Information Technology Reference
In-Depth Information
Note:
Although ISDN has been around for many years, the industry is moving toward
using broadband technologies such as cable, DSL and public wireless with IPsec VPNs.
ISDN remains as an effective WAN solution only if broadband is not available.
Digital Subscriber Line
Digital subscriber line (DSL) is a technology that provides high-speed Internet data serv-
ices over ordinary copper telephone lines. It achieves this by using frequencies that are not
used in normal voice telephone calls.
The term xDSL describes the various competing forms of DSL available today.
ADSL is the most popular DSL technology and is widely available. The key to ADSL is
that the downstream bandwidth is asymmetric or higher than the upstream bandwidth.
Some limitations include that ADSL can be used only in close proximity to the local
DSLAM, typically less than 2 km. The local DSLAM, or digital subscriber line access
multiplexer, allows telephone lines to make DSL connections to the Internet. Download
speeds usually range from 768 kbps to 9 Mbps, and upload speeds range from 64 kbps to
1.5 Mbps. The customer premises equipment (CPE) refers to a PC along with DSL modem
or DSL router that connects back to the network access provider (NAP) DSLAMs.
The ADSL circuit consists of a twisted-pair telephone line that contains their information
channels:
Medium-speed downstream channel
■
Low-speed upstream channel
■
Basic telephone service channel
DSL splitters are used to separate basic telephone service from the ADSL modem/router
to provide service even if the ADSL signaling fails.
■
Although DSL is primarily used in the residential community, this technology can also be
used as a WAN technology for an organization. However, keep in mind that because this
is a public network connection over the Internet, it is recommended that this technology
be used in conjunction with a firewall/VPN solution back into your corporate enterprise
network. The high speeds and relatively low cost make this a popular Internet access
WA N t e c h n o l o g y.
Cable
Broadband cable is a technology used to transport data using a coaxial cable medium
over cable distribution systems. The equipment used on the remote-access side or cus-
tomer premises is the cable modem, and it connects to the Cable Modem Te r m i n a t i o n
System (CMTS) on the ISP side. The Universal Broadband Router (uBR) or CMTS pro-
vides the CMTS services, which for ward traffic upstream through the provider's Internet
connections.
Cable modems support data, voice, and video TCP/IP traffic. Generally, cable modems are
and corporate teleworkers.
