The subject of communications really begins with the situation shown to the right. Here is an entity called the Source and one called the User- located remotely from the Source. The Source generates Information and the User desires to learn what this Information is.

However, our attention will only be focused on the case illustrated where the Information is a sequence of binary digits, 0's and 1's, bits. Information in this case is termed data

It is absolutely impossible in the real world for the User to obtain the Information without the chance of error. The question then arises as to how to send the binary data stream from Source to User. A Transmission Medium is employed to transport the Information from Source to User. This medium is some physical entity that connects the information path from the source to the user.



The Transmission Medium has a set of properties described by physical parameters. The set of properties exists in a quiescent state. However, at least one of these properties can be stressed or disturbed at the Source end. This is accomplished by somehow imparting energy in order to stress the property.

This disturbance does not stay still, but affects the parts of the Transmission Medium around it. This disturbance then travels from the Source end to the User end. Consequently, energy imparted in creating the disturbance is thereby transferred from the Source end to the User end.

Finally, this disturbance or stressed property, can be sensed at the User end. It can be measured or received.

This propagation of a disturbance by the Transmission Medium is illustrated below: http://homework.uoregon.edu/pub/class/155/fig1-5.gif>

Examples of various kinds of transmission media:

  • Air - propogation is done via air presssure sound waves
  • EM field - EM radiation propogation to an antenna (e.g. radio waves)
  • Transmission line - propogation is done via voltage differences
  • Paper - you write on it producing alternating patterns of light and dark
  • Cylindrical glass tube - propogation is done via the intensity of light - this is a fiber optic cable
The rate that the media can be stressed or the rate at which the propogation can occur is nominally known as the bandwidth of the media. For instance, your band width for writing down words or typing with your thumbs is quite low. Bandwidth is measure in units of bits transmitted per second

Bandwidth or total data rate depends both on the frequency of the data transmission , data sent at higher frequencies have more bandwidth (but are also subject to more interference) but more importantly on the physical chararacteristics of the transmission medium



A twisted pair of wires is the first kind of medium. Twisting increases the probability that both wires are effected equally by noise. One wire carries the signal, the other wire is ground. The number of twists per units length determines the overall quality.

Twisted pair is distance limited and needs amplifiers or repeaters every few km. It is also suscepitble to interference and noise.



The evolution of this technology in terms of telephone and ethernet cables is shown here. Some college campuses (e.g. Univ. of Wisconsin) still have Cat 3. The UO is all Cat 5 with some Cat 6 now appearing. Note the large difference in bandwidth.



The next improvent is coaxial cable and that is what Cable TV is based on. Cable TV would never work over twisted pair because the bandwidth is too low.



This cable with its inner conductor is greatly shielded from noise, which is why you get a better TV picture. This cable still suffers from distance limitations so requires amplifier and repeater infrastructure.

The principal advantage of Coaxial Cable lies in its much better shielding than twisted pairs. It is also capable of providing higher bandwidth (up to 1 Gig) by transmission information at a higher frequency than can be done over twisted pair.

The next evolution is to optical fiber. Optical fiber will transmit information via light through the principle of total internal reflection

When light goes from a more dense to a less dense medium refraction occurs and the amount of light that is refracted depends on the angle of incidence between the two media. So its possible, as illustrated below, to have a critical angle which results in NO transmission to the less dense medium and complete reflection back in the more dense medium



So an optical fiber cable looks like this:



The benefits of fiber are large and include:

  • Greater bandwidth (100 gigs easily)
  • Lower loss (attenuation) along the single chain (see below)
  • much greater repeatear spacing (due to low loss)
  • excellent shielding
  • can tranmit at very high frequencies 1014-15 hz. - data rate is directly related to frequency


Hence fiber optics is the next generation backbone of the Internet Indeed this may cause the future collapse of the Internet

The entire situation with data communications then devolves to the model illustrated below.

The fundamental problem of communications then becomes a design problem. The combination of Transmitter, Transmission Medium and Receiver is termed the communication link or the data link. The disturbance launched into the Transmission Medium by the Transmitter is usually referred to as the input data signal. The resulting disturbance at the Receiver is termed the output data signal. In the context of our discussion the fundamental problem of communications is to design a data link appropriate for connecting a given Source-User pair.

Most exercises in obtaining the design solution usually begin with choosing a Transmission Medium to meet the general requirements of the Source-User pair. That is, the data link design process pivots on choosing the Transmission Medium. Every Transmission Medium has constraints on its operation, on its performance. It is these constraints that really decide which Transmission Medium will be employed for the data link design.

Source of constraints on transmission through various media:

  • Attenuation: loss of signal amplitude as the signal interacts with the transmission medium and suffers losses from either dispersion or from heta. The amount of attenutation depends on total distance travelled. This is why many transmission media require "boosters" or relays.

    In general conductive media (e.g. cooper cable, coax cable, etc) have relatively high attenutation rates. For instance, this means that a single Cat 5 ethernet cable can not have a free run longer than 600 feet.

  • Interference: It is some extraneous signal that is usually generated outside of the Transmission Medium. Somehow it gets inside of the Transmission Medium. It realizes its effect usually by adding itself to the propagating signal. Though, sometimes it may multiply the propagating signal. This is usually a limiting constraint when propogating signals electromagnetically through the air (e.g. wireless networking).

  • Frequency losses: higher frequency (shorter wavelength) signals tend to be lost in most transmision media (usually as heat) as they have a larger rate of interaction with the material that compromises the transmission medium than lower frequency (longer wavelength) signals.

The basic engineering issue is that, while one can encode more information in higher frequencies, those frequencies are subject to higher losses as the signal progates through the transmission media. In general, this is a significant problem and if we only had standard copper cable as the transmission media, it would be virutally impossible for the Internet to exist as it presently does.

There is really only one way to overcome these limitiations and that is to sue fiber optic media in which the losses, primarily due to collision other atoms within the tranmission media, are greatly minimized.

The figure below shows loss (attenuation) per km on the Y axis as a function of the frequency of the signal. Ordinary telephone cable, for instance, can't even transmit high frequencies and its subject to significant losses even at the highest frequencies it can tansmit. Fiber optics (far right of the diagram) not only allow very high frequency transmission, the losses remain relatively low at all frequencies of transmission. Its magic.

In addition, fiber optics suffer relatively low attenuation losses meaning that signals can be propogated over relatively large distances:

Fiber optic cable is the unchallenged winner in the Transmission Medium sweepstakes when it comes to attenuation, interference and bandwidth.

The current world record for transmission via these optical means was recently set at just over 1 TerraBit per second