Integrity constraints to be enforced
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1. WAN port associated with a BACKUP link has to be asynchronous.

2. If primary link is dialing type, backup will also be of dialing type.
   If primary link is answering type, backup will also be of answering type.
   This constraint will be enforced by ROUCON.

3. On a dialing end, backup port WAN 3 can be shared by two ports WAN 1 and
   WAN 2. But if either WAN 1 or WAN 2 is answering, then WAN 3 cannot be 
   shared for backup.

4. A warning message can be issued, if both primary link and backup link
   are defined with same phone numbers.

5. User need not give any phone number for primary link. (WAN 1 or WAN 2)
   But phone number is required for backup link if it is a dialing end.
   If the user has not given it, a warning message can be issued.


