Norbert Hartl

mostly brackets and pipes

DSL to VDSL: More Than One Letter Upgrade

A few weeks ago our telecommunication provider offered us to upgrade from DSL to VDSL. Before we had a typical combined line consisting of ISDN channels for telephony and DSL for internet. This line had a bandwidth of 16 MBit/s. To me it appeared as just a one letter upgrade at first . I thought they do a slightly different sort of DSL to get higher bandwidth. The new VDSL should have a bandwidth of 50 MBit/s upgradable to 100MBit/s. But I didn’t know…

I was glad that all of the installed boxes became obsolete with the upgrade. We had a typical installation which is made up of 5 boxes: splitter, NTBA, A/B, DSL modem and router. It’s a big mess just to be able to use telephony and internet. The telecommuncation provider delivers a mixed frequency set consisting of ISDN and DSL into the flat. The splitter divides those frequencies into two. The ISDN related ones are put into NTBA (german) to make ISDN available. But all the years ISDN equipment was so expensive that nearly nobody owns an ISDN telephone. So the only thing you put into the NTBA is a A/B transformer which is an Adapter for analog telephones. The other output of the splitter is connected to a DSL modem and this is connected to a DSL router box. All of this was replaced by two boxes: a VDSLmodem and a router box.

Now I was curious how this works. A little research showed me that they really did it: From now on everything that is delivered to my flat is IP based. The output of the modem is just IP traffic that is fed into the box. And the router has plugs for analog telephones. From an installation point of view this little black box is just great! But….I don’t really like black boxes because I want to know how this works in detail. My further research unveiled a much more clever design of the use cases telephony and internet. Those two just converged into one technology: The telephony is now pure Voice over IP. So telephony is just another IP based service and there could be more. I found some references that Video on demand is planned to be another service delivered in the same fashion as telephony. That would mean that the “internet” can be seen as another “general data” service.

It makes sense in a way that different services are likely to be dealt be different devices especially if you are thinking about your home. This all can be done on a pure IP basis but it also has some ambiguities. Regarding ISDN there was a similar problem: How do you know if the incoming call is voice, fax or data? ISDN transmits a service indicator with the call. ISDN is a bus system so all of the devices are listening at the same thing at the same time. You could have a telephone and a fax answering machine for the same telephone number. The incoming call carries the service indicator so the fax answers a fax service and a telephone answers (rings) a voice service. Someting similar exists in the VDSL world.

The services are divided into different VLANs. A VLANis created by using some bytes of the packet payload to differentiate between the networks. Switches that are configured for virtual networks can filter packets according to the vlan tag to different physical ports thus making those networks real separated networks. That is good to know. It is basically one technology to deal with and using a vlan capable switch I’m able to handle different services with different boxes. I just need to know what vlan tag numbers are used by my provider.