Wireless and Mobile Networks

Question 1

What is the mobility challenge?

Answer 1

Handling mobile user who changes point of attachment to network.

Question 2

What elements make up a wireless network?

Answer 2

Wireless hosts
Base stations
Wireless link

Question 3

What is a wireless host?

Answer 3

Laptop, smartphone

Run applications

May be stationary or mobile.

Question 4

What is a base station?

Answer 4

Typically connected to wired network.

Relay - responsible for sending packets between wired network and wireless host(s) in its area

EG: Cell towers, wifi access points.

Question 5

What is a wireless link?

Answer 5

Typically used to connect mobile(s) to base stations.

Also used as backbone link

Multiple access protocol coordinates link access.

Various data rate due to distance.

Question 6

What is infrastructure mode?

Answer 6

Base station connects mobiles into wired network.

Handoff: mobile changes base station as they move.

Question 7

What is ad hoc mode?

Answer 7

No base stations, nodes can only transmit to other nodes within link coverage

Nodes organise themselves into a network: routing among themselves

Question 8

Why is the signal weaker in wireless than in wired connections?

Answer 8

Path loss as it propagates through matter

Interference from other wireless network frequencies.

Question 9

What is SNR?

Answer 9

Signal-to-noise ratio.

Larger SNR - easier to extract signal from noise - good.

Question 10

What is BER?

Answer 10

Bit-Error Rate

the rate at which errors occur in the transmission of digital data.

Question 11

What are the tradeoffs between SNR and BER?

Answer 11

Given physical layer: Increase power, increase SNR, decrease BER

Given SNR: Choose physical layer that meets BER requirement, giving highest throughput.

SNR may change with mobility.

Question 12

What is the hidden terminal problem?

Answer 12

Anything that can block the signal means that hosts may not be able to see each other and therefore, not able to detect collisions.

Question 13

What are the speeds of the various wireless LAN?

Answer 13

802. 11b: 2.4-5 GHz unlicensed spectrum. up to 11Mbps
803. 11a: 5-6GHz range, up to 54Mbps
804. 11g: 2.4-5 GHz range, up to 54 mbps
805. 11n: Multiple antennae, 2.4-5 GHz range, up to 200 Mbps

Question 14

What is the architecture of Wireless LAN?

Answer 14

Wireless host communicates with base station.

Question 15

What is the Basic Service Set (BSS)?

Answer 15

Cell in infrastructure mode contains

Wireless hosts
Access point (AP) base station

Ad hoc mode: Hosts only.

Question 16

What is association in WiFi?

Answer 16

802.11B: 2.4GHz-2.48GHz spectrum divided into 11 channels at different frequencies.
Access point admin chooses frequency. Interfering possible if channel is the same as neighboring AP

Question 17

How do hosts associate with an access point?

Answer 17

Scans channels, listening for beacon frames containing AP’s name (SSID) and MAC address.

Selects AP to associate with

May perform authentication

Will run DHCP to get IP address.

Question 18

What is passive scanning?

Answer 18

Beacon frames sent from Access Points

Association request frame sent from host to access point.

Association response frame sent from selected access point to host.

Question 19

What is active scanning?

Answer 19

Probe request frame broadcast from host

Probe response frames from access point.

Association request frame sent host to selected access point.

Association response frame sent from selected AP to host.

Question 20

How does WiFi handle multiple access?

Answer 20

Use CSMA/CA - sense before transmitting

Has no collision detection, difficult to sense collisions when transmitting due to weak received signals. G

Question 21

How does CSMA/CA work?

Answer 21

If sense channel idle for DIFS then transmit entire frame (No Collision detection)

If sense channel busy, start random backoff time. Transmit when channel idle and timer expires.

If frame received ok, return ACK after SIFS.

Question 22

How else does WiFi avoid collisions?

Answer 22

Transmit Request to send packet to base station using CSMA

Packet is small, so shouldn’t collide.

Base station broadcasts clear-to-send in response..

CTS heard by all frames

Question 23

How are 802.11 frames structured?

Answer 23

Frame cntrl | duration | addr 1 | addr 2 | addr 3 | seq cntrl | addr 4| payload | CRC

2 | 2 | 6 | 6 | 6 | 2 | 6 | 0-2312 | 4

Question 24

What do the different addresses mean in the 802.11 frames?

Answer 24

addr 1: MAC address of wireless host or AP to receive this frame.

addr2: MAC address of wireless host or AP transmitting this frame.
addr3: Mac address of router interface to which AP is attached
addr4: Used only in Ad Hoc mode.

Question 25

What are the advanced capabilities of WiFi?

Answer 25

Rate adaptation

Power Management

Question 26

How does rate adaptation work?

Answer 26

Base station, mobile dynamically change tranmission rate as mobile moves.

Beacons only transmit to nodes within a certain range.

Question 27

How does power management work?

Answer 27

Node tells AP that it’s going to sleep until next beacon frame.

Beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent.

Nodes will stay awake if AP-to-mobile frames to be sent.

Question 28

What is the impact of wireless on the higher layer protocols?

Answer 28

Packet loss/delay due to bit errors and handoff

TCP interprets loss as congestion, will decrease congestion window un-necessarily

Delay impairments for real-time traffic.