Many telephone companies offer a digital subscriber line (DSL) connection, an almost fully digital, dedicated (no phone number) connection. DSL represented the next great leap forward past ISDN for telephone lines. A physical DSL connection manifests as just another PSTN connection, using the same telephone lines and RJ-11 jacks as any regular phone line.
DSL comes in a number of versions, but the three most important to know for the CompTIA Network+ exam are Symmetric DSL (SDSL), Asymmetric DSL (ADSL), and the newer Very High Bitrate DSL (VDSL). SDSL lines provide the same upload and download speeds, making them excellent for those who send as much data as they receive, although SDSL is relatively expensive (VDSL is a new form of SDSL—see ...

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