Wide-area monitoring and control (WAMC) systems based on synchrophasor data streams are becoming more and more significant to the operation of the smart power grid. Reliable and secure communication, and higher quality of service (very low latency, high availability, etc.) of data are crucial to the success of WAMC systems. However, the IEEE standard for synchrophasor data communication (IEEE Standard C37.118.2-2011) does not place any restrictions on the choice of transport layer protocols. In light of this, we examine the communication between synchrophasors [phasor measurement units (PMUs)] and phasor data concentrators to analyze potential security vulnerabilities present at the transport layer, and investigate the advantages and disadvantages of both the TCP and UDP protocols, respectively, with an emphasis on security issues. Demonstrations of attacks related to these security vulnerabilities are shown in lab environment and underlying mechanisms are analyzed to determine the capabilities attackers to succeed with them.