Question: What happens when a SERVICE REQUEST message is received with a protocol error in the network side?
Option 1: The network sends a SERVICE ACCEPT message.
Option 2: The network stays in PMM-CONNECT mode.
Option 3: The network initiates the GMM common procedures.
Option 4: The network returns a SERVICE REJECT message with one of the specified reject causes.
Explanation: According to the text, if a SERVICE REQUEST message is received with a protocol error, the network shall return a SERVICE REJECT message with one of the following reject causes: #96: Mandatory information element error; #99: Information element non-existent or not implemented; #100: Conditional IE error; #111: Protocol error, unspecified. The network stays in PMM-IDLE mode.