Transcript Datalink+Wifi

Datalink Layer
Purpose
understand principles behind link layer services:
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error detection, correction
sharing a broadcast channel: multiple access
link layer addressing (IEEE 802.xx)
Framing (Header)
local area networks: Ethernet, VLANs
instantiation, implementation of various link layer
technologies (Ethernet, fast-Ethernet, Wifi, ppp)
Addressing
• Most protocol based upon IEEE 802.x standrds
– Media Access Control (MAC) address:
• function: used ‘locally” to get frame from one interface
to another physically-connected interface (same
network, in IP-addressing sense)
• 48 bit MAC address (for most LANs) burned in NIC
ROM, also sometimes software settable
e.g.: 1A-2F-BB-76-09-AD
Address Resolution Protocol (ARP)
SRC
DEST
IP
111.111.111.111
222.222.222.222
MAC
74-29-9C-E8-FF-55
E6-E9-00-17-BB-4B
SRC
DEST
IP
111.111.111.111
222.222.222.222
MAC
1A-23-F9-CD-06-9B
49-BD-D2-C7-56-2A
B
R
A
111.111.111.111
74-29-9C-E8-FF-55
222.222.222.222
49-BD-D2-C7-56-2A
222.222.222.220
1A-23-F9-CD-06-9B
111.111.111.112
CC-49-DE-D0-AB-7D
111.111.111.110
E6-E9-00-17-BB-4B
222.222.222.221
88-B2-2F-54-1A-0F
A : arp-broadcast ‘Who have 111.111.111.110 ?
Only R: reply ‘I have 111.111.111.110’ – which mean on MAC E6-E9-00-17-BB-4B
A: make frame with Ip dest 222.222.222.222 and mac dest E6-E9-00-17-BB-4B
R: arp-broadcast ‘Who have 222.222.222.222 ?
Only B: reply ‘I have 111.111.111.110’ – which mean on MAC E6-E9-00-17-BB-4B
R: make frame with Ip dest 222.222.222.222 and mac dest E6-E9-00-17-BB-4B
WiFi – Concepts (802.11xxx standards)
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Wireless hosts
 laptop, smartphone
 run applications
 may be stationary (non-mobile) or mobile
Base station
 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
Wireless link
 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
Characteristics of selected wireless links
Data rate (Mbps)
200
54
5-11
802.11n
802.11a,g
802.11b
4
1
802.11a,g point-to-point
4G: LTWE WIMAX
3G: UMTS/WCDMA-HSPDA, CDMA2000-1xEVDO
802.15
.384
2.5G: UMTS/WCDMA, CDMA2000
.056
2G: IS-95, CDMA, GSM
Indoor
Outdoor
10-30m
50-200m
Mid-range
outdoor
Long-range
outdoor
200m – 4 Km
5Km – 20 Km
Wireless Link Characteristics
– Decreased signal strength: radio signal attenuates as it
propagates through matter (path loss)
– Interference from other sources: standardized wireless
network frequencies (e.g., 2.4 GHz) shared by other
devices (e.g., phone); devices (motors) interfere as well
– Multipath propagation: radio signal reflects off objects
ground, arriving ad destination at slightly different times
– Hidden terminal problem: terminal A can se B and B can
see C
BUT A can not see C
– Fading problem: terminal A can not se C and C can see A
BUT at a terminal in the middle B signals from A and C
interfer