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Cable Modem Explained: How to Swap an ISP-Provided Gateway with One

If you’re using cable Internet, check your bill. You might be paying around $15/month “equipment rental” fee. If so, replacing the ISP-provided equipment with a cable modem of your own will eliminate that monthly cost.

Over time the saving will more than pay for the hardware you need to buy upfront.

Using your modem brings about other advantages, too. You then can pick a router, or a mesh system, with features and performance grades to your liking. And, in many cases, retailed hardware can improve connection speeds.

(Some of you might be happy with your ISP-supplied hardware and don’t mind paying the extra for the convenience and some potential added perks. That’s fine with me. But in that case, you should check out my take on how to get the best out of that gateway instead.)

See also  Double NAT vs Single NAT: How to Best Handle an (ISP-Provided) Gateway

This post will walk you through the process of replacing cable-provider-supplied equipment with your own. But first, let find out what a cable modem is.

Motorola MB8600 Cable Modem
The back of a typical cable modem. Note the coaxial connector.

Dong’s note: I originally published this post on February 15, 2018, and updated it on January 18, 2021, with substantial additional relevant information.

Cable modem explained: What is DOCSIS anyway?

A modem is a device that works both as a modulator and a demodulator. It converts service signals into computer data signals and vice versa. If the service is cable TV, then we have a cable modem.

Cable modems use the DOCSIS standard, which is an acronym for data over cable service interface specifications.


Extra: Cable Internet vs Fiber-optic

DOCSIS is a major contender of GPON, short for Gigabit passive optical networks, or Fiber-optic as we generally call it.

(Yes, there’s also the landline-phone-based digital subscriber line Internet or DSL, but it’s generally inferior in bandwidth and reliability. So, many DSL providers have been moving to GPON as the replacement.)

GPON is part of the fiber to the premises (FTTP), a.k.a fiber to the homes (FTTH), broadband delivery approach. Recently, it’s more often called PON since it can deliver Multi-Gig broadband.

The messy acronyms aside, Fiber-optic means you get fast Internet thanks to the fact that the optical data line runs (almost) directly from the provider to your home.

Indeed, this type of broadband delivers high speeds in both directions (upload and download). The downside is it’s expensive — new wiring required — and has a single point of failure. If a line is cut or broken, the Internet can be down for a large population.

On the other hand, DOCSIS is much more affordable since it uses the existing wiring for cable TV — the infrastructure is already there. It also works like a cobweb and, therefore, is resilient. When a cable breaks, that affects only a few families, if at all.

The biggest shortcoming of DOCSIS is that it has lopsided connection speeds — the upload tends to be less than one-tenth of the download. That’s the case with all cable connections, including mine.

With that, let’s continue with our main topic, the Cable modem.


Cable modem speeds: DOCSIS 3.0 vs DOCSIS 3.1

Currently, the world is using versions 3.0 and 3.1 of the DOCSIS standard. (So, there’s no need to worry about the earlier revisions.)

DOCSIS’s specifications can be very confusing. For one, it changes depending on the region. For example, the same modem specs might mean different speeds in the U.S. than they do in the E.U.

Also, it involves many technicalities, like channels, streams, QAM, and so on. I’m not getting into the details of all these in this post, nor should you care about them.

DOCSIS 3.0: Stream channels matter

In a simplified way, with DOCSIS 3.0, you can think of a cable modem’s speeds via the number of stream channels it can handle. There are downstream channels (for download) and upstream channels (for upload). More stream channels, or channels for short, translate into faster speeds.

Each modem comes with an indicator of the number of channels it can handle via two numbers. For example, the Netgear CM600 is a 24×8 modem. It has 24 downstream channels and 8 upstream channels.

In the U.S., the DOCSIS 3.0 standard delivers about 40Mbps per channel for download and 4Mbps for upload– these are ballpark numbers that vary from one provider to another. As a result, the CM600 caps at 960Mbps download and 32Mbps upload.

Generally, DOCSIS 3.0’s number of channels max out at 32×8. So a top-notch modem of this standard has cap speeds of some 1.3Gbps download. And that’s DOCSIS 3.0’s top bandwidth.

It’s important to note that just because a modem supports a certain performance grade (represented by the number of stream channels) doesn’t mean it will work at that grade. That depends on the service provider’s end (and the Internet plan you pay for).

And a provider generally loves to use as few channels as possible. The more stream channels, the more expensive equipment they need.

DOCSIS 3.1: Top-tier DOCSIS 3.0 is the base

And that brings us to DOCSIS 3.1. This standard delivers a higher speed per stream channel. This version needs fewer channels to deliver the same bandwidth and now has the cap of some 10Gbps in theory — that’s some 10x of version 3.0.

Though the actual speed varies from one vendor to another, a low-end DOCSIS 3.1 modem can generally deliver at least the same download speed as a top-tier 32×8 DOCSIS 3.0 counterpart.

The speed of DOCSIS 3.1 is so high that vendors now omit the stream channel numbers. Instead, they call the modem DOCSIS 3.1 and its cap speed, be it Gigabit or Multi-Gig. The Netgear CM2000, for example, is a DOCSIS 3.1 2.5Gbps modem.

In other words, the stream channel numbers, such as 32×8 or 24×8, are only relevant in DOCSIS 3.0, where most modems cannot deliver Gigabit Internet. Starting with 3.1, Gigabit is the minimum, and Multi-Gig is a new norm.

To sum up, DOCSIS 3.1 starts at the place where DOCSIS 3.0 maxes out. And generally, most, if not all, DOCSIS 3.1 modems can function as 32×8 DOCSIS 3.0 ones. But it’s ultimately the Internet provider that decides which modem works and at what speed.

(DOCSIS 3.1 includes other benefits, but they are generally irrelevant from the consumers’ end.)

Real-world cable (download) speeds

No matter how fast a cable modem’s advertised speed is, its actual ceiling speed is always its LAN port. This is the port you connect to a router’s WAN port.

See also  Internet or Wi-Fi Speed Test: How You Can Figure Out the Correct Numbers

That said, all cable modems with a Gigabit LAN port will cap at 1Gbps or lower.

Some modems can deliver Multi-Gig broadband speeds. In this case, they must have a Multi-Gig port (be it 2.5Gbps, 5Gbps, or 10Gbps) or features WAN Link Aggregation where you can combine two 1Gbps ports into a single 2Gbps WAN connection.

And, of course, to enjoy Multi-Gig broadband, you need to have a router supporting similar speed grades on the WAN side. After that, the speed must be available from the provider’s end. And, finally, you’re willing to pay for it.

The point is, there’s no need to get a modem that can deliver a faster speed than you pay for. However, it never hurts to have a top-tier modem if you want to upgrade your broadband later.

DOSIS3.0 vs DOCSIS 3.1: Which to get

In most cases, it doesn’t matter which you get if you have a sub-Gigabit cable Internet plan. However, keep in mind that if your broadband is slower than 300Mbps, it might require a DOCSIS 3.0 modem. So check with your provider.

On the other hand, with a Gig+ and Multi-Gig Internet plan, DOCSIS 3.1 is a must. On top of that, you might want to get a cable that supports Link Aggregation if that’s how your provider delivers Multi-Gig.

Here’s my simple rule to determine which type of modem to get based on your Internet download speed:

  • 500Mbps or slower: Get a DOCSIS 3.0 modem.
  • 500Mbps to sub-Gigabit: Either will do.
  • Gigabit for faster (Gig+, Multi-Gig): DOCSIS 3.1

With that, let’s move on to how to replace that ISP-provided gateway with your own modem and be happier.

How to replace the ISP-Provided gateway with your own modem (and router)

This part is based on an Xfinity Internet plan, but the process is similar if you use any other residential cable Internet service, such as Time Warner Cable, Cox, Charter, and so on.

While this is about replacing an existing gateway, it also applies when you want to upgrade a modem or set up a new service. There are three steps.

A. Identify your current cable Internet’s modem/gateway

If you’re about to replace a provider’s equipment, there are two things to keep in mind.

First, are you currently using a modem or a gateway? Most users have a gateway instead of just a modem. If so, they will need a new Wi-Fi router in addition to a new modem, or they can get a retail gateway. A network needs a router — a gateway combines a modem and a router into a single box.

Second, do you also use phone service with your cable internet provider? If so, you will need a phone-capable modem/gateway, which is typically more expensive.

B. Equipment to get

There are many options for retail modems, routers, and gateways. You’ll need a set of a modem and a router, or a single gateway.

(If you also have a TV plan from your provider, the Internet modem has nothing to do with that. TV generally uses separate hardware.)

AmpliFi Alien
The AmpliFi Alien is a cool and super-user-friendly home Wi-Fi 6 router.

What cable modem to get

Modems are simple devices and tend to work similarly. The only difference between them is the reliability. The problem is to know that you have to use the modem for a long time to find out.


Extra: Dong’s recommended cable modems

So, it’s generally hard to review cable modems — it requires a lot of time. But below are a few I’ve personally had experience with over a (relatively) long period. Obviously, these are not the only options, but any will make a safe choice.

A. Best modem for a sub-Gigabit connection: The Netgear CM600

Netgear CM600 Cable Modem's Rating

8.7 out of 10
Netgear CM600 Cable Modem 4
Performance
9/10
Features and Setup
8/10
Value
9/10

Pros

Fast and reliable performance for sub-Gigabit broadband

Supported by multiple cable providers

Affordable and relatively compact

Clear status lights, useful web interface

Cons

No Gig+ or Multi-Gig support

No WAN Link Aggregation

See also  Netgear CM600 Review: An Excellent Sub-Gigabit Cable Modem

B. Best modem for a Gigabit connection: The Motorola MB8600

Motorola MB8600 DOCSIS 3.1 Cable Modem's Rating

8.5 out of 10
Motorola MB8600 Cable Modem 8
Performance
9/10
Features and Setup
8/10
Value
8.5/10

Pros

Fast and reliable performance

Clear status lights

DOCSIS 3.1

Supported by multiple cable provider s

Optional WAN Link Aggregation and multi-Static-IP support

Cons

No Multi-Gig network port

A bit bulky

See also  Motorola MB8600 Review: The Go-to Gigabit DOCSIS 3.1 Cable Modem

C. Best modem for a Multi-Gig connection: The Netgear CM2000

Netgear Nighthawk Multi-Gig Cable Modem CM2000's Rating

8.2 out of 10
Netgear CM2000 Nighthawk Multi Gig Cable Modem Side
Performance
9/10
Features and Setup
8.5/10
Value
7/10

Pros

Fast and reliable performance, up to 2.5Gbps of broadband speed

Supported by multiple cable providers

Relatively compact with clear status light

Comprehensive web interface

Cons

Expensive

Single LAN port, no WAN Link Aggregation

See also  Netgear CM2000 Nighthawk Review: A Reliable Multi-Gig Cable Modem

Generally, if you have a sub-Gigabit broadband connection, it really doesn’t matter what modem you get — be it a DOCSIS 3.0 or 3.1. Just make sure you get one that delivers the speed you pay for, and the provider supports you.

(Here’s the list of the approved modems and gateways for Comcast Xfinity. Note: You need to enter your home address or zip code or sign in with your account.)

The key here is to get what you need, it doesn’t hurt to get the fastest modem out there, but if you don’t have an internet plan that requires it, you’re just wasting your money.

If you need a phone-capable modem, there are fewer options. In my experience, either the ARRIS Surfboard T25 or the NETGEAR CM500V is an excellent choice.

Keep in mind that a phone-capable modem will work even when you do not have a phone service plan. So get one if you intend to add a phone service later.

By the way, an used or refurbished modem will work the same as a new one. Just make sure you get one that’s in good physical shape. Most importantly, make sure the device has been removed from the old owner’s account if you plan to use it with the same Internet provider. (You can check with the provider using the device’s MAC address.)

Once you’ve settled on a modem, it’s time to pick a router.

What’s the best Wi-Fi router or mesh system for cable Internet?

All routers, or mesh systems, work with any internet source. So if you get a good one, it’ll be good no matter who your provider is and what type of connection you have.

What’s also always true is the fact the router is the one that decides the performance of your home network. So, pick one that fits your needs and budget.

Not sure what you need? This post on how to pick the best router for your situation will help. Living in a large home? Check out this post on mesh Wi-Fi systems.

Or, you can follow the general direction:

Or pick one of the following lists:

Retail gateways are available, too

An ideal setup is a cable modem and a router. However, if you don’t want more than one box at your internet drop, you can get a gateway (router/cable modem combo) of your own.

Keep in mind, though, that if you get a gateway, you’ll have to replace the entire thing when you want to upgrade your network, and not just the modem or the router portion of it.

The ASUS CM-32 Cable Modem Wifi Router can be a great choice if you don’t have phone service. It’s essentially the RT-AC88U plus a built-in cable modem.

On the other hand, if you have phone service, the ARRIS Surfboard SVG2482AC worked well in my experience. Again, a phone-capable gateway will work even when you do not have a phone service plan.

If you live in a large property and need a mesh-ready gateway, Netgear’s Orbi CBK40 or Netgear CBR750 are excellent options. They are available as single gateways or mesh systems. Neither supports phone service, however.

C. How to set up new cable Internet equipment

No matter what you get, be it a simple cable modem, a phone-capable modem, or a gateway, the setup process is the same. You need to activate your equipment on the provider’s network.

These steps, by the way, apply only to when you want to activate a residential Internet plan. If you have a business plan, you’ll likely need to call the provider.

Cable Internet setup: What you need

To activate a piece of new equipment, you need two things.

  • A computer with a network port. If you have one of those laptops that only have Wi-Fi, get an Ethernet dongle for this job. Or, if you have an existing Wi-Fi router, you can use its Wi-Fi network — more on this below.
  • Your cable account information (account number, phone number used when signed up, home address, etc.) or the login information. In my experience with Comcast, the Xfinity username and password are enough.

Steps to replace / install a cable Internet cable modem / gateway

These relatively specific steps apply to those with a Comcast Xfinity Internet plan. If you use a different provider, the process varies though it is likely similar.

Netgear CM600 Cable Modem 14
A typical cable hardware setup: Connect the modem (left) to the service line (white cable) and the router’s WAN port to the modem (black cable).
1. Replace the old hardware

Remove the existing gateway or cable modem. (Make sure you return the equipment afterward to have the rental fee removed from your account.)

2. Connect the new hardware

This is an important part, and I’ll describe it in great detail to include different scenarios. In real life, it just takes a few minutes. Here goes:

a. Connect the service coaxial cable into your new cable modem or gateway securely.

b. Use a network cable and connect your computer to a LAN port of the cable modem (or gateway). If it has more than one port, you can use any of them.

A couple of things to note:

  • Alternatively, if you get a new modem and have an existing router that has already been set up, you can connect the router’s WAN port to the modem, and then your computer to the router via its Wi-Fi or LAN port. Don’t do this if you have a brand-new router that’s not yet initialized. You can’t activate a modem using a router at its default factory settings. That’s because many routers require a live Internet connection (one with an already-activated modem) before you can set it up.
  • Make sure your computer and router (or gateway) have the default (auto) DNS settings, meaning it will use the DNS servers of the Internet service provider. The good news is that’s always the case unless you have changed the DNS manually. Using customized DNS will cause the activation process to fail. So use the default one and you can change that later.

c. Now plug everything into power and wait for the equipment to boot up. Typically, this will take just about one minute, but you can give it a few minutes to make sure.

Motorola MB8600 Cable Modem 9
Note the Internet light on the cable modem (second from bottom). It’s supposed to be on solid (green, blue, or white) when your broadband connection works well.
A bit of troubleshooting

Look at the lights on the cable modem (or gateway). You want to wait till the Online light (sometimes labeled as Signal or Sync or Data) is on solid.

The color of this light varies depending on the modem, but it tends to be white, blue, or green, and it almost always takes the symbol of a little globe. If this light doesn’t turn on, that means there’s no Internet signal. Check to make sure the coaxial cable is plugged in correctly, or call the provider.

3. Activation

Typically, you can now call the provider’s tech support. Give them the MAC address of the modem (or gateway) and get it activated that way. Or you can do that yourself.

Comast Mobile App Suggestion
You can skip Comcast Xfinity’s mobile app coercion by clicking the link at the bottom of the page.

Here are the steps for self-activation:

On your connected computer, launch a browser (such as Firefox or Chrome), you will automatically get to the activation page.

If not, try going to any website, and you’ll automatically reach that page. Or, if you use Comcast, navigate to this page: https://register.be.xfinity.com/activate.

Comcast might show a page suggesting that you download the Xfinity mobile app so you can finish the job on your phone. I’d recommend skip that and continue with the web browser. You’re really close — it’ll be a waste of time to download the app, sign in with it, etc.

In this case, choose to log in with your Comcast account and follow the onscreen instruction shown in the screenshots below.

Comcast Activation
Comcast Activation

If you don’t have an account or don’t remember the password, you can use the account number instead and go through a few other verification steps. After that, the new equipment will then be activated automatically and will restart itself.

Netgear CM600 Cable Modem 6
You can find the MAC address on the back or bottom of a cable modem. It’s a string of numbers and letters following the word MAC.

The final step

And that’s it. Now your new equipment is ready. If you just activated a cable modem and have a new router, you can now plug the router’s WAN port into the modem’s LAN port and follow this guide to set up your home network.

Using your own modem, you have the option of replacing the router — and upgrade your home network — whenever you’d like. And when you do, note that you should restart the cable modem after you plug the new router into it for the two to work well with each other.

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38 thoughts on “Cable Modem Explained: How to Swap an ISP-Provided Gateway with One”

  1. Good information. Thank you!. However, owning your own modem isn’t perfect: My ISP refuses to update the firmware on my cable modem or do any line debugging because they don’t own the modem (even when it’s identical to their rental modem). How do you handle this situation, or is a regulatory loophole that we simply have to live with?

    Reply
    • You can update the firmware yourself, Joe, via the modems interface. Most of the time, you won’t need to do that at all.

      Reply
      • I have to disagree. For combined modem/router devices, you may be able to update the router portion of the firmware, but NOT for the modem section. And the user cannot update any firmware at all for cable modems alone. This process was set many years ago to prevent people from stealing internet service. Unfortunately, it also gives the ISP full control of your device and the reason to deny line debugging if they chose.

        Reply
        • Interesting. I actually didn’t know that. But generally, folks get a modem to get connected, not to debug or update the firmware. If that’s something really important to you, I’d recommend getting the ISP-supplied hardware. There are other benefits, too, as mentioned in this post.

          Reply
  2. Hi Dong,
    I understand that we have more choices and control if we provide our own hardware instead of renting from Comcast. But I’m wondering if you know what the comparable performance is with their recent Xfi XB7 gateway for gigabit service. Do you have a sense of how much we would have to spend to get a gateway or modem/router combo that would perform better?

    I like your sentiment of not over-paying now for “future-proofing”, so would that make the argument to lease from Comcast and get a new gateway from them every 2 years or so?

    Thanks for helping us all!

    Reply
    • Any of the modems I recommended in this post will outdo the XB7 in broadband speed, Mike. After that, you can get any router or mesh system of your choice. Links in the post, too.

      Reply
  3. Dong Ngo,

    My router is currently located in my living room next to my modem and I’m trying to extend my network wirelessly to a back bedroom so I can set up my home office. I know I should replace my router anyway because it’s most likely rented. However, I need to be able to connect my work phone to a router via ethernet cable. So I’m trying to find a router/mesh system that I can connect an access point (With an ethernet port) in the back bedroom without having to run an ethernet cable from the router location in living room under the house (Older ranch with a crawl space) that will allow my phone to work effortlessly. My work phone system utilizes a phone answering software program on our computers so I have to be connected thru an ethernet cable connection. TYIA

    Reply
  4. Hi Dong, I love your thorough and informative postings and reviews.
    I now understand the cable modem situation.
    I have Verizon FIOS Giga service (modem). Is there any alternative modem/router for people with that service?

    Thanks, Reuven

    Reply
  5. Something your readers may want to consider, when looking replacing the Xfinity gateway with their own equipment. To make a long story short, some high bandwidth customers could end up paying more.

    Unlimited internet is $50/ mo
    If a customer has 300mb/s or faster connection they can get xfi advantage which includes XB6 or XB7 gateway AND unlimited data for only $25/mo. A savings of $25/ MO. At $300/yr savings one can justify picking up vert high high end router/mesh system.

    Reply
    • You forget the rental fee, Sky. And you don’t need unlimited, you can just pay for the extra amount when/if you go over the limit. The data cap is a scam anyway.

      Reply
    • The prices have changed and the differences are less. For those who need unlimited Internet the cost is now only $30 a month for unlimited and leasing a modem at $14 a month and unlimited is $11 a month for For a total of$25 it is called Xfi complete. The difference is now less. But the cost does end up as being $5 more per month if unlimited is necessary For the privilege of buying your own equipment. Unfortunately turning the cost savings of owning your own equipment upside down. But there is clearly some benefits to owning your own equipment, Namely performance and control.

      Unfortunately wether you believe unlimited is a scam or not is irrelevant for this part of the conversation. Could be a fruitful conversation otherwise.

      Reply
  6. Hi, I have Arris TG3482G provided to me by Xfinity but can’t change the DNS settings which I would like to do. I bundle cable, internet & telephone service with Xfinity so if I’m understanding currently I need to buy both a router and modem? Or do I only need a gateway device? (Sorry, I have very limited knowledge on this stuff)

    Reply
  7. Hey Dong! I have the Motorola (Arris) SBG6782-AC Combo Modem/Router with built-in MoCA back in 2014, based on your excellent recommendation. Is there an updated version of this now? Complete with the built-in MoCA (which I love)?? Thank you!

    Reply
    • That one should still work fine, Thomas. I think eventually, there will be a Wi-Fi 6 version, but right now, there’s no update yet, as far as I know.

      Reply
  8. I am moving into a 600sq ft apartment soon. I will have 300mbps internet speed and want to use a wired connection for my gaming console and desktop computer. Streaming music and video will be most of the wifi load. I’m planning to purchase a 600mbps modem but when searching for a router I want something with high speeds but a large coverage area isn’t necessary. Do you have any recommendations?

    Reply
  9. You’re correct, during the setup, the gateway, or router, need to use the DNS of Comcast, which is the default, unless you have changed it. In this case you need to change it back for the setup.

    Reply
  10. I replaced my leaxed Xfinntiy Arris gatewary router with amy own Motorola MB8600 modem. Eliminated the double NAT problem my network was having with the Arris as well as giving me download speeds somewhat in excess fo my plan’a 300 MBS (the Arris was givinng less than 100 MBS).

    As far as self-activation goes, it worked when I went to Comcast activation via the Motorola site but not if I went directly to Comcast. I had no problem with having two home locations on my account. I signed in with my id and password and was given the choice of which location I was doing the activation from. I did have to change my DNS settings to “automatic”.from the dedicated Goolgle DNS server addresses I had been using.. changed back after activation.

    Reply
  11. Hey Dong,
    I’ve always been partial to the Arris cable modems. What is your reasoning for recommending the TP-Link? Better reliability or just personal brand preference?

    Reply
    • It was just the cheapest at the time I wrote the piece. I also had used many of them all were reliable. Modems are relatively simple devices and they are very similar. And of them will do. Just pick one that supports the speeds of your plan. 🙂

      Reply
  12. I guess this doesn’t apply to FIOS. I’ve got an ethernet cable coming right out of the FIOS unit right into my N66U. I do need to piggyback off the FIOS router for Video On Demand and Guides to work, but hey it gives me three more ports!

    Reply
    • No, the post applies to cable only. However, I think you can get your own FIOS box, too, and replace the vendor’s box with it. I haven’t tried that, however, since we don’t have that option where we live.

      Reply
  13. Also, depending on which region of the country you are in, if you supply your own modem, you may be eligible for a $2.50 “customer supplied equipment” credit per month. So net savings of $12.50/mo adds up quick.

    Reply
    • If by service you mean the Wi-Fi, then you should just add an access point. But it’s better if you replace the Comcast box with your old modem and router.

      Reply
  14. Dong Ngo,

    I have the Linksis CM-8 cable modem. I have it connected to my Asus RT-AC3100 and RT-AC1900P connected via AiMesh network.

    Thanks

    Reply

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