Technologism Essay, Research Paper

The Internet is a network of networks that interconnects computers around

the world, supporting both business and residential users. In 1994, a

multimedia Internet application known as the World Wide Web became

popular. The higher bandwidth needs of this application have highlighted

the limited Internet access speeds available to residential users. Even at 28.8

Kilobits per second (Kbps)—the fastest residential access commonly

available at the time of this writing—the transfer of graphical images can be

frustratingly slow.

This report examines two enhancements to existing residential

communications infrastructure: Integrated Services Digital Network (ISDN),

and cable television networks upgraded to pass bi-directional digital traffic

(Cable Modems). It analyzes the potential of each enhancement to deliver

Internet access to residential users. It validates the hypothesis that upgraded

cable networks can deliver residential Internet access more cost-effectively,

while offering a broader range of services.

The research for this report consisted of case studies of two commercial

deployments of residential Internet access, each introduced in the spring of

1994:

? Continental Cablevision and Performance Systems International (PSI)

jointly developed PSICable, an Internet access service deployed over

upgraded cable plant in Cambridge, Massachusetts;

? Internex, Inc. began selling Internet access over ISDN telephone

circuits available from Pacific Bell. Internex’s customers are residences and

small businesses in the “Silicon Valley” area south of San Francisco,

California.

2.0 The Internet

When a home is connected to the Internet, residential communications

infrastructure serves as the “last mile” of the connection between the

home computer and the rest of the computers on the Internet. This

section describes the Internet technology involved in that connection.

This section does not discuss other aspects of Internet technology in

detail; that is well done elsewhere. Rather, it focuses on the services

that need to be provided for home computer users to connect to the

Internet.

2.1

ISDN and upgraded cable networks will each provide different functionality

(e.g. type and speed of access) and cost profiles for Internet connections. It

might seem simple enough to figure out which option can provide the needed

level of service for the least cost, and declare that option “better.” A key

problem with this approach is that it is difficult to define exactly the needed

level of service for an Internet connection. The requirements depend on

the applications being run over the connection, but these applications are

constantly changing. As a result, so are the costs of meeting the applications’

requirements.

Until about twenty years ago, human conversation was by far the dominant

application running on the telephone network. The network was

consequently optimized to provide the type and quality of service needed for

conversation. Telephone traffic engineers measured aggregate statistical

conversational patterns and sized telephone networks accordingly.

Telephony’s well-defined and stable service requirements are reflected in the

“3-3-3″ rule of thumb relied on by traffic engineers: the average voice call

lasts three minutes, the user makes an average of three call attempts during

the peak busy hour, and the call travels over a bidirectional 3 KHz channel.

In contrast, data communications are far more difficult to characterize. Data

transmissions are generated by computer applications. Not only do existing

applications change frequently (e.g. because of software upgrades), but

entirely new categories—such as Web browsers—come into being quickly,

adding different levels and patterns of load to existing networks.

Researchers can barely measure these patterns as quickly as they are

generated, let alone plan future network capacity based on them.

The one generalization that does emerge from studies of both local and wide-

area data traffic over the years is that computer traffic is bursty. It does not

flow in constant streams; rather, “the level of traffic varies widely over

almost any measurement time scale” (Fowler and Leland, 1991). Dynamic

bandwidth allocations are therefore preferred for data traffic, since static

allocations waste unused resources and limit the flexibility to absorb bursts

of traffic.

This requirement addresses traffic patterns, but it says nothing about the

absolute level of load. How can we evaluate a system when we never know

how much capacity is enough? In the personal computing industry, this

problem is solved by defining “enough” to be “however much I can afford

today,” and relying on continuous price-performance improvements in digital

technology to increase that level in the near future. Since both of the

infrastructure upgrade options rely heavily on digital technology, another

criteria for evaluation is the extent to which rapidly advancing technology

can be immediately reflected in improved service offerings.

Cable networks satisfy these evaluation criteria more effectively than

telephone networks because:

? Coaxial cable is a higher quality transmission medium than twisted

copper wire pairs of the same length. Therefore, fewer wires, and

consequently fewer pieces of associated equipment, need to be

installed and maintained to provide the same level of aggregate

bandwidth to a neighborhood. The result should be cost savings and

easier upgrades.

? Cable’s shared bandwidth approach is more flexible at allocating any

particular level of bandwidth among a group of subscribers. Since it

does not need to rely as much on forecasts of which subscribers will

sign up for the service, the cable architecture can adapt more readily

to the actual demand that materializes.

? Telephony’s dedication of bandwidth to individual customers limits

the peak (i.e. burst) data rate that can be provided cost-effectively.

In contrast, the dynamic sharing enabled by cable’s bus architecture

can, if the statistical aggregation properties of neighborhood traffic

cooperate, give a customer access to a faster peak data rate than the

expected average data rate.

2.2 Why focus on Internet access?

Internet access has several desirable properties as an application to

consider for exercising residential infrastructure. Internet technology is

based on a peer-to-peer model of communications. Internet usage

encompasses a wide mix of applications, including low- and high-

bandwidth as well as asynchronous and real-time communications.

Different Internet applications may create varying degrees of

symmetrical (both to and from the home) and asymmetrical traffic

flows. Supporting all of these properties poses a challenge for existing

residential communications infrastructures.

Internet access differs from the future services modeled by other studies

described below in that it is a real application today, with growing

demand. Aside from creating pragmatic interest in the topic, this factor

also makes it possible to perform case studies of real deployments.

Finally, the Internet’s organization as an “Open Data Network” (in the

language of (Computer Science and Telecommunications Board of the

National Research Council, 1994)) makes it a service worthy of study

from a policy perspective. The Internet culture’s expectation of

interconnection and cooperation among competing organizations may

clash with the monopoly-oriented cultures of traditional infrastructure

organizations, exposing policy issues. In addition, the Internet’s status

as a public data network may make Internet access a service worth

encouraging for the public good. Therefore, analysis of costs to provide

this service may provide useful input to future policy debates.

3.0 Technologies

This chapter reviews the present state and technical evolution of

residential cable network infrastructure. It then discusses a topic not

covered much in the literature, namely, how this infrastructure can be

used to provide Internet access. It concludes with a qualitative

evaluation of the advantages and disadvantages of cable-based Internet

access. While ISDN is extensively described in the literature, its use as

an Internet access medium is less well-documented. This chapter

briefly reviews local telephone network technology, including ISDN

and future evolutionary technologies. It concludes with a qualitative

evaluation of the advantages and disadvantages of ISDN-based Internet

access.

3.1 Cable Technology

Residential cable TV networks follow the tree and branch architecture.

In each community, a head end is installed to receive satellite and

traditional over-the-air broadcast television signals. These signals are

then carried to subscriber’s homes over coaxial cable that runs from the

head end throughout the community

Figure 3.1: Coaxial cable tree-and-branch topology

To achieve geographical coverage of the community, the cables

emanating from the head end are split (or “branched”) into multiple

cables. When the cable is physically split, a portion of the signal power

is split off to send down the branch. The signal content, however, is not

split: the same set of TV channels reach every subscriber in the

community. The network thus follows a logical bus architecture. With

this architecture, all channels reach every subscriber all the time,

whether or not the subscriber’s TV is on. Just as an ordinary television

includes a tuner to select the over-the-air channel the viewer wishes to

watch, the subscriber’s cable equipment includes a tuner to select

among all the channels received over the cable.

3.1.1. Technological evolution

The development of fiber-optic transmission technology has led cable

network developers to shift from the purely coaxial tree-and-branch

architecture to an approach referred to as Hybrid Fiber and Coax(HFC)

networks. Transmission over fiber-optic cable has two main advantages

over coaxial cable:

? A wider range of frequencies can be sent over the fiber, increasing

the bandwidth available for transmission;

? Signals can be transmitted greater distances without amplification.

The main disadvantage of fiber is that the optical components required

to send and receive data over it are expensive. Because lasers are still

too expensive to deploy to each subscriber, network developers have

adopted an intermediate Fiber to the Neighborhood (FTTN)approach.

Figure 3.3: Fiber to the Neighborhood (FTTN) architecture

Various locations along the existing cable are selected as sites for

neighborhood nodes. One or more fiber-optic cables are then run from

the head end to each neighborhood node. At the head end, the signal is

converted from electrical to optical form and transmitted via laser over

the fiber. At the neighborhood node, the signal is received via laser,

converted back from optical to electronic form, and transmitted to the

subscriber over the neighborhood’s coaxial tree and branch network.

FTTN has proved to be an appealing architecture for telephone

companies as well as cable operators. Not only Continental

Cablevision and Time Warner, but also Pacific Bell and Southern New

England Telephone have announced plans to build FTTN networks.

Fiber to the neighborhood is one stage in a longer-range evolution of

the cable plant. These longer-term changes are not necessary to provide

Internet service today, but they might affect aspects of how Internet

service is provided in the future.

3.2 ISDN Technology

Unlike cable TV networks, which were built to provide only local

redistribution of television programming, telephone networks provide

switched, global connectivity: any telephone subscriber can call any

other telephone subscriber anywhere else in the world. A call placed

from a home travels first to the closest telephone company Central

Office (CO) switch. The CO switch routes the call to the destination

subscriber, who may be served by the same CO switch, another CO

switch in the same local area, or a CO switch reached through a long-

distance network.

Figure 4.1: The telephone network

The portion of the telephone network that connects the subscriber to

the closest CO switch is referred to as the local loop. Since all calls

enter and exit the network via the local loop, the nature of the local

connection directly affects the type of service a user gets from the

global telephone network.

With a separate pair of wires to serve each subscriber, the local

telephone network follows a logical star architecture. Since a Central

Office typically serves thousands of subscribers, it would be unwieldy

to string wires individually to each home. Instead, the wire pairs are

aggregated into groups, the largest of which are feeder cables. At

intervals along the feeder portion of the loop, junction boxes are placed.

In a junction box, wire pairs from feeder cables are spliced to wire pairs

in distribution cables that run into neighborhoods. At each subscriber

location, a drop wire pair (or pairs, if the subscriber has more than one

line) is spliced into the distribution cable.

Since distribution cables are either buried or aerial, they are disruptive

and expensive to change. Consequently, a distribution cable usually

contains as many wire pairs as a neighborhood might ever need, in

advance of actual demand.

Implementation of ISDN is hampered by the irregularity of the local

loop plant. Referring back to Figure 4.3, it is apparent that loops are of

different lengths, depending on the subscriber’s distance from the

Central Office. ISDN cannot be provided over loops with loading coils

or loops longer than 18,000 feet (5.5 km).

4.0 Internet Access

This section will outline the contrasts of access via the cable plant with

respect to access via the local telephon network.

4.1 Internet Access Via Cable

The key question in providing residential Internet access is what kind of

network technology to use to connect the customer to the Internet For

residential Internet delivered over the cable plant, the answer is

broadband LAN technology. This technology allows transmission of

digital data over one or more of the 6 MHz channels of a CATV cable.

Since video and audio signals can also be transmitted over other

channels of the same cable, broadband LAN technology can co-exist

with currently existing services.

Bandwidth

The speed of a cable LAN is described by the bit rate of the modems

used to send data over it. As this technology improves, cable LAN

speeds may change, but at the time of this writing, cable modems range

in speed from 500 Kbps to 10 Mbps, or roughly 17 to 340 times the bit

rate of the familiar 28.8 Kbps telephone modem. This speed represents

the peak rate at which a subscriber can send and receive data, during

the periods of time when the medium is allocated to that subscriber. It

does not imply that every subscriber can transfer data at that rate

simultaneously. The effective average bandwidth seen by each

subscriber depends on how busy the LAN is. Therefore, a cable LAN

will appear to provide a variable bandwidth connection to the Internet

Full-time connections

Cable LAN bandwidth is allocated dynamically to a subscriber only

when he has traffic to send. When he is not transferring traffic, he does

not consume transmission resources. Consequently, he can always be

connected to the Internet Point of Presence without requiring an

expensive dedication of transmission resources.

4.2 Internet Access Via Telephone Company

In contrast to the shared-bus architecture of a cable LAN, the telephone

network requires the residential Internet provider to maintain multiple

connection ports in order to serve multiple customers simultaneously.

Thus, the residential Internet provider faces problems of multiplexing

and concentration of individual subscriber lines very similar to those

faced in telephone Central Offices.

The point-to-point telephone network gives the residential Internet

provider an architecture to work with that is fundamentally different

from the cable plant. Instead of multiplexing the use of LAN

transmission bandwidth as it is needed, subscribers multiplex the use of

dedicated connections to the Internet provider over much longer time

intervals. As with ordinary phone calls, subscribers are allocated fixed

amounts of bandwidth for the duration of the connection. Each

subscriber that succeeds in becoming active (i.e. getting connected to

the residential Internet provider instead of getting a busy signal) is

guaranteed a particular level of bandwidth until hanging up the call.

Bandwidth

Although the predictability of this connection-oriented approach is

appealing, its major disadvantage is the limited level of bandwidth that

can be economically dedicated to each customer. At most, an ISDN

line can deliver 144 Kbps to a subscriber, roughly four times the

bandwidth available with POTS. This rate is both the average and the

peak data rate. A subscriber needing to burst data quickly, for example

to transfer a large file or engage in a video conference, may prefer a

shared-bandwidth architecture, such as a cable LAN, that allows a

higher peak data rate for each individual subscriber. A subscriber who

needs a full-time connection requires a dedicated port on a terminal

server. This is an expensive waste of resources when the subscriber is

connected but not transferring data.

The Internet is a network of networks that interconnects computers around

the world, supporting both business and residential users. In 1994, a

multimedia Internet application known as the World Wide Web became

popular. The higher bandwidth needs of this application have highlighted

the limited Internet access speeds available to residential users. Even at 28.8

Kilobits per second (Kbps)—the fastest residential access commonly

available at the time of this writing—the transfer of graphical images can be

frustratingly slow.

This report examines two enhancements to existing residential

communications infrastructure: Integrated Services Digital Network (ISDN),

and cable television networks upgraded to pass bi-directional digital traffic

(Cable Modems). It analyzes the potential of each enhancement to deliver

Internet access to residential users. It validates the hypothesis that upgraded

cable networks can deliver residential Internet access more cost-effectively,

while offering a broader range of services.

The research for this report consisted of case studies of two commercial

deployments of residential Internet access, each introduced in the spring of

1994:

? Continental Cablevision and Performance Systems International (PSI)

jointly developed PSICable, an Internet access service deployed over

upgraded cable plant in Cambridge, Massachusetts;

? Internex, Inc. began selling Internet access over ISDN telephone

circuits available from Pacific Bell. Internex’s customers are residences and

small businesses in the “Silicon Valley” area south of San Francisco,

California.

2.0 The Internet

When a home is connected to the Internet, residential communications

infrastructure serves as the “last mile” of the connection between the

home computer and the rest of the computers on the Internet. This

section describes the Internet technology involved in that connection.

This section does not discuss other aspects of Internet technology in

detail; that is well done elsewhere. Rather, it focuses on the services

that need to be provided for home computer users to connect to the

Internet.

2.1

ISDN and upgraded cable networks will each provide different functionality

(e.g. type and speed of access) and cost profiles for Internet connections. It

might seem simple enough to figure out which option can provide the needed

level of service for the least cost, and declare that option “better.” A key

problem with this approach is that it is difficult to define exactly the needed

level of service for an Internet connection. The requirements depend on

the applications being run over the connection, but these applications are

constantly changing. As a result, so are the costs of meeting the applications’

requirements.

Until about twenty years ago, human conversation was by far the dominant

application running on the telephone network. The network was

consequently optimized to provide the type and quality of service needed for

conversation. Telephone traffic engineers measured aggregate statistical

conversational patterns and sized telephone networks accordingly.

Telephony’s well-defined and stable service requirements are reflected in the

“3-3-3″ rule of thumb relied on by traffic engineers: the average voice call

lasts three minutes, the user makes an average of three call attempts during

the peak busy hour, and the call travels over a bidirectional 3 KHz channel.

In contrast, data communications are far more difficult to characterize. Data

transmissions are generated by computer applications. Not only do existing

applications change frequently (e.g. because of software upgrades), but

entirely new categories—such as Web browsers—come into being quickly,

adding different levels and patterns of load to existing networks.

Researchers can barely measure these patterns as quickly as they are

generated, let alone plan future network capacity based on them.

The one generalization that does emerge from studies of both local and wide-

area data traffic over the years is that computer traffic is bursty. It does not

flow in constant streams; rather, “the level of traffic varies widely over

almost any measurement time scale” (Fowler and Leland, 1991). Dynamic

bandwidth allocations are therefore preferred for data traffic, since static

allocations waste unused resources and limit the flexibility to absorb bursts

of traffic.

This requirement addresses traffic patterns, but it says nothing about the

absolute level of load. How can we evaluate a system when we never know

how much capacity is enough? In the personal computing industry, this

problem is solved by defining “enough” to be “however much I can afford

today,” and relying on continuous price-performance improvements in digital

technology to increase that level in the near future. Since both of the

infrastructure upgrade options rely heavily on digital technology, another

criteria for evaluation is the extent to which rapidly advancing technology

can be immediately reflected in improved service offerings.

Cable networks satisfy these evaluation criteria more effectively than

telephone networks because:

? Coaxial cable is a higher quality transmission medium than twisted

copper wire pairs of the same length. Therefore, fewer wires, and

consequently fewer pieces of associated equipment, need to be

installed and maintained to provide the same level of aggregate

bandwidth to a neighborhood. The result should be cost savings and

easier upgrades.

? Cable’s shared bandwidth approach is more flexible at allocating any

particular level of bandwidth among a group of subscribers. Since it

does not need to rely as much on forecasts of which subscribers will

sign up for the service, the cable architecture can adapt more readily

to the actual demand that materializes.

? Telephony’s dedication of bandwidth to individual customers limits

the peak (i.e. burst) data rate that can be provided cost-effectively.

In contrast, the dynamic sharing enabled by cable’s bus architecture

can, if the statistical aggregation properties of neighborhood traffic

cooperate, give a customer access to a faster peak data rate than the

expected average data rate.

2.2 Why focus on Internet access?

Internet access has several desirable properties as an application to

consider for exercising residential infrastructure. Internet technology is

based on a peer-to-peer model of communications. Internet usage

encompasses a wide mix of applications, including low- and high-

bandwidth as well as asynchronous and real-time communications.

Different Internet applications may create varying degrees of

symmetrical (both to and from the home) and asymmetrical traffic

flows. Supporting all of these properties poses a challenge for existing

residential communications infrastructures.

Internet access differs from the future services modeled by other studies

described below in that it is a real application today, with growing

demand. Aside from creating pragmatic interest in the topic, this factor

also makes it possible to perform case studies of real deployments.

Finally, the Internet’s organization as an “Open Data Network” (in the

language of (Computer Science and Telecommunications Board of the

National Research Council, 1994)) makes it a service worthy of study

from a policy perspective. The Internet culture’s expectation of

interconnection and cooperation among competing organizations may

clash with the monopoly-oriented cultures of traditional infrastructure

organizations, exposing policy issues. In addition, the Internet’s status

as a public data network may make Internet access a service worth

encouraging for the public good. Therefore, analysis of costs to provide

this service may provide useful input to future policy debates.

3.0 Technologies

This chapter reviews the present state and technical evolution of

residential cable network infrastructure. It then discusses a topic not

covered much in the literature, namely, how this infrastructure can be

used to provide Internet access. It concludes with a qualitative

evaluation of the advantages and disadvantages of cable-based Internet

access. While ISDN is extensively described in the literature, its use as

an Internet access medium is less well-documented. This chapter

briefly reviews local telephone network technology, including ISDN

and future evolutionary technologies. It concludes with a qualitative

evaluation of the advantages and disadvantages of ISDN-based Internet

access.

3.1 Cable Technology

Residential cable TV networks follow the tree and branch architecture.

In each community, a head end is installed to receive satellite and

traditional over-the-air broadcast television signals. These signals are

then carried to subscriber’s homes over coaxial cable that runs from the

head end throughout the community

Figure 3.1: Coaxial cable tree-and-branch topology

To achieve geographical coverage of the community, the cables

emanating from the head end are split (or “branched”) into multiple

cables. When the cable is physically split, a portion of the signal power

is split off to send down the branch. The signal content, however, is not

split: the same set of TV channels reach every subscriber in the

community. The network thus follows a logical bus architecture. With

this architecture, all channels reach every subscriber all the time,

whether or not the subscriber’s TV is on. Just as an ordinary television

includes a tuner to select the over-the-air channel the viewer wishes to

watch, the subscriber’s cable equipment includes a tuner to select

among all the channels received over the cable.

3.1.1. Technological evolution

The development of fiber-optic transmission technology has led cable

network developers to shift from the purely coaxial tree-and-branch

architecture to an approach referred to as Hybrid Fiber and Coax(HFC)

networks. Transmission over fiber-optic cable has two main advantages

over coaxial cable:

? A wider range of frequencies can be sent over the fiber, increasing

the bandwidth available for transmission;

? Signals can be transmitted greater distances without amplification.

The main disadvantage of fiber is that the optical components required

to send and receive data over it are expensive. Because lasers are still

too expensive to deploy to each subscriber, network developers have

adopted an intermediate Fiber to the Neighborhood (FTTN)approach.

Figure 3.3: Fiber to the Neighborhood (FTTN) architecture

Various locations along the existing cable are selected as sites for

neighborhood nodes. One or more fiber-optic cables are then run from

the head end to each neighborhood node. At the head end, the signal is

converted from electrical to optical form and transmitted via laser over

the fiber. At the neighborhood node, the signal is received via laser,

converted back from optical to electronic form, and transmitted to the

subscriber over the neighborhood’s coaxial tree and branch network.

FTTN has proved to be an appealing architecture for telephone

companies as well as cable operators. Not only Continental

Cablevision and Time Warner, but also Pacific Bell and Southern New

England Telephone have announced plans to build FTTN networks.

Fiber to the neighborhood is one stage in a longer-range evolution of

the cable plant. These longer-term changes are not necessary to provide

Internet service today, but they might affect aspects of how Internet

service is provided in the future.

3.2 ISDN Technology

Unlike cable TV networks, which were built to provide only local

redistribution of television programming, telephone networks provide

switched, global connectivity: any telephone subscriber can call any

other telephone subscriber anywhere else in the world. A call placed

from a home travels first to the closest telephone company Central

Office (CO) switch. The CO switch routes the call to the destination

subscriber, who may be served by the same CO switch, another CO

switch in the same local area, or a CO switch reached through a long-

distance network.

Figure 4.1: The telephone network

The portion of the telephone network that connects the subscriber to

the closest CO switch is referred to as the local loop. Since all calls

enter and exit the network via the local loop, the nature of the local

connection directly affects the type of service a user gets from the

global telephone network.

With a separate pair of wires to serve each subscriber, the local

telephone network follows a logical star architecture. Since a Central

Office typically serves thousands of subscribers, it would be unwieldy

to string wires individually to each home. Instead, the wire pairs are

aggregated into groups, the largest of which are feeder cables. At

intervals along the feeder portion of the loop, junction boxes are placed.

In a junction box, wire pairs from feeder cables are spliced to wire pairs

in distribution cables that run into neighborhoods. At each subscriber

location, a drop wire pair (or pairs, if the subscriber has more than one

line) is spliced into the distribution cable.

Since distribution cables are either buried or aerial, they are disruptive

and expensive to change. Consequently, a distribution cable usually

contains as many wire pairs as a neighborhood might ever need, in

advance of actual demand.

Implementation of ISDN is hampered by the irregularity of the local

loop plant. Referring back to Figure 4.3, it is apparent that loops are of

different lengths, depending on the subscriber’s distance from the

Central Office. ISDN cannot be provided over loops with loading coils

or loops longer than 18,000 feet (5.5 km).

4.0 Internet Access

This section will outline the contrasts of access via the cable plant with

respect to access via the local telephon network.

4.1 Internet Access Via Cable

The key question in providing residential Internet access is what kind of

network technology to use to connect the customer to the Internet For

residential Internet delivered over the cable plant, the answer is

broadband LAN technology. This technology allows transmission of

digital data over one or more of the 6 MHz channels of a CATV cable.

Since video and audio signals can also be transmitted over other

channels of the same cable, broadband LAN technology can co-exist

with currently existing services.

Bandwidth

The speed of a cable LAN is described by the bit rate of the modems

used to send data over it. As this technology improves, cable LAN

speeds may change, but at the time of this writing, cable modems range

in speed from 500 Kbps to 10 Mbps, or roughly 17 to 340 times the bit

rate of the familiar 28.8 Kbps telephone modem. This speed represents

the peak rate at which a subscriber can send and receive data, during

the periods of time when the medium is allocated to that subscriber. It

does not imply that every subscriber can transfer data at that rate

simultaneously. The effective average bandwidth seen by each

subscriber depends on how busy the LAN is. Therefore, a cable LAN

will appear to provide a variable bandwidth connection to the Internet

Full-time connections

Cable LAN bandwidth is allocated dynamically to a subscriber only

when he has traffic to send. When he is not transferring traffic, he does

not consume transmission resources. Consequently, he can always be

connected to the Internet Point of Presence without requiring an

expensive dedication of transmission resources.

4.2 Internet Access Via Telephone Company

In contrast to the shared-bus architecture of a cable LAN, the telephone

network requires the residential Internet provider to maintain multiple

connection ports in order to serve multiple customers simultaneously.

Thus, the residential Internet provider faces problems of multiplexing

and concentration of individual subscriber lines very similar to those

faced in telephone Central Offices.

The point-to-point telephone network gives the residential Internet

provider an architecture to work with that is fundamentally different

from the cable plant. Instead of multiplexing the use of LAN

transmission bandwidth as it is needed, subscribers multiplex the use of

dedicated connections to the Internet provider over much longer time

intervals. As with ordinary phone calls, subscribers are allocated fixed

amounts of bandwidth for the duration of the connection. Each

subscriber that succeeds in becoming active (i.e. getting connected to

the residential Internet provider instead of getting a busy signal) is

guaranteed a particular level of bandwidth until hanging up the call.

Bandwidth

Although the predictability of this connection-oriented approach is

appealing, its major disadvantage is the limited level of bandwidth that

can be economically dedicated to each customer. At most, an ISDN

line can deliver 144 Kbps to a subscriber, roughly four times the

bandwidth available with POTS. This rate is both the average and the

peak data rate. A subscriber needing to burst data quickly, for example

to transfer a large file or engage in a video conference, may prefer a

shared-bandwidth architecture, such as a cable LAN, that allows a

higher peak data rate for each individual subscriber. A subscriber who

needs a full-time connection requires a dedicated port on a terminal

server. This is an expensive waste of resources when the subscriber is

connected but not transferring data.