Data Phone Network - Solutions & Applications

Data Phone Network - Solutions & Applications

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How DOCSIS systems work?

DOCSISPosted by ERMURAT 2015-04-13 21:36:27

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 performance.

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 upon time.

The DOCSIS 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.


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 important.

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 System.

Understanding the Different DOCSIS Specifications

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.

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