Hello everyone!
Today, we will continue to learn the key technologies of ADSL2 and VDSL.
ADSL2 Standard
ADSL2 is developed based on ADSL and has been established in June 2002 (G.992.3)
The ADSL2 frequency band division is similar to the current ADSL (the upstream and downstream frequency bands are both up to 1104 kHz). Theoretically, the maximum downstream rate can reach 12 Mbps and the maximum upstream rate can reach 1.2 Mbps.
In G.992.3 Annex | and J, the support for the full digital loop mode is added. Annex 1 is applicable to the scenario where the adjacent route pair is POTS and the Annex J is applicable to the scenario where the adjacent route pair is ISDN
G.992.3 Annex Lis the so-called long-distance ADSL2 or READSL2
Comparison Between ADSL2+ and ADSL
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 of ADSL connections.
The ADSL2 standard implements traffic-based power control. When a large amount of data needs to be transmitted, for example, during a 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 page 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 specifies a fully 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 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 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,...to 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) 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 provide 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 ADSL 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.
ADSL2plus uses a wider frequency (tone 32-511) and more Sub-bands (512) to support a maximum downstream rate of 24 Mbps.
3. ADSL ADSL2, and ADSL2+ Spectrum distribution
4. Rate comparison between ADSL2 and ADSL2+
5. Longer transmission distance and lower power consumption
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 a 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 is 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 NR. In addition, the 2 bits on the pilot tones can increase the bandwidth by 8 Kbps.
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 0–12dB.)
6. 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 receiving 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.
7. 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.
8. 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 environmental changes.
9. Rate binding
To provide different QoS for 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.
10. 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 sublayer is encapsulated and messages between sub-layers are defined. In this way, devices from different vendors can communicate with each other.
VDSL Overview
The preceding two types of DSL technologies, ADSL and SHDSL, have a maximum transmission rate of 8 Mbps. VDSL which is short for a very high-speed digital subscriber line is a new generation of high-speed DSL technology.
VDSL can reach a maximum transmission rate of 52 Mbps on common twisted pairs. It provides various transmission rates and multiple working modes, including symmetric and asymmetric transmission, to meet the requirements of different customers. Because the transmission rate of VDSL is high, the twisted pairs used in VDSL are shorter, usually from 300 m to 1 km. The length of the twisted pairs is inversely proportional to the transmission rate.
Compared with ADSL, VDSL uses a higher frequency band than PSTN and ISDN frequencies on twisted pairs. Therefore, VDSL is compatible with existing traditional telephone services as well as ISDN services. VDSL also uses passive filters as signal splitters to support voice and ISDN transmission. The basic working principles of VDSL are similar to that of ADSL.
VDSL is a video and data transmission technology over the voice frequency band. It provides multiple working modes and can transmit data at a high rate within a short distance. This technology enables telecom operators to use existing twisted pairs to transmit broadband services, such as VoD and high-speed Internet access.
Currently, the internationally recognized VDSL system complies with the American standard formulated by T1E1.4 and the European standard formulated by ESTI TM6. Both standards adopt the G.998 standard and DMT/CAP modulation modes. The difference is that the American standard adopts the G.998 standard for baseband planning to provide the highest rate of 22 Mbps, while the European standard adopts the G.998 standard for baseband planning to provide the highest rate of 14 Mbps.
Relationship Between the VDSL Rates and Distances Defined by the ITU
Although the transmission rate of VDSL is high, the transmission distance is far less than that of ADSL. This is because high-speed transmission requires complex modulation modes, dense constellation encoding, and outstanding channel characteristics, but the length of transmission lines can pose negative impacts on channel performances.
At the early stage of VDSL design, the ITU divides VDSL transmission capabilities into long-, medium-, and short-distance transmissions in asymmetric and symmetric transmission modes.
Reference Model of the VDSL System
This is the reference model of the VDSL system. Basic modules include VDSL devices at the CO and subscriber sides. Devices at both sides are connected by twisted pairs through their own splitters. The other end of the CO device is connected to an ONU, and the other end of the subscriber device is connected to a network device or a small LAN.
VTU-O is short for the VDSL transceiver unit at the ONU and has the same function as the CO-side modem ATU-C in ADSL. VTU-R is short for the VDSL transceiver unit at the remote and has the same function as the subscriber-side modem ATU-R in ADSL.
Network interfaces defined by applications: Different data interfaces are used to aggregate data from upper-layer protocols to form unified data flows to be transmitted in fast and slow channels as required. Data flows are sent to the lower layer for framing. At the same time, application-independent data frames from the lower layer are split and used by different application interfaces and protocols.
In VDSL, a splitter similar to that in ADSL is used to separate data signals from traditional voice/ISDN signals.
Development History of VDSL2
The International Telecommunication Union(ITU) recently completed the formulation of a new technical specification that enables global telecom operators to provide the "super" triple services integrating video, Internet, and voice services, with a rate 10 times faster than that of ordinary ADSL.
ITU-T recommendations on VDSL2 enable telecom operators to provide high-definition television(HDTV)，video-on-demand(VoD), videoconferencing, high-speed Internet access, and advanced VolP voice services through standard copper telephone lines， giving telecom operators
the capability to compete with cable providers and satellite operators.
Advantages of VDSL2
The rate can reach 10 times the average ADSL rate, and the maximum rate can reach 100 Mbps at 0 distance, effectively Supporting triple play of video, Internet, and voice services.
VDSL2 not only has the long-distance transmission capability of the ADSL2+ technology but also increases the data transmission rate of the VDSL from 70 Mbps (downstream) /30 Mbps (upstream) to 100 Mbps (downstream) /100 Mbps (upstream). To achieve such a high transmission rate within 350 meters, the working frequency of VDSL2 is increased from 12 MHz to 30 MHz.
To meet the access requirements of medium and long-distance loops, the transmit power of VDSL2 is performance is similar to that of ADSL in long-distance transmission scenarios.
Thanks for reading!
