ADSL Interface Overview

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. The typical bandwidths of ADSL, ADSL2, and ADSL2+ circuits are defined in Table 44.

Table 44: Standard Bandwidths of DSL Operating Modes

Operating Modes	Upstream	Downstream
ADSL	800 Kbps — 1 Mbps	8 Mbps
ADSL2	1 — 1.5 Mbps	12 —14 Mbps
ADSL2+	1 — 1.15 Mbps	24 —25 Mbps
ADSL2+ Annex M	4 Mbps	25 Mbps

SRX210 devices support ADSL, ADSL2, and ADSL2+, which comply with the following standards:

For Annex A:
- ITU G.992.1 (ADSL)
For Annex A only:
- ANSI T1.413 Issue II
- ITU G.992.3 (ADSL2)
- ITU G.992.5 (ADSL2+)
For Annex M:
- ITU G.992.3 (ADSL2)
- ITU G.992.5 (ADSL2+)
For Annex B:
- ITU G.992.1 (ADSL)
- ITU G.992.3 (ADSL2)
- ITU G.992.5 (ADSL2+)
For Annex B only
- ETSI TS 101 388 V1.3

The ADSL Mini-PIM is supported on SRX210 devices. The ADSL Mini-PIM facilitates a maximum of ten virtual circuits on SRX210 devices.

SRX210 devices with Mini-PIMs can use PPP over Ethernet over ATM (PPPoEoA) and PPP over ATM (PPPoA) to connect through ADSL lines only.

ADSL Systems

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 17 and Figure 18.

Figure 17: Typical ADSL IP DSLAM Topology

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Figure 18: Typical ADSL ATM DSLAM Topology

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ADSL2 and ADSL2+

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).

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.

ATM CoS Support

Certain class-of-service (CoS) components for Asynchronous Transmission Mode (ATM) are provided to control data transfer, especially for time-sensitive voice packets. The ADSL Mini-PIM on the SRX210 device provides extended ATM CoS functionality to provide cells across the network. You can define bandwidth utilization, which consists of either a constant rate or a peak cell rate, with sustained cell rate and burst tolerance. By default, unspecified bit rate (UBR) is used because the bandwidth utilization is unlimited.

The following ATM traffic shaping is supported for the SRX210 device:

Constant bit rate, (CBR)	CBR is the service category for traffic with rigorous timing requirements like voice, and certain types of video. CBR traffic needs a constant cell transmission rate throughout the duration of the connection.
Variable bit rate non-real - time (VBR-NRT)	VBR-NRT is intended for sources such as data transfer, which do not have strict time or delay requirements. VBR-NRT is suitable for packet data transfers.
Unspecified bit rate (UBR)	UBR is ATM’s best-effort service, which does not provide any CoS guarantees. This is suitable for noncritical applications that can tolerate or quickly adjust to loss of cells.

The ability of a network to guarantee class of service is related in the way in which the source generates cells and also on the availability of network resources. The connection contract between the user and the network will thus contain information about the way in which traffic will be generated by the source.

A set of traffic descriptors is specified for this purpose. The network provides the class of service for the cells that do not violate these specifications. The following are the traffic descriptors specified for an ATM network:

Peak cell rate (PCR) - Top rate at which traffic can burst.
Sustained cell rate (SCR) - Normal traffic rate averaged over time.
Maximum burst size (MBS) - The maximum burst size that can be sent at the peak rate.
Cell delay variation tolerance (CDVT) – Allows the user to delay the traffic for a particular time duration in microseconds to follow a rhythmic pattern.
Note: CDVT is not supported on J Series devices.

For traffic that does not require the ability to periodically burst to a higher rate, you can specify a CBR. You can configure VBR-NRT for ATM interfaces, which supports VBR data traffic with average and peak traffic parameters. VBR-NRT is scheduled with a lower priority and with a larger sustained cell rate (SCR) limit, allowing it to recover bandwidth if it falls behind.