Inside that white shell

So we now know there's relatively little to be learned about the device on the software side. What about the hardware itself? Again, our semiconductor-possessed sensibilities wouldn't be happy unless we got to see what's at this thing's core. I was eager to tear open the box and void some warranties like nobody's business, until I discovered something while doing my homework online that made me stop. Several users on the AT&T Forums reported that they were called by tech support because their MicroCell's tamper switch had been triggered. You read that right, the MicroCell has a built in physical tamper switch. They're that serious about you not getting into this thing. Rather than render the box useless before I even got to use it, I decided on another route.

Instead, I got the FCC ID off the bottom of the box (it's MXF-3GFP980217) and found the FCC filing online. Luckily, the internal photos of the device are freely available and of marginally passable quality. The most important photo is this one, of the board's topside with the heatsink removed:

That's some horrendous barrel distortion, FCC cameraman
 
I've tried locating the physical tamper switch and can't quite find it, though I'm certain it's there somewhere. Starting from the left, we can see the GPS chipset (RoyalTek) and the external connector just above it. There's also a trace to the Cirocomm internal GPS antenna which is the white square protruding from the board at far left. Running along the top of the board are the two ethernet connectors, reset pin, and DC power in. On the left we can also see two relatively thick traces for the antenna, likely Rx and Tx for the air interface. Interestingly, there are some connector headers also visible at the base of where those traces emerge from the amplifiers. Just south of the GPS RoyalTek chip is a flash TSOP, a Ralink RT2150F chip, and some DRAM up top. Searching for that particular Ralink chip revealed nothing. Given that Ralink traditionally makes wireless access point SoCs and wireless adapters, it's possible this is the device's radio. It's also possible that this is handling routing and ethernet for that "priority mode" configuration mentioned earlier.
 
 
Moving right, there's something we don't see every day - except in the domain of high speed signal processing - a Xilinx Spartan 3A FPGA sitting right alongside the platform's SoC. If you squint, you can make out picoChip under that thermal paste, along with 302. It's highly likely this board is using the picoChip PC302 SoC. Remember that 4 simultaneous phone limitation earlier? The PC302 is the only 3GPP Release 7 WCDMA (HSPA) SoC baseband picoChip makes with that implicit restriction, so it's very likely this is what we're seeing here. There's some RAM alongside at right, but it's impossibly hard to make out the markings. 
 
 
The PC302 is the core of the MicroCell, incorporating a 400 MHz ARM11 processor as well as hardware accelerator support for IPsec, ethernet, and everything you need for a femtocell. Unsurprisingly, picoChip calls this a WCDMA Femto Access Point (FAP) supporting up to 4 users, and 21 Mbps HSDPA, 5.7 Mbps HSUPA. Keep in mind here that although the controller supports a physical layer data rate at those speeds, the air interface, protocol, and implementation limit the speeds further. We'll see that in practice, speeds are lower.
Inside The MicroCell: Networking Inside The MicroCell: Security
POST A COMMENT

62 Comments

View All Comments

  • MyTechLife2 - Tuesday, April 06, 2010 - link

    Thanks for the in-depth review! I got my MicroCell yesterday and have been testing it thoroughly. I've also experienced problems on the opposite side of the house from the MicroCell with deteriorating call quality, dropped calls during hand-off and even after hand-off. I call 611 so my test calls are free (I didn't get a MicroCell calling plan). There's an extensive thread on the AT&T forums about dropped calls http://forums.wireless.att.com/t5/AT-T-3G-MicroCel...

    A few initial observations:

    • It runs on 1900Mhz, even though AT&T also has 850Mhz spectrum here. I use the SignalLoc BlackBerry app to check this and other call stats.

    • In my 2-story 2100 ft. house, the MIcroCell is located on the northernmost upstairs end of the house. Sometimes on the southernmost downstairs end of the house it will often switch from the MicroCell to the cell network. Locking my phone to 3G only seems to have minimized that.

    • I had a situation where I was on the MicroCell, but it was showing "SOS only". Putting the MicroCell in the DMZ on the router and power-cycling the MicroCell seems to have fixed that for now.

    • 2 of our 3 phones had to be power-cycled before they would recognize and switch over to using the MicroCell.

    • In the far side of my house while while standing still, signal strength will fluctuate from -110 (worst) to -70 (best) on my BlackBerry Bold 9700. About every 1/2 second It fluctuates down about 1 bar until it reaches 1 bar, then right back up to 5 bars. Seems like some power shifting going on.

    • The 3G antenna seems to be somewhat directional. I seem to get the best coverage on the far end of the house when the front of the device is pointed in that direction. I tried laying the unit horizontally with the top of it facing the far end of the house and that killed signal strength there. I was hoping to get some down-tilt in the antenna pattern to reach the far end downstairs, but that wasn't the way to do it.

    Did you experiment with placement of the device?
    Reply
  • mobius147 - Sunday, April 25, 2010 - link

    My unit covers one or two rooms at the most. In attempting to purchase additional units in our area ATT will refuse to sell more than one unit per household. Reason given is that the units will not
    handoff to one another. Still looking for a way to cover more of the house.
    Does anyone have information on the units mentioned in the review that are used in the
    Apple stores.
    My uplink data rate seems capped at 50 Kb or so and downlink about 1.5 Mb. Adequate for phone
    but "light" for data compared to a standard cell site.
    Im sure more folks have come up against the limited coverage. Interested to hear possible
    remedies.
    Reply
  • Raajah - Saturday, December 11, 2010 - link

    Wonder if anyone has noted the fact that Microcell looses the wireless connection each time a phone call is received or made from a landline conneted. To be sure, I need to mention that we have dsl from Bellsouth as well as the land line from the same company. There seems to be a conflict that occurs probably because voice calls are carried on the same line connected to dsl service. In this situation it is not worth buying the Microcell. Reply
  • SonarTech - Monday, December 20, 2010 - link

    There are actually two tamper traps inside the AT&T Microcell - one on the front of the PCB, and once on the back. The FCC pictures don't show the full tamper mechanism installed on the board, only the empty headers. The top-side tamper trap is located to the upper-right of the Ralink chip, U37, and consists of three jumper headers sitting side-by-side. During assembly, a gray plastic frame sits over this header. After that frame has been installed, three jumpers are installed on top of the frame. One or two of those jumpers aren't actually jumpers, however - they're just chunks of plastic that looks like jumpers. When the case is installed, two long plastic hooks built into the sides of the case "lock" into the sides of that gray plastic frame. Once locked in place, there's no easy way to remove them. If you attempt to open the case, you'll pull either the front-side or back-side tamper jumpers off the headers and you won't know what order to replace them. The odds of you getting them back in the same order they came off is very low. Once the unit is powered up, the jumpers get checked. If the pattern doesn't match what the unit expects, it goes into a tamper lock-down which, as far as I can tell, is a permanent condition requiring factory JTAG to clear.

    The rear (bottom) tamper header is located about 3 inches away from the top-side tamper header, near the unpopulated pads of D25. Again, it's a set of three 2-pin jumpers sitting side-by-side. It's also the only jumpers on the bottom-side of the board.

    Although very tedious, it IS possible to ascertain the correct positioning of the top-side jumpers (and blanks) with the case installed, but you have to know exactly where to look and what to look for.

    The bottom-side jumpers, however, are nearly impossible to see through the installed case, and I haven't yet found an easy way to record their values before opening the case.

    If you have no intentions of returning the unit for warranty service, the easiest way to get inside this case without bricking the unit would be to cut the case open, taking extra care around the clips that lock into the gray plastic tamper traps.

    Besides - you'll do significant (and very obvious) damage to the case just trying to get it open. You'll break at least 4 clips (that are designed to break, by the way) and you'll likely distort the plastic so it won't go together the right way again anyway.

    If you need nice hi-res pictures of the board, top and bottom, I have them - with the tamper traps installed.

    SonarTech
    Reply
  • SonarTech - Monday, December 20, 2010 - link

    One last update: The top-side tamper header is screened as J15, and the bottom-side header is J16. Both have traces that feed right into the Ralink RT2150F chip (U37). Reply
  • ivioo - Monday, April 11, 2011 - link

    I was wondering if you can show me where the tamper jumpers go. I'm pretty sure I know where 1 jumper goes but I have 2 jumpers left that I don't know where they go and then the 2 dummy ones Reply
  • vsa1977 - Thursday, March 03, 2011 - link

    This article seems to be well written based on facts. I however am an end user and have little knowledge on how this stuff works.

    This article states that "GPS is critical for getting very precise and accurate timing signals for the radio without expensive clock hardware". Would it make any difference in terms of performance if I get an external GPS antenna and connect to this Microcell? I have the microcell placed in a closet since my networking components are deployed there and the GPS at that location is pretty weak.

    Calls using the microcell is not often clear and I lose the signal all together maybe once in two weeks. I then have to resync-lock with GPS by taking it closer to the window and then back into the closet again.

    Is there any way to increase the range of the microcell by using any kind of external boosters or gain antennas? I do not see any options to connect any external antennas.

    I would appreciate if I get an answer on the external GPS antenna's effect on performance.
    Reply
  • akaken - Wednesday, July 20, 2011 - link

    you dont have to pay that 19.99 charge for unlimited microcell calling but what it does is give you UNLIMITED MINUTES WHILE YOU ARE ON YOUR MICROCELL your plan minutes are not used while on your microcell. Reply
  • MSSS - Monday, September 26, 2011 - link

    In an urban area near UC Berkeley (20min from SF) we are in a "dead zone" with irregular tower access.

    With the Microcell, we get constantly dropped calls and cannot be heard by the person on the other end of the line, often after a minute or two.

    Apparently, the microcell keeps handing off the calls to a mystery tower nearby, and cutting off calls.

    here’s what ATT says:

    In researching your issue we are not finding any network issues and see that the issue is with the micro cell handing off to towers in the area which will often cause the call to disconnect.

    They offer NO solution - they will replace the microcell, just in case it's the problem. But we are frustrated.

    One forum suggests moving the microcell to a room with bad reception:

    http://forums.att.com/t5/3G-MicroCell/add-option-t...
    Re: add option to disable handoffs completely to prevent dropped calls
    Options
    07:42:21 AM - 05-17-2010
    I've stumbled upon an option - put the MicroCell in a room with poor signal from the macro network.
    Over the past 6 weeks I've tried the MicroCell in 5 different rooms in my home in an attempt to keep it from handing off the the macro network prematurely. Placing it in the room with the worst signal from the macro network has solved the problem.
    The only drawback is that now the MicroCell will not in-call handoff to the macro network when leaving my home, moving down the street. The call always drops now. When it was in other rooms in my home it would successfully hand-off to a nearby cell about 1/2 the time. While a drawback, I've rarely found the need to continue a conversation while leaving my home.
    Reply
  • PointTechScottsdale - Wednesday, March 07, 2012 - link

    Just wanted to post a note to thank you for this great dissection of the microcell. Your work and writings are by far the best that I have been able to find on the web over the last six months.

    You had just the right amount of detail about just the issue I was looking for. Thanks and keep up the good work!

    Elliott

    --
    Point Technologists
    Scottsdale, AZ
    (480) 744-6464
    Reply

Log in

Don't have an account? Sign up now