Understanding VDSL Interfaces
Working Mode
- Asynchronous Transfer Mode (ATM): ATM cells are transmitted over VDSL lines.
- Packet Transfer Mode (PTM): Ethernet frames are transmitted over VDSL lines.
Ethernet frames do not need to be fragmented into ATM cells. 1483B/1483R encapsulation, ATM Adaptation Layer 5 (AAL5) encapsulation, and ATM cell encapsulation are not required, enabling Ethernet services to be transmitted more rapidly than in ATM mode.
When the device is deployed as a CPE, a VDSL interface on the device must work in the same mode as the VDSL interface on the DSLAM.
Interface Activation
Activation of a VDSL interface refers to the training between a DSLAM and a CPE. During the training, the two devices check the line distance and status and negotiate parameters such as the transmission standard, upstream and downstream line rates, and specified noise margin. If the training succeeds, the VDSL interface on the CPE is activated and can transmit data.
After the device starts, its VDSL interface is automatically activated. The VDSL interface remains activated as long as its VDSL line works properly. To set line parameters, deactivate the VDSL interface on the CPE. After setting parameters, activate the interface to make configurations take effect.
Uplink Parameters
Uplink parameters for a VDSL interface include the transmission standard, bit exchange status, seamless rate adaptation status, and trellis coding status.
Transmission Standard
Before configuring the device to transmit services on a VDSL line, configure the transmission standard for a VDSL interface on the device. Table 18-2 lists the transmission standards supported by the device.
The device functions as a CPE and must be configured with the same transmission standard as the DSLAM. You are advised to configure the device to automatically adapt to the transmission standard of the DSLAM. By doing this, the device can automatically select the same transmission standard as the DSLAM from G.DMT, ADSL2, AnnexL, ADSL2+, AnnexM, AnnexJ, AnnexA, AnnexB, T1.413, and VDSL2.
Transmission Standard |
Description |
|---|---|
G.DMT (G.992.1) |
Provides an upstream frequency band from 25 kHz to 138 kHz, a downstream frequency band from 138 kHz to 1.104 MHz, a maximum of 1 Mbit/s upstream transmission rate, and a maximum of 8 Mbit/s downstream transmission rate. |
ADSL2 (G.992.3) |
Provides faster transmission rates by improving the modulation rate, coding gain, and initialization state machine, reducing the frame overhead, and using enhanced signal processing methods. ADSL2 can provide a maximum of 1 Mbit/s upstream transmission rate and 12 Mbit/s downstream transmission rate. |
AnnexL |
Defines reach extended ADSL2 (AnnexL for short) in ADSL2 recommendation, which uses a narrower frequency band and optimizes the power spectral density (PSD) mask to provide long-distance transmission. |
ADSL2+ (G.992.5) |
Expands the frequency band to 2.208 MHz and provides a maximum of 1 Mbit/s upstream transmission rate and 24 Mbit/s downstream transmission rate. |
AnnexM |
Expands the upstream frequency band of ADSL2 or ADSL2+ and provides a maximum of 2 Mbit/s upstream transmission rate. |
AnnexJ |
Expands the upstream frequency band of ADSL2 or ADSL2+ and provides a maximum of 3078 Kbit/s upstream transmission rate. |
AnnexA |
Applies AnnexA to ADSL over POTS and to be compatible with POTS services. |
AnnexB |
Applies AnnexB to ADSL over ISDN and to be compatible with ISDN services. |
T1.413 |
Provides a maximum of 800 kbit/s upstream transmission rate and 8 Mbit/s downstream transmission rate. |
VDSL2 |
Provides a maximum of 50 Mbit/s upstream transmission rate and 100 Mbit/s downstream transmission rate. NOTE:
A VDSL interface in ATM mode does not support VDSL2 transmission standard. |
VDSL2 35B |
Provides a maximum of 40 Mbit/s upstream transmission rate and 350 Mbit/s downstream transmission rate. |
The performance of a VDSL interface depends on external factors, such as DSLAM line card types, DSLAM software version, line noise, line length, and temperature.
Bit Exchange
During line activation, every sub-channel calculates the signal-to-noise ratio (SNR) and the number of bits to be transmitted. After the line is activated, the SNR may change because the external environment changes. For example, the SNRs of some sub-channels decrease, whereas the SNRs of others increase. After a long period of time, the line may be disconnected.
Allows sub-channels with low SNRs to switch some bits that they cannot transmit to sub-channels with higher SNRs.
Reduces the transmit power of sub-channels with higher SNRs to increase the sending rate and SNRs of sub-channels with lower SNRs.
These measures reduce the bit error rate without triggering line negotiation.
Seamless Rate Adaptation
To prevent the line disconnection when the external environment changes, the bit exchange function adjusts bit distribution or the power among sub-channels without deactivating the line. However, the bit exchange function cannot change the line rate. When the line environment becomes worse, the bit exchange function just allows the line to be activated at a smaller rate. When the line environment becomes better, the bit exchange function cannot adjust the rate and therefore, wasting line resources.
Seamless rate adaptation solves this problem by dynamically adjusting the line rate without reactivating the line.
Trellis Coding
Trellis coding increases the SNR by using an algorithm with a high coding efficiency. After trellis coding is enabled, the line activation rate is greatly increased.