A+ Second Edition Genetic Computer School Lesson 7 Data Communication Devices

A+ Second Edition Genetic Computer School Overview  Modem Concept  Analog Modem  Cable Modem  Digital Data Service (DDS)  Digital Subscriber Line (DSL)  Asymmetric DSL (ADSL)  Integrated Systems Digital Network (ISDN)  Pulse Code Modulation (PCM)

A+ Second Edition Genetic Computer School Modem Concept  A modem is a device used by the PC to communicate over a phone line (Public-Switched Telephone Network – PSTN).  The modem contains firmware that has a set protocols and instructions needed to convert data so that it can be transported over phone lines to a receiving modem on the other end.  Modem stands for modulation and demodulation.  What it actually does is to convert digital data of the computer to analog used by the phone line.

A+ Second Edition Genetic Computer School Modulation and Demodulation Modulation The process is known as modulation when the modem converts digital data to an analog signal. Demodulation The process is known as demodulation when the modem at the receiving end converts the analog signal back to digital data.

A+ Second Edition Genetic Computer School Analog Modems They are used to connect to the PSTN (Public-Switched Telephone network )

A+ Second Edition Genetic Computer School Cable Modem

A+ Second Edition Genetic Computer School Digital Data Service (DDS)  It provides a direct point-to-point dial up connection usually between two points of the same organization.  A DDS line does not use modems, using a CSU/DSU (customer service unit/data service unit) at each end of the line instead.  DDS data travels along 4-wire copper lines in the form of signals, which are regenerated at key points in the path.  It reduces error and ensures a clear transmission.  The primary benefit to the DDS line is that it is dedicated. It is still a dialup line with dialup speeds, but others a clear end-to- end channel for the user.  By contrast, analog signals are merely amplified, meaning data can be lost and distortion noise increased to disastrous levels.  It is still a dialup line with dialup speeds, but others a clear end-to-end channel for the user.

A+ Second Edition Genetic Computer School Digital Subscriber Line (DSL)  It is a very high-speed connection that uses the same wires as a regular telephone line.  It is an emerging technology that delivers high-bandwidth to homes and businesses using their ordinary POTS copper telephone lines.

A+ Second Edition Genetic Computer School Some Advantages of DSL  You can leave your Internet connection open and still use the phone line for voice calls.  The speed is much higher than a regular modem  DSL doesn't necessarily require new wiring; it can use the phone line you already have.  The company that offers DSL will usually provide the modem as part of the installation.

A+ Second Edition Genetic Computer School Disadvantages of DSL  ADSL connection works better when you are closer to the provider's central office.  The connection is faster for receiving data than it is for sending data over the Internet.  The service is not available everywhere.

A+ Second Edition Genetic Computer School Technology Used by DSL Modem  Discrete Modulation Technology (DMT)  Carrierless Amplitude Modulation (CAP)  Multiple Virtual Line (MVL)

A+ Second Edition Genetic Computer School DSLs  Symmetric Digital Subscriber Line, Single-line DSL (SDSL)  Consumer DSL (CDSL)  G.lite, DSL Lite, Splitterless ADSL, Universal ADSL  ISDN DSL (IDSL)  Rate-Adaptive DSL (RADSL)  Unidirectional DSL (UDSL)  Asymmetric DSL (ADSL)  High bit-rate DSL (HDSL)  Very high data-rate DSL (VDSL)

A+ Second Edition Genetic Computer School Symmetric Digital Subscriber Line Single-line Digital Subscriber Line (SDSL)  It is a form of Digital Subscriber Line (DSL) service.  It possesses all of the common characteristics of DSL, including an "always on" combination of voice and data services, availability limited by physical distance, and high speed access compared to analog modems.  It provides equal bandwidth for both uploads and downloads.  It supports data rates up to 3,088 Kbps.

A+ Second Edition Genetic Computer School Consumer DSL (CDSL)  It is a version of DSL.  It is slower than ADSL. (1 Mbps downstream, probably less upstream).  It has the advantage that a "splitter" does not need to be installed at the user's end.

A+ Second Edition Genetic Computer School G.Lite DSL Lite Splitterless ADSL Universal ADSL  It is essentially a slower ADSL.  It doesn't require splitting of the line at the user end but manages to split it for the user remotely at the telephone company. This saves the cost of what the phone companies call “the truck roll.”  It provides a data rate from Mbps to 6 Mpbs downstream and from 128 Kbps to 384 Kbps upstream.

A+ Second Edition Genetic Computer School ISDN DSL (IDSL) It is somewhat of a misnomer since it's really closer to ISDN data rates and service at 128 Kbps than to the much higher rates of ADSL.

A+ Second Edition Genetic Computer School Rate-Adaptive DSL (RADSL)  It is an ADSL technology in which software is able to determine the rate at which signals can be transmitted on a given customer phone line and adjust the delivery rate accordingly.  It delivers from 640 Kbps to 2.2 Mbps downstream and from 272 Kbps to Mbps upstream over an existing line.

A+ Second Edition Genetic Computer School Unidirectional DSL (UDSL) It is a proposal from a European company. It's a unidirectional version of HDSL.

A+ Second Edition Genetic Computer School Asymmetric DSL (ADSL)  It provides greater bandwidth for downloads at the expense of upload speeds.  It is the most common form of DSL used in home networking.  It is designed to support the typical home user who frequently downloads large amounts of data from Web sites and P2P networks but upload relatively less often.  It works by allocating a majority of the available phone line frequencies for communication of downstream traffic.  It possesses all of the characteristics one associates with DSL, including "high-speed" service, an "always on" combination of voice and data support, and availability and performance that is limited by physical distance.  It is technically capable of up to 6 Mbps (roughly 6000 Kbps).

A+ Second Edition Genetic Computer School ADSL

A+ Second Edition Genetic Computer School High bit-rate DSL (HDSL)  One of the earliest forms of DSL  It is used for wideband digital transmission within a corporate site and between the telephone company and a customer.  The main characteristic of HDSL is symmetrical: an equal amount of bandwidth is available in both directions, downloads and uploads, offering data rates up to 2,048 Kbps.  It requires multiple phone lines to accomplish this.

A+ Second Edition Genetic Computer School Very high data-rate DSL (VDSL)  It is a developing technology that promises much higher data rates over relatively short distances (between 51 and 55 Mbps over lines up to 1,000 feet or 300 meters in length).  It's envisioned that VDSL may emerge somewhat after ADSL is widely deployed and co-exist with it.  It is currently going through a standards issue, so it isn't widely deployed yet.

A+ Second Edition Genetic Computer School DSL Summary Table (1)

A+ Second Edition Genetic Computer School DSL Summary Table (2)

A+ Second Edition Genetic Computer School DSL Summary Table (3)

A+ Second Edition Genetic Computer School Carrierless Amplitude/Phase (CAP) CAP modulation was the original approach for modulation of a DSL signal. CAP is closely related to quadrature amplitude modulation (QAM). CAP operates by dividing the signals on the telephone line into three distinct bands:  0 to 4 KHz - Voice conversations  25 to 160 KHz - Upstream channel (from the user back to the server)  240 KHz to 1.5 MHz - Downstream channel (from the server to the user) This system, with the three channels widely separated, minimizes the possibility of interference between the channels on one line, or between the signals on different lines.

A+ Second Edition Genetic Computer School Discrete MultiTone (DMT) It is a method of separating a DSL signal so that the usable frequency range is separated into 256 frequency bands (or channels) of kHz each. DMT uses the fast Fourier transform (FFT) algorithm for modulation and demodulation. Dividing the frequency spectrum into multiple channels allows DMT to work better when AM radio transmitters are present. Within each channel, modulation uses quadratude amplitude modulation (QAM). By varying the number of bits per symbol within a channel, the modem can be rate-adaptive.

A+ Second Edition Genetic Computer School Integrated Systems Digital Network (ISDN) It is another standard for transmitting digital signals over PSTN copper wires It allows voice and data to be transmitted simultaneously across the world using end-to-end digital connectivity. It is designed to carry voice, data, images, video, everything you could ever need. It is also designed to provide a single interface (in terms of both hardware and communication protocols) for hooking up your phone, your fax machine, your computer, your videophone, your video-on- demand system (someday), and your microwave. ISDN service requires an adapter, called an inverse multiplexor, at each end of the transmission.

A+ Second Edition Genetic Computer School Two Basic Types Of ISDN Service  Basic Rate Interface (BRI)  Primary Rate Interface (PRI)

A+ Second Edition Genetic Computer School Pulse Code Modulation (PCM)  It is a digital scheme for transmitting analog data.  The signals in PCM are binary.  Using PCM, it is possible to digitize all forms of analog data, including full-motion video, voices, music, telemetry, and virtual reality (VR).  The output of a PCM is a series of binary numbers, each represented by some power of 2bits.  At the destination (receiver end) of the communications circuit, a PCM converts the binary numbers back into pulses having the same quantum levels as those in the modulator. These pulses are further processed to restore the original analog waveform.