CSMAMacLayer Class Reference
[macLayer - MAC layer modules]

MAC module which provides non-persistent CSMA. More...

#include <CSMAMacLayer.h>

Inherits BasicMacLayer.

Inheritance diagram for CSMAMacLayer:

[legend]Collaboration diagram for CSMAMacLayer:

[legend]List of all members.

Public Member Functions
	Module_Class_Members (CSMAMacLayer, BasicMacLayer, 0)
virtual void	initialize (int)
	Initialization of the module and some variables.
virtual void	finish ()
	Delete all dynamically allocated objects of the module.
virtual void	handleLowerMsg (cMessage *)
	Handle messages from lower layer.
virtual void	handleUpperMsg (cMessage *)
	Handle messages from upper layer.
virtual void	handleSelfMsg (cMessage *)
	Handle self messages such as timers.
virtual void	handleLowerControl (cMessage *msg)
	Handle control messages from lower layer.
virtual void	receiveBBItem (int category, const BBItem *details, int scopeModuleId)
	Called by the Blackboard whenever a change occurs we're interested in.
Protected Types
typedef std::list< MacPkt * >	MacQueue
enum	States { RX, CCA, TX }
	MAC states. More...
Protected Member Functions
virtual void	scheduleBackoff ()
	schedule a backoff
Protected Attributes
States	macState
	kepp track of MAC state
RadioState::States	radioState
	Current state of active channel (radio), set using radio, updated via BB.
int	catRadioState
	category number given by bb for RadioState
double	rssi
	Last RSSI level.
int	catRSSI
	category number given by bb for RSSI
double	busyRSSI
	RSSI level where medium is considered busy.
double	slotDuration
	Duration of a slot.
double	difs
	Maximum time between a packet and its ACK.
SingleChannelRadio *	radio
	cached pointer to radio module
MacQueue	macQueue
	A queue to store packets from upper layer in case another packet is still waiting for transmission..
unsigned	queueLength
	length of the queue
cMessage *	backoffTimer
	Timer for backoff (non-persistent CSMA).
cMessage *	minorMsg
	Multi-purpose message.
unsigned	txAttempts
	count the number of tx attempts
unsigned	maxTxAttempts
	maximum number of transmission attempts
double	bitrate
	the bit rate at which we transmit
unsigned	initialCW
	initial contention window size
DroppedPacket	droppedPacket
	Inspect reasons for dropped packets.
int	catDroppedPacket
	plus category from BB
int	nicId
	publish dropped packets nic wide

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Detailed Description

MAC module which provides non-persistent CSMA.

This is an implementation of a simple non-persistent CSMA. The idea of nonpersistent CSMA is "listen before talk". Before attempting to transmit, a station will sense the medium for a carrier signal, which, if present, indicates that some other station is sending.

If the channel is busy a random waiting time is computed and after this time the channel is sensed again. Once the channel gets idle the message is sent. (State of the channel is obtained from SnrEval via Blackboard.)

An option of this module is to use a queue. If a packet from the upper layer arrives although there is still another packet waiting for its transmission or being transmitted the new packet can be stored in this queue. The length of this queue can be specified by the user in the omnetpp.ini file. By default the length is 0. If the queue is full or there is no queue (length = 0) new packet(s) will be deleted.

Todo:

Inform upper layers about the full queue!

ATTENTION: Imagine the following scenario:

Several stations receive a broadcast request packet, usally exactly at the same time. They will all try to answer at exactly the same time, i.e. they all will sense the channel at exactly the same time and start to transmit because the channel was sensed idle by all of them. Therefore a small random delay should be built into one/some of the upper layers to simulate a random processing time!

The TestApplLayer e.g. implements such a processing delay!

Author:

Marc Löbbers, Yosia Hadisusanto, Andreas Koepke

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Member Enumeration Documentation

enum CSMAMacLayer::States [protected]

MAC states. The MAC states help to keep track what the MAC is actually trying to do -- this is esp. useful when radio switching takes some time. RX -- MAC accepts packets from PHY layer TX -- MAC transmits a packet CCA -- Clear Channel Assessment - MAC checks whether medium is busy

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Member Function Documentation

void CSMAMacLayer::handleLowerMsg (  cMessage *  msg  )  [virtual]

Handle messages from lower layer. Compare the address of this Host with the destination address in frame. If they are equal or the frame is broadcast, we send this frame to the upper layer. If not delete it. Reimplemented from BasicMacLayer.

void CSMAMacLayer::handleSelfMsg (  cMessage *  msg  )  [virtual]

Handle self messages such as timers. After the timer expires try to retransmit the message by calling handleUpperMsg again. Implements BasicLayer.

void CSMAMacLayer::handleUpperMsg (  cMessage *  msg  )  [virtual]

Handle messages from upper layer. First it has to be checked whether a frame is currently being transmitted or waiting to be transmitted. If so the newly arrived message is stored in a queue. If there is no queue or it is full print a warning. Before transmitting a frame it is tested whether the channel is busy at the moment or not. If the channel is busy, a short random time will be generated and the MacPkt is buffered for this time, before a next attempt to send the packet is started. If channel is idle the frame will be transmitted immediately. Implements BasicMacLayer.

void CSMAMacLayer::receiveBBItem (  int  category, const BBItem *  details, int  scopeModuleId )  [virtual]

Called by the Blackboard whenever a change occurs we're interested in. Update the internal copies of interesting BB variables Reimplemented from BasicModule.

void CSMAMacLayer::scheduleBackoff (   )  [protected, virtual]

schedule a backoff A non-persistent CSMA ideally uses a fixed backoff window, but it could also increase this window in a linear fashion, or even exponential -- here, a linear increase is used. Overwrite this function to define another schedule function. It also checks the retransmission counters -- if you overwrite it, do it with care.

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Member Data Documentation

double CSMAMacLayer::difs [protected]

Maximum time between a packet and its ACK. Usually this is slightly more then the tx-rx turnaround time The channel should stay clear within this period of time.

unsigned CSMAMacLayer::maxTxAttempts [protected]

maximum number of transmission attempts The packet is discared when this number is reached.

cMessage* CSMAMacLayer::minorMsg [protected]

Multi-purpose message. Keeps track of things like minimum time of clear channel (important for atomic acks) and race condition avoidance.

double CSMAMacLayer::slotDuration [protected]

Duration of a slot. This duration can be a mini-slot (that is an RX-TX-turnaround time) but in general it should be the time it takes to send a packet plus some guard times (RX-TX-turnaround + a minimum inter-packet space).

unsigned CSMAMacLayer::txAttempts [protected]

count the number of tx attempts This holds the number of transmission attempts, since no Acks are used this is the way how we can empty our queue even with overloaded channels.

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The documentation for this class was generated from the following files:

• CSMAMacLayer.h
• CSMAMacLayer.cc

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