The Amazing New AT&T 3G MicroCell, Part II

Most home users won’t have any problem installing the AT&T 3G MicroCell. However, in a few cases, there can be router and network issues that can create a temporary roadblock. This article discusses those potential problems, including mine, and how to solve them. (For the full review of the AT&T 3G MicroCell, see Part I.)

A Simple Home Network

To better understand what’s going on with the AT&T 3G MicroCell and how it works, let’s first take a look at some basic home network configurations. Figure 1 is a very simple home network. Many home users have something like this, and the only thing simpler would be to direct connect a single PC to a cable or DSL modem — which is also common.


Figure 1

  • CM = Cable Modem, but could be a DSL Modem
  • WR = Wireless router

In this case, the cable modem issues a DHCP address to the wireless router on the WAN side, assigned by the ISP. By default, the LAN address, out of the box, of most home routers is*. In turn, the router issues internal addresses to the devices attached, like Macs and PCs, in the range of and up. How many devices supported is a function of the feature set/cost of the router. To enable hard wire connections, there are typically several Ethernet ports on the back of the router.

Note that the cable modem only has to issue a single DHCP address to the router. For the vast majority of home systems, that’s all that’s required of the cable modem.

Out of the box, most consumer grade routers leave essential ports open, that is, the firewall, if it has one, is turned off by default. That’s a concession to home users who may want to attach devices, like the AT&T 3G MicroCell, but don’t understand how to configure the firewall, close and open ports on their router. That’s probably a major security snafu in America, but it’s done for the sake of happy consumers and brisk router sales. Understanding all this is key #1 to trouble shooting devices you may attach to your router.

A More Advanced Home Network

Before I go on, I want to clear up something that may trouble some users. In those cases where the number of ports on the back of the router, standard or wireless, is insufficient, it’s convenient to attach a switch. (Hubs do almost the same thing, but they’re obsolete.) An eight port Gigabit switch is not expensive, and provides many more ports for things like a Blu-ray player, AirPort Extreme, PogoPlug, etc. Connecting devices to the downstream switch is the same, logically and electrically, as attaching to the router. That’s key #2 to understanding your home network.


Figure 2

  • CM = Cable Modem, but could be a DSL Modem
  • WR = Wireless router
  • GbS = Gigabit Switch

Note that every device on this network in Figure 2 still gets its DHCP address from the router.

Attaching the AT&T 3G MicroCell

You have two choices for where to place the AT&T 3G MicroCell. Call them configurations.

  1. Plugged into the router or a downstream switch.
  2. Placed between the cable modem and the router.

Configuration #1

AT&T, in the User Manual, calls this Option A. Using this configuration allows a) the 3G MicroCell to obtain its DHCP address from the router and b) depends on a certain configuration of the router. Namely, the firewall, if turned on, needs to have some ports left open. Here’s the specification from the 3G MicroCell User Manual, page 5.


Ports required for 3G MicroCell from Page 5 of User Manual

If you don’t want to turn on the firewall in your router, then carefully open these ports, then one way to proceed (and maintain good security) is to simply leave the router’s firewall turned OFF, but turn it ON on your Mac(s) or PC(s) for protection. For a Mac, see System Preferences -> Security -> Firewall. Here’s what Option A can look like when you have several devices plugged into a switch. See Figure 3.



Figure 3 (Option A)

  • CM = Cable Modem, but could be a DSL Modem
  • WR = Wireless router
  • GbS = Gigabit Switch
  • 3GM = AT&T 3G MicroCell

If you don’t have a switch, then you’ll plug the 3G MicroCell directly into your router. Doing all this, AT&T’s Option A, should get you easily up and running with the 3G MicroCell.

Configuration #2

In my case, I have a business-class SonicWall hardware router and firewall. (It’s has no Wi-Fi capability, so I plug an AirPort Extreme into my switch and use “bridge” mode.) I have no intention of opening up any ports, ever. If you are of like mind or are running a small business with a firewall that you want locked down, you will have to place the 3G MicroCell upstream of the router so it can have unfettered access to the Internet. This is what AT&T calls Option C. See Figure 4.


Figure 4 (Option C)

  • CM = Cable Modem, but could be a DSL Modem
  • WR = Wireless router
  • GbS = Gigabit Switch
  • 3GM = AT&T 3G MicroCell

Here is where I got into trouble, and others may as well. During initial set up and testing, when I had a problem with Option A, AT&T technical support suggested Option C. (This was before I realized the 3G MicroCell required certain ports open on my router. As I said, not gonna happen.) The 3G MicroCell worked fine, but all my Macs went off the air. That was not a network configuration I liked, and that’s why at the end of day one, I still didn’t have a functioning 3G MicroCell unit combined with a working home network. I preferred to keep my network working, so I stayed with Option A overnight.

Problem Resolution

The key #3 to understanding this was when I called Comcast on the evening of day one and asked how many DHCP licences my Scientific Atlanta DPC2100 cable modem could issue. The quick, concise answer was “one.” **

Right away I realized what was wrong with Option C. The 3G MicroCell, being the first device downstream, was slurping up the only DHCP address the cable modem had at its disposal. Anything downstream, like my router, was starved. See Figure 4 above.

I started to suspect that the “Computer” port on the 3G MicroCell was a simple pass-through, unable to issue any downstream addresses. Later, AT&T confirmed that in an e-mail.

Then I asked if Comcast could provision my cable modem to issue two DHCP licences. They said, “Sure. It’s an extra $4.95/month.” I agreed, and within 5 minutes, Comcast re-provisioned my cable modem remotely and rebooted it. As soon as that happened, both the 3G MicroCell and my router could each obtain a DHCP license. The 3G light on the Microcell glowed solid green, I had the M-Cell indicator on my iPhone, and all my household computers were back on the Internet. Problem solved.

I logged onto my router and copied down the WAN IP.  Then I used the Network in /Applications/Utilities and fed it to the “Lookup” tab.  Sure enough, my SonicWall had been given that second Comcast address: 6115 IN PTR

(Some numbers obscured for privacy.)

Finally, for completeness sake, AT&T discusses Option B, which is really just a simpler version of Option C.


Figure 5 (Option B)

The same discussion above applies because the downstream Mac also needs a DHCP address/license.

End of the Line

Home networks can be complicated, and no vendor can predict all the possible configurations. If the AT&T 3G Microcell is plugged into a router (or downstream switch), which is typical, the user has to know a little about how the router and its firewall is configured. If the MicroCell is upstream, connected to the cable or DSL modem, then one has to understand how many DHCP licenses may be required and work with the ISP as I did. Either way introduces potential gotchas.

To AT&T’s credit, they’ve made a rational assumption about how most consumer routers work out of the box. Option A will likely work. However, in the case of Option C, the user has to be alert about how the cable or DSL modem issues DHCP licenses, and that could trip up an inexperienced home user who connects a downstream router via the pass through port on the 3G MicroCell. Perhaps AT&T has discovered that most cable modems are provisioned differently than mine was, but it still should have been discussed briefly in the user manual, even if in fine print. Afterall, Comcast is a fairly large ISP.

So that’s what I learned, and that’s how I got my AT&T 3G MicroCell running on my home network with a severely locked down router. For a time, my office looked like the wiring guts of a starship bridge, but you know I loved every moment.


I had some additional question for AT&T after Part I was published. AT&T answered via e-mail.

Q: If I have a MicroCell plus unlimited MicroCell service on my account and then go to someone else’s house with a MicroCell who does not have unlimited service, how are my calls treated?

A: “The individual’s line (mobile number) has the unlimited plan. So if you have unlimited calling, up can enjoy that on another user’s MicroCell — so long as you have approved access to it by the owner.”

Q:  What is the radiated power of the MicroCell, and how does it compare to, say, a Wi-Fi router?

A: “AT&T 3G MicroCell meets all FCC standards for electronic devices.  The device is very efficient and actually has an RF output lower than most Wi-Fi routers.  AT&T 3G MicroCell has a maximum output of 5 mW which is well below typical Wi-Fi router maximum output of 100 mW”. 

Q: The product FAQ (see below) answers a question about “What if I move?” and says “…your device can be moved to another location provided it is within the AT&T wireless authorized service area.” That would make it tough if I moved to a location that isn’t served by AT&T. Is it an FCC rule?

A: “The 3G MicroCell must operate in a licensed AT&T wireless footprint.”

The second answer wasn’t what I was looking for, but that’s all AT&T is willing to say, officially. The interesting part is that some people have apparently, erroneously concluded, from that rule, that the device needs to be able to “see” a cell tower to operate. That’s not true, according to AT&T.  My own take is that the home location must simply be within AT&T’s nominal footprint of coverage. That will be checked when you purchase the 3G MicroCell. I surmise it’s a geographic rather than a signal issue.


AT&T 3G MicroCell FAQ

AT&T 3G MicroCell User Manual


* Another possible set of internal, non-routable addresses are - and, rarely, - But the - range is most commonly used in consumer routers.

**No doubt Comcast provisions their cable modems this way because most of their customers connect the cable modem directly to a router and only one license is required. If we want more, we pay more. I don’t know how other ISPs and their DSL/cable modems handle this. Let me know if you know.