rf

Class RF

java.lang.Object

rf.RF

All Implemented Interfaces:

java.lang.Runnable

public class RF
extends java.lang.Object
implements java.lang.Runnable

The RF class models a broadcast-based wireless system, but does so on a traditional network by exchanging UDP packets. The layer models collisions properly (packets involved in collisions are not delivered to the MAC layer), and provides the carrier-sense facility required to implement a CSMA-like system. The RF layer knows nothing about addressing, packet structures, etc. It simply takes a collection of bytes from the caller and broadcasts them. Similarly, incoming transmissions from any and all sources are collected and delivered, as long as they were not involved in a collision.

The RF class uses multicast UDP packets to communicate with other "hosts", which should be forwarded by most routers, and uses a scheme that allows multiple RF instances to be running simultaneously on a computer with a single IP address. All RF instances should see one another's traffic, so multiple instances on a single PC as well as other instances sharing the same network with the PC will behave as if they're in the same collision domain. By default, the UDP multicast address 224.0.1.38 is used (with group 1301), though a different address can be specified to the constructor.

Version:

v2.0

Author:

Brad Richards

Field Summary

Fields

Modifier and Type	Field and Description
static int	aCWmax The maximum size of the collision window
static int	aCWmin The minimum size of the collision window
static int	aMPDUMaximumLength The maximum number of bytes allowed in an RF packet
static int	aSIFSTime The SIFS inter-frame spacing time, in milliseconds
static int	aSlotTime The slot time, in milliseconds
static int	dot11RetryLimit The maximum nubmer of retransmission attempts

Constructor Summary

Constructors

Constructor and Description
RF(java.io.PrintWriter output, java.lang.String mcastAddr) The constructor initializes the simulated RF transceiver and prepares it for use.

Method Summary

Modifier and Type	Method and Description
long	clock() The RF layer's local clock.
boolean	dataWaiting() This non-blocking method returns true if the RF layer has one or more packets to deliver to the caller.
boolean	inUse() Inspects the shared channel to see if it's currently being used.
byte[]	receive() This method blocks until an RF packet arrives, then returns its contents as a byte array.
void	run() Starts executing the RF thread, which watches for incoming RF packets and queues them for delivery.
int	transmit(byte[] data) This routine takes data from the MAC layer and broadcasts it to all units on the same subnet.
void	transmitAdmin(byte[] msg) Do not use this.

Methods inherited from class java.lang.Object

equals, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Field Detail

aSIFSTime

public static final int aSIFSTime

The SIFS inter-frame spacing time, in milliseconds

See Also:

Constant Field Values

aSlotTime

public static final int aSlotTime

The slot time, in milliseconds

See Also:

Constant Field Values

aCWmin

public static final int aCWmin

The minimum size of the collision window

See Also:

Constant Field Values

aCWmax

public static final int aCWmax

The maximum size of the collision window

See Also:

Constant Field Values

aMPDUMaximumLength

public static final int aMPDUMaximumLength

The maximum number of bytes allowed in an RF packet

See Also:

Constant Field Values

dot11RetryLimit

public static final int dot11RetryLimit

The maximum nubmer of retransmission attempts

See Also:

Constant Field Values

Constructor Detail

RF

public RF(java.io.PrintWriter output,
          java.lang.String mcastAddr)

The constructor initializes the simulated RF transceiver and prepares it for use. As it runs, the RF layer will print information about channel use to the PrintWriter passed in as argument (unless it's null). An RF instance initializes its internal clock when created. Times, reported in milliseconds, are not directly comparable across RF transceivers. It is safe to create multiple RF instances on a single computer, and they (along with any remote RF layers on the same network) will all behave as though they're part of the same collision domain.

Parameters:

output - If not null, RF layer will print diagnostic information to this PrintWriter.

mcastAddr - String containing the desired multicast IP address. Defaults to 224.0.1.38 if this argument is null.

Method Detail

transmitAdmin

public void transmitAdmin(byte[] msg)

Do not use this. Seriously.

clock

public long clock()

The RF layer's local clock. Measures time in milliseconds on this machine, but there's no guarantee that the time on this machine is in synch with the RF clock values on any other machine. (In fact, it's almost certainly the case that it's not in synch with other machines.)

Returns:

Current time in milliseconds

inUse

public boolean inUse()

Inspects the shared channel to see if it's currently being used. This method does not block, and returns true if at least one host is currently broadcasting, false otherwise.

Returns:

true if the channel is in use

dataWaiting

public boolean dataWaiting()

This non-blocking method returns true if the RF layer has one or more packets to deliver to the caller. Since the RF layer is broadcast based, and has no notion of addresses, it will deliver any intact packet that it observes on the channel. In other words, this method might return true even if the packet wasn't intended for the MAC layer that called dataWaiting().

Returns:

true if the RF layer has accumulated one or more packets

receive

public byte[] receive()

This method blocks until an RF packet arrives, then returns its contents as a byte array. The RF layer queues arriving packets for delivery, so it's possible that receive will return immediately with the contents of the packet at the head of the queue. If blocking is unacceptable, only call receive() once dataWaiting() reveals that a packet has arrived.

Returns:

the contents of the next RF packet

run

public void run()

Starts executing the RF thread, which watches for incoming RF packets and queues them for delivery. User code should never have to call this explicitly, as the constructor creates a separate thread that invokes run().

Specified by:

run in interface java.lang.Runnable

transmit

public int transmit(byte[] data)

This routine takes data from the MAC layer and broadcasts it to all units on the same subnet. It attempts to transmit all of the data in they array of bytes, and returns the number of bytes actually transmitted.

Parameters:

data - The array of bytes to transmit

Returns:

number of bytes sent