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Note: Services Routers with ADSL PIMs can use PPP over Ethernet (PPPoE) and PPP over ATM (PPPoA) to connect through ADSL lines only, not for direct ATM connections. |
Asymmetric digital subscriber line (ADSL) technology is part of the xDSL family of modem technologies that use existing twisted-pair telephone lines to transport high-bandwidth data. ADSL lines connect service provider networks and customer sites over the "last mile" of the network—the loop between the service provider and the customer site.
ADSL transmission is asymmetric because the downstream bandwidth is typically greater than the upstream bandwidth. A typical ADSL circuit uses bandwidths of 1.5 Mbps to 2.0 Mbps downstream and 16 Kbps upstream. Depending on the length of the copper wire, an ADSL link can have up to 6.1 Mbps downstream and 64 Kbps upstream.
All Services Routers except the J2300 router support ADSL, ADSL2, and ADSL2+, which comply with the following standards:
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Note: Services Routers with ADSL PIMs can use PPP over Ethernet (PPPoE) and PPP over ATM (PPPoA) to connect through ADSL lines only, not for direct ATM connections. |
ADSL links run across twisted-pair telephone wires. When ADSL modems are connected to each end of a telephone wire, a dual-purpose ADSL circuit can be created. Once established, the circuit can transmit lower-frequency voice traffic and higher-frequency data traffic.
To accommodate both types of traffic, ADSL modems are connected to plain old telephone service (POTS) splitters that filter out the lower-bandwidth voice traffic and the higher-bandwidth data traffic. The voice traffic can be directed as normal telephone voice traffic. The data traffic is directed to the ADSL modem, which is typically connected to the data network.
Because twisted-pair wiring has a length limit, ADSL modems are typically connected to multiplexing devices. DSL access multiplexers (DSLAMs) can process and route traffic from multiple splitters. This typical ADSL configuration is shown in Figure 16.
Figure 16: Typical ADSL Topology
The ADSL2 and ADSL2+ standards were adopted by the ITU in July 2002. ADSL2 improves the data rate and reach performance, diagnostics, standby mode, and interoperability of ADSL modems.
ADSL2+ doubles the possible downstream data bandwidth, enabling rates of 20 Mbps on telephone lines shorter than 5,000 feet (1.5 km).
First-generation ADSL standards require fixed 32-bit overhead framing on all ADSL packets. On long lines with low rates of 128 Kbps, the overhead represents 25 percent of the available bandwidth. ADSL2 standards allow the overhead per frame to be a programmable value between 4 Kbps and 32 Kbps, to provide up to 28 Kbps more bandwidth for payload data.
ADSL2 uses seamless rate adaptation (SRA) to change the data rate of a connection during operation with no interruptions or bit errors. The ADSL2 transceiver detects changes in channel conditions—for example, the failure of another transceiver in a multicarrier link—and sends a message to the transmitter to initiate a data rate change. The message includes data transmission parameters such as the number of bits modulated and the power on each channel. When the transmitter receives the information, it transitions to the new transmission rate.
On a J-series Services Router, the ADSL link is employed over an Asynchronous Transfer Mode (ATM)-over-ADSL interface. Although the interface type is at, the physical interface is ADSL. ATM-over-ADSL and ATM-over-SHDSL interfaces can be configured with the properties associated with traditional ATM interfaces, including virtual circuit and path information and ATM encapsulation.