1. Are the nodes fixed? in other words,
2. Is the sequence mentioned by you a..b....c...d..e..f remain same always?

If the sequence varies, then its a different ball game! Thats where the network rediscovery and allied readjustments come. If they are fixed in location, then you could very well implement the following.

1. I assume node 'a' is the farthest from PC, and node 'f' is the nearest.

2. Give ID to every node: starting a=1 and f=6.

3. The Network layer (which is a piece of code written by you tailored for this application) will "forward" the packet to next higher numbered node, if, the packets are from a node whose ID is smaller than the self.

This means, node 3 will forward the packets if the received packet is from node 2 or node 1.
4. If your sensor locations are fixed, then you can form your own network layer. I have mentioned just an example above.

Try the following link, they have ready-to-interface CC2420 modules:
http://www.rfsolutions.co.uk/acatalog/...odule.html

Cheers,
Vignesh