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The Question is:
NOTE: this is not another 'can I route LAT' question.
What is the minimum bandwidth required for LAT to work over a dial-up
connection? In Ask the Wizard question number 1578 you mention "a
sufficiently low latency of the connection" related to bridging LAT between
LAN's; is there a specific latency value y
ou can cite? I've configured a bridge circuit with 28.8 modems. I can make an
NCP connection to the remote site (after a few unsuccessful attempts) but the
connection is lost after just a few seconds (with the specific error message
above displayed)-not
e the modem link is up the whole time. I can repeat the process with the same
results. I've looked at the LATCP set node /keepalive and set node
/retransmit parameters, but neither seem like they would solve the problem.
Is there a "listen for response
timeout" parameter in LAT? Obviously, I could replace the 28.8 modems with
56K modems or even an ISDN connection, but I would like to understand the
problem better before I throw money at it.
The Answer is :
This OpenVMS Wizard is surprised you have gotten as far as you have with
this configuration. The expectation is Ethernet bandwidth and Ethernet
latencies. Specifically, a bandwidth of 10 megabits per second, or larger,
is expected.
This means that the Local Area Terminal (LAT) transport and the cluster
System Communications Services (SCS) protocol and related LAN protocols
are expected to operate with bridges and brouters and switches with E3-
or T3-class communications connections, or higher bandwidth. Can you
bridge a LAN with less bandwidth? Quite probably. Can you bridge with
this 28 kilobit link? Clearly not.
When the link bandwidth saturates, the link will also tend to back-up
before the failure, as message retransmissions are attempted. These
retransmissions, of course, serve to increase the load. The results
are not surprising.
How low can you LAN-limbo? That depends on your local application
requirements and your expectations.
Protocols that can be routed will also tend to tolerate network
interruptions -- though routed protocols also tend to fail similarly;
as the retransmissions inevitably overload the link(s).
The usual approach to sizing the link is to assume a routed protocol,
and to calculate the amount of data and the timing requirements for
the data. From this, you can determine if a a high-bandwidth and
high-latency solution -- a delivery truck filled with disks, for
example -- or a low-bandwidth low-latency solution -- a 28.8 kilobit
dial-up modem link -- will suffice, or of something else is required.
Alternatives can include Virtual Private Networks (VPNs; basically
using a fraction of an established high-bandwidth link) and such
schemes as ADSL/DSL, ISDN, ATM, X.25, and other technologies.
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