Question 1

Serial communications basics: what is the minimum number of conductors required to send data in one direction over a simple serial link (assume asynchronous RS-232-style signaling and no hardware flow control)?

Accepted Answer

Introduction: Real-world serial links are built from conductors that carry data and reference signals. The absolute minimum you need depends on directionality (simplex vs. full duplex) and whether you rely on hardware handshaking lines. This question asks for the minimum number of wires to send data in one direction over a simple asynchronous serial connection like RS-232, with no hardware flow control. Given Data / Assumptions: Simplex one-way transmission (source to sink only). Asynchronous character framing (start/stop bits) removes the need for a separate clock wire. No RTS/CTS, DTR/DSR, or other hardware flow-control leads are used. Ground reference is required for reliable signaling. Concept / Approach: In asynchronous RS-232-style links, the transmitter places a voltage on the TxD conductor relative to a common signal ground. Because the receiver needs a reference, at least one return/ground conductor must be present. Therefore, for one-way data transmission, two conductors suffice: TxD and signal ground. Full-duplex requires a third for RxD, and hardware flow control adds more wires, but these are not strictly required to send data one-way. Step-by-Step Solution: 1) Identify the data path: TxD carries the bits from source to destination.2) Provide a voltage reference: a shared signal ground is necessary.3) Conclude that the minimum is two conductors (TxD + GND) for simplex sending. Verification / Alternative check: Practical lab tests with USB-to-TTL or RS-232 adapters confirm that a two-wire hookup (Tx and GND) can send characters; adding Rx enables full duplex; adding RTS/CTS introduces hardware flow control but is optional for the minimal link. Why Other Options Are Wrong: 1 wire: lacks a reference ground, leading to undefined voltage levels. 4 or 6 wires: include lines that are helpful (Rx, RTS/CTS, DTR/DSR) but not strictly necessary for one-way sending. None of the above: incorrect because two wires are sufficient. Common Pitfalls: Assuming full duplex or hardware flow control is mandatory; forgetting that asynchronous serial embeds timing with start/stop bits, avoiding a dedicated clock wire. Final Answer: 2 conductors (TxD and signal ground).

Question 2

Computer-based electronic message and discussion systems: which platform historically offered a common forum for users to post messages, exchange ideas, and participate in ongoing threaded discussions at their convenience?

Accepted Answer

Introduction: Long before modern social platforms, organizations and hobbyist communities used computer-based services to exchange messages asynchronously. The question asks you to identify the system that provides a shared forum where users can sign in at any time, post messages, and take part in ongoing, threaded discussions. Given Data / Assumptions: Users do not have to be online simultaneously (asynchronous access). Messages are posted to common areas visible to many participants. Discussions persist over time and can be revisited. Concept / Approach: A bulletin board system (BBS) provides public message boards, private mailboxes, file areas, and topic-oriented forums. It is distinct from e-mail (primarily one-to-one or one-to-many messaging without public threads by default) and from conferencing systems, which are synchronous and require all parties to be present simultaneously (teleconference or videoconference). Step-by-Step Solution: 1) Match features: persistent public forums and threads are core BBS features.2) Contrast with e-mail: asynchronous but not typically a shared public forum by default.3) Contrast with conferencing: synchronous, not post-and-return asynchronous. Verification / Alternative check: Classic BBS software (e.g., WWIV, FidoNet-connected boards) and later web forums exhibit the exact behavior described: topic boards, user logins, and threaded discussions. Why Other Options Are Wrong: E-mail: primarily mailbox-based, not a shared public forum. Teleconferencing/Videoconferencing: real-time meetings, not asynchronous threads. None of the above: incorrect because BBS matches perfectly. Common Pitfalls: Equating any messaging tool with a public forum; overlooking the persistence and thread structure central to BBS platforms. Final Answer: Bulletin board system (BBS).

Question 3

In the OSI model, which statement best reflects a core function provided by the Network layer (Layer 3) rather than the Data Link layer?

Accepted Answer

Introduction: The OSI model separates responsibilities so that each layer solves a specific part of the end-to-end communication problem. This question checks whether you can distinguish what truly belongs to the Network layer (Layer 3) versus what is confined to the Data Link layer (Layer 2). Given Data / Assumptions: Layer 2 (Data Link) handles framing, link-local addressing, and error detection over a single hop. Layer 3 (Network) handles logical addressing, routing, and end-to-end packet delivery across multiple hops. X.25 provides an example of Layer-3 virtual circuits (PLP) across a packet network. Concept / Approach: The Network layer’s hallmark is inter-network delivery: routing between subnets and often providing a virtual-circuit abstraction (as in X.25). Data Link focuses on a single link segment (framing, MAC addressing). Therefore, a description that centers on end-to-end virtual circuits for packet-switched services belongs to Layer 3, not Layer 2. Step-by-Step Solution: 1) Option A describes frame exchange across one link — a Layer-2 role.2) Option B references the X.25 interface at the network layer, but the phrasing is ambiguous; the crux is still Layer-3 packet service, not link framing.3) Option C explicitly states a virtual-circuit interface for packet-switched service, which is a canonical Layer-3 responsibility in X.25-style stacks.4) Choose Option C as the clearest, most representative Network layer function. Verification / Alternative check: In X.25, PLP (Packet Layer Protocol) provides virtual circuits across the network, whereas LAPB (Data Link) manages a single hop. This confirms that end-to-end virtual-circuit service belongs to the Network layer. Why Other Options Are Wrong: A: Layer-2 framing procedures, not Layer-3 routing/service. B: References X.25 but is imprecise; the defining Layer-3 function is captured more directly by C. D: Cannot be correct because A is Layer-2. E: False; C is correct. Common Pitfalls: Confusing single-link responsibilities (Layer 2) with multi-hop end-to-end services (Layer 3); assuming “All of the above” when one choice clearly belongs to a different layer. Final Answer: Offering a virtual-circuit interface to a packet-switched service for end-to-end delivery.

Question 4

At the Data Link layer, which mechanism is typically used to detect transmission errors on a link?

Accepted Answer

Introduction: Reliable communication starts with detecting corrupted frames on each hop. The Data Link layer appends error-detection information so that receivers can discard damaged frames and request retransmission at an appropriate layer. This question asks you to identify the standard mechanism used for detection at Layer 2. Given Data / Assumptions: Focus is strictly on error detection at the Data Link layer. Common Layer-2 technologies include Ethernet, PPP, HDLC, and others. We are not being asked about forward error correction used on some physical links. Concept / Approach: Most Data Link protocols use a frame check sequence computed via a Cyclic Redundancy Check (CRC). The transmitter computes the CRC over the frame; the receiver recomputes and compares. A mismatch signals a corrupted frame. Other techniques listed either serve different purposes or are used at different layers. Step-by-Step Solution: 1) Identify Layer-2 detection technique: CRC/FCS is ubiquitous in Ethernet, HDLC, PPP.2) Distinguish from non-detection items: bit stuffing is framing; equalization is a physical-layer channel technique; Hamming codes are primarily error-correction codes used in memory/storage and select links, not the default for Layer-2 frame checks.3) Select CRC as the correct answer. Verification / Alternative check: Ethernet uses a 32-bit CRC (FCS). HDLC/PPP use FCS variants (often CRC-16/CRC-32). These are classic examples of Layer-2 error detection in practice. Why Other Options Are Wrong: Bit stuffing: a framing method to avoid flag patterns within data, not error detection. Hamming codes: error correction; not the typical Layer-2 detection mechanism. Equalization: mitigates channel distortion at the Physical layer. None: incorrect because CRCs are standard. Common Pitfalls: Confusing framing (bit stuffing) with integrity checks; thinking that any error-related term implies detection at Layer 2. Final Answer: Cyclic redundancy codes (CRC/FCS).

Question 5

Internet architecture principles: which statement is incorrect regarding routing knowledge, peer-layer delivery, and address usage across layers?

Accepted Answer

Introduction: The Internet stack separates concerns across layers: link-level physical/MAC addressing, network-level logical addressing, transport ports, and application names. This question asks you to spot the one statement that is incorrect by carefully examining how addressing is actually used across layers and how routing knowledge is distributed. Given Data / Assumptions: Routers exchange and maintain routing information; hosts typically use default gateways and limited route knowledge. Peer layers communicate abstract “objects” (service data units) that are reconstructed at the destination. Upper layers use more than just IP addresses (e.g., ports, names, URIs); link layers handle physical addresses. Concept / Approach: Evaluate each claim. It is reasonable to assume routers know routes and hosts start with minimal information (default gateway, on-link routes). Layering is designed so layer-n at the destination reassembles the same layer-n object (modulo permitted transformations like compression/encryption). However, asserting that layers at and above the Internet layer use only IP addresses is too strong: applications primarily use domain names/URLs; transport relies on ports plus IP; security may use identities/certificates. Therefore option C is the incorrect overgeneralization. Step-by-Step Solution: 1) Option A: Plausible — hosts often depend on gateways for routing beyond local subnets.2) Option B: Aligns with the peer-layer principle — the destination peer reconstructs the same SDU.3) Option C: Overclaims that upper layers use only IP addresses, ignoring ports, names, and identities; thus it is incorrect.4) Therefore, select C as the incorrect statement. Verification / Alternative check: Everyday practice: users connect via DNS names, not raw IPs; transport endpoints are 4- or 6-tuples including IPs and port numbers; link layers still map to MAC addresses via ARP/ND, confirming the layered split. Why Other Options Are Wrong (as incorrect choices): A reflects common host/gateway behavior. B expresses the essence of peer-layer delivery. D/E are distractors; exactly one statement (C) is incorrect. Common Pitfalls: Accepting “only IP addresses” at upper layers; forgetting the role of ports (Transport) and names (Application). Final Answer: Option C is incorrect because upper layers use more than just IP addresses (e.g., ports, DNS names).

Question 6

Inter-network communication: devices on one network can communicate with devices on another network by using which component?

Accepted Answer

Introduction: Communication across subnet boundaries requires a device that understands logical addressing and forwarding between distinct networks. This question asks for the generic component that enables traffic to move from one IP network to another. Given Data / Assumptions: Two different IP networks or subnets must exchange traffic. End hosts do not share a broadcast domain or link-layer addressing scheme. A device must make routing decisions based on logical addresses. Concept / Approach: The traditional term “gateway” (commonly equated with a router in IP networks) denotes the device that forwards packets between networks. While “file server,” “utility server,” and “printer server” provide application-level services within a network, they do not perform Layer-3 routing/forwarding between networks. Step-by-Step Solution: 1) Identify the layer required: Layer 3 (Network) for routing between subnets.2) Map to device role: a gateway/router examines IP headers and forwards to the next hop.3) Exclude application servers that do not route packets. Verification / Alternative check: Client default gateway settings on hosts point to the local router, confirming that gateways enable off-subnet communication. Why Other Options Are Wrong: File/Utility/Printer servers provide services but do not route. None of the above is false because “Gateway” is correct. Common Pitfalls: Confusing “gateway” (router) with “application gateway” servers; assuming any server can forward packets between networks without routing capability. Final Answer: Gateway (i.e., a router) enables communication between different networks.

Question 7

Packet Switch Stream (PSS): in which country and approximate launch period was PSS first introduced as a public packet-switched data service?

Accepted Answer

Introduction: Packet Switch Stream (PSS) was the United Kingdom’s public packet-switched data network service offered by the national telecom operator. Knowing its launch window helps place X.25-era services on a historical timeline relative to earlier US research networks and later commercial offerings. Given Data / Assumptions: PSS refers specifically to the UK service name, not generic packet switching research efforts. Early 1960s US efforts (1961) relate to theoretical groundwork, not a UK public PSS service. Sources commonly cite the early 1980s, with service becoming available circa 1981. Concept / Approach: Distinguish between research milestones (e.g., early US packet-switching theory and ARPA-related work) and the commercial/public rollout branded “PSS” in the UK. The relevant fact is the UK launch timeframe, which aligns with X.25 network deployment and EDI/host access adoption across enterprises in the early 1980s. Step-by-Step Solution: 1) Eliminate US-centric dates for PSS since the service name is UK-specific.2) Choose the UK early-1980s option; multiple histories place first availability around 1981.3) Prefer the more precise 1981 over a rounded “1980.” Verification / Alternative check: Contemporary telecom references and archives list British Telecommunications PSS as being introduced around 1981–1982, matching the early-1980s commercialization of X.25 services in Europe. Why Other Options Are Wrong: US in 1961: not PSS; refers to early research timelines, not the UK service. UK in 1980: close but typically cited as 1981. US in 1961 & UK in 1981: mixed and partly irrelevant; PSS is a UK service. None of the above: incorrect because “UK in 1981” fits best. Common Pitfalls: Confusing research milestones with commercial service launch; accepting approximate dates without checking the specific branded service and country. Final Answer: The UK in 1981.

Question 8

At the Data Link layer, which technique is typically used to detect errors in frames carried over a physical link?

Accepted Answer

Introduction: Link-level reliability depends on quickly identifying corrupted frames so higher layers can recover efficiently. The canonical mechanism used by common Layer-2 protocols is an error-detecting code appended to each frame. This question asks you to select that mechanism. Given Data / Assumptions: Focus: error detection at Layer 2 (not correction). Examples: Ethernet, HDLC, PPP, and related protocols. Physical channel impairments can flip bits; the receiver must detect this. Concept / Approach: A Cyclic Redundancy Check (CRC) generates a Frame Check Sequence (FCS). The sender computes an FCS over the frame; the receiver recomputes and compares. A mismatch indicates corruption, prompting discard and possible retransmission by an upper-layer protocol or link-level procedure, depending on the technology. Step-by-Step Solution: 1) Identify common Layer-2 integrity method: CRC/FCS.2) Separate from non-detection techniques: bit stuffing is framing; equalization is Physical-layer signal conditioning; Hamming is error correction not typically used for general Layer-2 detection.3) Choose Cyclic Redundancy codes. Verification / Alternative check: Ethernet uses a 32-bit CRC; HDLC/PPP use CRC-16/CRC-32 variants, confirming CRC/FCS as the standard Layer-2 detection approach. Why Other Options Are Wrong: Bit stuffing: prevents flag emulation within payloads; not detection. Hamming codes: provide correction; not the typical Layer-2 detection method in these protocols. Equalization: Physical-layer channel equalization, not a Layer-2 integrity check. None: false because CRC is correct. Common Pitfalls: Confusing framing and signal-processing terms with integrity checking; assuming that any error-related term equals Layer-2 detection. Final Answer: Cyclic Redundancy codes.

Question 9

You are given a Class C network (default /24). You must create five subnets, and each subnet must support at least 25 usable host addresses (network and broadcast excluded). Which subnet mask should you configure to meet both constraints while keepi…

Accepted Answer

Introduction / Context: Subnetting a Class C (default /24) network requires balancing two goals: (1) ensuring each subnet has enough usable host addresses and (2) producing at least the required number of subnets. This question asks for a subnet mask that yields at least five subnets while providing a minimum of 25 usable host addresses per subnet. Given Data / Assumptions: Starting block: Class C, default mask /24 (255.255.255.0). Required subnets: at least five. Required hosts per subnet: at least 25 usable addresses. Usable hosts = total addresses per subnet − 2 (network and broadcast). Concept / Approach: For a mask /n, each subnet contains 2^(32 − n) total addresses and 2^(32 − n) − 2 usable addresses. The smallest subnet that still supports ≥ 25 hosts maximizes how many subnets can be carved from the /24. We will evaluate /27, /28, and finer masks and count how many such subnets fit within /24. Step-by-Step Solution: Check /28 (255.255.255.240): total 16, usable 14 → not enough for 25.Check /27 (255.255.255.224): total 32, usable 30 → meets the ≥ 25 requirement.How many /27 subnets fit into a /24? 2^(27 − 24) = 2^3 = 8 subnets → satisfies the need for five.Finer masks (/29, /30, etc.) reduce hosts below 25 and are invalid; larger masks like /26 waste capacity and reduce the number of available subnets compared with /27. Verification / Alternative check: The /27 subnets in a /24 start at .0, .32, .64, .96, .128, .160, .192, .224. Each has 30 usable hosts, which comfortably exceeds the target of 25. You can assign any five of these ranges to satisfy the requirement and still have room for growth. Why Other Options Are Wrong: 255.255.255.0 (/24): No subnetting performed; only one subnet. 255.255.255.240 (/28): Only 14 usable hosts; below 25. 255.255.255.248 (/29): Only 6 usable hosts. 255.255.255.252 (/30): Only 2 usable hosts; point-to-point only. Common Pitfalls: Forgetting to subtract 2 for network/broadcast, mixing total vs usable counts, or choosing an oversized subnet (e.g., /26) that unnecessarily reduces the number of available subnets. Final Answer: 255.255.255.224

Question 10

In the OSI reference model, which layer is responsible for providing reliable, error-free delivery of data between end systems (end-to-end), including sequencing and retransmission when needed?

Accepted Answer

Introduction / Context: The OSI model decomposes communication into seven layers, each offering distinct services. While several layers participate in reliability, the one classically tasked with end-to-end reliable, error-free delivery—handling sequencing, flow control, and retransmission—is the Transport layer. Given Data / Assumptions: We are using the OSI model as a conceptual framework. “Error-free delivery” here means end-to-end reliability as seen by applications. Mechanisms include segmentation/reassembly, sequencing, acknowledgments, and retransmission. Concept / Approach: The Transport layer (Layer 4) establishes logical end-to-end connections and can present a reliable byte stream (e.g., like TCP in the TCP/IP suite). Although the Data Link layer also performs error detection and reliable delivery, its scope is hop-to-hop on a single link, not end-to-end across the network. The Network layer focuses on routing and addressing, not reliability guarantees. Step-by-Step Solution: Identify the target service: end-to-end error-free delivery, not just local link reliability.Map this service within OSI: Layer 4 (Transport) provides sequencing, flow control, and retransmission.Confirm that lower layers (Data Link, Network) have different scopes: hop-level reliability and routing.Therefore, the correct OSI layer for the described responsibility is the Transport layer. Verification / Alternative check: In TCP/IP, TCP is often mapped to the Transport layer and implements these very features: sequence numbers, acknowledgments, window-based flow control, and retransmission timers. Why Other Options Are Wrong: Data Link: Provides frame-level reliability on a single link; not end-to-end across routers. Network: Routes packets between networks; typically offers best-effort delivery without retransmission. Session: Manages dialog control and synchronization for application exchanges, not raw data reliability. None of the above: Incorrect because the Transport layer fits. Common Pitfalls: Confusing hop-to-hop frame reliability with end-to-end transport reliability, and assuming “Network” does retransmissions (in classic OSI, it does not). Final Answer: Transport layer

Question 11

Convert the 32-bit binary Internet address 10000000 00001010 00000010 00011110 into dotted-decimal form. Choose the correct dotted-decimal representation.

Accepted Answer

Introduction / Context: IPv4 addresses are 32-bit values typically shown in dotted-decimal notation as four octets. Converting from binary to dotted-decimal requires splitting the bit string into four 8-bit groups and converting each to base 10. Given Data / Assumptions: Binary address: 10000000 00001010 00000010 00011110. Each group of 8 bits corresponds to one dotted-decimal octet. No subnet masks or class considerations are required—pure conversion only. Concept / Approach: The algorithm: (1) split the 32 bits into four octets, (2) convert each 8-bit binary number to decimal using positional weights (128, 64, 32, 16, 8, 4, 2, 1), and (3) join with dots. Step-by-Step Solution: Octet 1: 10000000 = 128.Octet 2: 00001010 = 8 + 2 = 10.Octet 3: 00000010 = 2.Octet 4: 00011110 = 16 + 8 + 4 + 2 = 30.Combine: 128.10.2.30. Verification / Alternative check: Quick mental check: a leading 1 followed by 7 zeros is 128; small set bits in the second (8 and 2), third (2), and fourth (16+8+4+2) octets corroborate 10, 2, and 30 respectively. Why Other Options Are Wrong: 148.20.2.30, 164.100.9.61, 210.20.2.64: Do not match the binary octet conversions. None of the above: Incorrect because 128.10.2.30 is valid and matches exactly. Common Pitfalls: Mis-grouping bits (not on 8-bit boundaries) or arithmetic slips when adding powers of two. Always write out the weight positions to avoid errors. Final Answer: 128.10.2.30

Question 12

Which of the following is NOT a standard synchronous data link protocol? (Choose the data-link option that is specifically asynchronous or otherwise not a synchronous link protocol.)

Accepted Answer

Introduction / Context: Data link protocols can be broadly categorized as synchronous (timing derived from a shared clock; frames sent in fixed timing) or asynchronous (characters framed with start/stop bits without a shared clock). Recognizing these categories is essential in legacy WAN and serial communication contexts. Given Data / Assumptions: We are comparing data-link protocols by their timing method. Focus is on whether a protocol is a standard synchronous link protocol. We will select the option that is specifically not synchronous at the data-link level. Concept / Approach: SDLC (Synchronous Data Link Control) is, by definition, synchronous. SLIP (Serial Line Internet Protocol) encapsulates IP over serial lines using asynchronous character framing (start/stop bits) and lacks features like error detection and addressing—it is not a synchronous data link protocol. SMTP is an application-layer email transport and not a data-link protocol at all, but the prompt directs us to pick the data-link option that is not synchronous. Step-by-Step Solution: Identify synchronous vs asynchronous: SDLC → synchronous; SLIP → asynchronous.Exclude non–data-link distractors (SMTP, PAS unspecified) per the instruction to choose the data-link option.Select SLIP as the data-link mechanism that is not synchronous. Verification / Alternative check: SLIP historically runs over RS-232 lines using asynchronous framing; PPP later replaced SLIP with richer features (LCP, authentication) but remained compatible with asynchronous serial operation. Why Other Options Are Wrong: SDLC: Canonical synchronous link protocol. SMTP: Application-layer email protocol; not a link protocol (outside the comparison domain). PAS: Not a standard data-link protocol; distractor. None of the above: Incorrect because SLIP clearly is not synchronous. Common Pitfalls: Confusing OSI layers; comparing application protocols with link protocols leads to category errors. The safest approach is to restrict the comparison to data-link candidates. Final Answer: SLIP

Question 13

Which medium access control (MAC) technique is specifically designed for bus or tree topologies, using a token that is passed logically among stations on the shared medium?

Accepted Answer

Introduction / Context: Medium access control determines how multiple stations share a common transmission medium without talking over each other. Different physical topologies often inspired distinct MAC strategies—rings, buses, and trees each posed unique coordination challenges. Given Data / Assumptions: Physical topology considered: bus/tree (e.g., coaxial bus or a logical tree). We need a MAC technique tailored to that topology. Token-based coordination vs contention (CSMA) are classic alternatives. Concept / Approach: Token Bus (IEEE 802.4) implements a logical ring over a physical bus or tree. A token circulates among stations in a predefined logical order; only the token holder transmits. This prevents collisions while retaining the cabling simplicity of a bus/tree. Token Ring, by contrast, assumes a physical (or concentrator-based) ring. CSMA (as in classic Ethernet) is contention-based rather than token-controlled. Step-by-Step Solution: Match topology: bus/tree → look for a MAC that overlays a logical ring on a bus.Token Bus fits: token passing without physical ring cabling.Conclude Token Bus is the intended MAC technique. Verification / Alternative check: Historical standards: IEEE 802.4 explicitly targeted industrial environments with bus cabling, providing deterministic access via token passing. Why Other Options Are Wrong: Token Ring: Designed for ring topology; not bus/tree. CSMA: Contention-based; may be used on bus but is not the token-based method asked for. MAC (generic): Not a specific technique. None of the above: Incorrect because Token Bus exists and matches. Common Pitfalls: Equating “bus = CSMA only.” While CSMA dominated Ethernet, token bus was specifically defined for bus/tree with deterministic access. Final Answer: Token Bus

Question 14

What is the key difference between TCP and UDP in the Internet protocol suite with respect to connection semantics and service model?

Accepted Answer

Introduction / Context: Applications choose between TCP and UDP based on their needs for reliability, ordering, flow control, and latency. Understanding the fundamental service models helps you select the right transport for web, streaming, gaming, or control applications. Given Data / Assumptions: Transport protocols compared: TCP and UDP. We focus on connection semantics and service characteristics. No assumptions about specific applications beyond common usage patterns. Concept / Approach: TCP (Transmission Control Protocol) is connection-oriented: it establishes a logical connection (three-way handshake), provides reliable delivery (ACKs, retransmissions), ordered byte stream, and flow/congestion control. UDP (User Datagram Protocol) is connectionless and message-oriented (datagram): it sends independent packets without built-in reliability or ordering, minimizing overhead and latency. Step-by-Step Solution: Identify TCP features: connection setup, reliable, ordered stream, congestion control.Identify UDP features: no connection, no guaranteed delivery/order, simple header.Map to the option that states “UDP is a datagram service whereas TCP is a connection-oriented service.” Verification / Alternative check: Typical mappings: HTTP/HTTPS over TCP for reliable delivery; DNS queries commonly over UDP for speed (with TCP fallback for truncation); live media sometimes uses UDP-based protocols to avoid retransmission delays. Why Other Options Are Wrong: UDP connection-oriented and TCP datagram: Reversed and incorrect. TCP Internet vs UDP ATM: UDP is also an Internet transport; ATM is a different technology stack. All/None of the above: Do not fit the accurate characterization. Common Pitfalls: Assuming UDP is “unreliable” in all senses; reliability can be built at the application layer. Conversely, assuming TCP is always best; latency-sensitive apps may prefer UDP. Final Answer: UDP is a datagram service whereas TCP is a connection-oriented service

Question 15

Under the historical classful IPv4 scheme, what is the valid first-octet range for a Class A unicast address (excluding the reserved 0 and the loopback block)?

Accepted Answer

Introduction / Context: While modern networks use CIDR, many exam questions still reference classful addressing. Class A networks historically had a /8 default mask and a distinct first-octet range. Knowing the range also clarifies special-use blocks such as loopback. Given Data / Assumptions: Class A default mask: /8. First octet defines the class in classful addressing. Loopback (127.0.0.0/8) is excluded from ordinary unicast. Concept / Approach: Class A first-octet range is 0–127 in binary terms (leading bit 0). However, the 0.0.0.0 block is reserved (this host/network) and 127.0.0.0/8 is loopback. Therefore, valid assignable Class A unicast addresses have first octet 1 through 126 inclusive. Step-by-Step Solution: Recall classful ranges: A = 0–127, B = 128–191, C = 192–223.Remove reserved: 0 (special), 127 (loopback).Remaining valid unicast first octet for Class A is 1–126. Verification / Alternative check: The dotted-decimal blocks beginning with 10.x.y.z (private Class A) sit within this 1–126 range; operating systems treat 127.x.y.z as loopback, reinforcing the exclusion. Why Other Options Are Wrong: 1–127: Includes 127 (loopback). 128–191: That is Class B. 192–223 and 192–255: Those ranges overlap Classes C and beyond; not Class A. Common Pitfalls: Forgetting the special-case 127/8 loopback block and accidentally including it in the Class A usable range. Final Answer: 1 - 126

Question 16

On a Class B network, using a subnet mask with an octet value of 248 (i.e., 255.255.248.0 → borrowing 5 bits in the third octet), how many subnets are available under the traditional classful rule set?

Accepted Answer

Introduction / Context: Before CIDR and modern practices, classful subnetting calculated the number of subnets based on bits borrowed from the host portion and often applied the “no all-zeros, no all-ones” rule to subnet IDs. This question uses that legacy approach. Given Data / Assumptions: Base class: Class B (default /16). Subnet mask: 255.255.248.0 → /21 (i.e., 16 + 5 bits borrowed). Legacy rule: subtract 2 subnets (all-zeros and all-ones subnets not used). We are counting subnets, not hosts per subnet. Concept / Approach: Borrowed bits = 5 (since 248 = 11111000 in the third octet). Under the traditional rule, number of subnets = 2^(borrowed bits) − 2 = 2^5 − 2 = 30. (Modern networks typically allow using all subnets, giving 32, but classical exam questions often expect the legacy 30.) Step-by-Step Solution: Determine borrowed bits: from /16 to /21 → 5 bits.Compute classic count: 2^5 − 2 = 30 subnets.Note: Hosts per /21 subnet = 2^(32 − 21) − 2 = 2046 usable (not asked, but useful context). Verification / Alternative check: Binary increment of the third octet in steps of 8 (since 248 leaves 3 host bits) shows distinct subnet IDs: .0, .8, .16, …, up to .248, which corresponds to 32 possible patterns; subtracting the two special patterns yields 30 subnets. Why Other Options Are Wrong: 2 or 6: Far too small; borrowing 5 bits yields many more subnets. 62 or 126: These match subtract-2 results for borrowing 6 or 7 bits, not the 5-bit case. Common Pitfalls: Mixing modern “use all subnets” practice (32) with the legacy subtract-2 convention (30), and confusing host-bit math with subnet-bit math. Final Answer: 30

Question 17

For interactive, conversational applications (e.g., terminals, chatty protocols, remote sessions), which type of communication line is best suited to support simultaneous two-way exchange?

Accepted Answer

Introduction / Context: Interactive processing (e.g., remote shells, live customer support, collaborative editing) depends on rapid exchange in both directions. The physical/logical properties of the communication line directly affect responsiveness and user experience. Given Data / Assumptions: Application style: conversational, request/response with frequent back-and-forth. Goal: minimize latency and avoid turn-taking delays. Bandwidth needs may be modest; directionality is critical. Concept / Approach: Full-duplex lines allow simultaneous bidirectional transmission, so the user can send while receiving without collision or enforced silence periods. Simplex permits only one-way flow; half-duplex alternates directions, introducing turn-around delays; “narrowband/mixed band” are vague with respect to directionality and often insufficient descriptors of interactive suitability. Step-by-Step Solution: Identify the requirement: continuous two-way interaction.Map to line capability: only full-duplex supports simultaneous send/receive.Conclude full-duplex lines best suit interactive processing. Verification / Alternative check: Think of phone conversations (full-duplex) versus walkie-talkies (half-duplex with push-to-talk). Interactive computing ideally mirrors the phone model to optimize responsiveness. Why Other Options Are Wrong: Simplex: One-way only; cannot support back-and-forth interaction. Narrowband / mixed band: Do not specify duplex properties; may still be half/single-direction. None of the above: Incorrect because full-duplex is a well-known solution. Common Pitfalls: Confusing bandwidth with interactivity; higher bandwidth does not fix half-duplex turn-taking delays. Duplex capability is the key requirement. Final Answer: full-duplex lines

Question 18

Within the OSI network architecture, which layer is responsible for routing packets across internetworks, choosing paths, and handling logical addressing?

Accepted Answer

Introduction / Context: The OSI model assigns distinct responsibilities to each layer. Determining which layer handles routing and logical addressing clarifies how packets move hop by hop across heterogeneous networks to reach their destinations. Given Data / Assumptions: We are discussing layer responsibilities in the OSI framework. Routing implies selecting next hops and end-to-end paths through multiple networks. Logical addressing refers to network-layer addresses (e.g., IP addresses). Concept / Approach: The Network layer (Layer 3) provides logical addressing and routing. It encapsulates segments from the Transport layer into packets, adds source/destination network addresses, and uses routing algorithms/protocols to forward packets across routers. Data Link handles local delivery on a single link; Transport provides end-to-end reliability and flow control but not routing decisions. Step-by-Step Solution: Identify the task: routing across networks and logical addressing.Map to OSI layers: Layer 3 owns these responsibilities.Confirm distinction from Layer 2 (framing/MAC) and Layer 4 (reliability/ports). Verification / Alternative check: In TCP/IP, IP (Internet Protocol) is the Layer-3 analogue, and routers operate here using protocols like OSPF, IS-IS, and BGP for path selection. Why Other Options Are Wrong: Data Link: Deals with MAC addresses and local link delivery, not inter-network routing. Transport: Manages end-to-end transport services; does not select network paths. Session: Coordinates dialog control; not a routing function. None of the above: Incorrect because Layer 3 clearly performs routing. Common Pitfalls: Assuming Transport “routes” because it is end-to-end. Transport rides atop the routing decisions made at the Network layer. Final Answer: Network layer

Question 19

In digital communications and computer networks, the error-detecting code in which a remainder (computed by dividing the bit sequence being checked by a predetermined binary polynomial) is appended to the data is known as:

Accepted Answer

Introduction: Catching transmission errors efficiently is a cornerstone of reliable networking and storage. Among the most widely used techniques is a method where the sender computes a remainder by dividing the message bits by a fixed binary polynomial and appends this remainder to the frame. This question asks you to identify that specific error-detecting code. Given Data / Assumptions: An error-detecting code is required for frames or blocks of bits sent over a potentially noisy channel. The code is produced as the remainder of a polynomial (modulo-2) division of the data bits by a predetermined generator polynomial. The receiver repeats the same division to verify integrity. Concept / Approach: The technique described is the cyclic redundancy check (CRC). In CRC, the data bits are treated as coefficients of a polynomial over GF(2). The transmitter divides this polynomial by an agreed generator polynomial and appends the remainder (CRC) to the data. At the receiver, the same division is applied to the combined data+CRC; a zero remainder indicates that no detectable error has occurred. CRCs are excellent at detecting common error patterns (single-bit flips, bursts up to a certain length) with modest overhead and very fast hardware or software implementations. Step-by-Step Solution: 1) Identify the operation: compute remainder = data(x) mod G(x), where G(x) is the generator polynomial.2) Append the remainder (CRC field) to the frame before transmission.3) On reception, compute (data(x) * x^k + CRC) mod G(x) and check if the result equals 0.4) If remainder is non-zero, declare a detected error and discard or request retransmission. Verification / Alternative check: Ethernet frames, many link-layer protocols, and storage formats (e.g., disk sectors) use CRCs because they are more powerful than simple parity or checksums for burst-error detection while remaining lightweight to implement. Why Other Options Are Wrong: Checksum: adds words under integer arithmetic; it is not a polynomial remainder and offers weaker detection for certain patterns. Error detecting code: is generic and does not name the specific polynomial remainder method. Error rate: a performance metric, not a coding method. None of the above: incorrect because CRC exactly matches the description. Common Pitfalls: Confusing CRC with parity or checksum; only CRC is defined by modulo-2 polynomial division using a generator polynomial such as CRC-16 or CRC-32. Final Answer: Cyclic redundancy check

Question 20

In basic data communications, what is the name of the device used to connect two computers over a telephone line by converting digital data to analog waveforms and back again?

Accepted Answer

Introduction: Before ubiquitous broadband, many computers communicated via public switched telephone networks. A specific endpoint device converted digital bits into analog audio tones suitable for the voice band and then reversed the process at the far end. Understanding this legacy but still-relevant device helps ground key concepts of modulation and demodulation. Given Data / Assumptions: Communication medium: voice-grade telephone line. End systems: digital computers that produce and consume bits. Need: translate between digital domain and analog channel. Concept / Approach: The correct device is a modem (from modulator-demodulator). The transmitter side modulates a carrier (varying amplitude, frequency, or phase), and the receiver side demodulates to recover the original bitstream. Although modern broadband uses different physical layers, the conceptual role of modems persists in DSL, cable, cellular, and satellite systems where digital data rides on passband signals. Step-by-Step Solution: 1) Identify the mismatch: digital bits vs. analog telephone channel.2) Use a device that modulates at the sender and demodulates at the receiver.3) Name that device: modem.4) Validate by typical standards (e.g., V.90, V.92) historically used over PSTN lines. Verification / Alternative check: Legacy dial-up Internet connections used modems on both ends; today, cable and cellular modems perform analogous functions with more advanced waveforms. Why Other Options Are Wrong: Tape: storage medium, not a communication interface. Bus: internal computer interconnect concept, not a phone-line interface device. Cable: a passive medium; it does not perform modulation/demodulation. None of the above: incorrect because modem fits precisely. Common Pitfalls: Confusing the medium (telephone line) with the active device (modem) that makes the line usable for digital data. Final Answer: modem

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