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about 3000 characters per second. This means that the
channel can handle all data from the 2790 adapter, assuming
adequate data service is provided by System/360. In addi
tion, deferred data can be recovered from the disk at the
same time. Factors that affect the actual data rate include:
rate of data reception from terminals, the amount of data
that consists of sorted transactions, and interference from
other devices on the multiplexer channel.
If the 2715 Model 2 is used, data transfer rates are
affected by the line speed. The maximum data-transfer
rate at 4800 bits per second is 600 characters per second;
however, the actual rate is likely to be much less because of
line turnarounds, responses, etc. The channel speed will be
limited primarily by the rate of data reception from the
terminals. Of course, if the input rate from the 2790
adapter exceeds the rate of the channel, data must be stored
on disk for later recovery when channel time is available.
On-Line IOff-Line. There are two critical factors that affect
channel speed. One is the frequency of the request for data;
the other is the amount of terminal activity at the time of
the request.
The 2715 disk priorities are so structured that writes
have priority over reads. If there is a large volume of data
on the disk (infrequent requests), the disk will have more
seek activity to service both reads and writes, thus reducing
channel speed.
If the request is made during a high-traffic period, nearly
all disk time can be spent putting data on the disk and the
read operation will proceed very slowly. Therefore, for best
results, the on-line operation for data recovering should be
scheduled at a low-traffic period. .
In this mode of operation, the sort speed could also be
reduced, since the writes to store data have priority over
disk operations for sorting.
On-Line for Inquiry, On-Line/Off-Line for Other Traffic.
In this mode of operation, all data is classified as deferred,
except for inquiry type transactions. Since all deferred data
is placed on the 2715 integral disk, this mode has the advan
tage that data can be recovered from the disk in case of
system failure. However, data is blocked prior to being
placed onto the disk, with the partial blocks being accumu
lated in core storage. When deferred-data mode is set, data
is recovered from the disk when no priority data is queued
for the channel. If large blocks of data are accumulated
before deferred-data mode is set, the channel throughput
will be slowed because of the added disk activity. Normally,
with deferred-data mode set, there would be little data on
the disk, but throughput will still be somewhat slower. If
there are large volumes of sorted data, the channel speed
could be slowed as sort-disk operations have priority over
read-disk operations. This mode also implies that the disk
must be re-initialized more frequently, since nearly all data
is placed on disk and the limit is reached more quickly.
Transmission Control Unit
55
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