[SI-LIST] Question regarding the differential impedance

I have a general question regarding the differential impedance of traces which 
dont have any plane above or beneath them at all.

Lets say it is 20 layer board and I am routing 100ohm differential pair on any 
of the layer but there is absolutely no plane above or beneath the traces. What 
will be the trace width and spacing I will be using for these traces.
These are the traces for T1/E1 from the connector to the line side of the 
transformer.

Do I have to calculate that by defining all of the layers as signal layers? 
Any suggestions

Thanx
Kaps

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