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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, the 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 fewer 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 are 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 the 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 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 bitstream 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 interleaved 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 interleaving 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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