Transmission Media Essay Example

through the network using a combination of electronic devices (such as network boards and hubs) and transmission media (such as cables and radio) until they reach the desired destination computer.

All signals transmitted between computers consist of some form of electromagnetic (EM) waveform, ranging from radio frequencies through microwave and infrared light. Different media are used to transmit the signals, depending on the frequency of the EM waveform.

The electromagnetic spectrum consists of several categories of waveforms, including radio frequency waves, microwave transmissions, and infrared light.

Radio frequency waves often are used for LAN signaling. Radio frequencies can be transmitted across electrical cables (twisted-pair or coaxial) or by using radio broadcast transmission.

Microwave transmissions can be used for tightly focused transmissions between two points. Microwaves are used to communicate between Earth stations and satellites, for example, and they also are used for line-of-sight transmissions on the earth’s surface. In addition, microwaves can be used in low-power forms to broadcast signals from a transmitter to many receivers.

Cellular phone networks are examples of systems that use low-power microwave signals to broadcast signals.

Infrared light is ideal for many types of network communications. Infrared light can be transmitted across relatively short distances and can be either beamed between two points or broadcast from one point to many receivers. Infrared and higher frequencies of light also can be transmitted through fiber-optic cables.

Characteristics of Transmission Media

Each type of transmission media has special characteristics that make it suitable for a specific type of service.

• Cost
• Installation requirements
• Bandwidth
• Band Usage (Baseband or
  

Broadband)

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 understood by using an analogy to water hoses. 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 bandwidth of two gallons per minute. A four-inch fire hose, however, might have a bandwidth that exceeds 100 gallons per minute.

Data transmission rates frequently are stated in terms of the bits that can be transmitted per second. An Ethernet LAN theoretically can 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.

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 bandwidth 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 choose the channel you want to watch by selecting its frequency. 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.

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 sufficient 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 only to partial capacity. With the proper equipment, a single fiber can support hundreds of megabits—or even a gigabit or more—of data.

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. De-multiplexing refers to recovering the original separate channels from a multiplexed signal.

Multiplexing and de-multiplexing are performed by a multiplexer (also called a mux), which usually has both capabilities.

Frequency-Division Multiplexing

Frequency-division multiplexing (FDM) works by converting all data channels to analog form. Each analog signal can be modulated by a separate frequency (called a carrier frequency) that makes it possible to recover that signal during the de-multiplexing process. At the receiving end, the de-multiplexer can select the desired carrier signal and use it to extract the data signal for that channel.

FDM can be used in broadband LANs (a standard for Ethernet also exists). One advantage of FDM is that it supports bidirectional signaling on the same cable.

Time-Division Multiplexing

Time-division multiplexing (TDM) divides a channel into time slots that are allocated to the data streams to be transmitted. If the sender and receiver agree on the time-slot assignments, the receiver can easily recover and reconstruct the original data streams.

TDM transmits the multiplexed signal in baseband mode. Interestingly, this process makes it possible to multiplex a TDM multiplexed signal as one of the data channels on an FDM system.

Conventional TDM equipment utilizes fixed-time divisions and allocates time to a channel, regardless of that channel’s level of activity. If a channel isn’t busy, its time slot isn’t being fully utilized. Because the time divisions are programmed into the configurations of the multiplexers, this technique often is referred to as synchronous TDM.

If using the capacity of the data medium more efficiently is important, a

more sophisticated technique, statistical time-division multiplexing (StatTDM), can be used. A stat-mux uses the time-slot technique but allocates time slots based on the traffic demand on the individual channels.

Notice that Channel B is allocated more time slots than Channel A and that Channel C is allocated the fewest time slots. Channel D is idle, so no slots are allocated to it. To make this procedure work, the data transmitted for each time slot includes a control field that identifies the channel to which the data in the time slot should be assigned.

Attenuation

Attenuation is a measure of how much a signal weakens as it travels through a medium. This book doesn’t discuss attenuation in formal terms, but it does address the impact of attenuation on performance.

Attenuation is a contributing factor to why cable designs must specify limits in the lengths of cable runs. When signal strength falls below certain limits, the electronic equipment that receives the signal can experience difficulty isolating the original signal from the noise present in all electronic transmissions.

The effect is exactly like trying to tune in distant radio signals. Even if you can lock on to the signal on your radio, the sound generally still contains more noise than the sound for a local radio station.

Electromagnetic Interference

Electromagnetic interference (EMI) consists of outside electromagnetic noise that distorts the signal in a medium. When you listen to an AM radio, for example, you often hear EMI in the form of noise caused by nearby motors or lightning. Some network media are more susceptible to EMI than others.

Crosstalk is

a special kind of interference caused by adjacent wires. Crosstalk is a particularly significant problem with computer networks because large numbers of cables often are located close together with minimal attention to exact placement.