Part 2: Maximizing LAN Bridge Data Rates


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Last month I talked about LAN connectivity and data rates. I covered several ways to verify that the Internet service provider is delivering the full access rate you think you should have. I discussed using a LAN bridge to connect a remote site (such as a transmitter) to the station. It's important to remember that all the links you use for this connection are in series with one another and that ultimately the speed will be limited by the slowest link in the chain.

At your HQ you may have a T-1 to your ISP, but if your LAN bridge to the transmitter site is limited to 512kb/s (as an example) then the speed you measure will never exceed (indeed it will be somewhat lower) 512kb/s.

So let's say now that you've studied the response time of your gateway, and that the speed to an outside speed test site is consistent with what you would expect on your remote LAN. You find that connecting to the Internet seems slow. Trying various URLs yields nothing. What could be the trouble?

As you are likely aware, a big part of using the World Wide Web (does anyone call it that anymore?) is making use of DNS (Domain Name Server) to resolve a URL into an actual IP address that your host then uses as a destination IP address when it builds up packets that go out over the Internet. When you open a browser and enter a URL, a certain amount of time goes by before a response comes from whatever DNS is used by your local computer. Some DNS servers respond faster than others, and if you happen to be pointed at a slow one, then this can slow down your entire experience on the Web.

There again you can do a little research in figuring this out (unless you know it already).

In that same command window, at the prompt type in . Sift through that information and you will see what DNS addresses are in use. After finding that you can ping using that DNS address, and see how short and consistent the amount of ping time is. You'll also notice in the ping results a column labeled TTL, which means time to live. Every time your packets traverse a router, that number is decremented by one. My default TTL for pings is 64 when packets are sent, so I can just subtract the TTL number I get back from 64 to see how many routers my packets have traversed. The lower, the better; it means you are that much closer to the DNS server you nominally use. So, ideally you'll pick a DNS server with a fast ping time, and a TTL that is decremented minimally. Alternatively, you can try another DNS server all together just to see if your overall results are faster. Try looking at DNSserverlist.org (www.dnsserverlist.org) or alternatively use Google's free public DNS of 8.8.8.8 or even Level 3's DNS at 4.2.2.3. I find those to be much faster than the one my ISP normally uses. Change your DNS on your computer if the address is static, or at the DHCP server on your LAN.


Irwin is transmission systems supervisor for Clear Channel NYC and chief engineer of WKTU, New York. Contact him at doug@dougirwin.net.




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