6: Wireless

Question 1

what is a base station?

Answer 1

 typically connected to
wired network
 relay - responsible for
sending packets
between wired network
and wireless host(s) in
its “
area
”
• e.g. cell towers,
802.11 access
points

Question 2

what is a wireless link?

Answer 2

typically used to
connect mobile(s) to
base station
 also used as backbone
link
 multiple access protocol
coordinates link access
 various data rates,
transmission distance

Question 3

how does a wireless link differ to a wire link?

Answer 3

decreased signal strength: radio signal attenuates
as it propagates through matter (path loss)
interference from other sources: standardised
wireless network frequencies (e.g. 2.4 GHz) shared
by other devices (e.g. phone); devices (motors,
drills, microwave ovens) interfere as well
multipath propagation: radio signal reflects off
objects ground, arriving ad destination at slightly
different times

Question 4

what is SNR?

Answer 4

signal to noise ratio
• larger SNR – easier to
extract signal from noise (a
“good thing”)

Question 5

what is the hidden terminal problem?

Answer 5

Hidden terminal problem
 B, A hear each other
 B, C hear each other
 A, C can not hear each other
means A, C unaware of their
interference at B
e.g. mountains

Question 6

what is signal attenuation?

Answer 6

Signal attenuation:
 B, A hear each other
 B, C hear each other
 A, C can not hear each other
interfering at B

Question 7

what is 802? 802.11?

Answer 7

a family of IEEE standards for dealing with LANs and MANs

11 is wifi

Question 8

what is a BSS

Answer 8

Basic Service Set (BSS)
(aka “cell”) in
infrastructure mode
contains:
• wireless hosts
• access point (AP):
base station
• ad hoc mode: hosts
only

Question 9

what is ad hoc mode?

Answer 9

no AP (access point/base station), just hosts hopping along each other

Question 10

what is passive scanning? active scanning?

Answer 10

passive, the base stations send beacon frames to the user, in active the user probes a request firest

Question 11

how can a mobile device save power?

Answer 11

power management
 node-to-AP: “I am going to sleep until next
beacon frame”
• AP knows not to transmit frames to this node
• node wakes up before next beacon frame

Question 12

how does the problems of wireless affect the upper layers? logically (1) and performance-wise (4)

Answer 12

• logically, impact should be minimal …
• best effort service model remains unchanged
• TCP and UDP can (and do) run over wireless, mobile
• … but performance-wise:
• packet loss/delay due to bit-errors (discarded packets,
delays for link-layer retransmissions), and handoff
• TCP interprets loss as congestion, will decrease
congestion window un-necessarily
• delay impairments for real-time traffic
• limited bandwidth of wireless links