TCP/IP .NET application detect client active or not?

Hi All,

now i am developing network service application. there are two
application. one is server mode and one is client. when the server
application starts, i establish connection between server and client.
client application send raw string (e.g. name|value1|value2|date) to
server when user press button in client app. but my problem is when
client application is hanging, server side is still connected. so how can
i know client machine is active or not. What kind of technique is suitable
for this kind of situation. (should i use WCF? is it more effective or same).

pls, help me.

best rgds,
df