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Last mile (telecommunications)
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==Existing last mile delivery systems== ===Wired systems (including optical fiber)=== Wired systems provide guided conduits for Information-Carrying Energy (ICE). They all have some degree of shielding, which limits their susceptibility to external noise sources. These transmission lines have losses which are proportional to length. Without the addition of periodic amplification, there is some maximum length beyond which all of these systems fail to deliver an adequate S/N ratio to support information flow. Dielectric [[Fiber to the x|optical fiber]] systems support heavier flow at higher cost. ====Local area networks (LAN)==== Traditional wired [[local area network]]ing systems require copper coaxial cable or a twisted pair to be run between or among two or more of the nodes in the network. Common systems operate at 100 Mbit/s, and newer ones also support 1000 Mbit/s or more. While length may be limited by [[collision detection]] and avoidance requirements, signal loss and reflections over these lines also define a maximum distance. The decrease in information capacity made available to an individual user is roughly proportional to the number of users sharing a LAN. ====Telephone==== In the late 20th century, improvements in the use of existing copper telephone lines increased their capabilities if maximum line length is controlled. With support for higher transmission bandwidth and improved modulation, these [[digital subscriber line]] schemes have increased capability 20-50 times as compared to the previous [[voiceband]] systems. These methods are not based on altering the fundamental physical properties and limitations of the medium, which, apart from the introduction of [[twisted pair]]s, are no different today than when the first telephone exchange was opened in 1877 by the Bell Telephone Company.<ref name="nxtbook">{{cite web|url=http://www.nxtbook.com/nxtbooks/natoa/journal_2009spring/index.php#/14|title=NATOA Journal - Spring 2009}}</ref> The history and long life of copper-based communications infrastructure is both a testament to the ability to derive new value from simple concepts through technological innovation β and a warning that copper communications infrastructure is beginning to offer [[diminishing returns]] for continued investment.<ref name="nxtbook" /> However one of the largest costs associated with maintaining an ageing copper infrastructure is that of truck roll<ref>{{Cite news|url=https://multi-link.net/how-much-does-a-service-truck-roll-cost-your-company/|title=How much does a service "truck roll" cost your company? β’ Multi-Link Inc.|date=2015-04-16|work=Multi-Link Inc|access-date=2017-05-23|language=en-US}}</ref> - sending engineers to physically test, repair, replace and provide new copper connections, and this cost is particularly prevalent in providing rural broadband service over copper.<ref>{{Cite web|url=http://www.ispreview.co.uk/index.php/2016/05/aaisp-struggles-cost-keeping-uk-rural-broadband-alive.html|title=UK ISPs Struggle with the Cost of Keeping 20CN Rural Broadband Alive - ISPreview UK|last=Jackson|first=Mark|website=www.ispreview.co.uk|date=25 May 2016 |access-date=2017-05-23}}</ref> New technologies such as G.Fast and VDSL2 offer viable high speed solutions to rural broadband provision over existing copper. In light of this many companies have developed automated cross connects (cabinet based automated distribution frames) to eliminate the uncertainty and cost associated with maintaining broadband services over existing copper, these systems usually incorporate some form of automated switching and some include test functionality allowing an ISP representative to complete operations previously requiring a site visit (truck roll) from the central office via a web interface.<ref>{{Citation|last=UTEL (United Technologists Europe Limited)|title=RoboCab - Full cabinet automation (Auto PCP / AMDF)|date=2017-03-03|url=https://www.youtube.com/watch?v=klHZG4ybmxU |archive-url=https://ghostarchive.org/varchive/youtube/20211212/klHZG4ybmxU| archive-date=2021-12-12 |url-status=live|access-date=2017-05-23}}{{cbignore}}</ref> In many countries the last mile link which connects landline business telephone customers to the local [[telephone exchange]] is often an [[ISDN30]] which can carry 30 simultaneous telephone calls. ====CATV==== Community antenna television systems, also known as [[cable television]], have been expanded to provide bidirectional communication over existing physical cables. However, they are by nature shared systems and the spectrum available for reverse information flow and achievable [[signal-to-noise ratio|S/N]] are limited. As was done for initial unidirectional TV communication, cable loss is mitigated through the use of periodic amplifiers within the system. These factors set an upper limit on per-user information capacity, particularly when many users share a common section of cable or [[access network]]. ====Optical fiber==== {{Further|Optical fiber}} Fiber offers high information capacity and after the turn of the 21st century became the deployed medium of choice ("[[Fiber to the x|Fiber to the ''x'']]") given its scalability in the face of the increasing bandwidth requirements of modern applications. In 2004, according to Richard Lynch, Executive Vice President and Chief Technology Officer of the telecom giant [[Verizon]], the company saw the world moving toward vastly higher bandwidth applications as consumers loved everything broadband had to offer and eagerly devoured as much as they could get, including two-way, user-generated content. Copper and coaxial networks would not β in fact, could not β satisfy these demands, which precipitated Verizon's aggressive move into [[Fiber to the x|fiber-to-the-home]] via [[FiOS]].<ref>{{cite web|url=http://www22.verizon.com/Content/ExecutiveCenter/Richard_Lynch/ftth_conference_expo/ftth_conference_expo.htm|title=Verizon Leadership Executive Biographies - Verizon}}</ref> Fiber is a [[future-proof]] technology that meets the needs of today's users, but unlike other copper-based and wireless last-mile mediums, also has the capacity for years to come, by upgrading the end-point optics and electronics without changing the fiber infrastructure. The fiber itself is installed on existing pole or conduit infrastructure and most of the cost is in labor, providing good regional [[economic stimulus]] in the deployment phase and providing a critical foundation for future regional commerce. [[fixed line|Fixed copper lines]] have been subject to theft due to the value of copper, but optical fibers make unattractive targets. Optical fibers cannot be converted into anything else, whereas [[Copper#Recycling|copper can be recycled without loss]]. ===Wireless delivery systems=== [[Mobile CDN]] coined the term the ''''mobile mile'''' to categorize the last mile connection when a wireless system is used to reach the customer. In contrast to wired delivery systems, wireless systems use unguided waves to transmit ICE. They all tend to be unshielded and have a greater degree of susceptibility to unwanted signal and noise sources. Because these waves are not guided but diverge, in free space these systems are [[attenuation|attenuated]] following an [[inverse-square law]], inversely proportional to distance squared. Losses thus increase more slowly with increasing length than for wired systems, whose loss increases [[exponential decay|exponentially]]. In a free space environment, beyond a given length, the losses in a wireless system are lower than those in a wired system. In practice, the presence of atmosphere, and especially obstructions caused by terrain, buildings and foliage can greatly increase the loss above the free space value. Reflection, refraction and diffraction of waves can also alter their transmission characteristics and require specialized systems to accommodate the accompanying distortions. Wireless systems have an advantage over wired systems in last mile applications in not requiring lines to be installed. However, they also have a disadvantage in that their unguided nature makes them more susceptible to unwanted noise and signals. Spectral reuse can therefore be limited. ====Lightwaves and free-space optics==== Visible and infrared light waves are much shorter than radio frequency waves. Their use to transmit data is referred to as [[free-space optical communication]]. Being short, light waves can be focused or collimated with a small lens/antenna, and to a much higher degree than radio waves. Thus, a receiving device can recover a greater portion of the transmitted signal. Also, because of the high frequency, a high [[Bit rate|data transfer rate]] may be available. However, in practical last mile environments, obstructions and de-steering of these beams, and absorption by elements of the atmosphere including fog and rain, particularly over longer paths, can greatly restrict their use for last-mile wireless communications. ====Radio waves==== Radio frequencies (RF), from low frequencies through the microwave region, have wavelengths much longer than visible light. Although this means that it is not possible to focus the beams nearly as tightly as for light, it also means that the aperture or "capture area" of even the simplest, omnidirectional antenna is significantly larger than that of a lens in any feasible optical system. This characteristic results in greatly increased [[attenuation]] or "path loss" for systems that are not highly directional. Actually, the term [[path loss]] is something of a misnomer because no energy is lost on a free-space path. Rather, it is merely not received by the receiving antenna. The apparent reduction in transmission, as frequency is increased, is an artifact of the change in the aperture of a given type of antenna. Relative to the last-mile problem, these longer wavelengths have an advantage over light waves when omnidirectional or sectored transmissions are considered. The larger aperture of radio antennas results in much greater signal levels for a given path length and therefore higher information capacity. On the other hand, the lower carrier frequencies are not able to support the high information bandwidths, which are required by Shannon's equation when the practical limits of S/N have been reached. For the above reasons, wireless radio systems are optimal for lower-information-capacity broadcast communications delivered over longer paths. For high-information capacity, highly-directive [[point-to-point (telecommunications)|point-to-point]] over short ranges, wireless light-wave systems are the most useful. ====One-way (broadcast) radio and television communications==== Historically, most high-information-capacity broadcast has used lower frequencies, generally no higher than the UHF television region, with television itself being a prime example. Terrestrial television has generally been limited to the region above 50 MHz where sufficient information bandwidth is available, and below 1,000 MHz, due to problems associated with increased path loss, as mentioned above. ====Two-way wireless communications==== Two-way communication systems have primarily been limited to lower-information-capacity applications, such as audio, facsimile, or [[radioteletype]]. For the most part, higher-capacity systems, such as two-way video communications or terrestrial microwave telephone and data trunks, have been limited and confined to UHF or microwave and to point-point paths. Higher capacity systems such as third-generation cellular telephone systems require a large infrastructure of more closely spaced cell sites in order to maintain communications within typical environments, where path losses are much greater than in free space and which also require omnidirectional access by the users. ====Satellite communications==== For information delivery to end users, satellite systems, by nature, have relatively long path lengths, even for low earth-orbiting satellites. They are also very expensive to deploy and therefore each satellite must serve many users. Additionally, the very long paths of geostationary satellites cause information latency that makes many real-time applications unfeasible. As a solution to the last-mile problem, satellite systems have application and sharing limitations. The ICE which they transmit must be spread over a relatively large geographical area. This causes the received signal to be relatively small, unless very large or directional terrestrial antennas are used. A parallel problem exists when a satellite is receiving. In that case, the satellite system must have a very great information capacity in order to accommodate a multitude of sharing users and each user must have large antenna, with attendant directivity and pointing requirements, in order to obtain even modest information-rate transfer. These requirements render high-information-capacity, bi-directional information systems uneconomical. This is one reason why the [[Iridium Satellite LLC|Iridium]] satellite system was not more successful. ====Broadcast versus point-to-point==== For terrestrial and satellite systems, economical, high-capacity, last-mile communications requires point-to-point transmission systems. Except for extremely small geographic areas, broadcast systems are only able to deliver high S/N ratios at low frequencies where there is not sufficient spectrum to support the large information capacity needed by a large number of users. Although complete "flooding" of a region can be accomplished, such systems have the fundamental characteristic that most of the radiated ICE never reaches a user and is wasted. As information requirements increase, broadcast [[wireless mesh]] systems (also sometimes referred to as [[microcell]]s or nano-cells) which are small enough to provide adequate information distribution to and from a relatively small number of local users require a prohibitively large number of broadcast locations or points of presence along with a large amount of excess capacity to make up for the wasted energy. ===Intermediate system=== Recently a new type of information transport midway between wired and wireless systems has been discovered. Called [[Single-wire transmission line#E-Line|E-Line]], it uses a single central conductor but no outer conductor or shield. The energy is transported in a plane wave which, unlike radio does not diverge, whereas like radio it has no outer guiding structure. This system exhibits a combination of the attributes of wired and wireless systems and can support high information capacity [[broadband over power lines|utilizing existing power lines]] over a broad range of frequencies from [[Radio frequency|RF]] through [[microwave]]. ===Line aggregation=== [[Link aggregation|Aggregation]] is a method of bonding multiple lines to achieve a faster, more reliable connection. Some companies{{weasel inline|date=December 2015}} believe that ADSL aggregation (or "bonding") is the solution to the UK's last mile problem.<ref>{{cite web|url=http://uk.reuters.com/article/pressRelease/idUS92087+05-Jan-2009+BW20090105?symbol=INAP.O|archive-url=https://archive.today/20120716191534/http://uk.reuters.com/article/pressRelease/idUS92087+05-Jan-2009+BW20090105?symbol=INAP.O|url-status=dead|archive-date=July 16, 2012|title=Internap Chosen by Sharedband to Help Bring High-Speed Internet to New Business and Residential Customers|publisher=Reuters|date=January 5, 2009}}</ref>
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