14.1 Implementation

The procedures and functions described in this section can be found in ~ns/errmodel.{cc, h}.

Error model simulates link-level errors or loss by either marking the packet's error flag or dumping the packet to a drop target. In simulations, errors can be generated from a simple model such as the packet error rate, or from more complicated statistical and empirical models. To support a wide variety of models, the unit of error can be specified in term of packet, bits, or time-based.

The ErrorModel class is derived from the Connector base class. As the result, it inherits some methods for hooking up objects such as target and drop-target. If the drop target exists, it will received corrupted packets from ErrorModel. Otherwise, ErrorModel just marks the error_ flag of the packet's common header, thereby, allowing agents to handle the loss. The ErrorModel also defines additional Tcl method unit to specify the unit of error and ranvar to specify the random variable for generating errors. If not specified, the unit of error will be in packets, and the random variable will be uniform distributed from 0 to 1. Below is a simple example of creating an error model with the packet error rate of 1 percent (0.01):

        # create a loss_module and set its packet error rate to 1 percent
        set loss_module [new ErrorModel]
        \$loss_module set rate_ 0.01

        # optional:  set the unit and random variable
        \$loss_module unit pkt            # error unit: packets (the default);
        \$loss_module ranvar [new RandomVariable/Uniform]

        # set target for dropped packets
        \$loss_module drop-target [new Agent/Null]

In C++, the ErrorModel contains both the mechanism and policy for dropping packets. The packet dropping mechanism is handled by the recv method, and packet corrupting policy is handled by the corrupt method.

        enum ErrorUnit { EU_PKT=0, EU_BIT, EU_TIME };

        class ErrorModel : public Connector {
        public:
                ErrorModel();
                void recv(Packet*, Handler*);
                virtual int corrupt(Packet*);
                inline double rate() { return rate_; }
        protected:
                int command(int argc, const char*const* argv);
                ErrorUnit eu_;          /* error unit in pkt, bit, or time /
                RandomVariable* ranvar_;
                double rate_;
        };

The ErrorModel only implements a simple policy based on a single error rate, either in packets of bits. More sophisticated dropping policy can be implemented in C++ by deriving from ErrorModel and redefining its corrupt method.