CompTIA A_ Certification All-In-One Exam Guide, Seventh Edition - Michael Meyers [443]
The physical connections for ISDN bear some similarity to analog modems. An ISDN wall socket usually looks something like a standard RJ-45 network jack. The most common interface for your computer is a device called a terminal adapter (TA). TAs look much like regular modems, and like modems, they come in external and internal variants. You can even get TAs that are also hubs, enabling your system to support a direct LAN connection.
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NOTE Another type of ISDN, called a primary rate interface (PRI), is composed of twenty-three 64-Kbps B channels and one 64-Kbps D channel, giving it a total throughput of 1.5 megabits per second. PRI ISDN lines are also known as T1 lines.
DSL
Digital subscriber line (DSL) connections to ISPs use a standard telephone line, but special equipment on each end to create always-on Internet connections at blindingly fast speeds, especially when compared with analog dial-up connections. Service levels vary around the United States, but the typical upload speed is ~768 Kbps, while download speed comes in at a very sweet ~3+ Mbps!
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NOTE The two most common forms of DSL you’ll find are asynchronous (ADSL) and synchronous (SDSL). ADSL lines differ between slow upload speed (such as 384 Kbps, 768 Kbps, and 1 Mbps) and faster download speed (usually 3–7 Mbps). SDSL has the same upload and download speeds, but telecom companies charge a lot more for the privilege. DSL encompasses many such variations, so you’ll often see it referred to as xDSL.
DSL requires little setup from a user standpoint. A tech comes to the house to install the DSL receiver, often called a DSL modem (Figure 25-18), and possibly hook up a wireless router. The receiver connects to the telephone line and the PC (Figure 25-19). The tech (or the user, if knowledgeable) then configures the DSL modem and router (if there is one) with the settings provided by the ISP, and that’s about it! Within moments, you’re surfing at blazing speeds. You don’t need a second telephone line. You don’t need to wear a special propeller hat or anything. The only kicker is that your house has to be within a fairly short distance from a main phone service switching center, something like 18,000 feet.
Cable
Cable offers a different approach to high-speed Internet access, using regular cable TV cables to serve up lightning-fast speeds. It offers faster service than most DSL connections, with a 1–10 Mbps upload and 6–50+ Mbps download. Cable Internet connections are theoretically available anywhere you can get cable TV.
Figure 25-18 A DSL receiver
Figure 25-19 DSL connections
Cable Internet connections start with an RG-6 or RG-59 cable coming into your house. The cable connects to a cable modem that then connects to a NIC in your PC via UTP Ethernet cable. Figure 25-20 shows a typical cable setup. One nice advantage of cable over DSL is that if you have a TV tuner card in your PC, you can use the same cable connection (with a splitter) to watch TV on your PC. Both DSL and cable modem Internet connections can be used by two or more computers if they are part of a LAN, including those in a home.
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NOTE The term modem has been warped and changed beyond recognition in modern networking. Both DSL and cable fully digital Internet connections use the term modem to describe the box that takes the incoming signal from the Internet and translates it into something the PC can understand.
Figure 25-20 Cable connections
LAN
Most businesses connect their internal local area network (LAN) to an ISP via some hardware solution that Network+ techs deal with. Figure 25-21 shows a typical small-business wiring closet with routers that connect the LAN to the ISP. You learned all about wiring up a LAN in Chapter 23, “Local Area Networking,” so there’s no need to go through any basics here. To complete a LAN connection to the Internet, you need to add a second NIC or a