problem with the data transmitted from ethernet

I assume you have the user's guide for the board. I think you're referring to this board:

ftp://ftp.altera.com/outgoing/devkit/cycloneiii_3c120_dev_kit-v9.0.2.exe

The board uses a Marvell 88E111 10/100/1000 ethernet PHY. This uses a GMII interfaces. Looking at the example projects, these are the corresponding pins:

// ethernet interface

enet_mdc, // mdio clock

enet_mdio, // mdio data

enet_rx_clk, // receive clock

enet_tx_en, // transmit enable

enet_gtx_clk, // 125MHz transmit clock (for gigabit)

enet_rxd, // receive data

enet_txd, // transmit data

enet_rx_dv, // receive data valid

enet_resetn // reset (negated)

Jake