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ARRIS Group, Inc > 
Item#: 9SIA9B53SP0561

ARRIS SurfBoard SB6121 DOCSIS 3.0 Cable Modem

  • Compatible with Time Warner, Comcast, Charter, Cablevision, and others
  • Gigabit Ethernet connectivity
  • DOCSIS 3.0-certified (EuroDOCSIS 3.0-based)

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  • Overview
  • Specifications
  • Warranty & Returns
  • Reviews


SB6121 SURFboard Cable Modem RJ-45

Building upon the success of the SB6120, The ARRIS SURFboard SB6121 delivers your complete personal media experience, at lightning-fast broadband speed. It harnesses the power of DOCSIS 3.0 technology to bond up to four downstream channels and four upstream channels - providing you advanced multimedia services with data rates of more than 100Mbps in each direction. That makes gaming, shopping, downloading, working, high-quality voice and video conferencing, and peer-to-peer networking applications far more efficient than ever before.



SB6121 Easy to Set Up and Use
  • Plug-and-play installation
  • Front panel LEDs indicate status and simplify troubleshooting
  • Multi-language user guides
  • Supports standard internet software
  • User-friendly online diagnostics

  • Stylish and space-saving enclosure
  • Offers innovative, high-bandwidth data and multimedia services to customers
  • Backward-compatible to DOCSIS / EuroDOCSIS 1.x and 2.0

Advanced Services Ready
  • DOCSIS 3.0-certified (EuroDOCSIS 3.0-based)
  • Channel bonding of up to four downstream and four upstream channels
  • 1GHz capable tuners
  • Supports IPv4 and IPv6 to expand network addressing capabilities
Versatile and Convenient
  • Compatible with Windows, Macintosh, and UNIX computers.
  • Gigabit Ethernet (RJ-45) data port enable flexible, high-speed connectivity with Auto Negotiate and Auto MDIX

Reliable and Secure
  • Enhanced security: supports Advanced Encryption Services (AES) traffic encryption
  • Remotely configurable and monitorable using SNMP and TFTP

As part of the ARRIS DOCSIS 3.0 Ultra-Broadband family of products, the SB6121 includes an enhanced tuner that supports up to a 1GHz downstream input, which allows operators to increase the frequency spectrum for deployment of new high-value service - such as bandwidth on-demand, commercial services, interactive gaming, and IPTV. The SB6121 features a 10/100/1000 Base-T Ethernet (RJ-45) port, as well as intuitive, easy-to-read front-panel operational status LEDs.

What Is DOCSIS 3.0?

With the DOCSIS 3.0 specification, cable Internet providers can take advantage of the extra capacity provided by channel-bonding technology in order to offer ultra-fast broadband services. Channel bonding allows parallel data pipes to act as a single broadband conduit. With each channel capable of providing around 40Mbps of data, the ARRIS SURFboard SB6121's four downstream channels can provide a theoretical maximum of 160Mbps of throughput.

Currently, cable providers max out at around 50Mbps of downstream data throughput, but DOCSIS 3.0 will enable cable Internet providers to continue to improve the speed of their networks.

System Requirements
The SB6121 is compatible with the following operating systems:
  • Windows 7
  • Windows Vista, Service Pack 1 or later
  • Windows XP, Service Pack 2 or later
  • Mac 10.4 or later (Ethernet connection only)
  • UNIX (Ethernet connection only)
  • Linux (Ethernet connection only)

What's in the Box

ARRIS SURFboard SB6121, Ethernet cable, Installation sheet


Learn more about the ARRIS Group, Inc SB6121




Cable Modem
1 x RJ-45, Gigabit Ethernet
Cable Interface
75ohm F-connector
DOCSIS 3.0, 2.0 and 1.1, Euro-DOCSIS 3.0
Max Transmission Rate
Up To 172 Mbps Down and 131 Mbps Up


Data Protocol: TCP/IP

Power: 9W (nominal)

Input Power: 105 to 125 VAC, 60 Hz

-Modulation: 64 or 256 QAM

-Capture Bandwidth: 100 MHz (edge to edge)

-Maximum Theoretical Data Rate:
DOCSIS: 171.537 Mbps (4 channels) / 42.884 (single channel)@ 256 QAM at 5.36 Msym/s
EuroDOCSIS: 222.464 Mbps (4 channels) / 55.616 (single channel)@ 256 QAM at 6.952 Msym/s.

DOCSIS: = 24 MHz
EuroDOCSIS: = 32 MHz.

-Symbol Rate:
DOCSIS: 64 QAM 5.057 Msym/s; 256 QAM 5.361 Msym/s

EuroDOCSIS: 64 QAM 6.952 Msym/s; 256 QAM 6.952 Msym/s.

-Operating Level Range: –15 to 15 dBmV

-Security: DOCSIS 3.0 Security (BPI+, EAE, SSD)

-Network Management: SNMP v2 & v3

-Provisioning Supports: IP addressing using IPv4 and/or IPv6 (dual stack).

-Modulation: QPSK and 8, 16, 32, 64, 128 QAM

-Maximum Channel Rate
DOCSIS 131.072 Mbps (4 channels) / 32.768 Mbps (single channel): @ 128 QAM at 6.4 MHz
EuroDOCSIS 131.072 Mbps (4 channels) / 32.768 Mbps (single channel): @ 128 QAM at 6.4 MHz.

-Channel Width 200 kHz, 400 kHz, 800 kHz, 1.6 MHz, 3.2 MHz, 6.4 MHz

-Symbol Rates: 160, 320, 640, 1280, 2560, 5120 ksym/s

-Frequency Range: DOCSIS 5-42 MHz (edge to edge), EuroDOCSIS and optional DOCSIS 5 to 65 MHz (edge to edge)

Compatibility PC: 90496, Pentium, or later; Windows Vista, 2000, XP or 7 or Linux with Ethernet connection (older versions of Windows, although not specifically supported, will work with this cable modem).
Macintosh: Power PC or later; OS 9 or higher, Ethernet connection.
UNIX: Ethernet connection.
Home Networking: Ethernet router or wireless access point.


Package Contents
Ethernet Cable
Power Supply
Software License & Regulatory Card
Product Guide (Install Sheet)
Warranty, Returns, And Additional Information
  • Warranty
  • Limited Warranty period (parts): 2 years
  • Limited Warranty period (labor): 2 years
  • Read full details

Customer Reviews of the ARRIS Group, Inc SB6121

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  • Vance M.
  • 5/24/2016 9:38:50 PM

4 out of 5 eggsGreat if you are on 100Mbps Connection, Bad if your on 140+.

Pros: I was getting 90Mbps on my 75Mbps connection

Cons: When I upgraded to a 200Mbps connection, I only saw 133Mbps tops on this modem model. I tried 3 different ones.

Other Thoughts: DOCSIS 3.0 requires a minimum of 4 - 38Mb streams. This model cable modem is the minimum. I will shop for a modem with 5-6 streams or more since DOCSIS 3.0 has no maximum number of streams.

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4 out of 5 eggsNo longer supported

Pros: Was a good modem,had no problems until Comcast decided not to support it

Cons: Purchased it last June (2016) and now Comcast will no longer support this modem says it is at End of Life.

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  • Michael D.
  • 5/6/2016 11:11:28 AM
  • Ownership: more than 1 year
  • Verified Owner

3 out of 5 eggsFine while it lasted

Pros: It's a DOCSIS 3.0 modem. It worked fine for about two years (with Cox) and got good download speeds.

Cons: After about two years, it started dropping its connection until rebooted, about every month or so.
After just over three years, it died. It still powers on, but won't connect.

Other Thoughts: Unfortunately, I haven't yet found a cable modem with a warranty longer than two years, and apparently that's for a reason.

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  • Steven H.
  • 4/18/2016 9:15:55 PM
  • Ownership: more than 1 year
  • Verified Owner

2 out of 5 eggsGetting massive packet loss after 1/1.25 years

Pros: Was great while it worked.

Cons: Very unreliable after about a year of use. Try proving packet loss for an RMA though... >.> Getting sometime 5 Mbps, sometimes 70+ Mbps and on average about 30% packet loss even directly connected and troubleshooting with the ISP

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  • Daniel D.
  • 3/8/2016 1:50:15 AM
  • Ownership: more than 1 year
  • Verified Owner

5 out of 5 eggsWhat's not to like?

Pros: all the time...
-Simple and sleek design.

Cons: -Non

Other Thoughts: -Would recommend this to any one who subscribes to cable based internet service.
-Would buy this again.

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  • Danielius U.
  • 2/19/2016 9:01:37 AM
  • Ownership: 1 month to 1 year
  • Verified Owner

5 out of 5 eggsWorks Great

Pros: Fast internet and it works with the router i bought

Cons: set up is always a drag but that's not the product's fault

Other Thoughts: Got this with TP-LINK Archer C5 AC1200 Dual Band Wireless AC Gigabit Router and worked without a hiccup

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  • Zach J.
  • 2/7/2016 9:24:21 AM
  • Ownership: more than 1 year
  • Verified Owner

1 out of 5 eggsWorked well for 2 years.

Pros: - Worked well for my 100Mbps service. Solid downstream and upstream.

Cons: - Modem started failing after the 2 year warranty but less than 3 years of use. Modem will stop responding every day or so, and for the past few days has needed to be restarted every few hours. Support line couldn't do anything about the hardware since my warranty is up. I wish the problem would go away.

Other Thoughts: - Too many connections can overwhelm the modem. I'm talking about hundreds of connections during heavy P2P. Perhaps my expectations are too high.

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  • Matthew F.
  • 2/6/2016 10:27:31 PM
  • Ownership: more than 1 year
  • Verified Owner

2 out of 5 eggs

Pros: Worked well while it lasted.

Cons: Died after using it for 2 years 3 months.

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  • Darrell T.
  • 1/2/2016 8:19:44 AM
  • Ownership: more than 1 year
  • Verified Owner

1 out of 5 eggsConfusion about support on comcast

Pros: Worked fine for some time then...................

Cons: About September I started dropping service and having to reboot several time a day. Comcast still lists this device as compatible with business starter speed but if you look up "End of Life Devices" the Arris 6121 is on the list. When you click on the link there it shows this device as no longer supported as of September 2015. (It seems to make a difference if it s a "retail device" or not.

Other Thoughts: I would not recommend buying this device. I would recommend going to your providers website to find a unit that meets speeds higher than what you think you will ever need.

0 out of 1 people found this review helpful. Did you? Yes No

  • Steven K.
  • 12/27/2015 11:03:57 PM
  • Ownership: more than 1 year
  • Verified Owner

5 out of 5 eggsOne of the Best Cable Modems on the Market

Pros: Myself and friends have each been using SB6121 modems for years. Each one has worked flawlessly. The SB6121 hands down has been the best cable modem available for years, and the most recommended by Comcast technicians. This is the highest rated modem that Comcast has a full certification on. It is DOCSIS 3.0 and supports Internet download speeds up to 172 Mbps. When one friend replaced his older SurfBoard modem with this one he gained about 1.5 Mbps of download speed on Speedtest.

Cons: I only have one complaint, and for the majority of people this will probably never happen for them. I have a device that is not-Gigabit speed rated that is plugged into the SB6121. When you have a non-Gigabit device plugged into the SB6121, the bottom indicator light (Ethernet port connection indicator light) blinks orange instead of being solid blue. The blinking is annoying to me, because the modem is on my desk. Again, most people probably would not have a non-Gigabit device plugged into the the SB6121, so this is not something that would occur for most people.

Other Thoughts: Stop paying outrageous cable modem rental fees to ISPs. For what most ISPs charge to rent a cable modem, you could own this modem in just half a year's time.

The SurfBoard SB6141 is the newer model above the SB6121 and is rated for download speeds up to 300 Mbps. The SB6141 is one star certified by Comcast, where the SB6121 has the maximum three star certification. This means Comcast has fully tested the SB6121, and it passed all three certification levels. If you are going to have Internet service below 172 Mbps, the SB6121 is the safer bet to go with if you are concerned about any compatibility or use issues. You can find out more details about the certification levels on Comcast's website.

I recommend using a modem over a gateway device. A gateway is just a modem and router bundled into one device. It's better to use a separate modem and separate router. Gateways have a higher failure rate than modems. Also, having a separate modem and router offers more flexibility in system setup and troubleshooting.

I always recommend using a firewall, whether it's in a home or office setting. With so much personal and financial information on your computer and transferred online, it's worth the extra security. You should also always use a full Internet security suite, not just an anti-virus program. And yes, you can use software firewalls, like those included with Internet security suites, and a hardware firewall together. It's just another layer of security and protection.

System Information:
Intel i7-3770K
Zalman CNPS9900MAX
Arctic Silver 5
EVGA GeForce GTX 670 SC 4GB
4 x 8GB G.Skill Ripjaws Z DDR3 1600
Corsair Force GS 240GB SATA3 SSD for SSD Caching
2 x Seagate Constellation ES.3 2TB SATA3 HDD in RAID 1
2 x ASUS BD-RE Burner
Corsair AX1200i 1200W PSU
Cooler Master HAF X Full Tower Case
Windows 7 Ultimate 64-Bit
4 x ASUS VS247H-P 23.6" LED Backlight Monitor

2 out of 2 people found this review helpful. Did you? Yes No

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Item#: 9SIA9B53SP0561
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