Speed of WAL send/receive when replicating

1/ Description:

I have a cluster include two machines

• Master: postgresql 9.3 run on Centos


• Slave : postgresql 9.3 run on Centos

Master ----> Slave : relpica from Master to Slave by asynchronous method (M send WAL, S receive WAL)

2/ Question:

How can I monitor (catch) speed of WAL (ex: 1MB/s) is sent from Master to Slave ?