Microcell Solution! Finally!

Contributor

Microcell Solution! Finally!

I bought a Microcell a couple weeks ago, and have had nothing but trouble setting it up.  Went through everything with ATT tech support as well as my ISP.  10+ hours later they tell me that everything is OK from their end, but no luck. 3G light just kept flashing; no activation(error FTC193). Checked router settings all according to specs.

 

Finally found this guy who suggested that the MTU size of 1492, as suggested, was causing dropped packets. I'll keep looking for the link. Anyway, set the MTU size to 1400, up and working 15 min later.  Finally!

Message 1 of 7 (3,493 Views)
Guru

Re: Microcell Solution! Finally!

Glad you're up and running, but just out of curiosity, who is your ISP and what type service to you have?  DSL, cable, etc.?

 

I'm trying to understand why you'd need to change your MTU.  Everyone I know around here, including me, is using the default MTU of 1500.  The problem I see with changing your MTU, if that's required to get your MC working, is it might have a negative impact on general performance on the Internet.

 

You can't argue with success but I like to try to understand the technical nitty gritty.

Message 2 of 7 (3,443 Views)
ACE - Master

Re: Microcell Solution! Finally!

 


orex wrote:

I bought a Microcell a couple weeks ago, and have had nothing but trouble setting it up.  Went through everything with ATT tech support as well as my ISP.  10+ hours later they tell me that everything is OK from their end, but no luck. 3G light just kept flashing; no activation(error FTC193). Checked router settings all according to specs.

 

Finally found this guy who suggested that the MTU size of 1492, as suggested, was causing dropped packets. I'll keep looking for the link. Anyway, set the MTU size to 1400, up and working 15 min later.  Finally!


That's interesting. AT&T told me that default MTUs are set to 1500 and to get the MicroCell to work properly, you had to drop it to 1492. I have DSL which connects to an AEBS and then the MicroCell.  There is no MTU setting to change on the AEBS so I had to institute port forwarding, as suggested by support, to get my MicroCell to work when it failed after an update last month. My network is 100% wireless to the MicroCell is the only wired connection. I think support just throws things out there because sometimes they work, and sometimes they don't. I never had to do any sort of port forwarding before so I think this last round of updated screwed up a bunch of systems because everyone's is slightly different.

 

___________________________________________________________

MicroCell Technical Guide by Otto Pylot


I am not an AT&T employee. For additional help, please send a PM to ATTCustomerCare

*The views and opinions expressed on this forum are purely my own. Any product claim, statistic, quote, or other representation about a product or service should be verified with the manufacturer, provider, or party.
Message 3 of 7 (3,436 Views)
Contributor

Re: Microcell Solution! Finally!

My ISP is Fairpoint Communications.  When I called their tech support, they set up my DSL modem as a bridge through to my WRT54G router, which is now set up as a PPPoe connection.DHCP is enabled on the router which is using the DD-WRT firmware. I placed the MC in the DMZ to make sure that the router was not blocking ports, and disabled any kind of filtering or blocking.  No luck till I changed the MTU.  I found this discussion http://www.dd-wrt.com/phpBB2/viewtopic.php?t=83006&highlight=femtocell, which while not directly addressing the problem, made me think of adjusting the MTU. As far as speed goes, I haven't noticed any difference. I will try moving the MTU back up until the MC quits working; we'll see what happens.

Message 4 of 7 (3,418 Views)
Highlighted
Guru

Re: Microcell Solution! Finally!

So here's what the whole MTU thing is all about.

 

MTU stands for Maximum Transmission Unit. It represents the size of the largest possible packet, in bytes, that can be sent over a communications link. Various communications links have various maximum sized MTUs. In general, if you're sending bulk data across the Internet, you want to send it in as large a packet as you can to minimize overhead. The problem is, if a packet is on its way from one point to another and encounters a router/link with a smaller MTU, the router in the middle will respond either by fragmenting the packet (cutting it into two pieces and sending those as separate packets) or just rejecting it and returning an ICMP must fragment error (or perhaps even dropping it). The problem with fragmenting the packet is that two packets are twice as likely as one to not make it to the destination, and if you drop one fragment of a whole packet, then the whole packet will be dropped. In addition, many firewalls refuse to accept fragments as they can, under certain circumstances, be used to attempt to subvert security.

 

The MTU of straight up Ethernet networks is 1500 bytes. Where it gets a little tricky is PPPoE. PPPoE adds 8 bytes of overhead to Ethernet (as part of its framing), so the MTU over a PPPoE link is 1492.

 

TCP includes functionality to attempt to probe the path between two nodes to discover the maximum sized packet that will make it through without being fragmented. Because of this, you typically don't need to worry about TCP.

 

The problem with the Microcell is that it uses IPSEC encapsulated in UDP. If its packets wind up needing to be fragmented along the way, then that's a Bad Thing(tm), for all the reasons mentioned.

 

Because PPPoE is so common for DSL, AT&T likely set the MTU on the IPSEC links at 1492 to insure against problems.

 

Somewhere along the way, that mutated to advice for folks to set the MTU to 1492. This, undoubtedly, is not necessary. The correct thing they should be saying is to not set the MTU *below* 1492. That would result, potentially, in the data from the Microcell being fragmented, which would be a bad thing. But if you've got a standard cable modem, the MTU is 1500 and you should leave it there. MTUs *higher* than 1492 are not a problem. It's only when the MTU drops *below* 1492 that there's a potential problem.

 

Now, AT&T could, as part of their protracted Microcell foreplay, perform a path MTU discovery and set up the IPSEC tunnels accordingly. With a name like Cisco behind them, you'd think they would have thought of that.

 

In short, I can't imagine that lowering the MTU to 1400 is what made the Microcell start working. And I'd have to guess it's probably going to wind up reducing the Microcell's reliability down the road.

 

Message 5 of 7 (3,414 Views)
ACE - Master

Re: Microcell Solution! Finally!

Nice explanation and introduction to Internet Protocol 101 Smiley Wink My DSL modem is in bridged mode but my address is dynamic so there's no PPoE for me. I could probably disable my port forwarding with the AEBS but everything works well now and it just affects the MC so I'll leave well enough alone, for now. I agree about the disinformation coming from AT&T about the MTU but considering the source........

___________________________________________________________

MicroCell Technical Guide by Otto Pylot


I am not an AT&T employee. For additional help, please send a PM to ATTCustomerCare

*The views and opinions expressed on this forum are purely my own. Any product claim, statistic, quote, or other representation about a product or service should be verified with the manufacturer, provider, or party.
Message 6 of 7 (3,392 Views)
Contributor

Re: Microcell Solution! Finally!

I have just gotten the MicroCell today from ATT and have not had successful activation as of yet I did the setup and waited the 90 mins with it NOT activating sucessfully. I Tried the other option for setting it up and still not working so i guess I will have to call Tech Support.

 

I am using cable modem with Wifi router and a phone connection thru i-net as well using MetroCast Communications in Phila. MS


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