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DSL
Digital Subscriber Line (DSL) has a “last mile” solution similar to ISDN: using existing copper pairs to provide digital service to homes and small offices. DSL has found more widespread use due to higher speeds compared with ISDN, reaching speeds of 10 megabits and more.
Common types of DSL are Symmetric Digital Subscriber Line (SDSL, with matching upload and download speeds), Asymmetric Digital Subscriber Line (ADSL, featuring faster download speeds than upload), and Very High Rate Digital Subscriber Line (VDSL, featuring much faster asymmetric speeds). Another option is HDSL (High-data-rate DSL), which matches SDSL speeds using two pairs of copper; HDSL is used to provide inexpensive T1 service.
Symmetric DSL is also called Single-Line DSL. An advantage of ADSL is that it allows the simultaneous use of a POTS line, often filtered from the DSL traffic. Generally, the closer a site is to the Central Office (CO), the faster the available service.
Table 5.9 summarizes the speeds and modes of DSL.
| Type | Download Speed | Upload Speed | Distance from CO |
|---|---|---|---|
| ADSL | 1.5–9 Mbps | 16–640 Kbps | 18,000 feet |
| SDSL | 1.544 Mbps | 1.544 Mbps | 10,000 feet |
| HDSL | 1.544 Mbps | 1.544 Mbps | 10,000 feet |
| VDSL | 20–50+ Mbps | Up to 20 Mbps | < 5000 feet |
Cable Modems
Cable modems are used by Cable TV providers to provide Internet access via broadband cable TV. Cable TV access is not ubiquitous but is available in most large towns and cities in industrialized areas. Broadband, unlike baseband, has multiple channels (like TV channels), so dedicating bandwidth for network services requires dedicating channels for that purpose. Cable modems provide a compelling “last mile” solution for the Cable TV companies: they have already invested in connecting the last mile, and the Internet service offers another revenue stream based on that investment.
Unlike DSL, Cable Modem bandwidth is typically shared with neighbors on the same network segment.
Callback and Caller ID
Callback is a modem-based authentication system. When a callback account is created, the modem number the user will call from is entered into the account. The user later connects via modem and authenticates. The system hangs up and calls the user back at the preconfigured number.
Caller ID is a similar method: in addition to username and password, it requires calling from the correct phone number. Caller ID can be easily forged: many phone providers allow the end user to select any Caller ID number of their choice. This makes Caller ID a weak form of authentication.