Your Internet service provider tells you that your service should provide "up to" 8 Mbps downstream. But then a Web-based speed tester tells you you're getting less than that. Who's to blame here?
To figure that out, you have to look at four components of your Internet connection: your local area network (LAN), the gateway (typically a router) that connects your LAN to your broadband modem, the modem itself, and the wide area network (WAN) that connects that modem to your ISP.
(What's that you say? You haven't tested your broadband speed? You really should. DSLReports.com has a long list of them. To use one correctly, make sure that nothing else is happening on your network or over your Net connection--no automated backups, no file downloads, no streaming video, not even any e-mailing. If at all possible, plug the computer directly into your broadband modem.)
The Local Network
Because you control your own LAN, it's a good place to start troubleshooting a problem with your broadband. (In addition to the advice that follows, you should also consult Troubleshooting AirPort Interference and Improving AirPort's Range.)
Upgrade Your Switches: An Ethernet switch is like a phone exchange; data traffic from your computers and other devices is routed through the switch to its destination. If you've got a lot of local traffic or a fast broadband connection, an older switch can slow you down.
First, make sure all your switches support Gigabit Ethernet. If you have an AirPort Extreme base station, that means making sure it was released sometime after September 2007. All Time Capsules support Gigabit Ethernet. If your Ethernet switch doesn't, upgrading is relatively inexpensive: about $40 to $50 for a five-port switch.
Watch the switch to figure out whether your network is working at Gigabit Ethernet rates. Almost all switches have a color LED that indicates the Ethernet link speed: it'll show orange or yellow for 10 or 100 Mbps, and green for 1 Gbps. If you expect Gigabit speed but see orange or yellow, you need to check the cable (as described in the next section).
Some switches also have lights that indicate collisions on the network. Ethernet collisions occur when multiple devices start "talking" at the same time. When such a light flickers, that's also an indication of cabling problems.
Check Your Cables: To begin with, make sure you have the right cables. Gigabit Ethernet requires Category 5E cable (for short runs) or Category 6 (for long runs). Most cables have their type printed repeatedly in small letters along their length. Neither type of cable is expensive: a 14-foot Category 6 patch cable is about $8.00 at Cyberguys.com, for instance. If you have the wrong cable, replace it.
Ethernet cables can also be surprisingly fragile, especially after years of use. If you suspect a cable problem, inspect your cables for bends or kinks. An Ethernet line should never be folded over itself; if you coil it, make sure the loops are several inches in diameter. The insulation should be smooth and rounded, with no twist marks or stretching. If you see these signs of wear, replace the cable. And if you're storing Ethernet cable, don't coil it tightly. You should especially avoid "ribbon wrapping" a cable--tightly wrapping a loose end of a coiled cable around the middle of the coil to keep it together. Rather, you should roll cable loosely, letting it fall into a loop.
You can use your Ethernet switch to test your wires: First check the switch's activity lights when a cable you know to be good is connecting the computer and the switch. Then connect the cable you want to test, and check the lights again. If the suspect cable causes the switch's status light to change color when data is flowing, ditch the cable.
Make Sure You're Using 802.11n: The most common cause of problems with wireless networks is mixing older Wi-Fi equipment with more-modern hardware.
The latest 802.11n equipment has a net throughput of 30 to 150 Mbps; 802.11g (which was used in AirPort Extreme hardware from 2003 to mid-2006) and 802.11a (included in first-generation Intel Core Duo systems) have a net throughput of about 25 Mbps; and 802.11b is, of course, even slower. (Net throughput is the amount of real data that gets through, as opposed to raw rate throughput, which includes the bandwidth consumed by error correction, signaling data, and other network overhead.)
So even if you have a nice, new 802.11n gateway, any 802.11g or 802.11b equipment on your network can slow down your Net connection. If the connection isn't that fast to start with (say 10 Mbps or slower), this older equipment may not affect performance. But if your broadband connection is 10 Mbps or faster, you'll want to replace older Wi-Fi equipment.
Apple has made that easier by enabling its newest AirPort base stations to use two wireless bands (2.4GHz and 5GHz) at the same time. This allows you to segment your network into fast and slow sections: your 802.11g and 802.11b hardware can use the 2.4GHz band, while your faster equipment can communicate over the 5GHz channel. (See "Understanding Wi-Fi's two spectrum bands".)
You can upgrade older Macs with 802.11n adapters. These adapters work only in the 2.4GHz band, but you'll still boost your network speed. Newer Technology has some particularly inexpensive USB, PC Card, and PCI Card adapters that will work with OS X 10.3.9 and later.
Speed-Test Your Wireless Network: Interference can also slow down your wireless network. If a computer or another device is connected to a nearby base station but the data rate is less than the maximum (11 Mbps for 802.11b, 54 Mbps for 802.11g, 130 Mbps for 802.11n using the 2.4GHz band, and 270 Mbps for 802.11n using 5GHz with wide channels), interference may be the culprit.
To find out how quickly a Wi-Fi adapter is connecting to a base station, launch AirPort Utility, select your base station, and click on Manual Setup. Then select the Advanced tab, and click on the Logs & Statistics button. In the Wireless Clients tab, you should see the unique MAC (Media Access Control) address of each adapter and the speed and standard that it's using to connect.
To identify which client is which, click on the DHCP Clients tab, which shows the Bonjour name or the DHCP Client ID for each. Alternatively, you can open the adapter settings in the Network preference pane, click on the Advanced button, and then click on TCP/IP. The AirPort ID is the MAC address that you'll see in the list. (You can also use this method to test how fast your network works with a given laptop or mobile device at various places in a home or an office.)
The second Internet-connection component that can slow down your Internet connection is the gateway. Typically, that's a router, such as an AirPort base station, connected to your broadband modem.
Double-Check Your NAT: If your service is 30 Mbps or faster, and your gateway is configured to hand out private network addresses to your local network, that gateway might become a bottleneck for data.
The problem is that network address translation (NAT), which lets multiple devices on a local network share a single gateway IP address, requires constant computation. Every hunk of data passing in or out needs to have the addresses rewritten. If that data is coming in at high speeds, your gateway's NAT may not be able to keep up.
If you have a superfast Internet account, the workaround is to pay for static IP addresses from your ISP and manually assign one to each of your Macs. You can then turn off NAT in your gateway.
Disable WDS: If you connect multiple base stations via Wi-Fi using the Wireless Distribution System (WDS)--in Apple base stations released starting in 2007, this functionality is called Extend A Wireless Network--you could be shortchanging your bandwidth. When WDS is turned on, each byte of data sent has to be retransmitted for each base station. So if you have two base stations, your network's bandwidth is cut in half; if you have three, it's cut in thirds; and so on. Disabling WDS and instead using Ethernet or powerline networking to connect your base stations should restore your full speed.
The biggest problem with modems is that they get old.
Make Sure Your Modem Is Modern: If you haven't bought a broadband modem or received a new one from your ISP within the last three years, call your ISP to make sure the one you have is up-to-date enough to handle your connection.
Some early DSL modems still in use don't support the faster speeds of newer DSL lines--their slower CPUs can throttle downloads. Newer hardware can work faster.
Similarly, older cable modems that support the DOCSIS 1.0 standard won't be able to keep up with networks that encode data with DOCSIS 2.0 (or, as is increasingly the case, DOCSIS 3.0). Check your cable modem's manual to see which standard is in use.
Your ISP might swap your old modem for a new one at no cost. If you're renting the modem, you should (politely) demand that upgrade. If your initial contract term is up, you can often renew and get a new modem at no cost. If you're within a contract term, suggest that you're thinking of switching providers; you could very well get a new modem in response.
Check Your Connections: Because a broadband modem has one input for a phone line or a coaxial cable, a bad hunk of cable or a wobbly plug can mean inconsistent service, with frequent drops or slowdowns.
Watch the Lights: As with your gateway, keep an eye on your modem's status lights if the network slows down. One DSL modem I had crashed every time a Mac connected to its Web interface, but I'd never have known that if I hadn't been watching its status lights. If something like that happens, call your ISP to start troubleshooting. (Mine was unable to solve the problem; I ended up switching ISPs.)
The Wide Area Network
If you've checked your local network, gateway, and modem, and they all seem to be up to snuff, you should check one more potential source of Internet slowdowns: the wires between your modem and the Internet.
Check Your Cable Again: The cable from the back of your broadband modem to the nearest phone or cable wall jack can suffer from the same problems as any Ethernet cabling on your local network. Check it for bends and kinks, and make sure the insulation shows no signs of wear. If it does, replace it. With coaxial cable, make sure the hard-to-turn hexagonal nut around the connector is as tight as it can be. To be sure, lift the cable to release tension while turning the nut.
Call Your ISP: If you suspect that the line outside your house is causing connection problems, call your ISP and ask for a line test. Such tests aren't as useful as they might be: they can detect only whether your modem is responding. (If you have DSL, your provider may also be able to check the impedance on the line and use that to figure out if it is achieving the proper signal-to-noise ratio for your line speed.)
Watch the Weather: At some point, a wire leaves your house and connects to a demarcation point at a utility pole or a buried line. That connection is typically exposed to the elements. If your bandwidth sags when the wind blows, call your ISP for a service call to check the wire. When service is erratic, calling your ISP is especially useful; a technician may be able to test the line and witness a problem. (Keep in mind that if the ISP doesn't find a problem, you may have to pay for a service call.)
Call an Electrician: If you're certain that there's a problem between your modem and the Internet, but you're also certain that the problem doesn't lie between your modem and the wall jack or between your house and the ISP, you may have a problem inside your walls. (For example, I once had a wasps' nest in an exterior wall, and the insects ate the insulation on the cable coming into the house.) Indoor wiring is your responsibility; if you're sure that's where the problem lies, you'll have to either hire a professional (in some states, that means an electrician) or pull some new wire yourself.
[Glenn Fleishman writes constantly about networking, mostly wireless, and is the author ofTake Control of Your 802.11n AirPort Network(Take Control Ebooks).]
This story, "Troubleshoot Your Internet Connection" was originally published by Macworld.