TRANSMISSION MEDIA

Each type of transmission media has special characteristics that make it suitable for a specific type of service. You should be familiar with these characteristics for each type of media:

Cost 

Installation requirements

Bandwidth

Band usage (baseband or broadband)

Attenuation

Immunity from electromagnetic interference

These characteristics are all important. When you design a network for a company, all these factors play a role in the decision concerning what type of transmission media should be used. 

Cost


Installation Requirements

Installation requirements typically involve two factors. One is that some transmission media require skilled labor to install. Bringing in a skilled outside technician to make changes to or replace resources on the network can bring about undue delays and costs. The second has to do with the actual physical layout of the network. Some types of transmission media install more easily over areas where people are spread out, whereas other transmission media are easier to bring to
clusters of people or a roaming user.

Bandwidth
In computer networking, the term bandwidth refers to the measure of the capacity of a medium to transmit data. A medium that has a high capacity, for example, has a high bandwidth, whereas a medium that has limited capacity has a low bandwidth Bandwidth can be best explained by using water hoses as an analogy. If a half-inch garden hose can carry water FLow from a trickle up to two gallons per minute, then that hose can be said to have a band width of two gallons per minute. A four-inch ?re hose, however, might have a bandwidth that exceeds 100 gallons per minute. Data transmission rates are frequently stated in terms of the bits that can be transmitted per second. An Ethernet LAN theoretically can

Band Usage (Baseband or Broadband)

The two ways to allocate the capacity of transmission media are with baseband and broadband transmissions. Baseband devotes the entire capacity of the medium to one communication channel. Broadband enables two or more communication channels to share the band width of the communications medium. Baseband is the most common mode of operation. Most LANs function in baseband mode, for example. Baseband signaling can be accomplished with both analog and digital signals. Although you might not realize it, you have a great deal of experience with broadband transmissions. Consider, for example, that the TV cable coming into your house from an antenna or a cable provider is a broadband medium. Many television signals can share the bandwidth of the cable because each signal is modulated using a separately assigned frequency. You can use the television tuner to select the frequency of the channel you want to watch. This technique of dividing bandwidth into frequency bands is called frequency-division multiplexing (FDM) and works only with analog signals. Another technique, called time-division multiplexing (TDM), supports digital signals. Both of these types of multiplexing  contrasts the difference between baseband and broadband modes of operation.

Multiplexing
Multiplexing is a technique that enables broadband media to support multiple data channels. Multiplexing makes sense under a number of circumstances:   When media bandwidth is costly. A high-speed leased line, such as a T1 or T3, is expensive to lease. If the leased line has sufIcient bandwidth, multiplexing can enable the same line to carry mainframe, LAN, voice, video conferencing, and various other data types. When bandwidth is idle. Many organizations have installed
FIber-optic cable that is used to only partial capacity. With the proper equipment, a single ?ber can support hundreds of
megabits-or even a gigabit or more-of data per second.  When large amounts of data must be transmitted through low-
capacity channels. Multiplexing techniques can divide the original data stream into several lower-bandwidth channels, each of which can be transmitted through a lower-capacity medium. The signals then can be recombined at the receiving end.

Multiplexing refers to combining multiple data channels for transmission on a common medium. Demultiplexing refers to recovering the original separate channels from a multiplexed signal. Multiplexing and demultiplexing are performed by a multiplexer (also called a mux), which usually has both capabilities.

transmit 10 million bits per second and has a bandwidth of 10 megabits per second (Mbps). The bandwidth that a cable can accommodate is determined in part by the cable's length. A short cable generally can accommodate

greater bandwidth than a long cable, which is one reason all cable designs specify maximum lengths for cable runs. Beyond those limits, the highest-frequency signals can deteriorate, and errors begin to occur in data signals. You can see this by taking a garden hose and snapping it up and down. You can see the waves traveling down the hose get smaller as they get farther from your hand. This loss of the wave's amplitude represents attenuation, or signal degradation.

One main factor in the purchase decision of any networking component is the cost. Often the fastest and most robust transmission media is desired, but a network designer must often settle for some thing that is slower and less robust, because it more than suf?ces for the business solution at hand. The major deciding factor is almost always price. It is a rare occasion in the ?eld that the sky is the limit for installing a network. As with nearly everything else in the computer field, the fastest technology is the newest, and the newest is the most expensive. Over time, economies of scale bring the price down, but by then, a newer technology comes along.