Interview Questions for Communication and Data Link Systems

The OSI (Open Systems Interconnection) model is a conceptual framework that standardizes the functions of a telecommunication or computing system without regard to its underlying internal structure and technology. It divides network communication into seven distinct layers, each responsible for a specific aspect of data transmission.

The Data Link Layer (Layer 2) sits between the Physical Layer (Layer 1) and the Network Layer (Layer 3). Its primary role is to ensure reliable data transfer between two directly connected nodes. This involves framing data into data link layer packets (frames), providing error detection and correction mechanisms, and managing access to the physical medium. Think of it as the postal service within a single city: it handles packaging, addressing, and local delivery, but doesn’t worry about routing to other cities (that’s the Network Layer’s job).

• Framing: The data link layer structures the data into manageable units called frames, adding headers and trailers for control information.
• Error Detection and Correction: It employs techniques like checksums or CRC (Cyclic Redundancy Check) to detect errors introduced during transmission and potentially correct them.
• Media Access Control (MAC): In shared media environments (like Ethernet), the data link layer manages access to the physical medium using protocols like CSMA/CD or CSMA/CA (explained in the next question).
• Flow Control: It regulates the rate of data transmission to prevent overwhelming the receiving node.

For example, imagine sending a letter. The data link layer ensures the letter is correctly addressed (MAC address), packaged into an envelope (framing), and checked for damage before delivery (error detection).

Both CSMA/CD (Carrier Sense Multiple Access with Collision Detection) and CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) are protocols used in local area networks (LANs) to manage access to a shared physical medium. The key difference lies in how they handle collisions.

CSMA/CD is a ‘listen-before-talk’ protocol where nodes listen to the medium before transmitting. If a collision is detected (two or more nodes transmit simultaneously), transmission is stopped, and a random backoff period is implemented before attempting retransmission. It’s relatively simple but can lead to inefficiency due to collisions and retransmissions. Think of it like a group of people shouting at once—they notice the chaos, stop, and try again later.

CSMA/CA employs a mechanism to avoid collisions rather than detect and recover from them. Nodes use a Request to Send (RTS) and Clear to Send (CTS) handshake mechanism to reserve the channel before transmitting. This reduces collisions significantly. It’s more complex but generally offers better performance, particularly in wireless environments where collision detection is challenging. It’s more like a well-organized meeting where people take turns to speak, preventing overlaps.

CSMA/CD is primarily used in traditional wired Ethernet networks, while CSMA/CA is frequently used in Wi-Fi networks.

Modulation techniques determine how information is encoded onto a carrier signal for transmission. Different techniques offer varying advantages and disadvantages:

• Amplitude Shift Keying (ASK): Information is encoded by varying the amplitude of the carrier wave. Simple to implement but susceptible to noise and less efficient.
• Frequency Shift Keying (FSK): Information is encoded by varying the frequency of the carrier wave. More robust to noise than ASK but less bandwidth-efficient.
• Phase Shift Keying (PSK): Information is encoded by shifting the phase of the carrier wave. More bandwidth-efficient than ASK and FSK and offers better noise immunity.
• Quadrature Amplitude Modulation (QAM): Combines ASK and PSK, encoding information using both amplitude and phase. Offers high bandwidth efficiency but is more susceptible to noise.

Advantages and Disadvantages Summary:

• Higher-order modulation techniques (like QAM) offer higher data rates but are more sensitive to noise.
• Lower-order techniques (like ASK and FSK) are more robust to noise but offer lower data rates.
• The choice of modulation depends on factors like the available bandwidth, required data rate, noise level, and complexity of implementation.

For instance, high-speed internet services often utilize QAM due to its high bandwidth efficiency, while simpler systems might employ FSK for its robustness in noisy environments.

Error detection and correction are crucial in data link communication to ensure data integrity. Errors can occur during transmission due to noise or other impairments. Error detection mechanisms identify the presence of errors, while error correction methods attempt to rectify them.

Error Detection: This involves adding redundant information (checksums, CRC, etc.) to the transmitted data. The receiver then recalculates the check value and compares it to the received one. A mismatch indicates an error.

Error Correction: More complex than detection, error correction techniques allow the receiver to reconstruct the original data even in the presence of errors. This typically involves adding more redundancy and using sophisticated algorithms to identify and correct the errors.

Imagine sending a package. Error detection is like verifying the package’s seal to confirm its integrity. Error correction involves including extra parts or instructions to repair any damage during transit.

Several error-detecting codes exist, each with its strengths and weaknesses:

• Parity Check: A simple technique that adds a single bit (parity bit) to ensure the number of 1s in the data is either even (even parity) or odd (odd parity). Detects single-bit errors but not all multiple-bit errors.
• Checksum: Calculates a sum of data words and adds it as a checksum. Detects many errors but not all.
• Cyclic Redundancy Check (CRC): A powerful technique that uses polynomial division to generate a check value. Detects bursts of errors and is widely used in networking protocols.
• Hamming Code: An error-correction code that adds multiple check bits to enable the detection and correction of single-bit errors.

When to use which code:

• Parity checks are suitable for simple applications where error rates are low and the overhead of more complex codes is undesirable.
• Checksums provide better error detection capability than parity checks and are often used in file transfers.
• CRC is the preferred method in networking due to its high efficiency in detecting burst errors.
• Hamming codes are used when error correction is required and the added redundancy is acceptable.

The choice depends on the required reliability, acceptable overhead, and complexity constraints.

A MAC (Media Access Control) address is a unique identifier assigned to a network interface controller (NIC) for communication at the data link layer. Think of it as the physical address of a network device, similar to a street address for a house. It’s a 48-bit address, typically represented as six pairs of hexadecimal digits (e.g., 00:16:3E:00:00:01).

MAC addresses are used to identify devices on a LAN and ensure that data is delivered to the correct recipient. Routers don’t use MAC addresses for routing across networks; they use IP addresses (explained later).

Routers and switches are both networking devices that connect multiple networks or devices, but they operate at different layers of the OSI model and have distinct functions.

Switches operate at the data link layer (Layer 2). They use MAC addresses to forward data within a single network. They learn MAC addresses by examining the source and destination MAC addresses in the frames they receive and build a MAC address table. This allows them to forward frames only to the port connected to the intended recipient, improving efficiency compared to hubs.

Routers operate at the network layer (Layer 3). They use IP addresses to forward data between different networks. They determine the best path to route packets based on routing tables and IP address information. Routers are essential for connecting different LANs or the internet.

Analogy: Imagine a city. A switch is like a local post office delivering mail within the city using street addresses (MAC addresses). A router is like a national postal service hub routing mail between cities using zip codes (IP addresses).

In the context of the data link layer, a frame is the fundamental unit of data transmission. Think of it like an envelope for your data. Just as an envelope contains a letter and an address, a frame encapsulates the data payload with header and trailer information necessary for transmission across a physical link. The header contains information such as source and destination addresses, frame type, and error detection/correction codes. The trailer often includes a Frame Check Sequence (FCS) for error detection.

For example, imagine sending a file across a network. The file’s data is broken down into smaller chunks, and each chunk is encapsulated into a frame before being transmitted. This allows for efficient and reliable data transfer. Each frame is independently processed and handled by the data link layer.

Network topologies describe the physical or logical arrangement of nodes (computers, devices) in a network. Here are four common types:

• Star Topology: All nodes connect to a central hub or switch. This is common in home and small office networks because it’s relatively simple to manage and troubleshoot. If one node fails, the rest continue to function. Imagine spokes radiating from the hub at the center of a wheel.
• Bus Topology: All nodes are connected to a single cable (the bus). Data is transmitted along the bus to all nodes. It’s inexpensive but can be unreliable as a single cable failure affects the entire network. Think of it like a hallway with doors leading to different rooms (nodes).
• Ring Topology: Nodes are connected in a closed loop. Data travels in one direction around the ring. Each node receives the data and passes it to the next. It’s efficient for local networks but a single node failure can disrupt the entire network. Visualize it like a track where data packets are passed around in a circle.
• Mesh Topology: Nodes are interconnected with multiple paths. This provides redundancy and high reliability. If one path fails, data can be routed through another. It’s robust but complex and expensive to implement. Think of a spiderweb, with many interconnected strands.

A Cyclic Redundancy Check (CRC) is an error detection code used to detect accidental changes to raw data during transmission. It works by treating the data as a polynomial and dividing it by a generator polynomial. The remainder of this division is appended to the data as the CRC. The receiver performs the same division and checks for a zero remainder. If the remainder is non-zero, an error has occurred.

Imagine you’re sending a message – the CRC acts like a checksum, a small piece of extra information calculated from the entire message. If the message is corrupted in transit, the receiver’s CRC calculation will be different, indicating an error. CRCs are widely used because they are efficient and effective at detecting various types of errors, making data transmission more reliable.

Flow control in the data link layer manages the rate of data transmission between sender and receiver to prevent data loss or buffer overflow. It ensures that the sender doesn’t overwhelm the receiver with data faster than it can process it. If the receiver’s buffer is full, it can send a signal to the sender to stop sending data until the buffer has space. Methods include sliding window protocols (discussed further in ARQ protocols) and throttling techniques.

Think of it like a water faucet: flow control ensures that the water flow (data) matches the capacity of the recipient (receiver). Without flow control, the receiver could be flooded with data, leading to dropped packets and data corruption.

Automatic Repeat reQuest (ARQ) protocols handle reliable data transmission by requesting retransmission of lost or damaged frames. Three common types are:

• Stop-and-wait ARQ: The sender sends one frame at a time and waits for an acknowledgment (ACK) before sending the next. If the ACK is not received within a timeout period, the frame is retransmitted. It’s simple but inefficient for high-bandwidth links.
• Go-back-N ARQ: The sender can transmit multiple frames before waiting for ACKs. If an error is detected, all frames from the erroneous one onwards are retransmitted. It is more efficient than stop-and-wait but can waste bandwidth retransmitting correctly received frames.
• Selective Repeat ARQ: Similar to Go-back-N, but only the erroneous frames are retransmitted. This is the most efficient ARQ protocol but more complex to implement.

These protocols use ACKs and timeouts to ensure reliable delivery. Imagine Stop-and-Wait like a postal service sending one letter at a time and waiting for confirmation, Go-back-N like sending a batch and resending if one is lost, and Selective Repeat like only sending the lost letter in the batch.

TCP (Transmission Control Protocol) and UDP (User Datagram Protocol) are both used in the transport layer (above the data link layer), but they have different characteristics:

• TCP: Connection-oriented, reliable, ordered delivery. It provides error checking, flow control, and congestion control. It’s suitable for applications requiring reliable data transfer, such as web browsing and file transfer.
• UDP: Connectionless, unreliable, unordered delivery. It’s faster and more efficient than TCP because it doesn’t have the overhead of error checking and flow control. It’s suitable for applications where speed is prioritized over reliability, such as streaming video and online gaming.

Think of TCP as a registered mail service ensuring delivery and order while UDP is like sending a postcard – it’s faster but may not arrive or arrive out of order.

Channel coding adds redundancy to data to improve its reliability during transmission over a noisy channel. It involves encoding the data using specific algorithms to add error-correcting or error-detecting bits. These extra bits allow the receiver to detect and correct errors caused by noise or interference.

Imagine you’re sending a message through a crowded room. Channel coding is like repeating key parts of the message or using a code that allows the recipient to decipher the message even if some words are missed. This improves the chance of the message being understood correctly, even in a noisy environment. Different coding schemes exist, offering varying levels of error correction capability. The choice depends on the characteristics of the communication channel and the desired level of reliability.

Antennas are crucial components in any communication system, responsible for radiating and receiving electromagnetic waves. Different antenna types are designed to optimize performance based on frequency, application, and physical constraints.

• Dipole Antennas: These are basic, resonant antennas consisting of two conductive rods of equal length. They are relatively simple and inexpensive, often used in applications like radio broadcasting (e.g., AM/FM radio) and Wi-Fi.
• Yagi-Uda Antennas (Yagi Antennas): These are directional antennas consisting of a driven element and multiple parasitic elements (directors and reflectors). They offer high gain and directivity, making them suitable for point-to-point communication and satellite TV reception. Think of your rooftop satellite dish—that’s a type of Yagi antenna.
• Patch Antennas: These are planar antennas etched onto a substrate, often found in mobile devices and wireless applications. They are compact, low-profile, and can be easily integrated into devices. Your smartphone’s built-in Wi-Fi antenna is likely a patch antenna.
• Horn Antennas: These antennas use a flared waveguide to radiate electromagnetic waves. They provide good directivity and are commonly used in microwave communication systems and radar applications.
• Microstrip Antennas: These are printed circuit board (PCB) antennas that are compact, lightweight and easily integrated into electronic devices. They find widespread use in mobile phones, GPS receivers, and RFID tags.

The choice of antenna depends heavily on the specific application. For instance, a high-gain directional antenna is preferable for long-range communication, while a compact omnidirectional antenna is suitable for mobile devices requiring coverage in all directions.

Signal propagation, the way a signal travels through a medium, is influenced by several factors. Understanding these factors is critical in designing reliable communication systems.

• Frequency: Higher frequencies experience greater attenuation (signal loss) and are more susceptible to obstacles. Lower frequencies can penetrate obstacles more easily but may have lower bandwidth.
• Distance: Signal strength decreases with distance, often following an inverse square law. The further the signal travels, the weaker it becomes.
• Obstacles: Buildings, trees, and even the Earth’s curvature can obstruct or reflect signals, leading to signal fading or shadowing. This is why you might experience weaker Wi-Fi signals in certain parts of your home.
• Atmospheric Conditions: Rain, snow, and fog can absorb or scatter radio waves, particularly at higher frequencies. Temperature inversions can also affect propagation.
• Multipath Propagation: Signals can travel along multiple paths to the receiver, causing constructive and destructive interference. This leads to fading and distortion.
• Terrain: Hills and mountains can block or reflect signals, affecting coverage. This is a major consideration in cellular network design.

Effective communication system design requires careful consideration of all these factors to ensure reliable signal transmission and reception.

Frequency hopping spread spectrum (FHSS) is a spread-spectrum modulation technique where the carrier frequency hops randomly among a set of predefined frequencies. This makes it resistant to jamming and interference.

Imagine a conversation in a crowded room. If you spoke on one frequency (pitch) consistently, others might easily overhear or interrupt. In FHSS, you rapidly change your pitch (frequency) making it harder for anyone to intercept or interfere.

Here’s how it works:

• Frequency Hopping Sequence: A pseudo-random sequence determines the order in which frequencies are used.
• Dwell Time: The time the signal remains on each frequency.
• Synchronization: Both the transmitter and receiver must use the same hopping sequence and dwell time to successfully decode the signal.

FHSS is used in applications where robustness against interference is crucial, such as Bluetooth and some military communication systems. The random hopping pattern makes it difficult for intentional jamming signals to effectively disrupt communication.

The signal-to-noise ratio (SNR) is a measure of the strength of a signal relative to the background noise. It’s expressed in decibels (dB). A higher SNR indicates a stronger signal compared to the noise.

Think of trying to hear a conversation in a noisy room. A high SNR is like hearing the conversation clearly despite the background noise. A low SNR is like struggling to hear the conversation because the noise is too loud.

SNR is crucial because it directly affects the quality and reliability of communication. A low SNR leads to errors, bit errors, and unreliable data transmission. In data communication, a minimum SNR is needed for successful data decoding. Different modulation schemes have different SNR thresholds for reliable operation. For instance, higher-order modulation schemes require higher SNR.

Multiple access protocols allow multiple users to share the same communication medium simultaneously. Here are three common types:

• Time Division Multiple Access (TDMA): Users share the same frequency channel but transmit at different time slots. Think of it like taking turns speaking in a conversation. GSM cellular networks use TDMA.
• Frequency Division Multiple Access (FDMA): Users are assigned different frequency channels. This is like having separate phone lines—each user has their own dedicated frequency. Traditional analog cellular networks use FDMA.
• Code Division Multiple Access (CDMA): Users share the same frequency and time but are distinguished by unique codes. It’s like many people speaking at once, but each person uses a different language (code). This allows multiple users to transmit simultaneously without interference. CDMA is used in some cellular networks and Wi-Fi.

Each protocol has its advantages and disadvantages. TDMA is efficient in terms of bandwidth usage, FDMA is simpler to implement, and CDMA offers better resistance to interference. The choice depends on the specific application requirements.

Time division multiplexing (TDM) is a technique that allows multiple signals to share a single communication channel by dividing the channel’s time into slots. Each signal is transmitted in its assigned time slot.

Imagine a highway with multiple lanes. TDM is like assigning each lane to a different car (signal). Each car gets its own time slot to travel on the highway (channel). While only one car can use a lane at any given time, multiple cars can use the highway simultaneously.

TDM is commonly used in digital telephony systems, where multiple phone calls are transmitted over the same physical wire. Each call is assigned a time slot, allowing many conversations to occur simultaneously.

My experience encompasses various communication protocols, including Ethernet, Wi-Fi, and Bluetooth.

• Ethernet: I’ve worked extensively with Ethernet networks, troubleshooting connectivity issues, configuring network switches and routers, and implementing VLANs (Virtual LANs) to segment network traffic. I understand the different Ethernet standards (e.g., 10BASE-T, 100BASE-TX, 1000BASE-T) and their respective capabilities. My experience includes working with both wired and wireless Ethernet (Power over Ethernet).
• Wi-Fi: I have experience in designing and configuring Wi-Fi networks, selecting appropriate access points, managing radio frequency (RF) interference, and optimizing network performance. I am familiar with various Wi-Fi standards (e.g., 802.11a/b/g/n/ac/ax) and their characteristics, including throughput, range, and security features. I have also worked with Wi-Fi mesh networks to improve coverage in large areas.
• Bluetooth: I have experience in integrating Bluetooth modules into devices, developing applications that utilize Bluetooth communication (e.g., data transfer, audio streaming), and troubleshooting connectivity issues. My understanding of Bluetooth profiles (e.g., A2DP, HFP, SPP) allows me to design and implement robust and efficient Bluetooth-enabled products.

Through these experiences, I’ve developed a strong understanding of the intricacies of each protocol, including their strengths, weaknesses, and best practices for implementation. My knowledge extends beyond basic configuration to include optimization for performance, security, and scalability.

Network security at the data link layer focuses on ensuring the integrity and confidentiality of data transmitted between directly connected nodes. While higher layers (like the network or transport layers) handle broader security concerns, the data link layer provides foundational protection. This primarily involves techniques like MAC address filtering (restricting access based on physical addresses) and link encryption (protecting data in transit using encryption algorithms).

MAC address filtering acts like a bouncer at a nightclub, only allowing devices with pre-approved MAC addresses to access the network. This can prevent unauthorized devices from connecting. For example, a company might only allow devices with specific MAC addresses registered in its internal database to connect to its Wi-Fi network.

Link encryption, on the other hand, scrambles the data so that even if intercepted, it remains unreadable without the decryption key. Think of it as sending a message in a coded language – only the recipient with the codebook (key) can understand it. This is commonly used in VPNs and secure wireless networks.

It’s important to note that data link layer security is just one piece of the overall security puzzle. A robust security architecture requires multiple layers of protection across different network layers.

Troubleshooting a communication link failure is a systematic process. My approach involves a series of steps, starting with the most basic checks and progressing to more advanced diagnostics.

• Physical Inspection: First, I visually inspect cables for physical damage (cuts, bends, loose connectors). This is often the easiest fix! I’ll also check that cables are securely connected at both ends.
• Connectivity Tests: I use tools like ping and traceroute (tracert on Windows) to verify connectivity to the remote node. Ping checks for basic reachability, while traceroute shows the path taken by packets, helping to pinpoint the location of the failure.
• Signal Strength and Quality: For wireless links, I’ll assess signal strength and quality using specialized tools or built-in utilities. Interference, distance, or obstacles can significantly affect performance.
• Layer 2 Diagnostics: I use tools that operate at the data link layer, such as analyzing the MAC address table of switches to determine if the failing node is properly learned and if bridging is functioning correctly. I may also use protocol analyzers (like Wireshark) to capture and analyze network traffic for errors or unusual patterns.
• Higher Layer Checks: If the data link layer checks are fine, I’ll move up to higher layers (network, transport) to investigate potential issues like routing problems, port conflicts, or application-specific errors.

For example, if ping fails, it indicates a complete lack of connectivity at the IP layer or below. If ping works but other applications fail, the problem likely lies in a higher layer protocol or the application itself.

I have extensive experience using various network monitoring tools, both open-source and commercial. These tools are essential for maintaining network health and performance. My experience includes:

• Nagios/Icinga: For system and network monitoring, these tools provide comprehensive alerts and dashboards. I’ve used them to monitor server uptime, network connectivity, and resource utilization.
• Zabbix: Another powerful monitoring system allowing for flexible configuration and customization. I’ve used Zabbix to track various network metrics, including bandwidth usage and packet loss.
• SolarWinds: A suite of tools designed specifically for network management. Their capabilities extend beyond basic monitoring to include performance analysis and troubleshooting assistance.
• Wireshark: A crucial protocol analyzer for deep packet inspection. This is invaluable for diagnosing network issues at the data link and higher layers, allowing me to examine individual packets for errors or anomalous behavior.

My experience extends to integrating these tools into automated systems, enabling proactive problem detection and alerting.

Data integrity in communication systems ensures that data remains accurate and unchanged during transmission and storage. Several techniques are crucial for achieving this:

• Checksums/CRC: These algorithms generate a small value based on the data. The receiver calculates the same value and compares it to the transmitted value. Any mismatch indicates corruption.
• Error Detection and Correction Codes (e.g., Hamming Codes): These codes add redundant data to the message to allow for detection and correction of errors. They are more robust than checksums but require more overhead.
• Data Encryption: While primarily for confidentiality, encryption also aids data integrity. Tampering with encrypted data is easily detectable due to the encryption/decryption process.
• Digital Signatures: These provide authentication and integrity verification. A digital signature is like a tamper-evident seal; any alteration would invalidate the signature.

For example, when transferring files, using checksums like MD5 or SHA-256 verifies that the received file matches the original. If the checksums don’t match, it implies data corruption during transmission.

My experience encompasses a wide variety of data link cabling, each with its own characteristics and applications. This includes:

• Twisted-Pair Cables (Cat5e, Cat6, Cat6a): These are the workhorses of many networks, offering various bandwidth capabilities depending on the category. Cat6a offers higher bandwidth and is often used for high-speed data transmission.
• Fiber Optic Cables (Single-mode, Multi-mode): Used for long-distance or high-bandwidth applications, fiber optic cables offer superior performance and immunity to electromagnetic interference compared to copper cables. Single-mode fibers are used for longer distances, while multi-mode fibers are suitable for shorter distances.
• Coaxial Cables: While less common in modern data networks, coaxial cables are still used in some applications, such as cable television networks.

Choosing the right cabling type depends on factors such as distance, bandwidth requirements, and environmental conditions. For example, fiber optics are preferred for long runs in harsh environments due to their resilience to electromagnetic interference and signal attenuation.

Network performance analysis involves identifying bottlenecks and inefficiencies within a network to optimize its performance. My approach involves a multi-faceted strategy:

• Monitoring Tools: I leverage tools like SolarWinds, PRTG, or Nagios to gather network performance metrics, including bandwidth utilization, latency, packet loss, and CPU/memory usage of network devices.
• Protocol Analysis: Using Wireshark, I can capture and analyze network traffic to identify specific protocol issues or applications causing congestion.
• Network Topology Mapping: Understanding the network’s physical and logical structure is vital. Tools like SolarWinds Network Topology Mapper help in visualizing the network and pinpointing potential trouble spots.
• Performance Baselines: Establishing baselines allows me to detect deviations and anomalies. This requires ongoing monitoring and data analysis.
• Root Cause Analysis: Once performance issues are identified, I conduct root cause analysis to pinpoint the underlying factors contributing to the problem. This could involve examining application logs, device configurations, or environmental factors.

For example, if bandwidth utilization is consistently high during specific times, it may indicate a need for network upgrades or optimization of bandwidth-intensive applications.

Quality of Service (QoS) in data link communication involves prioritizing certain types of network traffic over others to ensure that critical applications receive the necessary bandwidth and resources. Imagine a highway system – QoS is like having designated lanes for emergency vehicles, ensuring they reach their destination quickly even during heavy traffic.

QoS mechanisms at the data link layer often involve marking packets with priority levels. Switches and routers can then use this information to prioritize the forwarding of high-priority packets. Techniques include:

• Traffic Shaping: Limiting the rate at which certain types of traffic enter the network.
• Traffic Policing: Monitoring traffic and discarding or dropping packets that exceed pre-defined thresholds.
• Priority Queuing: Assigning different queues to different traffic types, giving priority to certain queues.

For example, in a VoIP system, QoS ensures that voice traffic is prioritized over less time-sensitive data traffic, minimizing latency and ensuring clear communication. Without QoS, voice calls might experience interruptions or poor quality during periods of high network congestion.