How DOCSIS systems work?
The term DOCSIS,
itself an acronym for Data Over Cable Service Interface Since its initial
release in 1997, DOCSIS has been better known as the technology behind cable
broadband services offering transparent IP traffic between the Ethernet
interface and user interface.
Different variants of the DOCSIS standard are
used by different broadband providers, which may confuse some consumers and put
some on quest to understand their broadband options better.
There are three
different levels of DOCSIS specifications, 1.x, 2.x, and 3.x, each one tends to
improve performance and/or offer additional features over the last. This means that 3.x tends to be a better
standard than 2.x, which in turn is generally better than 1.x.
There are a few
exceptions to this, and of course a working definition of ‘better’ would
probably be helpful. Some areas serviced
by second-tier cable companies still use older DOCSIS 1.x systems, but most
top-tier providers using 3.x, but tend to charge more money for greater
For the purposes of
fostering a productive discussion on the subject, greater features and
performance will generally be considered ‘better’ than great value. After all, prices for the same level of
service tend to fall over time and thus value is relative and highly dependent
specification can be broken down into at least two layers, the physical layer
(PHY) and the media access control (MAC) layer
DOCSIS Physical Layer
The physical layer
is the easiest to understand as it refers to things that people can see and
touch, in this case wiring and routing equipment. The physical layer also specifies of the
frequency at which data is transmitted over the wires of a cable modem system
and DOCSIS-compliant network. The faster
the transmission speeds, the greater the performance tends to be, but there are
limitations on distance. Limitations on
distance restrict the areas in which DOCSIS-based cable modem services can be deployed
their speed, and their pricing.
DOCSIS MAC Layer
The MAC layer is
used to handle the massive packet switching requirements of a cable network and
ensure that there are fewer traffic jams caused by signals collision.
In effect, the MAC
layer is something of a traffic-cop that helps maximize the performance of a
network. Not all MAC layers are created
equal, however, and understanding how MAC affects a major network is
There are dozens,
perhaps hundreds of homes within a housing sub-division that are serviced by
dedicated DOCSIS networking hardware provided by the broadband service provider
Think of the
problem faced by network architects this way: imagine data on a DOCSIS network transmitted
by each user at the same time on the same frequency channel.
Nobody can receive
the data correctly as there is a mix up of the signals. The only way to avoid
this chaos is the transmitter of the modems to obey to certain rules to
transmit, and they do not always work 100% of the time.
There is no
guarantee that only a single sub-division is serviced by one terminal, but for
the purposes of simplification, the example should suffice. Each device has its own specific address and
name, referred to as an IP address and MAC address respectively.
The MAC address is
used for intra-network signal timing rather than actual data transmission and
reception. The central DOCSIS manager
called CMTS will make rules in order to allow the transmission from one user at
a time using the MAC address and a centralized timing system.
CMTS is the hard and brain of the DOCSIS
Understanding the Different
The original DOCSIS
1.0 standard offered support for a single channel, a trend that continued up
until the introduction of the most recent DOCSIS standard, DOCSIS 3.0. The differences between DOCSIS 1.0 and 1.1
are mostly academic, and relate to the number of consumers that can be serviced
and their range from hardware operated by a cable provider. The official throughput for DOCSIS 1.x
systems was limited to a usable 38 Mbps downstream and approximately 9 Mbps of
upstream. These rates are shared amongst
multiple consumers in most cases, and are practical. Some hosts provide higher specs, specifically
42.88 Mbps downstream and 10.24 Mbps upstream, but these fail to calculate
network overhead and are not necessarily honest figures.
As competition with
DSL and other forms of broadband services, a new standard was needed to deliver
greater speeds to a greater number of consumers who were believed to be using
upstream and/or downstream numbers as a guide to making purchasing decisions.
A standard that
offered substantially higher bandwidth and that standard became known as DOCSIS
2.0 and eventually DOCSIS 2.0 + IPv6.
DOCSIS 2.0 actually kept the already impressive downstream speeds, but
tripled the upstream performance to 27 Mbps.
The logic behind this decision was simple: take a 100 customer region as
an example. How many of these consumers
are downloading at full-speed at any given moment? If individual download speeds are capped at 9
Mbps, then 4 could be using every last iota of network performance. Of course, what are the chances that
consumers or businesses could even find something that would tax 9 Mbps of
downstream for any sustained duration?
It quickly became
apparent that usage patterns pointed out that downstream speeds were already
sufficient, but more customers could be serviced by increasing upstream speeds;
the disparity between DOCSIS 1.x’s 38 Mbps downstream and 9 Mbps upstream was
too great, but DOCSIS 2.x’s ratio of 38/27 Mbps was more desirable. Of course, the advances made by DSL providers
and fiber optics would eventually cause the birth of DOCSIS 3.0.
What makes DOCSIS
3.0 different from its predecessors is that it is able to support multiple
channels and bind them together to increase performance. More channels means greater speed, and there
is a 4-channel minimum requirement for DOCSIS 3.0 approved hardware. Each channel offers a familiar 38 Mbps
downstream and 27 Mbps upstream, but there are no limits to how many channels
can be used. This opens up a lot of
performance possibilities, a great example of which is the 100 Mbps DOCSIS 3.x
service available from most ISP to the business customers, in selected areas.
Of course, a DOCSIS
3.0 cable modem with 4 channel support is theoretically capable of downstreams
greater than 100 Mbps, but it is only a matter of time until faster services
are deployed that will be capable of utilizing more channels effectively. Customers would do well to match the right
DOCSIS 3.0 cable modem to the appropriate service, or risk paying for a
broadband service that they are not fully capable of utilizing.