Besides the two major connections mentioned above -- leased line and ethernet WAN there are many other WAN connections such as Satellite, Wireless, DSL and Cable.
For the CCENT, we only care about DSL and Cable, these two types of WAN services are very popular for home users as well as businesses.
Digital Subscriber Line (DSL) -- DSL has a distance limitations of 18,000 ft, and not universally available in all geographic locations. Unlike leased line, which covers long distance, DSL technology only creates a relatively short distance WAN link between telco customer and ISP. DSL 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 (upstream) 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 service can be incrementally added in any area.
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| Home DSL link |
The above picture shows an example of DSL internet connection for a home owner named Jim. At Jim's house, there is a twisted-pair copper line, which can be used for normal telephone voice connection and fax. This copper line could terminate at several wall plates, often with RJ-11 ports, or other fancy devices such as NTBA on the picture. The same copper line can also terminate at a DSL-modem, allows Jim's PC to access internet via the DSL modem. A filer often is used to prevent interfering among electronic devices. Outside Jim's house, the copper line called subscriber line, 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 (Jim's, Mike's, Bob's...) are multiplexed into a single, high-capacity link by the use of a DSL access multiplexer (DSLAM). 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. The DSLAM split out the data over to the router which connects to the ISP. Typically, this link between CO and ISP is a T3 (DS3) line, with data rates up to 8.192 Mbps. The DSLAM also split out the voice signals. 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. ADSL is cost effective for typical home owners, because they generally download more content than upload content during web surfing.
- Symmetric DSL (SDSL): Provides the same bandwidth for both downstream and upstream.
The following video introduces how the DSL modem works for the home users.
Cable Internet -- The structure of cable internet is almost the same as DSL internet.
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| cable Internet |
Cable Internet only creates a relatively short distance WAN link between telco customer and ISP. Cable Internet uses existing coaxial lines that transmit cable television to transport high-bandwidth data, such as multimedia and video, to service subscribers. Cable Internet technology allows use of the local-loop line for normal cable TV (CATV) connection and an always-on connection for instant network connectivity. It place upload (upstream) and download (downstream) data transmissions at frequencies different than TV signal, allowing both TV and data transmissions to occur simultaneously on a Cable service.
On the CATV company side, the CATV company splits out the data over to the router which connects to the ISP. They also split out the TV signals from the video dishes for distribution.
The only difference is: the DSL twisted-pair telephone lines are replaced with coaxial cable from the CATV company, and the DSL modem has been replaced by a cable modem.
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
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