The ns Manual (formerly ns Notes and Documentation)1 - NM Lab at ...
The ns Manual (formerly ns Notes and Documentation)1 - NM Lab at ... The ns Manual (formerly ns Notes and Documentation)1 - NM Lab at ...
“real-time”. Each of these are described below. The scheduler runs by selecting the next earliest event, executing it tocompletion, and returning to execute the next event.Unit of time used by scheduler is seconds. Presently, the simulator issingle-threaded, and only one event in execution at any given time. If more than one event are scheduled to execute at thesame time, their execution is performed on the first scheduled – first dispatched manner. Simultaneous events are not reorderedanymore by schedulers (as it was in earlier versions) and all schedulers should yeild the same order of dispatchinggiven the same input.No partial execution of events or pre-emption is supported.An event generally comprises a “firing time” and a handler function. The actual definition of an event is found in ~ns/scheduler.h:class Event {public:Event* next_; /* event list */Handler* handler_; /* handler to call when event ready */double time_; /* time at which event is ready */int uid_; /* unique ID */Event() : time_(0), uid_(0) {}};/** The base class for all event handlers. When an event’s scheduled* time arrives, it is passed to handle which must consume it.* i.e., if it needs to be freed it, it must be freed by the handler.*/class Handler {public:virtual void handle(Event* event);};Two types of objects are derived from the base class Event: packets and “at-events”. Packets are described in detail inthe next chapter (Chapter 12.2.1). An at-event is a tcl procedure execution scheduled to occur at a particular time. This isfrequently used in simulation scripts. A simple example of how it is used is as follows:...set ns_ [new Simulator]$ns_ use-scheduler Heap$ns_ at 300.5 "$self complete_sim"...This tcl code fragment first creates a simulation object, then changes the default scheduler implementation to be heap-based(see below), and finally schedules the function $self complete_sim to be executed at time 300.5 (seconds)(Note thatthis particular code fragment expects to be encapsulated in an object instance procedure, where the appropriate reference to$self is correctly defined.). At-events are implemented as events where the handler is effectively an execution of the tclinterpreter.4.2.1 The List SchedulerThe list scheduler (class Scheduler/List) implements the scheduler using a simple linked-list structure. The list iskept in time-order (earliest to latest), so event insertion and deletion require scanning the list to find the appropriate entry.Choosing the next event for execution requires trimming the first entry off the head of the list. This implementation preservesevent execution in a FIFO manner for simultaneous events.39
4.2.2 the heap schedulerThe heap scheduler (class Scheduler/Heap) implements the scheduler using a heap structure. This structure is superiorto the list structure for a large number of events, as insertion and deletion times are in O(log n) for n events. Thisimplementation in ns v2 is borrowed from the MaRS-2.0 simulator [1]; it is believed that MaRS itself borrowed the code fromNetSim [14], although this lineage has not been completely verified.4.2.3 The Calendar Queue SchedulerThe calendar queue scheduler (class Scheduler/Calendar) uses a data structure analogous to a one-year desk calendar,in which events on the same month/day of multiple years can be recorded in one day. It is formally described in [6],and informally described in Jain (p. 410) [15]. The implementation of Calendar queues in ns v2 was contributed by DavidWetherall (presently at MIT/LCS).4.2.4 The Real-Time SchedulerThe real-time scheduler (class Scheduler/RealTime) attempts to synchronize the execution of events with real-time.It is currently implemented as a subclass of the list scheduler. The real-time capability in ns is still under development,but is used to introduce an ns simulated network into a real-world topology to experiment with easily-configured networktopologies, cross-traffic, etc. This only works for relatively slow network traffic data rates, as the simulator must be able tokeep pace with the real-world packet arrival rate, and this synchronization is not presently enforced.4.2.5 Precision of the scheduler clock used in nsPrecision of the scheduler clock can be defined as the smallest time-scale of the simulator that can be correctly represented.The clock variable for ns is represented by a double. As per the IEEE std for floating numbers, a double, consisting of 64 bitsmust allocate the following bits between its sign, exponent and mantissa fields.sign exponent mantissa1 bit 11 bits 52 bitsAny floating number can be represented in the form (X ∗2 n ) where X is the mantissa and n is the exponent. Thus the precisionof timeclock in ns can be defined as (1/2 ( 52)). As simulation runs for longer times the number of remaining bits to representthe time educes thus reducing the accuracy. Given 52 bits we can safely say time upto around (2 ( 40)) can be represented withconsiderable accuracy. Anything greater than that might not be very accurate as you have remaining 12 bits to represent thetime change. However (2 ( 40)) is a very large number and we donot anticipate any problem regarding precision of time in ns.4.3 Other MethodsThe Simulator class provides a number of methods used to set up the simulation. They generally fall into three categories:methods to create and manage the topology (which in turn consists of managing the nodes (Chapter 5) and managing the links(Chapter 6)), methods to perform tracing (Chapter 26), and helper functions to deal with the scheduler. The following is a listof the non-topology related simulator methods:40
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4.2.2 the heap scheduler<strong>The</strong> heap scheduler (class Scheduler/Heap) implements the scheduler using a heap structure. This structure is superiorto the list structure for a large number of events, as i<strong>ns</strong>ertion <strong>and</strong> deletion times are in O(log n) for n events. Thisimplement<strong>at</strong>ion in <strong>ns</strong> v2 is borrowed from the MaRS-2.0 simul<strong>at</strong>or [1]; it is believed th<strong>at</strong> MaRS itself borrowed the code fromNetSim [14], although this lineage has not been completely verified.4.2.3 <strong>The</strong> Calendar Queue Scheduler<strong>The</strong> calendar queue scheduler (class Scheduler/Calendar) uses a d<strong>at</strong>a structure analogous to a one-year desk calendar,in which events on the same month/day of multiple years can be recorded in one day. It is formally described in [6],<strong>and</strong> informally described in Jain (p. 410) [15]. <strong>The</strong> implement<strong>at</strong>ion of Calendar queues in <strong>ns</strong> v2 was contributed by DavidWetherall (presently <strong>at</strong> MIT/LCS).4.2.4 <strong>The</strong> Real-Time Scheduler<strong>The</strong> real-time scheduler (class Scheduler/RealTime) <strong>at</strong>tempts to synchronize the execution of events with real-time.It is currently implemented as a subclass of the list scheduler. <strong>The</strong> real-time capability in <strong>ns</strong> is still under development,but is used to introduce an <strong>ns</strong> simul<strong>at</strong>ed network into a real-world topology to experiment with easily-configured networktopologies, cross-traffic, etc. This only works for rel<strong>at</strong>ively slow network traffic d<strong>at</strong>a r<strong>at</strong>es, as the simul<strong>at</strong>or must be able tokeep pace with the real-world packet arrival r<strong>at</strong>e, <strong>and</strong> this synchroniz<strong>at</strong>ion is not presently enforced.4.2.5 Precision of the scheduler clock used in <strong>ns</strong>Precision of the scheduler clock can be defined as the smallest time-scale of the simul<strong>at</strong>or th<strong>at</strong> can be correctly represented.<strong>The</strong> clock variable for <strong>ns</strong> is represented by a double. As per the IEEE std for flo<strong>at</strong>ing numbers, a double, co<strong>ns</strong>isting of 64 bitsmust alloc<strong>at</strong>e the following bits between its sign, exponent <strong>and</strong> mantissa fields.sign exponent mantissa1 bit 11 bits 52 bitsAny flo<strong>at</strong>ing number can be represented in the form (X ∗2 n ) where X is the mantissa <strong>and</strong> n is the exponent. Thus the precisionof timeclock in <strong>ns</strong> can be defined as (1/2 ( 52)). As simul<strong>at</strong>ion ru<strong>ns</strong> for longer times the number of remaining bits to representthe time educes thus reducing the accuracy. Given 52 bits we can safely say time upto around (2 ( 40)) can be represented withco<strong>ns</strong>iderable accuracy. Anything gre<strong>at</strong>er than th<strong>at</strong> might not be very accur<strong>at</strong>e as you have remaining 12 bits to represent thetime change. However (2 ( 40)) is a very large number <strong>and</strong> we donot anticip<strong>at</strong>e any problem regarding precision of time in <strong>ns</strong>.4.3 Other Methods<strong>The</strong> Simul<strong>at</strong>or class provides a number of methods used to set up the simul<strong>at</strong>ion. <strong>The</strong>y generally fall into three c<strong>at</strong>egories:methods to cre<strong>at</strong>e <strong>and</strong> manage the topology (which in turn co<strong>ns</strong>ists of managing the nodes (Chapter 5) <strong>and</strong> managing the links(Chapter 6)), methods to perform tracing (Chapter 26), <strong>and</strong> helper functio<strong>ns</strong> to deal with the scheduler. <strong>The</strong> following is a listof the non-topology rel<strong>at</strong>ed simul<strong>at</strong>or methods:40
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