The first step in auditing a data communication system is to determine the business use and types of messages to be transmitted. This is because the auditor needs to understand the purpose, scope, and objectives of the data communication system, as well as the nature, volume, and sensitivity of the data being transmitted. This will help the auditor to identify the risks, controls, and audit criteria for the data communication system.
Traffic volumes and response-time criteria, physical security for network equipment, and the level of redundancy in the various communication paths are important aspects of a data communication system, but they are not the first step in auditing it. They depend on the business use and types of messages to be transmitted, and they may vary according to different scenarios and requirements. References: CISA Review Manual (Digital Version), [ISACA Auditing Standards]

The best document for an IS auditor to use in detecting a weakness in segregation of duties is a process flowchart. A process flowchart is a diagram that illustrates the sequence of steps, activities, tasks, or decisions involved in a business process. A process flowchart can help detect a weakness in segregation of duties by showing who performs what actions or roles in a process, and whether there is any overlap or conflict of interest among them. The other options are not as useful as a process flowchart in detecting a weakness in segregation of duties, as they do not show who performs what actions or roles in a process. A system flowchart is a diagram that illustrates the components, functions, interactions, or logic of an information system. A data flow diagram is a diagram that illustrates how data flows from sources to destinations through processes, stores, or external entities. An entity-relationship diagram is a diagram that illustrates how entities (such as tables) are related to each other through attributes (such as keys) in a database. References: CISA ReviewManual (Digital Version), Chapter 3, Section 3.2

The most significant risk from this observation is that access rights may not be removed in a timely manner. If the process for removing access for terminated employees is not documented, there is no clear guidance or accountability for who, how, when, and what actions should be taken to revoke the access rights of the employees who leave the organization. This could result in delays, inconsistencies, or omissions in removing access rights, which could allow terminated employees to retain unauthorized access to the organization's systems and data. This could compromise the security, confidentiality, integrity, and availability of the information assets. References:
* CISA Review Manual (Digital Version)
* CISA Questions, Answers & Explanations Database

A system electronic log is the most useful source of information for an IS auditor to review all access attempts to a video-monitored and proximity card-controlled communications room. A system electronic log can provide accurate and detailed records of the date, time, card number, and status (success or failure) of each access attempt. A system electronic log can also be easily searched, filtered, and analyzed by the auditor to identify any unauthorized or suspicious access attempts.
A manual sign-in and sign-out log is not as reliable or useful as a system electronic log, because it depends on the honesty and compliance of the users. A manual log can be easily manipulated, forged, or omitted by the users or intruders. A manual log also does not capture the status of each access attempt, and it can be difficult to verify the identity of the users based on their signatures.
An alarm system with CCTV is not as useful as a system electronic log, because it only captures the events that trigger the alarm, such as unauthorized or forced entry. An alarm system with CCTV does not provide a complete record of all access attempts, and it can be affected by factors such as camera angle, lighting, and resolution. An alarm system with CCTV also requires more time and effort to review the video footage by the auditor.
A security incident log is not as useful as a system electronic log, because it only records the incidents that are reported by the users or detected by the security staff. A security incident log does not provide a comprehensive record of all access attempts, and it can be incomplete or inaccurate depending on the reporting and detection mechanisms. A security incident log also does not capture the details of each access attempt, such as the card number and status.
References:
* ISACA CISA Review Manual 27th Edition (2019), page 247
* ISACA CISA Certified Information Systems Auditor Exam ... - PUPUWEB

A computer-assisted technique is the most helpful method for an IS auditor to determine whether duplicate vendor payments exist on a complex system with a high volume of transactions. A computer-assisted technique is a tool or procedure that can be used to perform audit tests or procedures on data stored in electronic form. Examples of computer-assisted techniques include data analysis software, query tools, scripting languages, and specialized audit software. A computer-assisted techniquecan help an IS auditor to identify and extract duplicate payments from a large data set, perform calculations and comparisons, and generate reports and summaries. A computer-assisted technique can also provide more accuracy, efficiency, and coverage than manual methods.
Stratified sampling, statistical sampling, and process walk-through are not as helpful as a computer-assisted technique for this purpose. Stratified sampling is a sampling method that divides the population into subgroups based on certain characteristics and selects samples from each subgroup. Statistical sampling is a sampling method that uses probability theory to determine the sample size and selection criteria. Process walk- through is a review technique that involves following a transaction or process from start to finish and observing the inputs, outputs, controls, and documentation. These methods may be useful for other audit objectives, but they are not as effective as a computer-assisted technique for detecting duplicate payments in a complex and high-volume system. References: ISACA Frameworks: Blueprints for Success, [ISACA Glossary of Terms]