Due to the recent APCN2 undersea cable outage, I read quite a bit of complaints from subscribers and thought I should write a little educational entry on how the Internet works.

What are you really paying for?

When you buy a connection to your ISP, you are merely buying the link from your home/office to your ISP. Theoretically speaking, ADSL users should have the first advantage over Cable Modem users as the Cable Modem sits on a shared topology.

The ugly truth about the bandwidth test.

However, this isn’t much of a concern as your nearby POP (Point of Presence) would usually have enough bandwidth to take care of this so subscribers always get near maximum bandwidth up to this point; StarHub users should be familiar with the annoying Bandwidth Test that always seem to report excellent bandwidth. If it doesn’t, your physical line might be faulty.

Going international.

Beyond that, it gets a little more complicated. Data from a subscriber travel through some tens of kilometers of fiber optic cables, then to some routers and switches within your ISP. When it reaches the border – the part of the ISP that connects to the “outside world” (other ISPs, known as peers), the data goes in all directions, e.g. if the subscriber requests a site in China, it might go through Hong Kong, then to China.

Well, that’s for data heading out. It’s a different story when the data returns from China.

Return beyond an ISP’s control.

When you make a request for a website, it isn’t as simple as it looks. Your browser sends a request to a server, and the server will need to return that site you requested. When a piece of data returns, it might not take the same path it went out with. Why? Because you cannot control how others would choose to send your data.

Using the site in China as an example, if the ISP in China prefers to send return traffic via Taiwan, then to Japan and finally to Hong Kong, there’s nothing much your ISP can really do about it. At the time of this writing, there’s approximately 300,000 Internet routes, so it’s not a viable effort to look into every route possible.

No knowledge of link size or cost.

Adding to this problem, the Border Gateway Protocol that is used by ISP routers to communicate with each other only takes into consideration hop count and does not consider link bandwidth or cost, so a potentially nearer but smaller bandwidth link might be preferred over a further but larger bandwidth link. Most ISPs use a technique known as AS-prepend to make a particular link more preferred than others, but this technique has its limits especially after traveling many hops around the Internet.

The real cost of bandwidth.

A typical 1Gbps “international” link costs an ISP tens of thousands of dollars a month. This works out to approximately tens of dollars per Mbps of bandwidth. A typical SingNet 10Mbps Home ADSL costs $46.90 a month and comes bundled with a free ADSL wireless gateway, maybe an iPod, etc. If you did the math, there’s no way SingNet would be making a profit, duh!

No such thing as bandwidth end-to-end.

An ISP would have multiple links overseas. Major outbound links in Singapore head towards Hong Kong, Japan, Malaysia and Taiwan. Because traffic bound for different sites might take different paths, guarantying end-to-end bandwidth will theoretically require an ISP to buy the same amount of bandwidth they sold a subscriber on every link to every single location on planet Earth – so that at any point a subscriber would be able to download from any site at full speed.

In reality, this will never happen. Subscribers are never online downloading at full speed 24 hours a day, so ISPs will only buy bandwidth sufficient to sustain decent traffic.

Even if an ISP buys full bandwidth from an oversees peer, it cannot guarantee if the oversees peer has bought enough bandwidth upstream.

Then why do corporate customers pay so much for so little?

Two problems arise from selling large, cheap bandwidth to consumers.

The first is fairly simple to understand – your bandwidth gets oversold, i.e. your ISP buys less bandwidth than it sells. A typical corporate provider would buy more bandwidth than a consumer provider to cater for traffic peaks and surges.

The second problem is caused by fair-sharing. A subscriber with 3Mbps might potentially be hogging a continuous 3Mbps with P2P while another with 10Mbps hardly surfs the net. If the ISP only has 5Mbps, it means the second user gets 2Mbps – first come, first served. Selling less bandwidth means the bandwidth is controlled even before the subscriber goes “international”.

The Internet is based on mutual relationships.

In the real world, the site you access may be slow due to many other reasons other than your ISP, such as a congested server or a cyber attack. So there’s really no such thing as end-to-end guaranteed bandwidth. If you ever hear the term guaranteed bandwidth, it usually means guaranteed Tier 1 bandwidth – the direct link your ISP buys from an upstream peer.

Think about it, the Internet is really made up of networks based on mutual relationships. If an ISP decides not to buy a link to a specific location, then that location will not be accessible.

Be a smart consumer. Buy only what you need.

So the real truth behind all the marketing is to buy only what you need. Your subscribed bandwidth is only going to be as good as your phone line, coaxial cable, or fiber cable (in the near future) to your ISP.