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- Subject: N-1-4-040.92
-
- What's the response time ?
-
- by Frode Greisen <Frode.Greisen@uni-c.dk>
- and Greg Lloyd <glloyd@frors12.bitnet>
-
- Most network providers give a very good response time and availability,
- both for interactive work, file transfer and electronic mail. That is,
- on the days when everything is working well.
-
- Users develop an intuitive feeling of the quality of the network which
- may be somewhat dependent on temper. Some people tend to remember the
- normal days when things went fine while others concentrate on that
- mail which stayed in a postmaster's mailbox for a week or two before it
- was manually forwarded or the mail that never got through, either because
- it really was lost or because the recipient cancelled it by mistake.
-
- In order to get an objective picture, EARN has been monitoring its
- network's performance along three scales for the last five months.
- These are link availability, link file-queues and round trip times
- (RTTs) for both files and interactive messages. These measurements are
- designed to provide not only technical performance data for the
- network, but also to evaluate the quality of the network as perceived
- by a typical end-user.
-
- EARN monitors its network links, file-queues and message RTTs down to
- its international level. That is, each member country subscribed to the
- EARN Association has designated one international node that acts as that
- country's gateway into the international network. A subset of these
- international nodes have been selected as the EARN backbone and make up
- the EARN Core nodes. The remaining international nodes are allocated
- into regions, each region being serviced by a specific EARN Core node.
-
- In addition to collecting figures relating solely to its own
- international backbone, round trip time figures are also recorded for
- EARN's transatlantic links with the BITNET network. These files
- traverse a section of the BITNET backbone, cross the Atlantic and
- enter the EARN backbone and are subsequently returned to the USA.
-
- File RTTs are measured down to an inter-regional level (across the EARN
- backbone). They are measured for two different file sizes; the first is
- 50 records files (representative of a typical piece of electronic mail)
- and the second, 1001 records files (representative of a medium sized
- data file). Every hour around the clock, seven days a week, these files
- are transmitted and returned from each EARN core site to every other
- EARN core site over the backbone. The time in seconds taken for this
- round trip is recorded and used to calculate the averages displayed
- below.
-
- Avg RTTs Avg RTTs
- (1001 recs) (50 recs)
- (seconds) (seconds)
-
- June '92 525 423
- July '92 564 459
- August '92 521 502
- September '92 464 307
- October '92 410 362
-
- The above figures show the round trip times for differently sized files
- to traverse and return over the EARN backbone, EARN's transatlantic
- links to the USA and a section of the BITNET backbone. In ideal
- conditions, these figures range from 12 to 19 seconds for 1001 files
- and 6 to 8 seconds for 50 records files. But as can be seen, the cases
- where a node is down over the week-end or when any of the intermediary
- links between sender and receiver has a failure seriously affects the
- average.
-
- The numbers above are for two hops over the backbone. Now, the average
- number of hops between any two of the 3389 EARN/BITNET nodes in 48
- countries is 6.7. So if we divide the measurements by two and multiply
- by 6.7 we get the likely average transit time for an e-mail of around
- 20 minutes. This is not perfect, and we are working to improve it, but
- it is better than postal services.
-