Channel Separation
Early versions of VDSL will use frequency division multiplexing to separate
downstream from upstream channels and both of them from basic telephone
service and ISDN (shown in Figure 15-6). Echo cancellation may be required for
later-generation systems featuring symmetric data rates. A rather substantial
distance, in frequency, will be maintained between the lowest data channel and
basic telephone service to enable very simple and cost-effective basic
telephone service splitters. Normal practice would locate the downstream
channel above the upstream channel. However, the DAVIC specification reverses
this order to enable premises distribution of VDSL signals over coaxial cable
systems.
Figure 15-6: Early versions of VDSL will use FDM to separate downstream from
upstream channels and both of them from basic telephone service and ISDN, as
this example shows.
Forward Error Control
FEC will no doubt use a form of Reed Soloman coding and optional interleaving
to correct bursts of errors caused by impulse noise. The structure will be
very similar to ADSL, as defined in T1.413. An outstanding question is whether
FEC overhead (in the range of 8%) will be taken from the payload capacity or
added as an out-of-band signal. The former reduces payload capacity but
maintains nominal reach, whereas the latter retains the nominal payload but
suffers a small reduction in reach. ADSL puts FEC overhead out of band.
Upstream Multiplexing
If the premises VDSL unit comprises the network termination (an active NT),
then the means of multiplexing upstream cells or data channels from more than
one CPE into a single upstream becomes the responsibility of the premises
network. The VDSL unit simply presents raw data streams in both directions. As
illustrated in Figure 15-7, one type of premises network involves a star
connecting each CPE to a switching or multiplexing hub; such a hub could be
integral to the premises VDSL unit.
In a passive NT configuration, each CPE has an associated VDSL unit. (A
passive NT does not conceptually preclude multiple CPE per VDSL, but then the
question of active versus passive NT becomes a matter of ownership, not a
matter of wiring topology and multiplexing strategies.) Now the upstream
channels for each CPE must share a common wire. Although a collision-detection
system could be used, the desire for guaranteed bandwidth indicates one of two
solutions. The first invokes a cell-grant protocol in which downstream frames
generated at the ONU or farther up the network contain a few bits that grant
access to specific CPE during a specified period subsequent to receiving a
frame. A granted CPE can send one upstream cell during this period. The
transmitter in the CPE must turn on, send a preamble to condition the ONU
receiver, send the cell, and then turn itself off. The protocol must insert
enough silence to let line ringing clear. One construction of this protocol
uses 77 octet intervals to transmit a single 53-octet cell.
Figure 15-7: This figure shows examples of termination methods in passive and
active networks.
The second method divides the upstream channel into frequency bands and
assigns one band to each CPE. This method has the advantage of avoiding any
MAC with its associated overhead (although a multiplexor must be built into
the ONU), but either restricts the data rate available to any one CPE or
imposes a dynamic inverse multiplexing scheme that lets one CPE send more than
its share for a period. The latter would look a great deal like a MAC
protocol, but without the loss of bandwidth associated with carrier detect and
clear for each cell.
VDSL Issues
VDSL is still in the definition stage; some preliminary products exist, but
not enough is known yet about telephone line characteristics, radio frequency
interface emissions and susceptibility, upstream multiplexing protocols, and
information requirements to frame a set of definitive, standardizable
properties. One large unknown is the maximum distance that VDSL can reliably
realize for a given data rate. This is unknown because real line
characteristics at the frequencies required for VDSL are speculative, and
items such as short bridged taps or unterminated extension lines in homes,
which have no effect on telephony, ISDN, or ADSL, may have very detrimental
affects on VDSL in certain configurations. Furthermore, VDSL invades the
frequency ranges of amateur radio, and every above-ground telephone wire is an
antenna that both radiates and attracts energy in amateur radio bands.
Balancing low signal levels to prevent emissions that interfere with amateur
radio with higher signals needed to combat interference by amateur radio could
be the dominant factor in determining line reach.
A second dimension of VDSL that is far from clear is the services environment.
It can be assumed that VDSL will carry information in ATM cell format for
video and asymmetric data communications, although optimum downstream and
upstream data rates have not been ascertained. What is more difficult to
assess is the need for VDSL to carry information in non-ATM formats (such as
conventional Plesiochronous Digital Hierarchy [PDH] structures) and the need
for symmetric channels at broadband rates (above T1/E1). VDSL will not be
completely independent of upper-layer protocols, particularly in the upstream
direction, where multiplexing data from more than one CPE may require
knowledge of link-layer formats (that is, ATM or not).
A third difficult subject is premises distribution and the interface between
the telephone network and CPE. Cost considerations favor a passive network
interface with premises VDSL installed in CPE and upstream multiplexing
handled similarly to LAN buses. System management, reliability, regulatory
constraints, and migration favor an active network termination, just like ADSL
and ISDN, that can operate like a hub, with point-to-point or shared-media
distribution to multiple CPE on-premises wiring that is independent and
physically isolated from network wiring.
However, costs cannot be ignored. Small ONUs must spread common equipment
costs, such as fiber links, interfaces, and equipment cabinets, over a small
number of subscribers compared to HFC. VDSL therefore has a much lower cost
target than ADSL because VDSL may connect directly from a wiring center or
cable modems, which also have much lower common equipment costs per user.
Furthermore, VDSL for passive NTs may (only may) be more expensive than VDSL
for active NTs, but the elimination of any other premises network electronics
may make it the most cost-effective solution, and highly desired, despite the
obvious benefits of an active NT.
Table of Contents
Digital Subscriber Line
Background
Asymmetric Digital Subscriber Line (ADSL)
ADSL Capabilities
ADSL Technology
ADSL Standards and Associations
ADSL Market Status
Very-High-Data-Rate Digital Subscriber Line (VDSL)
VDSL Projected Capabilities
VDSL Technology
Line Code Candidates
Channel Separation
Forward Error Control
Upstream Multiplexing
VDSL Issues
Standards Status
VDSL's Relationship with ADSL
Digital Subscriber Line
Background
Digital Subscriber Line (DSL) technology is a modem technology that uses
existing twisted-pair telephone lines to transport high-bandwidth data, such
as multimedia and video, to service subscribers. The term xDSL covers a number
of similar yet competing forms of DSL, including ADSL, SDSL, HDSL, RADSL, and
VDSL. xDSL is drawing significant attention from implementers and service
providers because it promises to deliver high-bandwidth data rates to
dispersed locations with relatively small changes to the existing telco
infrastructure. xDSL services are dedicated, point-to-point, public network
access over twisted-pair copper wire on the local loop ("last mile") between a
network service provider (NSP's) central office and the customer site, or on
local loops created either intra-building or intra-campus. Currently the
primary focus in xDSL is the development and deployment of ADSL and VDSL
technologies and architectures. This chapter covers the characteristics and
operations of ADSL and VDSL.
Asymmetric Digital Subscriber Line (ADSL)
ADSL technology is asymmetric. It allows more bandwidth downstream---from an
NSP's central office to the customer site---than upstream from the subscriber
to the central office. This asymmetry, combined with always-on access (which
eliminates call setup), makes ADSL ideal for Internet/intranet surfing,
video-on-demand, and remote LAN access. Users of these applications typically
download much more information than they send.
ADSL transmits more than 6 Mbps to a subscriber, and as much as 640 kbps more
in both directions (shown in Figure 15-1). Such rates expand existing access
capacity by a factor of 50 or more without new cabling. ADSL can literally
transform the existing public information network from one limited to voice,
text, and low-resolution graphics to a powerful, ubiquitous system capable of
bringing multimedia, including full motion video, to every home this century.
Figure 15-1: The components of a ADSL network include a telco and a CPE.
ADSL will play a crucial role over the next decade or more as telephone
companies enter new markets for delivering information in video and multimedia
formats. New broadband cabling will take decades to reach all prospective
subscribers. Success of these new services will depend on reaching as many
subscribers as possible during the first few years. By bringing movies,
television, video catalogs, remote CD-ROMs, corporate LANs, and the Internet
into homes and small businesses, ADSL will make these markets viable and
profitable for telephone companies and application suppliers alike.
ADSL Capabilities
An ADSL circuit connects an ADSL modem on each end of a twisted-pair telephone
line, creating three information channels---a high-speed downstream channel, a
medium-speed duplex channel, and a basic telephone service channel. The basic
telephone service channel is split off from the digital modem by filters, thus
guaranteeing uninterrupted basic telephone service, even if ADSL fails. The
high-speed channel ranges from 1.5 to 6.1 Mbps, and duplex rates range from 16
to 640 kbps. Each channel can be submultiplexed to form multiple lower-rate
channels.
ADSL modems provide data rates consistent with North American T1 1.544 Mbps
and European E1 2.048 Mbps digital hierarchies (see Figure 15-2) and can be
purchased with various speed ranges and capabilities. The minimum
configuration provides 1.5 or 2.0 Mbps downstream and a 16 kbps duplex
channel; others provide rates of 6.1 Mbps and 64 kbps duplex. Products with
downstream rates up to 8 Mbps and duplex rates up to 640 kbps are available
today ADSL modems accommodate Asynchronous Transfer Mode (ATM) transport with
variable rates and compensation for ATM overhead, as well as IP protocols.
Downstream data rates depend on a number of factors, including the length of
the copper line, its wire gauge, presence of bridged taps, and cross-coupled
interference. Line attenuation increases with line length and frequency and
decreases as wire diameter increases. Ignoring bridged taps ADSL performs as
shown in Table 15-1.
Figure 15-2: This chart shows the speeds for downstream bearer and duplex
bearer channels.
Table 15-1: Claimed ADSL Physical-Media Performance Data rate (Mbps) Wire
gauge (AWG) Distance (feet) Wire size (mm) Distance (kilometers)
Although the measure varies from telco to telco, these capabilities can cover
up to 95% of a loop plant, depending on the desired data rate. Customers
beyond these distances can be reached with fiber-based digital loop carrier (DLC)
systems. As these DLC systems become commercially available, telephone
companies can offer virtually ubiquitous access in a relatively short time.
Many applications envisioned for ADSL involve digital compressed video. As a
real-time signal, digital video cannot use link- or network-level error
control procedures commonly found in data communications systems. ADSL modems
therefore incorporate forward error correction that dramatically reduces
errors caused by impulse noise. Error correction on a symbol-by-symbol basis
also reduces errors caused by continuous noise coupled into a line.
ADSL Technology
ADSL depends on advanced digital signal processing and creative algorithms to
squeeze so much information through twisted-pair telephone lines. In addition,
many advances have been required in transformers, analog filters, and
analog/digital (A/D) converters. Long telephone lines may attenuate signals at
1 MHz (the outer edge of the band used by ADSL) by as much as 90 dB, forcing
analog sections of ADSL modems to work very hard to realize large dynamic
ranges, separate channels, and maintain low noise figures. On the outside,
ADSL looks simple---transparent synchronous data pipes at various data rates
over ordinary telephone lines. The inside, where all the transistors work, is
a miracle of modern technology. Figure 15-3 displays the ADSL
transceiver-network end.
Figure 15-3: This diagram provides an overview of the devices that make up the
ADSL transceiver-network end of the topology.
To create multiple channels, ADSL modems divide the available bandwidth of a
telephone line in one of two ways---frequency-division multiplexing (FDM) or
echo cancellation---as shown in Figure 15-4. FDM assigns one band for upstream
data and another band for downstream data. The downstream path is then divided
by time-division multiplexing into one or more high-speed channels and one or
more low-speed channels. The upstream path is also multiplexed into
corresponding low-speed channels. Echo cancellation assigns the upstream band
to overlap the downstream, and separates the two by means of local echo
cancellation, a technique well known in V.32 and V.34 modems. With either
technique, ADSL splits off a 4 kHz region for basic telephone service at the
DC end of the band.
Figure 15-4: ADSL uses FDM and echo cancellation to divide the available
bandwidth for services.
An ADSL modem organizes the aggregate data stream created by multiplexing
downstream channels, duplex channels, and maintenance channels together into
blocks, and attaches an error correction code to each block. The receiver then
corrects errors that occur during transmission up to the limits implied by the
code and the block length. The unit may, at the user's option, also create
superblocks by interleaving data within subblocks; this allows the receiver to
correct any combination of errors within a specific span of bits. This in turn
allows for effective transmission of both data and video signals.
ADSL Standards and Associations
The American National Standards Institute (ANSI) Working Group T1E1.4 recently
approved an ADSL standard at rates up to 6.1 Mbps (ANSI Standard T1.413). The
European Technical Standards Institute (ETSI) contributed an annex to T1.413
to reflect European requirements. T1.413 currently embodies a single terminal
interface at the premises end. Issue II, now under study by T1E1.4, will
expand the standard to include a multiplexed interface at the premises end,
protocols for configuration and network management, and other improvements.
The ATM Forum and the Digital Audio-Visual Council (DAVIC) have both
recognized ADSL as a physical-layer transmission protocol for UTP media.
The ADSL Forum was formed in December 1994 to promote the ADSL concept and
facilitate development of ADSL system architectures, protocols, and interfaces
for major ADSL applications. The forum has more than 200 members, representing
service providers, equipment manufacturers, and semiconductor companies
throughout the world. At present, the Forum's formal technical work is divided
into the following six areas, each of which is dealt with in a separate
working group within the technical committee:
ATM over ADSL (including transport and end-to-end architecture aspects)
Packet over ADSL (this working group recently completed its work)
CPE/CO (customer premises equipment/central office) configurations and
interfaces
Operations
Network management
Testing and interoperability
ADSL Market Status
ADSL modems have been tested successfully in more than 30 telephone companies,
and thousands of lines have been installed in various technology trials in
North America and Europe. Several telephone companies plan market trials using
ADSL, principally for data access, but also including video applications for
uses such as personal shopping, interactive games, and educational
programming.
Semiconductor companies have introduced transceiver chipsets that are already
being used in market trials. These chipsets combine off-the-shelf components,
programmable digital signal processors, and custom ASICs (application-specific
integrated circuits). Continued investment by these semiconductor companies
has increased functionality and reduced chip count, power consumption, and
cost, enabling mass deployment of ADSL-based services.
Very-High-Data-Rate Digital Subscriber Line (VDSL)
It is becoming increasingly clear that telephone companies around the world
are making decisions to include existing twisted-pair loops in their
next-generation broadband access networks. Hybrid fiber coax (HFC), a
shared-access medium well suited to analog and digital broadcast, comes up
somewhat short when used to carry voice telephony, interactive video, and
high-speed data communications at the same time. Fiber all the way to the home
(FTTH) is still prohibitively expensive in a marketplace soon to be driven by
competition rather than cost. An attractive alternative, soon to be
commercially practical, is a combination of fiber cables feeding neighborhood
optical network units (ONUs) and last-leg-premises connections by existing or
new copper. This topology, which is often called fiber to the neighborhood (FTTN),
encompasses fiber to the curb (FTTC) with short drops and fiber to the
basement (FTTB), serving tall buildings with vertical drops.
One of the enabling technologies for FTTN is VDSL. In simple terms, VDSL
transmits high-speed data over short reaches of twisted-pair copper telephone
lines, with a range of speeds depending on actual line length. The maximum
downstream rate under consideration is between 51 and 55 Mbps over lines up to
1000 feet (300 m) in length. Downstream speeds as low as 13 Mbps over lengths
beyond 4000 feet (1500 m) are also common. Upstream rates in early models will
be asymmetric, just like ADSL, at speeds from 1.6 to 2.3 Mbps. Both data
channels will be separated in frequency from bands used for basic telephone
service and Integrated Services Digital Network (ISDN), enabling service
providers to overlay VDSL on existing services. At present the two high-speed
channels are also separated in frequency. As needs arise for higher-speed
upstream channels or symmetric rates, VDSL systems may need to use echo
cancellation.
Figure 15-5: This diagram provides an overview of the devices in a VDSL
network.
VDSL Projected Capabilities
Although VDSL has not achieved ADSL's degree of definition, it has advanced
far enough that we can discuss realizable goals, beginning with data rate and
range. Downstream rates derive from submultiples of the SONET (Synchronous
Optical Network) and SDH (Synchronous Digital Hierarchy) canonical speed of
155.52 Mbps, namely 51.84 Mbps, 25.92 Mbps, and 12.96 Mbps. Each rate has a
corresponding target range:
Target Range (Mbps) Distance (feet) Distance (meters)
12.96-13.8
4500
1500
25.92-27.6
3000
1000
51.84-55.2
1000
300
Upstream rates under discussion fall into three general ranges:
1.6-2.3 Mbps.
19.2 Mbps
Equal to downstream
Early versions of VDSL will almost certainly incorporate the slower asymmetric
rate. Higher upstream and symmetric configurations may only be possible for
very short lines. Like ADSL, VDSL must transmit compressed video, a real-time
signal unsuited to error retransmission schemes used in data communications.
To achieve error rates compatible with those of compressed video, VDSL will
have to incorporate forward error correction (FEC) with sufficient
interleaving to correct all errors created by impulsive noise events of some
specified duration. Interleaving introduces delay, on the order of 40 times
the maximum length correctable impulse.
Data in the downstream direction will be broadcast to every CPE on the
premises or be transmitted to a logically separated hub that distributes data
to addressed CPE based on cell or time-division multiplexing (TDM) within the
data stream itself. Upstream multiplexing is more difficult. Systems using a
passive network termination (NT) must insert data onto a shared medium, either
by a form of TDM access (TDMA) or a form of frequency-division multiplexing (FDM).
TDMA may use a species of token control called cell grants passed in the
downstream direction from the ONU modem, or contention, or both (contention
for unrecognized devices, cell grants for recognized devices). FDM gives each
CPE its own channel, obviating a Media Access Control (MAC) protocol, but
either limiting data rates available to any one CPE or requiring dynamic
allocation of bandwidth and inverse multiplexing at each CPE. Systems using
active NTs transfer the upstream collection problem to a logically separated
hub that would use (typically) Ethernet or ATM protocols for upstream
multiplexing.
Migration and inventory considerations dictate VDSL units that can operate at
various (preferably all) speeds with automatic recognition of a newly
connected device to a line or a change in speed. Passive network interfaces
need to have hot insertion, where a new VDSL premises unit can be put on the
line without interfering with the operation of other modems.
VDSL Technology
VDSL technology resembles ADSL to a large degree, although ADSL must face much
larger dynamic ranges and is considerably more complex as a result. VDSL must
be lower in cost and lower in power, and premises VDSL units may have to
implement a physical-layer MAC for multiplexing upstream data.
Line Code Candidates
Four line codes have been proposed for VDSL:
CAP (carrierless amplitude modulation/phase modulation)---A version of
suppressed carrier quadrature amplitude modulation (QAM). For passive NT
configurations, CAP would use quadrature phase shift keying (QPSK) upstream
and a type of TDMA for multiplexing (although CAP does not preclude an FDM
approach to upstream multiplexing).
DMT (discrete multitone)---A multicarrier system using discrete fourier
transforms to create and demodulate individual carriers. For passive NT
configurations, DMT would use FDM for upstream multiplexing (although DMT does
not preclude a TDMA multiplexing strategy).
DWMT (discrete wavelet multitone)---A multicarrier system using wavelet
transforms to create and demodulate individual carriers. DWMT also uses FDM
for upstream multiplexing, but also allows TDMA.
SLC (simple line code)---A version of four-level baseband signaling that
filters the based band and restores it at the receiver. For passive NT
configurations, SLC would most likely use TDMA for upstream multiplexing,
although FDM is possible.