If two devices need to communicate, they usually perform a handshaking procedure, as specified by a protocol.

A typical sequence that would be followed when a computer wants to send data to a printer...

• The computer sends a signal to the printer
  ("Are you ready?")
• The printer sends an acknowledgement signal back to the computer.
  ("Yes, I am ready")
• The computer sends data to the printer.

On the Internet the TCP/IP protocol uses a three-way handshake...

When a client computer wants to establish a link to a server over the Internet...

• The client computer sends a synchronisation packet (SYN) to the server, part of this contains a randomly chosen sequence number x.
• The server receives this and replies by sending back an acknowledgement packet (SYN-ACK) containing the next number in the sequence (x+1), and a sequence number of its own (y)
• The client replies by sending an acknowledge packet (ACK) containing the next number in the sequence (y+1) back to the server.

At this point the connection is established and data transfer can take place.

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