Network Elements






Global Network Variables

Random Variate Generators


Branches are used to model activities. Only at branches are explicit time delays prescribed for entities flowing through the network. Activities emanating from QUEUE nodes are referred to as service activities. Service activities restrict the number of concurrent entities flowing through them to be equal to the number of servers represented by the activity.
  1. Branches starting on a QUEUE node are service activities

    ACTIVITY (N)/A,DUR,PROB; DUR = activity duration; any network variable or random variate PROB = probability that transaction will move along this branch N = number of parallel servers A = integer label , 1 < = A < = 100

    	Elevator Example
    	Column:  1	7
    			CREATE, 10, 7;
    			ACTIVITY, 3;
    			QUEUE (1);
    				ACTIVITY (1)/3, 9, 0.75, EXIT;
    				ACTIVITY (1)/3, 8.5, 0.25;
    		EXIT	TERMINATE, 100;

  2. Branches not starting on a QUEUE node are regular activities

    PROB COND = Fortran-like logical condition involving network variables;
    transaction will move along this branch if condition is true

Basic nodes for traffic generation and control

  1. Create node - creates entities and routes them into the system over activities that emanate from the CREATE node

    mnemonic    definition/comments      default
    TF time of first creation 0.0 TBC time between creations infinity can be any network variable MA mark attribute None MC maximum number of infinity transactions created M M-number
    The M-number on the output side of a non-queue node specifies the maximum number of emanating activities which are taken
    1. If there are any probabilistic branches, draw a random number to select a single probabilistic branch
    2. Test conditional branches in the order that they occur and select additional activities to start until the M-number is exhausted
  2. Terminate node - terminates (destroys) entities (reclaims disk space used to store information about the entity terminated). The TERMINATE node is used to destry or delte entities from the network.

    mnemonic    definition/comments                  default
    TC termination counter for this exit point infinity
  3. Queue node - queue in which entities wait (are queued) A QUEUE is a location in the network where entities wait for service. When an entity arrives at a QUEUE node, its disposition depends on the status of the server that follows the QUEUE node. If the server is idle, the entity passes through the QUEUE node and goes immediately into the service activity. If no server is available, the entity waits in the QUEUE node until a server can process it. When a server becomes available, the entity will automatically be taken out of the queue and service will be initiated.

    When an entity arrives at a QUEUE node which is at its capacity, its disposition must be determined. This decision is based on a specification at the QUEUE node as to whether the entity should balk or be blocked. In the case of balking, the entity can be routed to another node of the network. When an entity is blocked by a QUEUE node, it waits until a free space is available in the queue. The activity which just served the entity that is blocked is also considered as blocked. A blocked entity will join the queue when a space is available.

    mnemonic    definition/comments                  default
    IQ initial # in queue 0 QC queue capacity infinity IFL file #, 1 < = IFL < = 100
  4. Accumulate node - entities accumulate at this node; once the specified number have accumulated, an entity is released that represents the group of entities that "accumulated"

    mnemonic    definition/comments                  default
    FR number of accumulated 1 transactions for first release of node SR transaction count required on 1 subsequent releases SAVE save criterion : FIRST LAST LAST, HIGH(I), LOW(I), SUM, MULT
  5. Goon node - node used to connect activity branches; provides linkage for activities in series. The GOON node is used in the modeling of sequential activities since 2 consecutive activity statements are used to model parallel activities. Also probabilities and/or deterministic branching can occur following GOON nodes.

    GOON, M;

Operations on network variables

  1. Assign node - assigns values to entity or network variables. The ASSIGN node is used to prescribe values to the attributes of an entity passing through the ASSIGN node or to the prescribed values to the system variables that pertain to the network in general.

    ASSIGN, VARI = EXP1, VAR2 = EXP2, ..., M;
    mnemonic               definition/comments 
    VAR1, VAR2, ... One of the SLAM network variables: ATRIB(I), XX(I), SS(I), DD(I), II EXP1, EXP2, ... Arithmetic expression involving at most 10 of the following: Network variables, random variables, constants; No parentheses
  2. COLCT node - tabulate statistics on observations taken during the simulation. The COLCT (collect) node is used to collect statistics on five types of variables: 1) time of all arrivals, 2) time of first arrival, 3) time between arrivals, 4) interval statistics, and 5) a SLAM II variable. For each of the above 5 types of variables, estimates for the mean and standard deviation of the variable are obtained. In addition, a histogram of the values collected at a COLCT node can be obtained.

    mnemonic            definition/comments     default
    TYPE observation to be recorded: FIRST LAST node arrival times BETWEEN INTERVAL - time in system SLAM network variable ID 16 character output label BLANK NCEL number of interior cells in 0 histogram to be printed HLOW upper limit of unbounded 0.0 cell on the left HWID width of interior cells; class 1.0 interval for empirical frequency distribution
  3. Select node - used to provide branching into a set of queues; determines which queue an entity will enter based on the queue selection rule being used

    mnemonic            definition/comments          default
    QSR Queue selection rule looking forward POR to destination-queues BLOCK Block the service activity ending at no blocking; node SLBL if all destination queues transaction destroyed are full BALK(NLBL) Balk to node NLBL if all destination no balking; queues are full transaction destroyed QLB1, QLB2, ... Labels of queue nodes to which ERROR transactions are to be routed QSR options POR Priority given in preferred order CYC Cyclic priority RAN Random priority LAV priority given to Queue which has had the largest average number of entities in it to date SAV priority given to queue which has had the smallest average number of entities to date LNQ priority to queue with largest number of entities in queue SNQ priority given to queue with smallest number of entities in queue

Simulation control statements

  1. GEN statement -
          IXQT, IPIRH, ISMRY, IPR;
    mnemonic    definition       options                  default
    NAME Analyst's name 20 alphanumeric characters Blanks PROJECT Project name 20 characters Blanks MONTH Month number 1 DAY Day number 1 YEAR Year number 2001 NNRNS Number of runs 1 ILIST Request for Y or N Y input listing IECHO Request for echo Y or N Y summary report IXQT Request for Y or N Y execution IPIRH Request for Y or N Y intermediate results heading ISMRY Request for Y or N Y summary reports IPR Print option 72, 130 130
  2. LIMITS statement - defines the maximum number of entities and entity attributes (used to assign memory required)
    mnemonic   definition                        default
    MFIL largest file # 1 MATR maximum # of attributes per entity 0 MNTRY maximum # of entries in all files 0
  3. PRIORITY statement - defines the queue priority rule to be used
    mnemonic   definition                        default
    IFL file number None RNK file ranking: LVF(I); FIFO HVF(I); FIFO; LIFO
  4. Seeds statement - assigns a starting seed for a random number stream
    mnemonic   definition                             default
    ISEED integer seed; negative seed yields complimentary sequence of random numbers ISTRM Stream number R reinitialization option (Y or N) N
  5. Initialize statement - defines when the simulation begins and stops
    mnemonic   definition                    default
    TTBEG beginning time of run 0.0 TTFIN finishing time of run 10 sup 20
  6. Monitor statement - defines when "monitoring" information is collected; useful for debugging and better understanding a simulation
    mnemonic   definition         options                      default
    OPT monitor option SUMRY - print summary report None FILES - list contents of files STATES - list SS(.), DD(.) vars CLEAR - clear statistical arrays TRACE - trace transaction flow TFRST time of 1st report 0.0 TBTWN time between reports 10 sup 20
  7. Simulate statement - indicates that network simulation statements follow
  8. FIN - signals end of SLAM statements