android - IM client and servers -

can explain how make basic im client old pc server, client can on ios or android? , please explain how work im , servers simultaniously.

for android use google cloud messaging , similar messaging service ios.

when client send's message, should sent gcm via upstream service , gcm push message server. server house deviceid's of client's , can push message whenever required client making api call gcm message , deviceid.
check documentation gcm , clear idea.
there other services gcm can work android ios

reason using such service ensures messaging full proof. if client not having internet connectivity or switched off, shall receive message on getting internet connection. not have handle troubles!