This program is for measuring 1) the link correlation and 2) empirical number of transmissions in reliable broadcast.

Scenario: 
1) 1 sender and 3 receivers, which forms three links.
2) Sender side:
	Broadcast a packet every 2 seconds.
	The first NUM_FOR_CORR packets are for measuring pairwise link correlations. After that, the sender should cover all receivers with NUM_FOR_TX packets and record the actual number of transmissions.
3) Receiver side:
	For packets with a seqno. less than NUM_FOR_CORR, just output bool strings (100101...) to the serial port, where a 1 denotes a received packet and a 0 denotes a missing packet.
	For packets with a seqno. larger than NUM_FOR_CORR, request for the missing packets (as it should receive a packet in every 2 second).
	
Other aspects:
1) the radio power level is set to be 1.
2) the packet transmit interval is 2 seconds.

msg types:
1) data packets(broadcast): nodeid, seqno.
2) req packets(unicast): nodeid, pseqno.

variables:
1) rseq: newest received seq.
2) tseq: the seqno. to transmit.