DSL/ADSL Modem

Technology, concept and troubleshooting

Objective

Describing DSL modem. Comparison to voiceband modem. Hardware components. Service features. Manufacturers. Common issues and troubleshooting.

DSL Modem

ADSL modem or DSL modem is a device used to connect a single computer or router to a DSL phone line, in order to use an ADSL service. Like other modems it is a type of transceiver. It is also called a DSL Transceiver or ATU-R. The acronym NTBBA (network termination broad band adapter, network termination broad band access) is also common in some countries.

DSL Modem

Some ADSL modems also manage the connection and sharing of the ADSL service with a group of machines: in this case, the unit is termed a DSL router or residential gateway.

ADSL Modem A basic network using ADSL modem

DSL vs Voiceband modem

Most DSL modems are external to the computer and wired to the computer's Ethernet port, or occasionally its USB port, whereas voiceband modems are usually built inside the computer.

DSL modems use frequencies from 25 kHz to above 1MHz (see Asymmetric Digital Subscriber Line), in order not to interfere with voice service which is primarily 0-4 kHz. Voiceband modems use the same frequency spectrum as ordinary telephones, and will interfere with voice service - it is usually impossible to make a telephone call on a line which is being used by a voiceband modem.

DSL vs Voiceband modem

DSL modems vary in data speed from hundreds of kilobits per second to many megabits, while voiceband modems are nominally 56K modems and actually limited to approximately 50 kbit/s.

DSL modems exchange data with only the DSLAM to which they are wired, which in turn connects them to the Internet, while most voiceband modems can dial directly anywhere in the world.

DSL vs Voiceband modem

DSL modems are intended for particular protocols and sometimes won't work on another line even from the same company, while most voiceband modems use international standards and can "fall back" to find a standard that will work

Hardware components

A DSL modem requires the following for its operation

Power supply: Transformer and capacitor Data connection and power circuitry (for example,

USB, Ethernet, PCI) DSL digital data pump DSL analog chip and line driver Micro controller Filter

Service Features

Apart from connecting to an ADSL service, many modems offer additional integrated features:

ADSL2 or ADSL2+ support Router functionality that includes Network Address

Translation (NAT) to share a single IPv4 address. An 802.11b, 802.11g or 802.11n wireless access point A built-in switch (typically 4 ports) Virtual Private Network termination Dynamic Host Configuration Protocol (DHCP) server Dynamic DNS (Domain Name System) clients Voice over IP functionality including Quality of Service

(priority control for data flows between users)

Manufacturers

2Wire Netgear Belkin D-Link Linksys Siemens

Common issues and troubleshooting

Problems accessing the Internet can occur due to:

a) Problems with the computer being used

b) Failures of the DSL service to operate correctly

c)  General problems within the Internet

d)  Disconnected or faulty cables

Common issues and troubleshooting

Solution to such common problems include

1. Checking the modem activity light.

2. Checking the physical connection between the various devices.

3. Checking the hardware functionality

4. Checking the security software such as anti viruses and firewalls.

Common issues and troubleshooting

Using techniques such as obtaining a manual ip address from the internet service provider.

Checking for proper connectivity of the default gateway and the DNS server by pinging.

Note : Sometimes very basic troubleshooting steps such as restarting the hardware or unplug and replacing of the various cables can resolve the issue.

Thank You