Sensormetrix
Basingstoke Road
Spencers Wood
Reading
Berkshire RG7 1PW
UK
+44(0)118 976 9023
+44(0)118 976 9020
sales@sensormetrix.co.uk
www.sensormetrix.co.uk
sales
fax
email
web
A member of the Olancha Group Ltd
Registered in England No. 4004881
VAT Registration No. GB164 5653 45
EM01b Websensor
Instruction Manual
33
2. If you are not able to execute commands using “Internet explorer” please
make the following changes to turn off caching in IE.
Go to “tools”, “internet options”. Then under Temporary Internet Files tab, click on
settings….then click on the radio button for “Every visit to the page”. Finally, click
on ok.
Websensor connection via router
:
Q
Why doesn’t the Websensor need a subnet mask and default gateway?
:
A
Neither a subnet mask nor default gateway is needed because the
Websensor only responds to HTTP requests. The incoming packet contains
the necessary information needed for the Websensor to return data. As in
Q4, we know that incoming TCP/IP packets include the source IP/MAC
addresses. Websensor will never initiate any TCP/IP communication.
:
Q
How can I retrieve data from a Websensor?
:
A
The Websensor has a WWW service built in (mini web-server). You can use
any web browser to access the Websensor.
:
Q
Will a Websensor initialize any kind of TCP/IP communication?
:
A
No. The Websensor will not initialize any TCP/IP communication. It only listens
on port 80, waiting for other computers to send a HTTP request.
:
Q
How does the Websensor return data to the computer which sends the HTTP
request?
:
A
Every TCP/IP packet will include the destination IP/MAC, and source IP/
MAC addresses. When the Websensor responds to a request (within the
same subnet), it simply swaps the source and destination IP/MAC addresses.
(During the very first transmission when the PC knows the Websensor IP
address, but not the Websensor MAC address, it will issue an ARP request
which contains the Websensor’s IP address and use ‘FF’s’ to fill-in the
Websensor’s MAC address. The Websensor performs an ‘ARP respond’ to
broadcast its IP address and associated MAC address.)
:
Q
How does the Websensor return data between subnets?
:
A
In the case where a packet passes through a router, the router will substitute
its own MAC address in place of the originating source MAC address. The
source IP/MAC addresses can be retrieved from any incoming TCP/IP
packet. The Websensor will use the modified IP/MAC address to return its
data. This is further explained in the examples following the Q & A section.
Please Reference the Figure on page 34
