Last mile


Last mile

The last mile is the final leg of delivering connectivity from a communications provider to a customer. Usually referred to by the telecommunications and cable television industries, it is typically seen as an expensive challenge because "fanning out" wires and cables is a considerable physical undertaking. In countries employing the metric (as opposed to the imperial) measurement system, the phrase "last kilometre" is sometimes used. Because the last mile of a network to the user is also the first mile from the user to the world, "first mile" is sometimes used.

To solve the problem of providing enhanced services over the last mile, some firms have been mixing networks for decades. One example is Fixed Wireless Access, where a wireless network is used instead of wires to connect a stationary terminal to the wireline network.

Various solutions are being developed which are seen as an alternative to the "last mile" of standard incumbent telecommunications providers: these include WiMAX and BPL (Broadband over Power Line) applications.

Existing delivery system problems

The increasing worldwide demand for rapid, low latency and high volume communication of information to homes and businesses has made economical information distribution and delivery increasingly important. As demand has escalated, particularly fueled by the widespread adoption of the Internet, the need for economical high speed access by end-users located at millions of locations has ballooned as well. As requirements have changed, existing systems and networks which were initially pressed into service for this purpose have proved to be inadequate. To date, although a number of approaches have been tried and used, no single clear solution to this problem has emerged. This problem has been termed "The Last Mile Problem".

As expressed by Shannon's equation for channel information capacity, the omnipresence of noise in information systems sets a minimum signal-to-noise ratio requirement in a channel, even when adequate spectral bandwidth is available. Since the integral of the rate of information transfer with respect to time is information quantity, this requirement leads to a corresponding minimum energy per bit. The problem of sending any given amount of information across a channel can therefore be viewed in terms of sending sufficient Information-Carrying Energy (ICE). For this reason the concept of an ICE "pipe" or "conduit" is relevant and useful for examining existing systems.

The distribution of information to a great number of widely separated end users can be compared to the distribution of many other resources. Some familiar analogies are:

*blood distribution to a large number of cells over a system of veins, arteries and capillaries
*water distribution by a drip irrigation system to individual plants, including rivers, aqueducts, water mains etc.
*Nourishment to a plants leaves through roots, trunk and branches

All of these have in common conduits which carry a relatively small amount of a resource a short distance to a very large number of physically separated endpoints. Also common are conduits supporting more voluminous flow which combine and carry the many individual portions over much greater distances. The shorter, lower volume conduits which individually serve only one or a small fraction of the endpoints, may have far greater combined length than the larger capacity ones. These common attributes are shown to the right.

The high capacity conduits in these systems tend to also have in common the ability to efficiently transfer the resource over a long distance. Only a small fraction of the resource being transferred is either wasted, lost or misdirected. The same cannot necessarily be said of the lower capacity conduits. One reason for this has to do with the efficiency of scale. These conduits which are located closer to the endpoint, or end-user, do not individually have as many users supporting them. Even though they are smaller, each has the overhead of an "installation"; obtaining and maintaining a suitable path over which the resource can flow. The funding and resources supporting these smaller conduits tend to come from the immediate locale. This can have the advantage of a "small government model". That is, the management and resources for these conduits is provided by local entities and therefore can be optimized to achieve the best solutions in the immediate environment and also to make best use of local resources. However, the lower operating efficiencies and relatively greater installation expenses, compared with the transfer capacities, can cause these smaller conduits, as a whole, to be the most expensive and difficult part of the complete distribution system.

These characteristics have been displayed in the birth, growth and funding of the Internet. The earliest inter-computer communication tended to be accomplished with direct wireline connections between individual computers. These grew into clusters of small Local Area Networks (LANs). The TCP/IP suite of protocols was born out of the need to connect several of these LANs together, particularly as related to common projects among the defense department, industry and some academic institutions. DARPAnet (Defense Advanced Research Projects Agency network) came into being to further these interests. In addition to providing a way for multiple computers and users to share a common inter-LAN connection, the TCP/IP protocols provided a standardized way for dissimilar computers and operating systems to exchange information over this inter-network. The funding and support for the connections among LANs could be spread over one or even several LANs. As each new LAN, or subnet, was added, the new subnet's constituents enjoyed access to the greater network. At the same time the new subnet made a contribution of access to any network or networks with which it was already networked. Thus the growth became a mutually inclusive or "win-win" event.

In general, economy of scale makes an increase in capacity of a conduit less expensive as the capacity is increased. There is an overhead associated with the creation of any conduit. This overhead is not repeated as capacity is increased within the potential of the technology being utilized. As the Internet has grown in size, by some estimates doubling in number of users every eighteen months, economy of scale has resulted in increasingly large information conduits providing the longest distance and highest capacity backbone connections. In recent years, the capacity of fiber-optic communication, aided by a supporting industry, has resulted in a great deal of raw capacity, so much so that in the United States there exists a large amount of "dark fiber"; installed fiber which is not being used because it is excess to current needs.

This excess backbone capacity exists in spite of the trend of increasing per-user data rates and overall quantity of data. Initially, only the inter-LAN connections were high speed. End users used existing telephone lines and modems which were capable of data rates of only a few hundred bit/s. Now almost all end users enjoy access at one hundred or more times those early rates. But in spite of this great increase in user traffic, the high capacity backbones have kept up and information capacity and rate limitations almost always occur near the user. The economy of scale along with the fundamental capability of fiber technology have kept the high capacity conduits adequate but have not solved the appetite of the home users. The last mile problem is one of economically serving an increasing mass of end-users with a solution to their information needs.

Economical information transfer

Before considering the characteristics of existing last mile information delivery mechanisms, it is important to further examine what makes information conduits effective. As the Shannon-Hartley theorem shows, it is a combination of bandwidth and signal-to-noise ratio which determines the maximum information rate of a channel. The product of the average information rate and time yields total information transfer. In the presence of noise, this corresponds to some amount of transferred energy. Therefore the economics of information transfer may be viewed in terms of the economics of the transfer of ICE. Effective last mile conduits must:
#Deliver signal power, S — (must have adequate signal power capacity).
#Low loss (low occurrence of conversion to unusable energy forms - presumaby also low packet loss).
#Support wide transmission bandwidth.
#Deliver high signal-to-noise ratio (SNR) — low unwanted-signal (Noise) power, N.
#Provide nomadic connectivity.

In addition to these, a good solution to the last mile problem must provide each user:
#High availability and reliability.
#Low latency, latency must be small compared with required interaction times.
#High per-user capacity.
##A conduit which is shared among multiple end-users must provide a correspondingly higher capacity in order to properly support each individual user. This must be true for information transfer in each direction.
##Affordability, suitable capacity must be financially viable.

Existing last mile delivery systems

Wired systems (including dielectric guides)

Wired systems provide guided conduits for ICE. They all have some degree of shielding which limits the 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 adequate S/N to support information flow.

;Local area networks (LAN)

Traditional wired local area networking systems require copper coaxial cable or 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 the maximum length may be limited by collision detection and avoidance requirements, signal loss and reflections over these lines also set 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.

;TelephoneIn 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. Together with CATV, these systems provide the bulk of end-user broadband Internet connections in many countries large and small.

;CATVCommunity Access Cable Television Systems, also known simply as "cable", 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 S/N are limited. As was done for the initial unidirectional (TV) communication, cable loss is mitigated through the use of periodic amplifiers within the system. These factors set an upper limit on the per-user information capacity, particularly when there are many users sharing a common section of cable.

;Optical fiber

Fiber is an excellent medium with respect to information capacity but is not readily available to most end users. It is generally laid underground in conduits, requiring a relatively expensive installation which is currently prohibitive for most users or overhead along existing rights-of-way. Until this situation changes, other media must be utilized to economically solve the last mile problem.

Wireless delivery systems

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 have attenuation which is inversely proportional to distance squared. This means that losses increase more slowly with increasing length than for wired systems. In a free space environment, beyond some length, the losses in a wireless system are less 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 over and above the free space value. Reflections, refraction and diffraction of these 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 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. The use of this light to transmit data is referred to as free space optics. Being short, light rays can be focused or collimated with a small lens/antenna and to a much higher degree than radio waves. In free space, a greater portion of the transmitted signal can be recovered by a receiving device. Also because of the high frequency, a high 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 like fog and rain, particularly over longer paths, can greatly restrict their use for last mile wireless communications. Longer (redder) waves suffer less obstruction but may carry lesser data rates. See RONJA.

;Radio waves

Radio frequencies (RF), from low frequencies through the microwave region, have wavelengths much longer than light. While 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, omni directional antenna is greatly 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. In actuality the term path loss is something of a misnomer since no energy is actually 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 actually 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 wave when omni-directional 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, once the practical limits of S/N have been reached.

For the above reasons, wireless radio systems have the advantage of being useful for lower information capacity, broadcast communications over longer paths and while wireless light wave systems are most useful for high information capacity, highly directive point-to-point, short range communications.

;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 1000 MHz, due to problems associated with increased path loss as mentioned above.

;Two-way wireless communications

Two way communications systems have primarily been limited to lower information capacity applications, such as audio, facsimile or radio teletype. 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, 3G, 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 omni-directional 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 unusable. 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 size, with attendant directivity and pointing requirements, in order to obtain even modest information rate transfer. These requirements render high information capacity, bidirectional information systems uneconomical. This is a reason that the Iridium satellite system was not more successful.

;Broadcast versus point-to-point

For both 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 large amounts of S/N at low frequencies where there is not sufficient spectrum to support the large information capacity needed by a large number of users. While 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 microcells 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 which is midway between wired and wireless systems has been discovered. Called 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 while like radio, has no outer guiding structure. This system exhibits a combination of the attributes of wired and wireless systems and can support high information capacity utilizing existing power lines over a broad range of frequencies from RF through microwave. See BPL (Broadband over Power Line).

Courier

Wizzy Digital Courier is a project to distribute useful data to places with no Internet connection. Primarily for e-mail, it also carries web content (stored locally in a web cache).

In an implementation of a sneakernet, its delivery mechanism is USB flash drive. The USB stick uses the UUCP protocol, carrying information to and from a better-connected location - perhaps a school or local business, which acts as the dropoff for Email, and fetches web content by proxy. The email and web content is re-packaged as a UUCP transaction, and ferried back on the USB flash drive.

ee also

* access network
* Ethernet in the First Mile
* local loop
* wireless local loop
* WiMAX


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