I am the system administrator. I am suspecting some unwanted data transfers in some of the computers under my lan. This also effects the overall performance of the network. Is there anyway by which i can check the transfered data over a particular interval. I am not supposed to install any applications on any other computers than mine. I have the router access. I can use tools like ettercap dsniff wireshark etc.
If you switches / routers support SNMP you could poll them with software like Cacti and make all kind of graphs.
Router / Switches have this functionality. Netflow can do that from a decent managed switch, including analysing where the traffic flows. Same router wise - or from any decent firewall.
Hard to give any more information on your hardware layout no answer is possible.
ettercap etc. are useless unless you put your switch into a mode mirroring all traffic to am anagement port - which then will make things SLOW.
I personally would go with netflow, but that would likely be a little too profesional (i.e. pricey) for a one time job.
One quicky answer would be to use the output from "ethtool -S eth0" which will give you statistics on this number of packets and/or bytes in and out of the port. The statistics available will be dependent on the device and driver - e.g. r8169 gives different info than e1000. You could set up periodic data logging of the port and watch the data rate over time.