Telecommunications is a term used to describe any type of long distance communication techniques. In the digital age, telecommunications describes the use of electronic devices that facilitate communications between people, computers, and other machines. There are several technologies used today that enable these communications and allow them to perform efficiently. The basis behind these technologies is the process of converting analog signals, or signals that can have an infinite number of values that encapsulates the data stream, to digital signals, signals that generally can only have a value of zero or one, and digital signals to analog signals.
There are many different devices in use that convert the different signal types and transmit the data. Some of the devices are used to facilitate longer distance communications and others are used to allow short distance communications. An example of the digital-to-analog conversion process is the use of a data modem to convert the digital signals that a computer generates to an analog signal that can be transmitted over the twisted pair telephone lines. This is also an example of analog-to-digital conversion as the modem does double duty by decoding the information coming in across the telephone lines into data that is usable by the computer. Other types of modems include Digital Subscriber Line (DSL) modems, Cable Modems, and satellite radio modems. All of these devices convert digital signals into analog for transmission and when they are receiving information they convert the analog signals back to digital for computer processing.
The techniques used to convert the digital signals to analog signals and back to digital vary with the application of the network. In many cases the same information gets converted from digital to analog and back multiple times and using multiple methods before it reaches its final destination. Cable modems are a ubiquitous form of this process and can be found in many homes across the United States and the world. Cable internet is made possible by utilizing the bandwidth allotted for one or more cable TV channels and modulating the signal to allow for internet data transmission as well. Utilizing this method it is possible to achieve speeds between 56.6K and 10mbps (Kiniry 1998). The cable modem houses both a modulator and a demodulator which is how the term “modem” (MOulator DEModulator) came about. The signal is received and demodulated to allow for the information to be used on an IP based network, and the data from the computers is converted from IP based data to the modulated carrier signal to be sent out across the cable network.
Analog signals have been around for a long time and are used more frequently than most people may think. There are different types of analog signals that many of us are familiar with: AM (amplitude modulation), FM (frequency modulation), and PM (phase modulation) which is not very popular. These signals are most commonly used for today’s radio broadcasts but they have also been used in the transference of data as well. QAM (quadrature amplitude modulation) is considered more of a digital signal but it uses two AM signals which is the reason why it is mentioned here.
AM (AMPLITUDE MODULATION). This was one of the first analog signals created and used by the first radio stations. One of the advantages of AM signals is that they are very easily detected. “There have even been reports of people hearing some nearby radio station from their stainless steel kitchen sink” (Olofson, 2007). This also means that the components used to make the receivers are cheap which also made it popular. One of the biggest disadvantages of the AM signal is that it picks up a lot of noise or distortion. According to Goleniewski, “As the signal moves across the distance, it loses power and becomes impaired by factors such as moisture in the cable, dirt on a contact, and critters chewing on the cable somewhere in the network” (2007). When the signal reaches an amplifier any noise accumulated in the signal is also amplified and sent to the next location. Another disadvantage is that is has a low bandwidth which does not allow for much data to be sent. It also has a high rate of error reaching its destination.
FM (FREQUENCY MODULATION). FM is an improvement to AM in that it does not allow as much noise to come into the signal. “The great merit of FM over AM is that FM allows us to suppress the effects of noise at the expense of bandwidth”(Sharma, Mishra, & Saxena, 2010). This is accomplished by keeping the amplitude of the wave constant and varying the frequency of the wave. A disadvantage is that it is restricted by barriers in the way of the signal. This is because it uses higher frequencies than AM. The components are also more complicated so are more expensive to make.
PM (PHASE MODULATION). PM is very similar to FM. It is also very resistant to noise but it prone to more data loss and requires more expensive equipment,
QAM (QUADRATURE AMPLITUDE MODULATION). The advantage of QAM is that it uses two signals over one channel. This also allows for a greater increase in sending data along a QAM signal. The drawback to this is that it does tend to pick up more noise than other modulation techniques.
The specific modulation techniques that are used in a 56K modem, Asymmetrical Digital Subscriber Line and Wi-Fi are as follows:
56K MODEM: The modulation techniques used to make modems work faster is phase-shift keying (PSK) and quadrature amplitude modulation (QAM). The modem modulates the digital signal to an analog signal (Brain, 2000).
ADSL: The standard modulation technique for an Asymmetrical Digital Subscriber Line (ADSL) is discrete multi-tone (DMT) line coding. This technique has many advantages (Baker, n.d.):
rate adaptation as a function of the signal-to-noise ratio (SNR).
adjustable bit rates
inherent immunity to impulse noise and radio frequency interference (RFI).
This standard was chosen by the ANSI T1E1 Committee.
WI-FI: The standard modulation technique for Wi-Fi is (InformIT, n.d.):
Frequency hopping spread spectrum (FH or FHSS)
Direct sequence spread spectrum (DS or DSSS)
This specifies support for 1-Mbps and 2-Mbps data rates.
T(X) AND SONET DIGITAL HIERARCHY
T(X) or also known as T carrier services is an “A series of digital communication services provided by telcos for high-speed permanent voice and data connections” (“Definition Of T-Carrier Network Encyclopedia”, 2013). T carrier was originally developed in the 1960’s by Bell in order to provide digital transmission over analog voice communications. T(X) is the most commonly used digital circuits in North America because of the ability to “to connect telephone exchange switching equipment within the telco’s central office (CO)” (“Definition Of T-Carrier Network Encyclopedia”, 2013). T(X) has units that it’s defined by and some of those go from T-1 carrier to T-4 carrier.
“Units of the T-Hierarchy
DS-0 (64Kbps); Basic unit
T-1 (a.k.a.DS-1) (1.544 Mbps)
Allows 24 simultaneous 64 kbps channels which transport data or voice messages using PCM
T-2 (6.312 Mbps) multiplexes 4 T-1 circuits
T-3 (44.376 Mbps); 28 T-1 Capacity
T-4 (274.176 Mbps); 178 T-1 Capacity (672 DS-0 Channels)
Fractional T-1, (FT-1) offers a portion of T-1″ (John Wiley & Sons Inc. , 2005).
SONET or Synchronous Optical Network “SONET defines optical signals and a synchronous frame structure for multiplexed digital traffic” (“A Brief Overview Of Sonet Technology”, 2005). SONET is basically a ANSI for optical fiber transmission in Gbps range. What most people think of when it comes to SONET is SDH which can at the same time be easily interconnected with each other. SDH or Synchronous Digital Hierarchy is similar to that of SONET but it’s used in Europe by the ITU-T (International Telecommunication Union Telecommunication Standardization Sector). With SONET, it’s usually used in North America whereas SDH is used everywhere else around the world.
“Both SONET and SDH are based on a structure that has a basic frame format and speed. The frame format used by SONET is the Synchronous Transport Signal (STS), with STS-1 as the base-level signal at 51.84 Mbps. An STS-1 frame can be carried in an OC-1 signal. The frame format used by SDH is the Synchronous Transport Module (STM), with STM-1 as the base-level signal at 155.52Mbps. An STM-1 frame can be carried in an OC-3 signal. Both SONET and SDH have a hierarchy of signaling speeds. Multiple lower-level signals can be multiplexed to form higher-level signals. For example, three STS-1 signals can be multiplexed together to form an STS-3 signal, and four STM-1 signals multiplexed together to form an STM-4 signal” (“A Brief Overview Of Sonet Technology”, 2005).
A Brief Overview of SONET Technology. (2005). Retrieved from http://www.cisco.com/en/US/tech/tk482/tk607/technologies_tech_note09186a0080094313.shtml#sonetbasics
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