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We consider a variant of M/M/1 where customers arrive singly or in pairs. Each single and one member of each pair is called primary; the other member of each pair is called secondary. Each primary joins the queue upon arrival. Each secondary is delayed in a separate area, and joins the queue when “pushed” by the next arriving primary. Thus each secondary joins the queue followed immediately by the next primary. This arrival/delay mechanism appears to be new in queueing theory. Our goal is to obtain the steady-state probability density function (pdf) of the workload, and related quantities of interest. We utilize a typical sample path of the workload process as a physical guide, and simple level crossing theorems, to derive model equations for the steady-state pdf. A potential application is to the processing of electronic signals with error free components and components that require later confirmation before joining the queue. The confirmation is the arrival of the next signal.

The M/M/1 arrival/delay mechanism considered in this paper was introduced by Hlynka [

Here we analyze the model using a level crossing approach, and derive: the steady-state pdf of the server workload; probability that the system is empty; probability that the server is idle when a secondary is being delayed; expected busy period (as defined in Hlynka [

Section 2 specifies the M/M/1 variant and sample path structure. Section 3 derives the model equations for the steady-state pdf of the workload, and specifies related quantities. Section 4 uses the model equations to obtain relevant constant terms, and gives a numerical example of the steady-state pdf of workload.

Singles (primaries) arrive at the system at Poisson rate and pairs (pair = primary + secondary) arrive at the system at Poisson rate; let. When a primary arrives at the system, it immediately joins the M/M/1 queue, either alone or just behind a secondary that was being delayed. When a secondary arrives at the system it splits from its primary and is delayed in a separate area outside the queue. The delayed secondary is “pushed” by the next arriving primary to join the queue, followed immediately by the new primary. (Thus the delay of each secondary before joining the queue is distributed as exponential-.) Customers in the queue are served one at a time at exponential rate in firstcome-first-served order. When a primary joins the queue alone, the server workload is increased by exponential-. When a primary and secondary join the queue simultaneously, the server workload is increased by Erlang (2,), i.e., the sum of two independent exponential-’s. All secondaries join the queue simultaneously with (next) primaries. The number of secondaries being delayed in the system at any instant is either 0 or 1.

Define the state of the system as

where W(t) = server workload at time, if zero secondaries are being delayed, if one secondary is being delayed. The state with zero customers in the system is denoted by. The state when the server is idle and one secondary is being delayed is denoted by. Let denote an exponential- random variable, and denote an Erlang random variable.

Technique of “Lines and Sheets (or Pages)” We utilize a technique of “lines and sheets (or pages)” to picture the state space and a sample path in it (e.g., see Brill [

We select an arbitrary continuous subset in each sheet, having one boundary as a fixed level in the state space of, e.g., ,. We use this concrete physical picture as a guide to balance the samplepath exit and entrance rates of the selected state-space subsets. Simple level-crossing theorems (e.g., Brill [

Consider the sample path of in

At time 0 the system is empty (state). A single arrives and the SP (sample path) jumps to level on page 0; the arrival immediately starts service. The SP decreases at rate A pair arrives, the primary joins the queue and the secondary is delayed; the SP jumps since the server workload includes only those customers in the queue, and transits to page 1 (delayed secondary present).

A single arrives and pushes the delayed secondary to join the queue; the single joins just after it. The SP jumps and transits to page 0 (no delayed secondary present).

A single arrives and joins the queue; the SP jumps and remains on page 0. A pair arrives; the primary joins the queue and the split-off secondary becomes delayed. The SP jumps and transits to page 1. A pair arrives, the primary pushes the delayed secondary into the queue and joins just after it; the new secondary becomes delayed. The SP jumps and remains on page 1. The SP hits level 0 from above, and remains in state for a time distributed as exponentia- (server idle). A single arrives, pushes the delayed secondary into the queue and follows just after it. The SP jumps by and transits to page 0. Finally the SP hits level 0 from above and enters (system empty).