EE 1’imes - ADSL Teolnology Exddned, Pat 2: Getting totheApplication Layer
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`ADSL Technology Explained, Part 2: Getting to the
`Application Layer
`Louis Litwin, Michael Pugel, Rob
`Rhodes, and John Richardson
`4/2/2001 12:11 PM EDT
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`ADSL Technology Explained, Part 2: Getting to the Application Layer
`The main physical (PHY) layer structure of asymmetric digital subscriber line (ADSL)
`technology, as explored in part one of this article, involves a modulation scheme known as
`discrete multitone (DMT). In this second part, we examine other PHY layer components,
`move on to the handshake and initialization process of ADSL, and finally end with a look at
`the network and application layer protocols.
`
`The phone line interface in ADSL is an analog circuit, but the signal processing for a DMT
`signal is done digitally, so an interface is needed between these two circuits. An analog-to-
`digital converter (ADC) is connected in the receive path of the hybrid to interface
`downstream signals at the home, while a digital-to-analog converter (DAC) is used to
`connect into the transmit path for upstream data.
`
`Multicarrier systems contain many simultaneous signals. If each signa|'s peak amplitude is
`represented by x, and all signals simultaneously reach peak signal level, the resulting level
`would be x*20IogN, where N is the number of signals. The signals in a DMT system have a
`statistical nature — they can be considered uncorrelated random processes (that is, their
`cross-correlation is equal to zero). The possible peak amplitude may be large, but the
`probability of this level occurring is low.
`
`The peak-to-average ratio (PAR) is used to define the ratio between a signal's peak level
`and its average level over time. Most multicarrier systems use a modified definition for PAR
`that is based on the statistical likelihood of exceeding a certain peak level (such as the
`probability of clipping in the DAC output). For 256 subcarriers and a clipping probability of
`
`10'7, the PAR value is around 5 (14 dB). The PAR value partially determines the operating
`parameters for the ADC and DAC.
`
`The important parameters associated with the design of the converter for ADSL include the
`PAR factor mentioned above, the number of bits per subcarrier, and the required signa|-to-
`noise ratio (SNR). Typical DACs use 10-b resolution, a level considered acceptable for up
`to 8 b per subcarrier.
`
`The ADC must take into account all of these parameters, plus additional bits of resolution
`for input noise (receiver SNR is lower than transmitter SNR) and for echo energy. Typically
`one to two extra bits are employed in the ADC.
`
`Other ADSL components
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`EE Times - ADSL Technology Explained, Part 2: Getting to the Application Layer
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`The POTS splitter is a passive three-terminal circuit which is key to the coexistence of the
`existing phone service and ADSL service. The splitter contains a common terminal (from
`the phone plant end of the service), a low-pass filter to the POTS side, and a high-pass
`filter to the ADSL side. The filters are designed to diplex the signals onto the outside phone
`line and have stopband impedance characteristics that minimize the effect of changing
`from "on hook" to "off hook" condition.
`
`The residence modem also contains several other blocks. A hybrid simultaneously
`connects the transmitter and receiver to the same copper wire. The hybrid contains the line
`conditioning and gain control for proper operation over a range of signal levels.
`
`The digital circuits connected to the DAC and ADC converters contain the signal processing
`and memory necessary to perform the demodulation and data conversion to get the
`information from the phone wire to the home device.
`
`The equipment in the central office (CO) is similar to the equipment used in the home
`except it is structured as a bank of modems. These modems, one for each home in use,
`along with some network and phone interfacing equipment, comprise the device known as
`a digital subscriber loop access multiplexer (DSLAM).
`
`Phone systems employ tricks to extend the usable range of their system. One such trick is
`to employ load coils in loops that potentially extend far from the CO (more than 3 miles or
`16 kft). The copper wire will attenuate frequencies in the voice band at these distances,
`degrading operation. However, the revised resistance design (RRD) distance may not have
`been reached yet, so switching operation is still possible.
`
`In order to maintain voice service operation, load coils are added in series with the line at
`periodic distance intervals. These coils are used to compensate for the effect of the cable
`capacitance through the voice band region on the line. As a result, the frequency response
`above the voice band (ADSL spectrum) rolls off at an accelerated rate. Load coils typically
`need to be removed from phone lines that will carry ADSL services. In some cases the
`plant must be re-engineered to compensate for the missing load coils.
`
`ADSL has evolved to the point of receiving attention at a standards level. In 1998,
`agreement was reached on a set of standards for ADSL. G.992.1 is part of a suite of
`standards (the G.99x.x series) covering several DSL systems as well as protocols and
`tests. Key PHY layer specifications are outlined in Table 1.
`
`Table 1: G.992.1 PHY Layer Parameters
`
`Overall symbol rate
`Number of carriers per DMT
`
`Subcarrier spacing
`
`Cyclic prefix length
`Operational modes
`FDM mode frequency range
`
`|4 kHz
`symbo|256
`
`4.3125 kHz
`
`|32 samples
`IFDM or echo cancelled
`64 to 1100 kHz
`
`Echo cancelled mode frequency range
`
`13 to 1100 kHz
`
`Number of bits assigned per subcarrier l0 to 15 (no bits assigned to 64k QAM)
`Synchronization
`Pilot tone at subcarrier 64, f= 276 kHz
`
`Number of subcarriers per DMT symbol
`l4 samples
`Cyclic prefix length
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`EE Times - ADSL Technology Explained, Part 2: Getting to the Application Layer
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`The PHY layer specification outlines power levels and spectral masks that must be
`maintained.
`
`The lower three to six subcarriers are set to a gain of ‘'0'’ (turned off) to permit operation of
`POTS, with the addition of a splitter at the home entry point of the phone line.
`
`It is important to note that these standards encompass only a framework for operation.
`Individual networks and providers are free to adapt their system within that framework. The
`standards provide a set of boundaries for equipment manufacturers.
`
`In the beginning
`
`When an ADSL termination unit-residence (ATU-R), or modem, is first connected to a DSL
`network, it goes through a fairly extensive initialization process. This process identifies and
`qualifies both the capabilities of the network equipment and of the underlying physical
`infrastructure.
`
`The initialization process consists of four major phases. The first phase is a handshake
`using the G.994.1 or G.hs protocol. (G.hs is a precursor to the G.992.1 specific initialization
`and is used by other DSL and telecommunication devices.). The remaining three phases -
`transceiver training, channel analysis, and exchange - are covered directly in the G.992
`standard and apply specifically to standards-based ADSL networks.
`
`The G.994.1 handshake is used to determine the nature and capabilities of the endpoints
`(such as an ADSL modem) and to indicate which protocol will be used for the remainder of
`the initialization. The signaling method used for the handshake interchange is designed to
`be robust to address channel characteristics that could be atrocious. Biphase shift keying
`(BPSK) modulation is used to modulate multiple single-tone subcarriers, all carrying the
`same data.
`
`The subcarriers used are selected based on the typical impairments likely to be present in
`a given global region. The handshake has several possible variants, but, fundamentally,
`the two endpoints exchange a message which contains information about the endpoint
`type, and a number of related subparameters such as the frequency range and number of
`DMT subcarriers supported.
`
`The second phase of initialization is transceiver training. Receivers at each end of the line
`acquire the DMT symbol stream, adjust receiver gain, perform symbol timing recovery, and
`train any equalizers. There is an optional echo cancellation training step that can also be
`performed during this phase, but the specification does not define the training signal to be
`used.
`
`Characterization
`
`The transmitter power at each end of the line is set to a predetermined level at this phase,
`allowing a preliminary estimate of loop attenuation by the receivers. The received upstream
`power level is reported back to the ATU-R transmitter to allow limited power level
`adjustment (attenuation), if needed, to meet spectral mask requirements. The training
`phase is conducted with all available upstream and downstream subcarriers modulated,
`using two of the four constellation points of a QPSK constellation.
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`EE Times - ADSL Technology Explained, Part 2: Getting to the Application Layer
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`In the third phase, the transceivers exchange capability information and perform detailed
`channel characterization. For example, the ADSL termination unit-CO (ATU-C) specifies
`the minimum SNR margin for the system and whether it can support functions such as
`trellis coding and echo cancellation. Similar information is exchanged about the ATU-R.
`Although some of these same parameters were exchanged during the G.994 handshake,
`the handshake message parameters are used to gather information only and are not
`necessarily used for the connection.
`
`During this third phase, both transceivers attempt to measure specific channel
`characteristics such as unusable subcarriers, loop attenuation on a per subcarrier basis,
`SNRs, and any other channel impairments that would affect the potential transmitted bit
`rates. Based on the discovered channel characteristics, the ATU-C makes the first offer of
`the overall bit rates and coding overhead that will be used for the connection.
`
`Four possible rates are offered, in decreasing order of preference. In the current release of
`the ADSL standard, the ATU-C completely controls the final bit rate. All subcarriers are
`modulated simultaneously with the same information. The primary tool for channel
`measurement is a pseudo-random bit sequence.
`
`Setting the rates
`
`The last phase of the initialization sets the final overall transmission rates in both the
`upstream and downstream directions for the connection. These final rates are determined
`based on calculated channel parameters measured during the channel analysis phase,
`and are not necessarily the same as the preliminary rates offered during that phase.
`
`As the ATU-C controls data rates, if the ATU-R cannot support any of the offered rates,
`both terminals will return to the beginning of the initialization process. Otherwise the ATU-R
`responds with the rate it can support.
`
`Since ADSL uses multiple orthogonal subcarriers, each subcarrier can be assigned a
`modulation format (number of bits per subcarrier) and relative gain independently. The
`ATU-C assigns bits and gains for the downstream direction while the ATU-R assigns the
`upstream parameters.
`
`The resulting assignment maximizes the amount of traffic that can be carried over the PHY
`layer. The last part of the exchange phase is a synchronized transition from the highly
`robust BPSK and QPSK modulations used during the initialization to the full traffic rate
`modulations (such as higher-order QAM) assigned during the exchange phase. At the
`conclusion of the initialization steps, the system is ready to pass higher-layer traffic.
`
`ADSL and ATM
`
`Along with TCP/IP, ATM, with its key layers and its relationship with ADSL, is one of the key
`enabling technologies for broadband applications using ADSL.
`
`ADSL uses a framed transport structure in which frames are encoded and modulated into
`DMT symbols. The ADSL frames can be further grouped into superframes, each consisting
`of 68 frames. An ADSL frame is sent every 250 microseconds, therefore a superframe is
`sent every 17 ms.
`
`In full-rate ADSL, a frame can be broken down further into two parts, each being 125
`microseconds. These two parts can be classified as the fast data path and the interleaved
`data path. The fast data path could have a higher bit error rate (BER) because the
`interleaver is not used to mitigate the effects of impulse noise. However, the removal of the
`interleaver significantly reduces the latency of this data path (which is well suited for time-
`sensitive information such as interactive audio and video).
`
`ATM, which allows data to be sent asynchronously, uses cells consisting of 53 B of
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`EE Times - ADSL Technology Explained, Part2: Getting to the Application Layer
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`information. The cells‘ small size allows the efficient multiplexing of data from multiple
`sources. ATM is connection-oriented - once a connection is established, the connection
`carries traffic that meets the quality of service (Q08) requirements requested by the source
`and destination.
`
`The ATM and ATM adaptation layer (AAL) are the two most important of the several layers
`in the ATM protocol stack. The ATM layer is responsible for the definition of logical
`connections through the network. Logical connections in ATM are known as virtual circuits
`(VCs).
`
`VCs represent fundamental ways of switching in an ATM network; a VC is established
`between two end users on the network. Bundles of VCs are called virtual paths (VPs) and
`they share the same end-point. A single VP carries the cells from multiple VCs, and the
`cells are subsequently switched together.
`
`The AAL is responsible for inserting higher-layer information into cells to be transported
`over the network. It consists of two sublayers called the segmentation and re-assembly
`(SAR) sublayer and the common part convergence sublayer (CPCS). The SAR sublayer
`segments the upper-layer protocol data units (PDUs) into 48-B cell payloads (SAR PDUs).
`The SAR PDUs are then passed to the ATM layer to form a complete cell.
`
`The CPCS is responsible for performing functions for different classes of service. These
`are referred to as AAL1-5. A summary of the types of services is shown in Figure 1.
`
`AAL5 is most often used for connectionless Internet traffic, as it allows the entire 48 B of
`cell payload to transport data with minimal overhead. A 10% overhead is typical when
`transporting Internet traffic over ATM.
`
`Figure 2 illustrates a basic ADSL network architecture representing the connection
`between the service provider and the customer. An ATM connection is set up between the
`ADSL modem and a termination point in the back-end network. This connection is referred
`to as a permanent virtual circuit (PVC).
`
`The deployment model for ADSL is based on the current dial-up system, which uses point-
`to-point protocol (PPP) to support network services such as authentication and client
`addressing. The ADSL Forum recommends this PPP-over-ATM-over-ADSL model, and it
`has become the standardized method for accessing data networks over ADSL. For the
`ever-popular encapsulation method of transmission, the Internet Engineering Task Force
`(IETF) has defined a method called RFC-2364 for PPP encapsulation over AAL5.
`
`A DSLAM is used in all instances of ADSL deployments to aggregate traffic. With this
`device, the data from the ADSL modems is statistically multiplexed onto a common
`upstream link that interfaces to an ATM network. The ATM switch then routes the cells to
`their destination based on the cell header information. This destination is an IP router that
`
`reassembles the data cells into packets for transmission across the Internet.
`
`Advanced services
`
`ADSL is primarily deployed for Internet connectivity. Recently, other services have become
`available. For example, ADSL allows the service provider to offer multiple voice lines over a
`single copper pair. Instead of the voice being carried by POTS, it is now packetized directly
`in ATM and carried over the ADSL link. The voice packets are routed to a point in the
`service provider's network that interfaces through a gateway into the POTS network.
`
`ATM's Q08 capabilities allow the network to deliver acceptable voice-quality services. High-
`quality audio and video streaming are other potential services that can be offered using
`ADSL.
`
`The demand for bandwidth and high-value content and services is soaring in both the
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`http://www.eetimes.com/docum ent.asp?doc_id=1203659&print=yes
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`EE Times - ADSL Technology Explained, Part 2: Getting to the Application Layer
`
`home and office, and ADSL provides a robust and cost-effective mechanism to meet the
`demand. Existing infrastructures can often be reused, providing for fast and economical
`deployment.
`
`Because ADSL was designed to be consumer-friendly, it can coexist with POTS. By using
`existing voice circuits instead of a shared broadcast medium, ADSL can better individualize
`services. The roll-out of ADSL will continue to increase as business models are created to
`
`exploit the capabilities, both business and technical, that are available.
`
`Rob Rhodes is manager of the Communications Design group at Thomson Multimedia
`(Indianapolis, IN). The group is responsible for the development of the PHYinterfaces for
`satellite, cable, and DTV communications. He can be reached at rhodesr@tce.com.
`
`Mike Pugel is a principal member of the technical staff at Thomson Multimedia
`(Indianapolis, IN). He is currently working on advanced communication receiver concepts.
`He can be reached at pugeIm@tce.com.
`
`Louis Litwin is a member of the technical staff at Thomson Multimedia (Princeton, NJ).
`His focus is the development of wireless communication devices used for digital home
`networking and mobility applications. He can be reached at litwinl@tce.com.
`
`John Richardson is a member of the technical staff at Thomson Multimedia (Princeton,
`NJ). He is primarily involved in the areas of digital home networking and ADSL system
`development. He can be reached at richardsonj@tce.com.
`
`Resources
`
`1. Busby, M., Demystifying ATM/ADSL, Worldware Publishing, TX, 1998.
`
`2. Chen, W., DSL: Simulation Techniques and Standards Development for Digital
`Subscriber Line Systems, Macmillian, IN, 1998.
`
`. Cioffi, J., Silverman, P., and Starr, T., Understanding Digital Subscriber Line
`Technology, Prentice Hall, NJ, 1999.
`
`. Goralski, W., ADSL and DSL Technologies, McGraw-Hill, NY, 1998.
`
`. Rhodes, R., Pugel, M., Litwin, L., and Richardson, J., "Digital Subscriber Line
`Technology Tutorial," International Conference on Consumer Electronics, Los
`Angeles, CA, June 11, 2000 (invited tutorial).
`
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