Modern Wireless Networks at TU Berlin

Flashcards and summaries for Modern Wireless Networks at the TU Berlin

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In welchem Spektrum liegen die RF Frequenzen?

Exemplary flashcards for Modern Wireless Networks at the TU Berlin on StudySmarter:

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

Exemplary flashcards for Modern Wireless Networks at the TU Berlin on StudySmarter:

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

Exemplary flashcards for Modern Wireless Networks at the TU Berlin on StudySmarter:

What is collision avoidance?

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What are the challenges in Ad-hoc networks?

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

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What are the types of MANET protocols?

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

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Explain Flooding with control packets: route discovery


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

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What happens if a link is broken in DSR?

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How can the routing be further optimised?

(other protocol types)

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Exemplary flashcards for Modern Wireless Networks at the TU Berlin on StudySmarter:

Modern Wireless Networks

In welchem Spektrum liegen die RF Frequenzen?

3kHz bis 300GHz

Modern Wireless Networks

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

- 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)

Modern Wireless Networks

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

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

Modern Wireless Networks

What is collision avoidance?

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

Modern Wireless Networks

What are the challenges in Ad-hoc networks?

- 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...

Modern Wireless Networks

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

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

Modern Wireless Networks

What are the types of MANET protocols?

- 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


Modern Wireless Networks

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

- 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.

Modern Wireless Networks

Explain Flooding with control packets: route discovery


- 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

Modern Wireless Networks

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

reactive protocol: Dynamic Source Routing (DSR)


Modern Wireless Networks

What happens if a link is broken in DSR?

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

Modern Wireless Networks

How can the routing be further optimised?

(other protocol types)

Route Caching - learns route by all means all nodes know something about what the shortest route is (can be quicker, but can also lead to collisions and routes from the cache might become invalid)


proactive protocols for example add aging information to a route or only links that have been stable for some minimum duration are utilized


geographic routing: calculate next hop from physical distance

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