ATM over ADSL probe in Telecom Italia environment

Computer Networks 34 (2000) 965±980

www.elsevier.com/locate/comnet

ATM over ADSL probe in Telecom Italia environment Stanislav Milanovic a,*, Alessandro Maglianella b a

Serco Group plc, Via Luciano Manara 5, 00044 Frascati, Italy Telecom Italia, Via di Val Cannuta 250, 00166 Rome, Italy

b

Abstract This paper covers access network performance analysis by deploying asynchronous transfer mode (ATM) over ADSL within the Telecom Italia experimental department ``EXANET'' (experimental ATM network). In this scenario, data trac leaves customer site as ATM trac running over ADSL modem links, it is aggregated via digital subscriber line access multiplexer (DSLAM) at the central oce (CO) and then dropped onto the high-speed ATM switching fabric. The Italian telephone company was eager to use ADSL as a way to relieve congestion on their circuit-switched voice telephony system, increasingly bogged down by the growth of the Internet and by long data transmissions. Ó 2000 Elsevier Science B.V. All rights reserved. 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 public switched telephone networks (PSTNs). Although most of the developed world is criss-crossed with powerful long-distance ®bre-optic links, a bottleneck remains in the so-called ``last mile''. This local loop, feeding from the TelcoÕs (telecommunications company) central oces (COs) 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 *

Corresponding author.

8 Mb/s, ADSL is nearly 300 times faster than 28.8 K dial-up modems, and 70 times faster than paired 128 Kb/s integrated digital services network (ISDN). ADSL is ushering in a new era of multimegabit access, satisfying todayÕs desire for speed while paving the way for interactive multimedia applications [1]. 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 a downstream speed of 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 in®nite. Only the channel selected by the user is actually carried over the ADSL line. ADSL has a number of bene®ts:

1389-1286/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 9 - 1 2 8 6 ( 0 0 ) 0 0 1 6 6 - 3

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· 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 CO telephone switch upgrades as with ISDN. Unlike ISDN, ADSL networks entirely bypass 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 customers can continue to use an existing phone line even if the modem gets turned-o€, fails or is unplugged. The regular telephone service can continue unaffected by the state of an ADSL connection. Thus, a single ADSL line o€ers simultaneous channels for PCs, telephones and TVs. Furthermore, since each ADSL customer has their own dedicated copper line, access speed and bandwidth are una€ected by neighbouring users that access the Internet. Thus, it does not su€er 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 an 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. · An ADSL connection is always up and can also be used to provide a leased line. In fact, it is 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 o€ered services. Service providers are distinguishing themselves by o€ering various services such as online gaming rooms, video streaming chat rooms, online reference resources, Internet Protocol (IP) dial tone, disk backup and of course Internet access. The bene®ts 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 ®rst subscriber turns up. Telephone companies can install the common equipment necessary to provide ADSL service to any subscriber (remote or branch oce, small oce, home oce 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 CO, since all lines terminate in the oce, 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 di€erent rates in the upstream and downstream directions. The user sends data at one rate but receives data at up to 10 times the send rate. This send/receive rate asymmetry is e€ective for many typical applications where more data downloads than uploads. Such applications include WWW access, telecommuting, database queries, ®le transfers and broadcast video [3]. Business can use ATM/ ADSL for all their communications needs: Internet access, voice services, data services between sites and video services. By using ATM/ADSL, Telecom Italia can provide all of these services with guarantees for quality of service (QoS). Asynchronous transfer mode (ATM) simpli®es 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 di€erent 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 constant bit rate (CBR) 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 ¯ow control. Switched connections mean that users can separate private and public trac on the same physical connection with the certainty that the two would not be mixed up. With ¯ow control, the userÕs data is delivered reliably from end-toend without retransmissions. The result is higher throughput and lower costs: site-to-site communication will use services tari€ed on a usage basis. · Video services. A growing number of companies are using videoconferencing 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 (VBR). ATM has a VBR 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. 2. The local loop constraints Throughout the 100 yr of the 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's CO can pick up an unintended signal from car ignitions, hair dryers, power lines, neon signs or other

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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 signi®cant 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 di€erent 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 cross-talk. There are two very di€erent types of cross-talk in multi-pair access network cables: near-end cross-talk (NEXT) and far-end cross-talk (FEXT). NEXT is the result of a strong, nearby transmit source, leaking into a receiver through the coupling between pairs; your next-door neighbourÕs 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 a€ects any systems that transmit in both directions at once (e.g., echocancelling 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 e€ective span length, rather than loss. In principle, NEXT can 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.

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In addition to the transmission characteristics, we have to deal with the systemic impairments in the loop caused by decades of installation practices centred on 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 that actually deliver service to the subscriber (see Fig. 1). 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. It is important to note that 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 and 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 is an impairment that any high-speed distribution technology has to deal with. A twisted pair looks like a transmission line, and with bridge taps, it looks like a trans-

mission 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 re¯ections or nulls in the line response. Since ®nding 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. Other impairments are gauge changes and splices. Often, existing loops use two di€erent gauge wires for feeder and distribution. The most popular cable gauges are 26 AWG (0.4 mm) and 24 AWG (0.5 mm) and hence these are most commonly represented. Since feeder cables have higher pair counts, they often use ®ner wire to keep overall size, weight, and cost down. Each gauge pair may have di€erent 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 re¯ections. The 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

Fig. 1. Typical copper feeder/distribution loop.

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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. 3. ADSL modems ADSL depends on 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 ®lters, and A/D converters. On the outside, ADSL looks simple ± transparent synchronous data pipes at various data rates over ordinary telephone lines. However, on the inside, where all the transistors work, there is a miracle of modern technology. An ADSL circuit connects an ADSL modem at 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 Fig. 2). ADSL has a range of downstream and upstream speeds depending on distance: the highspeed downstream channel ranges from 1.544 (up to 5.5 km) to 8.448 Mb/s (up to 2.8 km) while the medium-speed duplex channel range from 16 to 640 Kb/s. All of these arrangements operate in a frequency band above POTS, leaving POTS service independent and undisturbed, even if a premise's 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 UnionÕs G.992.1 standard (full rate ADSL or G.dmt) for both FDM and echo cancellation, a ®lter called a POTS splitter front-ends an ADSL modem to split o€ 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 down-

Fig. 2. ADSL uses FDM or echo cancellation to divide the available bandwidth for services.

stream 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 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

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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. Echo cancellation uses bandwidth more eciently than FDM as the signals are both kept at the lowest possible frequencies (since cable loss and cross-talk noise both increase with frequency) and therefore achieves greater cable distance for a given bit rate. Compared 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 di€er 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 di€er 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. It is not easy to generate a modulated wave that carries amplitude and phase state changes. 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 ®nds 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 ecient version of QAM to ensure satisfactory performance for individual signal transmissions. DMT o€ers a multicarrier alternative to QAM. As high-frequency signals on copper lines su€er 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 fastFourier 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 speci®c 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. DMTÕs main advantage is the fact that it is the American National Standards Institute (ANSI), European Telecom Standards Institute (ETSI), and International Telecommunications Union (ITU) 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 acknowledgement 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 sucient interleaving to correct all errors created by impulsive noise events of some speci®ed duration. Error correction on a symbol by symbol basis also reduces errors caused by continuous noise coupled into a line. FEC techniques, such as Reed Solomon (RS), enable the receiver to detect and ®x errors to data packets without requiring the transmitter to retransmit packets. This is vital for the system to maintain speed [6]. An ADSL modem organises the aggre-

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gate 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 speci®c span of bits. The latency between the customerÕs 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 bu€er (which results in serialisation 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 to 60 ms each way but is typically set to around 20 ms which o€ers greater protection against impulsive noise. Hence, worse case round trip delay added would be 2  20 ˆ 40 ms. With the interleaver turned o€, the residual latency of standard ADSL modem is 2 ms, but some other modulation schemes can achieve less than this. As there is a necessary trade-o€ between signal quality and latency, compromises must be made. ANSI ADSL standard T1.413 provides a more detailed speci®cation for interleaver in the ADSL modem. ADSL modems cannot be used to connect directly from one modem to another at di€erent locations. An important piece of equipment is required at the telephone company's CO called a digital subscriber line access multiplexer (DSLAM). The function of the DSLAM is to terminate all the remote ADSL modems and routers into a single location and send the data trac 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 between them like any other form of switched communications.

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4. Objective ADSL will enable carriers to o‚oad data trac onto a separate packet or cell-switched overlay network. As a service provider, Telecom Italia can o€er this type of service as a managed network (OSI layer 2) where a customerÕs remote site is connected to its regional site or to its headquarter via a permanent virtual circuit (PVC). It is also possible to provide a managed service, where the router installed at a customerÕs premises is run by a 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 trac towards the service providerÕs IP backbone [7]. Based upon the ADSL Forum recommendations and relating technical report TR-002, the goal of the proposed access network con®guration is to evaluate the network performance of two integrated technologies, ATM and ADSL for the distribution of broadband multimedia and Internet services. 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 con®gurations: · 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 Internet service providers (ISPs) connected with high-speed links from Telecom ItaliaÕs CO. These high-speed links are part of Telecom ItaliaÕs 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 Telecom ItaliaÕs own regional broadband network to provide direct high-speed connectivity to the corporate network. This has the advantages of higher speed and greater security.

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· Local content. Locally hosted content can be delivered at high-speed without going through the Internet. Local content may be stored at the point-of-presence (PoPs) of the ISPs, content providers or the COs and regional operation centres of 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 increase 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 point-to-point protocol (PPP); any new broadband Internet access solution must take this architecture into account. Ideally, the broadband service model for accessing ISP service can reuse most of the networking, management and administrative infrastructure (such as IP address and domain name administration) and will not require a paradigm shift for the ISP. · 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 networkÕs 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 nonwork related reasons (such as entertainment)

·

·

·

·

·

·

without using 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 telecommuterÕs PC. 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. Security. Telecommuters and branch oces must be able to communicate with the enterprise in a fashion that supports authentication, authorisation 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 o€ered in audio and video on the Internet, and multicast is the preferred delivery mechanism. A ®rst-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 ®ts all'' paradigm. Di€erent classes of service are required to satisfy the di€erent needs of, for example, power users vs occasional users. Such di€erences 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 quality of service (QoS) to ensure their performance. QoS 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 subscriberÕs customer premises equipment (CPE) requires a transport

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mechanism, a means of controlling information ¯ow, and interfaces into the customerÕs CPE to carry information regarding network and service provider con®gurations. The user modem and PC must be automatically con®gured to match the ATM network characteristics set by the carrier. The network con®guration 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 userfriendly provider name. 6. ATM/ADSL networking The new focus on Internet and corporate LAN access has forced change in access network architectures. Recognising that the 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 recognised the power of ATM to support a future of mixed services (data, video, voice) and management of 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 trac [8]. ADSL modems accommodate 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 a near-term ADSL networks approach with 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 ®les and other resources available

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from the corporate network, again using IP. The original plans for ATM access networks envisaged 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 e€ort of providing service to a new customer, and would also permit the customer to freely roam among ISPs. PVCs will work well for early market entry ADSL networks; however, PVCs cannot be scaled as the network grows to support millions of customers with thousands of circuit changes every day. The long-term answer ± SVCs that connect and disconnect under customer control just like telephone calls ± has proved far more dicult and costly to implement than ®rst expected. In a re®nement to the ATM access network architecture, PPP over ATM was adopted by Telecom Italia since it meshed well with the protocols and operating methods of ISPs. For example, ISPs utilise AAA (authentication, authorisation, accounting) security servers in their access con®gurations. 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 autocon®guration functionality that high-speed remote access networks will demand. Essential operational functions can be delivered over ATM using features well established in PPP: · Authentication (password authentication protocol (PAP), challenge handshake authentication protocol (CHAP) and token-based systems); · Layer 3 address autocon®guration (e.g., domain name autocon®guration and 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);

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home (telecommuting) or from branch oces. To implement ADSL, the bottom line is that an ISP needs access to copper lines. An incumbent Telco, of course, has copper already. If the ISP is not a Telco then the ISP must gain access to copper. An ISP would actually need to lease a copper wire that runs from the customer to its PoP by contracting with the telephone company, and then re-sell access to the 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 becoming more and more common these days). If the ISP leased only a wire that runs from the customer into the CO building, then the ISP would need to install its own ADSL equipment within the CO building. Since the Telco owns the CO, an ISP would have to lease a co-location space.

· Encryption; · Compression; · Billing and usage metering. Adapting the PPP suite to ADSL can happen with little or no extra e€ort 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 the following subnetworks: the customer premise network, the access network, the regional network and the service provider networks. They are shown in Fig. 3. · 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

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

Fig. 3. End-to-end ADSL-based broadband network architecture.

LAN

Voice Network (PSTN)

Splitter

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· Regional broadband network. A regional broadband network, based on a SONET infrastructure, interconnects the COs within 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 ADSL termination unit-CO (ATU-C) 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 Fig. 3 shows them as separate pieces, the CO POTS splitter may be a part of DSLAM. In some cases, all this hardware may actually be contained on the voice switch line card itself, with the concentration of the trac 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 an important asset for network operators because it allows them to o€er di€erentiated 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 premises. Customer premises include residences, home oces and small business oces. Each may contain one or more PC (or workstation). 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 network interface card (NICs), one for connecting to an ADSL modem (or serving as the ADSL modem in the case of an internal ADSL modem card)

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and the other for the LAN. The ADSL modem on the customer premises is called the ADSL termination unit-remote (ATU-R). The POTS splitter is installed at the telephone company demarcation point. The POTS splitter serves to split o€ 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 premises 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. Testbed The testbed 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 (VoD) services (home shopping, games and movies on demand). For this purpose, the following network equipment is used (see Fig. 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), and thus reproducing both the AC and DC characteristics of the line. The DLS 400 contains hundreds of segments of cable simulation that can be matrixed together in various con®gurations and line lengths. In analogue terms, the DLS 400 reproduces cable accurately up to 1.5 MHz and in some con®gurations 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. · The Westell SuperVision CAP RADSL access system consists of a remote unit (ATU-R) located at the subscriberÕs premises and an access line module (ATU-C) located within the SuperVision DSLAM system at the central exchange of®ce. ATU-Cs are available with or without integral POTS splitters. The SuperVision system

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S. Milanovic, A. Maglianella / Computer Networks 34 (2000) 965±980 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

LAN

Console

Fig. 4. Testbed layout.

o€ers 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 outof-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 o€ers an ATU-R with a standard ATM25 interface that meets ATM forum speci®cations. 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 o€ers a high-speed copper highway in the local loop, enabling support of high-capacity interactive multimedia applications as well as a conven-

tional 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 con®gured to operate at the highest possible speed. The operator con®gures through the management system a maximum and minimum speed, as well as a required margin. Then the system selects the maximum upstream and downstream rates within this speci®ed range that are feasible based upon the speci®c loop and noise impairments. This innovative system will support applications such as high-speed Internet access, remote LAN access and VoD. · The BPX StrataCom switch emulating an ATM access switch at the CO. Connections are established via PVCs utilising PPP over ATM model. · An ADTECH AX4000 ATM test system was used for protocol analysis and testbed performance ®ne-tuning in the ATM environment. · A Cisco 7505 series router emulating content provider network for distributing WWW, Telnet

S. Milanovic, A. Maglianella / Computer Networks 34 (2000) 965±980

and FTP services to a telecommuter over ADSL carrier network. · A Cisco 3600 series router emulates a branch of®ce gateway to the external network. ATM 25 Mb/s network modules allows branch oces to take advantage of the higher bandwidth and QoS available with ADSL service. · A video server providing video content to subscriberÕs home via ADSL technology. VoD is a video streaming technology developed by NEC. Video streaming means that a client computer plays video as it receives it over the network from a video server. Rather than having to download a video ®le 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 motion picture expert group (MPEG) compressed format. This means that, upon request, the client will receive high de®nition 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 the 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 and the addition of more disk storage space and additional servers can cater for more VoD clients. The NEC VoD system allows viewers to pause, rewind and 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 NIC. 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. QoS of the video stream can be guaranteed to the PCs connected to an ATM network. However,

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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. High trac in the Ethernet environment results in continuous video with a slightly degraded quality [9]. · An NT server providing WWW, FTP and Telnet services. NT server and workstation boxes are equipped with an Oki ATM 25 Mb/s NIC each, while PC with AccessVision software for DSLAM management is equipped with Ethernet NIC. 8. Measurements and results Referring to the testbed con®guration illustrated in Fig. 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 7.1 Mb/s and under a constant upstream of 1.088 Mb/s in the presence of various noise signals: · white noise (signal due to irregular movement of electrons in conductor grid caused by thermal e€ects), · AMI (AM interference), · metallic noise (interference due to metallic oxide caused by phone wire corrosion), · cross-talk due to 1 adjacent ADSL line, · cross-talk due to 10 adjacent ADSL lines, · cross-talk due to 24 ADSL lines, · type A cross-talk due to 1 adjacent ADSL line, · type B cross-talk due to 1 adjacent ADSL line, · cross-talk due to 1 adjacent HDSL line, · noise provoked by two di€erent cross-talk signals: one is due to 1 adjacent ADSL line and the other is due to 1 adjacent HDSL line. Table 1 shows measured values relating to phone cable length for various wire diameters under different noise signals while keeping a constant downstream of 7.1 Mb/s and an upstream of 1.088 Mb/s. Data from Table 1 are illustrated in Fig. 5.

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S. Milanovic, A. Maglianella / Computer Networks 34 (2000) 965±980

Table 1 Noise impact on signal propagation over phone cable Phone wire diameter

White noise

AMI

Metallic noise

1 ADSL line

10 ADSL lines

24 ADSL lines

ADSL type A

ADSL type B

HDSL

ADSL + HDSL

é 0.63 mm 3 km of é 0.63 mm + the rest of é 0.4 mm é 0.4 mm

4400 m 4050 m

4350 m 4200 m

3850 m 3600 m

4400 m 4200 m

4400 m 4050 m

4350 m 3900 m

4400 m 3950 m

3800 m 3650 m

4350 m 3850 m

4000 m 3450 m

3000 m

3150 m

1850 m

3000 m

2650 m

2400 m

2450 m

1550 m

2350 m

2000 m

Fig. 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.

9. Conclusion The results of testing demonstrate that ATM/ ADSL is able to send data/voice/video services downstream/upstream at a high rate within a range of nearly 4.5 km over ordinary subscriber loops without any cell loss. However, there is a physical limit for copper cable due to corrosion or ageing of a junction that could make e€ective results inferior

compared to expected laboratory performance. 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 deployment by local telephone companies entering new markets to deliver information in video and multimedia formats. By bringing the Internet into homes and small businesses, along with corporate

S. Milanovic, A. Maglianella / Computer Networks 34 (2000) 965±980

LANs, movies, television, video catalogues and remote CD-ROMs, ADSL will make these markets viable and pro®table for telephone companies and application suppliers alike. In order for a mass-market broadband-access network to develop, it requires low costs and ¯exible services for market segments. By leveraging the worldÕs 800 million phone lines, ADSL o€ers the most viable option for providing virtually ubiquitous high-speed remote access and interactive multimedia transmission. The key issue for ADSL services is how to con®gure and manage concurrent connections from end-users to their multiple service destinations, end-to-end [10]. For the future, as ®bre moves gradually to the customers, another variant of ADSL±VDSL (veryhigh speed DSL) with capacities of up to 52 Mb/s will enhance even further the use of those simple copper pairs. It will provide a smooth upgrade path for operators, enabling them to transport ever more sophisticated services to ever more sophisticated users. In fact, it is likely that combinations of ®bre feeding access nodes and VDSL feeding premises over copper (so called ®bre to the neighbourhood, or FTTN) will be the next generation network topology for telephone companies in most countries of the world.

ATM AWG

Broadband CHAP

Core network Digital signal processing Downstream 1 foot [0 ] HDSL

Acknowledgements The authors would like to thank Mr. Fabio Romano, Manager with Telecom Italia, for his patience, support and encouragement.

Appendix A. Glossary ADSL AMI

Asymmetrical digital subscriber line (OSI layer 1 communication protocol) AM radio frequency interference is a type of narrowband interference. Narrowband interference a€ects a small number of frequencies over a longer period of time

IP IPX LANE L2F

L2TP

979

Asynchronous transfer mode (OSI layer 2 communication protocol) American wire gauge is a wire diameter speci®cation; the lower the AWG number, the larger the wire diameter A communication channel with a bandwidth in excess of 1.54 Mb/s 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 A combination of switching oces and transmission plant that connects switching oces together This is a process performed by a specialised microchip, which converts from analogue to digital signals at very high speeds Trac moving towards the customer premises 0.3048 m High bit-rate digital subscriber line is a symmetric technology, providing 1.544 Mb/s over two copper pairs and 2.048 Mb/s over three pairs Internet protocol (OSI layer 3 communication protocol) Internetwork packet exchange (OSI layer 3 communication protocol) ATM LAN emulation 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) A Layer 2 tunnelling protocol is an extension to the PPP protocol used for virtual private networks (VPNs)

980

Modulation

MPOA Multiplexer OC-3c OSI

PAP

PPTP

Protocol SONET S-VHS Upstream VCR

S. Milanovic, A. Maglianella / Computer Networks 34 (2000) 965±980

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 Multiprotocol over ATM A device that makes available the same communication line to be shared among various signals OC-3c (also known as STM-1) is 155.52 Mb/s ®ber interface Open system interconnection is a layered network model. For the OSI model, seven numbered layers indicate distinct networking functions within a communication environment 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'' 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 Set of rules being used between two entities while data communications takes place The physical interface function providing the capability for using FC, SC or ST connectors Super video home system Referencing the ¯ow of information from the subscriber to the CO (and beyond) Video cassette recorder

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, 21 April, 1998. [3] An interoperable end-to-end broadband service architecture over ADSL systems, ADSL Technology Briefs, 3Com, 3 June, 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, 3Com Corporation, 1998. [6] George Abe, Residential Broadband, Internetworking Technologies Handbook, 2nd ed., Cisco Press, Cisco Systems, 1999. [7] La promessa di ADSL: lÕalta velocit a per tutti, NetWorking Italia, May 1998. [8] Tom Starr, Kim Maxwell, ADSL Access Networks, May 1998. [9] 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.

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 as 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 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 ÔEXANETÕ (Experimental ATM Network) in Rome.