Sun Microsystems V2.0 User Manual download pdf (Page 31)

Program 2

RadiostreamConnection conn = (RadiostreamConnection)

Connector.open("radio://0014.4F01.0000.0007:100");

DataInputStream dis = conn.openDataInputStream();

DataOutputStream dos = conn.openDataOutputStream();

try {

String question = dis.readUTF();

if (question.equals("Hello up there")) {

dos.writeUTF("Hello down there");

} else {

dos.writeUTF("What???");

dos.flush();

}

} catch (NoRouteException e) {

System.out.println ("No route to 0014.4F01.0000.0007");

} finally {

dis.close();

dos.close();

conn.close();

}

Data is sent over the air when the output stream buffer is full or when a

flush()

is issued.

The

NoRouteException

is thrown if no route to the destination can be determined. The stream

accesses themselves are fully blocking - that is, the

dis.readUTF()

call will wait forever (unless a

timeout for the connection has been specified – see below).

Behind the scenes, every data transmission between the two devices involves an acknowledgement.

The sending stream will always wait until the MAC-level acknowledgement is received from the

next hop. If the next hop is the final destination then this is sufficient to ensure the data has been

delivered. However if the next hop is not the final destination then an acknowledgement is

requested from the final destination, but, to improve performance, the sending stream does not wait

for this acknowledgement; it is returned asynchronously. If the acknowledgement is not received

despite retries a

NoMeshLayerAckException

is thrown on the next stream write that causes a send or

when the stream is flushed or closed. A

NoMeshLayerAckException

indicates that a previous send has

failed – the application has no way of knowing how much data was successfully delivered to the

destination.

Another exception that you may see, which applies to both

radiostream

and

radiogram

protocols, is

ChannelBusyException

. This exception indicates that the radio channel was busy when the SPOT

tried to send a radio packet. The normal handling is to catch the exception and retry the send.

The radiogram protocol

The

radiogram

protocol is a client-server protocol that provides datagram-based IO between two

devices.

To open a server connection do:

RadiogramConnection conn = (RadiogramConnection) Connector.open("radiogram://:<portNo>");

where

portNo

is a port number in the range 0 to 255 that identifies this particular connection. The

connection is opened using the default radio channel and default PAN Id (currently channel 26,

PAN 3). The section Radio properties shows how to override these defaults.

To open a client connection do:

RadiogramConnection conn =

(RadiogramConnection)Connector.open("radiogram://<serveraddr>:<portNo>");

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Sun Microsystems V2.0 User Manual Page 31

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