Broadband for Techies

What is Capacity and Why Does it Matter?

The capacity of a communications network such as the Internet is defined as the total number of bits/sec that can be delivered to all subscribers concurrently (the average measured download speed multiplied by the total number of subscribers when all are downloading at once). This number is important because it limits the number of subscribers who can realize download/upload speeds that are those they have subscribed for during the network’s busy hours. To maximize revenue, it has become an Internet service provider (ISP) practice to oversubscribe networks, sometimes by as much as a factor of three. The result is that, during busy hours, data rates must be slowed and/or connections dropped in order to keep the network operating within its capacity. This is a frequently heard complaint in DVFiber’s underserved areas. Researchers working on future 6G wireless technology estimate the Internet’s current annual data load to be 57 exabytes/year—that is 57 followed by 18 zeros. They further estimate that this load will increase 100-fold to over 5,700 exabytes/year within the next decade. Even in areas able to handle the current data load, this growth will swamp networks based on current technologies. To bring our region to the right side of the information gap and keep us there during the coming period of explosive growth in data loads, DVFiber is building a network with reserve capacity far exceeding anticipated needs.

Communications network capacity is limited by the physical properties of the technologies used to transmit digital information and by U.S. law. Digital information is the sequences of bits (1s and 0s) that our devices’ software turns into useful entities—text, images, voice, video, etc. Bits are abstractions (see our FAQ definition) realized as states of binary devices, e.g., transistors which like light bulbs can be turned on or off to represent 1s or 0s. To transmit bits from a source device to a distant receiving device, bits must “ride” a carrier. Carriers take the form of propagating physical phenomena, like an electric current in a conducting media (wire), electromagnetic waves propagating in air or space, or light (electromagnetic waves in the visible frequency range) propagating in an optical fiber. All of them travel at light speed, but they differ significantly in the distance they can travel while remaining detectable. Information is “carried” as modulation, changes in the values of the physical properties of the carrying signal that represent bit values. Modulation may be as simple as turning the carrier signal on and off to represent 1s and 0s, or it may be more complex, using combinations of values of carrier properties to represent sequences of bits. In the case of electromagnetic waves using the public airways, FCC regulations also limit the capacity of those waves to “carry” digital information.

“Bandwidth” here refers to the frequency range between upper and lower analog frequencies of a modulated carrier. Modulation states are referred to as “symbols.” Carrier bandwidth must be at least twice the modulating bit stream symbol rate to transmit modulation with sufficient fidelity to recover the original bit stream at the receiver. In its simplest form, the modulating information bit rate can be no more than half the bandwidth of the transmitted signal (carrier and modulation). For more advanced multi-bit modulation techniques, the modulating information bit rate can be no more than half the bandwidth multiplied by the number of bits a symbol represents. Electrical currents are the most severely bandwidth-limited. Attenuation losses of modulated or changing current in a conducting media (wire or twisted pair wire) increase rapidly as the modulated carrier bandwidth increases. While high voltage-powered direct (unmodulated) current can travel many miles without significant loss, 10 Mbps bit rate transmission in a twisted pair wire is limited to approximately one mile (without boosting), and 100 Mbps bit rate transmission is limited to a very few hundreds of yards

Electromagnetic wave propagation expands over a spherical volume. The energy in the modulated wave diminishes with distance as the energy spreads over an expanding spherical surface. Attenuation losses are insignificant in air but very high in water. These losses are far less limiting than those of electrical current, but electromagnetic waves are subject to blockage by trees, terrain, and water in the form of storms. Given sufficient broadcast power, line-of-sight propagation to the horizon is the upper bound on range. However, the most severe form of limitation on bit rates, and therefore capacity, is the amount of the electromagnetic spectrum that the FCC licenses to communications companies. Bit rates cannot legally use more bandwidth than the spectrum segment licensed; for example, 19 GHz for Starlink and 14 GHz for 5G wireless systems. The symbol rate of these systems cannot legally be greater than half of their licensed bandwidth, and whatever bit rate can be squeezed into that symbol rate is the maximum bit rate a satellite or cell tower can transmit. And this rate is thus their capacity for serving all users connected to their network.

By stark contrast to electrical current, the attenuation losses of light-carrying modulation through an optical fiber are minimal, making it possible to transmit information over distances of 40 miles or more before boosting (reamplification) is needed. This makes fiber optic networks economicaly feasible. Fiber optic carriers are not subject to blockage by terrain or storms. The physical limit of bandwidth, the bandwidth of visible light, is 40,000 times that of the spectrum licensed to Starlink or 5G wireless systems and is not subject to FCC licensing. Current modulation technology enables a single fiber pair to carry a 100 Gbps bit stream. In-home optical to copper transceiver bit rates range from 10 Mbps to 10 Gbps, supporting any service tier that DVFiber may offer. Moreover, by dividing the light carrier into its different colors and modulating each color individually, it is possible to serve multiple subscribers at bit rates of as much as 10 Gbps. And this is on a per fiber basis; cables can contain upwards of 200 fibers. All of the 16,000 residences in the current DVFiber region can be served at 100 Mbps data rates with the capacity of only 16 fibers of a 200-fiber cable. If everyone wanted 1 Gbps service, 160 fibers would be required. And none of this is limited by FCC licensing.

So, what does this mean for the digital future of rural Vermont? Hybrid networks of fiber optic cables to neighborhoods, combined with twisted pair wire last-mile connections to homes using multiple sets of twisted pair wires, is the quickest and least expensive way to bring modern service to homes where broadband is defined as 25/3 Mbps service. But such an approach will surely be obsolete by decade’s end, and the 25/3 Mbps standard will soon give way to a 100/100 Mbps symmetrical service definition of broadband. Radio-based services, wireless, and Starlink are too limited by their FCC licenses to meet the coming demand for capacity in rural areas. Wireless ISPs plan to meet capacity requirement increases in urban areas with 6G technology and densely spaced cell towers utilizing advanced modulation techniques to squeeze a greater bit rate out of limited licensed bandwidth without interfering with neighboring towers. This is impractical in forested, hilly terrain and economically infeasible in sparsely settled areas such as ours in Vermont. Starlink and other possible low earth orbit (LEO) satellite systems could conceivably use the same 6G modulation schemes with highly advanced antennae technology to provide high capacity through non-interfering dense satellite coverage of our CUD’s area, but at a cost of maintaining a very large galaxy of satellites that has complexity which grows geometrically with its size. So, while conceivable, this is far from certain. Reported performance to date of the Starlink beta system is far from meeting 5G standards, and much less than the anticipated requirements for 6G. Fiber optics have none of these limitations. They can be thought of as just another type of wire that “carries” digital information at very high data rates over very long distances, thereby enabling the construction of very high capacity networks.

DVFiber is building an all fiber-to-the-premises (FTTP) network with proven fiber optic technology—with the capacity to not only meet today’s broadband requirements, but also to provide reserve capacity to meet anticipated growth in the coming decade and beyond. We will overbuild, but not oversubscribe! Barring a widespread natural disaster, the DVFiber network will provide 100% of the data rate for every customer’s subscribed service level (tier) 99.9% of the time.

–Chris Robbins, Jamaica

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