ADSL, ADSL Interface, and ADSL Settings in ScreenOS Devices

The following are the topics of ADSL Interface:

About ADSL

Traditional telephone lines use analog signals to carry voice service through twisted-pair copper wires. However, when using analog transmission, the service provider can use only a small portion of the available bandwidth. To work around this limitation, the service provider can use digital transmission to access a wider bandwidth on the same media, at the same time. Because the service provider separates analog and digital transmissions, you can use your telephone and connect the Internet with your computer at the same time on the same line.

At the service provider’s central office, the digital subscriber line access multiplexer (DSLAM) connects many DSL lines to a high-speed network such as an Asynchronous Transfer Mode (ATM) network. ADSL transmission is asymmetric because the rate at which you can send data (the upstream rate) is considerably less than the rate at which you can receive data (the downstream rate). ADSL is ideal for Internet access because most messages sent to the Internet are small and do not require much upstream bandwidth, while most data received from the Internet require greater downstream bandwidth.

You can use the ADSL port on the NetScreen-5GT ADSL security device to enable Internet access for a network—without adding additional phone lines, and without using an additional ADSL modem. For details on connecting and cabling the NetScreen-5GT ADSL, see the NetScreen-5GT ADSL User’s Guide.

About the ADSL Interface

The ADSL interface on the NetScreen-5GT ADSL security device uses ATM as its Transport Layer. The interface supports multiple permanent virtual circuits (PVCs), which are continuously available logical connections to the network, on a single physical line (the adsl1 interface). You can configure additional virtual circuits on the device by creating subinterfaces (such as adsl1.1, adsl1.2).

Before you can configure the adsl1 interface, however, you must obtain the DSLAM configuration details for the ADSL connection from the service provider, as detailed in ADSL Settings from the Service Provider.

ADSL Settings from the Service Provider

The service provider for ADSL Internet access must provide you with some details about the ADSL connection so you can configure the security device to connect to their servers. Not all service providers use the same implementation of ADSL; you might be given any combination of the ADSL parameters as described in Table 26.

Table 26: ADSL Settings

ADSL Parameters


Virtual Path Identifier and Virtual Channel Identifier (VPI/VCI)

The service provider identifies the virtual circuit on the DSLAM.

ATM encapsulation method (Multiplexing mode)

The ADSL interface on the security device supports the following ATM Adaptation Layer 5 (AAL5) encapsulations:

  • Virtual circuit (VC)-based multiplexing, in which each protocol is carried over a separate ATM virtual circuit.
  • logical link Control (LLC), which enables several protocols to be carried on the same ATM virtual circuit (default encapsulation method). This is the default option for the adsl1 interface on the NetScreen-5GT ADSL security device.

    The service provider must tell you the type of multiplexing used on the ADSL line.

Point-to-Point Protocol (PPP)

A standard protocol for transmitting IP packets over serial point-to-point links, such as an ATM PVC. The security device supports the following methods of transporting PPP packets:

  • PPP over Ethernet (PPPoE). RFC 2516 describes the encapsulation of PPP packets over Ethernet. For more information about PPPoE, see About Configuring PPPoE.
  • PPP over AAL5 (PPPoA). RFC 1483 describes the encapsulation of network traffic over AAL5. For more information about PPPoA, see Configuring a PPPoA Client Instance.

    If the service provider’s network uses PPPoE or PPPoA, the service provider must give you the username and password for the connection, the authentication method used, and any other protocol-specific parameters.

IP addresses

The service provider might give the network a static IP address or a range of IP addresses. The service provider should also give you the address of the DNS server to use for DNS name and address resolution.

Discrete multitone (DMT)

A method for encoding digital data in an analog signal. By default, the ADSL interface uses Auto Detect mode, in which it automatically negotiates the DMT operating mode with the service provider DSLAM. You can change the mode on the adsl1 interface to force the interface to use only one of the following DMT standards:

  • American National Standards Institute (ANSI) TI.413 Issue 2, which supports rates up to 8 Mbps downstream and 1 Mbps upstream.
  • International Telecommunications Union (ITU) G.992.1 (also known as G.dmt), which supports minimum data rates of 6.144 Mbps downstream and 640 kbps upstream.
  • ITU 992.2 (also known as G.lite), which supports up to data rates of 1.536 Mbps downstream and 512 kbps upstream. This standard is also called “splitterless DSL” because you do not have to install a signal splitter on your ADSL line (the service provider’s equipment splits the signal remotely).

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