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Mac Forum / General / Networking / June 2008WiFi frequencies
I have a Time Machine that I'm using as a base station and an Airport Express. They're both capable of 5 GHz. I also have a WiFi printer and a iPhone(*) on the network, but they can handle only 2.4 GHz. Does the Time Machine communicate on both frequencies, or does it revert to 2.4 GHz with the Airport Express because of the other devices? Similarly, the TM and the AEx both are 802.11n capable, but the printer and iPhone are only b/g capable; do the TM and AEx connect via the n protocol or do the other devices restrict everything to the g protocol? (*) Actually, I had an iPhone on the network until it got stolen, but I will again after the new ones come out.  SignatureSupport the troops: Bring them home ASAP. > I have a Time Machine that I'm using as a base station and an Airport > Express. They're both capable of 5 GHz. I also have a WiFi printer and [quoted text clipped - 6 lines] > and iPhone are only b/g capable; do the TM and AEx connect via the n > protocol or do the other devices restrict everything to the g protocol? Not sure about the frequency question, but AFAIK the slowest 802.x protocol on the network drags everything down to that level. SignatureWoodworking links and more at http://www.woodenwabbits.com -------------- I kill all messages from Google Groups. See http://improve-usenet.org for details. > Not sure about the frequency question, but AFAIK the slowest 802.x > protocol on the network drags everything down to that level. Even if there is no traffic between the slowest item and the base station? SignatureSupport the troops: Bring them home ASAP. > > Not sure about the frequency question, but AFAIK the slowest 802.x > > protocol on the network drags everything down to that level. > > Even if there is no traffic between the slowest item and the base > station? That's my understanding. I could be wrong. SignatureWoodworking links and more at http://www.woodenwabbits.com -------------- I kill all messages from Google Groups. See http://improve-usenet.org for details. > > > Not sure about the frequency question, but AFAIK the slowest 802.x > > > protocol on the network drags everything down to that level. [quoted text clipped - 3 lines] > > That's my understanding. I could be wrong. There is quite an interesting, though somewhat dense, interview from Aug '06 of Matthew Gast, author of "802.11 Wireless Networks: The Definitive Guide, Second Edition", by Glenn Fleishmann of Wifi Net News available here: <http://www.wifinetnews.com/archives/006925.html> If I understand what Matthew is saying, the answer to 'slow devices slowdown the entire wifi network vs oh no they don't' is 'yes, you're both right'. The n spec is exceedingly complex and the net speed of any wireless network depends entirely on the pragmatic hardware and firmware design decisions made by manufacturers. Thus, we could have single basestations which could handle all versions of 802.11 with no speed hit to n; but because such devices would be very expensive, manufacturers op for cheaper access points which do take a hit with slower wireless clients. I'd be interested if anyone out there with wifi design knowledge could comment on the interview and whether my conclusion is correct. --Fred > > Not sure about the frequency question, but AFAIK the slowest 802.x > > protocol on the network drags everything down to that level. > > Even if there is no traffic between the slowest item and the base > station? Yes, the base station doesn't know that there's no traffic until it listens using that protocol. When this question came up a while back, I ran some experiments with 802.11b and 802.11g. I may still have the results around if anyone's interested. SignatureTom Stiller PGP fingerprint = 5108 DDB2 9761 EDE5 E7E3 7BDA 71ED 6496 99C0 C7CF > > > Not sure about the frequency question, but AFAIK the slowest 802.x > > > protocol on the network drags everything down to that level. [quoted text clipped - 8 lines] > 802.11b and 802.11g. I may still have the results around if anyone's > interested. I'm quite interested. SignatureWoodworking links and more at http://www.woodenwabbits.com -------------- I kill all messages from Google Groups. See http://improve-usenet.org for details. > > > > Not sure about the frequency question, but AFAIK the slowest 802.x > > > > protocol on the network drags everything down to that level. [quoted text clipped - 4 lines] > > Yes, the base station doesn't know that there's no traffic until it > > listens using that protocol. But note, as shown below, there is a difference between "no traffic" and "radio off". > > When this question came up a while back, I ran some experiments with > > 802.11b and 802.11g. I may still have the results around if anyone's > > interested. > > I'm quite interested. I performed the following experiment with several combinations of 802.11b and 802.11g computers. Wrote test programs to send and receive blocks of arbitrary bytes through a TCP connection. The transmitter sent 8192 1024 byte blocks and closed the connection. The receiver looped receiving data until the connection was closed. The receiving computer was a 700 MHz G3 iMac connected to an AirPort Extreme base station via 100 Mb ethernet. The sending computers were a 367 MHz Clamshell G3 iBook equipped with the original AirPort card and a 1.2 GHz G4 iBook equipped with an Apple Extreme card. The network was WPA enabled and while there was no local traffic, it was not isolated from the internet. The tests were: 1. Base station in b/g mode. Send from 802.11g. 802.11b computer enabled but idle (AirPort on). 2. Base station in b/g mode. Send from 802.11b. 802.11g computer enabled but idle (AirPort on). 3. Base station in b/g mode. Send from 803.11g. 802.11b computer disabled (AirPort off). 4. Base station in g only mode. Send from 803.11g. 802.11b computer disabled (AirPort off). 5. Base station in b only mode. Send from 803.11g. 802.11b computer disabled (AirPort off). 6. Base station in b only mode. Send from 803.11b. 802.11g computer enabled but idle (AirPort on). Each test was repeated five times, the throughput results (Mb/sec) are: Test Max. Min. Avg. 1 15.4 14.5 15.14 2 4.9 4.8 4.84 3 27.2 26.0 26.76 4 27.2 23.8 25.92 5 5.4 5.3 5.32 6 4.9 4.8 4.88 Note: the similarities in test 3 and 4. If there is no 802.11b radio active, the network behaves pretty much as if the base station were operating in 802.11g mode only. I think the discrepancies in tests 5 and 6 are due to the speed differences in processors while computing the WPA encryption. I could be wrong about this. SignatureTom Stiller PGP fingerprint = 5108 DDB2 9761 EDE5 E7E3 7BDA 71ED 6496 99C0 C7CF >> > > > Not sure about the frequency question, but AFAIK the slowest 802.x >> > > > protocol on the network drags everything down to that level. [quoted text clipped - 65 lines] >differences in processors while computing the WPA encryption. I could >be wrong about this. Send ethernet packets and remove the bias of the upper network stack layers if you really want valid figures. CSMA/CD shouldn't have any preference to what is sent save for the size of what is sent. SignatureVAXman- A Bored Certified VMS Kernel Mode Hacker VAXman(at)TMESIS(dot)COM "Well my son, life is like a beanstalk, isn't it?" http://tmesis.com/drat.html > Send ethernet packets and remove the bias of the upper network stack layers > if you really want valid figures. CSMA/CD shouldn't have any preference to > what is sent save for the size of what is sent. That would be a different experiment. I was interested in sending data between two computers. SignatureTom Stiller PGP fingerprint = 5108 DDB2 9761 EDE5 E7E3 7BDA 71ED 6496 99C0 C7CF > > > Not sure about the frequency question, but AFAIK the slowest > > > 802.x protocol on the network drags everything down to that [quoted text clipped - 5 lines] > Yes, the base station doesn't know that there's no traffic until it > listens using that protocol. So if I'm streaming to the AEx, I should turn off the printer, and disconnect the iPhone from the network? SignatureSupport the troops: Bring them home ASAP. > > > > Not sure about the frequency question, but AFAIK the slowest > > > > 802.x protocol on the network drags everything down to that [quoted text clipped - 8 lines] > So if I'm streaming to the AEx, I should turn off the printer, and > disconnect the iPhone from the network? If the streaming data is choppy, I would certainly try it. SignatureTom Stiller PGP fingerprint = 5108 DDB2 9761 EDE5 E7E3 7BDA 71ED 6496 99C0 C7CF > > I have a Time Machine that I'm using as a base station and an Airport > > Express. They're both capable of 5 GHz. I also have a WiFi printer and [quoted text clipped - 9 lines] > Not sure about the frequency question, but AFAIK the slowest 802.x > protocol on the network drags everything down to that level. It's not quite that simple. Individual protocols still function at the speed determined by their protocols, but the base station has to time share its attention among the competing protocols. With less than full attention paid to any given protocol, the aggregate throughput for that protocol will be reduced. SignatureTom Stiller PGP fingerprint = 5108 DDB2 9761 EDE5 E7E3 7BDA 71ED 6496 99C0 C7CF > I have a Time Machine that I'm using as a base station and an Airport > Express. They're both capable of 5 GHz. I also have a WiFi printer and [quoted text clipped - 9 lines] > (*) Actually, I had an iPhone on the network until it got stolen, but I > will again after the new ones come out. Looking at my 802.11n Airport Extreme base station via Airport Utility, my WiFi Radio Modes are: 802.11n (802.11b/g compatible) 802.11n only (2.4 GHz) 802.11n (802.11a compatible) 802.11n only (5 GHz) While not direct evidence, I infer from this that if you are 802.11b/g compatible, then you are also operating at 2.4 GHz frequencies, which will limit the total throughput of your 802.11n transmissions. That is to say there is only so much you can push through the 2.4GHz channel assignments if you want to play nice with existing 802.11b/g base stations (like your neighbors). What I did at my home was setup 2 networks. My old Flying saucer Airport Extreme offering up 802.11b/g services. 802.11n Airport Extreme operating in 802.11n only 5 GHz mode. The old AEBS(b/g) connected to the new AEBS(n) via ethernet cable. The old AEBS(b/g) set up as a bridge letting the new AEBS(n) be the home router and connected to the DSL Modem. My AEBS(b/g) is also operating on a channel 1 (after using a AP Grapher to make sure no one else was using that channel). Bob Harris > I have a Time Machine that I'm using as a base station and an Airport > Express. They're both capable of 5 GHz. I also have a WiFi printer and > a iPhone(*) on the network, but they can handle only 2.4 GHz. > > Does the Time Machine communicate on both frequencies, or does it revert > to 2.4 GHz with the Airport Express because of the other devices? I have the current model Airport Extreme (802.11n with Gigabit Ethernet), and have set up a Time Capsule [not "Time Machine"]. They are identical as far as their wireless network configuration is concerned. The settings allow you to pick which frequency band it runs on. You can specify any of the following: 5 GHz (802.11n only) 5 GHz (802.11n with 802.11a compatibility) 2.4 GHz (802.11n only) 2.4 GHz (802.11n with 802.11b/g compatibility) You cannot have a base station operating on both 5 GHz and 2.4 GHz at the same time. If you have some devices that only support 2.4 GHz (802.11b/g) then your base station must be operating at 2.4 GHz. If you want best performance from 802.11n then it needs to be operating at 5 GHz, with wide channels. This rules out supporting 802.11b/g devices. In this situation, the best solution is to have two wireless base stations: one operates at 5 GHz and the other operates at 2.4 GHz. From my experiments, it seems my Airport Extreme doesn't want to let me create a WDS network (with multiple base stations cooperating to create a single network) at 5 GHz - it forces 2.4 GHz mode. My Airport Extreme and 802.11n Airport Express also offer "Extend a wireless network", while this isn't offered by my 802.11g Airport Express, so perhaps this is a new feature that only applies to 802.11n. > Similarly, the TM and the AEx both are 802.11n capable, but the printer > and iPhone are only b/g capable; do the TM and AEx connect via the n > protocol or do the other devices restrict everything to the g protocol? If there is a mixture of 802.11n and 802.11g devices on the network (all operating at 2.4 GHz), direct communication between two devices will use the best protocol supported by those devices, so they will exchange data as fast as possible. Two 802.1n devices will communicate using 802.11n. The catch is that some time must be reserved to communicate with the slower 802.11g devices, which means that communication between two 802.11n devices will not be able to use 100% of the network, reducing their average throughput. (I see that Tom Stiller has posted some detailed numbers for 802.11b/g mixed networks later in the thread; I'd expect a similar pattern for 802.11n mixed with 802.11g.) SignatureDavid Empson dempson@actrix.gen.nz > I have the current model Airport Extreme (802.11n with Gigabit > Ethernet), and have set up a Time Capsule [not "Time Machine"]. *doh* You're right, of course. > The settings allow you to pick which frequency band it runs on. You can > specify any of the following: [quoted text clipped - 3 lines] > 2.4 GHz (802.11n only) > 2.4 GHz (802.11n with 802.11b/g compatibility) Mine gives me only one option: 802.11n with 802.11b/g compatibility with no frequency mentioned. So it must be 2.4 GHz. The Airport Express likewise shows only that one option. > If you have some devices that only support 2.4 GHz (802.11b/g) then > your base station must be operating at 2.4 GHz. I shut down the printer, but I still get only that one option. > From my experiments, it seems my Airport Extreme doesn't want to let > me create a WDS network (with multiple base stations cooperating to > create a single network) at 5 GHz - it forces 2.4 GHz mode. Ah, that might be the reason. > My Airport Extreme and 802.11n Airport Express also offer "Extend a > wireless network", while this isn't offered by my 802.11g Airport > Express, so perhaps this is a new feature that only applies to > 802.11n. I just tried to switch to that mode, but couldn't get it to work, so I'm back to WDS network. SignatureSupport the troops: Bring them home ASAP. > I just tried to switch to that mode, but couldn't get it to work, so I'm > back to WDS network. Michelle- Did you update the AirPort Express (n) firmware? Run AirPort Utility.app, select the AirPort Express, and choose Check for Updates... under the Airport Utility menu. Fred > Michelle- > > Did you update the AirPort Express (n) firmware? > > Run AirPort Utility.app, select the AirPort Express, and choose Check > for Updates... under the Airport Utility menu. I guess that I must have because it says there are no updates available. SignatureSupport the troops: Bring them home ASAP. |
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