Tech Files
Daddy, What's a Duplex?
Anyone who's spent more than three minutes hanging out with a modem, network card (NIC), or sound card has no doubt run headlong into the magical term duplex or one of its two offshoots, full duplex and half duplex. In most cases, we simply ignore these terms, trusting in that magical beast called Plug and Play (or the other handy tool called ignorance is bliss) to deal with this duplex thingamabob. We pray that our little bit of hardware du jour will work, even though we chose not to deal with the duplex animal—whatever the heck it is.
The scary aspect of duplex is that in most cases you don't need to deal with it; this means that usually, the whole concept of a hardware device's duplexity (my word) is totally academic. So, arm in arm, we trot happily down the yellow brick road of PC repair, confident—nay, smug—that we wouldn't know a duplex device if it bit us.
Let's end this ignorance, shall we? I invite you to once again dive deep into that beautiful computer of yours, to discover what duplex means and how you can use it to make your PC run better ... or at least, to know why something isn't working when you know it should.
Duplex unveiled
Sound cards, NICs, and modems all share one very important characteristic: they both send and receive data. While most of us can easily visualize NICs and modems sending and receiving data, we forget that sound cards come with microphones and input ports to receive sound, and speakers for output.
So, given that all of these devices send and receive data, where does duplex come into play? That's easy. Duplex simply refers to a device's ability to send and receive data at the same time! A half-duplex device can only send or receive at any given moment. A full-duplex device can both send and receive simultaneously. The term duplex used alone means full duplex.
Is duplex capability something we can turn on or off? In most cases, the answer is no. You simply need to know whether a device is full or half duplex, to understand why certain things you ask a device to do may or may not work.
One great example is Internet phone applications like Microsoft's NetMeeting or Net2Phone. These applications enable you to talk to another user over the Internet using your sound card. Sounds great, right? Yeah, but if your sound card isn't capable of full duplex, forget it!
Many cheap sound cards on the market today are only half duplex. In most cases, duplex capability is clearly advertised on the box, as shown in Figure 1.
Figure 1: Full duplex support (click on all images for larger versions)
Certain sound applications provide a setting that allows you to use the full duplex capability of your hardware. For example, here is NetMeeting's Options dialog box, where you can set the application to use full duplex for simultaneous two-way conversations (Figure 2).
Figure 2: Setting up NetMeeting for full duplex
When you see the term full duplex with a modem, it usually refers to a voice modem, capable of handling advanced TAPI (Telephony Application Program Interface) functions like voice recordings. For regular data modems, there's neither a means nor a reason for you to mess with duplex settings.
NICs often have the ability to switch between full and half duplex communication. Which one you use depends on your network. Most modern networks using twisted pair cable have hubs that support both full duplex and older half duplex NICs. You'll want to use full duplex if at all possible, as it's much faster than half duplex.
Most NICs let you change their duplex settings in Device Manager. Try the Advanced tab under the device's Properties, but keep in mind that this may vary depending on your version of Windows.
Figure 3 shows the settings for a NIC. Note that it has an Auto Detect setting; this is usually sufficient, although some hubs require that you force the card to use either full or half duplex. In that case, always try full duplex first, only switching to half duplex if you encounter problems.
Figure 3: NIC Properties including Auto Detect option
Duplex doesn't stop with just these three types of devices. In fact, everything that performs I/O inside and outside of your PC uses either half or full duplex. Serial, FireWire, SCSI, IDE, and USB connections all use full duplex; only parallel connections use half duplex.
So, if someone asks you whether a generic device is half or full duplex, you can play the resident Duplex Super Genius and answer with confidence: unless it's parallel, it's capable of full duplex.
- Mike Meyers