Wireless Networks

Question 1

How does single hop with infrastructure work?

Answer 1

host connects to base station which connects to internet

Question 2

How does single hop with no infrastructure work?

Answer 2

no base station, no connection to internet

Question 3

How does multiple hops with infrastructure work?

Answer 3

hosts may have to relay through several wireless nodes to connect to internet

Question 4

How does multiple hops with no infrastructure work?

Answer 4

no base station, no connection to internet, may have to relay to reach other wireless node

Question 5

What are the SNR characteristics of a wireless link?

Answer 5

Larger SNR = easier to extract signal from noise

Question 6

What are the SNR vs BER tradeoffs for a wireless link?

Answer 6

given physical layer: increase power = increase SNR = decrease BER

given SNR: choose physical layer that meets BER giving highest throughput

Question 7

What is the worldwide interoperability for microwave access (WiMAX)?

Answer 7

• radio access technology providing broadband and long distance wireless data delivery
• supports QoS and multicasting
• for mobile environments provides high-bandwidth services
• low cost of deploying

Question 8

What is WiFi? (wireless fidelity)

Answer 8

• radio access technology designed to provide in-building broadband coverage
• doesn’t guarantee QoS
• not originally designed for high-speed mobility

Question 9

WiFi vs WiMAX, range

Answer 9

Range:
wifi = local network, few hundred metres
wimax = single antenna up to 60km

Question 10

WiFi vs WiMAX, scalability

Answer 10

Scalability:
wifi = LAN application so users scale from one to tens with one subscriber for each consumer premises equipment device (CPE)
wimax = efficiently supports from one to hundreds of CPEs, unlimited subscribers behind CPE

Question 11

WiFi vs WiMAX, bit rate

Answer 11

Bit rate:
wifi = 2.7bps/Hz (peak 54Mpbs in 20 Mhz channel)
wimax = 5bps/Hz (peak100Mbps in 20MHz channel)

Question 12

WiFi vs WiMAX, QoS

Answer 12

QoS:
wifi = doesn’t guarantee, no ubiquitous broadband
wimax = several levels, ubiquitous broadband

Question 13

What is bluetooth?

Answer 13

• family of radio access standards
• short range connectivity solution for hand held devices
• creates personal areas networks (PANs) with high security
• benefit of omni-directionality
• elimination of line of sight requirement of RF connectivity

Question 14

What is ZigBee?

Answer 14

• family of radio access standards
• provide networking solution for low data rate/low power consumption apps

Question 15

What are the 3 roles of a ZigBee network?

Answer 15

End device:
- FFD or RFD executing apps, doesn’t have child node

Router:
- FFD relaying messages, usually has child and parents

Coordinator:
- FFD controlling network, also gateway to external world, trust centre, access authenticator

Question 16

What is an FFD?

Answer 16

full function device

Question 17

What is an RFD?

Answer 17

reduced function device

Question 18

What is a wireless LAN architecture (WiFi)?

Answer 18

basic service set in infrastructure mode contains wireless hosts, access point base station

wireless host communicated with base station

Question 19

How do wireless networks avoid collisions?

Answer 19

CSMA/CA

Question 20

How does the sender in CSMA/CA work?

Answer 20

• if snese channel idle for DIFS transmit entire frame
• if sense channel busy, start random backoff time, counts down, while channel idle transmit when timer expires
• if no ack, increase random backoff interval and go back to sense channel

Question 21

How does the receiver work in CSMA/CA?

Answer 21

if frame received ok, return ack after SIFS

Question 22

What is SIFS?

Answer 22

short inter frame space

Question 23

What is DIFS?

Answer 23

distributed inter frame space

Question 24

What are some advanced capabilities of wireless networks?

Answer 24

Rate adaption: dynamically change transmission rate as mobile moves

Power management: agreement between mobile node and AP