ADSL Modem Types
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Contents |
PPP Encapsulation
In South Africa, we use PPPoE encapsulation. PPPoA is used in some other parts of the world.
| VPI: | 8 |
| VCI: | 35 |
Physical modem types
All ADSL modems are in fact bridges but we'll call them modems for the sake of your sanity.
USB
Avoid these like the plague. They may be (just) ok, sometimes, for Windows users, but for Linux geeks these are no good.
Exceptions:
- SR had a decent experience with the Sagem Fast@800 which uses the Eagle drivers (available in Debian)
PCI
These are rare, you probably don't even have to worry about coming across them...
Ethernet
These are the preferred modems for linux users. You attach them by ethernet cable to a NIC on your router.
Telkom sell a Marconi modem free with a 24month contract, but most CLUGgers will strongly recommend against signing such a contract with Telkom, as getting out of it would be very expensive. The Marconi modem is considered by most to be OK, but not excatly great, stable, or recommended
Ethernet modems can be used in several modes:
Router Mode
This makes the ADSL modem act as a router for your average home / SOHO network.
The modem does the PPPoE termination, and provides NAT to allow machines on your private network to access the Internet.
Generally it has a built in DHCP server to assign addresses to all your machines, and a DNS forwarder.
You can normally forward a few ports to an internal machine, allowing you to run basic servers.
While this is simple to set up, it isn't as flexable as bridged mode.
Half-Bridged Mode
Not many modem support this, as it is only useable for a single machine.
The modem does the PPPoE termination, and hands out 1 and only 1 address by DHCP to a machine on your network. This is the public internet IP address that the router received, so there is no NAT involved.
Naturally you will want some firewalling if you choose this option.
Bridged Mode
Almost all modems support this, and it is generally the most stable, as it is the simplest.
This is the true linux-geek mode. You will need to have a router (i.e. a Linux box or a WRT54g) running whenever you want internet access, but you get ultimate flexibility.
The modem simply bridges the PPPoE onto Ethernet and the router must terminate it (thus it knows when your connection goes up/down).
For security, connect the modem to a dedicated NIC on the router.
Your router must firewall itself, and do DHCP, DNS, NAT, and whatever else you want.
Recommeded Modems
- Billion Bipac 5102 family - Rock solid in Bridged mode. Available from the frogs.
