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What signal level should I consider usable for a good wireless link?

I get asked this question rather too often, so I’m posting my short answer here. The answer is rather more complex than it ought to be, and depends on a huge number of factors.

The most important is the receive sensitivity of your equipment. Many manufacturers fail to publish this data, but those that do will generally rate their radios by dBm at various data rates. As an example, let us take the venerable ORiNOCO Gold 802.11b “Classic” card. Its receive sensitivity is:

• -94 dBm at 1 Mbps
• -91 dBm at 2 Mbps
• -87 dBm at 5.5 Mbps
• -82 dBm at 11 Mbps

In theory this means, in order to operate at 11 Mbps, this card must be consistently receiving a minimum signal level of -82 dBm. Any less and it is likely to drop to one of the lower rates; if you get as low as -94 dBm then the connection may drop altogether. As I mentioned before, many manufacturers do not quote their receive sensitiviy for their adapters; if you have one of these, I suggest picking a conservative figure such as -76dBm at 11 Mbps, which is the number for the Belkin F5D6020.
The signal level you receive in an unobstructed environment depends on the transmitter power, the gain of the two antennas involved, and the distance between them, as well as any loss between the antenna and the radio at each end.

In practice, radio waves behave unpredictably in a number of ways. First, the signal will fade out due to multipath effects (radio waves that bounce off objects and increase or decrease the signal that you receive). The further the receiver is from the transmitter, and the more objects between them, the higher this effect will be. Walls, people, electronic equipment, rain/snow/ice/fog are all quite effective at decreasing your signal level. In a typical home or small office environment without too many obstructions, a 10dB variation in signal level is quite normal. So, if you are looking at a NetStumbler scan and the signal is consistently around -65 dBm, it could drop to -75 dBm when somebody comes over to talk to you.

Summary so far:
(Received signal) = (transmit power) – (loss between transmitter and antenna) + (transmit antenna gain) – (path loss) – (multipath and obstruction loss) + (receive antenna gain) – (loss between antenna and receiver)
In order to operate, (received signal) must be greater than (receiver sensitivity).

Another factor is noise. This is “background” radio-frequency junk that your receiver can “hear” but needs to reject. Sources of noise include other wireless networks, cordless phones, microwave ovens, radio hams, medical equipment, Like other radio phenomena, noise may be highly variable. Many wireless network adapters do not report noise, so if you’re using NetStumbler with them then you can’t even tell how much noise you have in your environment. A typical urban location these days might have an average noise level around -95 dBm. When you switch on the microwave oven or take a call on your 2.4GHz phone, this value will increase. I’ve seen a 2.4GHz phone produce -50 dBm of noise, which is enough to saturate some Wi-Fi radios and thus kill their connection completely.

Let’s take these concepts and combine them. In order to operate, the actual signal level at your receiver needs to be higher than the noise level. The actual signal level varies depending on signal fade, so if you measured -75 dBm one day, it might drop to -85 dBm occasionally. On most radios this is sufficient to make it drop to a lower data rate, and on some it will cause the connection to drop altogether. Likewise your background noise might be around -98 dBm, but then your neighbor takes a call on her cordless phone and it jumps to -78 dBm. With multipath effects, this is sufficient to make your connection drop randomly.

My conclusion, therefore, is:
Q: What signal level should I consider usable for a good wireless link?
A: Depends on your equipment and your environment.

How to clean a clogged port

If you’re having trouble getting USB or other devices to work with your computer, cleaning any grime from the connecting ports is a good first step.

Turn off the hardware first. If you can see that the port is severely clogged, say, with peanut butter or another thick, child-friendly substance, gently dig the gunk out with toothpicks. But remember: Ethernet, serial, and other jacks rely on fragile pins, so try to move in and out on the same path instead of swirling around the sides.

For less sticky situations, use a can of compressed air (actually pressurized gases) to blast out loose bits. These cans sell for £5 to £10 at any computer store. (Keep them away from children: The gases are dangerous to inhale and come out extremely cold.) Fire at the target in short bursts.

Finish cleaning with a cotton swab dipped in rubbing alcohol or electronics cleaner. Leave the device turned off for a couple hours to dry.

Snow Leopard vs Windows 7

Which operating system is best?

This is shaping up to be the autumn of new operating systems. The latest version of Mac OS X, Snow Leopard, ships to customers this Friday. Windows 7, the follow-up to the much-maligned Windows Vista, hits store shelves in late October. Neither operating system is going to drastically change the way you work.

Windows 7 builds upon Windows Vista and smooths out Vista’s rough spots, though it also brings a number of new end-user features to the table (such as the re-worked taskbar). Meanwhile, Apple’s focus with Snow Leopard mostly involves new under-the-hood technologies with subtle refinements and fixes. But there is plenty to say about how Apple’s next big cat and Microsoft’s lucky number stack up against each other.

Managing Your Files

Snow Leopard’s Finder and Windows 7’s Explorer have strikingly similar interfaces: Both have quick search fields in the upper-right corner, path bars (OS X’s is optional and can be switched on in the View menu), and sidebars giving you easy access to various common locations on your computer.

Windows 7 introduces a new feature to the mix: Libraries. A library is best defined as a way to view the contents of several folders all in one place. For example, the Pictures library pulls together the contents of the My Pictures and Public Pictures folders by default. You can add or change the folders tied to any particular library, of course.

Nothing in Snow Leopard directly compares with Windows’ libraries. The closest OS X feature is saved searches (known as Smart Folders), but a saved search pulls together files based on search criteria, not location. You can’t, for example, create a smart folder containing all photos from only two folders. On the other hand, Windows 7 libraries can’t be combined with saved search results.

Both Snow Leopard and Windows 7 allow a large icon view. Windows 7 supports icons up to 256-by-256 pixels. Snow Leopard one-ups Windows 7, though—the Finder can display icons up to a seemingly absurd 512-by-512 pixels (512-pixel icons were around in 10.5, but the Finder could not take advantage of them outside of Quick Look and Cover Flow view).

Quick Access

Some OS X apps can use the Dock’s pop-up menus to display application-specific information and provide easy access to frequently used commands. For example, if you right-click iTunes’ Dock icon in Snow Leopard, you’ll get a menu that lets you see what’s playing, play or pause your music, assign a rating to the current song, and control other simple iTunes commands.

With Windows 7’s re-tooled taskbar, Microsoft introduces a similar feature called jump lists. Jump lists can not only provide access to common commands (Windows Media Player‘s jump list has a Play command, for example), they also let you “pin” items to a specific list. For example, you can pin commonly-used folders to the Windows Explorer jump list and important documents to the WordPad jump list.

Snow Leopard doesn’t have any features that directly compare to the jump list’s pinning feature; instead, Mac users can use stacks in the Dock to provide quick access to folders and files (drag any folder to the Dock to create a stack). Stacks get a refresh in Snow Leopard: You can now view unlimited items in a stack using Grid view (thanks to the addition of scrollbars), as well as drill down into folders without having to open any Finder windows. You can also drag and drop any file into the Dock for quick access.

Previews

In addition to Leopard’s preview features (Quick Look, Cover Flow), Snow Leopard adds inline preview to the Finder’s icon view. When viewing a folder using 64-by-64 pixel icons or larger, mousing over your files will display preview and playback controls. If you mouse over an audio or video file, you’ll get a play button. If you mouse over a Word doc, a PowerPoint presentation, or PDF (among other common file formats), you’ll get forward and back arrows for paging through a document.

Windows 7 has an optional preview pane for use in any Explorer window. Select a file, and the preview will appear in the preview pane. Windows 7’s preview feature seems pretty basic compared to Snow Leopard’s (text loses all formatting, for example), but it’s better than nothing. Also, as was the case with Vista, folder icons in Windows 7 give you a peek at the folder’s contents.

PDF Reader Pro for iPhone

Snow Leopard arrives

Sneaking in a few days before its promised September release, the tune-up for Mac OS X Leopard costs £25 for current Leopard users, and packs just enough punch to be worth your money. Apple is careful to point out that Snow Leopard isn’t a complete system overhaul, but rather a collection of hundreds of smaller refinements to make Leopard run more gracefully. Hidden among smaller tweaks are some technical improvements that result in a smoother, easier-to-use Leopard with plenty for Mac fans to be excited about. The user interface and everyday tasks feel faster in general, although we didn’t notice a substantive improvement in application performance.

Even if you’re not a current Leopard user, the £129 package that includes Snow Leopard, iLife and iWork is a steal for the system upgrade and two of Apple’s major software suites — not to mention the long-pined-for inclusion of Microsoft Exchange compatibility. Finally you will be able to connect with Exchange Servers (without using Microsoft’s Entourage), but only if your company is using Microsoft Exchange 2007 — many still aren’t. Snow Leopard is offered on a single install disk (there are no separate, tiered pricing structures to worry about), and you’re getting every feature and technical enhancement available in a single install. Unfortunately, for those on PowerPC systems, Snow Leopard only works with Intel Macs.

Installation

Installation of Snow Leopard is dead simple and (according to Apple) up to 45 percent faster than Leopard using a newly designed installer that asks only one question during the process. On our test machine, the process took about an hour, including two automatic restarts. The default setting installs Snow Leopard without tampering with any of your saved files, music, photos or documents. Mostly we had no problems, but on one test machine we needed to reinstall the OS when it had trouble rebooting. Fortunately the new installer is designed for safely reinstalling the OS in the event you encounter any glitches during your initial installation. On our second try, the OS installed perfectly on our test machine and no files were harmed. PowerPC Macs are no longer supported with Snow Leopard, however: you will need an Intel-based Mac to install the latest Mac OS.

Those who want to do a Clean Install (starting fresh by deleting everything for minimal conflicts) still can, but unlike installations in previous versions of previous Mac OS X that offered the clean install as a primary option, you’ll need to use Disk Utility to first erase the volume, then run the install. Apple told us that not everyone knows what a clean install is and often chose it, not knowing that they would lose their files. We’re happy with that answer, as long as people still get the option in some form.

Apple also claims that Snow Leopard uses 7GB less space than Leopard because of better file compression paired with selective driver inclusion. According to Apple, Snow Leopard will locate any missing drivers on the web for you. We had no need of any special drivers during our tests.

New technologies

Apple says a few new technologies in Snow Leopard make it worthy of the upgrade alone, with several features that will boost performance. Because all new Macs come with 64-bit multi-core processors, multiple gigabytes of RAM, and high-powered graphics processing units, all the major applications in Snow Leopard — including the Finder — have been rewritten in 64-bit to take full advantage of the hardware (64-bit technology allows application developers to allocate more memory to complete tasks so that the software runs faster and more smoothly).

Apple has also added what it calls the Grand Central Dispatch (GCD) that manages data sent to multi-core processors in an effort to maximise performance; Apple says GCD will speed up any application task, from processing images in Photoshop to playing your favourite games. The addition of the GCD also takes away the need for software developers to spend as much time managing multi-core processors.

Another new technology in Snow Leopard is OpenCL, which allows software developers to tap into the power of any onboard video cards and their GPUs (Graphics Processing Units) for general-purpose computing without the addition of enormous amounts of code. Like the GCD, these are improvements that will mainly affect software developers. But hopefully it will mean more and better-performing software for users in the future.

To put some of these claims to the test, we decided to pit Mac OS X 10.5.8 Leopard against Mac OS X 10.6 Snow Leopard to see how these new technologies affected overall performance.

In our anecdotal tests of performance within the Snow Leopard user interface (UI), the operating system seems faster and more responsive than with Leopard. Finder, Stacks, Expose, launching apps and other everyday processes feel snappy. We didn’t, however, notice any improvement in application performance.

Overall, we saw a 2.5 percent slowdown in application performance from Leopard to Snow Leopard on our more processor-intensive performance tests — including our multimedia multitasking test, in which we measure the time for QuickTime to finish converting a short movie while iTunes is performing its own conversion of MP3 into AAC format in the background simultaneously. As this falls within our typical margin of error (5 percent), we saw no significant difference with application performance when moving from Leopard to Snow Leopard (see the Benchmarks tab for the performance charts.)

New features

Exposé

Snow Leopard includes a number of user interface improvements intended to make working with Mac OS X easier and more efficient. Exposé, Apple’s system for visually finding the window you want on a cluttered desktop, used to be relegated to the Function keys on your keyboard. Snow Leopard now makes Exposé accessible from the Dock; just click and hold on a Dock icon to see thumbnails of all the open windows in that application. Hitting the Tab key lets you cycle through the preview thumbnails of each open application. Using Exposé in the Dock is very natural and elegant, making us wonder why this wasn’t already a feature in Leopard.

Click and hold on an application icon in the Dock to bring up full thumbnails of open windows in an application.

The Dock

In addition to using Exposé to find the right window, you now also have the ability to drag files from one application to another using the Dock. Let’s say you want to add an image to an email, but your desktop is full of open windows. In Snow Leopard you can go to the image, drag it to the Mail icon in the Dock, and your email window will spring-load, allowing you to drop the image into place. Although the ability to drag and drop files in this fashion is nice, we’re not sure it’s much easier than attaching an image by browsing through your folders. Still, if you know the image is already on your desktop, it’s probably the faster method.

Stacks

Stacks has had a much-needed upgrade as well. In Leopard, Stacks only listed a certain number of files and applications, requiring you to go to a Finder window if your app wasn’t listed. Similarly, if you tried to open a folder in Stacks, you were sent to the Finder. In Snow Leopard, Stacks comes with a scroll bar so icons are still easy to read and anything can be launched out of the Dock. Folders are now accessible within Stacks as well, so you’ll be able to navigate to files within folders all without leaving the Stacks Window. These changes make Stacks much more useful than before and probably should have been available when Stacks was introduced.

Now you can scroll through your applications and documents (and even open folders in Stacks) without being sent to the Finder.

The Finder

Although the Finder itself has seen little in the way of interface tweaks, the way files behave in the Finder makes it easier to use. A zoom slider has been added to the lower right of Finder windows so you can zoom in on icons. An enhanced icon view has been added, letting you preview multipage documents and even play QuickTime movies without ever leaving the Finder window.

Preview now lets you preview almost any file, even if it was created with software you don’t have on your hard drive. This means common file types from Microsoft Excel, PowerPoint and even PDF files can all be previewed without owning the originating programs. As an added bonus, Preview in Snow Leopard provides accurate text selection to multi-column PDF files using artificial intelligence to infer the layout of each page. This means that Preview recognises that there are multiple columns in your document so you can select the text you want from any column.

You can now flip through the pages of multi-page PDF documents using the arrows that appear over PDF docs when you mouse over.

Safari 4

Safari 4 has been widely available for some time, but it offers a couple of new features when running in Snow Leopard. Safari 4 already includes Top Sites for viewing all your favourite sites as thumbnails for easy access and full history search, which lets you view your history in a Cover Flow-like interface. But in Snow Leopard, Safari is now crash resistant. This means that if a plug-in crashes, it won’t crash the whole browser. Simply refresh the page to try to load the plug-in again. Also, Safari checks to see if a site you are visiting is known to be fraudulent, is distributing malware or is known to be a phishing site, and then warns you if it is.

Top Sites, which was already available in Safari 4, lets you navigate to your most viewed web sites quickly.

QuickTime X

QuickTime X, Apple’s media player, has had some major tweaks in Snow Leopard. Now, when you play a movie and move your mouse outside the window, the interface fades away quickly to give you a more immersive video-viewing experience. When watching a movie, you can click the new Share button to convert your movie for iPod, iPhone or Apple TV, and QuickTime converts the video to work best on your chosen device. You also can now record video from your webcam, audio or just the action on your screen with a few clicks. Those with the iPhone 3GS will recognise the new trimming feature in QuickTime X, letting you grab just the video content you want.

QuickTime X probably received the most interface tweaks in the Snow Leopard update. The cleaned-up interface and autofade features look great (like most things Apple), but it’s more of an aesthetic improvement than anything else. The recording features for video, audio and screencast capturing are the big wins here and used to be offered only in QuickTime Pro. It’s good to see these features will be able to be used by a wider audience in Snow Leopard.

Trim your videos easily by clicking and dragging start and end points of the clip.

Exchange support

One of the main roadblocks for Mac users in a primarily Windows workplace has been the inability to connect with Microsoft Exchange servers. Most Mac users used Microsoft Entourage or available open-source options as a workaround, but it was never as smooth as connecting from a Windows machine with Microsoft Office. Snow Leopard now supports Microsoft Exchange Server 2007 out of the box so you can easily connect using Apple’s Mail app, grab global address lists in the Address Book and create meetings with contacts using iCal.

Apple has done more than simply give you the ability to connect, however. Common tasks like creating meetings, for example, are incredibly easy with intuitive controls. iCal lets you view work events and personal activities all in the same window (with easy controls to include or not include the information you want). The Apple Address Book works seamlessly across Mail and iCal so you can quickly bring up global address lists, add people to a meeting (including predesigned groups) and invitations will automatically be sent to each attendee. As an added bonus, if some attendees have scheduling conflicts with your proposed meeting time, iCal will automatically figure out the earliest available time that everyone is free. These are features already available in Microsoft’s Outlook for Windows, but in Snow Leopard the process feels much more intuitive.

File Quarantine

According to Apple, File Quarantine has also been refined in Snow Leopard. First introduced in Mac OS X 10.4 Tiger, File Quarantine checks for known malware signatures, and in Snow Leopard will now display an alert dialog if it finds a known offender. The dialog will tell users to move the offending file to the Trash. For example, a bogus version of iWork was circulated on the web a few months ago that contained malware. That particular malware is now automatically detected by File Quarantine in Snow Leopard.

Apple says that File Quarantine will be automatically updated via Mac OS X’s software update as new malware signatures are found in the wild. We had no way to test these features, but we are happy to see that Apple is taking steps to defend against malware as more people switch to Macs and the danger of new malware becomes more prevalent.

Universal access

Beginning with Mac OS X 10.4 Tiger, Apple included VoiceOver to help people who are blind or with impaired vision to better understand and interact with what’s happening on-screen. Apple continues to help visually impaired users in Snow Leopard by adding gesture support on multi-touch trackpads with easy-to-learn gestures to perform specific functions. We had mixed results with these features depending on the web page we visited, but mostly we found the features to be useful. The trackpad acts as the viewable area on the current window so you can tap to have window elements explained to you or swipe to move on to the next item in the window, for example. New features in Snow Leopard are particularly helpful when web browsing, with options like web page summaries to explain the various elements on a page you haven’t visited before, making it easier to get the information you want.

More than 40 different Braille displays (including wireless Bluetooth displays) are supported in Snow Leopard right out of the box, allowing visually impaired users to plug in and start computing immediately.

Other refinements

Some of the smaller refinements in Snow Leopard are noteworthy, affecting many of Apple’s core apps. iChat is now compatible with more routers, making video chat accessible to a wider range of users, and iChat Theater now offers 640 by 480 resolution, using only one-third the bandwidth it used before. New Chinese character input via the trackpad predicts which characters you’re creating and offers likely subsequent characters to speed up the process. A new text substitution feature in applications like iChat, Mail and TextEdit lets you create shortcuts for phrases you use frequently. The Services menu in Snow Leopard has been rewritten to include only the services relevant to the application or content you’re viewing. Core Location technology locates nearby Wi-Fi hot spots to find your location and automatically resets your time zone so wherever you are in the world, your Mac will be set to the correct time. These are all small refinements, but each one makes your Mac easier to use with smart features not found in other operating systems.

Conclusions

Mac OS X 10.6 Snow Leopard is not a complete system overhaul, but is instead a refinement of the current Leopard OS — some have gone so far as to call it a ’service pack’. We think the interface tweaks to Exposé, Stacks, the Finder, Mail and iCal make Snow Leopard more than just a service pack and worthy of the £25 upgrade price. We don’t like the fact that PowerPC users are unable to use Snow Leopard, but we understand that after three years with Intel, Apple is making a decision to continue moving forward with this technology.

The largest of the feature enhancements are probably reason enough for Intel Mac users to spend the money on Snow Leopard. Added enhancements such as video, audio and screen recording in QuickTime X were once only available to those who purchased QuickTime Pro (which cost about the same as this system upgrade). But the killer feature addition for Snow Leopard could be Exchange support out of the box — not even Windows 7 comes with Exchange support without buying Microsoft Office.

Overall, we think that Snow Leopard delivered almost everything Apple says it set out to do: it refined and enhanced Leopard to make it easier to use. Although the system performs well in everyday use, many of our tests indicate it’s slightly slower than the older version of Leopard in more intensive application processes. Still, we highly recommend upgrading for all the new features and Microsoft Exchange support.