Understanding ADSL Interfaces
Interface Activation
Activation of an ADSL 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 ADSL interface on the CPE is activated and can transmit data.
After the device starts, its ADSL interface is automatically activated. The ADSL interface remains activated as long as its ADSL line works properly. To set line parameters, deactivate the ADSL interface on the CPE. After setting parameters, activate the interface to make configurations take effect.
Uplink Parameters
Uplink parameters for an ADSL 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 an ADSL line, configure the transmission standard for an ADSL interface on the device. Table 17-2 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, and T1.413.
Transmission Standard |
Description |
|---|---|
G.DMT (G992.1) |
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 (G992.3) |
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+ (G992.5) |
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. |
T1.413 |
Provides a maximum of 1 Mbit/s upstream transmission rate and 8 Mbit/s downstream transmission rate. |
The performance of an ADSL 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.