This chapter provides a case
study that gives a practical example of how ITU standards for broadband are improving
access to the Internet, in particular, using digital subscriber line
(DSL) and cable modem technologies.
Although most people have
heard of broadband, few know exactly how they could begin to define it. Many
would probably associate broadband with a particular speed or set of services,
but in reality the term “broadband” is like a moving target. Internet speeds
are increasing all the time, and at each new advance, marketeers eagerly
emphasize just how blazingly fast the latest connection speeds are. It is
revealing to look back at advertisements for 14.4 and 28.8 kbit/s modems, for
example, just to see how every step is considered “blazing” at the time. It is
also important to acknowledge that broadband technologies are always changing,
and that a report like this necessarily reflects just a “slice in time”. One can therefore only
really talk about the “current” state of broadband, and make tentative
extrapolations, based on planned or incipient developments, that may or may not
come to fruition in the future.
The term broadband is
commonly used to describe recent Internet connections that are significantly
faster than earlier dial-up technologies, but it does not refer to a certain
speed or specific service. For instance, what was termed as a “fast” Internet
connection two years ago is now designated as “narrowband”. While the term
broadband is used to describe many different Internet connection speeds,
Recommendation I.113 of the ITU Standardization Sector (ITU-T) defines broadband
as a transmission capacity that is faster than primary rate ISDN, at 1.5 or 2.0
Mbit/s. However, this definition is not strictly followed. The OECD considers
broadband to correspond to transmission speeds equal to or greater than 256
In this chapter, we define
broadband as the last-mile connection to the end-user, although some of these
last-mile technologies may also be used for back-end infrastructure. Indeed, wireless technologies such as Wi-Fi
have been used to form backbone Internet connections in countries such as
Bhutan that do not have a developed wireline infrastructure. It is also
important to bear in mind that broadband speeds are only as fast as the slowest
portion of the network connecting them to the Internet. Therefore, the speeds
referred to here are considered to be maximum speeds that will in fact vary
according to type of available infrastructure and level of network congestion.
all that broadband may enable in terms of applications and services, the
availability of broadband depends primarily on infrastructure. Around
the world there does not appear to be a universally optimal broadband
technology. Rather, different broadband technologies seem suited to different
environments, with relative benefits depending largely on what they are used
for. This is corroborated by the fact that a technology that proves successful
in some countries may not work well in others, due to economic, cultural,
political, geographical, or other factors. Indeed, the medium of choice may
depend upon the legacy medium (where existent), the regulatory framework, and
the supporting institutional arrangements.
Although most of the
marketing literature differentiates broadband according to transmission speed
(bandwidth), there are in fact a number of different characteristics that help
determine the appropriateness of a particular broadband platform for a
particular application. These include latency (very important in on-line
gaming); burstiness (important for file-sharing applications); mobility and the
ability to interwork with other platforms.
While there is no
one-size-fits-all broadband technology, broadband can be said to fall into two
basic categories: wired (fixed-line) and wireless. This chapter discusses two
of the most popular fixed-line broadband technologies, highlighting some of
their benefits and drawbacks.
Broadband has tended to follow the evolutionary
pattern pioneered by traditional phone service; in other words, connections are
initially established over fixed lines and eventually become available over
wireless networks as technology develops. Where broadband is concerned, wired
connections account for the vast majority (over 98 per cent) of current
connections—although wireless technologies are starting to grow quickly.
For fixed-line connections, digital
subscriber line (DSL) technologies are the most popular worldwide, followed
closely by cable. By region, DSL is more common than cable in Asia and Europe
but the opposite is true in the Americas. While DSL and cable modem
technologies have largely been built on top of existing networks, some new
transmission technologies, such as fibre optic cables, have been gaining in
popularity as well (see Figure 7.1). These are explained in greater depth in
the sections that follow.
Source: ITU World Telecommunication Indicators Database.
Building upon the traditional
analogue system that formed the basic telephone network, the integrated
services digital network (ISDN) was the digital switched network technology
that first enabled improved quality and speed, not only for the transmission of
voice, but also of data and images. While ISDN offered a significant upgrade to
traditional copper phone lines, digital subscriber lines (DSL) have advanced
the technology and increased the speeds further still. A key advantage of DSL
technologies is that they use existing copper twisted pair wiring and do not
require new cabling as would say, fibre optics. DSL utilizes different
frequencies to split voice and data services over the same standard phone line.
Previously, phone networks only used a small portion of the available bandwidth
for voice traffic. However, DSL has taken advantage of the unused space on the
copper pair to include data traffic (see Figure 7.2). DSL speeds are
influenced by the distance between the subscriber and the local exchange, the
gauge of the phone wire, and the type of DSL technology.
A main benefit of DSL technologies is that they
offer a dedicated amount of bandwidth that does not vary with the number of
subscribers in an area. This is because each line functions like a complete
circuit to the central office of the operator. Cable and wireless technologies
can suffer from congestion when more and more users start using the allotted
bandwidth for an area. This makes DSL technologies ideal for home and business
use that need a certain amount of bandwidth available at any given time.
In its various forms, DSL is the most popular
broadband technology in the world. In 2002, 64 per cent of broadband countries
had more DSL lines than cable connections. The Republic of Korea led the world
in DSL penetration in 2002 with 13.4 subscribers per 100 inhabitants followed
by Iceland (8.4), Hong Kong, China (8.3), Taiwan, China (8.1), and Japan (5.5).
Importantly, the rate of DSL deployment often
hinges on whether the incumbent operator is willing or not to open the local
loop to competitors. This process is known as local loop unbundling (LLU).
Source: ITU, ITU World
Telecommunication Indicators Database.
DSL comes in several different
“flavours” that offer different benefits. Some are more suited for residential
use and others for business. Asymmetric DSL (ADSL), for instance, is a form of
DSL where more bandwidth is allocated to download than to upload. This makes it
ideal for web browsing and typical Internet usage, where downloading of large
files is more important than uploading, because it enables maximum speeds of
8-10 Mbit/s downstream and a maximum of 1 Mbit/s upstream. ADSL is available at
a maximum distance of 3 km from the local exchange. It is well suited to
residential use because it shares a single twisted copper pair with voice,
allowing users to use the telephone and surf the Internet simultaneously on the
same line. ADSL is the most popular form of DSL offered around the world and
the transceivers falling in this category are described in the ITU-T
Recommendations of the G.99x-series.
Originally, ADSL installations required a physical
splitter to separate out voice and data traffic. These installations had to be
performed by technicians and significantly increased the cost of installing an
ADSL line. However, G.lite allows for a splitter-free connection that simply
requires the modem to be plugged in, thus drastically reducing the expense and
difficulty of rolling out ADSL service. In
addition, this ITU Recommendation extends the reach of ADSL by sacrificing
speed; G.lite can reach 5.4 km but its maximum download speed is limited to 1.5
Mbit/s whole upload speeds are limited to 512 kbit/s. G.lite has been used to
connect areas that were previously inaccessible via standard ADSL. It has also
facilitated so-called “plug and play” installations, by users themselves.
Single pair high-speed
DSL (SHDSL) is defined in a recent ITU Recommendation
(ITU-T G.991.2) for
symmetric, high-speed DSL. SHDSL connections are best suited for servers (web,
FTP, file) and other business uses such as video conferencing that require high
speeds in both directions. SHDSL uses a copper pair to send and receive data
through two bands, which allow for speeds up to 2.3 Mbit/s in both directions.
By including a second copper pair, SHDSL speeds can reach 4.6 Mbit/s in each
direction. These speeds are possible over a 3 km range with data rates
attenuating for longer distances. The two SHDSL bands send data over the low
frequencies to extend the reach of the loop, making it impossible for SHDSL to
carry a voice channel (POTS or ISDN) like ADSL. This lack of voice capability imposes
significant installation costs in the local loop—a cost that is passed on to
the consumer through higher subscription costs. Therefore, SHDSL is more
suitable as a replacement for traditional leased lines for business (businesses
can usually absorb higher subscription rates than private users) rather than
for the consumer market.
In some parts of Europe, SHDSL is referred to
simply as SDSL, not to be confused with a different standard of the same name
used in North America, which is described below.
SDSL is a proprietary
standard mainly used in North America, but which even there is losing
popularity to SHDSL. SDSL offers equivalent traffic flow in each direction but,
like SHDSL, it cannot share the line with analogue signals, thus posing
significant installation/modification costs in the local loop. SDSL is best
suited to sites that require significant upload speeds such as web/FTP servers
and business applications. The capacity of SDSL is adjusted according to signal
quality and speeds and distance combinations ranging from 160 kbit/s over 7 km
to 1.5 Mbit/s over 3 km are offered. Higher speeds are possible by combining
multiple twisted pair wires.
and ITU-T G.992.4 (ADSL2plus)
are the sequels to the original ITU-T ADSL Recommendation, which is to date the
most successful broadband technology in the world, and enables improved speed,
reach, power consumption and other technical elements over the original
version. When developing the two new Recommendations, ITU-T was able to
incorporate feedback from service providers and end-users. ADSL2 can deliver 8‑12 Mbit/s
while extending the reach of the original ADSL by 300 metres.
The main improvements over the original ADSL standard
can be summarized as follows. The speed and reach increases in ADSL2 are
largely owed to improved performance on long lines in the presence of
improvements include addressing several technical issues that appeared with
original ADSL standards. The new Recommendations allow for the use of filters,
rather than splitters, at both ends of the connection. This offers cost
savings, obviating the need for a technician to install splitters at the home
or office. ADSL2 also introduces power management features into ADSL modems,
allowing for more cost-effective operation on both sides of the connection.
Original ADSL equipment remains in an always-on state, using the same amount of
energy regardless of whether data is being transferred or not. ADSL2 introduces
three power states for the equipment, each corresponding to need. The highest power levels are used during
large downloads as they allow for more bandwidth. Lower power levels are used during periods of inactivity,
allowing for energy savings both at the central office and at the customer
premises, all while maintaining an always-on connection.
The new ADSL2 Recommendation also realigns the
voice channels and offers providers the ability to combine multiple ADSL2 lines
for faster bandwidth to certain customers. In addition, ADSL2 systems can enter
an “all-digital” mode where voice channels are reassigned to data, similar to
SHDSL. This is especially important for business lines that may not need voice
services over the ADSL2 line.
ITU-T Recommendation G.992.5 (ADSL2plus or ADSL2+)
builds further on ADSL2, increasing the bandwidth by extending the usable
frequencies on the line. While both technologies use the same frequencies for
telephone calls, and uploading data, the download channel is extended from a
maximum of 1.1 MHz with ADSL2 to 2.2 MHz with ADSL2plus. This increases
download bandwidth from 8 Mbit/s with ADSL2 to 16 Mbit/s with ADSL2plus. These
speeds are possible over 1.5 km and higher speeds may even be possible.
VDSL (ITU-T G.993.1) is the latest form of DSL and offers the
fastest DSL speeds over short distances to date, at 52 Mbit/s of bandwidth over
a standard twisted pair cable. While the speed is much higher than other DSL
technologies, it comes at a price: decreased reach of the network. This makes
VDSL the optimal choice for branching out short distances from fibre
connections, rather than, for example, providing longer-range broadband to
VDSL was originally named VADSL but the “A” (for
asymmetric) was dropped because VDSL can support both symmetric and asymmetric
transport. These connections can be very fast because the physical distances
are kept very short, allowing for maximum throughput. As fibre optic networks
continue to move closer to communities around the globe, VDSL will become
increasingly important as a way to bridge the last-kilometre gap. The Republic of Korea has made extensive use
of VDSL technologies in apartment buildings around the country as a way to
share fibre connections arriving in the basement of apartments. Fibre connections
are difficult to install in places that require twists and turns, such as
apartment buildings. By using VDSL, which involves a twisted copper pair rather
than fibre, the short distances to each apartment can be covered.
While DSL is the most prevalent broadband technology
in the world, cable modem technology is not far behind (see Figure 7.1). Cable
broadband access may not be as prevalent in the world as DSL, but it dominates
in some markets with a fully developed cable television network. In 2002, the
Republic of Korea led the world in cable broadband connections with 7.7
subscribers per 100 inhabitants, while Canada (5.2), the Netherlands (4.9),
Switzerland (3.6) and Belgium (3.5) round out the top five economies.
Cable networks were originally designed for one-way
video transmission. Cable companies provided video that was sent, or broadcast,
over lines to subscribers’ homes. However, as the networks have evolved, new
equipment has made it possible to send data in both directions on a cable
network, (i.e. downloading and uploading from a household), thus making
Internet access over cable a viable solution. The physical cable network sends
different “channels” on separate blocks of 6 MHz frequencies along the
same cable. Originally, these channels each carried different television
channels until a method was developed to reserve unused channels and dedicate
them to Internet traffic. One channel sends data from the Internet to users
(6 MHz of frequency corresponds to roughly 30 Mbit/s) and another channel
is used to send data back on the Internet from households (see Figure 7.3).
These reserved channels are “broadcast” around the
network to all subscribers in a certain area. Each cable modem can recognize
which parts of the broadcast are destined for it and pull them off the network.
Cable modems are then able to send information back to the Internet by waiting
for their “turn to talk” on the response channel and essentially broadcasting
their request in quick bursts back to the central office of the cable company.
All cable subscribers in a small area share the
same channels to send and receive data, and the amount of bandwidth users
receive is directly tied to how much bandwidth their neighbours are using. If
no other users are using a cable node at a given time, cable subscribers may
theoretically have disposal of all of the combined bandwidth allotted to their
own and their neighbours’ homes. Conversely though, during heavy usage, cable
modem subscribers can see significant reductions in their bandwidth. Typically,
1.5 Mbit/s download speeds or higher can be expected over cable modems during
normal usage times. In order to protect against abuse, many cable companies
have restricted the upload bandwidth to 128 kbit/s in order to stop
high-bandwidth users who use more than their share from running server or
peer-to-peer applications on their home computers. Cable companies have also
found another way to increase the bandwidth of users in an area by simply
dedicating additional channels to data and dividing the number of users on a
Source: ITU-T Recommendation J.122, ITU-T Study Group 9.
Cable modem technologies are being
standardized in ITU-T
Study Group 9 based on
technologies originally developed by Cablelabs called DOCSIS (Data over Cable Service
Interface Specification). The
first generation of cable modems were built on ITU-T
Recommendation J.112, however
a new ITU-T
Recommendation J.122 was
approved at the end of 2002 and offers improvements to the existing standard
while maintaining backwards compatibility. The new standard improves the way
the cable modem broadcasts data back to the central office, allowing for more
economical use of existing bandwidth.
is another project, named IPCablecom, which
focuses on the delivery of real time services over cable television networks as
In their conversion to digital television,
cable television systems in many countries are provisioning very high-speed
bi-directional data facilities to support, among other payloads, those
utilizing the Internet Protocol (IP). These operators want to expand the
capability of this delivery platform to include bi-directional voice
communication and other time-critical services.
IPCablecom is a
project organized in ITU-T Study Group 9
(SG9) for the purpose of making progress towards the development of a
coordinated set of ITU-T Recommendations that will specify an architecture and
a set of integrated protocol interfaces that operate as a system to enable the
delivery of time-critical interactive services over cable television networks
using the Internet Protocol (IP).
The new set of Recommendations will address a
series of SG9
currently under study including Questions 6/9
(Conditional access methods and practices for digital cable distribution to the
home), 10/9 (Functional characteristics for the interconnection of cable
networks with the public switched network and other delivery systems), 12/9 (Cable Television delivery of advanced multimedia digital
services and applications that use Internet Protocols (IP) and/or packet-based
data) and 13/9 (Voice and Video IP
Applications over cable television networks).
The initial focus of work within IPCablecom
has been targeted at providing an integrated system for cable that can support
a wide variety of time-critical interactive services within a single zone. In
the IPCablecom architecture, a zone is defined as the set of devices (client,
gateway, and other) that are under the control of a single supervisory
function, which is referred to as a Call Management Server (CMS). Future work
will consider the study of issues associated with communication between zones
as well as issues associated with the support of intelligent client devices.
In order to meet the market requirements of
cable operators, the IPCablecom specifications are being developed on an
aggressive time schedule. The established and currently planned Recommendations
for development are listed below.
According to the ITU-T Recommendation J.160,
the IPCablecom architecture at a very high level has to connect with three
networks: HFC access network, managed IP network and PSTN. The system
architecture should describe the specifications of the functional components
and define the interfaces between these networks and IP-based cable television
networks. The reference architecture for IPCablecom is shown in Figure 7.4.
The Cable Modem HFC access network provides
high-speed, reliable, and secure transport between the customer premise and the
cable headend. This access network may provide all Cable Modem capabilities
including Quality of Service. The Cable Modem HFC access network includes the
following functional components: the Cable Modem (CM), Multimedia Terminal
Adapter (MTA), and the Cable Modem Termination System (CMTS).
Managed IP network serves several functions. First, it provides interconnection
between the basic IPCablecom functional components responsible for signalling,
media, provisioning, and quality of service establishment. In addition, the
managed IP network provides long-haul IP connectivity between other Managed IP
and Cable Modem HFC networks. The Managed
IP network includes the following functional components: Call Management Server
(CMS), Announcement Server (ANS), several Operational Support System (OSS)
back-office servers, Signalling Gateway (SG), Media Gateway (MG), and Media
Gateway Controller (MGC).
Both the Signalling Gateway (SG) and the
Media Gateway (MG) provide connectivity between the managed IP network and
An IPCablecom zone consists of the set of
MTAs in one or more Cable Modem HFC access networks that are managed by a
single functional CMS as shown in Figure 7.5. Interfaces between functional
components within a single zone are defined in the IPCablecom specifications.
Interfaces between zones (e.g., CMS-CMS) have not been defined and will be
addressed in future phases of the IPCablecom architecture.
An IPCablecom domain is made up of one or
more IPCablecom zones that are operated and managed by a single administrative
entity. An IPCablecom domain may also be referred to as an administrative
domain. Interfaces between domains have not been defined in IPCablecom and are
for further study.
Table 7.1 below lists the set of IPCablecom
Recommendations established or that are currently planned for development along
with the current status of each document.
Approved February 02
Defines architecture framework for IPCablecom
networks including all major system components and network interfaces
necessary for delivery of IPCablecom services.
Defines the audio and video codecs necessary to
provide the highest quality and the most resource-efficient service delivery
to the customer. Also specifies the performance required in client devices
to support future IPCableCom codecs. and describes suggested methodology for
optimal network support for codecs.
Network-Based Call Signalling
Approved February 02
Defines a profile of the Media Gateway Control
Protocol (MGCP) for IPCablecom embedded clients, referred to as the
Network-based Call Signalling (NCS) protocol. MGCP is a call signalling
protocol for use in a centralized call control architecture, and assumes
relatively simple client devices.
Defines the QoS Architecture for the “Access”
portion of the PacketCable network, provided to requesting applications on a
per-flow basis. The access portion of the network is defined to be between
the Multimedia Terminal Adapter (MTA) and the Cable Modem Termination System
(CMTS). The method of QoS allocation over the backbone is unspecified in
Defines the concept of Event Messages used to
collect usage for the purposes of billing within the IPCablecom
Internet Signalling Transport Protocol (ISTP)
Approved February 02
Amended May 03
Defines the Internet Signalling Transport Protocol
(ISTP) for IPCablecom PSTN Signalling Gateways. ISTP is a protocol that
provides a signalling interconnection service between the IPCablecom network
control elements (Call Management Server and Media Gateway Controller) and
the PSTN C7 Signalling network through the C7 Signalling Gateway.
Describes the framework in which IPCablecom MIBs
(Management Information Base) are defined. It provides information on the
management requirements of IPCablecom specified devices and functions, and
how these requirements are supported in the MIB. It is intended to support
and complement the actual MIB documents, which are issued separately.
MTA Device Provisioning
Defines the MIB module which supplies the basic
management objects for the MTA Device.
Defines the MIB module which supplies the basic
management object for the NCS protocol.
Approved February 02
Defines the Security architecture, protocols,
algorithms, associated functional requirements and any technological
requirements that can provide for the security of the system for the
PSTN Gateway Call Signalling
Approved February 02
Amended May 03
Defines a Trunking Gateway Control Protocol (TGCP)
for use in a centralized call control architecture that assumes relatively
simple endpoint devices. TGCP is designed to meet the protocol requirements
for the Media Gateway Controller to Media Gateway interface defined in the
Management Event Mechanism
Approved February 02
Defines the management Event Mechanism that
IPCablecom elements can use to report asynchronous events that indicate
malfunction situations and notifcation about improtnt non-fault situation.
Embedded - MTA Device Specification
Approved February 02
Specifies minimum device requirements for embedded
multimedia terminal adapters in the areas of physical interfaces, power
requirements, processing capabilities and protocol support.
Interdomain Quality of Service
Approved February 02
Defines an architectural model for end-to-end
Quality of Service for IPCablecom Inter-and Intra-Domain environments.
Approved July 02
Defines the architecture and protocols that are
required for playing announcements in Voice-over-IP (VoIP) IPCablecom
IPCablecom MIB for Management Event Mechanism
Defines the MIB (Management Information Base) for
Management Event Mechanism that IPCablecom elements can use to report to
management systems and/or local logs asynchronous events.
IPCablecom CMS subscriber provisioning
planned for June 03
Defines the interface used between Call Management
Server (CMS) and Provisioning Server for the exchange of service
IPCablecom CMS to CMS signalling
Approved May 03
Describes the CMS to CMS Signalling protocol
intended for use by a CMS to communicate with another CMS in order to
support packet-based voice and other real-time multimedia applications.
Telecommunications for disaster relief
Approval planned for May 04
Defines Telecommunications for disaster relief over
IPCablecom networks and its interworking across different networks.
popularity of the Internet and other IP-based networks has increased requirements
for telecommunications capacity and bandwidth which has driven much innovation
in telecommunication access and transport networks. Some examples include
leveraging copper wire “last-mile” networks through DSL technologies,
re-architecturing of cable networks to support IP services and advances in
optical networking technologies. This chapter has reviewed some of the ITU-T
standards that are being developed and deployed to improve access to the
Internet and other IP-based networks.