ATM over ADSL Probe in Telecom Italia Environment
Authors: Stanislav Milanovic , Alessandro Maglianella
Serco Group plc, Via Luciano Manara 5, 00044 Frascati, Italy
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
Keywords: Internet, Residential Broadband, ADSL Access Network, ATM/ADSL Integration, Quality of Service.
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 . ADSL is simply a data 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
• 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 . 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.
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• 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 . 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:
• 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
• 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
• 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 we