What is the distribution of service in a M/M/1 airport simulation queue?

[EvLer]

I hope this is the right 'subforum' to post my question, if not, sorry, I was not sure...
What kind of model is airport simulation of a queue (planes land and take off)
with one runway (one server only). I am sure about the first and the last parameter but not about the 'middle': M/(const)/1? What does 'distribution of service' exactly mean? When is it constant, i.e. can it be constant in this case?
Thanks for your help.

 

[Damned charming :)]

I think the const refers to the fact that a plane needs a non random service time. Maybe once it lands it spends exactly 20 minutes on the ground.
Here was a page were everything is defined.
http://www.geocities.com/qtsplus/ModelOverview.htm