Lower Layers Part 1 Flashcards
What are the three ideal goals when linking network elements?
To exchange data:
1. In any chosen amounts
2. At any chosen speed
3. With zero error rates
Why can’t networks achieve zero error rates and infinite speeds?
Data travels at bounded speeds through networks with finite capacity and experiences non-zero error rates due to real-world constraints.
What metrics are used to characterise a flow of data?
- Bandwidth: Volume of data per unit time (commonly Mbps).
- Latency: Delay in transmission (usually milliseconds).
- Error rate: Errors per gigabit or gigabits per error.
Why is “speed” not always the best measure of network performance?
Speed alone can be misleading, like comparing a car at 60mph to a lorry at 60mph.
It’s more about bandwidth (data capacity) and how much can be transmitted without loss.
How has data transmission capacity evolved over time?
Commodity servers have increased from 10Mbps to 40Gbps in 30 years.
This represents a 4000x increase in bandwidth capacity.
What is latency in networking?
Latency is the time it takes for a bit, or group of bits, to travel through the network.
It includes delays caused by the speed of light, clocking the packet at each end, and delays at intermediate stages due to packet size and bitrate.
Why does the speed of light impose a lower bound on latency?
The speed of light determines the minimum time required for data to travel over long distances, and this is constant. It dominates other delays over long distances.
What is Bit Error Rate (BER), and what causes it?
Bit Error Rate (BER) measures errors in data transmission. Causes include:
1. Data loss due to interference.
2. Cosmic rays, impulse noise, cable, and connector issues.
How does CPU power affect error correction in 2024?
CPU power is cheap, so adding more or longer error detection and correction is rarely costly.
This enables better error handling without significant overhead.
What prediction did Ian make in the slides about memory errors in the next few years?
The prediction is that memory errors in CPU caches will become a significant problem as memory ages and systems handle larger data volumes.
This is especially true as systems process increasingly larger data volumes, requiring caches to handle more intense workloads.
What are the current measurements for measuring volume, latency, and error rates? (long-haul networks)
- Volume: Gigabits or terabits per second.
- Latency: Measured in milliseconds.
- Undetected error rates: 1 bit per terabyte.
Why has latency become a significant issue despite bandwidth improvements?
While bandwidth has increased by 3–5 orders of magnitude (10^3–10^5), latency has not improved much in the last 30 years, making it a major bottleneck today.
What trade-offs do different types of data require? (voice, file transfers and streaming)
- Voice: Tolerates low bandwidth and high errors but cannot tolerate high latency.
- File Transfers (e.g., OS images): Prioritise reliability above all else.
- Streaming and recordings: Tolerate some buffering, unlike live content.
For voice, file transfers and streaming, what are their requirements for latency and error rates?
- Voice and live sport require low latency but tolerate higher error rates.
- Streaming and recordings tolerate latency but require low error rates.
- File transfers and OS images prioritise reliability (low errors) over latency.
Why is latency due to the speed of light significant for long distances?
Because even with infinite bandwidth, latency caused by the speed of light is unavoidable and becomes dominant over long distances, like from London to California.
What is circuit switching, and how did it work in old telephone systems?
Circuit switching connected physical wires so that one microphone was continuously linked to a speaker at the other end using amplifiers and multiplexors.
What are the problems with circuit switching?
- Inefficient: It ties up a duplex circuit even if one or both parties are silent.
- Complexity: Multiplexing was very complicated and expensive, especially with 1950s technology.
- Reliability: Treating the wire as a radio with multiple carriers (for increased efficiency, FDM) was difficult to do reliably.
What is packet switching, and how does it work?
Packet switching divides data into small units called “packets,” adds identifying information, and switches packets over the network to their destination.
What happens when no data is being sent in packet switching?
When no data is being sent, no (or at least few) resources are consumed.
What are the advantages of packet switching over circuit switching?
- Multiplexing happens in the time domain instead of the frequency domain.
- Resources are not wasted when no data is sent.
- You achieve a statistical gain on bandwidth (because it efficiently shares resources among multiple users)
- Data can be re-routed around failed switches, improving resilience.
What is the concept of the Time-Division Multiplexing (TDM) in data streams?
The time domain refers to buffering multiple data streams at a lower rate (e.g., 8kbps) and sending them together at a higher rate (e.g., 16kbps).
The line alternates usage, resulting in latency due to waiting for all bits to arrive.
Does the Time-Division Multiplexing (TDM) approach for data streaming prioritise efficiency or latency?
It prioritises efficiency over latency.
What is the alternative to Time-Division Multiplexing (TDM), and why is it often preferred?
The alternative is frequency domain multiplexing, where data streams are sent simultaneously at different frequencies.
Time domain is often preferred because it is simpler to implement with modern digital electronics and reduces complexity.
Is frequency domain multiplexing faster than time domain?
No, FDM does not make data travel faster.
When is frequency domain better for data streaming?
FDM is better when you have:
- Continuous, independent streams that need to be sent in parallel (e.g., analogue signals like radio stations).
- Sufficient bandwidth to allocate separate frequencies to each stream.
When is time domain better for data streaming?
TDM is better for:
- Digital networks where efficiency matters, and data can be sent in chunks.
- Situations where you can tolerate minor delays while waiting to buffer data (e.g., file transfers).
How does frequency domain multiplexing distinguish signals?
FDM distinguishes signals by their frequency. Each signal is modulated to a different frequency (carrier), allowing multiple signals to share the same medium simultaneously. On the receiving side, filters separate the signals by their frequencies for decoding.
What real-world example illustrates frequency domain multiplexing?
An example is two lighthouses flashing messages in Morse code:
- One uses a red light, and the other uses a green light.
- Filters, like coloured lenses on a telescope, separate the two signals so the messages can be read independently.
How does frequency modulation (FM) work?
Frequency modulation (FM) works by encoding data into a carrier signal by varying its frequency. This allows the data to “ride” on the carrier and be transmitted efficiently over a medium.
Since each signal has its dedicated frequency range and transmits data independently within that range, all signals can flow simultaneously and continuously over the medium. This makes FDM ideal for things like radio and TV broadcasting, where many channels operate at once without interruptions.
What are the key differences between Frequency Domain Multiplexing (FDM) and Time Domain Multiplexing (TDM)?
- FDM does not introduce systematic latency, as signals are transmitted simultaneously on different frequencies.
- TDM introduces latency because data is divided into time slots and sent sequentially, but it efficiently uses bandwidth.
- FDM is harder to engineer robustly and uses less available capacity for real signals.
- TDM is commonly used in networking, whereas FDM is still used in analogue systems like musicians’ monitoring systems. (analogue form (e.g., sound waves or video signals) and are divided by frequency. Unlike digital systems that encode data into discrete 0s and 1s)
What is statistical gain in networking?
Statistical gain refers to the practice where Internet Service Providers (ISPs) oversubscribe bandwidth, assuming that not all users will use their full line rate simultaneously.
For example, selling 10Mbps to 100 users but provisioning only a fraction, such as 5% of the total (contention ratio: 1:20).
This allows efficient resource use but may lead to congestion during peak usage.
What are the main problems with packet-based communication?
- Packets can get lost, delayed, or re-ordered during transmission.
- Data larger than a single packet must be split into packets and reassembled upon arrival.
- Missing packets require waiting for a completed sequence before the data can be used.
What is a virtual circuit in networking?
- A virtual circuit is a connection where the network sets up a path between endpoints.
- The endpoints request the connection, and the network assigns a token to identify it.
- All packets in the connection follow the same route.
- The virtual circuit is torn down when the connection is no longer needed.
What are the key features of virtual circuits?
- The network maintains knowledge of all active connections.
- Packet ordering and loss/duplication are managed but not always guaranteed.
- The user does not need to handle details, although checksums are useful to ensure data integrity.
What is a potential issue with virtual circuits if code crashes?
- If the code managing a virtual circuit crashes, the network may not receive a signal to properly shut down the connection.
- This can leave the virtual circuit active unnecessarily, consuming resources until the network cleans it up.
What is a datagram service?
A service where each packet contains complete addressing information and is treated as a separate item. The network routes packets without needing to know about connections.
What Ian annotation highlights how datagram services send data?
“Datagram services send data across wires and hopes.”
What are Netheads vs Bellheads?
People who mistrust the controllers / government vs people who are closely assigned with the government.
What is the Bellheads’ approach to traffic management?
Bellheads use virtual circuits to shape and groom traffic, adding value with complex protocols.
What is the problem with marking data as priority, as noted in the Ian annotation?
“Everyone would just mark all data as priority.”
What solutions and risks are suggested for preventing all data from being marked as priority?
Solutions: price or flow control.
Risks: government involvement, money scams, or user annoyance.
What problem does layering solve in networking?
Layering allows programs to operate over different types of networks without requiring radical changes.
What is layering?
Layering is the concept of dividing a network into a series of layers or “stacks,” where each layer provides services to the layer above and uses services from the layer below.
How does layering simplify networking for programs?
Layering allows network programs to operate over different types of networks without requiring radical changes, ensuring flexibility and abstraction.
What is an example where layering might not fully apply?
Video streaming might face challenges due to specific network requirements like bandwidth and latency.
What is a solution to enabling flexibility across different network types?
The solution is layering, which provides abstraction and separation of tasks.
What real-world tools rely on the concept of layering?
Python request package and Ethernet are examples that use layered network concepts to interact with different network services.
What flexibility does layering offer?
Layering allows the swapping of one layer with another implementation as long as it obeys the same interface, enabling modularity and flexibility.
How are specific tasks handled independently in a network system?
By using layers to provide a clean, defined interface between components.
How is data handled efficiently in layering systems?
Instead of copying and modifying large buffers, layers often share a large buffer pool with headers and footers added at each end.
What was the OSI model, and where did it originate?
The OSI (Open Systems Interconnect) model was a European alternative to TCP/IP in the USA, aimed at standardizing networking protocols.
Why did the OSI model fail to gain traction?
The OSI model came long before any successful implementations and was competing with existing proprietary systems and predicted standards like TCP/IP.
What impact did the OSI model have despite its failure?
The OSI model achieved traction as a conceptual model, showing how computer networks should or could work, even if practical implementations were limited.
What is the DoD model, and how does it compare to the OSI model?
The DoD model is a simplified version of the OSI model, and is used to compare TCP/IP architecture with OSI proposals. Unlike the OSI model, practice and implementation came first.
What is the unofficial IETF slogan, and what does it signify?
The IETF slogan is “Rough consensus and running code,” signifying that innovations prioritize practical, working implementations over theoretical models.
How does the DoD model differ from the OSI model in structure?
The DoD model has fewer layers:
1. Process (Application)
2. Host to Host (Transport, TCP/UDP)
3. Internet (IP)
4. Link (Ethernet, FDDI…) and Physical layers combined.
The OSI model has seven layers, including additional layers like Presentation and Session.
What is the key concept of the DoD model in networking?
The key concept of the DoD model is the end-to-end connection provided by TCP, where data flows between two end systems across a network. Each layer (Process, Host-to-Host, Internet, Link, and Physical) contributes to enabling this connection.
What is the role of the Internet (IP) layer in the DoD model?
The Internet (IP) layer is the only protocol layer responsible for routing packets between networks, ensuring they reach the correct destination.
What does the DoD model demonstrate about the role of routers in networking?
Routers operate at the Internet (IP) and Link (Ethernet) layers, facilitating the forwarding of data between networks while the end-to-end TCP connection is maintained between the two end systems.
How does the DoD model ensure end-to-end communication between systems?
The DoD model uses the TCP connection at the Host-to-Host layer to provide reliable end-to-end communication, while the Internet layer handles addressing and routing, and the Link layer handles data transmission.
Which layers are involved in the DoD model, and what are their functions?
The layers are:
- Process/Application: Handles applications and user data.
- Host-to-Host: Manages transport (TCP/UDP) for end-to-end communication.
- Internet: Handles IP addressing and routing.
- Network access: (link & physical) Manages Ethernet or other data link protocols, and actual hardware transmission of bits.
What does the Host-to-Host layer ensure in the DoD model?
The Host-to-Host layer ensures reliable transport of data between end systems, establishing, managing, and terminating TCP connections.
Why is the DoD model’s end-to-end connection important for communication?
The end-to-end connection ensures data integrity, reliable delivery, and ordered transmission of packets across a network, which is crucial for applications requiring accuracy and consistency.
What are the roles of TCP and UDP in the transport layer?
TCP provides reliable, sequenced delivery for streams of data, while UDP is used for sending individual packets without guaranteeing delivery or order.
How does the transport layer function in the DoD model?
The transport layer moves bytes between end systems and ensures communication, offering properties like reliability, sequencing, and delivery without needing to understand the underlying network topology.
Why is TCP preferred for streams of data and UDP for individual packets?
TCP is preferred for streams of data because it guarantees reliability and order of delivery. UDP is better for packets where speed and simplicity are prioritized over delivery guarantees.
What properties does the transport layer in the DoD model guarantee?
The transport layer ensures reliability, sequenced delivery, and communication between multiple entities, while being unconcerned with the content or meaning of the bytes.
What is the primary function of the Internet/Network layer in the DoD model?
The Internet/Network layer moves packets between end systems and decides which link to use to get data closer to its destination, without offering reliability guarantees.
Why does the Internet/Network layer not handle multiple applications?
The Internet/Network layer focuses only on packet movement, while separating data between multiple applications is the responsibility of the layer above.
What protocols are associated with the Internet/Network layer in the DoD model?
The protocols associated with this layer include IPv4 and IPv6.
What is the purpose of the Link layer in the DoD model?
The Link layer moves data between one network element and the next, handling tasks like packetisation, addressing, and protocol-specific functions.
Why is the Link layer still significant despite Ethernet’s dominance?
Although Ethernet is now dominant, the Link layer remains relevant because, historically, IP had to run alongside other protocols like ATM and coexist with them for decades.
What is the role of the Physical Layer in a network?
The Physical Layer handles the actual encoding of data to shift bits over a distance, using mediums such as co-axial cables, twisted pair, fibre optics, or radio.
What happens to packets as they pass through different layers of a network?
Each layer adds its own header (and sometimes a trailer) to the packet as it moves through the network layers.
What security measure can be added during packet encapsulation?
Padding can be added for security reasons when encapsulating packets.
What components are typically included in a fully encapsulated packet?
A fully encapsulated packet includes:
- Ethernet Header
- IP Header
- TCP or UDP Header
- Data
- TCP or UDP Trailer
- IP Trailer
- Ethernet Trailer
Why does padding add security during packet encapsulation?
Padding can add security by preventing attackers from deducing the size of the original data, which could otherwise reveal sensitive information. It also ensures packets meet minimum size requirements to avoid issues during transmission.
What is the main difference between the DoD and OSI models?
The main difference is that the OSI model splits the transport layer’s equivalent into three distinct layers: Presentation, Session, and Transport, while the DoD model combines all three into a single transport layer.
What is the role of the Presentation layer in the OSI model?
The Presentation layer handles data encoding, such as converting integers to a neutral format that can be universally understood.
What is the function of the Session layer in the OSI model?
The Session layer manages relationships between multiple connections, such as restarting a failed file transfer or ensuring connection continuity.
Do IPv4 and IPv6 do the same job?
Yes. They only differ in address length.
Why are the extra layers in the OSI model considered unnecessary according to the slide?
Experience and history suggest that the extra layers are unnecessary because applications require services that are too specific to provide as generic functions, making the additional layers redundant.
How did OSI’s focus on telcos contribute to its failure?
OSI was influenced by telcos trying to protect existing business models and capital plant, leading to over-engineering and a lack of practical implementations.
Why is abstraction considered hard in networking?
Abstraction is hard because higher layers closer to the application may require services that are a poor fit for the lower layers, such as writing complex transport services for datagrams or inefficient virtual circuit handling for single datagrams.
Why does TCP/IP offer a strong transport service compared to OSI?
TCP/IP stabilised over 40 years and integrates the transport service efficiently into one layer, unlike OSI, which splits transport functionality into Session, Presentation, and Transport layers.
What responsibility does the implementor have in making TCP/IP work?
The implementor must ensure TCP and UDP work fully over the lower layers they propose, as without TCP and UDP, an IP stack cannot function, and most applications would fail.
What is the main advantage of TCP/IP that makes it widely adopted?
The main advantage is that there is exactly one network layer, IP.
TCP/IP offers one simple transport service for connections (TCP), and one for datagrams (UDP), making it easy to implement and ensuring compatibility with most applications.
What was a key reason OSI failed to interwork properly?
OSI had two different network layers: CONS for connection-oriented (virtual circuit) services and CLNS for connectionless (datagram) services, which caused incompatibility between infrastructures.
Why did OSI try to be “efficient” but fail in its approach?
OSI tried to provide multiple transport services ranging from thin layers for virtual circuits to complex ones for datagrams, leading to unnecessary complexity.
What are the two ways equipment can be layered in a network?
Equipment can be layered in two ways:
1. Different pieces of equipment perform different jobs in the stack.
2. Different layers can exist within the same piece of equipment for easier design, construction, and security.
What layers do “hosts” understand in network hardware?
Hosts understand the transport layer and application layer, acting as general-purpose computers.
What function does a “router” perform in network hardware?
A router operates at the IP Layer and moves packets between different links, such as Ethernet and long-haul serial links.
Why did none of OSI’s transport services interwork successfully?
In practice, OSI implementations only used a subset of the alternatives, particularly in transport layers, which led to failures in interworking and the need for “transport relays.”
What role does a “switch” play in network hardware?
A switch operates at the Link Layer and switches Ethernet packets between links. It connects devices within a local network.
What is the primary difference between switches and routers?
Switches operate at the Link Layer, handling Ethernet frames, while routers operate at the Internet (IP) Layer, moving IP packets between networks.
What has caused the blurring of roles between switches and routers?
Switches are becoming “L3 aware” by making switching decisions based on IP headers, and modern routers often integrate switches into them.
What are hosts capable of in modern networking?
Hosts can understand all layers of the networking model, making them capable of acting as routers.
What are the three planes within networking equipment?
The three planes are:
1. Management plane - Handles GUI/CLI interfaces, configuration, and reporting.
2. Control plane - Makes decisions about routing policies, usually running in software.
3. Data plane - Ships the actual traffic, often using specialised hardware or software.
What is the function of the management plane?
The management plane is responsible for running the GUI/CLI, error reporting, and device configuration, typically implemented in software like Linux.
What role does the control plane play in networking equipment?
The control plane makes routing decisions and manages policies. It usually runs in software, often in real-time.
What is the purpose of the data plane?
The data plane handles the actual movement of traffic through the network, often using special-purpose hardware or network processors.
What are “planes” in networking, and are they physical components?
Planes in networking refer to conceptual divisions of tasks within a device.
What is the “stack of protocols” in networking, coloquially?
The stack of protocols includes:
- Applications that do real work.
- Transport that carries bytes between applications.
- Network that carries packets between transport endpoints.
- Link that carries packets over single cables.
- Physical, which consists of the cables.
What roles do hosts, routers, and switches play in networking?
- Hosts handle applications and transport layers.
- Routers handle the network layer.
- Switches handle the link layer.
What are the three components within equipment in networking?
The three components are:
- Management: Configures and monitors equipment.
- Control: Sets up switching and routing decisions.
- Data: Carries packets (bytes) between interfaces.
What are LANs, MANs, and WANs in networking and how much area do they cover geographically?
- LAN (Local Area Network): Covers small areas like buildings or campuses.
- MAN (Metropolitan Area Network): Covers city-level networks.
- WAN (Wide Area Network): Covers large geographical areas.
What are PANs, and how are they used today?
PANs (Personal Area Networks) are small-scale networks, such as those using Bluetooth for personal devices.
Why are LAN and WAN technologies said to be rapidly converging?
Because very few pure LAN or pure WAN technologies remain today due to advancements in networking.
What limited the development of WANs in Europe historically?
Telco monopolies and the need for government permission limited WAN development in Europe.
What made the requirements for LANs historically simpler than WANs?
LANs were simpler because they focused on smaller, localized networks with fewer technical and regulatory restrictions.
What are the characteristics of a LAN, as described in the slide?
A LAN can be set up with minimal planning and works by broadcasting data to connected stations. Addresses are unstructured and simply need to be distinct.
What is an example of a WAN requirement that differs from a LAN?
A WAN requires planning, structured addresses, and protocols to locate devices far outside the range of a broadcast.
Why were early LANs not considered true networks as we think of them today?
Early LANs mostly connected terminals to timeshare computers, transmitted simple data like keystrokes, and operated at slow speeds (e.g., 9600 baud about 9600 bits per second).
What technologies were used in early LANs, and what were their limitations?
Early LANs used technologies like serial lines (“RS232”). They had very limited data transmission, focusing only on basic tasks such as screen updates.
What job did early LANs perform in addition to connecting terminals?
Early LANs were also used for tasks that WANs performed but achieved them faster.
