Networking & Data Transmission in Networking
The first step when building a network, even a worldwide network such as the Internet, is to connect two hosts
together.To enable the two hosts to exchange information, they need to be linked together by some kind of physical media.
Computer networks have used various types of physical media to exchange information,
• electrical cable. Information can be transmitted over different types of electrical cables. The most common
ones are the twisted pairs (that are used in the telephone network, but also in enterprise networks) and the
coaxial cables (that are still used in cable TV networks, but are no longer used in enterprise networks).
Some networking technologies operate over the classical electrical cable.
• optical fiber. Optical fibers are frequently used in public and enterprise networks when the distance between
the communication devices is larger than one kilometer. There are two main types of optical fibers
multimode and mono-mode. Multitude is much cheaper than monomode fiber because a LED can be
used to send a signal over a multimode fiber while a mono mode fiber must be driven by a laser. Due to the
different modes of propagation of light, mono mode fibers are limited to distances of a few kilometers while
multitude fibers can be used over distances greater than several tens of kilometers. In both cases, repeaters
can be used to regenerate the optical signal at one endpoint of a fiber to send it over another fiber.
• wireless. In this case, a radio signal is used to encode the information exchanged between the communicating
devices. Many types of modulation techniques are used to send information over a wireless channel.
Networking & Data Transmission in Networking
Time-sequence diagrams are usual when trying to understand the characteristics of a given communication scheme. When considering the above transmission scheme, is it useful to evaluate whether this scheme allows the two communicating hosts to reliably exchange information ? A digital transmission will be considered as reliable when a sequence of bits that is transmitted by a host is received correctly at the other end of the wire. In practice, achieving perfect reliability when transmitting information using the above scheme is difficult. Several problems can occur with such a transmission scheme.
The first problem is that electrical transmission can be affected by electromagnetic interferences. These interferences can have various sources including natural phenomenons like thunderstorms, variations of the magnetic field, but also can be caused by interference with other electrical signals such as interference from neighboring cables, interferences from neighboring antennas, … Due to all these interferences, there is unfortunately no guarantee that when a host transmit one bit on a wire, the same bit is received at the other end.
With the above transmission scheme, a bit is transmitted by setting the voltage on the electrical cable to a specific value during some period of time. We have seen that due to electromagnetic interferences, the voltage measured by the receiver can differ from the voltage set by the transmitter. This is the main cause of transmission errors.
However, this is not the only type of problem that can occur. Besides defining the voltages for bits 0 and 1, the above transmission scheme also specifies the duration of each bit. If one million bits are sent every second, then each bit lasts 1 microsecond. On each host, the transmission (resp. the reception) of each bit is triggered by a local clock having a 1 MHz frequency. These clocks are the second source of problems when transmitting bits over a wire. Although the two clocks have the same specification, they run on different hosts, possibly at a different temperature and with a different source of energy. In practice, it is possible that the two clocks do not operate at exactly the same frequency. Assume that the clock of the transmitting host operates at exactly 1000000 Hz while the receiving clock operates at 999999 Hz. This is a very small difference between the two clocks. However,
when using the clock to transmit bits, this difference is important. With its 1000000 Hz clock, the transmitting host will generate one million bits during a period of one second. During the same period, the receiving host will sense the wire 999999 times and thus will receive one bit less than the bits originally transmitted. This small difference in clock frequencies implies that bits can “disappear” during their transmission on an electrical cable.