Upload
elvin-fox
View
214
Download
0
Embed Size (px)
Citation preview
Lecture 27
University of Nevada – RenoComputer Science & Engineering Department
Fall 2015
CPE 400 / 600Computer Communication Networks
Prof. Shamik SenguptaOffice SEM 204
[email protected]://www.cse.unr.edu/~shamik/
Link layer in wireless
Link Layer 5-2
link access in wireless domain # wireless (mobile) phone subscribers
now exceeds # wired phone subscribers!
computer networks: laptops, palmtops, PDAs, Smart Phones promise anytime wireless Internet access!
It has really been a wireless revolution decade…with more to come Wireless is no longer a luxury but a
necessity
WLAN Market: WiFi
0
1
2
3
4
5
$-b
il
2001 2002 2003 2004 2005
Forecast Sales of Wi-Fi Equipment(Source: InfoTech Trends)
Source: Pyramid Research
Worldwide WLAN Infrastructure Shipments (Source: Gartner)
0
1
2
3
4
5
6
7
Mil
lio
ns o
f U
nit
s
Source: AirTight Networks
WLAN growing exponentially
IEEE 802.11 Wireless LAN 802.11b
2.4 GHz unlicensed spectrum
up to 11 Mbps 802.11g
2.4 GHz range up to 54 Mbps
802.11a 5 GHz range up to 54 Mbps
802.11n: multiple antennae 2.4 / 5 GHz range up to 200 Mbps
all use CSMA/CA for multiple access all have infrastructure and ad-hoc network versions
What else? 802.11 ac – builds on 802.n – provides 80-160MHz channels 802.11ad – 60GHz mmwave spectrum 802.11af – Super Wi-Fi
802.11 LAN architecture
wireless host communicates with base station base station = access
point (AP) Basic Service Set (BSS)
(aka “cell”) in infrastructure mode contains: wireless hosts access point (AP): base
station ad hoc mode: hosts
only
BSS 1
BSS 2
Internet
hub, switchor routerAP
AP
Basic Service Set (BSS)
BSS
Extended Service Set (ESS) BSS’s with wired Distribution System (DS)
BSS
BSS
Distribution
System
802.11: Channels, association 802.11b: 2.4GHz-2.485GHz spectrum divided
into 13 channels at different frequencies AP admin chooses frequency for AP interference possible: channel can be same
as that chosen by neighboring AP!
host: must associate with an AP scans channels, listening for beacon frames
containing AP’s name (SSID) and MAC address selects AP to associate with will typically run DHCP to get IP address in
AP’s subnet
IEEE 802.11: multiple access problem 802.11: CSMA - sense before transmitting
don’t collide with ongoing transmission by other node
Certain differences from Ethernet LAN in wired domain
802.11: no collision detection! difficult to receive (sense collisions) when transmitting
due to weak received signals (fading)• Signal strength falls off rapidly with distance • Signal strength may weaken due to obstacles• Medium “air” shared among many users (not just WiFi users)
can’t detect all collisions in any case: hidden terminal problem
Wireless interference
Hidden terminal
Goal: CSMA/C(ollision)A(voidance)
“Open” Wireless Medium
S1
S2
R1
R1
S1 R1 S2
How does the medium access work in WLAN?
Access methods DCF CSMA/CA (mandatory)
• collision avoidance via exponential backoff• Minimum distance (IFS) between consecutive packets• ACK packet for acknowledgements (not for broadcasts)
DCF with RTS/CTS (optional)
• Distributed Foundation Wireless MAC• avoids hidden terminal problem
PCF (optional)
• access point polls terminals according to a list
Contention Based
Contention Free
Distributed Coordination Function (DCF) Point Coordination Function (PCF)
802.11 – MAC Priorities
defined through different inter frame spaces SIFS (Short Inter Frame Spacing)
• highest priority, for ACK, CTS, polling response PIFS (PCF IFS)
• medium priority, for time-bounded service using PCF
DIFS (DCF, Distributed Coordination Function IFS)
• lowest priority, for asynchronous data service, competing stations
t
medium busy SIFS
PIFS
DIFSDIFS
next framecontention
access if medium is free DIFS
DIFS = SIFS + (2 * Slot time)
WLAN access scheme details
Sending unicast packets station has to wait for DIFS before sending data receivers acknowledge at once (after waiting for SIFS) if the
packet was received correctly automatic retransmission of data packets in case of
transmission errors
t
SIFS
DIFS
data
ACK
waiting time
otherstations
receiver
senderdata
DIFS
contention
Contention for channel When the other stations find the channel idle, they
would like to transmit their own packets Contention for channel
If all the waiting stations attempt at once, this will surely result in collision Some CA scheme is necessary Backoff intervals can be used to reduce collision probability
t
SIFS
DIFS
data
ACK
waiting time
otherstations
receiver
senderdata
DIFS
contention
Backoff Interval When transmitting a packet, choose a backoff interval in
the range [0,cw] cw is contention window
Count down the backoff interval when medium is idle Count-down is suspended if medium becomes busy
When backoff interval reaches 0, transmit packet
data
wait
B1 = 5
B2 = 15
B1 = 25
B2 = 20
data
wait
B1 and B2 are backoff intervalsat nodes 1 and 2
Assume cw = 31
B2 = 10
Backoff Interval The time spent counting down backoff intervals is a part
of MAC overhead Choosing a large cw leads to large backoff intervals and
can result in larger overhead Choosing a small cw leads to a larger number of collisions
(when two nodes count down to 0 simultaneously)
Since the number of nodes attempting to transmit simultaneously may change with time, some mechanism to manage contention is needed IEEE 802.11 DCF: contention window cw is chosen
dynamically depending on collision occurrence Follows Binary exponential backoff algorithm
Binary Exponential Backoff (BEB) in DCF
Even before the first collision, nodes follow BEB
Initial backoff interval (before 1st collision) [0,7]
If still packets collide, double the collision interval [0,15], [0,31] and so on…
Avoiding collisions (more)idea: allow sender to “reserve” channel rather than
random access of data frames: avoid collisions of long data frames
sender first transmits small request-to-send (RTS) packets to BS using CSMA RTSs may still collide with each other (but they’re
short) BS broadcasts clear-to-send CTS in response to RTS CTS heard by all nodes
sender transmits data frame other stations defer transmissions
avoid data frame collisions completely using small reservation packets!
Collision Avoidance: RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
Wireless, Mobile Networks 6-21
framecontrol
durationaddress
1address
2address
4address
3payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
802.11 frame: addressing
Address 2: MAC addressof wireless host or AP transmitting this frame
Address 1: MAC addressof wireless host or AP to receive this frame
Address 3: MAC address(dependent on frame control field)
Address 4: used only in ad hoc mode
Wireless, Mobile Networks 6-22
InternetrouterH1 R1
AP MAC addr H1 MAC addr R1 MAC addr
address 1 address 2 address 3
802.11 frame
R1 MAC addr H1 MAC addr
dest. address source address
802.3 frame
802.11 frame: addressing
Wireless, Mobile Networks 6-23
framecontrol
durationaddress
1address
2address
4address
3payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
TypeFromAP
SubtypeToAP
More frag
WEPMoredata
Powermgt
Retry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
duration of reserved transmission time (RTS/CTS)
frame seq #(for RDT)
frame type(RTS, CTS, ACK, data)
802.11 frame: more
Frame Control field
Protocol Version: zero for 802.11 standard
Type= frame type: data, management, control Subtype = frame sub-type
ToDS: when bit is set indicate that destination
frame is for DS FromDS:
When bit is set indicate frame coming from DS
Data Link Layer 5-24
Frame Control field
Retry: Set in case of retransmission frame
More fragments: Set when frame is followed by other
fragment Power Management
bit set when station go Power Save mode (PS)
More Data: When set means that AP have more
buffered data for a station in Power Save mode
Data Link Layer 5-25
Address Field Description
Addr. 1 = All stations filter on this address.Addr. 2 = Transmitter Address (TA), Identifies transmitter to address the ACK frame to.Addr. 3 = Dependent on To and From DS bits.Addr. 4 = Only needed to identify the original source of WDS (Wireless Distribution System) frames
ProtocolVersion
Type SubTypeToDS
RetryPwrMgt
MoreData
WEP Rsvd
Frame Control Field
Bits: 2 2 4 1 1 1 1 1 1 1 1
DSFrom More
Frag
To DS
0
0
1
1
From DS
0
1
0
1
Address 1
DA
DA
BSSID
RA
Address 2
SA
BSSID
SA
TA
Address 3
BSSID
SA
DA
DA
Address 4
N/A
N/A
N/A
SA
Type field descriptions
Type and subtype identify the function of the frame: Type=00 Management Frame
Beacon (Re)Association
Probe
Type=01 Control FrameRTS/CTS ACK
Type=10 Data Frame
ProtocolVersion
Type SubTypeToDS
RetryPwrMgt
MoreData
WEP Rsvd
Frame Control Field
Bits: 2 2 4 1 1 1 1 1 1 1 1
DSFrom More
Frag
Type and subtypes
Data Link Layer 5-28
Type and subtypes
Data Link Layer 5-29
Type and subtypes
Data Link Layer 5-30
RTS/CTS frames
Data Link Layer 5-31
Lecture 27
University of Nevada – RenoComputer Science & Engineering Department
Fall 2015
CPE 400 / 600Computer Communication Networks
Prof. Shamik SenguptaOffice SEM 204
[email protected]://www.cse.unr.edu/~shamik/
Numerical Problems Practice
Wi-FiHidden Node ProblemWi-Fi with RTS/CTS
See handouts for numerical problems: Lec27_numerical problems.pdf
Announcements Dec. 2: Quiz 4
Dec. 7: Sample final exam in class. Discussion.
Dec. 9: Prep. Day. No class.
Dec. 14: Final Exam: 10.15am – 12.15pm
Link Layer 5-34