Modern Wireless Networks at TU Berlin | Flashcards & Summaries

Lernmaterialien für Modern Wireless Networks an der TU Berlin

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TESTE DEIN WISSEN

What are the problems with radio propagation?
(What is weakening the signal?)

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TESTE DEIN WISSEN

- Basic loss over a certain distance 

=> path loss


- Obstacles in the propagation path 

=> shadowing, slow-fading


- Multi-path propagation 

=> frequency-selective fading


- Mobility of transceivers 

=> time-selective fading


- Interference


- Noise (thermal, atmospheric, man-made noise)

Lösung ausblenden
TESTE DEIN WISSEN

What is the difference between schedule- and contention-based medium access? In which case should which one be used?

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TESTE DEIN WISSEN

Schedule-based MAC:
- a schedule exists, regulating which participant may use which resource at which time (TDMA component)
- schedule can be fixed or computed on demand => usually: mixed
advantage: usually collisions, overhearing, idle listening are no issues, very efficient at high load
disadvantage: time synchronization needed!


Contention-based MAC:

- risk of colliding packets is deliberately taken
- hope: coordination overhead can be saved, resulting in overall improved efficiency
- mechanisms to handle/reduce probability/impact of collisions required


Rule of the thumb: 

high regularity => schedule based, 

very irregular traffic => contention based

Lösung ausblenden
TESTE DEIN WISSEN

What is collision avoidance?

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TESTE DEIN WISSEN

based on virtual carrier sense:

short control packets are send for reservation:
- RTS = Request-to-Send: 

sender requests the right to send from a receiver
- CTS = Clear-to-Send: 

the receiver grants the right to send as soon as it is ready to receive


- signaling packets contain: sender address, receiver address, packet duration
- if data packets too small, sending RTS-CTS not beneficial
- Collision may occur for control packets, but they are short => lower collision cost

-"MACAW"  sends an acknowledgement after data transmission

Lösung ausblenden
TESTE DEIN WISSEN

How does MIPv6 work?

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TESTE DEIN WISSEN

- MIPv6 can solve triangular routing problem

- new extension header (contains home adress)

- FA is no longer needed: MH is now always the endpoint of the tunnel


- two options: 

1) MIPv4 Biderictional tunneling

- similar to IPv4 -> still inefficient

2) Route optimisation

- therefore the packet gets a additional IPv6 header that saves the home adress and replaces the home adress for transmission with the destionation adress

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TESTE DEIN WISSEN

What are the challenges in Ad-hoc networks?

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TESTE DEIN WISSEN

- Asymmetric capabilities: transmission ranges, battery live, speed of movement varies between nodes

- Asymmetric responsibilities: only some nodes may route packets, some nodes may act as leaders of nearby nodes (e.g., cluster head)

-Traffic characteristics may differ in different ad-hoc networks: bit rate, timeliness constraints, reliability requirements...

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TESTE DEIN WISSEN

What is a new performance criterium for Ad-hoc networks?

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TESTE DEIN WISSEN

route stability despite mobility -> not necessarily the shortest path but stable over time

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TESTE DEIN WISSEN

What are the types of MANET protocols?

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TESTE DEIN WISSEN

- Proactive protocols constantly maintain and detect paths to all possible destinations

->high overhead, low latency
- Reactive protocols save overhead and set-up paths only when needed

->low overhead, high latency
- Hybrid protocols combine both aspects


Lösung ausblenden
TESTE DEIN WISSEN

What are the problems with flooding for data delivery? And what are solutions?

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TESTE DEIN WISSEN

- flooding generates vast numbers of duplicate packets


1. hop count: lifetime of a packet is limited after a certain amount of hops the package is discarded - problem: choice of hop counter


2. Keep track of which packets have been flooded: source router put a sequence number in each packet, source checks in its dictionary if it has seen the packet already.

Lösung ausblenden
TESTE DEIN WISSEN

Explain Flooding with control packets: route discovery


Lösung anzeigen
TESTE DEIN WISSEN

- flood packets to find a route

1. route discovery

route request packet (RREQ) contains

< target address, initiator address, route record, rquest ID >

each nodes saves a list with 

<initiator adress, request ID>


a node appends itself to the route record if it hasn't seen the PREQ before


if the node is the target, it uses the route record to send a route reply back to the initial node

Lösung ausblenden
TESTE DEIN WISSEN

What is the algorithm dor flooding of control packets called? And what type of protocol does it belong to?

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TESTE DEIN WISSEN

reactive protocol: Dynamic Source Routing (DSR)


Lösung ausblenden
TESTE DEIN WISSEN

What happens if a link is broken in DSR?

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TESTE DEIN WISSEN

A route error (RERR) is send and nodes hearing that request update thier route cache to remove all invalid routes related to the broken link

Lösung ausblenden
TESTE DEIN WISSEN

In welchem Spektrum liegen die RF Frequenzen?

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TESTE DEIN WISSEN

3kHz bis 300GHz

Lösung ausblenden
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Q:

What are the problems with radio propagation?
(What is weakening the signal?)

A:

- Basic loss over a certain distance 

=> path loss


- Obstacles in the propagation path 

=> shadowing, slow-fading


- Multi-path propagation 

=> frequency-selective fading


- Mobility of transceivers 

=> time-selective fading


- Interference


- Noise (thermal, atmospheric, man-made noise)

Q:

What is the difference between schedule- and contention-based medium access? In which case should which one be used?

A:

Schedule-based MAC:
- a schedule exists, regulating which participant may use which resource at which time (TDMA component)
- schedule can be fixed or computed on demand => usually: mixed
advantage: usually collisions, overhearing, idle listening are no issues, very efficient at high load
disadvantage: time synchronization needed!


Contention-based MAC:

- risk of colliding packets is deliberately taken
- hope: coordination overhead can be saved, resulting in overall improved efficiency
- mechanisms to handle/reduce probability/impact of collisions required


Rule of the thumb: 

high regularity => schedule based, 

very irregular traffic => contention based

Q:

What is collision avoidance?

A:

based on virtual carrier sense:

short control packets are send for reservation:
- RTS = Request-to-Send: 

sender requests the right to send from a receiver
- CTS = Clear-to-Send: 

the receiver grants the right to send as soon as it is ready to receive


- signaling packets contain: sender address, receiver address, packet duration
- if data packets too small, sending RTS-CTS not beneficial
- Collision may occur for control packets, but they are short => lower collision cost

-"MACAW"  sends an acknowledgement after data transmission

Q:

How does MIPv6 work?

A:

- MIPv6 can solve triangular routing problem

- new extension header (contains home adress)

- FA is no longer needed: MH is now always the endpoint of the tunnel


- two options: 

1) MIPv4 Biderictional tunneling

- similar to IPv4 -> still inefficient

2) Route optimisation

- therefore the packet gets a additional IPv6 header that saves the home adress and replaces the home adress for transmission with the destionation adress

Q:

What are the challenges in Ad-hoc networks?

A:

- Asymmetric capabilities: transmission ranges, battery live, speed of movement varies between nodes

- Asymmetric responsibilities: only some nodes may route packets, some nodes may act as leaders of nearby nodes (e.g., cluster head)

-Traffic characteristics may differ in different ad-hoc networks: bit rate, timeliness constraints, reliability requirements...

Mehr Karteikarten anzeigen
Q:

What is a new performance criterium for Ad-hoc networks?

A:

route stability despite mobility -> not necessarily the shortest path but stable over time

Q:

What are the types of MANET protocols?

A:

- Proactive protocols constantly maintain and detect paths to all possible destinations

->high overhead, low latency
- Reactive protocols save overhead and set-up paths only when needed

->low overhead, high latency
- Hybrid protocols combine both aspects


Q:

What are the problems with flooding for data delivery? And what are solutions?

A:

- flooding generates vast numbers of duplicate packets


1. hop count: lifetime of a packet is limited after a certain amount of hops the package is discarded - problem: choice of hop counter


2. Keep track of which packets have been flooded: source router put a sequence number in each packet, source checks in its dictionary if it has seen the packet already.

Q:

Explain Flooding with control packets: route discovery


A:

- flood packets to find a route

1. route discovery

route request packet (RREQ) contains

< target address, initiator address, route record, rquest ID >

each nodes saves a list with 

<initiator adress, request ID>


a node appends itself to the route record if it hasn't seen the PREQ before


if the node is the target, it uses the route record to send a route reply back to the initial node

Q:

What is the algorithm dor flooding of control packets called? And what type of protocol does it belong to?

A:

reactive protocol: Dynamic Source Routing (DSR)


Q:

What happens if a link is broken in DSR?

A:

A route error (RERR) is send and nodes hearing that request update thier route cache to remove all invalid routes related to the broken link

Q:

In welchem Spektrum liegen die RF Frequenzen?

A:

3kHz bis 300GHz

Modern Wireless Networks

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