---
# System prepended metadata

title: SoSo 2023 Mobnet

---

# SoSo 2023 Mobnet

## Wireless Communications

1.1

a)
Shadowing: Power fluctation due to obstacles on propagation path
Scattering: Signal encounters small object -> signal is redirected in many directions
 

b) Slow & Fast Fading
Fast: varies quickly with frequency -> constructive/descructive interference (due to multipath)
Slow: varies slowly with frequency -> shadowing/obstructions

1.2
a)
Path loss describes the decrease in signal strength over increasing distance and frequency

b)
higher frequency => lower wavelength => smaller antennas

c)
1. 

lambda = c / f = 3 * 10^8 m/s / 6*10^6 Hz = 50m


2. FSPL = 48dB, PL=?


1.3

a) Time Division Multiplexing
(+) high channel utilization
(-) needs precise time synchronization / needs guards time between slots

b) Code Division Multiplexing
(+) bandwidth efficient -> all STAs use whole bandwidth
(-) high computation complexity when demultiplexing the signal

## 2 IEEE 802.11

2.1

a) 


![image](https://hackmd.io/_uploads/ByVl-DqoT.png)


b)
- carrier sense
- exp. backoff with contention window



2.2

a)
Station schedules a new packet after success with backoff stage 0. The backoff is chosen from (0, cw_min) as cw_min

b)
Decrement backoff when channel is sensed idle (3 -> 2)

c)
Unsuccessful transmission, i-tes collision window, choose 0 from (0, cw_i)

d)
Unsuccessful transmission, maximum cw is already reached and not further incremented

e)
successful transmission, reset cw to cw_min, reset i to 0


2.3
a)
- collision probability increases -> packets need to be rescheduled and transmitted again, backoff increases on each collision => higher delays 


b)
- cw is rather small => backoff not diverse enough -> higher probability that multiple stations send at the same time

c)
- RTS/CTS throughput almost insensitive to the number of stations
- only RTS packets collide -> shorter framelength -> less collision time
- 50: threshold reached

## mmWave

3.1

a)
- very high frequency -> higher path loss
- mmWaves uses directed antennas -> LoS can be obstructed

b)
- RTS/CTS recieve omnidirectional => range too low
- quasi omnidirectional recieve may miss some RTS from not covered directions

3.2

a) 
- 60 GHz waves behave like optical waves -> scattering, reflection, ...

b)
- Multi-beam: transmit over multiple beams => dynamically navigate around obstacles
- -> Requirement: Possibility to maintain multiple beams

- Multi-hop: use other nodes to route around the blockage
- -> Requirement: keep routing info about each relay path

3.3

a)
- Directional antennas used
- focus signal => increase signal strength -> improve quality & range

b)
- determine optimal beam to target (optimal parameters)


## Low Power Protocols

4.1 
a)

- BT: uninterrupted continious data streams
- BLE: bursty/short communication

b)
- BT: wireless headphones
- BLE: Wearables (fitness watch)

4.2

a) 1 - 2

b)
- BT: all nodes in piconet
- BLE: each master-slave pair


4.3

a)

Cskip: Address determined based on Topology (Depth, maxRouter per Node, max Children per node)
- each child has address space which it can divide to their children

Stochastic:
- Address is randomly chosen by parent and broadcasted 
- re-chosen if already in use


b)
(+) implies topology -> faster routing, guranteed conflict free
(-) Tree only supports depth <= 5 (> 5 does not fit in 16-bit address space)



## LoRaWAN

5.1

(a)
1. FOpts Field: piggyback commands -> short commands
2. FPorts: Dedicated Message with FPort=0 -> longer commands


(b)
-

(c)

-

5.2

(a) each region has different RF regulation
(b) Dwell Time

## MANETs

6.1

(a)

![image](https://hackmd.io/_uploads/BkpFgO9j6.png)

(b)
Route discovery: 
- send RREQ to all neighbours (broadcast), rebroadcasted once per node (except destination node, doesnt broadcasts)
- Packets are dropped if already received
- Intermediate nodes save S, D, next hop, DSN, hop count in routing table
Route reply: unicast RREP from destination to source

c)
- store previous hop on route towards S
- delete entry on timeout


6.2

AODV: uniform, destination-based, reactive

DSR: uniform, topology-based, reactive

LAR: non-uniform, topology-based, reactive

6.3

Only Multipoint Relays (MPR) forward messages -> less overhead
Selection:
- nodes that have the only connection to a 2-Hop neighbour 
- nodes that have max. number of 2-Hop neighbours


Selection criterion:
- any 2-hop neighbor must be covered by a MPR
- minimize number of MPRs


## Cellular Networks

7.1

NCHO:
- link quality monitored by BS (assumes symm. link)
- BS makes decision

MAHO:
- link quality monitored by BS & MS
- MS sends measurements periodically to BS
- BS makes decision


7.2
(+) more capacity due to spacial reusal
(-) more handovers required

7.3
B
- relative signal strength with threshold (T2)
- if current signal strength is below T2 and signal strength of other BS is higher than that of the current one -> handoff

C
- relative signal strength with hysteresis (h)
- handoff if signal strength of other BS > current BS'es signal strength + hysteresis




















