September 16, 2014 Jessica Sims

Know Your DOCSIS: Get Up To Speed!

The term DOCSIS, itself an acronym for Data Over Cable Service Interface Specification, is something that many techies and broadband junkies talk about nearly endlessly, often in excited and/or argumentative tones.  Since its initial release in 1997, DOCSIS has been better known as the technology behind cable broadband services.  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.  The existence of multiple DOCSIS standards makes for a slightly steeper learning curve, which unfortunately impairs the ability of consumers to make the best possible purchasing decision.  After all, how can one decide which path to broadband is best for them if they do not even understand the fundamental terminology being used?

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.

DOCSIS Gets Physical

The DOCSIS specification can be broken down into at least two layers, the physical layer (PHY) and the media access control (MAC) 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.

Meet DOCSIS’s Other Layer, the MAC

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 colliding.  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.  Unfortunately, understanding the MAC layer is easier said than done.

Think of the problem faced by network architects this way: imagine data on a DOCSIS network as a car, and the network itself to be the layout of streets in a major metropolitan area, but dominated by a single major avenue onto which all traffic merges at some point.  Now imagine this complex crisscross of city streets accelerated via time lapse photography, at a pace that turns each minute into a fraction of a second; one side of a traffic light bursts then stops followed by vehicles traveling in the opposite direction.  The only thing that brings order to what would otherwise be chaos is the system of traffic lights that the vehicles attempt to obey, and they do not always work 100% of the time.  This is not very different from how networks function, even DOCSIS-based systems.

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.  There is no guarantee that only a single sub-division is serviced by one terminal, but for the purposes of education, 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.  This would make MAC addresses similar to traffic lights that exist at each customer’s cable modem router and the cars akin to packets of data.  The central computer system in any city the controls the traffic lights would be the DOCSIS hardware devices that are deployed across the country, serving consumers and businesses alike.

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 surged post-Y2K, 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 Comcast is offering 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.

23 comments on “Know Your DOCSIS: Get Up To Speed!”

  1. Karl D. says:

    Curious as to when this article was written.
    I subscribe to Comcast/Xfinity’s “Performance” level cable internet. Have been using a Motorola SB4200 (DOCSIS 1) for several years up to the present, without any problems, that I could detect. Have been getting phone/email messages from Comcast/Xfinity, lately, that I MAY need to upgrade to a DOCSIS 3 modem to get the full speed of internet sevice. I went ahead and bought a DOCSIS 3 modem (Motorola SB6121). Have been using it for a couple days, now. Cannot detect any difference in performance. BUT when I test the speed at websites such as or I now get readings of 4.5 – 5.1 Mbps download speed. When I ran the same tests a couple days ago, on the OLD DOCSIS 1 modem I would get readings in the 20Mbps+ range.

    Wondering if the faster readings on the OLD modem, were faulty, because of the older technology in the modem. OR is there really some benefit to keeping the old modem? Is there a way to reliably test the TRUE speed of the different modems?


  2. Fred says:

    I’m researching for same reason – got same notice from Xfinity. I have DOCSIS 2 modem and am getting 19mbps downstream and 5mbps upstream – which is pretty close to my 22mbps / 5mbps plan. Doesn’t seem to be a need to upgrade to DOCSIS 3, unless you’re in the new fastest Xfinity plan – Extreme 50.

  3. Bill says:

    I have Docsis 1.0 Linksys and I’m getting 27.92 download and 4.59 upload. Do I need to upgrade??

  4. Neil Stapley says:

    Comcast recently changed out my modem to a Arris Touchstone WBM760. The technician had mentioned that Docsis 3.0 modems tend work to have a minimum speed I think he said 20mps download. Well we are paying for 12mps regular speedtests comeback with 25mps download and 4mps upload. Would I still be getting 25mps if I dropped the service to 6mps.

  5. Chris says:

    No you wouldn’t get the same speeds. Comcast would throttle back your download speeds. What you are seeing in the brief speed test is what Comcast calls speed boost. It is a temp boost to your download speeds. But it goes back to your regular speeds during longer downloads. ie; gaming, movies, music downloads etc…

  6. Mark (secured2k) says:

    Max Speeds (US)
    DOCSIS 1.x (1 Channel) – 38 Down; 9 Up
    DOCSIS 2.0 (1 Channel) – 38 Down; 27 Up (Might have IPv6 support)
    DOCSIS 3.0 (1 Channel) – 38 Down; 27 Up (Must support IPv6)
    DOCSIS 3.0 (4/4 Channels) – 152 Down; 108 Up (38 x 4 and 27 x 4)

    You are fine to use a DOCSIS 1.x modem if you don’t need IPv6 and your service level is lower than 38/9 Mb/s Down/Up.

    If your service level is higher for the upload (Or PowerBoost would “boost” you over the DOCSIS 1.x limits), you need at least a DOCSIS 2.0 modem to enjoy the extra bandwidth.

    DOCSIS 3.0 is for any case where your bandwidth (with/without PowerBoost) could be higher than 38 Down and 27 Up as the spec combines channels (to no defined limit) to allow for more bandwidth.

    You can probably assume newer spec’d modems will support previous DOCSIS specs and will handle connections and bandwidth better just because the technology has evolved.

    However, the cable registration (to bring you online) involves downloading a configuration file that tells the modem how much bandwidth is allowed. In order to not get a generic configuration file, your cable provider must support your cable modem so a configuration file with your correct level of service can be applied to the modem.

    I have used 2 DOCSIS 3 modems and I am in an area that supports DOCSIS 3 with 4 bonded downstream channels and 3 upstream channels at once. My cable provider has been upgrading the same level of service over the past 10+ years and most recently, I have been moved from 30Mb/s down and 6 Mb/s up to 50 Mb/s down and 10 Mb/s up. My DOCSIS 2 modem can’t handle the new upgraded speeds so I was limited to lower bandwidth. The cable provider also didn’t update the old DOCSIS 2.0 modem configuration file so even if DOCSIS 2 supported 38/27 Down/up, the modem config file was stuck at a 30/6 Down/Up bandwidth. Both of my DOCSIS 3.0 modems (which were supported by my cable provider) had no issue reaching 55Mb/s down and 10.5 Mb/s up. The first one was just a modem and the 2nd one replaced the modem with an all-in-one cable modem, wireless gateway, router, and added VOIP phone support.

  7. Rick says:

    I just upgraded (so I thought) from a Motorola SB5120 DOCSIS 2.0 to a Cisco Model DPC3010 DOCSIS 3.0 modem (on Cox Communications (Orange County Ca.) approved list) and my speeds slowed down! (per ~ same servers).

    Went from 33/6 Mbps down to 24/7 Mbps ~ I am confused…

  8. Santos Murillo says:

    I’m purchasing 25mps from eyefinity and trying to figure out what modem and router to get. Not too concerned about the router (it will be docsis 3.0 but wondering about the speed of the modem. Does it make sense to purchase a modem that has 802.2ac capacity when I only get 25mps throughput?

  9. bob says:

    I have Comcast 50mps service (blast).

    I currently rent the Comcast issued Gateway/modem. I think the OEM is Arris.
    Anyhow, not exactly thrilled with several performance issues like wireless range and only 2.4 ghz band.
    I was wondering about buying a Motorola 6141 modem and a router of choice.
    Anyhow, Iread something about binding two 6141 Motorolas together to get 2x speed? Is that possible and if so, how do you do it?

    • dave says:

      @BOB There is no reason to bind two 6141 modems together for 50Mb service, as ONE of them alone is more than you need, you might as well save some money and get a 6121 instead of 6141 and you’ll still be set for 100Mb, let alone half that.

      As for the wifi, sure get a different wifi router if you like, though in many cases it’s not the frequency as much as the placement of it and choosing an uncongested channel, though units with multiple external antennas are better than those with only internal wire elements or PCB traces, and the same is true for antennas on the client devices using the wifi. No router will make up for a terrible client system performance and your clients pretty much dictate which 802.11 standard you’ll use since many don’t have upgradable wifi and/or it’s costly to do so.

      • dave says:

        Actually a better router will go a long way to make up for poor client performance because the TX/RX is seldom equal. Clients usually upload far less than they download and the typical phone, laptop, tablet, usually does have inferior performance to a mid grade or better wifi router, AND in many cases you can’t even upgrade the client device (reasonably, if it’s portable. If it’s not portable then stop messing around with wifi and go GbE).

      • dave says:

        Not necessarily true. On many ISPs they cap the per-channel throughput much lower than the theoretical max a channel can handle. For example on my ISP’s system, a 6121 with only 4 channels, can do 50Mb but can’t do 100Mb, because they cap each channel below 25Mb to force better use of spectrum.

        Additionally a router can make up for poor client system performance. Remember that the typical internet usage scenario is much more data down than up, so getting a stronger, higher quality signal to the clients for download can be a lot more important than having resends for the uploads due to a poorer client generated signal.

        PS – all these “daves” are not the same person, lol, just coincidence.

    • dave says:

      Yes that is possible, but it makes no sense to do it. You would contact the cable co and get a 2nd isp account for the 2nd modem, cannot have two modems registered to the same account. Then you “may” need a 2nd cable line ran into the residence for isolation purposes, or to put a line isolator after each and remove the one at the junction box, then aggregating the two modems would happen with an additional piece of hardware you add, whether it be a custom programmable router or through multiple network links in to the same PC set up as a bridge for the rest of the LAN.

      Why it makes no sense is you can just get a faster single ISP plan for more money in most areas. However, yes you can get your own modem and a better router and it makes sense to do it just to stop paying a monthly rental fee to the ISP, let alone the potential for better wifi rate and range.

  10. TomT says:

    How do you tell a docsis 2.0 from a 3.0? I have an RCA DCM425 modem but can’t find any markings on it to indicate which version it is. Any idea how to determine this?

  11. dave says:

    Meh, you’d mostly be getting higher results in speed test /benchmarks or P2P filesharing. For most people the other end of the internet is the bottleneck even on DOCSIS2.

  12. Anderson,SGT says:

    As a network engineer for the ARMY and in the civilian world, Dave and Mark hit the nail on the head. Although you can team 2 separate providers (if you have that option in your area). A big plus is to talk to the installer and get them to put you on a new circuit and go with a 6121 or for $20 more get the 6141 for future upgrades. Have fun with your network and setup a cache proxy… Remember were doing a full circle back to the old ways… (Term serv = Dumb Term) & MPLS … just food for thought as I once “shot gun” my 14.4 modem on 3 phone lines to get across the FCC barrier 🙂

  13. Trent says:

    The last post hit the nail!
    (Term serv = Dumb Term) & MPLS

    I can’t believe I am diving into MPLS again, but it’s happening.

  14. Gul says:

    Would this modem have a phone line connection as well as cable?

  15. Gunther Steinberg says:

    As a long time PC user (33 yrs) I know little of the mechanics of the Internet data packet moving.
    My PC uses a SB6121 Modem (DOCSIS 3.0) and a Trendnet 652BRP wireless Router.
    On the PC i get download speed that vary from 54 to 90 Mbps and the service rarely goes down.
    An iMac (about early 2006) and an iPadAir2 both use the wireless function of the router.
    Their DSL Reports data shows download values that range between < 1Mbps to 30Mbps.
    The problem I have that at certain times of the day, at meal times and ~5-7 PM, both the iMac and iPad fail to get a signal from the wireless router. The settings say I am connected but the devices cannot connect. If I wait long enough, many minutes, the URL site I was trying to get does appear.
    Apparently the Link Rate has recovered.
    Tried to help the problem with a Wireless Extender, but this was no help at all. But, what I noticed was the the Link Rate LED would change from Green to Yellow or RED, when the computers failed to connect. As soon as the LED went back to green, DSLReports/Speedtest would give me 25-30Mbps again. – The BufferBloat grades were always D or F with latency at 450 ms.
    A Comcast Tech assured me that with today's technology, the number of people on the cable into this area would not make a difference. However he ha no explanation for the great variation of download speeds 90 to50 or es on the PC (Ethernet connection) and 25 to 4 Mbps or less for the iMac and iPadAir2 located about 20 feet away (upstairs). – Somehow I doubt the Tech assurances.

    After reading a great deal about Bufferbloat and where the problem lies I have not become smarter.
    Any comments, copy to me, will be appreciated.

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