Refers to the party or device that originates a communication, signal, or transmission. Often contrasted with 受信側 (receiver side).
The sender's phone number was not displayed.
The sender and receiver must use the same communication protocol.
Also means sender/transmitter side, but 発信側 is more common for originating a call or message, while 送信側 is often used in data transmission.
Compound of 発信 (transmission, sending) and 側 (side).