Systems and Layers

Question 1

What is system design?

Answer 1

Computer systems have computation, storage and transmission resources. System design is putting these together into a harmonious whole.

Question 2

What is the goal of system design?

Answer 2

Maximizing a set of performance metrics given a set of resource constraints.

Question 3

Time constraints

Answer 3

Deadline for task completion, time to market and mean time between failures.

Question 4

Response time

Answer 4

Mean time to complete a task

Question 5

Throughput

Answer 5

Number of tasks completed per unit time

Question 6

Degree of Parallelism

Answer 6

Response time * throughput

Question 7

Space Constraints

Answer 7

Limit to available memory and limits to bandwidth.

Question 8

Computation Constraints

Answer 8

Amount of processing that can be done in unit time.

Question 9

Money Constraints

Answer 9

What components can be used, what prices users are willing to pay for a service and the number of engineers available to complete a task.

Question 10

Labour Constraints

Answer 10

Labour is the human efffor required to design and build a system nd constrains what can be done and how fast.

Question 11

Social Constraints - Standards

Answer 11

Forces designs to conform to sometimes non-sensical requirements. Underspecified standard can lead to faulty and non-interopable implementations.

Question 12

Social Constraints - Market Requirements

Answer 12

Backwards compatibility and the need to use a particular operating system.

Question 13

Scaling

Answer 13

A design constraint - centralised elements prevent the design from easily scaling

Question 14

Bottleneck

Answer 14

The most constrained element in the system.

Question 15

Balanced system

Answer 15

A system in which all resources are simultaneously bottlenecked.

Question 16

Multiplexing

Answer 16

Essentially sharing, trades time and space for money as user sees an increased response time and takes up space while waiting but the system costs less.

Question 17

Statistical Multiplexing

Answer 17

We can use statistical knowledge of how frequently users are active and for how long to minimise cost of link. Temporal - expect a task to be active only part of the time. Spatial - we expect only a fraction of tasks to be simultaneously active.

Question 18

Pipelining

Answer 18

Break tasks down into serially dependent subtasks so that each subtask only depends on previous one in execution chain. This allows stages of the process to be executed simultaneously.

Question 19

Batching

Answer 19

Groups tasks together to amortize overhead. This only works when overhead is nonlinear. Trading off reduced overhead for longer worst case response time and increased throughput.

Question 20

Hierachy

Answer 20

Recursive decomposition of a system into smaller pieces that depend only on parent for proper execution.

Question 21

Binding

Answer 21

Translation from an abstraction to an instance.

Question 22

Indirection

Answer 22

Automatic binding where translation table is stored in a well known place.

Question 23

Virtualization

Answer 23

Combination of indirection and multiplexing by referring to a virtual resource that hets matched to an instance at run time.

Question 24

Soft State

Answer 24

State that is deleted on a timer. If it is desierd to be kept then refresh. This allows for automatic clean up after failure but increases bandwidth requirements.

Question 25

Hysteresis

Answer 25

System may change state depending on whether a variable is above or below a threshold. If variable fluctuates near threshold this can lead to rapid fluctuations in system state. This can be fixed by using a state-dependent threshold or hysteresis.

Question 26

Separating Data and Control

Answer 26

Divide actions that happen once per dta transfer from actions that happen once per packet. This can increase throughput by minimising actions in data path but keeping control info in data element allows for per-packet QoS.

Question 27

Steps in Performance analysing and tuning

Answer 27

Measure, characterise workload, build a system model, analyse and implement.

Question 28

Protocol

Answer 28

A set of rules and formats that govern the communication between communicating peers including the set of valid messages and the meaning of each message.

Question 29

What does a protocol tell us?

Answer 29

The syntax of a message - what fields, what format.

Semantics - what does a message mean

Actions to take on receipt of a message.

Question 30

Protocol Layering

Answer 30

A protocol using another as a step in its execution e.g. packet transfer is one step in execution of a reliable file transfer protocol.

Question 31

Service Access Point

Answer 31

Interface between an upper layer and a lower layer

Question 32

Protocol Data Units

Answer 32

Packets exchanged between peer entities.

Question 33

Service Data Units

Answer 33

Packets handed to a layer by an upper layer.

Question 34

Protocol Stack

Answer 34

A set of protocol layers with each layer using the layer belwo and providing a service to the layer above. Allows for information hiding and change decoupiling.

Question 35

Importance of being layered

Answer 35

Breaks up a comlex problem into smaller manageable pieces. Allows for abstraction of implementation details allowing for the sepearation of implementation and specification. It also allows functionality to be reused.

Question 36

Problems with layering

Answer 36

Hidden information can be used to improve performance but layering hides information.

Question 37

Open Protocol

Answer 37

Details are publicly available and cahnges are managed by an organisation whose membership and transactions are open to the public.

Question 38

ISO OSI - Outline

Answer 38

Open System Interconnect

Reference Model - Architecture

It formally defines what is meant by a layer, a service etc.

Service architecture - Design

Describes the services provided by each layer and the service access point.

Protocol architecture - Implementation

Set of protocols that implement the service architecture.

Question 39

ISO OSI - Seven Layers

Answer 39

Physical - Coding Scheme, shapes and sizes of connectors, bit-level synchronization

Datalink layer - Defines frames (sets of bits that belong together), Idle tells us link not carrying a fram, Begin and End delimit a frame. In broadcast links also need MAC to ensure end-system only receives bits meant for it.

Network Layer - Logically concatenates a set of links to form the abstraction of an end-to-end link.

Transport Layer - Creates abstraction of error-controlled, flow-controlled and multiplexed end-to-end link.

Session-Layer - Provides full-duplex service, expedited data delivery (allows some messages to skip ahead in end system queues), synchronization allows users to place marks and roll back.

Presentation Layer - Hides data representation differences between applications. It can also encrypt data.

Application Layer - The set of applications that use the network.

Question 40

Why seven layers?

Answer 40

Need a top and bottom so 2. Need physical link so 3. Need end-to-end and hop-to-hop so need at least network and transport so 5 layers at least needed but 7 is excessive.