Overview of Broadband Networks

Characterizing the Internet as a cloud and a network of networks emphasizes how users benefit from the seamless integration of many different carrier facilities located throughout the world. The Internet design emphasizes convenience and simplicity even as complex network interconnections take place between equipment of different vintage and manufacturer. By agreeing to use a common set of operating standards, all ISPs have the ability to interconnect the equipment needed to transmit, switch, identify, label and deliver traffic. This means that users can access content located anywhere within the Internet cloud by using a universally agreed upon addressing system that uses easily remembered words, e.g., www.worldbank.org. These domain names translate into a sequences of numbers used by devices called routers to identify the origin and destination of traffic as well as the next network that will deliver traffic closer to its intended destination, or to the final destination. The Internet addresses of senders and recipients of traffic are located in the header portion of packets. Content is located in a separate portion of each packet commonly referred to as the payload.

Broadband networks combine high capacity transmission lines with devices such as routers that coordinate the delivery of traffic. By analogy think of the telecommunications lines as high capacity highways, or pipelines and routers as the traffic lights that manage the intersection of lines, i.e., their interconnection, and the routing of traffic that typically involves a switch from one transmission line to another, i.e., a handoff from one carrier’s network onto the network of another carrier.  

The Internet offers fast and reliable management of high capacity traffic bitstreams thanks to the reciprocal agreements among ISPs to share transmission, switching and routing duties.  Peering refers to an agreement between two ISPs to exchange Internet traffic typically with no payment if the traffic handed off to the other ISP roughly equals  the volume received from the other ISP.* For instances where ISPs exchange unequal traffic volumes, the carrier generating more traffic volume typically has to pay the other carrier for its comparatively greater traffic carriage, a financial transaction commonly referred to as transiting.

Internet traffic can quickly and seamlessly maneuver through the cloud using the networks of many ISPs. ISPs agree to interconnect their networks and have both technological means and financial incentives to secure a complete link from an end user upstream via his or her retail ISP and many other interconnected ISPs all the way to the source of desired content and back. The specific networks use at any time during the connection can change, because carriers subdivide Internet traffic into packets and the decision on which carrier network to use for each packet is made “on the fly,” i.e., as the packets are presented to a router. Routers typically use operating standards that switch individual packets on a “best efforts” basis that identifies which of possibly several networks are available and which individual network is most likely to deliver traffic quickly onward to another ISP, or to the retail ISP serving the end user requesting the content. 

Hierarchical Structure of the Internet

The Internet ecosystem divides into a number of separate network elements that combine to provide users with a complete link to and from sources of content. A retail ISP  serving end users provides the first connection that originates an upstream traffic flow used by subscribers to initiate a request for content, e.g., a query submitted to a search such as Google, or to upload their own content, e.g., uplinking a photograph to a social networking site such as Facebook. More than one retail ISP may offer broadband links to individual subscribers, but generally, in developing countries, end users opt to subscribe to only one carrier for all uplinking and downlinking services. 

In many locales consumers have a choice of technological options that include broadband provided by a telephone company that has retrofitted its voice network to provide data services, a cable television company that has reassigned a portion of its video content delivery capacity for data services, one or more terrestrial and satellite wireless carriers and possibly the electric power company. These carriers have limited opportunities to aggregate traffic and achieve operational efficiencies for the first and last kilometer link,* because eventually they must identify and deliver or receive traffic from each and every subscriber on an individualized basis.  However, retail ISPs can aggregate traffic for upstream delivery to other carriers and receive such aggregated traffic from these carriers.

The ability or inability to aggregate traffic has great significance on how much operational efficiency and cost savings an ISP can generate.  As traffic moves upstream from a retail ISP the carriers providing intercontinental and transoceanic transmission have the greatest ability to combine traffic onto the fastest and highest capacity transmission links available. This traffic aggregation function makes it possible for such carriers to accrue best possible scale efficiency and to become a part of the largest class of operators known as Tier-1 ISPs.* These ISPs provide long haul traffic delivery and typically only interconnect directly with other similarly-sized Tier-1 ISPs, on zero cost, peering terms. Smaller ISPs may also enter into peering agreements, but typically have to pay transit fees to Tier-1 ISPs for access to their long haul services. 

The Internet ecosystem can be visualized as a hierarchical pyramid with many comparatively small ISPs serving individual localities and regions, with fewer ISPs operating upstream as Tier-2 and Tier 1 carriers. 

Internet traffic also has characteristics that affect how ISPs configure their networks. Because most subscribers download more content than they upload Internet networks have to handle more downstream traffic. This asymmetrical traffic volume requires some ISPs, particular retail carriers, to allocate more capacity for downstream transmissions than for upstream flows. Similarly ISPs have to configure networks that can handle and quickly respond to significant variation in the total volume of traffic demand made by individual subscribers. Most Internet subscribers have “bursty” traffic requirements as they will require fast, high capacity downloading capability for a period of time after which they may impose no significant demands whatsoever. ISPs need to have the ability to accommodate high throughput demand, e.g., downloading a very large file containing video content, but also to reassign network resources when one subscriber completes a bursty demand for transmission capacity and starts to watch the downloaded content.

The Internet ecosystem will constantly change as new types of content and uses emerge. Current developing trends include increasing reliance on wireless broadband networks, the proliferation of applications designed for use of these networks and the rapid inclusion of Internet connected sensors, for instance, to monitor the health and performance of both people and devices. These trends will have a substantial impact on how planners design and configure future networks.  With increasing reliance on wireless networks ISPs will need to convince government regulators to reallocate and assign more radio spectrum for Internet access. A growing “Internet of Things”*means that our understanding of what the Internet can do will expand into an even larger ecosystem of people, devices and monitors. 

Another developing Internet trend reduces the hierarchical nature of interconnections and  increases the number of financial compensation arrangements available. When the Internet started only a few carriers participated and they generally had roughly equal volumes of traffic to exchange. Additionally these carriers did not have to pay close attention to traffic volumes, because governments typically subsidized their operations. Most governments have stopped or reduced subsidizing Internet development thereby prompting ISPs to pay closer attention to capital and operating costs, including whether an interconnecting carriers generates more traffic than it receives. ISPs  generating more traffic for carriage by another carrier now have to pay for such access. Faced with significantly higher interconnection costs, these carriers have explored new ways to interconnect networks.  

For example many smaller Tier-2 ISPs have agreed to interconnect directly rather than rely solely on higher capacity Tier-1 carrier networks.  Additionally ISPs of all sizes increasingly opt to interconnect and to store content at a centralized location commonly referred to as an Internet Exchange. Such co-location makes it possible for many ISPs to interconnect in the same premises. This promotes operational efficiency and reduce interconnection costs.

Very large suppliers or repositories of content also consider alternatives to relying on one or more ISPs to manage delivery. These ventures, such as Google, Facebook, and Youtube can secure and manage their own network routings closer to end users.  Some content providers have registered to secure an Autonomous System identifier that specifies routing options as though the content source operated as an ISP. In the alternative Netflix has under consideration the installation of high capacity storage units on subscriber premises that will contain hundreds of the most popular content thereby reducing the aggregate subscriber demand for immediately downloaded content.