Modem and DSL variations

Posted on August 3, 2009. Filed under: CCNA, ICND1 break down | Tags: , , , |


Besides the three major connections mentioned above — leased line, circuit-switched, packet-switched connections, there are many other WAN connections such as Satellite, Wireless, DSL and Cable.

For the CCENT and CCNA test, we only care about DSL and Cable, these two types of WAN services are very popular for home users as well as bussinesses.

Digital Substriber Line (DSL) — DSL technology is a circuit-switched connection technology that uses existing twisted-pair telephone lines to transport high-bandwidth data, such as multimedia and video, to service subscribers. DSL technology allows use of the local-loop line for normal telephone voice connection and an always-on connection for instant network connectivity. It place upload (uptream) and download (downstream) data transmissions at frequencies above this 4-kHz window, allowing both voice and data transmissions to occur simultaneously on a DSL service.

DSL has a distance limitations of 18,000 ft, and not universally available in all geographic locations. DSL service can be incrementally added in any area.

dsl-network

The above picture shows an example of DSL internet connection for a home user named Mike. At Mike’s house, there is a twisted-pair copper line, which can be used for normal telephone voice connection and fax. The same copper line allows Mike’s PC to access internet via a DSL modem. Sometimes, “microfilters” is used to prevent interfering among electronic devices. The copper line outside Mike’s house is the local phone line leading to the telephone company’s central office (CO). It has the spacial limit of 18,000 ft. At the central office,  many DSL subscribers’ lines (Mike’s, Jean’s, Bob’s…) are multiplexed into a single, high-capacity link by the use of a DSL access multiplexer (DSLAM), then connects to the ISP… Typically, this link between CO and ISP is a T3 (DS3) line, with data rates up to 8.192 Mbps. Note that whereas the voice channel covers the frequency range of 330 Hz to 3.3 kHz,  the data channel is above the frequency 4-kHz. Therefore, both voice and data transmissions can occur simultaneously on a DSL service. At the CO, voice lines are multiplexed via a PSTN switch, then connected to PSTN network.

There are basically two forms of DSL:

  • Asymmetric digital subscriber line (ADSL) — ADSL is designed to deliver more bandwidth downstream than upstream. Downstream rates range from 1.5 to 9 Mbps, whereas upstream bandwidth ranges from 16 to 640 kbps.
  • Symemetric DSL (SDSL): Provides the same bandwidth for both downstream and upstream.

ADSL include the following varieties:

  • ADSL
  • Consumer DSL (CDSL), also called G.Lite or G.992.2
  • Very-high-data-rate DSL (VDSL)

SDSL include the following varieties:

  • SDSL
  • High-data-rate digital subscriber line (HDSL)
  • ISDN DSL (IDSL)
  • Symmetric high-bit-rate DSL (G.shdsl)

The following video introduces how the DSL modem works for the home users.

Cable — Cable modems enable two-way, high-speed data transmissions using the same coaxial lines that transmit cable television. Cable modem access provides speeds superior to leased lines, with lower costs and simpler installation.

Here is a video introducing how Cable modem works.

Cable Modem vs DSL Modem, which one is better? It depends on your budget, the bandwidth you needed and the distance between you and the telephone company. The following video is a comparison of them.

There are mainly three types of Modem for home users in the market:

  • Analog modem — The device which connects a computer with analog telephone line by providing modulation and demodulation of incoming and outgoing data.
  • DSL modem 
  • Cable modem

This video talk about different network modems:

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