as an additional information, recently an opensource protocol(0x protocol) has been presented to enable free EC20 token treading over Ethereum blockchain. the white paper is available https://www.0xproject.com/whitepaper/0x_white_paper.pdfhttps://github.com/bellaj/Bitcoin_Ethereum_docs/blob/master/0x_white_paper.pdf
How it Works
0x protocol is a hybrid implementation of a few existing technologies, combining the off-chain functionality of state channels with on-chain settlement, which they refer to as “off-chain order relay with on-chain settlement” for making transactions. What does this mean? Orders are relayed off-chain and then settled on-chain, lowering the costs for market makers while simultaneously benefiting from the near-instantaneous settlement time of the Ethereum blockchain.