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August 24, 2006

Networking

My daughter entered fifth grade last week and, in her school, that means that she and her classmates were each issued a Dell Latitude laptop. They use them in school and then take them home and use them in preparing their homework as well.

Overall, I’m very pleased with the school laptop program. There are so many educational resources the kids can tap into. Tonight, her homework assignment involved looking up aurochs and woolly mammoths and other extinct large mammals. Even minor things, like her still-dodgy spelling, have shown marked improvement, thanks to the shear repetition of warnings from the spell-checker. On the other hand, there is something eery about watching a 10-year old give a PowerPoint presentation.

There was one thing, however, that I really dreaded: figuring out how to add her Windows laptop to our home network.

Last time I had to do that (for a guest’s Windows ME machine, which is to say it’s been a while), entailed hours of frustration. I never did succeed in getting the wireless card to associate with our closed network and getting past the %$#@ “Wizard”s to configure the ethernet interface with a static IP address was a major pain.

Anyway, this time things went quite a bit more smoothly.

  1. Her computer teacher happily supplied me with the MAC address of the wireless card.
  2. This time, the card had no trouble associating with a closed network; all I had to do was type in the SSID.
  3. After some emails back and forth, the teacher granted Admin privileges on the laptop and I was able to configure this wireless interface with a static IP address.
  4. Setting up the laptop to use our laser printer was slightly less obvious. This page from Microsoft is a complete lie. Microsoft discontinued AppleTalk support in Windows XP (it was there in Windows 2000). The solution was to turn on Printer Sharing on one of our existing computers, and then direct her machine to print to 
    http://192.168.x.x:631/printers/PRINT_QUEUE
    (a remote IPP printer).

So far, “supporting” this Windows machine been relatively painless. I’m keeping my fingers crossed.

Update (8/26/2006):

I knew that was too easy. It turns out that assigning a static IP address to the wireless interface applies to all wireless networks. There’s no evident way (at least, none evident to me) to configure it to use DHCP on the school’s wireless network, but a static IP address at home. I guess my short-lived honeymoon with Windows XP is officially over …

Update 2:

I’ve found what, apparently, passes for a solution in Windows XP. It’s kludgy and inconvenient (there’s a 2 minute delay, on startup or on waking from sleep, before her machine obtains an IP address at home), and doesn’t really address the general problem. But she will, at least, be able to connect to the internet both at home and at school.

The same week saw another change in our home network. After over a decade of trouble-free service, our gateway router (a dual-ethernet Power Computing PowerCenter Pro, with a 400 MHz G3 card, running MacOS 9.2 and IPNetRouter) coughed and sputtered and decided that it needed to start enjoying an active adult lifestyle.

Off I went to Frys and returned with a discounted Belkin F5D7231-4. It’s about as full-featured a router as one could imagine, with Appletalk-bridging, a stateful firewall, inbound port mapping, … The only feature I miss1 is DNS-forwarding.

I don’t miss the whir of the three fans in the PCP, though.

1 Like most modern routers, the Belkin firmware is Linux. So, if the need were pressing enough, I could try installing OpenWrt on it. However, it’s not clear how well this Belkin model is supported and the version of the Belkin firmware available online, 4.05.03 (source) is older than the version (5.01.05) installed on the router, so I’m not about to experiment.

Posted by distler at 11:49 PM | Permalink | Followups (10)

August 21, 2006

Coupled

For somewhat obscure reasons, I’ve been looking, recently, at a recent paper by Aharony, Clark and Karch. They consider the following problem. Say we have two CFT’s, C 1 , and C 2 . each of which has an AdS dual description. Each CFT has its own conserved, traceless, stress tensor. The AdS dual of C 1 ×C 2 is the disjoint union of the corresponding AdS spaces. (Aharony et al use the phrase “direct sum” instead of “disjoint union”, but one learns to make allowances …) The graviton, on each component of this disjoint union, couples in the usual way to the stress tensor of the corresponding boundary theory.

But now suppose we deform the theory by choosing a pair of relevant operators, O 1 , O 2 , (with scaling dimensions Δ 1 and Δ 2 =d−Δ 1 ) in the two theories and adding

(1)
ΔS=ϵ∫d dxO 1 (x)O 2 (x) For definiteness, let’s normalize the O i so that 〈O(0 )O(x)〉=1 ∣x∣ 2 Δ

(More generally, it’s interesting to contemplate a marginal or relevant perturbation of the erstwhile decoupled theory.)

What is the AdS dual of the deformed theory?

Obviously, by turning on ΔS, we are correlating the boundary conditions of the fields on the two formerly disjoint AdS spaces. So we should think of these two spaces as glued together at their common boundary. In the boundary field theory, we no longer have two seaparately conserved stress tensors. The total stress tensor, T=T 1 +T 2 is conserved, but T˜μ ν =c 2 T μν (1 )−c 1 T μν (2 )+ϵ(c 1 Δ 2 −c 2 Δ 1 )dη μνO 1 O 2 is not. Here c, defined by 〈T μν(0 )T ρσ(x)〉=c∣x∣ 4 (η μρη νσ+η μση νρ−2 dη μνη ρσ) is one measure of the central charge of the conformal field theory. Instead of being conserved, to leading order in ϵ, ∂ μT˜μ ν =K ν<