A discussion of consumer DSL, of which ADSL is a common offering, necessitates a discussion
of vendor claims regarding oversubscription and bandwidth contention. As you might know,
oversubscription occurs when the network is provisioned with greater potential demand than
could be serviced at any one time, under the reasonable assumption that use patterns and the
quantity of bandwidth demanded will never be 100 percent.
This assumption is very reasonable in many networks. Consider your network for a moment.
You might have 100 workstations connected to a switch with a single 100Mbps uplink to the
core. If each of the 100 workstations is connected at 100Mbps, the network would be oversubscribed
100:1. Consumer DSL network vendors commonly oversubscribe at ratios between 3
and 10 to 1, or 10:1.
Let’s suspend discussion of oversubscription for a moment and consider bandwidth contention.
DSL providers quickly point out that cable modem networks provide shared bandwidth from the
head end to a population of users. Think of this as shared Ethernet. They then add that their DSL
technology is more akin to switched Ethernet, where each user has no contention for bandwidth
from their router to the DSLAM.
On the surface it would appear that DSL is the superior technology, as many networkers have
migrated from the old shared network model to the superior switched network in Ethernet.
The marketing folks for DSL providers enjoy that analogy and relish in users choosing the
dedicated technology.
However, all is not as it appears. Although it is true that DSL dedicates bandwidth from the end user
to the head end at the Physical layer, we must return to oversubscription. I might have a dedicated
100Mbps Ethernet connection to my workstation, and Piper might have 100Mbps to her workstation,
but if we have a single 100Mbps uplink from the switch to our resource, we could expect
only 50 percent, or 50Mbps in this example, of throughput. So long as we have that consideration,
shared bandwidth is always a factor, even if the hop from my router to the head end is dedicated.
As such, cable modem’s shared technology (presented further in Chapter 28, “Remote Access with
Cable Modems and Virtual Private Networks”) is less of a concern than DSL providers would like.
Cisco contends that ADSL is best suited to video on demand and video conferencing; however,
in practice we would recommend against this generalization. The asymmetric nature of
ADSL is such that quality upstream video conferencing is unlikely if there is concurrent load.
Because video conferencing is typically a bidirectional experience, it would be overgeneralizing
to conclude that ADSL is the best solution. We justify their answer by simplifying the
scope and comparing ADSL to ISDN, analog (POTS), and other remote access technologies.
In this light, ADSL is the best solution. However, HDSL and other DSL flavors, discussed
later, might be better for your installation.