right-of-way be secured, trenches be dug, holes be bored under streets, poles be planted, conduits and manholes be placed, cables be pulled and spliced, amplifiers or repeaters be placed, etc. Such costs, clearly, are not trivial. Wireless systems, on the other hand, require that right-of-way be secured, antennae be placed (perhaps in orbit), and so on. While it is difficult to make hard-and-fast generalizations, the deployment of wired systems certainly speaks to a set of cost issues which often can be more problematic. Further, wired systems tend to be more susceptible to the forces of man (e.g., cable-seeking backhoes and train derailments) and nature (e.g., earthquakes and floods) than do their wireless counterparts.
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