Broadband Primer Part 1

(Not for nerd readers although you’re welcome to stick around and tell me what I got wrong in comments)

What’s broadband? Good Question. There is no consensus on the answer except that it involves a connection which is better than dialup. Most people use the term to mean a connection which can be setup for an indefinite period of time (persistent), which has enough bandwidth (discussed below), and low enough latency (also discussed below) and jitter (below) for whatever use they intend to make of the connection. In practice the requirements for minimal useful broadband keep ratcheting upward since Internet services are designed for users with about 50th percentile capabilities.

What’s bandwidth? That’s an easier question. Bandwidth (in its common but not engineering use) is a measure of how much data can be delivered over a connection in a given period of time. Usually bandwidth is quoted in bits per second (bps). The top speed of most dialup connections in the downlink direction (towards you) is 56 kilobits per second (a kilobit is a thousand bits).

Basic DSL (the broadband you get on your phone line) usually has a downlink speed (synonymous with the colloquial usage of bandwidth) of 768kbps (kilobits per second) but an uplink (from you) speed of only 128kbps.

Cable service these days often offers at least 3 megabits per second (a megabit is a million bits) down and 1.5 mbps (megabits per second) up.

Is a bit the same as a byte? (told you this wasn’t for nerds). No; a byte consists of eight bits. File sizes are usually measured in bytes so an 8 megabyte file has 64 megabits in it. In a perfect world (which assuredly doesn’t exist), it would take 64 seconds (plus a few more for some control bits) to download this 8 megabyte file over a connection which has 1 mbps of downlink bandwidth.

Then what DOES it mean that I pay for an x megabit connection if I can’t count on it to download x millions of bits per second? What AM I paying for? Why can’t I count on downloading at rated speed? Isn’t there any kind of “truth in bandwidth”? Starting with the last question first, no, there is no truth in bandwidth. Most vendors describe the MAXIMUM capacity of the link between you and them (not between you and the Internet) when they quote bandwidth. The fine print almost always says that experience will vary.

Why? First of all, many Internet links are actually shared even between the subscriber and the Internet Service Provider (ISP). Cable is shared; DSL is not; dialup is not; some radio connections are shared and others aren’t; satellite is shared on the downlink side only. So, if every user who is connected is trying to run at maximum speed on a party line, no one is gonna achieve maximum speed. If everyone gets on the freeway at once, no one gets to drive at the speed limit. Note that even when connections are nominally not shared like DSL, there are technical reasons why too many connections at once can still degrade service through various types of interference.

Second, even if you are not sharing the connection between your computer and your ISP, you are usually accessing web sites located somewhere on the Internet other than on the network of your own ISP. Those web sites are connected to the Internet through their own ISPs. And then there are intermediate ISPs (the Internet backbone) between your ISP and the ISP of the website you’re trying to download from. If your ISP has, for example, exactly one thousand customers each with one mbps of download capacity, the ISP’s connections to the rest of the Internet may total only 25 mbps (or less) even though all of you downloading together could theoretically use a gbps (a gigabit is a thousand million or one billion bits).

This isn’t fraud; it’s the way the Internet is built. The highways system wouldn’t work if every driveway disgorged a constant stream of cars. The phone system can’t handle more than a fraction of the phones being in use at once; it gives busy signals. The Internet doesn’t give busy signals; it just gets slow.

The third reason you may not get the speed you imagined you paid for is that the computer which runs the website you’re accessing is too busy to feed you data as fast as you think you ought to get it. Of course that computer also faces bottlenecks in any shared connections it has and on its ISP’s connection to the Internet backbone.

Fourth major reason for below-rated performance is the portion of the Internet backbone between your ISP and the ISP of your data provider may be congested. The Internet tends to route around congestion but TENDS is the operative word.

And a fifth reason, in case you still want one, may be the quality of your connection or some intermediate connection. A poor quality connection or even a very congested one will lose data. Most uses of the Internet have a way to request the retransmission of lost data but retransmission takes time and reduces the effective bandwidth available to you.

More detail on bandwidth here.

More tomorrow.

Comments

[Bruce]

Tom,

A good summary, but I'd correct one point (that often confuses even technical people).

DSL is not a dedicated connection between the user an the ISP. It is a dedicated connection between the DSL modem and the DSLAM (the telco equipment either in a central office or a neighbourhood node). However, the access control and ISP servers (even when the ISP is the local telco) are elsewhere in the network and the connection from the DSLAM to the ISP is shared. For some telcos, this is the biggest source of congestion in their network.

[Michael Birnbaum]

Hi Tom,

As you know, we are a small WISP in Central Vermont. We provide point-to-multipoint wireless Internet using radios in the 900 Mhz, 2.4 Ghz, and 5.8 Ghz bands. The "bandwidth" of our 2.4 Ghz radios, for example, is 83 Mhz with 10Mhz spacing per channel.

The "digital bandwidth" (data rate, or speed) is 5 Mbps. The "throughput" is something less than that due to overhead, etc.

The throughput to the subscriber is that which is actually delivered (after sharing and overhead costs are removed).

The throughput is limited by the cir/mir settings in the access point radios and the core routers.

Assuming that we were to purchase sufficient digital bandwidth from our supplier to handle the heaviest traffic, subscribers would receive the advertised maximum throughput, all the time. This is the "maximum information rate." We don't purchase that much capacity, of course, because much of it would lie unused, most of the time like a 64 lane highway. So, the "committed information rate" kicks in when the total traffic exceeds a preset limit. Cir settings are not usually made public, but I can say they are typically less than half of those of the mir (which are the typical advertised maximum rates). This knocks in to be fair to everyone, so that no individual users get to be hogs to the others' detriment. If we have purchased a reasonable amount of digital bandwidth, the cir only kicks in for short bursts, and the subscribers return to higher throughput again. The subscribers do get to see the advertised mir but only occasionally, during the busy parts of the day. The rest of the time they are sharing at a slower rate, but not as slow as that which causes the cir to kick in. During the quiet parts, they get full speed.

Another aspect of interest: Typically, offered downstream speeds are greater than upstream ones. Why not make it symmetrical? It's got to do with usage. It doesn't cost us any more to provide upstream than downstream digital bandwidth. Most users upload rarely in relation to their downloads. This is changing, though. As more and more users upload their videos to YouTube, for example, upstream needs will increase. We try to deliver the most usable product the economics dictate. Right now, we offer twice as much downstream speed as up. This leaves less purchased speed fallow from the subscriber's point of view. As the demands change, we will adjust. If digital bandwidth costs continue to drop, we can add the extra upstream speed without changing our subscriber rates. For individual business customers, we can set the cir/mir to symmetrical patterns now, too.

I am sure you know all of this. I thought it might be valuable to add the concepts of throughput, cir/mir, and symmetry to the discussion.