无线路由器常见问题

原文：http://www.smallnetbuilder.com/w ... -faq-the-essentials


SMALLNETBUILDER'S WIRELESS FAQ: THE ESSENTIALS
Updated 5/19/2011 with more FAQ!

I've written many articles about Wireless Networking that I often refer to when answering questions in the Forums. But rather than having to search the site again and again to unearth those gleaming nuggets of knowledge, I figured it would be easier on both of us for me to put the most important info in one place.

If you don't see your FAQ here, let me know. I can put it in the next Wireless FAQ article.

1. What speed can I expect from a wireless connection?

There are so many things that affect the performance of a wireless connection that this really is a "it depends" situation. But Table 1 attempts to summarize results from our actual testing in a residential home environment.

These are maximum, best-case results, using a TCP/IP connection from a single client connected to a router with about 10 feet between the two and no other networks active.

Product Technology	Approximate Maximum Throughput (Mbps)
	20 MHz B/W	40 MHz B/W
802.11b	4-6	N/A
802.11g	20-23	N/A
Enhanced 802.11g (SuperG)	20-23	35-40
802.11n - 1 stream (w/ N client)	40	60
802.11n - 2 stream	70	100

Table 1: WLAN Technologies vs. Maximum Throughput

Note that N routers with "out-of-the-box" factory settings will provide speeds in the "20 MHz B/W" column. You need to change a setting on the router (and sometimes in the client) to achieve speeds in the "40 MHz B/W" column.

Also note that while 100 Mbps speeds are possible, highest speeds tend to run more in the 80 Mbps range than 100.

2. So why does Windows show me a "speed" of 130 / 300 Mbps?

This number is only distantly related to the actual throughput of your wireless connection. What it shows is whatever the wireless adapter's driver tells it to show, which is usually the link rate currently being used.

Figure 1: Pay No Attention To This Number

The link rate is also referred to as the PHY (or physical layer) rate, which is the maximum rate that bits will move across the network link. For a 10/100 Ethernet adapter, you will usually see a speed of 100 Mbps and for a gigabit NIC, you will see 1000 Mbps (if you are connected to a gigabit switch port).

The reasons for the big difference between the "link" and actual rates include the high overhead involved in wireless connections (lots of bits used for communicating information other than the actual data you are trying to send or receive) and data retransmission (due to the inherent unreliability of a wireless connection). So maximum usable throughput is a much lower percentage of the link rate, usually 50% or less.

For more information on how to find the real speed of your wireless network connection, see 5 Ways To Fix Slow 802.11n Speed.

3. Will switching to N improve my wireless range?

In general, 802.11n products will not increase your wireless LAN range, no matter what the companies trying to sell you their wares say. They can, however, provide higher speeds at a given distance, as long as signal strength is relatively strong.

In other words, if you're getting a weak signal in a particular spot with your current G router and card, you'll still get a weak signal with an N router and card. But you might get a bit more speed, perhaps enough to provide basic connectivity. But not enough for good video streaming.

In locations where you get a medium-to-strong signal with G equipment, switching to N will give you a 2 to 3X (maybe more) speed boost.

4. Will switching to an N router improve the speed and range of my G devices?

No. When G devices connect to an N router, they do so at only G speeds (54 Mbps maximum link rate). See Question 3 for the range answer.

5. Is it ok to use G devices with an N router?

Sure. N routers fully support G (and even B) clients. This is called a mixed WLAN. If you have an especially busy wireless network, with both B/G and N clients active at the same time, you may see the speed of both the G and N devices drop.

If you're doing long file transfers or downloads or watching high-def video, you definitely will see a performance degradation. But if you're just web browsing, chatting or emailing, you might not notice.

If you do see a slowdown, you can just convert your old G router to an access point (see How To Convert a Wireless Router into an Access Point) and have your G devices connect to it, instead of the N router.

See Add, Don't Replace When Upgrading to 802.11n for a full explanation.

6. I want to stream HD video wirelessly. What N router should I buy?

Simply put, none. There is no product on the market today that can reliably deliver trouble-free HD content over a wireless connection, even if the distance is just a few rooms. You really need to use an Ethernet connection. 10/100 is fine. You don't need Gigabit Ethernet.

7. But I can't run Ethernet. Is there any way I can use wireless streaming?

Success with using wireless to stream multimedia content is extremely dependent on the bandwidth requirements of the content you are trying to stream.

Music generally requires in the 100s of Kbps and will be ok using even slower 802.11g connections. 1080p HD video, on the other hand can require 20+ Mbps of throughput with peak throughput twice that. So even 802.11n might not provide a trouble-free experience.

You should first profile the bandwidth of typical content you'll be streaming by running Netmeter or LAN Speed Test on a PC that is playing the stream. Be sure you run the stream long enough and include fast-action scenes, which can really spike throughput.

One you know the requirements, then you'll have a much better idea of what you're up against. The lower the bit rate of the stream, the more likely you'll have a good experience.

Wireless, especially 802.11n, is not as reliable as Ethernet and has higher speed variation. It also has periodic "dropouts" where throughput drops virtually to zero for over a second. Unless you have a lot of buffering in your player, you'll see problems in the video.

Read through these articles for more background:
Video Streaming Need To Know: Part 1- Encoding, Bit Rates and Errors
Video Streaming Need To Know: Part 2 - The Real World
HD Streaming Smackdown: Draft 11n vs. Powerline

8. Do I have to use a "matching" N card and router for best performance?

In the early "draft N" days, the answer was yes. The kinks were still being worked out of the technology and different chipset manufacturers frequently implemented optimizations that worked best between their AP and client chipsets.

These days, it's just not practical to match APs / routers and clients because all mobile devices come with wireless built in and you have little to no control over the adapter used.

9. But if I do have a choice, can the wireless adapter I use make a difference in performance?Updated 14 June 2010

Yes, but the difference isn't huge. A roundup test of nine wireless clients showed the Intel Wi-Fi Link 5300 as the overall winner. It's also the adapter we use as our standard wireless test client, but this in no way influenced the testing.

The Intel adapter is a mini-PCI-e board. But there are PCI-e to PCI and PCI-e to USB adapters that you can use. HWTools.net is one site that carries them.

10. Will a Dual-Band router connected to a dual-band client go twice as fast?

No. Dual-band clients have one radio. So even if you have a two-radio dual-band router (often calledsimultaneous dual-band), the client can connect to only one radio at a time.

And don't bother trying to use two adapters for higher speed. I don't know of any wireless adapters that support aggregation or bonding.

11. Why is the maximum "speed" of some wireless adapters and routers only 150 Mbps?

The rates you see manufacturers quote are maximum link rates, as explained in Q2.

Maximum link rate depends on the wireless standard and, with 802.11n, the number of streams and the bandwidth mode. It does not depend on frequency band (2.4 vs. 5 GHz). Table 2 has a quick rundown of maximum link rates. A full list of 802.11 rates is here.

Product Technology	Maximum Link Rate (Mbps)
	20 MHz B/W	40 MHz B/W
802.11b	11	N/A
802.11g	54	N/A
Enhanced 802.11g (SuperG)	54	108
802.11n - 1 stream	65	150
802.11n - 2 stream	130	300
802.11n - 3 stream	217	450

Table 2: Maximum 802.11 link rates

The rates you may see might be different, depending on options in the standards that different manufacturers use. That's why some two-stream N adapters will show only maximum link rates of 117 / 270 vs. 130 / 300, or why you may see only a maximum 135 Mbps link rate with some single-stream N products.

If you want a maximum link rate of 130 / 300, you need to have both client and AP that support that rate. If you see "150" in either the adapter or router / access point description or spec., you are getting a single-stream N router that will never deliver link rates higher than 150 Mbps.