13.3.3 Channel Class in C++

In C++, the class Channel extends the Connector object with several new methods to support a variety of MAC protocols. The class is defined as follow in channel.h:

   class Channel : public Connector {
   public:
        Channel();
        void recv(Packet* p, Handler*);
        virtual int send(Packet* p, double txtime);
        virtual void contention(Packet*, Handler*);
        int hold(double txtime);
        virtual int collision() { return numtx_ \> 1; }
        virtual double txstop() { return txstop_; }
                . . .
   };

The important methods of the class Channel are:

• txstop() method returns the time when the channel will become idle, which can be used by the MAC to implement carrier sense.
• contention() method allows the MAC to contend for the channel before sending a packet. The channel then use this packet to signal the corresponding Mac object at the end of each contention period.
• collision() method indicates whether a collision occurs during the contention period. When the Channel signal the end of the contention period, the MAC can use the collision() method to detect collision.
• send() method allows the MAC object to transmit a packet on the channel for a specified duration of time.
• hold() method allows the MAC object to hold the channel for a specified duration of time without actually transmitting any packets. This is useful in simulating the jamming mechanism of some MAC protocols.