All can't transmit at the same time! Alternative links should be used for communication to get the max throughput. This problem could be solved by appropriately using the MAC layer!!

My mistake..Its not "All transmitting simulataneously" but  "all have their data to transmit in regular intervals". Standard WSN protocols have lot of overheads as additional packets or additional bytes in the packets generated. MAC layer will only help for point to point communication but here we need to take the packet from the source to destination over many hops.

ARe you taking about transport and application layers? Standard protocols mainly concerned with PHY and MAC layer. additional packet overheads ( such as control packets, request, permission etc.) vary in all of the MAC standards. Any specific standard you are talking about?

Basic standard for WSN is defined by IEEE 802.15.4 protocol which details the PHY and MAC layer. Severals standards have been build upon this standard to provide APL and NWK support. It includes Zigbee , 6LowPAN etc. These help to support multiple topologies for system setup like mesh,tree etc.  As the layers increase the overhead from llayer specific headers also increase. Thus less space for actual data decrease. But thats not our concern.

I need NWK and APL layer, that may not comply to any existing standards, that is less complex and has less overheads. Also it should specifically support linear network over long distance. So we are talking about more hops here. Needless to say that hops will mean overheads(so as to discover routes, maintain routing tables etc.).

The WSN standards are quite general and addresses solutions for all types of topology. You can investigate a protocol called SOTDMA (Self Organized Time Division Multiple Access) where nodes self organize and can transfer data using a hop by hop mechanism as well.  This protocol can be of help to you.