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system memory because they occur across an I/O expansion bus. This means
that when shared RAM adapters are involved, the CPU spends a significant
amount of time doing the primitive task of moving data from point A to point B.
On lightly loaded servers providing traditional productivity applications such as
word-processing, spreadsheets, and print sharing, this is not really a problem.
But, for applications such as database or for more heavily loaded file servers,
this can be a major source of performance degradation.
The IBM Token Ring Network 16/4 Adapters I and II for MCA and ISA are
examples of shared RAM adapters.
1.7.2 Bus Master Adapters
Bus master adapters utilize on-board Direct Memory Access (DMA) controllers
to transfer data directly between the adapter and the system memory without
involving the system processor. The primary advantage of this architecture is
that it frees up the system processor to perform other tasks, which is especially
important in the server environment.
The IBM 16/4 Token-Ring bus master adapter/A:
This adapter was the first
generation of bus master LAN adapters from IBM It employed the 64KB on-board
adapter memory as a frame buffer which was used to assemble frames before
they were sent to the server or sent from the server to the network. The time
elasticity provided by this buffer allowed the token-ring chip set to complete its
processing and forwarding of the frame before the frame was lost; this is a
condition known as overrun (receive) or underrun (transmit).
This adapter was a 16-bit Micro Channel bus master capable of burst mode
DMA. Due to the 24-bit addressing capabilities of the adapter, it was limited to
using only the first 16MB of system address memory.
IBM LANStreamer Family of Adapter Cards:
The LANStreamer technology
employs a completely different design to previous IBM LAN adapters. The
LANStreamer utilizes a revolutionary chip set that is capable of processing
token-ring frames without using memory as a frame buffer. It does it
on-the-fly
as the frames are passing through the adapter. Therefore, the latency of
assembling frames from an on-card buffer is eliminated.
This low latency chip set is the key to the small-frame performance
characteristics of the LANStreamer adapter. The throughput for the
LANStreamer Token-Ring MC 32 Adapter/A is quite high relative to its
predecessors, especially for small frames. This is extremely important in
client/server environments where research has shown that the vast majority of
frames on the network are less than 128 bytes.
Another advantage of this technology is that since adapter memory buffers are
no longer required, the adapter is less expensive to produce.
A consequence of the high LANStreamer throughput is that the LAN adapter is
not usually the bottleneck in the system. Also, a side effect of using
LANStreamer technology could be the higher CPU utilization. This sometimes
happens because the LANStreamer adapter can pass significantly more data to
the server than earlier adapters. This corresponds to more frames per second
that must be processed by the server network operating system. Higher
throughput is the desired effect but what this also means is that the bottleneck
32 NetWare Integration Guide