Half Duplex, Full Duplex, Time Division Duplex, symmetric, asymmetric, and burstable network connections explained.
When you ask any Internet Service Provider about the speed of links they offer, they’ll probably rattle off some numbers, and then use one of the terms listed above. This can make choosing an Internet provider a lot more challenging, and sometimes result in disappointment.
Let’s say you’re looking for a ten megabit per second (10Mbps) link.
10Mbps Full Duplex (abbreviated “FDX”) means you have 10Mbps download and 10Mbps upload at the same time, all of the time. Some providers may call this a 20Mbps link.
10Mbps Half Duplex (abbreviated “HDX”) means that data can move across the link at 10Mbps, but only one end of the link can talk at a time. When both links try to talk at the same time a collision will occur and both ends will stop talking, wait a random amount of time, and then try again. A 10Mbps HDX link will usually feel like a 2Mbps link. There are some exceptions, but for typical web surfing and email access it’s best to avoid using a half duplex connection.
10 Mbps Time Division Duplex (abbreviated TDD) is half way between Full Duplex and Half Duplex. It’s a reasonable compromise. In a TDD link, the sum of the download speed and the upload speed will equal the link speed. For example, if you’re downloading at 7Mbps then you will have 3Mbps available for uploading; 3+7=10. If you are downloading at 2Mbps then you will have 8Mbps available for uploading. Using a TDD connection is typically less expensive than a full duplex link. It works well if you plan on downloading large files, processing them, then uploading them.
10Mbps Symmetric can mean one of two things. Either:
Typically, it will be the second option – which is equal to 5Mbps Full Duplex. Providers typically use the term “symmetric” so that they can inflate the numbers. Let’s face it, 10Mbps Symmetric sounds a lot nicer than 5Mbps Full Duplex – even though they are often the same thing.
Symmetric just means you have the same speed available up and down.
Most residential Internet connections are asymmetric. By definition, these links are different speeds in the up and down directions. DSL modems and cable modems will have specs like 5Mbps down and 1Mbps up or perhaps as high as 15Mbps down and 3Mbps up. Asymmetric connections are typically inexpensive when compared to either full duplex or time division duplex connections. As a general rule, most users will download a lot more content than they will upload. If you’re in the business of publishing websites or moving around large files you may be quite disappointed with this sort of connection.
Commercial Internet offerings are often labelled as “Burstable” which is perhaps the most ambiguous of all the terms so far. Let’s say you’ve been presented with an option for a 10Mbps link burstable to 50Mbps. The sales guy probably said that you can use up to 50Mbps and only pay for 10Mbps. It’s a good pitch, until you ask “how long can I sustain 50Mbps before it drops back down to 10Mbps?” Generally the answer will range from one to ten SECONDS. If you’re downloading anything larger than a few megabytes (like pictures from a cell phone) then you essentially have a 10Mbps link and a bunch of marketing fluff. Also keep in mind that your router will actually need to be able to handle the traffic at 50Mbps – even if only for a couple seconds. Essentially you end up with all of the benefits of a plain 10Mbps link, and all of the drawbacks of a 50Mbps link.