Japan has what is commonly known as an "unbundled" internet infrastructure when it comes to DSL and fiber optics. Long story short, you may be dealing with two companies to get your internet. NTT East/West, KDDI, etc who own the wires are called carriers, and they handle all the nitty-gritty of setting up your physical connection but they don't directly connect you to the internet.
One big exception to this rule are the fiber optic companies run by local power monopolies (Kyushu Electric`s BBIQ, Kansai Electric's EO, etc). Thus far, none of the big power companies have shown much interest in unbundling their services for use by independent providers.
Cable modems, on the other hand, are not unbundled so you have to purchase everything directly from one company. Similarly, wireless internet connections are not unbundled although small companies often lease network space from larger firms.
A provider (porobaida プロバイダ) is the company actually connects you to the internet. NTT and the like provide the modem and the connection jack and route the data to provider-owned servers co-located in the telecom's office.
The end result of Japan's unbundling policy is that independent providers will scramble to compete for your business and the resulting competition will lower prices. While this seems to have worked better in theory than in practice, more and more providers are entering the market. In order to get your business, they will often offer incentives such as reduced rates for multi-year commitments, free PS3s, and even lowering prices if your neighbors have the same service. Also, so long as your carrier is the same, there is no need to change physical equipment when changing providers.
One tricky part about dealing with multiple companies is figuring out who is responsible to fix what in the event of a problem. In short, your carrier is responsible for physical equipment (wires, modems, etc.) while your provider is responsible for maintaining your connection to the internet.
Providers also lay down terms and conditions for your internet usage such as how much bandwidth you can use per month, policies on file sharing, IP number assignment, and so on. It is possible for your provider to cut you off remotely even though you physical carrier connection is functioning.
Another tricky aspect of the internet system here is that different companies are in charge of different parts of your service. For instance, a provider maintains your actual net connection while NTT takes care of the entirety of your internet phone service and internet TV service, even though they use the same modem and wires. We will go into this in more detail in later posts.
And that's a wrap for now. Check back soon more great internet tips!
Other Posts in this Series#2 Carriers and Providers
#3 Shopping for Service
#4 Signing up
#6 Optional Services