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The Federal Aviation Administration (FAA) has established an operational plan for the future air traffic management (ATM) system, in which the controller pilot data link communications (CPDLC) is envisioned to evolve into digital messaging that takes on an ever increasing role in controller to pilot communications, significantly changing the way the National Airspace System (NAS) is operating. According to FAA, CPDLC represents the first phase of the transition from the current analog voice system to an International Civil Aviation Organization (ICAO) compliant system in which digital communication becomes the alternate and perhaps primary method of routine communication. The CPDLC application is an air traffic service (ATS) application in which pilots and controllers exchange messages via an addressed data link. CPDLC includes a set of clearance, information, and request message elements that correspond to existing phraseology employed by current air traffic control (ATC) procedures. These message elements encompass altitude assignments, crossing constraints, lateral deviations, route changes and clearances, speed assignments, radio frequency assignments, and various requests for information. The pilot is provided with the capability to respond to messages, to request clearances and information, to report information, and to declare/rescind an emergency. A "free text" capability is also provided to exchange information not conforming to defined formats. This paper presents simulated results of the aeronautical telecommunication application controller pilot data link communications over VHF digital link mode 3 (VDL mode 3). The objective of this simulation study was to determine the impact of CPDLC traffic loads, in terms of timely message delivery and capacity of the VDL Mode 3 subnetwork. It is an extension of the work performed in S. Bretmersky et al. (2004). The traffic model is based on DAG CNS Services Information Requirements Document (2003) and is used for generating air/ground messages with different priorities. Communication is modeled for several sectors in the en route domain of the Cleveland Air Route Traffic Control Center (ZOB ARTCC).