Sensoray 2518 - Manual (Page 9)

Sensoray Model 2518/2519 Ethernet Smart A/D™ 6 Network Interface
improvements can be realized by implementing only
those protocols that are needed for a particular
application.
In light of this, Smart A/D™ servers implement only the
protocols that are fundamental to providing
high-performance I/O services and diagnostics on a
dedicated LAN.
3.4.1 Address Resolution Protocol (ARP)
ARP is used to establish the mapping between IP
addresses and their corresponding physical Ethernet
(MAC) addresses. ARP exists at the boundary between
internet protocols and physical network; it is not
technically part of the internet protocols. ARP
transactions are transparent to the application process.
When a client first connects to a Smart A/D™ server it
doesn’t know the server’s MAC address; it only knows
the server’s IP address. In order to direct packets to a
specific server, the client must first determine the
destination MAC address. The following sequence of
events occurs when the client first sends data to a Smart
A/D™ server:
1. The client broadcasts an ARP request to the server’s
IP address. This is a message to the server, asking it
to respond with its MAC address.
2. The server responds by sending an ARP response
back to the requesting client. This message contains
the server’s MAC address. The server installs the
clients IP/MAC address pair into an address
resolution cache that it maintains in local memory.
3. The client extracts the server’s MAC address from
the ARP response and installs the server’s IP/MAC
address pair to its own address resolution cache.
4. Now that the server’s MAC address is known, the
client sends the data to the server.
The next time the client wishes to send data to the server,
it uses the cached address instead of executing another
time- and bandwidth-consuming ARP transaction.
3.4.1.1 ARP Implementation
With regard to address resolution caches, Smart A/D™
servers deviate in several ways from general-purpose
hosts. These deviations exist for the purpose of
providing the highest possible performance.
Specifically:
• General-purpose hosts maintain their address
resolution caches by expiring cache entries that have
not been accessed for some time interval, typically one
minute. Smart A/D™ servers expire cache entries only
upon reset.
• General-purpose hosts often employ large address
resolution caches to support the potentially large
number of hosts that might exist on a network. The
address resolution cache on Smart A/D™ servers is
limited to four entries.
3.4.2 Internet Protocol (IP)
IP is the foundation for all of the other TCP/IP protocols
employed on Smart A/D™ products. All ICMP and UDP
messages are encapsulated in IP datagrams.
3.4.3 Internet Control Message Protocol (ICMP)
Technically, ICMP is an error reporting mechanism. As
implemented on Smart A/D™ products, however, ICMP
serves as a diagnostic tool for testing reachability.
Smart A/D™ products support two ICMP message types:
Echo Request and Echo Reply. In response to receipt of
an Echo Request message, a Smart A/D™ server will
send an Echo Reply message. Smart A/D™ servers
never originate Echo Request messages.
Most general-purpose hosts have a “Ping” utility that can
be used to initiate ICMP echo transactions. This is an
important diagnostic tool for troubleshooting networks
that incorporate Smart A/D™ products, as the Smart
A/D™ servers can be “ping’ed” independent of any
application software.
3.4.4 User Datagram Protocol (UDP)
UDP is the transport layer mechanism by which
commands are sent to and sensor data is received from
Smart A/D™ boards. This protocol, combined with
application layer handshaking, provides a reliable,
high-performance message delivery service when used
on dedicated I/O networks.
3.5 Network Architecture
All networks that employ Smart A/D™ servers conform
to the classic client-server network model. In the
client-server network model, one or more clients access
the services of one or more servers
.
At least one hub is required in any 10BASE-T Ethernet
network that connects three or more hosts. Hubs provide
a means for connecting hosts to a network.