September 1998, Issue 98

TCP/IP Networking

TRANSFERRING DATA

We also need to discuss transferring data via TCP/IP. There are two Ethernet speeds, 10 and 100 Mbps.

Most100-Mbps cards automatically detect if the network they’re plugged into is 100 or 10 Mbps. At 10 Mbps, Ethernet can trans-fer almost 1 MBps of data in one direction.

However, you most likely won’t see that high of a throughput. In many cases, if there’s more than one host on the Ethernet, there will be some overhead in trying to access the media and deal with contention.

For Ethernet, this situation can be really bad. The utilization may be as low as 30% when Ethernet is saturated with tens of hosts.

That means all hosts together are only able to transfer about 300 kbps on a highly congested 10-MBps Ethernet. Of course, if only two hosts are involved, we should be able to achieve almost full utilization of the Ethernet.

Another problem is latency. To send data to another host, the data has to traverse the protocol stack, possibly get copied into the Ethernet card’s memory, be transmitted over the Ethernet, be received and copied form the receiving card, and passed back up the protocol stack on the receiver and then reverse this to get an acknowledgment back.

On a lightly loaded 10-MBps network with high-performance Ethernet cards, you might see latencies down to 1.0 ms. However, if the network gets loaded, the latency can easily reach 10–20 ms or more.

Basically, make sure the Ethernet is not very loaded if you expect high utilization or throughput. Also, this means there are upper bounds to what can be done with Ethernet as a device bus.