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+# Topic 1 Time order
+
+## Distributed system
+
+N nodes working together on a overall task.
+Interacting via communication network.
+
+Challenges include.
+
+- Bad communication channel
+- Global state is hard
+- Local clocks may not be in sync
+- Ordering of subtasks
+- Handling faults or malicous behavior
+
+Ways to implement ordering.
+
+- Clock synchronisation
+- Use of logical clocks
+- Algorithms that do not rely on order of events
+
+## Time and clocks
+
+Monotonicity
+:   Når et ur ikke kan gå tilbage i tiden. Altså hvis `t1 > t2`, så er t1 senere end t2.
+
+Local clocks have drift.
+To make them usefull, they must have bounded precision and accuracy.
+
+## Notions of time
+
+- UTC
+- Internal clocks
+    Often implemented with a counter function, which counts at a given frequency.
+- "Time-stamping"
+    Event ordering
+
+## Clock synchronization
+
+Uses measured round trip time to calculate the delay of the received time.
+
+Clients can either pull a server, or the other way around.
+
+### NTP
+
+Clients can pull times from another server.
+Builds a tree of servers.
+
+Represents time using number of seconds since 1900-01-01.
+Timestamps all messages, and timestamps of recent events.
+
+Accuracy is ~1ms for LAN and ~tens of ms for Internet.
+
+## Ordering
+
+Total order is if for all times `a` and `b`: `a > b or b > a`.
+So all times can be compared.
+
+Transisive relation
+:   If `a, b` can `b, c` can be compared then `a, c` can too.
+
+Irreflexive
+:   That `a, b` cannot be compared.
+
+If precise physical clocks are posible, total order can be achieved.
+
+Partial order implies that concurrent events can occur.
+
+## Partial order
+
+Important to be able to say a happened before b, as a could affect the result of b.
+
+Consistent clock
+:   If a happened after b then `C(a) < C(b)`. Where C translates event to time. *Strictly consistent* if the reverse is also true.
+
+Local logical clocks can do this internally in precesses and between processes.
+
+Causal order
+:   Her kigger man på om et event A kan have forårsaget B.
+
+## Scalar logical clocks
+
+Have a counter which is incremented before each event.
+Include this counter in each message sent.
+
+When a recipeint receives a message set its counter to either its own or the senders depending on which is largest.
+
+Scaler clocks are not strictly consistent, but can be used to define total order.
+
+### Vector based
+
+Each node keeps a vector of the logical clocks of all other nodes.
+
+# Topic 2 Algorithms and Consistency
+
+## Mutex locking
+
+Different solutions.
+
+Simple: A node can request a lock by sending a request, and locking nodes can object.
+Requires:
+
+- FIFO: message order between processes is preserved
+- Every messages is received
+- Each process can communicate with all other
+
+## Leader election
+
+Contralized control is useful in some algorithms.
+If done distributedly all nodes must be able to take this role.
+
+Requires a algorithm to elect leader.
+Can be done with mutex locking, but better methods exist.
+
+Nodes broadcast their role id(random) at a random time.
+If a node hasn't received at higher id, it broadcasts its own.
+
+## Transaction and consistency
+
+Transaction properties:
+
+- Atomicity (result is either commit og abort, not a half failure state)
+- Durability (persistent results)
+- Isolation (current transactions do not influence other transactions)
+- Consistency (transactions go from a consistent state to another)
+
+Global state cut
+:   Taking a snapshot of consistent state, not including transactions.
+
+Different cuts/snapshots.
+
+- Strongly consistent: No messages in transit
+- Consistent cut: Messages in transit not included
+- Inconsistent cut: Messages in transit are included
+
+## Computational model
+
+Includes read and write, FIFO queues, stacks etc.
+
+Can be implemented as:
+
+- Central storage
+- Master storage and copies
+- Fully symmetric distributed storage
+
+Consistency criteria
+:   correctness of result of distributed sequence of operations.
+
+### Linearizability
+
+A sequence of events are linearizable consistent if:
+
+- If a sequential order can give same result
+- This sequential order is consistent with the real order of events
+
+Requires global time, and limits concurrentcy.
+
+### Serializability
+
+- If the sequential order is consistent with the real order of operations on each node.
+
+# Topic 3 OSI
+
+Layered protocol to ease implementation of layers and adding new layers.
+As layers only talks to the layers above and under.
+
+1. Physical layer:
+    Defines physical things like voltage, frequency etc.
+    Different services which next layer relies on.
+2. Data link layer:
+    Breaks data from physical layer into packets, and handling error at the physical link.
+    Includes ethernet.
+3. Network layer:
+    Handles routing of packets across the network.
+    Where things go from single hop to many hops.
+    Implements the Internet with IP addresses.
+4. Transport layer:
+    Handles end to end connections, where ports live.
+    Where connections are implemented, such as in TCP.
+5. Session layer:
+    Handles session :-).
+    Things like *authorization* and *authentication*.
+6. Presentation layer:
+    Formatting of data, such as MIME, ASCII or PGP.
+7. Application layer:
+    Applications such as web browsers, ssh or email.
+
+## Connection oriented
+
+Simple communication can just send a packet with a destination on it.
+If the packet is lost it is just lost.
+
+A connection can be setup, packets can be sent and it cna be torn down.
+This allows for re-transmission and re-ordering of packets.
+
+# Topic 5 Mac and link layer
+
+Started with telephone switching where two are connected using a continues connection.
+When computers came packet switching was created for internet packets.
+
+These where combined when packet switching became fast enough.
+
+## Link layer
+
+Often comes in short burst
+
+Use *error correction* when retransmission or feedback channel is not available.
+
+### ARQ
+
+ARQ (Automatic Repeat Request) handles errors and retransmission.
+
+Uses different retransmission strategies:
+
+- Stop and wait: 
+    Vent på ACK før den næste packet skal sendes.
+- Go back N: 
+    Sender will keep a window of sent windows and will keep track of which sequence number has been received.
+    It will go back if missing.
+- Selective repeat/selective reject: 
+    Receiver selectively says which frames are missing.
+
+#### Hybrid ARQ
+
+Combination of error control schemes.
+
+Type-1
+:   Error detection and correction.
+
+Type-2
+:   On a good channel use pure ARQ, if not add more parity bits.
+
+## MAC
+
+Multiaccess media instead of point to point.
+Requires Media Access Control sublayer.
+
+Different way to have multiple devices talk on one line.
+
+### Static allocation
+
+Devide channel in portions and allocate these to devices.
+This can be done in different ways:
+
+- SDMA (space devision):
+    Segment using space, as with different directed antennas or wires.
+- TDMA (time devision):
+    Have devices transmission time slots.
+- FDMA (Frequency devision):
+
+Static allocation can be good with a small number of devices.
+But not to good if many devices or bursty traffic.
+
+#### Frequency hopping
+
+Combination of FDMA and TDMA.
+Can be done with fast hopping (for every symbol) or slow hopping (for every message).
+
+Can withstand interference, as multiple frequencies are used.
+
+Synchronization and discovery can be a challenge.
+
+This can be done by first sending a seed and offset.
+And then a new clock reference is sent with every packet.
+
+This requires that slaves can't got to sleep for too long.
+
+### Dynamic schemes
+
+#### Random schemes
+
+Direct transmission without contention phase, so collision are very possible.
+Suitable for short messages, which can afford to be retransmitted.
+
+##### Aloha
+
+Transmit immediadly, incase of collision retransmit after random time.
+
+Special case of stop and wait.
+
+**Slotted aloha** created slots where transmission is allowed.
+Not as many collisions, as only start can go wrong.
+
+##### CSMA
+
+Listen before talk, as listening is not super demanding.
+Does not relieve of collisions, because sending takes time.
+
+1-persistent
+:   Send if the channel is idle.
+
+Non-persistent
+:   Send if idle, if idle send. If not idle then wait for random period of time.
+
+Can deploy collision detection to stop the transmission if a collision happens.
+
+*Consensus reenforcement* what?
+
+#### Slotted systems
+
+Assign allowed transmission times to slots.
+
+Can use base station as reference, but distributed is much harder.
+
+#### Reservation schemes
+
+Devices signal they want to sent in a contention phase.
+The winning device will get to send a message collision free.
+
+Nice for long messages.
+
+#### Collision detection in radio
+
+Harder as with wire, as the signal is weaker throughout the range.
+Often not implemented in wireless systems.
+
+ACK is often required.
+
+#### Wireless problems
+
+*Hidden terminal* is when a collision happend between two temrinals which can't see each other.
+
+*Exposed terminal* is when a station can't transmit because the channel looks busy, but really transmission is possible.
+
+#### MACA
+
+Uses signalling packets for collision avoidance.
+Terminal sends RTS (Request) and receiver can answer with CTS if the channel is ready.
+
+Avoid hidden terminal, but not exposed terminal.
+As requests will collide at B.
+
+# Topic 6 WLAN
+
+Architecture in networks:
+
+- Station (STA):
+    Terminal with radio which communicates with access point.
+- Basic Service Set (BSS):
+    Group of stations using the access point
+- Access point:
+- Portal:
+    Bridge to other wires network
+- Distribution system:
+    Interconnected network to form one logical network.
+
+*Adhoc* network are only between stations directly.
+Important word is IBSS (Independend Basic Service Set), which is a group of stations using the same frequency.
+
+*Direct communication* is when a station functions as a access point.
+
+TODO der står en del om PCF i slides.
+
+## MAC
+
+Two different schemes:
+
+- Distributed Coordination Function (DCF):
+    Implementation of adhoc networks and all users contend to accessing the channel.
+- Point Coordination Function (PCF):
+    Is based on polling, performed by a AP.
+
+These want to achieve two *traffic services*.
+
+- Asynchronous Data Service (mandatory):
+    - Support for multicast and broadcast.
+    - Priority networks.
+- Time bounded service (optional)
+    - Implemented using PCF.
+
+## Carrier sensing
+
+Can be done in two different ways.
+
+- Physical carrier sensing:
+    Detects activity on the channel via signal strength.
+- Virtual carrier sensing:
+    Detects from received header information, where packets say how long they utilize the channel.
+
+The channel is marked busy if one of them indicate a busy channel.
+
+## NAV
+
+Network Allocation Vector is used by stations to predict the duration of the current transmission.
+
+## Performance
+
+Theoretical throughput calculates throughput deterministic analysis.
+Does not take collisions and channel errors into consideration.
+
+Multihopping in networks can give problems as it blocks many connections.
+
+TCP also gives poorer performance as it quickly breaks.
+
+A good solution would be multi-radio per station so forwarding can be done on another channel.
+
+## Transmission range
+
+Limited by 3 different things.
+
+- Transmission range:
+    Limited by the transmission power and radio propagation.
+- Physical carrier sensing range:
+    Is the range where another station.
+    This is larget than the transmission range.
+- Interference range:
+    Range which the transmission will cause interferenence on a receiver.
+
+## Access methods
+
+- DFWMAC-DCF CSMA/CA (mandatory):
+    - Collision avoidance with back-off mechanism.
+    - Using ACK for non broadcast packages.
+- DFWMAC-DCF w/ RTS/CTS (optional)
+- DFWMAC-PCF (optional):
+    Access point polls terminals according to a list.
+
+    Pull data in a superframe, which is started with a beacon from AP.
+
+# Topic 7 Bluetooth
+
+Bluetooth uses frequency hopping.
+
+
+Alternates RX and TX meaning master always transmit on odd time slots, and a slave transmit on even time slots.
+A slave can therefore not initilize a conversation itself.
+
+## Discovery
+
+Complex in bluetooth classic and takes some time.
+
+Bluetooth low energy has 3 advertisement channels where devices discover each other.
+
+## Different link types
+
+- Synchronous Connection Oriented for voice:
+    - Circuit switched with preallocated bandwidth.
+    - No retransmission
+    - Symmetric
+- Asynchronous Connection Less for data:
+    - Variable time slots
+    - Uses ARQ protocol
+    - Symmetric
+- eSCO for streaming:
+    - Retransmissions immediately after received slot.
+    - Symmetric or asymmetric
+
+Symmetric means that RX packets are the same type as TX.
+
+## Errors
+
+Can use different error correction schemes:
+
+- 1/3 FEC where each bit is 3 times.
+    Uses majority voting.
+- 2/3 FEC uses math to encode 10 bit code to 15 bit code. So 1 parity per 2 bits.
+
+But can also use ARQ schemes.
+
+## Energy states
+
+Active
+:   Channel is active and slave and master are kept in sync.
+
+Sniff
+:   Slave only listens for synchronization beacons at specific times, this entering low power mode.
+
+Hold
+:   Can be used if other actions must be performed such as participating in other networks.
+:   Here it will only listen for specific messages.
+
+Park
+:   Master parks slaves to support many devices is piconets.
+
+## Interference
+
+Bluetooth utilizes AFH (Adaptive frequency hopping) where is tries to avoid frequencies where many collision happen.
+
+# Topic 8 CAN
+
+Why use a fieldbus instead of dedicated wires:
+
+- Cost savings
+- Reduction of weight
+- Reliability
+- Easier fault diagnosis
+- Redundancy
+
+Fieldbusses often implement multiple OSI layers, and the layers 3 to 6 are non existent.
+
+## CAN
+
+CAN is good for noisy environments.
+
+CAN uses an single set of wires where everything is broadcast transmissions.
+
+Synchronization is required and resyncrhonization is done by looking at edges of signal.
+Because of this, long silent sequences should be avoided.
+This is done with bit shuffling of bits.
+
+Requires the idea of *dominant bits* which are explained in the question section together with addressing and priority.
+
+### Messages types
+
+- Data frame: a frame containing node data
+- Remote frame: a request for a specific ID
+- Error frame: transmitted by a node on error
+- Overload frame: used to create a delay if more time is needed
+
+# Spørgsmål
+
+If we answer a question nicely and quickly, we just get another question.
+We should not work hard to scretch time.
+
+## Time, synchronization and order
+
+1. **Explain the need for clock synchronisation by giving an example scenario. Provide an example synchronisation approach for 2 nodes and discuss its advantages/disadvantages**
+
+**Ide 1:**
+
+A distributed chat system requires must be able to preserve order of messages and their received times.
+Conversations often have questions which other can answer, which can be confusing if the order is skrewed up.
+
+It is also important for users to see when messages where sent.
+
+1. Alice sends a message to Bob.
+2. Bob will respond with a ACK.
+3. Alice sends the measured round-time to Bob.
+4. Bob sends the measured round-time to Alice.
+5. Alice and Bob independedly calculate the average delay between the two round times.
+6. The one behind sets it's clock to the most forward one.
+
+The has the advantage that it works without a central time server, however comes with many problems.
+
+- Alice and Bobs clocks will quickly come out of sync with the rest of world, making the timestamps useless for users.
+- This requires alot of extra traffic
+
+**Ide 2:**
+
+Many websites often have authenticated user interaction, where users log in to a session.
+This is often implemented with a server recording sessions and their expire time.
+
+If this is to be done distributed between multiple server, all servers must have 
+
+2. **Explain the concept of causal partial order using an example event diagram.**
+
+Svaret i notesbog.
+
+3. **Explain the motivation and realization of a logical clock using an example.**
+
+Multiple users add and remove text to a distributed document sharing program.
+
+When a users adds text it sends only the addition and where it happened to other clients.
+It is therefore important which order changes arive in as they depend of the previous state of the document.
+
+However it does not matter which time edits happen on so a real clock is not needed.
+Therefore a logical clock can be used to insure additions are applied in the right order.
+
+Shown in notebook.
+
+4. **Explain vector clocks and scalar logical clocks using an example and discuss their differences**
+
+Scalar clocks are implemented as a simple counter, counting up for each action.
+On receiving a message which a clock value a client takes the biggest.
+
+If two events have et same scalar clock value, its not possible to determine their order.
+Scalar clocks have eventual consistent time.
+
+Vector clocks have a counter for each node, which gives more insight into the order of messages.
+This has the advantage that one can see if one event caused another as it is causaly consistent.
+
+Se sammenligning notesbog.
+
+## Distributed exclusion, consistency
+
+5. **Explain the'distributed mutual exclusion' problem and compare it to a centralized approach.**
+
+When sharing resources like memory or devices between multiple nodes, it is often unwanted to have multiple nodes access it.
+For example if multiple nodes want to add 3 to a shared variable.
+
+Adding often consist of multiple operations:
+
+1. Retrieve
+2. Add locally
+3. Store
+
+This is an issue with multiple nodes wanting to add 3.
+Mutual exclusion is therefore needed, where a node locks the resource while operating on it.
+
+A centralized method is by having a locking variable which can be read and set atomicly, facilitated by an operating system.
+However this is not possible in a distributed system, as a centralized lock server is unwanted.
+
+Instead notes broadcast a lock request, which all other nodes must grant.
+A node can then only get the lock if all other nodes permit it.
+
+This comes at a much larger performance cost as the centralized approach.
+
+TODO here a contralized approach may refer to in distributed systems.
+
+6. **Give an example of a distributed read and write operation sequence and explain two different consistency criteria.**
+
+The example can be seen in notebook.
+
+Linearizability, order of events must be preversed on all nodes.
+So the example is not linearizable as no valid order can be found.
+
+`r_1` must read a `d` before `r_2` can an `a`, meaning `w_1` must happen after `r_1` which is not allowed.
+
+With sequantial order, time order at each node must be preserved (`w_1` must happen before `r_2`).
+So a valid order is.
+
+`w_1(a), r_2(a), w_2(d), r_1(d), w_3(e), r_3(e)`.
+
+## OSI model
+
+7. **What are the advantages of a layered architecture? Explain functionalities of different layers of OSI model.**
+
+Se det der er skrevet oppe i noterne.
+
+## Techniques used at Data Link Layer: ARQ and MAC concepts
+
+8. **Explain the three types of ARQ protocols.**
+
+Stop and wait is when the next packet can't be sent without a ACK from the previus.
+
+Go back N will implement a window of packets available to be sent, and will keep track of the latest ACKED packet.
+
+This allows for the receiver to sent multiple packets thus utilizing bandwidth.
+
+In selective repeat the sender will send many packets and will check which are received. When it will only retransmit the ones that where not ACKED.
+
+9. **Explain the difference between error correction and error detection. Give examples when it is preferable to use one or another, or a combination of them.**
+
+Error correction is when parity data is used to correct errors.
+This requires alot of extra parity data, used to reconstruct the original signal.
+
+Error detection is when terminals check for error but cant fix them.
+This is often done with CRC, and then not sending an ACK if a invalid packet was sent.
+
+Error detection can be done with much smaller overhead, but requires to a channel to send NACK or ACK.
+
+If this is not available error detection is a bit useless as a retransmission cant be signalled.
+
+Therefore error correction is better.
+Error correction is also better for channels with high latency as retransmission are expensive.
+
+10. **Give examples of MAC protocols with static channel allocation. Discuss their advantages anddisadvantages.**
+
+TODO
+
+Focus on FDMA and TDMA.
+Obvius advantage is that every thing is nice static and periodic.
+
+If dynamic requires a central entity which makes allocations.
+
+11. **Explain the principle behind a Random Access class of MAC protocols. Give an example of a such a protocol.**
+TODO 
+12. **Explain main features of Carrier Sense Multiple Access (CSMA) protocol, including the differencebetween non-persistent and 1-persistent versions.**
+TODO
+
+## WLAN IEEE 802.11 standard
+
+13. **Explain the Random backoff time mechanism used in the standard. Explain how prioritizationof different messages is realized.**
+
+If the medium is busy multiple devices can wait to send efter.
+A random backoff period is introduced to break this summetry.
+
+This is also called a *contention window* where devices fight to be able to send.
+Each devices chooses a random count number and counts down.
+The first one to reach 0 can start sending.
+
+Priority is implemented with the delay from end busy time to backoff window.
+Devices with a smaller delay will have an advantage in the contention window.
+
+The delays are as follows in order of smallest first:
+
+- SIFS (Short Inter Frame Spacing): for high priority such as ACK, CTS or polling responses.
+- PIFS (PCF IFS): For timebounded services implemented in PCF.
+- DIFS: Lowest priority, for asynchronous data.
+
+14. **Explainthe “handshaking” mode of operation (with RTS/CTS messages) and basic mode (without RTS/ CTS) and the associated trade-offs.**
+
+Handshake done before sending between A and B.
+
+A sends a request (RTS) to send to B, and B will respond (CTS) as soon as it is ready to receive.
+
+This enables other stations close to A or B to see that something is going to happen.
+This also solves the hidden terminal problem.
+
+However comes at a bandwidth reduction, and does not work well with multicast and broadcast.
+
+Good to use with large frames or when collisions are likely.
+
+## Bluetooth
+
+15. **Explain the principle of frequency hopping as well as its advantages and challenges in terms of channel sharing and coordination.**
+
+Frequency hopping is the combination of TDMA and FDMA.
+It has the timeslots of TDMA, but with each new time slot a new frequency is chosen.
+
+These frequencies are chosen a random meaning different networks can communicate in the same frequency range if they use a different random seed or offset.
+
+Frequency hopping has the advantage of being very resilient to interference, and the timeslots give it predictable behavior.
+
+However initial configuration is challenging as all nodes must agree on a clock offset and seed.
+Also clocks must be kept up to date as drifting can cause nodes to get out of sync.
+
+16. **Explain what is a piconet and how data is exchanged between a master and slaves (polling; time synchronization; slot structure; packet types).**
+
+Piconet is a small bluetooth network which allows multiple slaves to connect to the same master device.
+Each piconet may contain up to 7 simultaneous nodes, and many more parked slaves.
+
+The master controls all communication in the piconet, so no slave-to-slave communication is possible.
+Communication is done using polling where the master will poll a slave with a null packet and it can answer with data.
+
+Polling happens in time TDD (time division duplex) with 625 us size slots.
+Time slots are in the range 0 to 227-1, so each node does the same frequency hopping.
+
+Master sends at odd time slots and slaves at even, and because all traffic is controlled by master contention between nodes is avoided.
+
+Packet types:
+
+- ID packet
+- NULL, so only contains access code+header
+- POLL: used by master to force a response from a slave
+- FHS: Used for synchronization to exchange clock and ID information
+- DM1: Can carry payload
+
+## Field busses - CAN bus
+
+17. **Explain the role of the frame ID in the priority arbitration scheme used in CAN. Explain what is a dominant and a recessive bit. Show an example of bit sequencesof two frames with different IDs that are transmitted at the same time.**
+
+Many devices can share a single CAN bus, and priority is therefore beneficial.
+
+In a CAN bus each message is sent with a frame ID, which also functions as a priority.
+
+For example all messages belonging to brakes can have a low ID=2 (low is higher priority) while speedometer can have a higher ID=20.
+
+Other nodes can then listen in on a specific message ID they are interested.
+
+This is implemented with the idea of a *dominant bit*, where a line has a default state (recessive) until someone actively does something.
+So if one devices writes OFF and another writes ON the dominant bit decides what is read on the line.
+
+When a nodes wants to transmit a frame it will start by transmitting the frame ID.
+Other devices can then chime in with their own ID.
+If a node measures that its write dominated by another node it will stop.
+Nodes with lower ID will therefore win over ones with higher ID.
+
+```
+FRAME 23: 00000010111
+FRAME 3 : 00000000011
+```
+
+Here frame id=3 will win.
+
+## Network layer and routing
+
+18. **You regulate a flow of packets using a token bucket, feeding into a leaky bucket.**
+    Assume that the token bucket has a rate of 5 packets per second, and a capacity of 60 tokens.
+    The leaky bucket has a rate of 20 packets per second. Assume that the token bucket isempty.
+    200 packets has arrived. How long will it take before all packets have left?
+19. **Explain flooding and broadcast storm problem in ad hoc networks.**
+20. **Explain the difference between proactive and reactive routing approaches.**
+21. **Explain the main principles of Dynamic Source Routing protocol.**
+
+## Transport layer
+
+22. **Explain how TCP achieves reliable in-order delivery of TCP segments. What is the role of a receiver buffer and how is receiver buffer overflow prevented?**
+23. **Explain the congestion control mechanismof TCP (slow-start phase and congestion avoidance).**
+24. **What are the possible problems of executing TCP over a wireless technology? How can these be mitigated?**
+
+## Introduction to Fault Tolerance
+
+25. **A server node shows has a down-time of 20 hours per year. Show how to calculate the resulting availability Pr(Server operational). **
+    Extend your derivation to the case of aredundant structure of 3 servers. 
+    Show how to calculate its availability assuming independent faults.
+
+
+26. **Discuss advantages and disadvantages of cluster structures (that hide the redundancy to accessing nodes) as opposed to an architecture where failover is done via the Clients (such as RSerPool)**