Cable Modems vs. ADSL, Technology, Applications and
In the US, the two primary broadband
technologies competing for the home market are Cable Modems and
Asymmetric Digital Subscriber Loop (ADSL). There are a number
of myths associated with each of these technologies and this seminar
provides an understanding of the technologies and the strengths
and weakness of each.
In today's markets, ADSL and Cable Modem service can be purchased
for between $29.95 and $49.95 depending on the region of the country,
speeds being offered, and whether you own the modems or lease
them from the operator. As of the first quarter of 2006, there
are roughly three times as many cable modems in service compared
to ADSL and the trend appears to growing in favor of Cable Modems.
Cable Modems are being offered by the
Cable System operators and the perceived reliability of the network
is poor. Telephone Operators offer ADSL and their reputation for
ease of installation and customer service is poor. There are many
examples of both good and bad data services for each technology
depending on how the networks are constructed.
Cable Modem Technology
A typical Cable system shown in Figure
1, has a Headend, sometimes called the Master Head End (MHE),
that contains the major equipment for the network. It assembles
the program material from the many different video sources. It
also has the main link funneling data from the cable modem to
Figure 1. A typical Cable TV system
The Headend serves all customers in a
large geographical area and may typically be 100,000 subscribers.
The headend sends the assembled data and video to branch locations
around the geographic area to a facility known as a Distribution
Hub. A Distribution Hub contains the modulators to put a video
channel on a particular TV channel. (e.g. NBC goes on TV channel
12). It also contains the receivers for data that is sent from
The headend uses lasers to feed optical fibers that spread out
across the area. Each fiber will feed approximately 2,000 homes.
When the fiber reaches a neighborhood, it is converted from an
optical signal to an electrical signal and split into four different
paths feeding 500 homes. This combination of optical (fiber) and
electrical (coaxial) is called Hybrid Fiber Coax or HFC for short.
The data signal is carried just like
any other video signal through the system. Unlike the video signal,
the data system also requires a channel that goes back to the
head end since Internet users are interactive. The way the system
carries both the downstream (head end to subscriber) and upstream
(subscriber to head end) signals is by using separate frequencies
as shown in Figure 2.
Figure 2. Upstream and downstream data use different frequencies
on the same cable.
The lower frequencies are used to send
data from the Cable Modem to the Headend and the higher frequencies
are used to send data from the Headend to the Cable Modem. There
are many TV channels in the higher frequencies and there can be
multiple channels dedicated to data transmission verses video
transmission. Also each geographic area fed by a fiber can reuse
the data channel in a manner analogous to reusing a frequency
in cell phone network.
The acronym ADSL stands for Asymmetrical
Digital Subscriber Loop and this technology uses your existing
telephone line to transmit data between the Central Office and
your computer. Figure 3 shows the major components for this type
Figure 3. The major components of the ADSL network
There are two types of ADSL technology,
a full rate system and a "lite" system that is used
primarily for residential data services. In the home, all the
phones need to have a filter located at each phone so that these
phones don't cause interference with the higher frequencies being
used by the ADSL modem.
At the telephone Central Office, there
is a frequency splitter that separates the voice signal and sends
it the voice switch. A unit called a DSLAM (DSL Access Multiplexer)
is the unit that houses the data card that combines data from
many ADSL modems and forwards them to the Internet.
A standard voice signal on your telephone
line uses frequencies between 300 Hz. to 4,000 Hz. The phone line
can actually handle higher frequencies than that and these and
these higher frequencies are used to handle the upstream and downstream
data between the DSLAM and the ADSL modem. This is shown graphically
in Figure 4.
Figure 4. Frequency usage for an ADSL system.
As shown, a large packet overlaps the
time interval of the priority packet. The CMTS when controls this
process by informing the CM of the first packet that it is to
transmit part the first part of the first packet and then to stop
transmitting. The CM for the second packet is told to transmit
its priority packet and then the CM for the first packet is told
to continue transmitting.
The battle between broadband service
providers have made claims against the other that are usually
slightly true but not exactly what it appears. Here are a few
of these myths that have been presented to the public.
Myth #1 - ADSL has dedicated bandwidth
This is partially true in that the link
between the central office and the ADSL modem is used only by
that customer. After it arrives at the central office, it is immediately
combined with data from all the other ADSL subscribers in that
region and from that point on, all the links are shared amongst
Typically there are more links in an
ADSL network compared to a Cable Modem network because of restrictions
imposed on the phone company. So the myth may be true but total
irrelevant since it leads up to believe you'll have better performance.
The truth is either network can be designed well or poorly in
terms of how many subscribers share the links. In order to get
a network people can afford to use, the cost of the links must
be shared. Performance is an economic decision that your broadband
provider decides for you.
Myth #2 - Cable Modem systems can't provide guaranteed
Because the cable TV wire is shared by
many people, there is a perception that you can't get guaranteed
performance because your neighbors can "hog" the bandwidth.
The truth is that the cable operators individually set each modem
for some maximum level of performance and the user cannot exceed
their fair share.
In addition to the peak bandwidth limitation,
the cable modem standard can support telephony and other data
streams that require guaranteed performance. It is completely
up to cable operators on the way they design their networks and
these networks can have excellent performance or performance that
depends on the number of users.
Myth #3 - My Neighbor can read my Cable Modem data
There is no truth to this at all. Privacy
is very important and the system has been designed with good security.
There are two paths that need to be protected. The downstream
path is received by all authorized users. The control information
is necessary to keep the system synchronized. The data itself
is protected by a three layers of encryption and the encryption
keys are constantly changed.
The upstream path is almost impossible
to be received by your neighbor because of the way the cable system
is designed. First, your data is encrypted before being sent.
Also, your neighbor's modem only receives information in the 50-750
MHz. band while your modem only transmits in the 5- 40 MHz. band.
Assuming your neighbor got a special receiver the power level
received would be very small.
There is a 12 dB signal reducer from
the main distribution line to your home. To send to your neighbor
means the cable modem signal is reduced by 12 dB to get to the
main line and another 12 dB by your neighbors tap. This makes
it difficult to receive since it's a very weak signal. After a
short distance, the signal is converted to a fiber optic signal
and a neighbor on a different fiber would receive absolutely no
signal from your modem.
Myth #4 - The link costs of both technologies is
There cost of the modems for both technologies
are similar but for other elements of the system there is a cost
associated with having a dedicated connection. The discrepancy
is shown in Table 1 below.
Table 1. Equipment needed to Service 1,000 Customers
Pairs of wires
Besides having more components, ADSL also requires more links
before it can be connected to the Internet. The reason is that
the regulated phone companies are not allowed to process data
above layer 2. The data must go from the DSLAM central office
(CO) to another (CO) where the ATM switch is located. It moves
to another location to have the links for your Internet Service
Provider concentrated to a single connection and then onto another
link to your ISP. The ISP then goes to the Internet.
Cable Operators are the ISP and can avoid
several steps. In addition, the ISPs can cache the data so that
frequently used data is sent from local storage which speeds things
up. Again the phone company by law cannot cache the data.
ADSL does a good job of providing a reliable
service and can offer higher security of the links since it is
based on a very reliable backbone that is generally well managed.
Because of its higher cost structure and combining this with reliability
concerns, small business are a good target for this technology.
Cable System cover the residential areas
fairly well but there are many concentrated business areas that
do not normally have good cable TV connections. The reliability
of the network is not as good because of the philosophy of providing
a low cost, non critical, entertainment service. Also when a single
cable is cut it tends to affect far more people then when a telephone
cable is cut. This noticeably decreases the reliability of the
cable network. For residential Internet browsing, the performance
and cost tend to make this a more successful technology.
Impediments to Success
ADSL technology is not available in all
areas because of several technical issues.
ADSL data rates get slower the further the customer
is from the central office and generally can't be deployed
in distances greater than 15,000 feet.
Inside the big telephone cables, the presence
of multiple ADSL, ISDN or T1 connections causes mutual interference
limiting speed or availability of service.
Many geographic areas are beyond 15, 000 feet
and are served by optical fibers running to boxes (Digital
Loop Carrier boxes) located in the area needing service. Optical
fiber cannot transport ADSL and therefor the DSLAMs must be
located in these boxes. Many of the boxes are full and do
not have room for DSLAMs. These boxes must first be enlarged
before ADSL can be deployed.
DSLAMs are expensive and if only a few customers
in an area take the service, the cost per user becomes very
Cable System also have challenges for
The cable plant must be modernized to be Hybrid
Fiber Coax (HFC) with an active return path. Rural areas will
find it difficult to meet the economics for this upgrade.
It is also very expensive to do this upgrade but most of the
plants that are going to be upgraded, they are completed or
will be completed in the near future.
There are many quirks in the installation because
many home owners have put signal splitters and amplifiers
into their home wiring.
Electrical noise sources in the home can get
into the cable system and cause interference. Local radio
transmitters and CB radios are also sources of interference
if the plant is not properly isolated.
Cable operators have a poor reputation for service
and reliability of their networks.
The cable plant is not located in many business
ADSL generally is a more costly system but the
telephone companies have more reliable networks.
Cable Modem systems are generally cheaper and
faster than ADSL.
There are many myths that each side in the broadband
war are propagating.