ATM over ADSL Probe in Telecom Italia Environment
1)
2)
Authors:
Stanislav Milanovic , Alessandro Maglianella
1)
Serco Group plc, Via Luciano Manara 5, 00044 Frascati, Italy
2)
Telecom Italia, Via di Val Cannuta 250, 00166 Rome, Italy
Abstract:
This paper covers access network performance analysis by deploying ATM over ADSL within Telecom
Italia experimental department “EXANET” (Experimental ATM Network). In this scenario, data traffic
leaves customer site as ATM traffic running over ADSL modem links, gets aggregated via DSLAM
(DSL Access Multiplexer) at the central office (CO) and then dropped onto the high-speed ATM switching
fabric. Italian telephone company was eager to use ADSL as a way to relieve congestion on
circuit-switched voice telephony system increasingly bogged down by the growth of the Internet and long
data transmissions.
Keywords: Internet, Residential Broadband, ADSL Access Network, ATM/ADSL Integration, Quality of Service.
1. Introduction
The roadblock on the Information Superhighway is the current bandwidth limitations of PSTN (Public Switched
Telephone Network). Although most of the developed world is criss-crossed with powerful long-distance fibre-optic
links, a bottleneck remains in the so-called “last mile”. This local loop, feeding from the Telco’s (Telecommunications
Company) central offices to the customer premises, consists of copper twisted-pair phone lines. ADSL neatly
overcomes a number of limitations within the existing telephone network. ADSL is a modem technology that converts
an ordinary phone line into a high-speed digital pipe for ultra-fast access to the Internet and corporate networks while
also enabling real-time multimedia services. With downstream speeds as high as 8Mb/s, ADSL is nearly 300 times
faster than 28.8K dial-up modems and 70 times faster than paired 128 Kb/s ISDN (Integrated Digital Services
Network). ADSL is ushering in a new era of multimegabit access, satisfying today’s desire for speed while paving the
way for interactive multimedia applications [1]. ADSL is simply a date pipe and can be used to carry, amongst other
things, video images that are already compressed. Today's video compression technology is such that it demands
about 1 Mb/s for VCR quality pictures and 2-3 Mb/s for broadcast (superb) picture quality (for example DirectTV
satellite systems use about 3-4 Mb/s). ADSL runs at downstream speed up to 8 Mb/s and is therefore able to carry at
least one such video channel. Then, remote channel change in the CO is envisaged, making the number of accessible
channels infinite. Only the channel selected by the user is actually carried over the ADSL line. ADSL has a number of
benefits:
•
•
Copper bandwidth: ADSL exploits the unused spectrum capacity in ordinary phone lines, employing advanced
modulation techniques, coupled with complex algorithms of data compression and error correction to provide
bigger and faster digital pipes for high-speed remote access.
The same copper pair can be used for both telephone and ADSL service. A new separate line does not need to be
installed and there is no need for costly central office telephone switch upgrades as with ISDN. ADSL network
(unlike ISDN) entirely bypasses the telephone switches which enables service providers to deploy ADSL
technology more quickly. ADSL enables reliable services that will not interfere with telephone service in case of
power outages. If the power is out, so are ISDN lines and cable modems [2]. ADSL was designed so that customer
can continue to use an existing phone line even if modem gets turned-off, fails or is unplugged. The regular
telephone service can continue unaffected by the state of ADSL connection. Thus, a single ADSL line offers
simultaneous channels for PCs, telephones and TVs. Furthermore, since each ADSL customer has its own
dedicated copper line, access speed and bandwidth are unaffected by neighbouring users that access the Internet.
Thus it does not suffer the disadvantage of a cable modem where multiple users sharing a common coaxial cable
network will reduce the speed available to each user (the capacity has to be shared across all users). Dedicated
copper wire for each customer also gives ADSL the advantage over ISDN and cable modems in terms of security:
ISDN travels over the public telephone network and cable modems mainly use shared-access media. Moreover,
ADSL allows integration of the last-mile circuit into an end-to-end architecture protected by upper-layer
mechanisms like VPN tunnelling.
Page 1 of 16
•
•
•
ADSL connection is always up and it can be also used to provide a leased line. In fact, in its symmetrical form
(i.e. HDSL) that is the primary interest. This continuous connectivity frees users from having to dial up every time
they want to connect to the network.
ADSL is creating a new wave of service offerings. Service providers are distinguishing themselves by offering
various services such as: online gaming rooms, video streaming chat rooms, online reference resources, IP dial
tone, disk backup and of course Internet access. The benefits of the convergence of voice, video and data start to
shine with ADSL.
Pay as you go: investment tracks subscriber revenue. Investment matches revenue as it is not necessary to invest
heavily in network upgrades before the first subscriber is turned up. Telephone companies can install the common
equipment necessary to provide ADSL service to any subscriber (remote or branch office, small office, home office
or purely residential) anywhere in the serving area of that CO. It does not matter how geographically scattered the
demand is across the 10,000 or so lines typically served by a central office, since all lines terminate in the office,
and any of them can be easily connected to the ADSL equipment in the CO. As demand builds, more modules are
added to the common shelf, and more shelves are added as required.
ADSL gets its name from the asymmetry of how data is sent and received, providing different rates in the upstream
and downstream directions. The user sends data at one rate but receives data up to 10 times the send rate. This
send/receive rate asymmetry is effective for many typical applications where more data downloads than uploads. Such
applications include WWW access, telecommuting, database queries, file transfers, and broadcast video [3]. Business
can use ATM/ADSL for all of their communications needs: Internet access, voice services, data services among sites
and video services. By using ATM/ADSL, Telecom Italia can provide all of these services with guarantees for quality of
service. ATM simplifies Telco infrastructure by enabling them to provision and manage a single network. This, in turn,
reduces the amount of equipment and infrastructure that they have to maintain with consequent advantages for the
consumers of the services. The services that businesses require have very different requirements and ATM supports
them as well:
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Internet access: many companies already use ATM for Internet access.
Voice services: ATM can provision a CBR (Constant Bit Rate) service; the user hears good quality voice, without
satellite type delays, and the Telco does not have to add extra equipment, such as echo suppressers, to their
network.
Data among sites: ATM provides a switched connection technology with flow control. Switched connections mean
that users can separate private and public traffic on the same physical connection with the certainty that the two
won’t be mixed up. With flow control the user’s data is delivered reliably from end to end without retransmissions.
The result is higher throughput and lower costs: site to site communication will use services tariffed on a usage
basis.
Video services: A growing number of companies are using video conferencing and video distribution in their
business. Video quality is crucial. To provide constant video quality without annoying delays and resulting
jerkiness, the network needs to support a guaranteed but variable bit rate. ATM has a VBR (Variable Bit Rate)
service that is designed to support this.
There are a number of competitive technologies to ATM for any one of these services but only ATM has been designed
from its inception to support them all.
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2. The Local Loop Constraints
Throughout this 100 years of evolution of the network, one factor has remained constant: our telephone is still
connected to the network by a twisted pair of copper wires. The reason for this is simple economics: attempts to
replace it with more modern technology cost more than the revenue from a basic phone bill could support. Copper wire
is relatively inexpensive, it is in place, and it does the job.
All circuits in the network experience some form of noise. The twisted pair running from a home or business to the
phone company central office can pick up an unintended signal from car ignitions, hair dryers, power lines, neon signs
or other sources of electrical discharge. The age and quality of the cable also have a great deal to do with how much
noise is present. Older, squirrel-chewed cables with significant moisture ingress will have more noise than a new cable.
If we are going to reuse the existing twisted pair local loop for high speed service distribution, then we have to deal with
line attenuation (cable loss) which increases with line length and frequency and decreases as wire diameter increases.
Velocity of the signal through a wire is another function of frequency — the higher the frequency, the slower the signal.
All the different pairs of wire in a cable, up to several hundred or a few thousand, couple together and leak their signals
into adjacent pairs, a phenomenon called crosstalk. There are two very different types of crosstalk in multi-pair access
network cables: Near-End Crosstalk (NEXT) and Far-End Crosstalk (FEXT). NEXT is the result of a strong, near-by
transmit source, leaking into a receiver through the coupling between pairs; your next-door neighbour’s ADSL
transmitter coupling into the pair feeding your home and interfering with your receiver. FEXT is the result of a source
(or multiple sources) coupling into another pair and appearing at the opposite or far end of the cable along with the
desired signal on that pair. Its level is attenuated at least as much as the signal itself if both have travelled the same
distance, hence FEXT is not expected to be a problem for ADSL systems. NEXT affects any systems that transmit in
both directions at once (e.g. echo-canceling systems), and, where it occurs, it invariably dominates over FEXT since a
very strong signal, though weakly coupled, is appearing along with the desired signal which has been attenuated by
the entire length of cable. NEXT actually limits the effective span length, rather than loss. NEXT can in principle be
eliminated by not transmitting in both directions in the same band at the same time, separating the two directions of
transmission either into non-overlapping intervals in time or into non-overlapping frequency bands. This converts
duplex transmission into independent simplex transmissions, avoiding NEXT at the cost of a reduced bandwidth in
each direction. FDM ADSL avoids NEXT in this way.
In addition to the transmission characteristics, we have to deal with the systemic impairments in the loop caused by
decades of installation practices, centred around voice telephone service. The loop is divided into two parts: the feeder
and the distribution segments. Feeder cables are large, high pair count cables that leave the CO and head down major
corridors. Periodically, a certain number of pairs are dropped to a distribution frame and connected to distribution
cables, which actually deliver service to the subscriber (See Figure 1.).
Figure 1. Typical Copper Feeder/Distribution Loop
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The distribution cables travel up and down every street, and as they pass a home or small business, the drop wire is
attached to the cable. One thing is important to note: the pairs in a cable are never cut. When a subscriber orders
service, the drop wire is bridged onto the passing distribution cable. Similarly, pairs from distribution cables may be
bridged onto the feeder cable at the distribution frame. Thus provisioning service may consist of bridging an
unassigned distribution pair onto an unassigned feeder pair, then bridging on the drop wire. The theory is that if a
subscriber later terminates service, both the feeder and distribution pair can be used deeper into the loop for other
subscribers since neither was cut. The problem is that often the original bridge taps were not removed when service
was terminated. As subscribers add and delete lines, a feeder pair may acquire several bridge taps. This then is one
impairment that any high-speed distribution technology has to deal with. A twisted pair looks like a transmission line,
and with bridge taps, looks like a transmission line with one or more stubs of random length connected. While not a
particular problem at voice frequencies, at the rates necessary to support high-speed data services, these stubs can
cause frequency sensitive reflections or nulls in the line response. Since finding and removing bridge taps is an
expensive and time-consuming process, ADSL technology must deal with them in place if it is to be successful.
Another impairment is gauge changes and splices. Often, existing loops use two different gauge wires for feeder and
distribution. The most popular cable gauges are 26 AWG (~0.4mm) and 24 AWG (~0.5 mm) and hence these are most
commonly represented. Since feeder cables have a higher pair count, they often use finer wire to keep overall size,
weight, and cost down. Each gauge pair may have a different characteristic impedance and joining the two again
creates more impedance anomalies in our transmission line. Splices create minor discontinuities in
the line — yet another source of reflections.
Major problem in using the local loop for high-speed data services is loading. Loading is a process developed early in
the days of the network to extend the useful range of a loop. Loading coils (small inductors) are periodically inserted in
series with the loop. Working with the natural mutual capacitance of the twisted pair of wires, the coils form a tuned
circuit that reduces attenuation in the voice frequency range at the expense of trashing the frequency response outside
that range [4]. Since ADSL uses frequencies well above the voice region, lines with loading coils cannot be used for
ADSL technology without removing the coils. Telecom Italia have a process where the loading coil is removed from the
line. It is a cost overhead, but need not be a showstopper.
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3. ADSL Modem
ADSL depends upon advanced digital signal processing (DSP) and creative algorithms to squeeze so much
information through twisted pair telephone lines. In addition, many advances have been required in transformers,
analogue filters, and A/D converters. On the outside, ADSL looks simple — transparent synchronous data pipes at
various data rates over ordinary telephone lines. On the inside, where all the transistors work, there is a miracle of
modern technology.
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
(See Figure 2).
Figure 2. ADSL uses FDM or echo cancellation to divide the available bandwidth for services
ADSL has a range of downstream/upstream speeds depending on distance: the high-speed downstream channel
ranges from 1.544 Mb/s (up to 5,5Km) to 8.448 Mb/s (up to 2,8Km) while the medium-speed duplex channel range
from 16 kb/s to 640 kb/s. All of these arrangements operate in a frequency band above POTS, leaving POTS service
independent and undisturbed, even if a premises ADSL modem fails. To create multiple channels, ADSL modems
divide the available bandwidth of a telephone line using one of two methods: frequency division multiplexing (FDM) or
echo cancellation. Referring to the International Telecommunications Union’s G.992.1 standard (Full Rate ADSL
or G.dmt), for both, FDM and echo cancellation, a filter called a POTS splitter front-ends an ADSL modem to split off
4 kHz for voice service (referred to as plain old telephone service, or POTS) thus guaranteeing uninterrupted basic
telephone service, even if ADSL fails.
FDM assigns one band for upstream data and another band for downstream data. The downstream path is then
divided by time division multiplexing (TDM) 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 band and separates the two by means of
local echo cancellation, the same technique used by V.32 and V.34 modems. An ADSL receiver will see an incoming
signal that is both the incoming signal from the far end and the outgoing signal from the local transmitter. These are
mixed together at over the same frequency range. In other words, the received signal composes of not only the signal
to be recovered from the far end but also a local echo due to the local transmitter. The local echo must be accurately
modelled by DSP circuitry and then this replica echo is electronically subtracted from the composite incoming signal. If
done properly then all that is left behind is the incoming data from the far end ADSL system. The process of modelling
the echo is quite complicated since the echo varies depending on the connected cable type. DSP circuitry
automatically adapts to account for this.
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Echo cancellation uses bandwidth more efficiently than FDM as the signals are both kept at the lowest possible
frequencies (since cable loss and crosstalk noise both increase with frequency) and therefore achieves greater cable
distance for a given bit rate. Comparing to FDM, echo cancellation ADSL systems are more complex, more expensive
and only a few vendors have implemented them.
There are many ways to alter the high-frequency carrier signal that results in a modulated wave. For ADSL, there are
two competing modulation schemes: carrierless amplitude phase (CAP) modulation and discrete multi-tone (DMT)
modulation. CAP and DMT use the same fundamental modulation technique — quadrature amplitude modulation
(QAM) — but differ in the way they apply it. QAM, a bandwidth conservation process routinely used in modems,
enables two digital carrier signals to occupy the same transmission bandwidth. With QAM, two independent message
signals are used to modulate two carrier signals that have identical frequencies, but differ in amplitude and phase.
QAM receivers are able to discern whether to use lower or higher numbers of amplitude and phase states to overcome
noise and interference on the wire pair. Generating a modulated wave that carries amplitude and phase state changes
is not easy. To overcome this challenge, the CAP version of QAM stores parts of a modulated message signal in
memory and then reassembles the parts in the modulated wave. The carrier signal is suppressed before transmission
because it contains no information and is reassembled at the receiving modem (hence the word “carrierless” in CAP).
An emerging variation of CAP is Rate adaptive digital subscriber line (RADSL). Rate adaptive ADSL automatically finds
the fastest rate for a given line, enabling data service to all non-loaded loops. RADSL divides the transmission
spectrum into discrete sub-channels and adjusts each signal transmission according to line quality. At start-up, RADSL
also tests the quality of the access line and implements the most efficient version of QAM to ensure satisfactory
performance for individual signal transmissions. DMT offers a multicarrier alternative to QAM. Because high-frequency
signals on copper lines suffer more loss in the presence of noise, DMT discretely divides the available frequencies into
256 subchannels, or tones. DMT creates these channels using digital technique known as Discrete Fast-Fourier
Transform. As with CAP, a test occurs at start-up to determine the carrying capacity of each subchannel. Incoming
data is then broken down into a variety of bits and distributed to a specific combination of subchannels based on their
ability to carry the transmission. To rise above noise, more data resides in the lower frequencies and less in the upper
ones. DMT’s main advantage is the fact that it is the ANSI (American National Standards Institute), ETSI (European
Telecom Standards Institute), and ITU (International Telecommunications Union) standard. But DMT also has
drawbacks: it will initially be more costly than CAP, and it is very complex [5].
Many applications envisioned for ADSL involve digital compressed video, a real time signal which is much more
sensitive to errors but unsuited for error retransmission schemes used in data communications systems; digital video
cannot use link- or network-level error control procedures as a great deal of latency would be incurred by the round-trip
of negative acknowledgment of the original packet and retransmission of the second packet. To ensure very low error
rates compatible with compressed video, ADSL modems incorporate Forward Error Correction (FEC) with sufficient
interleaving to correct all errors created by impulsive noise events of some specified duration. Error correction on a
symbol by symbol basis also reduces errors caused by continuous noise coupled into a line. Forward error correction
(FEC) techniques, such as Reed Solomon (RS), enables the receiver to detect and fix errors to data packets without
requiring the transmitter to retransmit packets, which is vital for the system to maintain speed [6]. 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 users 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. The latency between the customer’s ADSL modem and the ADSL
line card in the CO depends on the line coding technique and the interleaving depth of the error correction scheme that
is programmed. The entire bit stream experiences delay because the bits must be taken in and out of the interleave
buffer (which results in serialization delay) and because Reed-Solomon calculations must be performed. The amount
of latency increases with the number of bits interleaved, and more burst protection means more latency. ADSL delay
time varies from 2-60 msec each way but is typically set to around 20msec which offers greater protection against
impulsive noise. Hence worse case round trip delay added would be 2x20msec=40msec. With the interleaver turned
off, the residual latency of standard ADSL modem is 2msec, but some other modulation schemes can achieve less
then this. Because there is a necessary trade-off of signal quality and latency, compromises must be made. ANSI
ADSL standard T1.413 provides a more detailed specification for interleaver in the ADSL modem.
ADSL modems cannot be used to connect directly from one modem to another at different locations. An important
piece of equipment is required at the telephone companies central office called DSLAM (Digital Subscriber Line Access
Multiplexer). The function of the DSLAM is to terminate all the remote ADSL modems and routers into a single location
and send the data traffic out to the Internet or another remote destination. However, if the customers subscribe to an
ADSL service and connect to the Internet or via a virtual private network, then they can communicate among them like
any other form of switched communications.
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4. Objective
ADSL will enable carriers to offload data traffic onto a separate packet or cell-switched overlay network. As a Service
Provider, Telecom Italia can offer this type of service as a Managed Network (OSI layer 2) where customer’s remote
site is being connected to its regional site or to its headquarter via PVC (Permanent Virtual Circuit). It is also possible
to provide a Managed Service where the router installed at a customer’s premises is run by Service Provider that can
implement a Virtual Private Network (integration with OSI layer 3): in this scenario, an ATM PVC ends up at a router
that sends traffic towards Service Provider’s IP backbone [7].
Basing on the ADSL Forum recommendations and relating technical report TR-002, the goal of proposed access
network configuration is to evaluate network performance of two integrated technologies — ATM and ADSL — for
broadband multimedia and Internet services distribution. Access network is that portion of a public switched network
that connects access nodes to individual subscribers. To successfully deploy ADSL, Telecom Italia envisioned support
for the following access configurations:
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The Internet
Providing high-speed Internet access is a key value for both home users and small businesses. The Internet is
accessed through one or more ISPs (Internet Service Provider) connected with high-speed links from the Telecom
Italia’s Central Office. These high-speed links are part of the Telecom Italia’s regional ATM network.
Corporate Networks
There are two ways to access corporate networks. One is to use an IP tunnelling mechanism through the Internet
(using PPTP, L2F, or L2TP) to reach the corporate network. This design obviates the need for dial-up modems at
the corporation while leveraging the Internet as the virtual private network. This, however, also means the
connectivity may be limited by the bandwidth of the Internet. A second method is to use the Telecom Italia’s own
regional broadband network to provide direct high-speed connectivity to the corporate network. This has the
advantages of higher speed and greater security.
Local Content
Locally hosted content can be delivered at high speed without going through the Internet. Local content may be
stored at the POPs (Point-of-Presence) of the ISPs, content providers or the COs and regional operation centres
of the Telecom Italia. Local content can be created locally (such as merchant services for retailing) or generated
remotely (Web content from the Internet cached in local servers).
Peer-to-peer Communication
The ability to interconnect consumers at high speed enables high quality peer-to-peer communication applications
such as video telephony or interactive gaming. Demand for this connectivity may ramp up more slowly than the
services described above, but aggressive pricing could materially accelerate this consumer use of ADSL.
5. Requirements
Telecom Italia have addressed the following functional requirements in order to enable a mass market for ADSL:
•
Easy Migration from existing ISP Access Infrastructure
Since ISPs already have an infrastructure to support dial-up access based on PPP (Point-to-Point Protocol), any
new broadband Internet access solution must take into account this architecture. Ideally, the broadband service
model for accessing ISP service can re-use most of the networking, management and administration infrastructure
(such as IP address and domain name administration) and will not require a paradigm shift for the ISP.
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Simultaneous Connectivity: Internet and Corporate network
A telecommuter working from home may need to access the Internet while connected to the corporate network.
There are two ways to allow such simultaneous connectivity. One way is to access the Internet through the
corporate network’s own Internet gateway. The other way is to support a separate Internet connection
simultaneously with the corporate connection. In many cases, the second way is more appropriate because it
allows the telecommuter to access the Internet directly for non-work related reasons (such as entertainment)
without using the corporate network resources. However, some corporations may not trust the simultaneous
connections because this may open a back door to the corporate network from the Internet through the
telecommuter’s PC.
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Multi-Protocol Support
Since not all corporations run IP exclusively, providing corporate connectivity requires interconnecting non-IP
networks over the ADSL access network. Hence, such connectivity involves protocol negotiation and address
assignment.
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Security
Telecommuters and branch offices must be able to communicate to the enterprise in a fashion that supports
authentication, authorization and privacy. Security is also important for connecting to the Internet since the ISP
already authenticates all user access. The ISP must identify users and provide them with the contracted level of
service.
Multicast
There has been an explosion of interest in IP multicast service. Live events are now commonly offered in audio
and video on the Internet, and multicast is the preferred delivery mechanism. A first-order requirement for the
ADSL access network is to deliver IP multicast service to homes and small businesses.
Multiple Service Class Support
It has become very clear that many services, including Internet access, cannot depend solely on a “one size fits
all” paradigm. Different classes of service are required to satisfy the different needs of, for example, power users
versus occasional users. Such differences in service class can be based on a variety of attributes, such as
maximum, average or minimum bandwidth.
Quality of Service Support
Real-time streaming audio and video applications have become increasingly popular, especially over the Internet.
These applications, and other real-time applications, require QoS to ensure their performance. Quality of service
also implies that the network can prevent aggressive or rogue users from consuming network bandwidth and
degrading the performance of other users.
Delivering automated service provisioning to residential or small-business subscribers
Automatic provisioning of the subscriber’s customer premises equipment (CPE) requires a transport mechanism,
a means of controlling information flow, and interfaces into the customer’s CPE to carry information regarding
network and service provider configurations. The user modem and PC must be automatically configured to match
the ATM network characteristics set by the carrier. The network configuration information can be used to support
an application that allows the user to automatically select from any service provider currently subscribed to.
Properly implemented, the user does not need to enter the ADSL equivalent of a phone number to access a
service, but will be able to use an automatically generated icon or a user-friendly provider name.
6. ATM/ADSL Networking
The new focus on the Internet and corporate LAN access has forced change in access network architectures.
Recognizing that the Internet Protocol (IP) dominates existing private networks and the Internet, telecommunication
companies planned to construct access networks to support IP to the customer premises. However, Telcos also
recognized the power of Asynchronous Transfer Mode (ATM) to support a future of mixed services (data, video, voice)
and management of Quality of Service (QoS), particularly the parameters of delay and variation of delay. Thus, ADSL
access networks evolved around a paradigm of ATM multiplexing and switching of paths that will carry IP and other
forms of user traffic [8]. ADSL modems accommodate Asynchronous Transfer Mode (ATM) transport with variable
rates and compensation for ATM overhead. ADSL and ATM together provide an ideal solution for multiple service
delivery.
Telecom Italia adopted near-term ADSL networks approach with Permanent Virtual Circuits (PVCs), and ATM-based
paths between a user terminal and an Information Service Provider such as an ISP or corporate LAN gateway. A PVC
is brought into existence by a network administrator following a service order and cannot be altered by the user.
A PVC to the Internet (via an ISP for example) provides access to all Internet resources through IP routing. By the
same token, a PVC to a corporate LAN provides access to all files and other resources available from the corporate
network, again using IP. The original plans for ATM access network envisioned the introduction of Switched Virtual
Circuit (SVCs) as the network grew in size and capabilities. A SVC connection is established in real-time in response to
signalling messages from the customer. SVCs would greatly reduce the effort to provision service to a new customer,
and would also permit customer to freely roam among ISPs. PVCs will work well for early market entry of ADSL
networks. However, PVCs cannot be scaled as a network grows to millions customers and thousands of circuit
changes a day. The long-term answer, SVCs that connect and disconnect under customer control just like telephone
calls, has proved far more difficult and costly to implement than first expected.
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In a refinement to the ATM access network architecture, Point-to-Point Protocol (PPP) over ATM was adopted by
Telecom Italia since it meshed well with the protocols and operating methods of Internet Service Providers. For
example, ISPs utilise AAA (authentication, authorisation, accounting) security servers in their access configurations.
The original function of the security servers was to provide for subscriber authentication. The security servers also
allow for extended service capabilities. The security servers are dependent on PPP session establishment to the
end-user. By adopting the same service access protocol as the lower speed dial-up users do today, service
convergence is enhanced for the ISP. Other connection models for ATM (such as Classical IP over ATM, LANE, and
MPOA) target campus environments and lack the security, session, and autoconfiguration functionality that high-speed
remote access networks will demand. Essential operational functions can be delivered over ATM using features well
established in PPP:
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•
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•
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Authentication (Password Authentication Protocol — PAP, Challenge Handshake Authentication Protocol —CHAP,
token-based systems),
Layer 3 address autoconfiguration (e.g. domain name autoconfiguration, IP address assignment by the destination
network),
Multiple concurrent destinations (i.e. multiple PPP sessions),
Layer 3 transparency (e.g. both IP and IPX are currently supported on PPP),
Encryption,
Compression,
Billing and usage metering.
Adapting the PPP suite to ADSL can happen with little or no extra effort and will accelerate delivery of an interoperable
service architecture. PPP over ATM is even more valuable because it adheres to the narrowband service model
currently driving the ISP business.
The end-to-end ADSL-based network architecture consists of following subnetworks: the customer premise network,
the access network, the regional network and the service provider networks. They are shown in Figure 3.
Content
Provider
Residential User /
Home Office
Central Office
phone line
ATM
Access
Switch
ISP
DSLAM
Internet
Splitter
Splitter
ADSL
modem
PC
phone line
Splitter
Regional
Broadband
Network
(ATM core
network)
Branch Office
Voice
Switch
ADSL
modem
Corporate
Network
ADSL
Router
Voice
DATA
LAN
Voice Network
(PSTN)
Splitter
Figure 3. End-to-end ADSL-based Broadband Network Architecture
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Service Provider Networks: The service provider networks include the ISP POPs, content provider networks and
corporate networks. An ISP POP is for connecting to the Internet and provides ISP services such as e-mail and
Web hosting. A content provider network consists of a server farm for distributing content. The corporate networks
are connected to the regional broadband network to allow remote access from a home (telecommuting) or from
branch offices. To implement ADSL, the bottom line is that an ISP needs access to copper lines. An incumbent
Telco, of course, has copper already. If ISP is not a Telco then ISP must gain access to copper. An ISP would
actually need to lease a copper wire that runs from a customer to its POP by contracting with the telephone
company, and then re-sell access to a customer. The DSLAM could be owned by the Telco and capacity leased to
the ISP. The backbone from the DSLAM in the CO would then connect into the ISP router at their POP. Therefore,
a customer can still connect to the ISP of his choice, however, Telecom Italia is an ISP itself (the phone operator
may be an ISP itself which is getting more and more common these days). If ISP leased only a wire that runs from
customer into the Central office building, then ISP would need to install its own ADSL equipment within the CO
building. Since CO is owned by the Telco, an ISP would have to lease a co-location space.
Regional Broadband Network: A regional broadband network, based on a SONET infrastructure, interconnects the
central offices in a geographical area. Telecom Italia have deployed ATM over this SONET infrastructure to provide
broadband connectivity among the COs.
Access Node: DSLAM which is a rack of ADSL modems, i.e. the ATU-C (ADSL Termination Unit - Central Office)
modules, with data multiplexed into the Telecom Italia's ATM backbone network interface/connection (OC-3c). This
resides in the CO due to the limited transmission range of ADSL. The access node performs the following
functions: line termination of the ADSL subscriber lines (ATU-C modules of the access node perform this function)
and concentration/multiplexing of the ADSL subscriber lines towards regional broadband network. Although
Figure 3 shows them as separate pieces, the central office POTS splitter may be a part of DSLAM, and, in some
cases, all this hardware may actually be contained on the voice switch line card itself with the concentration of the
traffic from many ADSL modems performed by a new common card in the switch line card shelf. A multiplexing
scheme that provides high concentration while guaranteeing the individually negotiated QoS is important asset for
network operators, because it allows them to offer differentiated services at a reasonable cost. Once voice over
ATM is widespread and the delays are acceptable, it could be possible to carry voice and data together to an ATM
network from DSLAM, instead of splitting the voice and submitting it to a voice switch; it would also be a neat way
of providing a second voice line.
Customer Premise: The customer premise includes residences, home offices and small business offices. Each
may contain one or more PCs (or workstations). Where there are multiple PCs, they reside on a LAN. The gateway
to the external network can be dedicated hardware (such as a router or ATM switch) or a PC-based access server
(acting as a router or proxy server). In the latter case, the PC server has two NICs (Network Interface Card), one
for connecting to an ADSL modem (or serving as the ADSL modem in the case of an internal ADSL modem card)
and the other for the LAN. The ADSL modem on the customer premise is called the ATU-R (ADSL Termination
Unit - Remote). The POTS splitter is installed at the telephone company demarcation point. The POTS splitter
serves to split off the lower portion of the frequency spectrum for voice, and separate the higher portion for data
into the ADSL modem (ATU-R). The functionality of the ATU-R is twofold: network termination of the ADSL
subscriber line at the customer premise and adaptation to the data interfaces from ADSL to the CPE side, which
can be a LAN interface or a NIC in a PC.
7. Test Bed
Test bed is based on the ATM/ADSL access network technology to provide the subscriber with broadband access to
the Internet, corporate networks and Video-on-Demand services (home shopping, games and movies on demand). For
this purpose, the following network equipment has been used (See Figure 4):
•
DLS 400 emulates various local loops for testing Asymmetrical Digital Subscriber Loop (ADSL) transmission
products and other high-speed digital subscriber loop technologies. It reproduces the exact characteristics of real
telephony cable using networks of passive discrete components (L,R,C), thus reproducing both the
A.C. and D.C. characteristics of the line. The DLS 400 contains hundreds of segments of cable simulation, which
can be matrixed together in various configurations and line lengths. In analogue terms, the DLS 400 reproduces
cable accurately up to 1.5 MHz and in some configurations up to 2.0 MHz. The DLS 400 comes complete with both
RS232C and IEEE-488 interfaces and associated cables. An IEEE-488 interface card for the IBM-PC is available
as an option.
Page 10 of 16
•
•
•
•
•
•
•
Westell SuperVision CAP RADSL access system consists of a remote unit (ATU-R) located at the subscriber's
premises and an access line module (ATU-C) located within the SuperVision DSLAM system at the central
exchange office. ATU-Cs are available with or without integral POTS splitters. The SuperVision system offers an
Integrated SNMP-based network management for the multiplexer. Management options include the AccessVision
element management system and SuperVision Administrator, a PC-based craft tool. Management system
connectivity is provided to SuperVision through either an ATM in-band connection via the ATM network interface or
an out-of-band 10BaseT Ethernet connection. Asynchronous craft port access is also provided. Stand-alone
ATU-Rs support a variety of applications such as Ethernet bridging, routing via PPP proxying and native ATM cell
relay. Westell also offers an ATU-R with a standard ATM25 interface, which meets ATM forum specifications. This
unit is ideal for cell relay services and is compatible with customer equipment that is already equipped with an
ATM25 interface. The SuperVision CAP RADSL access system offers a high-speed copper highway in the local
loop, enabling us to support high capacity interactive multimedia applications, plus conventional telephone service,
over a single copper pair. A key feature of this system is rate adaptation. The rate adaptive functionality allows this
ADSL system to be automatically configured to operate at the highest possible speed. The operator configures
through the management system a maximum and minimum speed, as well as a required margin, then the system
will select the maximum upstream and downstream rates within this specified range that are feasible based upon
the specific loop and noise impairments. This innovative system will support applications such as high-speed
Internet access, remote LAN access and video-on-demand.
BPX StrataCom switch emulating an ATM access switch at the central office. Connections are established via
PVCs utilizing PPP protocol over ATM model.
ADTECH AX4000 ATM Test System was used for protocol analysis and test bed performance fine-tuning in ATM
environment.
Cisco 7505 Series Router emulating content provider network for distributing WWW, Telnet and FTP services to a
telecommuter over ADSL carrier network.
Cisco 3600 Series Router emulates a branch office gateway to the external network. ATM 25 Mb/s network module
allows branch office to take advantage of the higher bandwidth and Quality of Service (QoS) available with ADSL
service.
Video server providing video content to subscriber’s home via ADSL technology. Video-on-Demand (VoD) is a
video streaming technology developed by NEC. Video Streaming means that a client computer plays video as it
receives it via the network from a video server. Rather than having to download a video file from an Internet or
Intranet server and then play it, the client can play the video as the computer receives it from the server. Video
content is stored on the video server disk array in MPEG (Motion Picture Expert Group) compressed format. This
means that, upon request, the client will receive a high definition 30 frames per second video that can be viewed
clearly as a full screen picture; the client also receives simultaneous digital stereo sound. The video server stores
movies and programmes via NEC real time encoder system within the control equipment and supplies them to
viewers on request. The control equipment includes content management, real time encoder software and
customer billing systems. Data can be input to the video server from S-VHS Video Cassettes, Live TV Feed, CD,
Magneto Optical or a Scanner. The VoD system is scaleable as more content or VoD clients can be catered for by
adding disk storage space and additional servers. The NEC VoD system allows viewers to pause, rewind, fast
forward the video content at will from a connected PC at home. High quality VoD can be distributed over Local and
Wide Area Networks in commercial and educational organisations. Video streams are delivered directly to the
client PC graphics card via the ATM or Ethernet Network Interface Card. The client PC is able to run normal
applications simultaneously with the video content. The only additional hardware required in the client PC is an
MPEG decoder card. Quality of Service (QoS) of the video stream can be guaranteed to the PCs connected to an
ATM network. However, the connectionless technology utilised by Ethernet networks can result in the delay of
video data so NEC has developed the QoS capability to throttle back the frame rate of the stream until bandwidth
becomes available. In the case of high traffic in the Ethernet environment, this results in continuous video with a
slightly degraded quality [9].
NT Server providing WWW, FTP and Telnet services. NT Server and Workstation boxes are equipped with Oki
ATM 25 Mb/s network interface card each, while PC with AccessVision software for DSLAM Management is
equipped with Ethernet NIC.
Page 11 of 16
Disk Array
ATM 25Mb/s
(Cat.5)
Workstation
AccessVision
Fiber optic
ATM 155Mb/s
Video Server
Fiber optic
ATM 155Mb/s
10BaseT (Cat.5)
phone line
MPEG
Magneto
Optical
Real Time
Encoder
Westell
SuperVision
DSLAM
BPX Stratacom
ATM Access
Switch
Fiber optic
ATM 155Mb/s
Splitter
phone line
Fiber optic
ATM 155Mb/s
Splitter
ADSL Modem
Video
DLS TestWorks 400
Local Loop Simulator
Scanner
ADSL Modem
ATM 25Mb/s
(Cat.5)
ADTECH
AX4000
ATM Test
System
CD
Control Equipment
ATM 25Mb/s
(Cat.5)
Router Cisco
3600 Series
Console
10BaseT (Cat.5)
NT Server
Services: WWW, FTP, Telnet
Router Cisco
7507 Series
Console
LAN
Figure 4. Test Bed Layout
8. Measurements and Results
Referring to test bed configuration, illustrated in Figure 4, performed measurements were focused on a maximum
length determination of the phone cable, with no cell loss during transmission, while varying a phone wire diameter
under a constant downstream of 7.1Mb/s and under a constant upstream of 1.088 Mb/s in presence of various noise
signals:
•
•
•
•
•
•
•
•
•
•
White noise (signal due to irregular movement of electrons in conductor grid caused by thermal effects),
AMI (AM interference),
Metallic noise (Interference due to metallic oxide caused by phone wire corrosion),
Crosstalk due to one adjacent ADSL line,
Crosstalk due to ten adjacent ADSL lines,
Crosstalk due to twenty four ADSL lines,
Type A crosstalk due to one adjacent ADSL line,
Type B crosstalk due to one adjacent ADSL line,
Crosstalk due to one adjacent HDSL line,
Noise provoked by two different crosstalk signals: one is due to one adjacent ADSL line and the other is due to one
adjacent HDSL line.
Table 1 shows measured values relating to phone cable length for various wire diameters under different noise signals
while keeping constant downstream of 7.1 Mb/s and upstream of 1.088 Mb/s.
Page 12 of 16
Phone wire
diameter
White
noise
AMI
Metallic
noise
1 ADSL
line
10 ADSL 24 ADSL
lines
lines
ADSL
type A
ADSL
type B
HDSL
ADSL
+
HDSL
Ø 0.63mm
4400m
4350m
3850m
4400m
4400m
4350m
4400m
3800m 4350m
4000m
3Km of
Ø 0.63mm
+
the rest of
Ø 0.4mm
4050m
4200m
3600m
4200m
4050m
3900m
3950m
3650m 3850m
3450m
Ø 0.4mm
3000m
3150m
1850m
3000m
2650m
2400m
2450m
1550m 2350m
2000m
Table 1. Noise impact on signal propagation over phone cable
Data from Table 1 are illustrated in Figure 5.
5000
4500
4000
3500
Phone cable length [m]
White noise
AMI
3000
Metallic noise
1 ADSL line
10 ADSL lines
2500
24 ADSL lines
ADSL type A
ADSL type B
2000
HDSL
ADSL + HDSL
1500
1000
500
0
Ø 0.63mm
3Km of Ø 0.63mm + the rest
of Ø 0,4mm
Ø 0.4mm
Phone cable type
Figure 5. Maximum phone cable length with no cell loss under a constant downstream
of 7.1 Mb/s and a constant upstream of 1.088 Mb/s
Page 13 of 16
9. Conclusion
Results of testing demonstrate that ATM/ADSL is able to send data/voice/video services under a high rate
downstream/upstream within a range of nearly 4.5Km over ordinary subscriber loops without any cell loss. There is,
though, a physical limit regarding copper cable due to corrosion or junction getting older, that could make effective
results inferior respect to laboratory expected performances. We can conclude that ADSL grants high throughput
performance for “the last mile” of the installed copper plant to end-users.
Future availability of ADSL will depend on the deployment by local telephone companies that enter new markets for
delivering information in video and multimedia formats. By bringing the Internet into homes and small businesses,
along with corporate LANs, movies, television, video catalogs and remote CD-ROMs, ADSL will make these markets
viable and profitable for telephone companies and application suppliers alike.
In order for a mass-market broadband-access network to develop, it requires low costs and flexible services for market
segments. By leveraging the world’s nearly 800 million phone lines, ADSL offers the most viable option for
providing virtually ubiquitous high-speed remote access and interactive multimedia transmission. Key issues of ADSL
service are how to configure and manage concurrent connections from end-users to their multiple service destinations,
end-to-end [10].
For the future, as fibre moves gradually to the customers, another variant of ADSL — VDSL (Very-high speed DSL)
with capacities of up to 52 Mb/s, will enhance even further the use of those simple copper pairs to provide a smooth
upgrade path for operator to enable them to transport ever more sophisticated services to ever more sophisticated
users. It is likely, in fact, that combinations of fibre feeding access nodes and VDSL feeding premises over copper
(so called Fibre to the Neighbourhood, or FTTN) will be the next generation network topology for telephone companies
in most countries of the world.
10. Acknowledgements
The authors would like to thank Mr Fabio Romano, Manager with Telecom Italia, for his patience, support and
encouragement.
Page 14 of 16
11. Glossary
ADSL
Asymmetrical Digital Subscriber Line (OSI layer 1 communication protocol).
AMI
AM radio frequency interference is type of narrowband interference. Narrowband interference
affects a small number of frequencies over a longer period of time.
ATM
Asynchronous Transfer Mode (OSI layer 2 communication protocol).
AWG
American Wire Gauge is a wire diameter specification; the lower the AWG number, the larger the
wire diameter.
broadband
A communication channel with a bandwidth in excess of 1.54 Mb/s.
CHAP
A PPP cryptographic challenge/response authentication protocol in which the cleartext password is
not passed over the line. This allows the secure exchange of a shared secret between the two
endpoints of a connection.
core network
A combination of switching offices and transmission plant that connects switching offices together.
Digital Signal
Processing
This is a process performed by a specialised microchip which converts from anhalog to digital
signals at very high speeds.
downstream
Traffic moving toward the customer premises.
1 foot [‘]
0.3048 meter [m].
HDSL
High Bit-Rate Digital Subscriber Line is symmetric technology, providing 1.544 Mb/s over two
copper pairs and 2.048 Mb/s over three pairs.
IP
Internet Protocol (OSI layer 3 communication protocol).
IPX
Internetwork Packet Exchange (OSI layer 3 communication protocol).
LANE
ATM LAN Emulation.
L2F
Layer 2 Forwarding. Access VPNs use L2F tunnels to tunnel the link layer of high-level protocols
(for example, PPP frames or asynchronous High-Level Data Link Control).
L2TP
A Layer 2 tunnelling protocol is an extension to the PPP protocol used for Virtual Private Networks
(VPNs).
Modulation
The process in which the characteristics of one wave or signal are varied in accordance with
another wave or signal. Modulation can alter frequency, phase, or amplitude characteristics.
MPOA
Multiprotocol over ATM.
Multiplexer
A device that makes available the same communication line to be shared among various signals.
OC-3c
OC-3c (also known as STM-1) is 155,52 Mb/s fiber interface.
OSI
Open System Interconnection is a layered network model. For the OSI model, seven numbered
layers indicate distinct networking functions within a communication environment.
PAP
A simple PPP authentication mechanism in which a cleartext username and password are
transmitted to prove identity. PAP is not as secure as CHAP because the password is passed in
"cleartext."
PPTP
Point-to-Point-Tunnelling Protocol is Microsoft networking technology that supports multiprotocol
virtual private networks (VPN), enabling remote users to access corporate networks securely across
PPP-enabled systems to dial into a local Internet service provider to connect securely to their
corporate network through the Internet.
Protocol
Set of rules being used between two entities while data communications takes place.
SONET
The physical interface function providing the capability for using FC, SC or ST connectors.
S-VHS
Super Video Home System
upstream
Referencing the flow of information from the subscriber to the central office (and beyond).
VCR
Video Cassette Recorder.
Page 15 of 16
12. References
[1] Martin Jackson, “ADSL for Broadband Access”, ADSL Forum, 1997.
[2] Martin Jackson, Stefan Knight, “At Work With ADSL: More Than Bandwidth”, Data Communications, April 21,1998.
[3] “ An Interoperable End-to-End Broadband Service Architecture over ADSL Systems”, ADSL Technology Briefs,
rd
3Com, June 3 , 1997.
[4] Jim Lane, “Personal Broadband Services: DSL and ATM”, Virata: Enabling Personal Broadband Solutions, 1998.
[5] “Delivering Broadband over Copper Wires: xDSL Local Loop Access Technology”, 3Com Technical Papers, 1998
3Com Corporation.
[6] George Abe, “Residential Broadband”, Internetworking Technologies Handbook (2nd Edition), Cisco Press
Publications, Cisco Systems Inc., Feb 27, 1999.
[7] “ La promessa di ADSL: l’alta velocità per tutti”, NetWorking Italia, May 1998.
[8] Tom Starr and Kim Maxwell, “ADSL Access Networks”, May 1998.
[9] Dr Glenn Wylie, “Video on Demand”, NEC Technology White Paper, March 1998.
[10] Mark Huntzinger, “Bringing broadband over the last mile”, TELECOM Interactive 97, Infrastructure Session.
Vitae
Stanislav Milanovic is a Network Engineer with Serco Group plc in Rome, Italy. His major responsibility is to provide
support in information technology services to the European Space Agency. Stanislav was working a Network Specialist
with Siemens Nixdorf Informatica and KPMG Consulting Group before joining Serco. His assignments included support
to the networking departments at Telecom Italia and Agip Petroli in Rome, Italy. Stanislav has received a B.Sc.
(Honors 1) in Electrical Engineering from the University of Belgrade, Faculty for Electrical Engineering, Department for
Electronics.
Alessandro Maglianella is a Project Manager with Telecom Italia in Rome, Italy. He is deploying the emerging
technologies within Telecom Italia environment. Alessandro is directly responsible for evaluation of the new
technologies in Telecom Italia experimental department 'EXANET' (Experimental ATM Network) in Rome.
Page 16 of 16