It is known that before Serbs came to Balkans they lived in today's Germany and Poland.They were known as "White Serbs" and just next to them "White Croats" lived(just like today where we are).
Their country was known as "White Serbia"(Bela Srbija) and on their west there was "White Croatia"(Bela Hrvatska).For example,Czar of Frankony(Frankonia) built the wall toward Serbian Czarhood and he called it "Limitus Sorbicus"("limitus"=border,"sorbicus"=I don't have to explain ;-) )
Small remaining part of Serbian people left there today is known as "Luzicki Srbi" or in German language "Lausitzer Sorben"(
http://www.sorben.de/).Today they have more similarities with Polish and German people then with us.They mixed with Baltic,German nations and Polish people,while we(current Serbs on south,Balkans) have certain influence of Vlahs,Illyrians and Romanians.
They call lands where they live like "Serbja"(Serbya) wich is the same semantic root as "Serbiya".Serbs before 20th century pronounced the name of our country like that-"Serbiya".
"Sorbs" is one of the variates for current modern name of the Serbs,probably the oldest one...and it could be the "original",as our current language is "spoiled" because of mixing up with other nations and influence of their languages.