On Sat, Jul 29, 2006 at 10:39:19PM +0200, Nathan Orenstein wrote: > I am confused by what my Internet bandwidth really is and how to measure it. > I noticed that I had some problems with slow Internet. I did not expect > this since > I am supposedly getting 2.5Gbps from Netvision.
Not on this planet. Maybe 2.5mbs (2.5 million BITS per second). That's the "raw" download speed. In plain English if you are lucky, you get a data bit around every .00000040 seconds (.40 microseconds). What that data bit means is a whole 'nother story. Data is sent in frames. A frames consist of header information, actual data and trailer information. Depending upon the proctol involved there can be start and stop indicators, length indicators, checksums, etc. You did not mention how you get to Netvision, aDSL uses a protcol called ATM (asynchronus transaction managment), which was designed for the transmission of voice between digital telephone switches. It uses a packet size (the useful data in a frame) of 56 BYTES, 448 bits. How much of those bits that mean anything to you, is far less. Cable modems use a different protocol. AFIK it's similar to token ring, so it has larger packets. This is good and bad. Larger packets means that you get more data with less overhead, smaller packets mean that you get less latency (the amount of time it takes for data to get from you to where it's going). Normally in Israel you use some sort of VPN (virtual private network) between you and your ISP. This is because both BEZEQ and HOT cannot legally connect you to the Internet and without a VPN, it becomes more difficult for your ISP to control and monitor your traffic. Using a VPN (or dialer as they are called here) causes all you packets to be sent via a "tunnel". Therefore packets sent to you are put into tunneling packets, and sent via the tunnel to you. They automaticaly get split apart and put back together several times before they end up at your ISP. The tunneling packets are usually sent via TCP. TCP checksums each packet on the way out and includes it in the packet. On the way in, each packet is checked to see if it matches. If it is a packet is sent back saying that the packet was received correctly. This is called an ACK packet. To save time, ACK packets are "windowed", they acknowlege a group of packets and only the missing ones are resent. If for some reason the ACK packet is not received, everything waits for a "time out" and then the whole window is resent. Your connection speed is controlled by how quicly you can decrypt (if they are encrypted), checksum, build an ACK packet and get it back to the other end. For example, my router, an 800mHz PIII does this a lot faster than a little stand alone router with a 200mHz ARM processor. If you are uploading, your ACK packets get held up your data packets, remember latency? More data, longer latency. Now, once the data goes out into the world, it is sent around using a protocol called IP (Internet Protocol). IP is a packet routing protcol. You stuff a packet in here and it pops out there. No one guarentees when a packet will pop out or if. There is also no guarentee that two packets sent in sequence will arrive in sequence. IP depends upon the fact that very few systems are directly connected to each other. Each packet is stored and forwarded when its turn comes up to the next node along the way. Each of these is called a hop. You can run traceroute and see how many hops a packet needs to get somewhere and how long it takes. Since the information provided by traceroute can be used to "hack" a network or for DOS (denial of service) attacks, many commercial sites block it. Because it's a store and forward network, your data only goes as fast as the slowest link along the way. The amount of time (latency) from here to there is the SUM of all the latencies along the way. Although NO ISP will actually guarantee how much data you will really get, they only claim that if you loose no data to retransmission, etc, you will get it between their gateway and yours. > I looked around on the Internet and found <http://www.abeltronica.com/> > which > allegedly allows me to measure my download and upload bandwidth. > Using the above I got as little as 500Mbps, up to 1.4Gbps as my > Internet download > speed, depending on time-of day and location to which the measurement > was made. I'll assume that you were confused about the numbers, but you are measuring the ammount of data per second that goes from their server, through several networks, to the U.S. hun of your ISP, usually in New York, via fibre optic cable under the Atlantic to the U.K., across the English channel, etc, etc, etc. Since traceroute only shows you IP connections, and most of the connections use some underlying protocol, you won't see them. For Netvision, you will see them at a host in Haifa and the next one will be in New York. The details of what happens in between are hidden from you. > I then contacted Netvision who asked me to download a large file from > their site > <http://cables2.netvision.net>. This download "settled" at about > 250Mbytes/sec in > the first third of the download -- Netvision said this proves that the > 2.5Gbps is > indeed being provided. Again mixed units and wrong ones. I'll assume you meant that the download speed from their host was 250 thousand bytes per second, or about 2 million bits per second. Considering there is overhead in the packets, additional overhead splitting them into frames, time needed for ACK packets, etc, this is pretty good, > > I am wondering if the Netvision-supplied bandwidth for Internet > connections is really > in the order of 1.3-1.4 Gbps, maximum, and the extra bandwidth being > supplied (i.e, the > difference between 2.5Gbps and the bandwidth measured through > abeltronica) is > not really useful to me in accessing the Internet. No, it's of no use at all except entertainment. Geoff. -- Geoffrey S. Mendelson, Jerusalem, Israel [EMAIL PROTECTED] N3OWJ/4X1GM IL Voice: (07)-7424-1667 Fax ONLY: 972-2-648-1443 U.S. Voice: 1-215-821-1838 Visit my 'blog at http://geoffstechno.livejournal.com/ ================================================================= To unsubscribe, send mail to [EMAIL PROTECTED] with the word "unsubscribe" in the message body, e.g., run the command echo unsubscribe | mail [EMAIL PROTECTED]
