Technical Foundationsof Residential Broadband
Residential Broadband, Second Edition
Author: George Abe
Publisher: Cisco Press (53)
This chapter covers the following topics:
DAVIC Reference Architecture
Noise Mitigation Techniquesd
Fiber Optic Transmission
The Limits of Audio/Visual Perception
MPEG-2 Compression and Systems
MPEG-4 Scene Description
This section develops a generic RBB networkthat is, a reference
model that incorporates the key features of RBB networks. A reference model is useful in
establishing vocabulary and pointing out parallel features among various technical
approaches. In addition, a reference model can help identify points of congestion
in end-to-end service delivery.
Our purpose is to identify the functions of three logical elements of an
end-to-end RBB network: 1. Core Network, 2. the Access Network, and 3. the
Home Network. The Core Network is covered in this chapter. The Access Network
is covered in Chapter 3, “Cable TV Networks”;
Chapter 4, “xDSL Access Networks”; Chapter
5, “FTTx Access Networks”; and Chapter
6 “Wireless Access Networks.” The Home Network is dealt
with in Chapter 7, “Home Networks.”
Useful reference models for RBB networks have been developed by the Digital
Audio Visual Council (DAVIC; www.davic.org)
and ATM Forum Residential Broadband Working
Group (ATMF RBB, www.atmforum.com).
The DAVIC Reference model was originally written in DAVIC 1.0 Specification
Part 02, System Reference Models and Scenarios, September 1995. The ATM
Forum Reference model is presented in the organization's document (BTD-RBB-01.01,
“Residential Broadband Baseline Document Draft,” February 1997).
Figure 2-1 depicts the end-to-end
schematic for the reference model. The Core Network, Access Network and
Home Network domains are segmented in this way to emphasize boundaries of
The Core Network provides high-performance bit pumps
to move bits between the content provider and the Access Network. It is the
repository for network management and application servers, such as Web servers
and video servers. Speed and manageability are critical in the Core Network.
The principle functions of the Core Network are listed here:
Switching, routing, and transmission
Handling concentration of traffic to the content provider
Handling service registration for multiple content providers
Multiplexing and switching for multiple Access Networks
Providing navigation aids and directory services
Enforcing Quality of Service (QoS)
Performing load balancing
Caching servers on behalf of the content providers.
The Access Network may also perform some of these functions. For example,
it is logical to place caching servers in the Access Network as well as the
The protocol options to consider in the Core Network are IP or MPEG
over Asynchronous Transfer Mode (ATM), packet mode over Sonet (POS), and IP
over Dense Wavelength Division Multiplexing (DWDM).
Figure 2-2 illustrates protocol
stacking for the ATM approach,
which has the (possible) current advantage of being conventional wisdom.
In this approach to Core Network design, IP and MPEG packets from the application
are segmented into ATM cells at the edge of the Core. ATM virtual circuits (VC) guide the packets through the Core to the
intended destinations. Key issues of ATM point mainly to signaling overhead
and the complexity of mapping IP addresses to ATM addresses. Call setup rates
for ATM are still in the low hundreds per second, which puts ATM at a disadvantage
with respect to pure IP networks. However, ATM does provide a partitioning
capability so that multiple users share the same Core (unbeknownst to each
other) with a high
degree of QoS.
This figure also points out that ATM is layered on top of SONET/SDH
networks, which in turn are layered on top of a fiber optic DWDM network.
POS advocates point out that the QoS and partitioning features of ATM
can be replicated at the packet layer; therefore, there is no need for ATM
However, some go further to say that SONET/SDH can be bypassed
as well. SONET/SDH provides timing and error protection. But timing would
be relatively less important for applications that can maintain strict timing
control at the packet layer and that provide for a rerouting capability. Therefore,
there is some discussion of an IP over DWDM architecture, which eliminates
the cost of the ATM switches and the SONET/SDH add/drop multiplexers (ADMs)
but potentially at some critical functionality loss.
These approaches are shown in Figure
If sophisticated QoS schemes are implementable in packet mode (pure
IP networks) cost-effectively, then the advocates of ATM and SONET/SDH will have
some difficult questions to answer. Time will tell.
The Access Network is the part of the carrier network
that touches the customer's premises. The Access Network is also referred
to as the local
line, or last
The principal functions of an Access Network are listed here:
Transmitting, switching, routing, and multiplexing traffic
from the consumer to the Core Network
Classifying traffic from consumers by QoS; that is, the Access
Network differentiates best-effort traffic from traffic with guaranteed bandwidth
Providing navigation aids and directory services
Caching servers on behalf of the content providers
Performing tunneling or packet encapsulation
Enforcing the MAC protocol
Enforcing packet filtering
Providing software updates to the Home Network
Providing measurements used for invoicing
These functions are performed cooperatively with the Core Network and
the Home Network, which means that there must be clear interfaces among these
At Issue: Network Intelligence
An important system design question is how network intelligence is to be optimally distributed
between the Core Network and the Access Network. Intelligence
here refers to processing of user signaling (for example, call setup requests).
[Quayle] argues that the Access Network should be relatively dumb and the
Core Network relatively intelligent. Also, large databases for invoicing and
resource management should be maintained in the Core Network. Because there
are relatively few Core Network nodes and a large number of Access Network
nodes, total system costs are optimized by concentrating the expensive (intelligent)
pieces in the Core Network.
Others argue that because the Core Network must move bits as fast as
possible, it must be simple, even dumb. Therefore, intelligence is pushed
to the periphery, even to the customer premises. The case for dumb
networks (overprovisioned, underengineered networks) and smart peripheral
equipment (and people) is argued most publicly by David
Isenberg). ATM switches, for example, are relatively simple,
with most of the hard software functions performed outside the switching
fabric. However, because some Access Networks require a Media Access
Control protocol, there is a requirement to interpret user signaling
in the Access Network. Hence, for these networks, there is some requirement
for some intelligence in the access. Whether or not both Core and Access
Networks will be dumb remains to be seen.
Finally, on a commercial matter, it may be the case that the Access
Network and the Core Network will be different companies. Each will assert
that intelligence should be put into its network, which boosts added value.
Many types of Access Networks are discussed widely in the popular and
trade press. Table 2-1 shows a list of the
Access Networks in operation or in contemplation, with associated standards
for telephony, video, and data. The acronyms will be discussed in due course;
we only attempt to indicate here which Access Networks could support which broad
class of service.
Table 2-1. Access Networks and Standards
Analog TV (NTSC, PAL, SECAM), digital TV using ATSC or DVB
OpenCable SCTE DVS
Data Over Cable Service Interface Specification (DOCSIS), DAVIC/DVB
Switched Digital Video (TBD)
Voice over ATM (VoATM) or Voice over IP (VoIP)
Switched Digital Video (TBD)
packet mode protocols such as the Point-to-Point Protocol Over Ethernet (PPPoE)
Switched Digital Video (TBD)
and other initiatives
Distribution Service (LMDS)
Multichannel Multipoint Distribution Service (MMDS)
Analog TV, digital TV using
Altitude Long Operation (HALO) aircraft
Powerline, use of electric power lines for data transmission
Though multiple types of Access Networks exist, there are three architectural
elements, which they share. These are the access node, the Optical Network
Unit, and the Network Termination (NT in Figure
The major aforementioned Access Networks are covered in subsequent chapters. HALOs, blimps, and powerline
are viewed as too experimental as of this writing. However, powerline transmission
deserves some mention because of its potential.
Powerline is the use of electric utility wires to transmit data. Because
computers and networking equipment need electricity, it makes sense to
consider using the electrical outlet for a data connection as well. The
issues with using powerlines have revolved around electrical emissions,
noise on the lines and cost. Also, it is not possible to pass data through
a transformer. However, changes in transformer design are giving rise
to the possibility of this new Access Network. Principal developers include
Northern Telecom), United Utilities
of Great Britain, and Sydkraft of Sweden (in conjunction with the Swedish
Internet Service provider, Tele2). Northern Telecom and United Utilities
maintain a powerline Web site at
A typical U.S. power grid supports only four to eight homes per transformer,
so powerline is viewed as expensive. But in Europe, each transformer supports
100 to 200 homes. A networking device at the transformer site is amortized
over a larger number of homes, thereby making powerline more cost-effective.
If noise and emission problems can be solved, this could represent an interesting
The access node (AN) serves some
or all of the following functions:
Modulating forward data onto the Access Network
Demodulating return path data
Enforcing the Media Access Control (MAC) protocol to arbitrate
access from multiple users onto the Access Network
Multiplexing traffic from the Access Network to the Core Network
Separating or classifying traffic prior to multiplexing onto
the Core Network, such as differentiating traffic that is subject to QoS guarantees
from traffic that receives best-effort support
Performing passive operations, such as splitting and filtering
Enforcement of the MAC protocol is one of the more complicated functions
of the AN. For some Access Networks, when multiple consumers vie for bandwidth
connections simultaneously, arbitration rules are required to determine who
gets to use the network at that moment. Without some controls, the Access
and Core Networks would be obliged to provide an indefinite amount of bandwidth
and connections. The MAC protocol arbitrates access to the Access Network.
It is enforced by the AN, which engages in a protocol exchange, or peers,
with the NIU on the customer premises. Important MAC protocols are token-passing
schemes, Slotted Aloha protocols, and Collision Sense Multiple Access protocols.
Examples of ANs are Cable Modem Terminal Server (CMTS) in the Multimedia
Cable Network System (MCNS) architecture for cable data modems (discussed
in Chapter 3), Digital Subscriber Line Access
Multiplexer (DSLAM) for concentration (discussed in Chapter
4), and the ATM Digital Terminal (ADT), referenced in the ATM Forum
RBB Baseline Document.
The function of the Optical Network Unit (ONU) is to terminate fiber and convert the
optical signal on fiber to electrical signals on wired networks. A major difference
among Access Networks is the proximity of the ONU to the consumer. The closer
the ONU is to the consumer, the higher the speed and usually the greater the
service reliability. Carriers and consumers alike would like to have fiber
as close as possible to the homeperhaps even inside the homeas
service called Fiber to the Home (FTTH).
Termination (NT) is a carrier-provided or customer-provided piece of equipment
that is located on the side of the home. Some of its functions are listed
Coupling home wiring and carrier wiring
Handling RF filtering
Performing media conversion
Handling security and interdiction
The NT is the legal and commercial demarcation point between the Access
Network and the consumer. This means that if something goes wrong on the network
side of the NT, the carrier is obliged to fix it. The NT is situated on the
customer premises, but many countries include it as a part of the Access Network.
In the United States, the NT is considered to be customer premises equipment
because the customer purchases it. In Europe, it is considered carrier property.
For purposes of this reference model, the NT is part of the carrier network
because it usually is under carrier control, even though the customer purchases
it. The fact that it can be purchased from a number of suppliers other than
the Access carrier is not relevant architecturally.
NT functions differ widely among Access Networks, but the main functions
are usually passive (coupling, splitting, grounding, RF filtering, and so
on). In some cases, the NT can have an active function, such as signaling.
For example, service activation might be enabled by signaling between the
carrier and the NT. This becomes an important concept for Fiber to the Home
(FTTH), which is discussed in Chapter 6.
Chapter 7 contains more discussion of various
Home Network components.
Proponents of specific Access Networks claim
that one Access Network would fit all applications. While it is technically
feasible that a single network could service all market drivers, there are
reasons why multiple Access Networks exist:
Embedded base Telephone companies and cable operators
have put billions of dollars into the ground. This represents investment
that cannot be easily replaced. The sensible thing is to leverage
Different applications Broadcast television lends
itself economically to shared media, such as air and cable. Retrofitting
broadcast media to accommodate point-to-point applications involves
software tricks. Similarly, retrofitting broadcast applications
over point-to-point media also involves software tricks.
Different population densities Some technologies are
more economically viable in urban areas than rural areas, and vice
Different geography Vast expanses of oceans and rivers
create a cost penalty for wired networks. On the other hand, obstacles
such as hills, buildings, vegetation, and heavy rainfall reduce
transmission quality for wireless networks.
Different business conditions In developing countries,
which are characterized by a lack of mature wired infrastructure,
broadcast technologies can create a fast infrastructure. Regulations
such as those imposing limits on market dominance of a single carrier
can foster or inhibit some technical options.
Entrepreneurship The availability of capital underwrites
a lot of development of various technologies.
Because of these variations, subsequent chapters consider a variety
of Access Networks to understand the strengths and weaknesses of each.
The Home Network consists of the following aspects:
The Network Interface Unit (NIU), essentially a modem
The Residential Gateway (RG), which adds network functionality
The Set Top Unit (STU), which performs application-specific
functions, such as decoding digital TV
The Terminal Equipment (TE), a television, personal computer,
or other device
Consumer premises distribution, either wired or wireless
In the home, the personal computer and consumer electronics industries
will wage a spirited battle for the consumer dollar. PCs, TV set-top boxes
(STB), and game stations will blur as each acquires some of the functions
of the others.
One of the primary architects of OpenCable, Michael
Adams, explains the key concepts of this initiative in his book
Broadband, Second Edition
by George Abe
Introduces the topics surrounding high-speed networks
to the home. It is written for anyone seeking a broad-based familiarity
with the issues of residential broadband (RBB) including product
developers, engineers, network designers, business people, professionals
in legal and regulatory positions, and industry analysts.