[asterisk-users] importance of crc4 in zaptel.conf?

[Louis-David Mitterrand]

Hello,

We have a TE410P connected to an EuroISDN E1 with these span 
definitions:

	span=1,1,0,ccs,hdb3
	span=2,1,0,ccs,hdb3
	span=3,1,0,ccs,hdb3
	span=4,1,0,ccs,hdb3

Why should we add "crc4" to these definitions? What does it do?

Thanks,