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HUAWEIs Access Network ADSL/ADSL2+

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Hi. To continue with the the XDSL technologies, this posts talks about the ADSL and ADSL2+

ADSL/ADSL2+ Technical Priciples Part II


Initializing the ADSL System


·        The number of available sub-carriers mentioned above depends on the result of channel analysis during the initialization of the ADSL system. This part describes the process of ADSL initialization in detail.


·        The purpose of ADSL initialization is to test the performance of the actual channel, coordinate the transmission configurations between the ATU-C and ATU-R (such as the upstream and downstream bandwidths and the number of sub-bands), and exchange various parameters to establish a usable communication link before the ATU-C and ATU-R start working.


·        The initialization process can be triggered by either the ATU-R or ATU-C.


·        In initialization triggered by the ATU-C, the ATU-C sends an activation request and waits for a response from the ATU-R upon system power-on, signals loss, or self-check completion. The ATU-C performs this process for a maximum of 2 times. If the ATU-R does not respond, the ATU-C waits for the ATU-R to send an activation request or waits for the network to send a retrial instruction.


·        In initialization triggered by the ATU-R, the ATU-R sends activation requests continuously upon power-on or self-check completion to start initialization.


·        The ADSL initialization process can be divided into four steps: activation request and acknowledgment, transceiver training, channel analysis, and parameter exchange.


·        In activation request and acknowledgment step, necessary handshake communication is performed to prepare for the initialization. Generally, initialization is triggered upon system power-on, signals loss, or self-check completion. In this step, the ATU-R and ATU-C transceivers are enabled and starts initialization handshake, complying with the G.hsprotocol mentioned above.


·        By means of transceiver training and channel analysis, the transceiver can determine the characteristics of signal transmission and related transmission parameters.


·        During parameter exchange, a local receiver exchanges its parameters with a remote transmitter to match the sending and receiving processes. The parameters to be exchanged include the number of bits modulated by each DMT subcarrier, transmission rate, and so on. To ensure optimal system performance, all parameters are determined based on transceiver training and channel analysis.


·        After the initialization is complete, the system enters the normal working state



Comparison Between ADSL2+ and ADSL

·        Better performance

·        Broadband test functions

·        Lower transmit power

·        The ADSL2+ provides a higher rate for subscribers in a short loop within 2.5 large and medium cities in China, more than 80% os subscribers loops is within the 2.5 km


·        The ADSL2 standard adopts an enhanced modulation mode, which can better reduce the impact of line noise on signals, obtain higher line encoding gain, and increase the connection rate. ADSL2 uses variable overhead bits with an overhead rate of 4–32 Kbps while the ADSL overhead rate is fixed at 32 Kbps. It can be considered that ADSL2 increases the speed by 50 Kbps and transmission distance by 200 m compared with ADSL (6% greater coverage area).


·        The ADSL2 standard requires the line noise and signal attenuation of each carrier in DMT mode to be tested to determine whether ADSL services can be provisioned on the line. It also requires real-time monitoring over ADSL connections.


·        The ADSL2 standard implements traffic-based power control. When a large amount of data needs to be transmitted, for example, during large file download, the line power increases to the standard ADSL power level L0. When a small amount of data or no data needs to be transmitted, for example, during web pages browsing, the power decreases to L2 and L3 specified in the ADSL2 power levels. Decreasing the line power can effectively reduce the crosstalk between line pairs.


Main Features of ADSL2+

1.   New running modes are added.

·         There are 3 ADSL running modes: ADSL over POTS (ADSL annex A in which the POTS service exists on the same line pair), ADSL over ISDN (ADSL annex B in which the POTS service exists on the same line pair), and ADSL annex C (ADSL in the TCM-ISDN crosstalk environment, which is mainly used in Japan). In addition to the preceding 3 modes, the following modes are added to the ADSL2/ADSL plus:


·         Annex I which specifies a full digital mode with a spectrum compatible with that in annex A (ADSL over POTS). In this mode, there is no POTS service on the line, the upstream spectrum is 3–138 kHz, the number of sub-bands is 31, and the upstream bandwidth is greater than 1 Mbps.


·         Annex J which specifies a full-digital mode with a spectrum compatible with that in annex B (ADSL over ISDN). In this mode, there is no ISDN service on the line (used when ADSL over ISDN coexists), the upstream frequency band is extended to 3–276 kHz, a maximum of 64 upstream sub-bands are supported, and the maximum upstream rate reaches 2.3 Mbps.


·         Annex M which extends the upstream bandwidth of ADSL over POTS. In this mode, the number of upstream sub-bands starts from 6, and increases to 32, 36, 40, 44, 63 depending on the bandwidth requirement. In this way, with the same total transmit power, Annex M achieves the upstream transmission rate of Annex J, and achieves the downstream transmission rate of Annex B in overlap and non-overlap modes.


·         Annex L (READSL2) which extends the transmission distance.


·        In addition, the ADSL standard supports only STM (such as PCM interface) and ATM (UTOPIA) interfaces, while ADSL2/ADSL plus also provides PTM (packet) interfaces to carry HDLC on ADSL in non-ATM transmission mode.


2.   Higher Transmission Rate

§  The modulation rate is improved, the encoding gain is improved, the frame header overhead is reduced, the initialization state machine is improved, and an enhanced signal processing algorithm is adopted.ADSL2 improves the modulation efficiency by making trellis encoding and 1-bit constellation encoding mandatory.

§  Less Overhead: in the first generation of DSL standard, the overhead is fixed to 32 Kbps. In ADSL2+, the overhead rate can be adjusted within 4-64 Kbps, which generates obvious effects in the case of a line transmission line.


§  ADSL2+ uses a wider frequency (tone 32-511) and more sib-bands (512) to support a maximum downstream rate of 24Mbps



3. Longer transmission distance

·        ADSL2/ADSL plus supports a transmission distance no less than 6.5 km at rates of 192 Kbps/96 Kbps.


·        ADSL2 supports the 1-bit constellation while ADSL supports a minimum constellation of 2 bits.


·        ADSL2 annex L adopts new spectrum division. When the distance exceeds 4 km, the sub-bands above tone 128 are turned off, and the transmit power of sub-bands with lower tones are increased to extend the transmission distance.


·        The frame overhead can be flexibly configured to provide a 28 Kbps bandwidth, which is very important in long-distance transmission.


·        The tone ordering and pilot tones determined by a receiver can reduce the probability of activation failures due to ADSL pilot tones with an excessively low SNR. In addition, the 2 bits on the pilot tones can increase the bandwidth by 8 Kbps.


4.   Lower power consumption

·         The noise margin is reduced by reducing the transmit power. In this way, unnecessary power consumption is saved while the system stability is ensured.


·        The new low-power mode L2 reduces the transmit power to 30% of the normal power when no data is transmitted. In L2 mode, the power is sufficient for transmitting only necessary management messages and synchronization signals (for example, 1-bit constellation). The power can be quickly restored when subscriber data is transmitted.


·        The CO and CPE of the ADSL2/ADSL plus provide the power cut back function in the range of 0–40 dB to effectively reduce the transmit power during normal operation. (In an ADSL system, only the CO has this function in the range of 0–12 dB.)


4.   More stable running and better spectrum compatibility

·         The receiver determines the tone ordering based on the channel analysis result and selects the best tone as the pilot tone, making ADSL connections more stable.


·         Tones are disabled during the training. The receive end tests the RF interference (RFI) signal distribution to avoid RFI and reduce the crosstalk to other line pairs.


·         Excellent dynamic adaptability: The enhanced bit swap dynamically changes the line rate.


·         Power cutback to a maximum of 40 dB in the receiver and transmitter reduces the near-end echo and crosstalk.


·         The receiver determines the pilot tone to prevent activation failures caused by line bridge connectors or AM interference.


·         The training process is shortened to quickly recover connection synchronization from errors.


6. Line diagnosis function

·         Supports the dual-end test function. The CO and CPE can be trained, and line parameters can be obtained through a dedicated line test process.





7. Dynamic rate adaptation

·        The Seamless Rate Adaptive (SRA) technology is used to resolve crosstalk and AM interference, and adjust the connection rate without being perceived by subscribers to adapt to environment changes.


8. Rate binding

·        To provide different QoSsfor different customers, ADSL2 adopts the IMA technology to bind two or more copper wires as an ADSL connection, which can flexibly increase the access rate.


9. Better interoperability

·        The ADSL2/ADSL plus divides the ADSL transceiver into multiple sub-layers according to functions.


§  Transmission protocol convergence sublayer(TPS-TC)

§  Physical medium convergence sublayer(PMS-TC)

§  Physical medium sublayer(PMD)

§  Management protocol convergence sublayer(MPS-TC) for network management interfaces


·        Each sublayeris encapsulated and messages between sub-layers are defined. In this way, devices from different vendors can communicate with each other.

With this posts we almost finish the themes for the XDSL technologies. The following links will leave you to the precedent posts:

HUAWEIs Access Network XDSL Principles and Applications

Created: 27 Minutes ago  8  0  0  0View the author #1

Hi. In this post we will discuss the ADSL/ADSL2+ Technical Principles

ADSL/ADSL2+ Technical Priciples


ADSL OverView


·        ADSL is a DSL technology with asymmetric transmission rates in the upstream and downstream directions. Here, upstream transmission refers to the transmission from the user side to the central office, and downstream transmission refers to the transmission from the central office to the user side.


·        The ADSL downstream transmission rate can reach up to 8 Mbps but the maximum upstream transmission rate is 640 Kbps. The downstream rate is far greater than the upstream rate.


·        The ADSL technology can transmit data signals and traditional analog voice signals at the same time on the same twisted pair.


·        ADSL is a widely used access technology because of its technical features and ease of use.

§  Firstly, asymmetric transmission of ADSL is of special significance. On one hand, in many DSL applications, users usually obtain a large amount of data from the backbone network, but transmit far less data to the backbone network. For example, when a user accesses the Internet and video on demand (VoD) services, a large amount of data needs to be downloaded at a high rate, but only some addresses and simple commands is sent to the backbone network. On the other hand, asymmetric transmission can greatly reduce near-end crosstalk.


§  Secondly, compared with other DSL technologies, ADSL makes it possible to provide traditional voice services in the same twisted pair at the same time. In this way, the

cost of cable routing is saved.


·        In October 1998, the ITU officially released the recommended ADSL standards. G.992.1 and G.992.2.

§  G.992.1 is also called G.dmt. It defines the full rate ADSL technical specifications. The maximum downstream transmission rate is 6.144 Mbps and the maximum upstream transmission rate is 640 Kbps.


§  G.992.2 is also called G.lite. It defines the ADSL technical specifications without using signal splitters. In this type of ADSL system, no splitter is required, which reduces the complexity and cost of device installation but brings about the side effect of reduced signal rates. The maximum downstream rate is 1.536 Mbps and the maximum upstream rate is 512 Kbps



G.dmt Standard ADSL System Composition


·        This is the structure of an ADSL system complying with the G.dmt standard. The ATU-R refers to the modem at the ADSL subscriber side. The ADSL transmission rate is high enough to support a home network or a small office LAN. The data sent from a PC first enters the home or office network. Then the ATU-R converts the data into signals that can be transmitted on the telephone line. To transmit data and voice signals on the same telephone line at the same time, the ATU-R and telephone are connected to the POTS splitter, and the hybrid transmission of data and voice signals on one twisted pair is implemented in different frequency bands.


·        After arriving at a CO, mixed voice and data signals are separated by a splitter at the CO side. Voice signals are transmitted through the telephone network, and data signals are transmitted through the high-speed data network. ATU-C refers to the ADSL CO modem. At the CO side, each subscriber has an independent splitter connected to the ATU-C. Therefore, a DSL network uses point-to-point private line transmission. After passing through the ATU-C, data is sent to the DSL access multiplexer (DSLAM) which aggregates multiple subscriber lines to transmit data streams at a higher rate. The DSLAM connects to the backbone network through high-speed network interfaces such as ATM or STM, and sends data to servers of network service providers through the high-speed data network. Currently, DSLAM devices are generally bound to ATU-C devices.


G.Lite Standard ADSL System Composition


·        In this figure, the upper part shows the ADSL system structure in the G.dmt standard, and the lower part shows the ADSL system structure in the G.litestandard. The difference is that G.liteADSL has no splitter, out-band signals become interference noise signals, and data and voice transmission interfere with each other. Due to the influence of the interference, the transmission rate of the G.liteis much lower than that of the G.dmt. In the ITU G.dmt standard, the maximum downstream and upstream transmission rates are 6.144 Mbps and 640 Kbps respectively. In the G.litestandard, the maximum downstream and upstream transmission rates are 1.536 Mbps and 512 Kbps respectively. It can be seen that the downstream transmission rate of the G.liteADSL is greatly reduced.


·        In the G.dmt standard, a splitter is used at the subscriber side to separate data and the voice signals by frequency to ensure that the two different types of signals can be transmitted in the same twisted pair. However, installing a splitter can be very complicated. Experienced technicians are required to install and commission the splitter, and the telephone lines may need to be reconstructed to some extent. Therefore, it is hoped that ADSL can be used without splitter. The G.litestandard is therefore formulated to implement ADSL with no splitter.



ADLS Modulation Technology

·        Common modulation methods used in DLS products:

§  Quadrature Amplitude Modulation: (QAM)

§  No Carrier amplitude/phase modulation (CAP)

§  Discrete multiTone (DMT)


·        QAM modulation uses a sine and cosine waves with the same frequency component to transmit information. All waves pass through a single channel at the same time. The amplitude of each waveform (including direction) represents a binary bit stream to be transmitted. CAP modulation is similar to QAM but has no carrier signal.


·        DMT combines QAM and FDM technologies. In 1995, ANSI T1.413 stipulates that ADSL line encoding adopts the DMT technology.


·        The DMT modulation and encoding technology improves the frequency utilization and can transmit signals with higher bit rates in a limited frequency band. It divides the entire channel into a maximum of 256 4.3125 kHz subcarriers, and implements 256-point constellation encoding in each discrete subcarrier according to the respective signal-to-noise ratio (SNR). In this way, one symbol in each subcarrier may represent 4 to 8 bits, greatly improving the spectrum utilization and enabling a higher ADSL transmission rate.



Signal Separation Technology


·        In ADSL, data and voice must be transmitted on separate frequency bands. ADSL uses a frequency band higher than 30 kHz, whereas common voice signals are in a frequency band lower than 4 kHz. Splitters are used to separate the data from voice by frequency band.


·        A splitter consists of a 3-port low-pass/high-pass filter group. The low-pass filter can filter allows only low-frequency voice signals to pass and suppresses the interference from data signals. The high-pass filter allows only high-frequency data signals to pass and suppresses the interference from voice signals. The voice and data signals are filtered by a splitter at the subscriber side and then transmitted on the same twisted pair in different frequency bands. Each subscriber-side splitter maps a CO-side splitter which separates the voice/data mixed signals transmitted on the twisted pair. A CO-side splitter also consists of a 3-port low-pass/high-pass filter group in which the high-pass filter separates data signals while the low-pass filter separates voice signals. After separation, voice signals are transmitted through the PSTN network, and data signals are transmitted through a dedicated high-speed data exchange network. In this way, data transmission is not restricted by the PSTN system, and can reach a rate much greater than 64 Kbps.



Interleaving Principle


·        The setting of interleaver parameters has a huge impact on the performance of the ADSL service. To facilitate the understanding of interleaving parameters, we introduce the working principles of an interleavering detail using a typical interleaving mode as an example.


·        First, let's see how an interleaverprocesses data at the transmit end. The elements 1, 2, 3, 4, 5, 6..., and 21 to be transmitted come out from a forward error correction (FEC) encoder, and are stored in a matrix with 3 rows and 7 columns row by row. After the matrix is full, elements are sent to the channel column by column. This is the interleaving process of an interleaver. At the receive end, how does the interleaver-interleave and restore the original data stream? The receive end writes the received data column by column to a matrix with the same size. After the matrix is full, the receive end reads the data row by row, restores the original data, and sends the data to the FEC for decoding. This is the de-interleaving process of an interleaver.


·        So, what is the benefit of an interleaverfor processing burst errors? Let's look at this example.

§  The sequence of the elements to be sent is 1, 2, 3, 4, 5, 6..., and21. Assuming that a burst interference with a length of 3 occurs during transmission and corrupts 3 consecutive bits, the elements reaching the receive end become 1, 2, x, x, x, 6, 7, 8, 9, 10..., and 21 if no interleaving is performed. Three consecutive errors in the received bit stream can result in an FEC failure.


§  If interleaving is performed, the sequence of elements reaching the transmit end becomes 1, 8, 15, 2, 9, 16, 3, 10, 17, 4, 11, 18, 5, 12, 19, 6, 13, 19, 7, 14, and 21.


§  After interleaving, if 3 consecutive bit errors occur and elements 15, 2, and 9 are affected, the sequence of elements received by the receive end after de-interleaving is 1, x, 3, 4, 5, 6, 7, 8, x, 10, 11, 12, 13, 14, x, 16, 17, 18, 19, 20, and 21. It can be seen that consecutive burst errors are separated into 3 non-consecutive errors by the interleaver. In this way, these errors can be corrected through FEC.


·         In this example, FEC can correct burst errors no longer than 3. The number of matrix rows is the interleaverdepth D (3 in this example), and the number of matrix columns is interleaverspan N (7 in this example) which is equal to the number of FEC codes.


·        An actual interleaverusually has larger D and N parameters. Assuming that the FEC sequence with N codes can correct B burst errors, a D x N interleavermatrix can correct burst errors with a maximum length of B x D. It can be seen that interleaving enhances the anti-interference capability and brings better system stability, but interleaving causes a delay. In actual application scenarios, whether to use interleaving and how to set interleaving parameters need to be determined based on service requirements.

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