Comparison of Remote Access Technologies

This chapter scratches the surface of how modems, ISDN, cable, and DSL work. Consumers choose between these options for Internet access all the time, and network engineers choose between these options for supporting their work-at-home users as well. So, it seems appropriate to close the chapter by listing some of the key comparison points for these options.

Comparison of Remote Access Technologies 465

The remote access technologies in this chapter provide services at Layer 1, and possibly Layer 2, of the OSI reference model. TCP/IP and all the associated higher-layer protocols (TCP, UDP, HTTP, FTP, Telnet, DNS, DHCP, and so on) can run over any of these access technologies; the differences lie in what is done at Layers 1 and 2. Figure 15-16 outlines the protocols used by each.

Figure 15-16 The OSI Reference Model and Remote Access Technologies

Applications

TCP or UDP

Modem Standards (v.x)

Applications

TCP or UDP

ISDN (I-430) Standards

Applications

TCP or UDP

ATM ; Ethernet xDSL Standards

Applications

TCP or UDP

IEEE 802.2 MCNS MAC

Upstream-QPSK, QAM-16

Downstream-QAM-64, QAM-256

Table 15-8 lists some of the main points for comparison of these technologies. Table 15-8 Comparison of Modems, ISDN, DSL, and Cable

Analog Modems

ISDN

DSL

Cable Modems

Transport

Telco line

Telco line

Telco line

CATV cable

Supports symmetric speeds?

Yes

Yes

Yes

No

Supports asymmetric speeds?

Yes

No

Yes

Yes

Speed ranges

56 kbps and lower

64 kbps per B channel

56 kbps to 2 Mbps

320 kbps to 40 Mbps

Table 15-8 Comparison of Modems, ISDN, DSL, and Cable (Continued)

Analog Modems

ISDN

DSL

Cable Modems

Degrades under higher loads?

No

No

No

Yes

Supports IP and associated higher-layer protocols?

Yes

Yes

Yes

Yes

Allows concurrent voice and data?

No

Yes

Yes

Yes

Always on?

No

No

Yes

Yes

Local loop distance issues

No

No

Yes; distance varies

No

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