PDF (double-sided) - Physics Department, UCSB - University of ...

structure kept in the Server that holds the collection of all relevant configuration
data.
As far as the LabRAD Protocol is concerned, a Context is nothing more than
a Cluster of two Words (ww). When forwarding Packets, the LabRAD Manager
makes the following alteration to a Packet’s Context before sending it on: For
incoming Packets where the first Context Word is 0, the LabRAD Manager sets
it to the ID of the source Module. For outgoing Packets where the first Context
Word is equal to the ID of the target Module, the Manager sets it to 0. Clients
will usually use 0 as the first Context Word. Servers will then receive Requests
“in a Context” where the first Word is equal to the ID of the Client that sent
the Request. If a Server needs to make secondary Requests to fulfill the original
Request, it can choose whether to make these in its own Context (first Word
equals 0) or in the Context of the original Request. The latter can make sense if
this Server simply assists Clients in the communication with an underlying Server.
The Client, for example, selects the device with the underlying Server and does
some initial setup and then calls the assistant Server to complete a more involved
part of the device setup.
The second Word of a Context can be chosen by the Client at will and allows
this Client to have multiple simultaneous Contexts open with any given Server.
Apart from being generally useful, this feature is indispensable for the imple-
149