While many carriers offer dedicated circuits to their customers, many do not dedicate the required bandwidth on their own network backbone. They will intentionally sell more bandwidth than their backbone can support on the assumption that only some of their customers will actually use their full bandwidth at any given time. This practice is known as oversubscription, and it is used to cut costs while sacrificing performance.

Oversubscription leads to less reliable connectivity with spontaneous periods of reduced network speed and a higher likelihood of outages. This is caused by increased congestion on the carrier’s backbone during high-activity times. Much like traffic on a highway, fiber optic cables have a fixed capacity and attempting to transmit more data than the medium can maintain leads to significantly slower speeds for everyone. Even while traffic between your office and the backbone matches your carrier’s promised speed, everything beyond that is a gamble.
To avoid the drawbacks of an oversubscribed network, you should choose an Internet Service Provider whose network is truly dedicated, like 365 Data Centers.