Time Warner Cable didn't take the steps to properly shield its boxes and/or cable system, it appears

Time Warner Cable Inc. (TWC) -- a company well known for its campaign to effectively outlaw municipal Wi-Fi offerings in North Carolina by paying off state politicians -- already has a reputation for controversy.  Now it's at the center of a very different problem as its Raleigh, North Carolina regional customers have been seeing signal interference rear its ugly head.  The issue seems to be triggered by the use of LTE antennas on Verizon Wireless (now a wholly owned subsidiary of Verizon Communications Inc. (VZ)) smartphones and tablets.
I. Interference Woes
Verizon is currently operating on the lower C-block in the region, which essentially occupies the space analog channel 54 used to occupy.  The first major carrier to deploy 4G LTE across the U.S., Verizon has the licenses that it bought at auction from the U.S. Federal Communications Commission (FCC).
Time Warner Cable carries the WRAL on a QAM (quadrature amplitude modulation) channel, 121-2.  It carries the HD signal of WRAZ Fox 50 on channel 121-5.  The Electronic Industries Association's (EIA) 542-B/-C/-D standards ('D' being the most recent) have defined what frequencies various digital cable channels have operated at.  For channel 121 video is carried on defined to be 775.25 MHz, QAM is at 777.00 MHz, and audio is carried at 779.75 MHz.

700 MHz block
A map of the 700 MHz upper and lower bands [Image Source: FCC]

In the U.S., both Verizon and AT&T, Inc. (T) have made use of the upper and lower parts of the 700 MHz block for their initial LTE efforts.  Spectrum is allocated on a regional basis via licenses purchased from the FCC.  Verizon currently occupies the two "C Block" chunks of the upper 700 MHz spectrum region.  These blocks can be found at 746–757 MHz and 776–787 MHz.
Clearly the second C-block (776-787 MHz) interferes with the signals used by channel 121.  And while it hasn't been reported it wouldn't be surprising to see interference on other channels that overlap the first Verizon C block (746-757 MHz).  Channels in this vulnerable region include 117-x channels (751 MHz; Univision, QVC, etc.) and 118-x (757 MHz; News 14) [source].  

It's the network
It's the network! -- Verizon Wireless

The onus here is not Time Warner Cable, as it is using the cable without license.  It is not illegal to use conducted signals (e.g. cable television signals routed to customer boxes) without a license -- in fact most of the cable TV industry does it.  Issues can be avoided if you properly shield your cabling and electronics to prevent it from not only being interfered with, but from interfering with licensed mobile signals.  But by the sound of it Time Warner Cable failed somewhere in the shielding department, and now its customers are paying the price.

II. TV Channels and the 700 MHz Block -- a Danger Zone

Generally assigning channels on 109-125 is a danger zone of sorts, giving the widespread use of the 700 MHz block by mobile carriers.  In most cases it is feasible to simply shift channels elsewhere.  In the long term Capitol Broadcasting Comp., Inc. -- the owner of WRAL and WRAZ Fox -- plans to shift its signals off the block, according to Innovation and Policy VP Sam Matheny.  But he warns, "It's going to take time."

Danger Zone
Using 700 MHz-region channels is a danger zone for cable -- and not in a good way.
[Image Source: FX]

In a blog post, he writes, "This is not just a Time Warner Cable issue, but is an issue for the entire cable industry."

Time Warner Cable has apologized to customers.  In a statement posted by WRAL, it writes, "We apologize for the inconvenience and are working on a solution that will resolve this problem definitively in the coming weeks."

Time Warner Cable boxesIn some cases replacing plastic-laden cable boxes with a better shield design that uses more metal can fix the problem.  Time Warner Cable has been using this approach in some cases. [Image Source: Bloomberg]

For now the cable provider has suggested customers take their phone in another room to prevent "hyper-local interference" with their cable box.  They say putting the phone in airplane mode or turning off 4G data may also stop the issue.
In a follow-up statement to Ars Technica, a company spokesperson states:

It's limited to the Raleigh market and WRAL...we expect to have this resolved in the next several weeks, we need to do some additional work to make sure the frequency to which we're moving WRAL will be visible to all customers. Obviously the change won't require any action for most customers, who watch WRAL via a set-top box or digital adapter. We haven't found the issue specific to any box or class of boxes.

Time Warner has reportedly been replacing some customers’ cable boxes.  
III. Undershielded Coaxial Cables May Also Carry Interference
While that approach may fix interference in some localized cases, primarily where customers have plastic boxes that lack significant metal shielding, it may not work for everyone.  Many cable box designs, including some that Time Warner Cable uses incorporate metal into the frame, creating a relatively strong shield against interference.  
For that reason many experts have speculated in some cases (particularly those where box replacement doesn't fix the problem) the interfering signals are actually leaking into poorly shielded coaxial cables that travel to the box.
 Coaxial cable -- premium
Premium coaxial cable is protected by an outer jacket, one or more braided shields, one or more foil shields, and a dielectric layer.  Cable companies often skimp on premium shielding as a cost savings, but those decisions have increasingly led to issues in the LTE era. [Image Source: Radio Range Extender]

That could explain why some users have been reporting identical interference issues that are anything but hyperlocalized.  

LTE tower
A nearby base-station could spill signal into undershielded coaxial cables. [Image Source: Wind River blog]

A customer by the username "F100" writes in a cable forum:

It's not just a cable box issue. I have had the problem also but I am using a tuner card and an HD Home Run network tuner to record in media center via clear QAM. No cable box. All of my coax is properly shielded and grounded. Distribution coax lines run underground in my neighborhood. Don't laugh but I still only have an old Verizon flip phone that I use for CDMA voice only. That is 800Mhz/1900Mhx bands I think. We don't have any LTE phones yet. My neighbors do though. WRAL shows CBS programming and I noticed it while watching a recording of "How I Met Your Mother."

I comes across as breakup from a weak signal. TWC has WRAL on QAM cable channel 121-2 as shown by the HD home run tuner setup. They are getting ingress into the plant somewhere from Verizon's 700mhz frequencies when they are trying to use the same frequency range in a closed plant system. I am getting it too so it's not just a cable box issue. It's further up in the coax plant.

Here's a video of the problem in action.

The problems appear isolated to Verizon and have thus far not affected AT&T subscribers, likely due to the fact that the Blocks AT&T operates on don't overlap local channels.

IV. No Interference Issues in Sight for AT&T in the Region

AT&T currently operates on the lower C Block -- at 710-716 MHz and 734-740 MHz.  It's currently in the process of reinforcing these by leasing licenses to the lower B Blocks of spectrum from Grain Management, LLC a Sarasota, Florida-based private equity firm that invests in the telecommunications sector.  Grain bought these blocks from Verizon Wireless, who had been forced to sell them off under the terms dictated by the FCC.

This sale and other spectrum sales allowed Verizon to wrap up a $3.9B USD purchase of Time Warner Cable, Comcast Corp. (CMCSA), Cox Communications, Inc., and Bright House Networks (an early $3.6B version of the deal involves Comcast getting $2.3B USD, TWC pocketing $1.1B USD, and Bright House earning $189M USD).  Verizon Wireless was willing to sell off the 700 MHz A and B blocks to placate the FCC, as it was primarily hunting for the cable companies' holdings in the so-called "Advanced Wireless Spectrum" (AWS) band (1700/2100 MHz).  

Verizon is working to transition its network off the 700 MHz band and onto chunks of the faster 1700 MHz band, which will allow it to deploy new services like voice-over-LTE (VoLTE).

The FCC tied a license swap between Deutsche Telekom AG (ETR:DTE) subsidiary T-Mobile USA with Verizon Wireless to the successful completion of the sale, further incentivizing Verizon Wireless to sell off the 700 MHz assets it was sitting on.  The T-Mobile deal involved a mutual swap of spectrum in markets that each carrier was week, allowing both to beef up their LTE offerings.  That deal gave T-Mobile licenses to cover 15 of the top 25 markets, home to about 60 million Americans.  Verizon received licenses to cover about 22 million Americans, plus an undisclosed sum of cash.

AT&T customers have experienced much less interference, as the carrier's spectrum blocks don't overlap Time Warner Cable's channels.  [Image Source: BGR]

Ultimately the deal was a boon not only to T-Mobile -- the nation's fastest growing carrier (or uncarrier?) -- it also benefited AT&T in some markets (such as the Raleigh one).

The Grain lease will give AT&T two effective 10 MHz bands (the actual band size is 12 MHz, but padding reduces it to 10 MHz).  These merged bands 704-716 MHz and 734-746 MHz overlap or closely neighbor channels 109-x through 111-x and 114-x through 116-x, respectively.  Fortunately, Time Warner does not appear to be making use of these channels.

That means not only should AT&T customers experience less interference in the region now, that trend should continue even after AT&T broadens its signals and pumps up speeds with leased spectrum in the region.  Of course AT&T customers may experience interference still on their cable boxes, if they have some pesky Verizon-subscribing neighbors or a Verizon basestation nearby.

Sources: CBC View, WRAL, ArsTechnica, Broadband DSL Forums

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