Unable to access values in nested dictionary for adding link in network

Question

I have a following dictionary

d={\'s1\': {\'s2\': {}, \'s3\': {}, \'s4\': {}}, \'s2\': {\'s5\': {\'bw\': 20, \'delay\': \'1ms\'}}, \'s3\': {\'s6\': {\'bw\': 20, \'de