The internet is, as its name suggests, a complex “network of networks.” And sending an email or accessing a webpage requires data to transit multiple networks, owned and operated by different internet service providers (ISPs). Policymakers working to improve the availability and affordability of high-speed internet service, or broadband, need to understand how data travels across the millions of miles of pipes, cables, wires, and other equipment owned by various ISPs between users across the country and around the world.
Three main components of broadband networks connect with each other to funnel data from point to point on the internet:
- The backbone consists of large fiber optic pipes, often buried deep underground, that are the main data routes on the internet and the primary path for internet traffic between countries.
- The middle mile is the part of a network that connects the backbone to the last mile and is also called “backhaul.” It typically involves fiber lines but can be wireless, and in some instances it may provide internet service directly to large customers, such as schools or health care centers.
- The last mile is the segment that connects a local ISP to a customer, such as via a cable line to the home. The type of technology used to deliver last-mile service is the primary determinant of the speed and quality of service available to consumers.
ISPs, which can be municipal utilities, electric and telephone cooperatives, or private companies such as cable or telephone companies, fall into three tiers based on how they transport and exchange data between networks. These classifications are defined by the geographic reach of the provider and whether they pay for “transit” on other providers’ networks.
- Tier 1: Large providers that own, operate, and maintain infrastructure, including the internet backbone. These ISPs, including AT&T, Deutsche Telekom, Lumen (CenturyLink), Verizon, and Zayo, have a global reach. Because they all carry roughly the same amount of data on their networks, the costs they each incur—and the fees they could charge one another—for exchanging data across networks are effectively the same. As a result, they exchange traffic at no cost according to mutually beneficial arrangements.
- Tier 2: Providers, typically large cable providers and multistate telecommunications companies such as Comcast, Cox, Frontier, and TDS, that exchange data for free with other providers in some parts of their network but purchase transit services, which allow them to move user data across another provider’s network, to reach other portions of the internet.
- Tier 3: Usually last-mile service providers or those that offer only direct connections to customers who must buy access to the broader internet, either through direct contracts with Tier 1 providers or by purchasing services from a Tier 2 provider that include connections to Tier 1 networks.
Contracts, known as interconnection or peering agreements, govern exchanges of data across different ISPs’ networks, allowing data to travel freely around the globe. The exchanges occur at internet exchange points (IXPs), which are typically large buildings where multiple carriers house equipment to link their networks and transfer data. Network switches within IXPs operate much like railroad switches, directing data from one ISP to another to ensure that it travels along the most direct route and to the correct destination.
That online content that we access on our computers and phones is largely generated by “edge providers,” typically large retail, social media, technology, or video streaming companies but also sometimes individuals that offer content such as websites, web applications, or web hosting services.
The transmission of this data is facilitated by content delivery networks (CDNs), systems of servers around the world that are typically owned by large technology firms such as Amazon CloudFront and Akamai and improve the efficiency of data transmission across the internet. CDNs function as data warehouses, storing copies of web content in various locations in order to shorten the time between when a user clicks a link and a webpage loads. CDNs link into the ISP-managed networks and expedite the transmission of data by “shipping out” the content from a nearby facility—rather than requiring it to travel from the edge provider’s headquarters—to the end user.
The internet speeds that customers experience are determined by the slowest link in this system—usually the last mile—and depend on two related network metrics:
- Bandwidth is the volume of data that a network can transmit, as measured in megabits per second (Mbps). More bandwidth provides a faster connection.
- Throughput is the amount of data that can pass through a communications system.
The relationship between them is akin to a road. Bandwidth is the number of lanes, and throughput is the amount of traffic. The wider the road, the more traffic it can carry at full speed before becoming congested and slowing down. The backbone is most similar to an interstate highway, offering high bandwidth, while the middle mile might be a major urban thoroughfare and the last mile would be more like a residential street.
However, congestion can occur at any part of the network. For example, a last-mile provider could have ample bandwidth to serve the local customer base, but traffic may be slowed by congestion at the connection point with the middle mile portion of the network.