Cisco LAN2LAN Software

LAN²LAN Technical Bulletin #12

A. This is because propagation delays on typical circuits are 1 millisecond per 100 miles. When you use the NetWare ping-pong file transfer protocol, each transaction suffers the round trip propagation delay. This delay can be further exaggerated because circuit miles can be more than twice as long as straight-line miles.

For example, a line from Oregon to New Jersey was routed through Seattle, Los Angeles, Chicago, and Ohio—a circuit distance of 5,402 miles. A round-trip propagation delay on this circuit is 108 milliseconds.

This means that, for such a line, if you use the NetWare ping-pong file transfer protocol, regardless of the line speed and data compression effectiveness, and ignore any other delays, fewer than 10 transactions a second can occur. If each transaction results in a payload of 512 bytes, the maximum possible transfer rate for a file on this line is approximately 5,000 bytes/second.

As an example, it took 40 seconds at 56 Kbps with data compression to execute PCONSOLE when the system was set up locally with no propagation delay. Because of the propagation delay, this time expanded to 170 seconds when connected 5,400 miles across country.

Solution:

Excessive delays can be alleviated if you use the large-packet feature provided by LAN²LAN. If you use Ethernet, the Packet Size used by your NICs is probably 1024. If you configure Packet Size to be 1024 on your LAN²LANs (see your user guide), you can expect to substantially lower your file-transfer time. The exact amount depends upon your line speed, use of compression, and circuit mile length of your line. If you are a Thomas Conrad ARCNET or Token Ring user, you possibly can increase the packet size even further.

A different solution to the problem will be provided by the Novell Packet Burst NLM when it becomes available.

A. If you need to investigate excessive delays, these suggestions are helpful:

• Calculate your expected time per file transaction (in milliseconds) which is given by:

        Request transmission time + Response transmission time + Two way
        Propagation delay + other delays
  

• Request transmission time is given by:

        File request size (34 bytes)/(line speed in bps/8) * 1000
  

• Response transmission time is given by:

        File response size (552 bytes)/(line speed in bps/8) * 1000
  

• Two-way propagation delay is given by:

       (Circuit miles one way * 2)/100
  

• Other delays = 5 milliseconds

In order to measure the time per file transaction, it is important to know how many transactions are involved. This can be determined with the Surveyor. Choose the LAN²LAN driver at the end nearest to your workstation, and choose the line statistics for the line to be used in the transfer. Clear the statistics with F8 and then, when no one else uses the line, run the required test. Record the number of blocks in each direction.

Divide the number of blocks by the number of seconds it took to transfer the file in order to establish the number of file transactions per second. Divide the number of transactions into 1000 to obtain the number of milliseconds per transaction. Compare this figure with your calculated figure to determine if there are unexplained delays.

It is also important to differentiate between a simple copy of an executable file, for example, COPY PCONSOLE.EXE C:, versus the execution of that file remotely, for example, PCONSOLE. In this example, the number of blocks required to copy the file is 500, but the number of blocks required to execute it up to the first screen is 1281.