Choice and Configuration of your DSL Modem

Consider your requirements carefully when selecting your DSL modem. The ‘free’ modem offered as part of your set-up package by your new ADSL service provider may not meet all your needs. Likewise, the most expensive one may provide capabilities you don’t need and thereby generate unnecessary expense. Remember: even if the DSL modem is provided for free: under most countries’ telecommunications laws, the DSL modem (as CPE = customer premises equipment) to be owned by you the customer: it’s yours, and if you change ADSL service provider you can ‘take it with you’! Since the modems are standard equipment, they will generally work on other providers networks.


When choosing a DSL modem, look out for:



DSL Modem feature


things to look out for

Basic DSL modem requirements

4          The modem should support standard ADSL (ITU-T recommendations G.992.1 and G.992.2)

4          Standard ADSL provides a downstream bitrate up to 8 Mbit/s and upstream up to at least 576 kbit/s

4          The modem you choose should support the bitrates you require now and the bitrates you foresee that you might require in future

4          ADSL2+: Since ADSL2+ is fast becoming the ‘standard’ method of providing ADSL amongst the ADSL service providers, I recommend that you ensure that any new modem you purchase is ADSL2+ compatible

Mandatory Connections – Every DSL modem should have at least three connections

4          Power connection

4          DSL network connection to DSL splitter or DSL filter and thus to ADSL network. This is usually a rectangular 8-pin RJ-45 socket

4          Ethernet 10/100baseT LAN connection for connecting your PC or LAN (RJ-45 socket also. [Ensure that your modem has at least one LAN cable connection, even if it also offers WLAN (IEEE 802.11)]

Essential Capabilities – you should ensure that your DSL modem has the following capabilities built in:

4          Router function

4          PPPoE (PPP over Ethernet) – this is essential in order your PC can establish the ADSL connection to your ADSL provider

4          DHCP server function (dynamic host control protocol) this may not be explicitly stated on the box, but is mostly included when a router is included)

4          NAT (network address translation) is an essential requirement of the router if you intend to connect more than one PC via a LAN to your ADSL connection. The NAT function converts the ‘private IP address’ used in the local LAN to the ‘public IP address’ issued by your ISP which you need when accessing the Internet.

4          Firewall function – your DSL modem should have at least a simple firewall function for preventing access to the files on your PC or LAN by unauthorised external Internet users

4          Web management interface – this allows you to access your DSL modem easily using an Internet browser software (e.g. Internet Explorer or Firefox) to configure your ADSL conenction for service and change settings (e.g. firewall settings) if required

Optional features – you may consider the following features to be advantageous

4          WLAN hub built-in

4          LAN switch built-in

4          Internet telephony function built-in


(Wireless LAN – otherwise known as WiFi or IEEE 802.11)

4          WLAN allows for wireless connection and networking of your PC and various computer peripherals (e.g. scanner, printer, external harddrive etc.)

4          Multiple 802.11 standards – be careful when purchasing your DSL modem/WLAN hub and WLAN cards for your computer equipment – to ensure that you purchase the same 802.11 WLAN standard for each device – either 802.11a, 802.11b or 802.11g

4          WLAN security – make sure not only that your DSL modem/WLAN hub can support WLAN security, but also that you remember to set it up. This ensures that only devices authorised and correctly configured by you with the appropriate passwords are able to access the Internet via your ADSL connection. You would be amazed how many people operate WLANs without proper security! (possible security standards: WEP –wireless equivalent privacy or WPA – WiFi protected access)

Internet telephony

VOIP (voice-over-IP)

4          SIP (session initiation protocol) is the protocol required for setting up (usually cheap) telephone calls across the Internet. Some DSL modems include a SIP client able to set up such calls. In order that the system works, your ADSL service provider or other third-party Internet Service Provider as VOIP operator must provide the corresponding SIP server

4          SIP soft client – it is possible to conduct VOIP (voice-over-IP) “Internet telephony” from any PC connected to the Internet and equipped with a “SIP soft client” (e.g. Skype client). However, the voice quality of such calls may be inadequate – particularly when you simultaneously ‘surf’ the Internet

4          DSL Modems offering SIP clients can typically be split into three classes:

                    I.      DSL modems offering an solution which fully integrates VOIP with your existing analogue or ISDN exchange line and which allow continued use of your existing telephone handsets. In this arrangement outgoing calls from your ‘normal’ phone benefit from the cheap rates of VOIP, while incoming calls to your existing known telephone number ring the same phone.

                II.      DSL modems offering a new VOIP telephone line in addition to your existing telephone line. A new (but standard) telephone handset may be required to make use of this extra line. In particular, the new line is likely to be attractive for cheap outgoing calls. Incoming calls to the new line are also possible – provided the caller knows your new VOIP telephone number. Solutions in this category do not always support adequate QOS (voice signal prioritisation in the upstream direction) to give good speech quality.

            III.      DSL modems offering VOIP but requiring the purchase of new special VOIP telephones. The quality of speech using such solutions is not usually very high.

4          Recommended: Solution I above when you wish to benefit from VOIP. The best DSL modem manufacturers for such a solution are typically national manufacturers (e.g. AVM FritzBoxFon in Germany).

4          Cisco/Linksys has not so far produced DSL modems with the best integrated VOIP solutions



When installing your DSL modem, I would recommend that you make provision for lightning protection (surge protection) on both the power supply connection (power surge protection) and the ADSL connection (telephone surge connection) to your network service provider. Without protection, you may find yourself having to buy a new DSL modem everytime a thunderstorm comes your way!  Experience has told me that this is a real risk!