5 - Sync and Clocks Flashcards

1
Q

Temporal Ordering

A

Exact time not needed but order is important

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2
Q

UTC

A

Universal Coordinated Time

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3
Q

International Atomic TIme (TAI) is based on

A

electron transition frequency of calcium 133

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4
Q

UTC = TAI - ????

A

leap seconds

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5
Q

Precision

A

Keep deviation within specified bound

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6
Q

Accuracy

A

Keep deviation from actual time within specified bound

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7
Q

Internal Sync

A

Sync clocks to improve precision but not neccessarily accuracy

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8
Q

Berkeley Algorithm

A

Time server sends time to all,
difference is returned,
average is calculated,
average - each computers difference is sent back to each computer

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9
Q

Negative Time adjustments are bad. Why?

A

Timestamps are not monotonically increasing and events on the same computer can have smaller timestamps than earlier events

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10
Q

Lamport Clock

A

Each process maintains and adjusts local counter.
Weak consistency

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11
Q

Lamport Clock: Event is within Pi

A

Ci is incremented by 1.

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12
Q

Lamport Clock: Message is sent by Pi

A

timestamp ts(m)=Ci is attached.

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13
Q

Lamport Clock: Message is sent by Pj

A

local counter Cj is set to max{Cj,ts(m)}; then Ci +1 before passing m to app

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14
Q

Weak Consistency Guarantee

A

a -> b => ts(a) < ts(b)

if a happened before b, always smaller timestamp but if a timestamp is smaller, not necessarily before b

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15
Q

Strong Consistency Guarantee

A

a->b <=> ts(a) < ts(b)

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16
Q

Vector Clock consistency

A

Strong

17
Q

GNSS

A

Global Navigation Satellite Systems

GPS etc

18
Q

GPS: What makes it work?

A

32 satellites with atomic clocks constantly send position and time.
Determine distance to several and compute intersection

19
Q

GPS: Computing distance

A

Multiply signal speed and transmission time

Transmission time = reception time - sending time

20
Q

Positioning: How many spheres/circles in 2D/3D?

A

3 circles in 2D
4 spheres in 3D

21
Q

Clock Deviation Problem

A

Internal Delay
Clocks not synced

22
Q

RBS: Definition

A

Internal sync in wireless networks where components are directly reachable

23
Q

RBS

A

Reference Broadcast Sync

24
Q

Showing that vector clocks fulfil strong consistency

A

Assume relation < from lecture. Consider ALL cases:
- Weak consistency follows from process clock increasing with every event and that the timestamp of reception is larger than sending. < is transitive.
- If B -> A then ts(B)<ts(A) and thus ts(A) /< ts(B). INCOMPLETE

25
Q

Where could vector clocks be useful?

A

Concurrent Events and dependencies between events.

Eg a faulty computation A, then B needs to be repaired too, but if A /-> B then B doesn’t need repair

26
Q

Total order multicast

A

Requires that messages be delivered in the same order as they would be if comms were instant.

27
Q
A