Question 1

You manage network 10.9.0.0 (class A block 10.0.0.0/8) with 73 existing subnets and plan to add 27 more next year (total 100). To simplify management, you want the maximum possible hosts per subnet while meeting the subnet count. Which subnet mask s…

Accepted Answer

Introduction / Context: Subnet design within private class A space (10.0.0.0/8) often balances the number of required subnets against the desire to keep many hosts per subnet. Borrowing more bits increases subnet count but reduces host capacity; the goal here is to borrow the minimum bits that satisfy the need for 100 subnets. Given Data / Assumptions: Base network: 10.0.0.0/8; given network ID is 10.9.0.0 within that block. Subnets required: 73 existing + 27 new = 100 total. Objective: maximize hosts per subnet while meeting 100 subnets. Concept / Approach: Let n be the number of borrowed bits. Subnet count = 2^n. Choose the smallest n such that 2^n ≥ 100. Hosts per subnet = 2^(host_bits) - 2. With a class A /8, host_bits initially = 24. We want minimal n for 100 subnets to keep host_bits as large as possible. Step-by-Step Solution: Compute n: 2^6 = 64 (insufficient); 2^7 = 128 (sufficient). So n = 7. New prefix = /8 + 7 = /15. Mask for /15 = 255.254.0.0. Host bits = 32 - 15 = 17 → usable hosts = 2^17 - 2 = 131070 per subnet (very large). Verification / Alternative check: Subnets increment by 2 in the second octet (since mask 255.254.0.0); this produces 128 subnets maximum—meeting the 100 requirement with room for growth while preserving maximal host counts for this choice of n. Why Other Options Are Wrong: 255.224.0.0 (/11), 255.240.0.0 (/12), 255.248.0.0 (/13), 255.252.0.0 (/14): each yields fewer than 100 subnets (2^(3..6) = 8..64), thus insufficient. 255.254.0.0 (/15) is the first mask that meets ≥100 subnets, keeping hosts per subnet as large as possible. Common Pitfalls: Forgetting to choose the minimal n that satisfies the subnet requirement; confusing classless allocation with classful defaults yet the math for borrowed bits remains identical; overlooking that /16 would halve host capacity vs /15 without being necessary for 100 subnets. Final Answer: 255.254.0.0

Question 2

Which of the following are examples of the client–server model in computer networks, where a client program requests a service and a server program responds?

Accepted Answer

Introduction / Context: The client–server model underpins most Internet applications. A client initiates a request, and a server listens and responds. Understanding which common protocols follow this pattern builds intuition about how applications interact over TCP/IP networks. Given Data / Assumptions: Protocols considered: TELNET, FTP, and DNS. Definition: client requests, server provides the requested service. Concept / Approach: TELNET clients connect to a TELNET server for remote terminal sessions. FTP clients connect to FTP servers for file operations. DNS resolvers (clients) query authoritative or recursive DNS servers to translate names to addresses. Each follows the core client–server pattern of request/response. Step-by-Step Solution: TELNET: client connects to server for interactive shell access. FTP: client authenticates and transfers files with an FTP server. DNS: client (resolver) sends queries to DNS servers and receives replies. Therefore, all listed protocols are examples of client–server. Verification / Alternative check: Port/service mappings show well-known server ports (e.g., 21 for FTP control, 23 for TELNET, 53 for DNS) with client ephemeral ports making requests, confirming client–server behavior. Why Other Options Are Wrong: Any single option alone is incomplete; all three are valid examples. None of the above: invalid since each listed protocol uses client–server communication. Common Pitfalls: Confusing peer-to-peer with client–server; mixing DNS client caching with authoritative service—both still follow a request/response model. Final Answer: All of the above

Question 3

Information systems built on common hardware, software, and network standards that promote interoperability and easy access for end users and their connected computers are referred to as what?

Accepted Answer

Introduction / Context: Enterprises prefer architectures that avoid vendor lock-in and allow different components to work together. Systems designed around widely accepted standards for hardware, operating systems, middleware, and networking are known for interoperability and ease of integration for end users and administrators. Given Data / Assumptions: Emphasis on common, widely adopted standards. Goal is interoperability and straightforward user access. Term sought is the overarching architectural approach, not a single protocol. Concept / Approach: “Open systems” describes environments that adhere to open, published standards and interfaces so that multi-vendor components interoperate. Examples include POSIX for operating systems, TCP/IP for networking, and standard data formats and APIs. This goes beyond naming a single protocol (like TCP/IP) or a device (like an internetwork processor). Step-by-Step Solution: Identify the architectural theme: interoperability via standards. Map to terminology: “open systems” captures the model. Distinguish from narrower terms (a specific protocol or device). Select “Open systems” as the best fit. Verification / Alternative check: Procurement guidelines often require open standards compliance to ensure portability and integration. Frameworks referencing OSI, POSIX, and TCP/IP stacks illustrate open systems in practice across vendors. Why Other Options Are Wrong: TCP/IP: a networking suite, not the full systems approach. Protocol: generic term, not the specific architectural concept. Internetwork processor: a device category (e.g., routers), not a system strategy. Standards-based architecture: close in spirit but the established term is “open systems.” Common Pitfalls: Equating “one standard” with “open systems” overall; ignoring that openness spans platforms, data formats, and interfaces, not just network protocols. Final Answer: Open systems

Question 4

According to CCITT (now ITU-T) Recommendation X.25, the public packet-switched interface is defined in terms of which three communication layers?

Accepted Answer

Introduction / Context: X.25 is a historic standard for public packet-switched networks. It predates widespread Internet use and describes how user equipment connects to a carrier’s packet network. Understanding which layers X.25 specifies clarifies its position within layered architectures. Given Data / Assumptions: We are referring to the original CCITT X.25 recommendations. The focus is on the user–network interface over public packet services. Concept / Approach: X.25 defines three layers: the physical layer (electrical/connector characteristics), the data link layer (LAPB for reliable link control), and the network layer (packet layer protocol for virtual circuits). Higher OSI layers (session, presentation, application) are outside X.25’s scope. Step-by-Step Solution: Identify the stack per X.25: physical + LAPB (link) + packet layer (network). Exclude transport/session/presentation/application; they are not specified by X.25. Select “Physical, data link, and network.” Verification / Alternative check: Documentation shows X.21/X.21bis for physical, LAPB (link) as Q.922-like reliability, and the packet layer protocol at network level for virtual calls and data transfer. Why Other Options Are Wrong: Application/presentation/session: too high in the OSI stack. Session/transport/network: includes layers not defined by X.25. Data link/network/transport: transport is not part of X.25 at the UNI. None: incorrect because the correct trio is well known. Common Pitfalls: Assuming every legacy standard maps one-to-one to OSI seven layers; forgetting LAPB’s role as the reliable link underpinning the packet layer in X.25 networks. Final Answer: Physical, data link, and network

Question 5

A perfect binary tree contains 2^m - 1 routers (one at each node). To send a message from router I to router J, the message first goes up to the root and then down to J. What is the mean router-to-router path length under this “via root” policy?

Accepted Answer

Introduction / Context: In a full (perfect) binary tree with 2^m - 1 routers, a “centralized” routing rule sends any message up to the root and then down to the destination. The hop count between two routers equals the depth of the source plus the depth of the destination (since the least common ancestor is always the root). We seek the mean hop length over all pairs under this policy. Given Data / Assumptions: The tree is perfect with m levels numbered 0..m-1; total routers = 2^m - 1. Depth(root) = 0; a node at level k has depth k. Path via root: hops(I → J) = depth(I) + depth(J). Routers are equally likely when computing the mean. Concept / Approach: Let D be the random variable “depth of a uniformly chosen node.” The mean path length is E[depth(I) + depth(J)] = 2 * E[D]. Hence compute the average node depth in a perfect binary tree: E[D] = (Σ k * 2^k from k=0 to m-1) / (2^m - 1). Use the identity Σ k * 2^k = 2^m * m - 2^(m+1) + 2, then double that value for the final mean path length. Step-by-Step Solution: Compute S1 = Σ 2^k (k=0..m-1) = 2^m - 1. Compute S2 = Σ k * 2^k (k=0..m-1) = 2^m * m - 2^(m+1) + 2. Average depth E[D] = S2 / S1. Mean path length L = 2 * E[D] = 2 * ( 2^m * m - 2^(m+1) + 2 ) / ( 2^m - 1 ) = 2 * ( 2^m * (m - 2) + 2 ) / ( 2^m - 1 ). Verification / Alternative check: Test small m. For m=1 (single node): L = 0 (formula gives 0). For m=2 (3 nodes): node depths {0,1,1}, E[D] = 2/3, L = 4/3; enumerating ordered pairs via root gives the same average, validating the expression. Why Other Options Are Wrong: 2 * (m - 2): linear approximation; ignores exponential node counts per level. 2 * (2m - 1): scales linearly with m but grossly overestimates for large m. m - 1: equals maximum depth, not the mean via root over all pairs. m / 2: heuristic guess; not derived from node distribution by depth. Common Pitfalls: Forgetting that most nodes lie near the leaves, increasing mean depth; computing undirected shortest path rather than the constrained “via root” path; averaging unordered pairs and then forgetting to account for ordered pairs (the mean is the same here because the sum is symmetric). Final Answer: 2 * ( 2^m * (m - 2) + 2 ) / ( 2^m - 1 )

Question 6

Which statement about data communication modes is incorrect in the context of online teleprocessing versus batch processing?

Accepted Answer

Introduction / Context: Teleprocessing refers to online, interactive computing over communication links, whereas batch processing handles jobs offline without interactive user sessions. Distinguishing synchronous/asynchronous transmission and duplex modes further clarifies how data moves across links. Given Data / Assumptions: Synchronous transmission uses a shared or embedded clock. Half-duplex permits both directions but not simultaneously. Teleprocessing emphasizes online interaction over networks. Concept / Approach: We evaluate each statement. The first three accurately describe common concepts. The fourth claims batch processing is preferred for telecom operations; this contradicts the essence of teleprocessing, which favors online, interactive processing for timely responses. Therefore, the incorrect statement is the one elevating batch mode over online operation in telecommunications contexts. Step-by-Step Solution: Check (a): Synchronous uses clocking derived/maintained with the data stream → correct. Check (b): Half-duplex allows two-way, not simultaneous → correct. Check (c): Teleprocessing marries telecom and data processing online → correct. Check (d): “Batch is preferred” for telecom → incorrect; online/interactive is preferred. Verification / Alternative check: Examples include online transaction processing, interactive terminals, and modern client–server applications—all requiring prompt, session-oriented communication incompatible with pure batch paradigms for primary operations. Why Other Options Are Wrong: (a) Correct definition of synchronous transmission. (b) Correct definition of half-duplex operation. (c) Correct description of teleprocessing. (e) “None” is wrong since (d) is demonstrably incorrect. Common Pitfalls: Confusing buffered/queued processing within online systems with batch processing; misinterpreting synchronous vs asynchronous at signal level versus application layer semantics. Final Answer: Batch processing is the preferred processing mode for telecommunication operations.

Question 7

A 4 kHz noiseless channel is sampled once every 125 microseconds (i.e., 8000 samples per second) to carry digital signals. If delta modulation is used (1 bit per sample), how many bits per second are actually sent?

Accepted Answer

Introduction / Context: Delta modulation is a simple form of analog-to-digital conversion in which each sample conveys only the sign of the change relative to the previous sample. Because each sample is encoded as a single bit, the bit rate equals the sampling rate. This question ties together channel sampling and the delta modulation bit budget. Given Data / Assumptions: Channel bandwidth mentioned: 4 kHz (contextual; sampling rate is explicitly provided). Sampling interval: every 125 microseconds → sampling rate = 8000 samples per second. Delta modulation uses 1 bit per sample. Concept / Approach: Bit rate in delta modulation = samples per second * bits per sample. With 8000 samples per second and 1 bit each, the transmitted bit rate is 8000 bits per second, i.e., 8 kbps (using kbps as 1000 bps in telecom shorthand). No additional framing or coding overhead is considered in the basic calculation. Step-by-Step Solution: Compute sampling rate: 1 / 125 microseconds = 8000 samples/second. Bits per sample (delta modulation): 1 bit/sample. Bit rate = 8000 * 1 = 8000 bps = 8 kbps. Therefore, the answer is 8 kbps. Verification / Alternative check: By comparison, PCM with 8 bits per sample at 8000 samples per second yields 64 kbps; delta modulation reduces this by a factor of 8 by using 1-bit coding per sample under ideal tracking conditions. Why Other Options Are Wrong: 32 kbps / 64 kbps / 128 kbps: correspond to multi-bit PCM scenarios (e.g., 4-, 8-, or 16-bit PCM), not delta modulation. 16 kbps: could match 2 bits/sample at 8 kHz (not delta modulation in its basic 1-bit form). Common Pitfalls: Confusing “bandwidth” with “bit rate” directly; forgetting the given sampling interval; mixing PCM and delta modulation bit budgets. Final Answer: 8 kbps

Question 8

What is the principal difference between synchronous and asynchronous serial transmission from a signaling perspective?

Accepted Answer

Introduction / Context: Serial links move bits sequentially. The receiver must know when to sample each bit. Two classic approaches exist: synchronous transmission, where timing is shared or recovered continuously, and asynchronous transmission, where timing is inferred per character with start/stop framing. Understanding this distinction is crucial for selecting link encapsulations and hardware interfaces. Given Data / Assumptions: Synchronous links rely on shared or embedded clocks (e.g., line coding enables clock recovery). Asynchronous links use start/stop bits per character without a continuous shared clock. Electrical levels and required bandwidth may be similar for comparable bit rates. Concept / Approach: In synchronous transmission, the receiver stays aligned with the sender’s bit timing—either via an explicit clock or by recovering the clock from the encoded data stream. In asynchronous transmission, each character begins with a start bit and ends with a stop bit, allowing the receiver to re-synchronize for each character without a separate continuous clock. Step-by-Step Solution: Identify the key differentiator: clock acquisition and maintenance. Synchronous: clock is shared/embedded; no per-character start/stop overhead. Asynchronous: no continuous shared clock; start/stop bits mark character boundaries. Therefore, statement (a) captures the core difference. Verification / Alternative check: Examples: HDLC/SDLC, Ethernet PHYs, and many WAN links are synchronous; classic RS-232 terminal connections are asynchronous with start/stop framing. Oscilloscope traces show continuous timing in synchronous lines versus character-framed bursts in asynchronous lines. Why Other Options Are Wrong: (b) Misstates asynchronous operation; it does not carry a separate continuous clock line in typical serial implementations. (c) Pulse height is an electrical parameter, not the defining distinction. (d) Bandwidth requirements depend on bit rate and framing overhead; not a fundamental categorical difference. (e) Incorrect because (a) is correct. Common Pitfalls: Confusing encoding schemes (e.g., NRZ, Manchester) with sync/async categories; assuming async always wastes large bandwidth—overhead depends on character size and stop bits chosen. Final Answer: The clocking is derived from or embedded in the data stream in synchronous transmission.

Question 9

In satellite communications engineering, which technique is specifically used to allocate fixed portions of a transponder's bandwidth and power when a channel plan follows fixed-assignment frequency-division multiplexing (FDM)?

Accepted Answer

Introduction: Satellite transponders support multiple simultaneous carriers. When a system planner decides to partition the transponder's spectrum into preassigned slices for different earth stations or services, the multiple-access method must match that fixed frequency plan. This question asks which technique corresponds to fixed-assignment FDM capacity allocation. Given Data / Assumptions: A satellite transponder provides a continuous RF bandwidth and finite power. Capacity is allocated using fixed-assignment frequency-division multiplexing (FDM). Multiple earth stations may transmit concurrently on different sub-bands. We are choosing the multiple-access method, not the modulation format of each carrier. Concept / Approach: Fixed-assignment FDM divides the available spectrum into non-overlapping channels. Each user (or carrier) occupies a distinct frequency slot. The corresponding multiple-access method is Frequency-Division Multiple Access (FDMA). FDMA describes how many users share the transponder simultaneously; their individual waveforms can use various modulations (AM, FM, FSK, QPSK, etc.) inside their assigned slices, but the sharing principle is frequency-division. Step-by-Step Solution: Identify resource partitioning: spectrum is split into fixed sub-bands => frequency-division sharing.Map to access method: frequency-division sharing across users => FDMA.Separate access vs modulation: AM, FM, FSK are modulation schemes, not multiple-access strategies.Conclude that the correct technique is FDMA for fixed-assignment FDM. Verification / Alternative check: In classic analog telephony over satellite, large FDM/FM carriers used FDMA at the transponder. In digital systems, multiple QPSK/8PSK carriers can also share a transponder via FDMA if frequency slots are preassigned. This confirms that FDMA is the multiple-access label for fixed FDM planning. Why Other Options Are Wrong: Amplitude modulation: waveform modulation type, not an access method. Frequency modulation: modulation method, not the sharing strategy. Frequency-shift keying: digital modulation, not a multiuser access technique. None of the above: invalid because FDMA matches fixed FDM allocation. Common Pitfalls: Confusing multiple-access schemes (FDMA/TDMA/CDMA) with modulation formats (AM/FM/FSK/QAM). Another pitfall is assuming FDM itself is the access method; FDM is the multiplexing plan, while FDMA names the multiuser access approach that uses that plan. Final Answer: Frequency-division multiple access.

Question 10

IEEE 802.5 (Token Ring) implements a collision-prevention method at the media-access level. Which mechanism prevents collisions in this standard?

Accepted Answer

Introduction: Local area network technologies differ primarily in how stations gain permission to transmit on a shared medium. IEEE 802.5 (Token Ring) was designed to avoid the need for contention by controlling access deterministically. This question asks which access mechanism prevents collisions in 802.5 networks. Given Data / Assumptions: The standard in focus is IEEE 802.5 (Token Ring). The medium is logically a ring, often implemented physically as a star with a Multistation Access Unit (MAU). The goal is collision prevention, not merely detection after a collision occurs. Concept / Approach: Token Ring uses a special short frame called a token. Only the station holding the token may transmit data frames. After transmission (or if it has nothing to send), the station releases the token to the next station in the ring. Because only one token exists and only the token holder can transmit, two stations cannot transmit at the same time, thereby preventing collisions by design. Step-by-Step Solution: Recognize the standard: IEEE 802.5 equals Token Ring.Understand MAC behavior: possession of the token grants transmit rights.Collision analysis: single token implies no simultaneous transmissions.Select the mechanism: token passing. Verification / Alternative check: Contrast with Ethernet's legacy CSMA/CD (Carrier Sense Multiple Access with Collision Detection), where collisions can occur and are handled probabilistically. Token Ring eliminates collisions by controlling access order with the token, often providing more deterministic latency under load. Why Other Options Are Wrong: CSMA/CD: used by classic Ethernet (IEEE 802.3), not Token Ring. Collision detection: a behavior of CSMA/CD, not 802.5. Time sharing: vague concept; not the defined MAC for 802.5. Switched repeaters: a hardware concept; not the MAC algorithm. Common Pitfalls: Confusing Token Ring with Ethernet or assuming all LANs rely on contention. Another pitfall is thinking that a ring implies round-robin without a token—802.5 specifically uses a token frame to manage access. Final Answer: Token passing.

Question 11

In data communications, which summation-based operation is applied to blocks of bits to verify integrity as an error-detecting code at the receiver?

Accepted Answer

Introduction: Error detection ensures that corrupted data is identified so that it can be retransmitted or discarded. One of the simplest integrity checks computes a summarized value over a block of bits and sends it with the data. The receiver recomputes the same value and compares the results to detect errors. This question asks for the name of that summation-based mechanism. Given Data / Assumptions: The mechanism operates over a block (e.g., a segment, packet, or frame payload). The transmitter computes a numeric summary and appends it. The receiver recomputes that summary and compares to the transmitted value. Concept / Approach: A checksum is produced by summing (often using 1's complement arithmetic or modular sums) all words in a block and then sending the complement or the final sum as the checksum field. The receiver performs the same summation; a mismatch indicates that one or more bits were altered in transit. While more robust methods like CRCs are widely used at the data link layer, checksums remain common at the transport layer (e.g., UDP/TCP) and in file formats due to their simplicity and speed. Step-by-Step Solution: Identify the family of techniques: summation over data words.Name the operation: checksum generation at sender, checksum verification at receiver.Differentiate from other signal/data concepts (codec, attenuation).Select the term: checksum. Verification / Alternative check: In practice, IP, TCP, and UDP include 16-bit checksums. Many firmware images and archive formats also use checksums for basic integrity assurance, confirming the term and its application. Why Other Options Are Wrong: Codec/Coder-decoder: devices or algorithms that convert between analog/digital or compress/decompress; not integrity checks. Attenuation: signal power loss over distance; unrelated to integrity verification. None of the above: incorrect because “checksum” precisely matches the description. Common Pitfalls: Assuming all error detection uses CRCs; forgetting that checksums are still prevalent at higher layers due to speed and ease of implementation. Final Answer: Checksum.

Question 12

Within the ITU-T X.25 suite, what does the X.25 recommendation specifically define for packet-switched public networks?

Accepted Answer

Introduction: The X.25 family standardized how end systems accessed public packet-switched data networks in the pre-Internet era. Understanding what X.25 actually specifies helps distinguish it from physical-layer signaling and service-dialing conventions. This question narrows the focus to the precise scope of X.25 itself. Given Data / Assumptions: X.25 addresses the interface between the user's device and the network. It comprises three layers up to the packet layer: Physical (often X.21), Link (LAPB), and Packet (PLP). Public data networks used virtual circuits (SVC/PVC) based on this interface. Concept / Approach: X.25 defines the DTE/DCE interface—where DTE (Data Terminal Equipment) such as hosts or PADs connect to DCE (network provider equipment). It specifies packet formats, procedures for virtual circuit establishment, flow and error control at the packet and link layers, and related parameters. It does not define asynchronous start–stop techniques, dial access methods, or mandate a single data bit rate; those belong to other recommendations or service-specific offerings. Step-by-Step Solution: Identify X.25's scope: end-system access to a PSDN via a DTE/DCE boundary.Confirm included elements: packet formats, PLP, LAPB, and physical mappings.Exclude non-scope items: generic dial access and asynchronous start–stop are separate topics.Conclude: X.25 specifies the DTE/DCE interface. Verification / Alternative check: Historical PSDN documentation consistently labels X.25 as the DTE–DCE access protocol suite for packet-switched networks, with X.21/X.21bis covering physical and dial-related aspects as needed. Why Other Options Are Wrong: Start–stop technique: asynchronous serial framing; outside X.25 scope. Dial access technique: governed by other recommendations/services. Data bit rate: X.25 supports multiple rates; it does not fix one rate. None of the above: incorrect because the DTE/DCE interface is correct. Common Pitfalls: Confusing X.25 (packet access) with underlying physical signaling or telephone-network dialing standards. Final Answer: DTE/DCE interface.

Question 13

Home networking scenario: to minimize hardware costs for a small study lab, which network topology historically requires the least hardware to set up and operate?

Accepted Answer

Introduction: Different LAN topologies impose different hardware requirements. A budget-constrained learner building a small practice network typically wants the fewest active components and the simplest cabling. This question asks which topology historically minimized hardware needs, especially in small peer-to-peer setups. Given Data / Assumptions: Very limited budget; minimal active equipment is desirable. Small number of nodes for study/testing purposes. We consider classic, widely taught topologies (bus, star, ring) and their common implementations. Concept / Approach: A bus topology (e.g., 10BASE2 thin coax or 10BASE5) connects all stations along a single shared cable segment with T-connectors/terminators. There is no central hub or switch required. By contrast, a star (including modern switched Ethernet) needs a central hub/switch and individual runs to each host. Token-Ring requires MAUs and specialized cabling. Therefore, in terms of pure component count and cost, the classic bus required the least dedicated hardware beyond the network interface cards and coax accessories. Step-by-Step Solution: List hardware per topology: bus (coax, T-connectors, terminators), star (hub/switch plus patch cables), ring (MAU plus ring cabling).Compare active components: bus has none; star/ring require active concentration.Choose the topology with the fewest devices: bus. Verification / Alternative check: Historical cost comparisons from early Ethernet eras show that thin-coax bus deployments avoided the central device cost entirely, making them attractive for small labs before inexpensive switches became ubiquitous. Why Other Options Are Wrong: Star / Switched Ethernet: needs a hub or switch. Ring / Token-Ring: needs MAUs and specialized cabling. Ethernet (switched star): modern default but not minimal hardware count. Common Pitfalls: Evaluating with today's commodity switch prices; historically, the question focuses on topology fundamentals and hardware count, where bus wins on minimalism. Final Answer: Bus topology.

Question 14

Wide Area Telecommunications Service (WATS): under which calling pattern does a WATS plan tend to be less expensive than a flat-rate long-distance plan?

Accepted Answer

Introduction: Wide Area Telecommunications Service (WATS) was a bulk long-distance offering that provided discounted rates within specified bands or calling zones. Organizations chose WATS when their traffic profile matched the plan's economics. This question asks you to identify the calling pattern under which a WATS arrangement typically saves money compared to a flat-rate approach. Given Data / Assumptions: WATS pricing discounts apply across a high volume of calls in defined regions. Flat-rate (or standard tariff) pricing applies per call, often with per-minute billing plus setup. We compare total monthly spend under different usage profiles. Concept / Approach: The core idea behind WATS is amortizing fixed or banded costs over many calls. When call volume (the number of calls) is large, the per-call and per-minute charges of standard tariffs accumulate quickly. WATS becomes economical because the effective cost per call drops as usage increases. Shorter calls also help avoid hitting duration thresholds where per-minute accumulations overtake the WATS allowance; frequent short calls spread the fixed WATS cost efficiently. Conversely, occasional long calls may not justify WATS versus conventional per-minute billing. Step-by-Step Solution: Model monthly cost under flat-rate: many calls * (setup + minutes * rate).Model WATS: fixed/banded charge amortized across total usage.Compare break-even: high call counts with modest durations favor WATS.Select the pattern that best aligns: many calls, relatively short durations. Verification / Alternative check: Historical procurement guidance recommended WATS for customer-service centers and sales operations making numerous short- to medium-length calls within certain distance bands, reinforcing the high-count, shorter-duration advantage profile. Why Other Options Are Wrong: Always less expensive: false; savings depend on usage profile. Small number of long calls: standard tariffs often cheaper. Never less expensive: contradicted by high-volume use cases. None of the above: incorrect because one pattern is advantageous. Common Pitfalls: Treating WATS as universally superior; ignoring call-setup charges and band restrictions; forgetting that extremely long durations can exhaust WATS allowances and flip the economics. Final Answer: Less expensive only when the number of calls is large and each call is relatively short.

Question 15

On Ethernet and Token Ring networks, the MAC (Media Access Control) address on a network interface card serves what purpose?

Accepted Answer

Introduction: Every network interface card (NIC) on broadcast networks such as Ethernet and Token Ring needs a low-level identifier for link-layer delivery. This identifier is the MAC address, which is central to switching, ARP/ND resolution, and frame delivery on a local segment. The question asks what a MAC address provides and how it is assigned. Given Data / Assumptions: Context: IEEE 802 networks (e.g., Ethernet, Token Ring). MAC addresses are 48-bit (or sometimes 64-bit in newer schemes) identifiers. Vendors receive Organizationally Unique Identifiers (OUIs) to allocate addresses. Concept / Approach: A MAC address is a physical (link-layer) address burned into the NIC by the manufacturer (often in firmware/EEPROM). Switches learn source MACs on ports and forward frames based on destination MACs. While some OSes allow software override (spoofing), the canonical definition remains a manufacturer-assigned physical address at Layer 2—not a logical address (Layer 3) and not a human-readable alias. Step-by-Step Solution: Distinguish layers: MAC = Layer 2 physical address; IP = Layer 3 logical address.Assignment: manufacturers assign MACs from their OUI blocks.Select statement that matches: manufacturer-assigned physical address. Verification / Alternative check: IEEE registration records tie OUIs to vendors; device labels and interface diagnostics show the burned-in address, confirming manufacturer assignment by default. Why Other Options Are Wrong: Logical address: describes IP, not MAC. Random at boot: not by default; although software can randomize for privacy, that is not the fundamental definition. Logical domain address: not a standard term at Layer 2. Alias for computer name: unrelated; names map to IPs via DNS, not MACs. Common Pitfalls: Confusing MAC with IP; assuming hostnames are directly tied to MACs; forgetting that MAC randomization is a privacy feature, not the default assignment method. Final Answer: A physical address that is assigned by the manufacturer.

Question 16

In data communications, which device aggregates simultaneous transmissions from multiple input/output (I/O) devices onto a single communication line?

Accepted Answer

Introduction: Many communication scenarios require combining several lower-rate streams for transport over a higher-capacity link. The device that performs this combination reduces cabling, increases efficiency, and lowers cost. This question asks you to identify that device from common networking terms. Given Data / Assumptions: Multiple I/O sources produce data that must share one line. The line supports higher capacity than each individual source requires. Terminology includes both accurate and distractor options. Concept / Approach: A multiplexer (MUX) combines multiple input signals onto a single output channel using time-division (TDM), frequency-division (FDM), or statistical multiplexing, depending on the technology. At the far end, a demultiplexer (DEMUX) separates the streams. While the term “concentrator” is sometimes used in telephony/data networks, “multiplexer” is the canonical device that aggregates signals at the physical/link level for a single line. Step-by-Step Solution: Identify requirement: many sources onto one higher-capacity medium.Recall device: multiplexer performs TDM/FDM/statistical combination.Select the standard term: multiplexer. Verification / Alternative check: Examples include T1/E1 multiplexers (time slots), optical WDM systems (wavelength slots), and statistical multiplexers for packet streams, all reaffirming the MUX role. Why Other Options Are Wrong: Concentrator: can refer to certain aggregation devices but is less precise than multiplexer for a single-line combination at the physical/link level. Modifier: not a standard communications device name. Full-duplex line: describes a link's capability, not an aggregating device. None of the above: incorrect because multiplexer is correct. Common Pitfalls: Confusing general aggregation terms with the specific MUX/DEMUX pair; assuming full-duplex implies aggregation (it does not). Final Answer: Multiplexer.

Question 17

Standards question: what is the canonical suite of protocols used by the Internet, most intranets and extranets, and many other IP-based networks?

Accepted Answer

Introduction: Network architectures rely on a stack of protocols that define how data is formatted, addressed, transported, routed, and delivered to applications. The global Internet, and by extension most intranets and extranets, standardize on a specific, interoperable protocol suite. This question asks you to name that suite unambiguously. Given Data / Assumptions: Scope includes the public Internet and typical enterprise IP networks. We are naming a protocol suite, not a single protocol or a hardware device. Distractors include generic or misapplied terms. Concept / Approach: The Transmission Control Protocol/Internet Protocol (TCP/IP) suite encompasses core protocols such as IP (routing/addressing), TCP and UDP (transport), and application protocols like HTTP, SMTP, and DNS. The OSI model is a conceptual framework, not a deployed suite. Terms like “protocol” and “internetwork processor” are generic or refer to hardware classes, not the comprehensive standards stack powering the Internet. Step-by-Step Solution: Identify the requirement: widely adopted, interoperable protocol suite.Map real-world usage: Internet and most private IP networks use TCP/IP.Eliminate conceptual frameworks or device terms (OSI, internetwork processor).Select TCP/IP as the correct answer. Verification / Alternative check: Every modern operating system ships with a TCP/IP stack; enterprise routers and switches are built to forward IP packets and support TCP/UDP-based applications, confirming ubiquitous use of the TCP/IP suite. Why Other Options Are Wrong: Protocol: too generic; not a suite name. Open Systems: vague phrase, not a protocol suite. Internetwork processor: hardware class (e.g., router), not a protocol suite. OSI: a reference model, not the de facto deployed suite. Common Pitfalls: Confusing the OSI model with practical stacks; thinking that a single protocol like HTTP represents the entire suite. Final Answer: TCP/IP.

Question 18

Geosynchronous satellite dynamics: which statement correctly describes the apparent position of a satellite in geosynchronous (specifically, geostationary) orbit relative to observers on Earth?

Accepted Answer

Introduction: Satellite orbits determine how a spacecraft appears to move across the sky to an observer on Earth. A special case, the geostationary orbit, is frequently used for communications and weather satellites because it simplifies ground antenna pointing. This question tests understanding of what observers see for satellites in geosynchronous (specifically, geostationary) orbit. Given Data / Assumptions: Geosynchronous means the satellite's orbital period equals Earth's rotational period (~24 hours). A geostationary orbit is a circular, equatorial geosynchronous orbit. Ground antennas are fixed (no continuous tracking) for geostationary satellites. Concept / Approach: In a geostationary orbit, the satellite orbits at approximately 35,786 km altitude in the equatorial plane with zero inclination and zero eccentricity. Because its angular velocity matches Earth's rotation and it is above the equator, the satellite appears motionless with respect to a point on the Earth's surface (fixed azimuth and elevation). This property allows continuous line-of-sight service with stationary ground dishes. Step-by-Step Solution: Define geosynchronous: orbital period = Earth's rotation period.Specialize to geostationary: circular, equatorial orbit yields no apparent drift.Infer observation: satellite appears fixed in the sky relative to the same ground point.Select the statement matching this behavior. Verification / Alternative check: Communications satellites such as those used for television and VSAT services park at longitudes on the geostationary belt. Ground antennas are pointed once and remain fixed, confirming the "appears stationary" property. Why Other Options Are Wrong: Fixed position to fully cover Earth: even from GEO, a single satellite cannot cover the entire Earth due to line-of-sight and elevation constraints. Moves faster than Earth: would not be geosynchronous and would not appear stationary. Moves simultaneously: meaningless description. None of the above: incorrect because one statement is correct. Common Pitfalls: Confusing geosynchronous with geostationary; assuming one satellite at GEO sees the entire globe; overlooking that non-zero inclination geosynchronous orbits trace an analemma (figure-eight) in the sky and are not strictly stationary. Final Answer: It remains in a fixed position relative to a point on Earth (appears stationary in the sky).

Question 19

In information theory and digital communications, the amount of uncertainty or surprise associated with a source of symbols is called what?

Accepted Answer

Introduction / Context: When engineers and computer scientists analyze data sources and messages, they often need a way to quantify uncertainty. Information theory provides a rigorous definition for how “surprising” or “unpredictable” a symbol stream is. This concept drives compression limits, error-control coding, and cryptography baselines. Given Data / Assumptions: We are discussing a system that emits symbols (e.g., bits, letters, packets). The question asks for the name of the quantity that measures uncertainty. No numerical calculation is required—only the correct term. Concept / Approach: The standard measure of uncertainty in a random variable (or symbol source) is entropy. For a discrete source with probabilities p_i, entropy H is defined as H = − Σ p_i * log2(p_i). Bandwidth, by contrast, is a physical-layer rate or frequency span; “loss” refers to attenuation or information loss; “quantum” is unrelated to the classic information-theoretic term in this context. Step-by-Step Solution: Identify the domain: information theory / communications.Recall the uncertainty metric name: entropy (measured in bits when using log base 2).Match the definition in the stem—“amount of uncertainty”—to the term “entropy.” Verification / Alternative check: If all outcomes are equally likely among N symbols, H = log2(N). For example, a fair coin (N = 2) has H = 1 bit per toss, which aligns with the intuitive “uncertainty” per outcome. If the source is biased or deterministic, entropy decreases toward zero, again matching our understanding of uncertainty. Why Other Options Are Wrong: Bandwidth: Measures capacity of a channel in Hz or bit/s, not uncertainty. Loss: A qualitative term for degradation; not a formal uncertainty measure. Quantum: Not the information-theoretic term here. None of the above: Incorrect because “entropy” is the standard name. Common Pitfalls: Confusing “information content” with “data rate.” Entropy is a property of the source distribution; achievable data rate depends on coding, channel, and protocol overheads. Final Answer: Entropy

Question 20

In legacy X.25 addressing (Network User Address, NUA), which digit positions are commonly treated as the “area code” within the national portion of the address?

Accepted Answer

Introduction / Context: X.25 networks used the X.121 numbering plan. Subscriber destinations were identified by a Network User Address (NUA). Understanding how the digits are structured—international versus national parts—helps interpret legacy addressing and routing on packet-switched public data networks. Given Data / Assumptions: NUA follows the X.121 format up to 14 decimal digits. Digits 1–4 are the DNIC (country + network code). The “area code” referenced is within the national (post-DNIC) portion. We select the digit positions typically used as the area indicator. Concept / Approach: In X.121, the first four digits (DNIC) identify the country and public data network. The remaining digits comprise the national terminal number (NTN). A common convention in many networks was to use digits 5–7 (the first three digits of the NTN) as an “area” or regional code, followed by subscriber/location specifics. While countries could define different NTN structures, “5–7” is the canonical exam convention for the area portion after the DNIC. Step-by-Step Solution: Partition address: DNIC = digits 1–4; NTN begins at digit 5.Identify the area subfield within the NTN: digits 5–7 in many national plans.Therefore select “5–7” for the area code positions. Verification / Alternative check: Many historical carrier documents describe DNIC (1–4) then national structure as area (5–7) + subscriber (8–12) + optional extensions. Differences existed across countries, but standardized training and exams typically adopt this mapping for consistency. Why Other Options Are Wrong: 1–4: DNIC (international portion), not the national “area.” 8–12: Often subscriber or site digits, not the area. 13–14: Typically extension/special-purpose digits, not area. None of the above: Incorrect because 5–7 is the conventional answer. Common Pitfalls: Assuming all countries used identical NTN layouts; remember that 5–7 is the standard exam model even if national implementations varied. Final Answer: 5–7

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