Demystifying Oversubscription

I think the concept that I have to explain the most as a consultant is oversubscription, which is the way that ISPs share bandwidth between customers in a network.

Most broadband technologies distribute bandwidth to customers in nodes. ISPs using passive optical networks, cable DOCSIS systems, fixed wireless technology, and DSL all distribute bandwidth to a neighborhood device of some sort that then distributes the bandwidth to all of the customers in that neighborhood node.

The easiest technology to demonstrate this with is passive optical fiber since most ISPs deliver nodes of only 32 people or less. PON technology delivers 2.4 gigabits of download bandwidth to the neighborhood node to share with 32 households.

Let’s suppose that every customer has subscribed to a 100 Mbps broadband service. Collectively, for the 32 households, that totals to 3.2 gigabits of demand – more than the 2.4 gigabits that is being supplied to the node. When people first hear about oversubscription, they think that ISPs are somehow cheating customers – how can an ISP sell more bandwidth than is available?

The answer is that the ISPs knows that it’s a statistical certainty that all 32 customers won’t use the full 100 Mbps download capacity at the same time. In fact, it’s rare for a household to ever use the full 100 Mbps capability – that’s not how the Internet works. Let’s say a given customer is downloading a huge file. Even if the ISP at the other end of that transaction has fast Internet, the signal doesn’t come pouring in from the Internet at a steady speed. Packets have to find a path between the sender and the receiver, and the packets come in unevenly, in fits and starts.

But that doesn’t fully explain why oversubscription works. It works because all of the customers in a node never use a lot of bandwidth at the same time. On a given evening, some of the people in the node aren’t at home. Some are browsing the web, which requires minimal download bandwidth. Many are streaming video, which requires a lot less than 100 Mbps. A few are using the bandwidth heavily, like a household with several gamers. But collectively, it’s nearly impossible for this particular node to use the full 2.4 gigabits of bandwidth.

Let’s instead suppose that everybody in this 32-home node has purchased a gigabit product, like is delivered by Google Fiber. Now, the collectively possible bandwidth demand is 32 gigabits, far greater than the 2.4 gigabits being delivered to the neighborhood node. This is starting to feel more like hocus pocus, because the ISP has sold 13 times the capacity that is available to the node. Has the ISP done something shady here?

The chances are extremely high that they have not. The reality is that the typical gigabit subscriber doesn’t use a lot more bandwidth than a typical 100 Mbps customer. And when the gigabit subscriber does download something, it does so quicker, meaning that the transaction has less of a chance of interfering with transactions from neighbors. Google fiber knows it can safely oversubscribe at thirteen to one because it knows from experience that there is rarely enough usage in the node to exceed the 2.4 gigabit download feed.

But it can happen. If this node is full of gamers, and perhaps a few super-heavy users like doctors that view bit medical files at home, this node could have problems at this level of oversubscription. ISPs have easy solutions for this rare event. The ISP can move some of the heavy users to a different node. Or the ISP can even split the node into two, with 16 homes on each node. This is why customers with a quality-conscious ISP rarely see any glitches in broadband speeds.

Unfortunately, this is not true with the other technologies. DSL nodes are overwhelmed almost by definition. Cable and fixed wireless networks have always been notorious for slowing down at peak usage times when all of the customers are using the network. Where a fiber ISP won’t put any more than 32 customers on a node, it’s not unusual for cable company to have a hundred customers.

Where the real oversubscription problems are seen today is on the upload link, where routine household demand can overwhelm the size of the upload link. Most households using DSL, cable, and fixed wireless technology during the pandemic have stories of times when they got booted from Zoom calls or couldn’t connect to a school server. These problems are fully due to the ISP badly oversubscribing the upload link.

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