Hello everyone!
Today, let's learn about xDSL.
xDSL Principles and Applications
xDSL Technology Overview
DSL Standard
Currently, there are many organizations working on DSL standards. The most important organizations are the American National Standards Institute (ANSI), the European Telecommunications Standards Institute (ETSI), and the International Telecommunication Union (ITU). In ANSI, the T1E1 committee is responsible for network interfaces, power, and protection, and the T1E1.4 working group is responsible for the standardization of DSL access. In ETSI, the TM6 workgroup is responsible for DSL access standards.
G.992.1 Annex A 一一 ADSL over POTS
The upstream and downstream Spectrums are 25-138 kHz and 138-1104 kHz respectively. ADSL of this type are classified into non-overlap and overlap ADSLs and are most commonly used.
G.992.1 Annex B 一一 ADSL over ISDN
Supports ISDN and ADSL on the same twisted pair.
G.992.1 Annex C
This is an ADSL in the G.961 (ISDN) crosstalk environment with a time division duplex. lt is mainly used in Japan
G.992.2: G.Lite or Splitterless ADSL
The upstream Spectrum is the same as that of Annex A. The downstream Spectrum is 138-552 kHz. The upstream and downstream transmission rates are 512 Kbps and 1536 Kbps respectively.
G.992.3
ADSL2 is developed based on ADSL and has been established in June 2002，
In G.992.3 Annex I and J, the support for the full digital loop mode are added. Annex I is applicable to the scenario where the adjacent route pair is POTS, and Annex j is applicable to the scenario where the adjacent route pair is ISDN.
G.992.3 Annex L is the So-called long-distance ADSL2 or READSL2，
G.992.5
Based on ADSL2, ADSL2+ extends the downstream frequency band from 1.104 MHz to 2.208 MHz, and the number of sub-bands increases from 256 to 512. Therefore, the downstream rate increases greatly.
G.993.2
The G.993.2 (VDSL2) transmission standard, as the Ultimate D9SL technology, is based on the Discrete Multi-tone Modulation (DMT) technology. In this standard, the ADSL2+ technology is used to provide long-distance transmission, and the maximum data transmission rate of VDSL is enhanced from 70/30 Mbps to symmetric 100 Mbps. To achieve such a high transmission rate in a range of 350 meters, the spectrum of VDSL2 has increased from 12 MHz to 30 MHz. In addition, the transmission power has increased to 20 dBm to meet the requirements for medium-and high-ring transmission.
In ITU-T, the standard numbering is G.993.2, which is a formal standard released by ITU-T in May 2005. To launch the VDSL2 standard, ITU formulated the RFC3728 MIB standard in October 2004.
ADSL/ADSL2+ Technical Principles
ADSL features
Upstream rate: 640 Kbps; downstream rate: 8 Mbps
ADSL: asymmetric digital subscriber line
The same twisted pair transmits voice and data at the same time.
ADSL standard
G.992.1(G.dmt)
G.992.2(G.lite)
T1.413
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 frequencies 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 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.lite standard is therefore formulated to implement ADSL with no splitter.
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.lite standard. The difference is that G.lite ADSL 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.lite is 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.lite standard, 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.lite ADSL is greatly reduced.
Signal Separation Technology
Introduction to the VDSL/VDSL2 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.
First, let's see how an interleaved processes 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 receiving end, how does the interleaver de-interleave and restore the original data stream? The receiving end writes the received data column by column to a matrix with the same size. After the matrix is full, the receiving 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 interleaved for processing burst errors? Let's look at this example.
l 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 receiving 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 bitstream can result in an FEC failure.
l 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.
l After interleaving, if 3 consecutive bit errors occur and elements 15, 2, and 9 are affected, the sequence of elements received by the receiving 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 interleaver depth D (3 in this example), and the number of matrix columns is interleaved span N (7 in this example) which is equal to the number of FEC codes.
An actual interleaver usually has larger D and N parameters. Assuming that the FEC sequence with N codes can correct B burst errors, a D x N interleaver matrix 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.
GPON MxU HSI Service Configuration Example
The purpose for initializing the ADSL system:
Before the ATU-Cand ATU-R start working, 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.
Initializing the ADSL System
The initialization process can be triggered by either the ATU-R or ATU-C.
The initialization process of 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 the 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 start initialization handshake, complying with the G.hs protocol 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.
Thanks for reading!
