Subscription rate is the difference between input and output bandwidth of each layer of switching. Network can be fully subscribed, oversubscribed, or undersubscribed. Fully subscribed network is having the same amount of downlink bandwidth as the uplink bandwidth for all the switches in the network. In other words, outgoing signal in the switch is equal to the incoming signal. Oversubscribed device is having larger downlink bandwidth than the uplink bandwidth (outgoing signal is bigger than the incoming). Undersubscribed switch would have less outgoing bandwidth than it receives.

 

Oversubscription is used for applications that are not too sensitive about packet loss and can tolerate network unavailability sometimes. However, oversubscription is not tolerated for the server access switch because of the traditional Ethernet network’s patterns.

 

Enterprise data servers were usually overprovisoned at all traffic levels, in order to have enough bandwidth for peak traffic requirements. Although it is a good approach for the customer, it is not cost-effective for the data center vendor. Development of new applications made it almost impossible to predict the peak values.

 

When the network is able to transmit data in all available directions at the full strength, it is called fairness. If the network is nonblocking the only way network can be congested is if the bandwidth limit is reached. If any output port is congested it would not affect performance of all other interfaces. At times when storage traffic is transmitted network should not drop data packets. This would allow to increase efficiency when transporting server to disk traffic.