- Contrary to popular opinion, the SCSI bus is more than just a disk
and tape bus. It's a widely-accepted, general-purpose interface
that specifically includes a group of commands for communications
devices, such as our scsiTerminal Servers. And with its high level
of market penetration (almost all systems today have a SCSI port),
it's an ideal choice for high- performance serial and parallel
expansion.
- Even with several fast disks on a system, there's usually plenty of
"air space" left on the SCSI bus. After all, the bandwidth of
SCSI-1 is 5 Mbytes per second, which is four times greater than
Ethernet. And SCSI-2 doubles the transfer rate to 10 Mbytes per
second. So to make the most efficient use of available bus time,
our products assemble traffic from multiple lines into packets for
transport across the SCSI bus. SCSI transaction rates are carefully
managed to yield both low overhead and good responsiveness.
- The result is that each scsiTerminal Server consumes only 5-8% of
SCSI bandwidth, and then only during heavy traffic to all ports.
Because each scsiTerminal Server uses only one SCSI address, they
can easily share the SCSI bus with other SCSI-based peripherals.
Their impact on SCSI performance is so minimal, they're almost
invisible. When performing file copies from SCSI disk to SCSI
disk, actual measurements show less than a 1% variance in elapsed
time when comparing no scsiTerminal Server (STS) traffic with
maximum STS traffic.
SCSI vs. Ethernet
- scsiTerminal Servers offer many advantages compared to Ethernet
terminal servers. scsiTerminal Servers are much easier to install.
There are no complex network configurations to deal with, and no
transceivers or messy ethernet cables to install. SCSI's simpler
design results in a much lower cost-per-port than Ethernet terminal
servers.
- The increased contention for limited Ethernet bandwidth can result
in serious I/O bottlenecks. Using scsiTerminal Servers reduces
network traffic, improving overall performance. And scsiTerminal
Servers consume significantly less CPU overhead than TCP/IP-based
terminal servers. The scsiTerminal Server uses a kernel driver, and
does not require additional overhead to handle individual TCP/IP
protocol stacks, daemons, and pseudo-TTYs.
- A major concern with Ethernet terminal servers is system security.
scsiTerminal Servers provide a much safer solution, allowing the
UNIX operating system on the host machine to handle all security
issues.
SCSI vs. Bus-boards
- scsiTerminal Servers are similar to SBus, EISA, and MicroChannel
serial boards. But there are some important differences that
translate into considerable savings. scsiTerminal Servers connect
to the host with a SCSI cable, saving valuable host system card
slots. By connecting serial ports externally, you can save
thousands of dollars by purchasing workstations with fewer (and in
some cases, zero) card slots.
- scsiTerminal Servers can also save time. You don't even have to
dismantle your system to install one. Not so with bus-based
multiport serial cards. And scsiTerminal Servers are portable. They
can be easily moved from one UNIX platform to another, protecting
your investment in serial I/O. With bus-boards, once you move to a
new platform, you'll probably have to incur the serial I/O expense
all over again.
- When expanding their number of ports, scsiTerminal Server users
only need to connect another unit to their SCSI chain. Up to seven
scsiTerminal Servers can be attached to a single SCSI bus,
providing hundreds of port options without using a single card
slot.
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