Digital Subscriber Line (DSL)
Digital Subscriber Line (DSL) has been the primary broadband technology employed by telephone companies (common carriers) for a number of years because it makes good use of existing dedicated telephone lines (typically copper). With DSL, a single telephone line is used to deliver both voice and high-speed data transmission. Providing two (2) services over a single (1) line is possible because the data transmission takes place over a different (higher) frequency than the voice service.
There are a number of variations or versions of DSL in the market (e.g., SDSL, ADSL, VDSL, etc.) The most common and less expensive version of DSL is Asymmetric Digital Subscriber Line (ADSL). As the name implies, this ‘asymmetric’ service provides download speeds that are different than the upload speeds. Download speeds are higher than upload speeds. Other versions of DSL include a symmetric version (SDSL) where the upload and download speeds are the same. SDSL and ADSL speeds range upward to 6 Mbps.
Using up to 7 different frequencies, very-high-bitrate DSL (VDSL or VHDSL) is one of the newer DSL technologies providing faster data transmission of up to 50 megabits per second (Mbps) downstream and 16 Mbps upstream over a single pair of copper wires (commonly referred to as the loop). With these faster speeds, VDSL is capable of supporting high bandwidth applications such as HDTV, as well as telephone services (Voice over Internet Protocol, or VoIP) and general Internet access, over a single connection.
As for speeds realized by DSL customers, the defining issue is distance from the telephone company’s central office (CO). Due to electrical resistance in the telephone wire, the farther a customer is from the CO, the weaker the signal—and therefore the slower the speed. It is commonly accepted with ADSL technology that broadband speeds (download in excess of 768 kilobits per second [kbps]) can be achieved up to approximately 10,000 feet (2 miles) from the nearest CO, although other factors such as wireline interference and network traffic can impact the speed consumers actually experience. Between 10,000 and 16,000 feet, speeds fall steadily to the point where they begin to match dial-up Internet service. Most customers cannot receive DSL if they live more than 16,000 feet (3 miles) from the nearest CO.
As the name suggests, cable broadband uses the cable television infrastructure. Strategically cable access is similar to the DSL approach used by telephone companies—the difference is that cable service makes good use of the cable TV company’s coaxial cable existing network while DSL service leverages an existing telephone company’s plant. The connections between the cable company office (called the ‘headend’ as opposed to the Central Office in a telephone company) and the customer’s premise is either a pure cable run or in more modern networks what’s called a hybrid fiber coaxial (HFC) facility (i.e., a network that uses both fiber and coaxial lines).
In discussions regarding cable broadband, you’ll often hear the term DOCSIS. Data over Cable Service Interface Specification (DOCSIS) is the international telecommunications standard that permits the addition of high-speed data transfer to an existing Cable TV (CATV) system. Due to the design of coaxial and fiber cable lines, cable speeds tend to be higher than traditional DSL speeds. Cable speeds currently range up to 100 Mbps. The maximum distance from the nearest headend that cable service can be offered is also typically much greater than with DSL service. However, cable lines are not nearly as ubiquitous as telephone lines. There may be installation charges for installing cable to new homes, and some homes may be too far from the nearest cable system for installation to be economically feasible.
Fiber optics are strands of optically pure glass that carry digital information as pulses of light. Each glass strand is surrounded by a material that reflects the light back into the glass core and a coating to protect it. Hundreds of thousands of these coated glass strands are bundled together to make the fiber optic cable that delivers the Internet to your home or business. One advantage of fiber optics is higher transmission speeds.
Fiber to the x (FTTx) is a generic term for any broadband network architecture that uses optical fiber to replace all or part of the traditional local loop used for last mile (the connection between the customer and the telephone company, cable company or ISP) transport. The variations (i.e., what the “x” refers to) depend on how far the fiber extends toward the home (or business). For example:
- FTTN (Fiber-to-the-Node): fiber is terminated in a street cabinet up to several kilometers away from the customer premises with the final connection being copper.
- FTTC (Fiber-to-the-Cabinet or Fiber-to-the-Curb): this is very similar to FTTN, but the street cabinet is closer to the user’s premises—typically within 300 m.
- FTTB (Fiber-to-the-Building or Fiber-to-the-Basement): fiber reaches the boundary of the building, such as the basement in a multi-dwelling unit, with the final connection to the individual living space being made via alternative means.
- FTTH (Fiber-to-the-Home): fiber reaches the boundary of the living space, such as a box on the outside wall of a home.
- FTTP (Fiber-to-the Premises): this term is used in several contexts—as a blanket term for both FTTH and FTTB, or where the fiber network includes both homes and small businesses.
With broadband, it’s primarily about speed. While the speeds of fiber optic and copper cables are both limited by length (i.e., distance from the central office and/or serving equipment), copper is much more sharply limited in this respect. Therefore, generally the further fiber extends into the network, the higher the resulting end-user (realized) speeds.
In a broad sense, wireless broadband access is either ‘fixed’ (transmission to/from a specific and stationary or static point) or ‘mobile’ (transmission to/from a device on the move). Consumer and business-level fixed wireless broadband is typically provided by companies known as Wireless Internet Service Providers (WISPs). WISPs employ networks of radios that transmit and receive broadband signals of up to 40 Mbps. Some of these radios will be placed on single purpose towers and other high structures (e.g., water towers, buildings, etc.) and some on homes or businesses.
Fixed wireless technology may include commonplace Wi-Fi wireless mesh networking techniques, or proprietary equipment designed to operate over open 900MHz, 2.4GHz, 4.9, 5.2, 5.4, 5.7, and 5.8GHz bands or licensed frequencies in the UHF or MMDS bands. A single radio in the network can serve multiple end users depending on the volume of traffic experienced (bandwidth used) and the provider’s oversell ratios. Oversell ratios (a strategy employed in nearly all broadband technologies) simply recognizes the fact that not all users are on the network at the same time. Fixed wireless providers typically operate in rural areas where DSL or cable broadband is not available (although there are exceptions where WISPs are taking advantage of unmet demand and service issues in more urban areas). At some point in their networks, the WISP will aggregate traffic and ultimately connect their radio-based facilities with an existing fiber or copper-based network, thereby gaining access to and from the Internet.
In most cases, fixed wireless access is what’s called ‘line of sight’ in that the transmission is dependent on a clear path from the radio on a home to the radio on the tower. Obstructions in this transmission path (e.g., seasonal foliage) can interrupt service. Fixed Wireless technologies are one of the most rapidly evolving of broadband technologies, with equipment providers announcing increased speed and reception capabilities on a regular basis. When evaluating Fixed Wireless, it’s important that people speak with their local provider to understand the capabilities of their current equipment and their plans for upgrading as the technology improves.
Mobile Wireless or Cellular Broadband
Mobile / cellular broadband covers a range of technologies employed by the likes of AT&T and Verizon Wireless to provide high-speed connections to end-user devices that are typically used on the move (e.g., smart phones, iPads, etc.).
Through the recent past, there have been two competing approaches to delivering mobile broadband service: Global System for Mobile Communications (GSM) and Code Division Multiple Access (CDMA). GSM dominates the market outside the US. Domestic CDMA carriers include Verizon and Sprint and whoever uses their networks (e.g., Virgin, Boost). Our GSM carriers include AT&T and T-Mobile and whoever uses their networks. There are also several smaller cellular companies on both networks.
Both of these technologies continue to evolve into higher speeds. An example in the GSM world is HSPA (High Speed Packet Access). In the CDMA world, an example is EVDO (Evolution, Data Only or Evolution, Data Optimized). Both continue to develop faster networks. These faster networks are often referred to as 3G or the most recent development, 4G, which include LTE (Long Term Evolution) and WiMAX (both under the umbrella of Orthogonal Frequency Division Multiplexing (OFDM—a technique for transmitting large amounts of digital data over a radio wave). The “G” simply stands for the 3rd or 4th “generation” of these broadband cellular networks. 3G speeds range upward to approximately 1.5 Mbps. 4G speeds range upward to 12 Mbps. However, even within the 3G and 4G categories, there are several “revisions” of the core technology with speeds and coverage constantly improving.
You will typically find satellite broadband access in our rural areas where other technologies have not yet been deployed due to cost and/or insufficient demand. Satellite Internet is provided through low earth orbit (LEO) satellites. Different types of satellite systems have different features and technical limitations, which can greatly affect their usefulness and performance in specific applications. Satellite download speeds run up to 2 Mbps, but the upload speed is much slower. Satellite broadband, because signals have to travel so far, also have much longer latency* rates than other broadband technologies. In addition, reliability is also questionable in bad weather (e.g., rain fade) or during sunspot activity.
* Latency is sometimes called the ‘round-trip time.’ It is the delay between requesting data and the receipt of a response, or in the case of one-way communication, between the actual moment a signal is broadcast and the time it is received at the destination.
Two of the more important elements of network performance for all broadband technologies are bandwidth and latency. The average person is more familiar with bandwidth because it’s the element advertised by service providers. However, latency matters equally to the end-user experience. Businesses use the term Quality of Service (QoS) to refer to measuring and maintaining consistent performance on a network by managing both bandwidth and latency in a coordinated fashion. Satellite users share satellite capacity to reduce the cost, while still allowing high peak bit rates when congestion is absent. There are usually restrictive time-based bandwidth allowances so that each user gets their share, according to their payment.
Broadband Over Power Lines
Broadband over power lines is a relatively new approach that permits high-speed Internet traffic to travel down standard high-voltage power lines. As with any emerging technology, there are a number of complex issues to be addressed. The primary issue with BPL is that power lines are inherently a very noisy environment. For example, every time a device turns on or off, it introduces a pop or click into the line, and energy-saving devices often introduce noisy harmonics into the line. The system must be designed to deal with these natural signaling disruptions and work around them. On the positive side, the power line infrastructure is widespread across the country.
WiMAX (Worldwide Interoperability for Microwave Access) is a technology that provides fixed and fully mobile Internet access. WiMAX is a contender for “last mile” connectivity in rural and emerging markets where laying fiber, cable or DSL is not cost effective. Many smaller Wireless Internet Service Providers (WISPs) use WiMAX or similar technologies to provision the Fixed Wireless service described above. Other companies are deploying WiMAX to provide mobile broadband or at-home broadband connectivity across whole cities or countries. In many cases this has resulted in competition in markets which typically only had access to broadband through an incumbent DSL (or similar) operator. Additionally, given the relatively low cost to deploy a WiMAX network (in comparison to GSM, DSL or Fiber-Optic) it is now possible to provide broadband in places where it might have been previously economically unviable.
WiMAX is a possible replacement candidate for cellular phone technologies such as GSM and CDMA, or can be used as an overlay to increase capacity. Current WiMAX speeds are upwards to 40 Mbps with expected updates to offer speeds of 1 gigabit per second (Gbps) for fixed WiMAX. Fixed WiMAX is also considered as a wireless backhaul technology for 2G, 3G and 4G networks in both developed and developing areas of the country.