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22
Plans for Ubiquitous Broadband Access to the National Information
Infrastructure in the Ameritech Region
Joel S. Engel
Ameritech
Statement of the Problem
The national information infrastructure (NII) can be visualized
as consisting of four components: (1) access transport from the
premises of the end user to an access node; (2) access nodes on an
intracity, intercity, and international network of networks; (3)
the internodal network of networks, including the internetwork
gateways; and (4) information repositories. The access nodes,
network, and information repositories—the last three
components—are common facilities; their costs are
predominantly proportional to usage and can be shared among many
users. In contrast, the access transport—the first
component—is a dedicated facility connecting the premises of a
single user; its cost is predominatly fixed, independent of usage,
and borne by that single user.
As a result, the access transport—popularly referred to as
the "last mile," although, in fact, it is typically a few miles in
length—presents the greatest challenge to the provision of a
ubiquitous, truly "national," wideband NII. Because the access
nodes and information repositories can be provided modularly and
grow with usage, they are already being put in place, by small
entrepreneurs as well as by large service providers. Since the
intracity, intercity, and international traffic can be multiplexed
into the traffic stream on the existing telecommunications network,
that portion of the ''information superhighway" already exists. The
major investment hurdle, then, is the deployment of ubiquitous
broadband access to individual user premises.
At first glance, it might appear that cable television systems
constitute such broadband access, but closer analysis reveals that
this is not so. Cable systems are predominantly one-way downstream,
some with minor upstream capability, usually relying on polling to
avoid interference among upstream signals. More constraining, they
are broadcast systems, with 50 or so video channels being
simultaneously viewed by several thousand users. The same cost
hurdle described above exists to upgrade these systems to provide
dedicated broadband access to individual users.
Currently available technologies for access transport span a
wide range of capabilities and costs. Since the costs cannot be
shared among users, at the low end of the range they are shared
among usages, particularly with voice telephony. Modems provide
dialup capability over standard telephone lines at speeds typically
up to 9,600 bits per second, with less common capability at 14,400
and 28,800 bits per second. Integrated services digital network
(ISDN) is rapidly becoming widely available, providing dialup
capability at 56, 64, and 128 kilobits per second. Various local
carriers, including the local exchange carriers, are offering frame
relay service at 56 kilobits per second and 1.5 megabits per
second, and switched multimegabit digital service at 1.5 and 45
megabits per second, with plans for migration to asynchronous
transfer mode (ATM) at OC-1 (45 megabits per second) and higher
SONET rates. However, all of these require a dedicated leased
digital line between the user's premises and the carrier's switch
of a capacity equal to the peak bit rate used. As a result, these
higher-speed services are cost effective only for large user
locations that generate sufficient traffic to fill them. The
challenge, again, is to provide cost-effective, high-speed access
to individual homes and small business locations.
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Deployment Plans for the Ameritech
Region
Ameritech has committed to providing broadband access in its
region, using a hybrid optical fiber and coaxial cable architecture
that can support a wide range of applications, including video
services similar to current cable television, expanded digital
multicast services, interactive multimedia services, and high-speed
data services. By providing a platform for a wide range of
applications, a cost-effective solution is achieved (economic data
are provided in a later section).
Construction is expected to begin in 1995 in the Chicago,
Cleveland, Columbus, Detroit, Indianapolis, and Milwaukee
metropolitan areas, ramping up to an installation rate of 1 million
lines per year by the end of 1995 and continuing at that rate to
deploy 6 million lines throughout the region by the end of the
decade. In December 1994, the Federal Communications Commission
granted approval for Ameritech to construct the first 1.256 million
lines, distributed among the metropolitan areas as follows:
•
Chicago—501,000,
•
Cleveland—137,000,
•
Columbus—125,000,
•
Detroit—232,000,
•
Indianapolis—115,000, and
•
Milwaukee—146,000.
Technical Architecture
The system architecture employs a hybrid transport network of
optical fiber and coaxial cable. Signals are delivered over
Ameritech's ATM network to video serving offices, each serving
100,000 to 150,000 customer locations. The signals are then
distributed on optical fiber to individual nodes, each serving a
total of 500 customer locations, not all of whom may actually
subscribe to the service. From each node, the signals are
distributed on four parallel coaxial cable systems, each serving
125 customer locations. With this architecture, the coaxial cable
network is less than 2,000 feet in length and contains, at most,
three amplifiers to any customer location.
The signal on both the optical fiber and the coaxial cable is a
broadband analog video signal. The initial deployment will have a
bandwidth of 750 megahertz, with capability for upgrade to 1
gigahertz when the reliability of such electronics becomes proven,
yielding 110 channels of standard 6 megahertz video bandwidth. The
allocation of these 110 channels to various applications is
flexible and will be adjusted to satisfy user needs. Based on
current estimates, approximately 70 of the channels will carry
analog video signals for applications similar to current cable
television, including basic and premium channels and pay-per-view.
The remaining, approximately 40, of the channels will be digitized
using 256 quadrature amplitude modulation, yielding a usable bit
rate of over 36 megabits per second on each channel. Approximately
30 of these digitized channels will be used for multicast services,
with multiple users viewing each transmitted program. Approximately
10 of the digitized channels will be used for switched interactive
services, for which each user requires a dedicated digital circuit
for the duration of the session.
On the digitized channels, the video signals will be compressed
using the MPEG-2 compression standard. Depending on the particular
application, each such signal will require a fraction of the
36-megabit-per-second or greater capacity. The signals will be
multiplexed at the video serving offices and demultiplexed by the
customer premises equipment, using the MPEG-2 transport layer
protocol.
In addition to the downstream capacity, the system will have an
upstream capability provided by up to 20 channels, each of
1.5-megabit-per-second capacity. Depending on local conditions of
noise and interference, it is expected that at least 15 of these
will be usable on each coaxial cable system serving 125 customer
locations.
The system is intended to be a platform for a wide range of
applications. Accordingly, the customer premises equipment may be a
set-top box for use with a television set or an adjunct to a
personal computer.
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System Capacity
As described above, each branch of the distribution network will
consist of an optical fiber to a node serving 500 customer
locations, with four coaxial cable buses from the node, each
serving 125 customer locations. Each branch will have a downstream
capacity of approximately 360 megabits per second for switched
interactive services.
The system is designed for economical growth in capacity as the
market expands and traffic increases. Initially, each fiber branch
will share a linear laser in the video serving office with two
other fibers, through a passive optical splitter, so that the
360-megabits-per-second downstream capacity will serve 1,500
customer locations, not all of whom may subscribe to the service.
When the traffic grows to exceed this capacity, the fibers can be
driven by independent lasers so that the 360 megabits per second
downstream capacity will serve 500 customer locations. When the
traffic requires it, up to four fibers can feed each node, one for
each coaxial cable bus, so that the 360 megabit per second
downstream capacity can serve 125 customer locations.
The downstream capacity will be assigned to the users on a
per-session basis, depending on the particular application. There
is some uncertainty about the bit-rate required for each
application. Human factors experiments and customer trials appear
to indicate that live programs other than athletic events,
compressed in real time and displayed on full-size television
screens, can be delivered at 3 megabits per second. Material that
is prerecorded and stored in compressed digital form can be
delivered at a lower bit-rate, since the compression is not
performed in real time. During the compression process, the results
can be observed, and the parameters of the compression algorithm
can be optimized to the particular material. Similarly, video
material that accompanies text and occupies only a portion of a
computer screen requires less resolution and can be delivered at a
lower bit-rate. Not all interactive applications will involve
video, and those that do will generally involve material that is
stored in compressed digital form and displayed in a window
occupying a portion of the screen, so that an average of 3 megabits
per second per session is probably a conservative estimate. At that
average bit-rate, the 360-megabits-per-second downstream capacity
could serve 120 simultaneous sessions.
Further, although the initial system will assign a fixed
bit-rate for the entire duration of the session, equal to the
bit-rate required for the most demanding segment, the capability
will exist for statistically multiplexing the signals and using a
bit-rate based on instantaneous requirement. This capability will
be employed if necessary. In that event, an average bit-rate per
session of 3 megabits per second would be quite conservative.
It is estimated that, during the peak usage period of the day,
15 percent of the subscribers will be using the service at the same
time. That would consume the entire 120-session capacity when 800
of the 1,500 customer locations became subscribers, equal to 53
percent market penetration. At that point, the next step in
capacity growth, sharing the capacity among 500 customer locations,
would be implemented. If the peak usage per subscriber turned out
to be higher than the 15 percent estimated, then the capacity
growth would be implemented at a lower number of users. This would
not affect the economic viability of the system, since it is the
total amount of usage that will generate both the need and the
revenue to support the increased capacity.
At the limit of the current architecture, the 120-session
capacity will be shared by the 125 customer locations on each
coaxial cable bus. That would support 96 percent simultaneous usage
at 100 percent market penetration. If that turned out to be
insufficient, because of multiple simultaneous users per customer
location, or higher bit-rate applications, statistical multiplexing
could be employed.
In addition to the downstream capability, each coaxial cable
will support at least 15 usable upstream channels at 1.5 megabits
per second each, and these will be multiplexed onto the fiber from
the node to the video serving office. Therefore, unlike the
downstream capacity, this upstream capacity will be shared by 125
customer locations from the start. These upstream channels must
utilize a time division multiple access protocol, which does not
allow for 100 percent "fill"; nevertheless, traffic studies
indicate that a single 1.5-megabit-per-second upstream channel
could easily support all of the digital video requirements,
including video on demand, of the 125 customer locations, leaving
the remaining 14 or more for the more interactive multimedia and
data applications.
There are two parameters of the interactive services that
determine the required upstream capacity: (1) the frequency (and
size) of the upstream messages generated by the user and (2) the
required latency, or speed of response to the message. Analyses of
the types of applications that are anticipated indicate that the
second
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parameter is controlling. If enough upstream capacity is
provided to assure that the transport delay contributes no more
than 100 milliseconds, it does not matter how frequently the user
generates upstream commands.
Of course, if the users were to generate large files for
transmission in the upstream direction, rather than the inquiry and
command messages typical of interactive services, this analysis
would not apply. But, by definition, such users would be generating
sufficient traffic to use one of Ameritech's other, more symmetric,
data offerings cost effectively, and would not be using the hybrid
fiber/coaxial cable system.
At a 100-millisecond latency, each upstream channel could
support approximately 20 simultaneous sessions, for a total of at
least 280 simultaneous sessions on the 14 or more upstream
channels. This is greater than the 125 simultaneous sessions
supportable by the downstream capacity that will only be generated
by 96 percent simultaneous usage at 100 percent market
penetration.
Economic Analysis
At this time, the FCC has approved construction of the first
1.256 million lines, and the economics for that initial phase,
which is extracted from Ameritech's application for construction
approval, is presented below.
Table 1 presents the economic data for the first 10 years. It is
important to note that these data are strictly for the transport
network; they do not include any costs or revenues for the
provision of "content."
Construction of the first 1.256 million lines is planned for
completion early in the third year. Both market penetration and
usage per customer are expected to grow throughout the period, as
shown by the revenue forecast. These revenues are for the transport
of all types of content, including broadcast television, video on
demand, interactive multimedia services, and high-speed data
access. The total costs each year consist of three components: (1)
the costs of constructing additional lines, which ends in the third
year; (2) the costs of adding equipment for additional customers
and additional usage per customer; and (3) the costs of providing
customer service.
The cash flow each year, revenues minus costs, is discounted to
Year 1 equivalent values and cumulated, and is presented in Table 1
as cumulative discounted cash flow (CDCF). As shown, the CDCF turns
positive in the eighth year. In actuality, as additional lines are
constructed in each of the six major metropolitan areas, building
on the initial base, economies of scale are expected to make the
economics even more favorable.
TABLE 1 Economic Data for Initial Phase of
Ameritech's Deployment of Broadband Access Transport
Infrastructure
Year
Customer locations passed (millions)
Revenues ($millions)
Costs ($millions)
Discounted cumulative cash flow ($millions)
1
0.5
0.963
131.2
(130.2)
2
1.058
47.9
162.5
(233.8)
3
1.256
84.2
85.8
(235.2)
4
1.256
115.5
65.8
(198.4)
5
1.256
137.4
75.4
(157.0)
6
1.256
154.0
71.1
(106.9)
7
1.256
162.9
65.1
(53.5)
8
1.256
176.3
66.4
0.8
9
1.256
191.1
68.3
55.6
10
1.256
200.2
48.1
117.0
Conclusions
The greatest challenge to the realization of a ubiquitous
wideband national information infrastructure, an NII that is truly
"national," is the provision of economical broadband access
transport to individual residences and small
businesses—popularly referred to as the "last mile." Access
transport to large business locations can be based on usage,
because such locations, as well as the nodes and internodal network
and the information repositories, are all shared facilities and are
used (and paid for) by multiple users. By contrast, access
transport
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to individual customer locations must be sized for the peak
requirements of the most demanding application, even though that
capacity is unused most of the time, and its cost must be borne by
that individual customer.
Although these facilities cannot be shared by multiple
customers, they can be shared among multiple services to the
individual customer. Ameritech has committed to providing broadband
access in its region, utilizing a hybrid optical fiber and coaxial
cable architecture, supporting a wide range of applications. These
include video services similar to current cable television,
expanded digital multicast services, and interactive multimedia
services, as well as high speed data services. By providing a
platform for a wide range of services, a cost-effective solution is
achieved.
Ameritech plans to begin construction in 1995, ramping up to a
rate of 1 million lines per year by year end and continuing at that
rate to deploy 6 million lines throughout the region by the end of
the decade. The FCC has granted approval for construction of the
first 1.256 million lines, in the Chicago, Cleveland, Columbus,
Detroit, Indianapolis, and Milwaukee metropolitan areas.
Representative terms from entire chapter:
coaxial cable
