North-south traffic

North-South Traffic or Communication (also, N-S, N/S, or N | S) denotes a direction of traffic flow into and out of a data center. 
Based on the most commonly deployed topology of systems within a data center, North-South traffic typically indicates data flow that either enters or leaves the data center from/to a system physically residing outside the data center, such as user to server. Southbound traffic is data entering the data center (through a firewall and/or other networking infrastructure); data exiting via the perimeter network devices is said to be northbound. The other direction of traffic flow is East-west traffic which typically indicates data flow within a data center.