Cable Modems

It would be unfair to present the DSL technologies without providing some space to discuss the
alternative: cable modems.
Cable modems
operate over the same cabling system that provides
cable television service; in other words, they use the same coax cable that is already used in the
homes with cable television. Most cable installations provide two cables—one for the television
and one for the data converter—but the signaling and the system are the same. This is accomplished
by allocating a television channel to data services. Bandwidth varies with each installation;
however, many installations provide up to 2Mbps in the downstream direction and
128Kbps to 256Kbps in the upstream one.
Downstream
is a common term for traffic from the
provider to the customer;
upstream
is return traffic. This type of asymmetric connection is sufficient
for most Internet users, because these users typically pull more information (bits) to their
machines than they send.
Detractors of cable modem technology are quick to point out that these installations are shared
bandwidth, similar to Ethernet, which results in contention for the wire among neighbors. This
shared bandwidth also introduces a security risk, in that network analysis is possible, although
vendors have addressed this concern with switching and encryption technology. This issue does
not exist in DSL because the local loop connection to the home is switched. In DSL, traffic is not
integrated until it reaches the central office, and at that point, the switch will forward only traffic
destined for the end station based on the Media Access Control (MAC) address. Basically, cable
modems are a shared technology—hub-based Ethernet versus switched. Along the same lines, a
cable modem is really a broadband Ethernet bridge to the cable.
There is a lot of confusion in the marketplace regarding oversubscription and
performance in the residential DSL and cable modem markets. DSL is usually
oversubscribed 10 to 1 at the central office; if a DS3 is used to link the DSLAM
to the Internet, as many as 300 homes could be connected to the DSLAM. None
of those users would be oversubscribed on their connection to the DSLAM.
Cable modems typically share bandwidth before the head end. As a result,
users contend for bandwidth both before and after the head end.
Network designers might wish to consider cable modems as part of a virtual private network
(VPN) deployment because the technology will not lend itself to the RLAN-type designs available
in DSL. Recall that an RLAN requires layer 2 isolation—a service not offered by cable modem
providers at present. This might change in the future if channels can be isolated to specific users.
This might be especially true in very remote rural areas, where cable is available and DSL is not.
Note: Cable modem technology, including the features of Data over Cable Service
Interface Specification (DOCSIS), is presented in Chapter 28, “Remote Access
with Cable Modems and Virtual Private Networks.”