A wireless power transmitter, comprising: a communication/control unit configured to: transmit a digital ping to solicit a response from a wireless power receiver and to wake up the wireless power receiver, and perform a negotiation for a first available power indicator with the wireless power receiver; and a power conversion unit configured to transmit a wireless power to the wireless power receiver by generating magnetic coupling in a primary coil according to the first available power indicator, wherein the wireless power transmitter is configured to: receive, from the wireless power receiver, a received power packet indicating a power value received by the wireless power receiver, transmit, to the wireless power receiver during a power transfer phase in which the wireless power is transmitted, a bit pattern response requesting a communication to change the first available power indicator in response to the received power packet, receive, from the wireless power receiver, a response signal indicating a permission for enabling the wireless power transmitter to transmit a packet, and transmit, to the wireless power receiver during the power transfer phase, the packet related to a second available power indicator after receiving the response signal, wherein the bit pattern response is composed of 8 bits, and wherein the bit pattern response is defined as a different pattern from a bit pattern for an 8-bit acknowledge, ACK, response, from a bit pattern for an 8-bit not acknowledge, NAK, response, and from a bit pattern for an 8-bit ND response. transmit a digital ping to solicit a response from a wireless power receiver and to wake up the wireless power receiver, and perform a negotiation for a first available power indicator with the wireless power receiver; and a power conversion unit configured to transmit a wireless power to the wireless power receiver by generating magnetic coupling in a primary coil according to the first available power indicator, wherein the wireless power transmitter is configured to: receive, from the wireless power receiver, a received power packet indicating a power value received by the wireless power receiver, transmit, to the wireless power receiver during a power transfer phase in which the wireless power is transmitted, a bit pattern response requesting a communication to change the first available power indicator in response to the received power packet, receive, from the wireless power receiver, a response signal indicating a permission for enabling the wireless power transmitter to transmit a packet, and transmit, to the wireless power receiver during the power transfer phase, the packet related to a second available power indicator after receiving the response signal, wherein the bit pattern response is composed of 8 bits, and wherein the bit pattern response is defined as a different pattern from a bit pattern for an 8-bit acknowledge, ACK, response, from a bit pattern for an 8-bit not acknowledge, NAK, response, and from a bit pattern for an 8-bit ND response. receive, from the wireless power receiver, a received power packet indicating a power value received by the wireless power receiver, transmit, to the wireless power receiver during a power transfer phase in which the wireless power is transmitted, a bit pattern response requesting a communication to change the first available power indicator in response to the received power packet, receive, from the wireless power receiver, a response signal indicating a permission for enabling the wireless power transmitter to transmit a packet, and transmit, to the wireless power receiver during the power transfer phase, the packet related to a second available power indicator after receiving the response signal, wherein the bit pattern response is composed of 8 bits, and wherein the bit pattern response is defined as a different pattern from a bit pattern for an 8-bit acknowledge, ACK, response, from a bit pattern for an 8-bit not acknowledge, NAK, response, and from a bit pattern for an 8-bit ND response.