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802.11n and Time Capsule


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802.11n and Time Capsule
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MBHockey
skates=grafs
 
Join Date: May 2005
Location: New York
 
2008-05-05, 19:33

I'm curious about 802.11n generally and also a specific question about my Time Capsule. What is the theoretical maximum of 802.11n? For g it's 54 mbps...but what for n? I've read that it's 248 mbps, but my 'link speed' in Network Utility is listed as 130 mbps...which brings me to my next question. I'm running my Time Capsule in mixed mode so my AirPort Express (g) can hop on it -- does this slow down my wireless lan? If i ran it in n-only mode, would my 'link speed' read 248 mbps (assuming i was close enough to the Time Capsule?)

And my second question is why does Apple do backups to Time Capsule in a disk image? I don't know much about disk images, but to me it seems that it's kind of putting all your eggs in one basket -- what happens if the image gets corrupted? ALL of your backups are then gone? What gives?!
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Eugene
careful with axes
 
Join Date: May 2004
Location: Hillsborough, CA
 
2008-05-05, 20:09

802.11n's theoretical maximum data rate is 540Mbps. In the real world if you have a perfect set-up you might see 150-200Mbps. In most situations I'd expect about 90Mbps or lower.
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PB PM
Sneaky Punk
 
Join Date: Oct 2005
Location: Vancouver, BC
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2008-05-05, 20:09

The theoretical max for 802.11n is 300MB/s (at least that's what I've seen listed on all the tech sits). Attaching lower speed devices will slow it down to the slowest device attached. So if a 802.11b device is attached, that is the top speed the network will operate at.

As for your other questions, no idea.
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Eugene
careful with axes
 
Join Date: May 2004
Location: Hillsborough, CA
 
2008-05-05, 20:14

Quote:
Originally Posted by PB PM View Post
The theoretical max for 802.11n is 300MB/s (at least that's what I've seen listed on all the tech sits). Attaching lower speed devices will slow it down to the slowest device attached. So if a 802.11b device is attached, that is the top speed the network will operate at.

As for your other questions, no idea.
Not quite, it has to actually be using the radiospace (transferring data). If it's just present it will only amount to a tiny bit of overhead.

The specification lists 540Mbps, which makes sense considering the modulation. "300Mbps" is common on retail products though.
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MBHockey
skates=grafs
 
Join Date: May 2005
Location: New York
 
2008-05-05, 20:27

Ah, ok thanks guys. That helps a lot.

Side note: Why is TC so slow at transferring files? When I'm doing a backup (right now it's just one large movie file -- 2.0 GB) and it's only going 1.0-2.0 Mb/sec. That's nowhere near the 130 mbps that my link speed is reporting...shouldn't it be closer to 15 Mb/sec? Or does the TC just not have enough horsepower to actually process data that quickly?
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Eugene
careful with axes
 
Join Date: May 2004
Location: Hillsborough, CA
 
2008-05-06, 01:03

Quote:
Originally Posted by MBHockey View Post
Ah, ok thanks guys. That helps a lot.

Side note: Why is TC so slow at transferring files? When I'm doing a backup (right now it's just one large movie file -- 2.0 GB) and it's only going 1.0-2.0 Mb/sec. That's nowhere near the 130 mbps that my link speed is reporting...shouldn't it be closer to 15 Mb/sec? Or does the TC just not have enough horsepower to actually process data that quickly?
The posted link speed is almost never representative of real-world throughput. Expect 90Mbps if the Time Capsule is fairly close to your client. Expect far worse if you have to go through a thick wall, around corners, up a floor, etc. I get ~30Mbps through two walls and at a straight line distance of about 50 feet for example.
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MBHockey
skates=grafs
 
Join Date: May 2005
Location: New York
 
2008-05-06, 05:11

Yeah that's the thing. I'm not very far at all, maybe 30 feet and through some walls. 1-2 Mbps seems quite low (AirPort reads 5 bars). I have a one bedroom apartment.
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veryamusing
Member
 
Join Date: Aug 2006
 
2008-05-06, 06:41

I've noticed the same slow transfer speed, MBHockey. My TC is downstairs and I'm upstairs (probably ~20 feet line of sight), but even our LAN-connected iMac crawls. I really can't figure why it takes so long, but it sucks. Just wanted you to know you're not alone...
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malcolm
Senior Member
 
Join Date: Feb 2005
Location: Canada
 
2008-05-06, 08:57

I just got a Time Capsule, and was curious about the very slow backup speed... phoned Apple Wireless help, and the guy had me get rid of my (year-old or so) Apple Express. This has sped things up considerably.
The fact that the TC is more powerful than the base station I used to have meant that, in fact, I didn't need the Airport Express anyway (to the the signal from my basement to my MBP here on the second floor). 4 bars without anything on the first floor... and fast now.
Cheers, Malcolm
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MBHockey
skates=grafs
 
Join Date: May 2005
Location: New York
 
2008-05-06, 23:09

Quote:
Originally Posted by Eugene View Post
The posted link speed is almost never representative of real-world throughput. Expect 90Mbps if the Time Capsule is fairly close to your client. Expect far worse if you have to go through a thick wall, around corners, up a floor, etc. I get ~30Mbps through two walls and at a straight line distance of about 50 feet for example.
How are you measuring the network throughput? I am using MenuMeters and in mixed b/g/n mode it's certainly not going anywhere near the 130 mbps (~16 Mb/sec) link speed being reported.
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Eugene
careful with axes
 
Join Date: May 2004
Location: Hillsborough, CA
 
2008-05-07, 02:29

Quote:
Originally Posted by MBHockey View Post
How are you measuring the network throughput? I am using MenuMeters and in mixed b/g/n mode it's certainly not going anywhere near the 130 mbps (~16 Mb/sec) link speed being reported.
I used iPerf to measure 802.11n throughput between clients, but not specifically with Time Capsule back-ups.
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MBHockey
skates=grafs
 
Join Date: May 2005
Location: New York
 
2008-05-11, 09:11

Eugene...would you happen to know why Apple decided to use disk images for the backups? I am worried that storing it all in a single 'file' is a bit of a bad idea. Am I worrying for nothing? It's just disk image corruption seems not so rare.
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Dave
Ninja Editor
 
Join Date: May 2004
Location: Bay Area, CA
 
2008-05-11, 14:36

Quote:
Originally Posted by MBHockey View Post
Eugene...would you happen to know why Apple decided to use disk images for the backups? I am worried that storing it all in a single 'file' is a bit of a bad idea. Am I worrying for nothing? It's just disk image corruption seems not so rare.
They only do that over the network. If you use Time Machine with a local drive it just copies the files. I'm not sure about why there's a difference though.
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fteoath64
Member
 
Join Date: Dec 2005
 
2008-05-12, 02:03

I stick the gigE for TM and it pushes about 8-9Mbytes/sec. On FastE, it drop to 6-7Mbytes/sec.
On the PB15, using WirelessG, it pushes a peak of 2Mbtyes/sec, not great but sufficient for my purposes. The only issue I have is that some TM backups takes a heck of a long time "preparing...". This is very annoying as it heats up the PB which I dislike. I gotta do more tests on MB as its 802.11n should kick close to 7Mbytes/sec speed ?. Not so sure on yet...
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MBHockey
skates=grafs
 
Join Date: May 2005
Location: New York
 
2008-05-23, 16:24

Quote:
Originally Posted by Dave View Post
They only do that over the network. If you use Time Machine with a local drive it just copies the files. I'm not sure about why there's a difference though.
Yeah, i'm aware of that (had been using a FW400 drive for TM backups previously). I just am curious why they switched to disk images over the air.
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