Transcript sample - Index of

S-72.1130 Telecommunication
Systems
Wireless Local Area Networks
Outline




LAN basics

Structure/properties of LANs
WLANs

Link layer services

Media access layer
 frames and headers
 CSMA/CA

Physical layer
 frames
 modulation
 Frequency hopping
 Direct sequence
 Infrared
Installation
Security
2
S-72.1130 Telecommunication
Systems
LAN Basics
What is a LAN?
Local area means:

Freedom from regulatory constraints at ISM Band (Industrial,
Science and Medical)

Short distance (~1km) between computers

Low cost

High-speed (10 Mb/s.. 10 Gb/s); support for TCP or UDP
type of communications

Flexible error control: in MAC and in upper levels

Computers move, machines have unique MAC address

MAC protocol takes care of optimizing throughput for the
expected services

Physical level takes care of physical transmission of packets
over a medium
4
Multiple Access Communications

Shared media basis for broadcast networks



Inexpensive: radio over air; copper or coaxial cable
M users communicate by broadcasting into medium
Key issue: How to share the medium?
3
2
4
1
Shared multiple
access medium
M

5
Approaches to Media Sharing
Medium sharing techniques
Static
channelization




Partition medium
Dedicated allocation
to users
Satellite
transmission
Cellular Telephone
Dynamic medium
access control
Scheduling




Random access
Polling: take turns
Request for slot in
transmission
schedule
Token ring
Wireless LANs




Loose coordination
Send, wait, retry if
necessary
Aloha
Ethernet
Bus Network



In a bus network, one node’s transmission traverses the entire
network and is received and examined by every node. The access
method can be :
 (1) Contention scheme : multiple nodes attempt to access
bus; only one node succeed at a time (e.g. CSMA/CD in
Ethernet 802.3)
 (2) Round robin scheme : a token is passed between nodes;
node holding the token can use the bus (e.g.Token bus 802.4)
Advantages:
 (1) Simple access method
C
D
A
B
 (2) Easy to add or remove
stations
D
term
term
Disadvantages:
- Line coded, serial data
 (1) Poor efficiency with high
- twisted pair or coaxial cable
network load
 (2) Relatively insecure, due to
11
the shared medium
term: terminator impedance
Typical Wired LAN



Transmission Medium
Network Interface Card
(NIC)
Unique MAC “physical”
address
Serial format
Ethernet
Processor
RAM
ROM
RAM
Reference: A. Leon-Garcia, I. Widjaja, Communication
Networks , Instructor's Slide Set
NIC implements MAC protocol &
physical port. Parallel interface to PC 12
IEEE 802-series of LAN Standards
802 standards free to
download from
http://standards.ieee.org
/getieee802/

hub
stations
hub
stations
hub
stations
WiMAX
hub
router
server
Demand priority: A round-robin (token ring) arbitration
method to provide LAN access based on message priority level
DQDB: Distributed queue dual buss, a ring network
13
S-72.1130 Telecommunication
Systems
IEEE 802 LAN Standard
The IEEE 802 LAN Standards
(http://www.ieee802.org/)
OSI Layer 3
Network
IEEE 802.2
Logical Link Control (LLC)
LLC
OSI Layer 2
(data link)
b: Wi-Fi
IEEE 802.3 IEEE 802.4 IEEE 802.5
IEEE 802.11
Carrier
Token
Token
Wireless
Sense
Bus
Ring
Ethernet
a b g
Physical Layers
- options: twisted pair, coaxial, optical, radio paths;
(not for all MACs above!)
Bus (802.3…)
Star (802.3u…)
MAC
OSI Layer 1
(physical)
Ring (802.5…)
15
IEEE 802 Layers
Logical Link Control (LLC) Sublayer

Utilizes services of HDLC (Highlevel Data Link Control)

Therefore, LLC SAPs separate
upper layer data exchanges =>
NIC applies different buffer
segments for each SAP (port)

LLC provides means to exchange
frames between LANs using
different MACs
IEEE 802.2
Logical Link Control (LLC)
LLC
b: Wi-Fi
IEEE 802.3 IEEE 802.4 IEEE 802.5
IEEE 802.11
Carrier
Token
Token
Wireless
Sense
Bus
Ring
abg
Ethernet
Medium Access Control Sublayer

Coordinates access to medium

Connectionless/Connection oriented
frame transfer service

Machines identified by
MAC/physical address (in NIC)

Broadcasts frames with MAC
addresses

Examples: CSMA/CD, CSMA/CA
Physical layers
Physical




MAC
PHY
level
Star, bus or ring topology
Cabling and electrical interfaces
Twisted pair, coaxial, fiber
Line coding (wired LANs) or
modulation (WLANs)
(More of HDLC in supplementary…)
16
S-72.1130 Telecommunication
Systems
IEEE 802 LAN Standard:
Logical Link Layer (LLC)
Logical Link Control Layer (LLC)




Specified by ISO/IEC 8802-2 (ANSI/IEEE 802.2)
Objective: exchange data between users across LAN using 802-based
MAC controlled link
Provides addressing and data link control (routing)
Independent of topology, medium, and chosen MAC access method
Data to higher level protocols
Info: carries user data
Supervisory: carries
flow/error control
Unnumbered: carries protocol
control data
Source
SAP
LLC’s Protocol Data Unit (PDU)
(SAP: Service Access Point)
18
HDLC Frame types



Information frames, or I-frames, transport user data
from the network layer. In addition they can also
include flow and error control information piggybacked on
data.
Supervisory Frames, or S-frames, are used for flow and
error control whenever piggybacking is impossible or
inappropriate, such as when a station does not have data
to send. S-frames do not have information fields.
Unnumbered frames, or U-frames, are used for various
miscellaneous purposes, including link management.
Some U-frames contain an information field, depending on
the type.
http://en.wikipedia.org/wiki/High-Level_
Data_Link_Control#I-Frames_.28user_data.29
21
LLC Services




A Unacknowledged connectionless service

no error or flow control - no ack-signal usage

unicast (individual), multicast, broadcast addressing

higher levels take care or reliability - thus fast for instance for TCP
B Connection oriented service

supports unicast only

error and flow control for lost/damaged data packets by cyclic
redundancy check (CRC)

Asynchronous balanced mode of HDLC: error control, sequencing,
flow control

Phases: Connection setup, data exchange, and release
C Acknowledged connectionless service
Problem: A workstation has a single, physical MAC address, how to separate
network or higher level service access? Ans: HDLC SAP addressing:

Can handle several logical connections, distinguished by their SAP (service
access points, next slides).



ack-signal used
error and flow control by stop-and-wait ARQ
faster setup than for B
HDLC : High-level Data Link Control
22
SAP Addressing
IEE802.11 (CSMA/CA)...
IEE802.11 (CDMA)...
ATM...
Reference: W.
Stallings: Data
and Computer
Communications,
7th ed
23
remember
encapsulation….
HTTP Request
TCP Header contains source &
destination port numbers
IP Header contains source and
destination IP addresses;
transport protocol type
TCP
header
HTTP Request
IP
header
TCP
header
HTTP Request
Ethernet
header
IP
header
TCP
header
HTTP Request
MAC
IP
port
data link
Ethernet Header contains source
& destination MAC addresses;
network protocol type
Traffic to the
target BSS / ESS
PHY layer transmits packet using a modulation method (DSSS, OFDM, IR,
FHSS)
FCS
S-72.1130 Telecommunication
Systems
IEEE 802 LAN Standard:
Media Access Control (MAC) Layer
Media Access Control:
Ways to Share a Medium

Medium sharing techniques
Static
channelization




FDMA,TDMA, CDMA
Uses partition
medium
Dedicated allocation
to users
Examples:

Satellite
transmission

Cellular
Telephone
Dynamic medium
access control
Scheduling



Medium sharing
required for multiple
users to access the
channel
Communications by

unicasting

multicasting

broadcasting
Random access
(contention)
Polling (take turns):
Token ring 802.5
Reservation systems:
Request for slot in
transmission schedule
802.4





Loose coordination
Send, wait, retry if
necessary
Aloha
CSMA/CD (Ethernet)
CSMA/CA (802.11
WLAN)
26
Selecting a Medium Access Control




Environment: Wired / Wireless?
Applications:
 What type of traffic?
 Voice streams? Steady traffic, low delay/jitter
 Data? Short messages? Web page downloads?
 Enterprise or consumer market? Reliability, cost
Scale:
 How much traffic can be carried?
 How many users can be supported?
Examples:
 Design MAC to provide wireless DSL-equivalent access for rural
communities
 Design MAC to provide Wireless-LAN-equivalent access to mobile
users (user in car travelling at 130 km/hr)
27
MAC Techniques in LANs




Contention

Medium is free for all

A node senses the free medium and occupies it as long as data packet
requires it

Example: Ethernet (IEEE 802.3 CSMA/CD)
Reservation (short term statistical access)

Gives everybody a turn

Reservation time depends on token holding time (set by network
operator)

For heavy loaded networks

Example: Token Ring/IEEE 802.5, Token Bus/IEEE 802.4, FDDI
Mixed

Flexible compromise: 802.11 WLANs
Reservation (long term)

Link reservation for multiple packets (whole session)

Example: scheduling a time slot: GSM using TDMA or FDMA
(uplink/dowlink)
28
Delay-Bandwidth Product

Delay-bandwidth product key parameter



Coordination in sharing medium involves using
bandwidth (explicitly or implicitly)
Difficulty of coordination comparable to delaybandwidth product
Simple two-station example


Station with frame to send listens to medium and
transmits if medium found idle
Station monitors medium to detect collision and
defers frame transmission if collision detection
Two-stations MAC example
30
Two-Stations MAC …
Two stations are trying to share a common medium
A
transmits
at t = 0
Distance d meters
tprop = d /  seconds
A
B
Case 1
A
B
Case 2
A detects
collision at
t = 2 tprop
A
B
A
B
B does not
transmit before
t = tprop & A
captures the
channel
B transmits
before t = tprop
and detects
collision after
receiving ack
from A
Efficiency of Two-Station
Example

Each frame transmission requires 2tprop of quiet time =>
number of bits wasted for access coordination: 2tpropR



R transmission bit rate
L bits/frame
time slots to be transmitted
Efficiency:  
actual time slots required for transmission

Reff
L
1
1
Efficiency   


L  2t propR 1  2t propR / L 1  2a
L
1
MaxThroughput  Reff 

R bits/second
L / R  2t prop 1  2a
Normalized
Delay-Bandwidth
Product
a
t prop
L/R
Propagation delay
Time to transmit a frame
R
A bit of history of Ethernet








1970 ALOHAnet radio network deployed in Hawaiian islands
1973 Metcalf and Boggs invent Ethernet, random access in wired net
1979 DIX (DEC + Intel + Xerox) Ethernet II Standard
1985 IEEE 802.3 LAN Standard (10 Mbps)
1995 Fast Ethernet (100 Mbps)
1998 Gigabit Ethernet
2002 10 Gigabit Ethernet
Ethernet is the dominant LAN standard
Metcalf’s Sketch
Ethernet MAC: CSMA/CD (802.3)
34
802.3 MAC of Ethernet (CSMA/CD)

CSMA/CD:
1. If the medium is idle, transmit; otherwise, go to step 2
2. If the medium is busy, continue listening (CS: carrier
sensing) until the channel is idle, then transmit
immediately
3. If a collision is detected (CD) during transmission,
transmit brief jamming signal to assure all stations
know about collision and then cease transmission
4. After transmitting the jamming signal, wait a random
time (back-off time), then attempt to transmit again
35
Typical MAC Efficiencies
Two-Station Example:

1
Efficiency 
1  2a

CSMA-CD (Ethernet) protocol:
1
Efficiency 
1  6.44a
If a<<1, then
efficiency close to
100%
As a approaches
1, the efficiency
becomes low
MAC protocol selection
criteria summarized








Delay-bandwidth product
Efficiency
Transfer delay
Fairness
Reliability
Capability to carry different types of traffic
Quality of service
Cost
802.3 Ethernet Standards: Link & Physical Layers
 many different Ethernet standards
 common MAC protocol and frame format
 different speeds: 2 Mbps, 10 Mbps, 100 Mbps,
1Gbps, 10G bps
 different physical layer media: fiber, cable
MAC protocol
and frame format
application
transport
network
link
physical
100BASE-TX
100BASE-T2
100BASE-FX
100BASE-T4
100BASE-SX
100BASE-BX
copper (twister
pair) physical layer
Ref: Kurose, Ross: Computer Networking
fiber physical layer
5: DataLink Layer
5-39
Throughput Performance of CSMA/CD
a  t prop R / L

   Reff / R  Reff L / RF
a : delay-bandwidth product
ρ:efficiency
Reff : aggregated bitrate
R F : frame rate
R: unloaded nominal bitrate
L: frame lenght (in bits)
 (Load)
We can see that in Ethernet transfer delays grow very fast as the
load approaches the maximum possible value for the given value of a
(tprop: one-way delay, R: signaling rate, L: frame length)
Reference: A. Leon-Garcia, I. Widjaja,
Communication Networks, 2nd ed
42
IEEE 802.3 MAC: Ethernet
MAC Protocol:
 CSMA/CD
 Slot (frame) duration is the critical system parameter






determines delay – throughput tradeoff
upper bound on time to detect collision
upper bound on time to acquire channel
upper bound on length of frame segment generated by
collision
effects retransmission scheduling
Truncated binary exponential back-off


for retransmission n: 0 < r < 2k, where k=min(n,10)
Give up after 16 retransmissions
IEEE 802.3 Original Parameters





Transmission Rate: 10 Mbps
Min Frame: 512 bits = 64 bytes
min. frame duration*: 512 bits/10 Mbps = 51.2 msec
 51.2 msec x 2x105 km/sec =10.24 km, 1 way
(= mini slot length)
Max Length: 2500 meters + 4 repeaters
Each x10 increase in bit rate, must be accompanied
by x10 decrease in distance (norm. bandwidth-delay
product remains thus constant)
t prop R 
a
L  a  d R

v L
d 
t prop 
v 
*mini slot
product must remain constant!
45
Typical Ethernet Deployment
Server farm
Server
Server
Server
Gigabit Ethernet links
Switch/router
Server
Ethernet
switch
100 Mbps links
Hub
10 Mbps links
Department A
Gigabit Ethernet links
Ethernet
switch
100 Mbps links
Server
Hub
10 Mbps links
Department B
Switch/router
Ethernet
switch
100 Mbps links
Server
Hub
10 Mbps links
Department C
S-72.1130 Telecommunication
Systems
IEEE 802.11 Wireless Local
Area Networks (WLANs)
Why WLANs?




Mobility
 Increases working efficiency and productivity
 Roaming support: extended on-line times
-> universal access & seamless services
No new wiring and installation on difficult-to-wire areas
 Offices, public places, and homes
 Factories, vehicles, roads, and railroads
Increased reliability - several networks & nodes secure links
 However, AAA (Authentication, Authorization, Accounting)
challenging
Reduced installation time
 No cabling time
 Easy setup
56
WLAN Technology Challenges




High data rates
 IEEE 802.11b supports rates up to 11 Mb/s (in practice 6
Mb/s), and 802.11g reaches up to 54 Mb/s, 802.11n ~ 100200 Mb/s (600 Mb/s theoretical rate)
Interference
 Working in ISM band means sharing the frequency bands
with microwave ovens, and Bluetooth. Modulation and MAC
design challenge
Security
 Original WEP (Wired Equivalent Privacy) algorithm is weak –
often not set ON by users, more efficient algorithms
developed later as WPA 2 (Wifi Protected Access)
Roaming, especially with GSM and UMTS would be desired
57
Requirements for
802.11 Wireless LAN Standard



Dynamic network management
 Stations movable and may be operated while moved
 addressing and association procedures
 interconnections (roaming)
License free operation
Wireless channel is unreliable
 error control
 security/secrecy
 Wireless channel is also the reason why access method
for 802.11 is CSMA/CA and not CSMA/CD
 Difficult to detect collisions in wireless environment
 External interference, especially at ISM
 Hidden terminal problem
CSMA/CA: Carrier Sense Multiple Access/Collision Avoidance
CSMA/CD: Carrier Sense Multiple Access/Collision Detection
58
802.11 WLAN Architecture Overview



LLC provides addressing and data link control
– common to all 802 LANs
IEEE 802.2
LLC
Logical Link Control (LLC)
802.11 MAC provides
b: Wi-Fi

Access to wireless medium
IEEE 802.3 IEEE 802.4 IEEE 802.5
 CSMA/CA (DCF)
IEEE 802.11
Carrier
MAC
Token
Token
Wireless
 Contention-free access (PCF)
Sense
Bus
Ring
abg

Joining the network (NAV, addressing) Ethernet

Services
Physical layers: DSSS, FHSS, IR ...
PHY
 Station service: Authentication,
privacy, MSDU* delivery
CSMA/CA: Carrier Sense Multiple Access
 Distributed system: Association**,
with Collision Avoidance
participates to data distribution
LLC: Logical Link Control Layer
MAC: Medium Access Control Layer
Three physical layers (PHY)
SS: Spread Spectrum

FHSS: Frequency Hopping Spread
FHSS: Frequency hopping SS
DSSS: Direct sequence SS
Spectrum (SS)
IR: Infrared light

DSSS: Direct Sequence SS
NAV: Network Allocation Vector
SAP: Service Access Point

IR: Infrared transmission
*MSDU: MAC service data unit
** with an access point in ESS or BSS
DCF: Distributed Coordination Function
PCF: Point Coordination Function
59
S-72.1130 Telecommunication
Systems
IEEE 802.11 Wireless Local
Area Networks (WLANs): Service Sets


802.11 networks can work in
 Basic service set (BSS)
 Extended service set (ESS)
BSS can also be used in ad-hoc
networking
Network
LLC
MAC
FHSS DSSS IR
Propagation
boundary
LLC: Logical Link Control Layer
MAC: Medium Access Control Layer
PHY: Physical Layer
FHSS: Frequency hopping SS
DSSS: Direct sequence SS
SS: Spread spectrum
IR: Infrared light
BSS: Basic Service Set
ESS: Extended Service Set
PHY
802.xx
IEEE 802.11 Architecture
Internet
Distribution
system
Station B
Station A
BSS 1
Basic (independent)
service set (BSS)
Access Point
BSS 2
Extended service set (ESS)
Portal: gateway access to other networks/Internet
61
Basic and Extended Service Sets

Basic Service Set (BSS) – tens of meters
Operate in Basic Service Area (BSA) that is much like the
area of cell in mobile communications
 BSSs may geographically overlap, be physically disjoint, or
they may be collocated (one BSS may use several
antennas)
 Ad-hoc or Infrastructure (nomadic) mode: Access
coordinated by the given instance of MAC
Extended Service Set (ESS)
 Multiple BSSs interconnected by Distribution System (DS)
 Each BSS is like a cell and stations in BSS communicate
with an Access Point (AP).
 Portals attached to DS provide gateways to access
Internet or other ESS


62
Distribution system (DS) services


DS provides distribution services:

Transfer MAC SDUs between APs in ESS (I)

Transfer MSDUs between portals & BSSs in ESS (II)

Transfer MSDUs between stations in same BSS (III)

Multicast, broadcast, or stations’s preference
ESS looks like a single BSS to LLC layer
Propagation
boundary
Internet
II
III
LLC: Logical Link Control Layer
MAC: Medium Access Control Layer
PHY: Physical Layer
FHSS: Frequency hopping SS
DSSS: Direct sequence SS
SS: Spread spectrum
IR: Infrared light
BSS: Basic Service Set
ESS: Extended Service Set
MSDU: MAC Service Data Unit
AP: Access Point
III
Distribution
system
Station B
IIIb
Station A
BSS 1
Basic (independent)
service set (BSS)
Access Point
I
BSS 2
Extended service set (ESS)
(Infrastructure mode)
Portal: gateway access to other networks/Internet
63
Infrastructure Network (& ESS)
Portal
Distribution System
Server
Gateway to
Portal the Internet
AP1
AP2
A1
B1
B2
A2
BSS A
BSS B
IEEE 802.11 Mobility


Standard defines the following mobility types:

No-transition: no movement or moving within a local BSS

BSS-transition: station movies from one BSS in one ESS to another
BSS within the same ESS

ESS-transition: station moves from a BSS in one ESS to a BSS in a
different ESS (continuos roaming not supported)
Especially: 802.11 don’t support roaming with GSM or 3G!
- Address to destination
mapping
- seamless integration
of multiple BSS
ESS 1
ESS 2
68
S-72.1130 Telecommunication
Systems
IEEE 802.11 Wireless Local
Area Networks (WLANs): Media
Access Protocol
Hidden Terminal Problem
(a)
C
A
Data Frame
A transmits data frame
B
(b)
Data Frame
B
A

New MAC: CSMA with Collision Avoidance
Reference: A. Leon-Garcia, I. Widjaja, Communication
Networks , Instructor's Slide Set
C senses medium,
station A is hidden from C
Data Frame
C
C transmits data frame
& collides with A at B
RTS: Request to Send
CTS: Clear to Send
70
CSMA with Collision Avoidance
(a)
B
RTS
C
A requests to send
(b)
CTS
B
CTS
A
C
B announces A ok to send
(c)
Data Frame
A sends
Reference: A. Leon-Garcia, I. Widjaja, Communication
Networks , Instructor's Slide Set
B
C remains quiet
RTS: Request to Send
CTS: Clear to Send
71
IEEE 802.11 Coordination Functions
Reference: W.
Stallings: Data
and Computer
Communications,
7th ed
72
Media Access Control in 802.11 WLANs


Distributed Wireless Foundation MAC (DWFMAC):
 Distributed access control mechanism (CSMA/CA)
 Optional centralized control on top (PCF)
MAC flavours provided by coordination functions:
 Distributed coordination function (DCF) - CSMA
 Contention algorithm to provide access to all traffic
 Asynchronous, best effort-type traffic
 Application: bursty traffic, add-hoc networks
 Point coordination function (PCF) – polling principle
 Centralized MAC algorithm
 Connection oriented
 Contention free
 Built on top of DCF
 Application: timing sensitive, high-priority data
73
IEEE 802.11 MAC (DWFMAC):
Timing in Basic Access
duration depends
on MAC load type
duration depends
on network condition+random
MAC frame: Control,
management , data + headers
(size depends on frame load and type)
Reference: W.
Stallings: Data
and Computer
Communications,
7th ed
PCF: Point Coordination Function (asynchronous, connectionless access)
DCF: Distributed Coordination Function (connection oriented access)
DIFS: DCF Inter Frame Space (minimum delay for asynchronous frame
access)
PIFS: PCF Inter Frame Space (minimum poll timing interval)
SIFS: Short IFS (minimum timing for high priority frame access as ACK, CTS,
MSDU…)
MSDU: MAC Service Data Unit
74
IEEE 802.11
MAC Logic
(DWFMAC)
IFS: Inter Frame Space (= DIFS, SIFS, or
PIFS)
DWFMAC: Distributed Wireless Foundation
MAC
Reference: W. Stallings: Data
and Computer Communications,
7th ed
75
Collisions, Losses & Errors


Collision Avoidance
 When station senses channel busy, it waits until
channel becomes idle for DIFS period & then
begins random back-off time (in units of idle slots)
 Station transmits frame when back-off timer
expires
 If collision occurs, recompute back-off over
interval
Receiving stations of error-free frames send ACK
 Sending station interprets non-arrival of ACK as
loss
 Executes back-off and then retransmits
 Receiving stations use sequence numbers to
identify duplicate frames
76
Carrier Sensing in 802.11 MAC

Physical Carrier Sensing
Analyze all detected frames
 Monitor relative signal strength from other sources
 Carrier sense threshold effects throughput! *
Virtual Carrier Sensing at MAC sublayer
 Source stations informs other stations of transmission
time (in msec) for an MPDU
 Carried in Duration field of RTS & CTS
 Stations adjust Network Allocation Vector (NAV) to
indicate when channel will become idle
Channel busy if either sensing is busy



*http://www.crhc.illinois.edu/wireless/papers/carrier-tech.pdf
Reference: A. Leon-Garcia, I. Widjaja, Communication
Networks , Instructor's Slide Set
77
Transmission of MPDU without RTS/CTS
DIFS
NAV: Network allocation vector
DIFS: DCF Inter Frame Space (async)
SIFS: SIFS: Short IFS (ack, CTS…)
RTS: Request to send
CTS: Clear to send
MPDU: MAC Protocol Data Unit
DCF: Distributed Coordination Function
PCF: Point Coordination Function
Data
Source
SIFS
ACK
Destination
DIFS
Other
NAV
Defer Access
Reference: A. Leon-Garcia, I. Widjaja, Communication
Networks , Instructor's Slide Set
Wait for
Reattempt Time
78
Transmission of MPDU
with RTS/CTS
NAV: Network allocation vector
DIFS: DCF Inter Frame Space (async)
SIFS: SIFS: Short IFS (ack, CTS…)
RTS: Request to send
CTS: Clear to send
MPDU: MAC Protocol Data Unit
DCF: Distributed Coordination Function
PCF: Point Coordination Function
DIFS
RTS
Data
Source
SIFS
CTS
SIFS
SIFS
Ack
Destination
DIFS
NAV (RTS)
Other
NAV (CTS)
NAV (Data)
Reference: A. Leon-Garcia, I. Widjaja,
Communication Networks , Instructor's Slide Set
Defer access
RTS: Request to Send
CTS: Clear to Send
79
PCF Frame Transfer
Fixed super-frame interval
TBTT
Contention-free (CF) repetition interval
SIFS
B
PIFS
SIFS
SIFS
SIFS SIFS
D2+Ac
k+Poll
D1 +
Poll
CF
Contention period (DCF)
End
U2+
ACK
U1+
ACK
Reset NAV
NAV
CF_Max_duration
D1, D2 = frame sent by point coordinator
U1, U2 = frame sent by polled station
TBTT = target beacon transmission time
B = beacon frame
NAV: Network allocation vector
DIFS: DCF Inter Frame Space (async)
SIFS: SIFS: Short IFS (ack, CTS…)
RTS: Request to send
CTS: Clear to send
MPDU: MAC Protocol Data Unit
DCF: Distributed Coordination Function
PCF: Point Coordination Function
CF: Contention-Free
80
Point Coordination Function





PCF provides connection-oriented, contention-free service
through polling
Point coordinator (PC) in AP performs PCF
Polling table up to implementor
Contention free period (CFP) repetition interval
 Determines frequency with which CFP occurs
 Initiated by beacon frame transmitted by Point
Coordinator (PC) in AP
 During CFP stations may only transmit to respond to a
poll from PC or to send ACK
All stations adjust Network Allocation Vector (NAV) to
indicate when channel will becomes idle
Reference: A. Leon-Garcia, I. Widjaja, Communication
Networks , Instructor's Slide Set
81
MAC Frame Functions



Management frames
 Station association & disassociation with AP (this
establishes formally BSS)
 Timing & synchronization
 Authentication & deauthentication (option for
identifying other stations)
Control frames
 Handshaking
 ACKs during data transfer
Data frames
 Data transfer
Reference: A. Leon-Garcia, I. Widjaja, Communication
Networks , Instructor's Slide Set
82
S-72.1130 Telecommunication
Systems
IEEE 802.11 Wireless Local
Area Networks (WLANs):
Physical Level
802.11 WLAN bands and technologies


IEEE 802.11 standards and rates

IEEE 802.11 (1997) 1 Mbps and 2 Mbps (2.4 GHz band ) [FH, DS]

IEEE 802.11b (1999) 11 Mbps (2.4 GHz band) = Wi-Fi [QPSK]

IEEE 802.11a (1999) 6, 9, 12, 18, 24, 36, 48, 54 Mbps (5 GHz
band) [OFDM]

IEEE 802.11g (2001 ... 2003) up to 54 Mbps (2.4 GHz) backward
compatible to 802.11b [OFDM]
IEEE 802.11 networks work on license free Industrial, Science,
Medicine (ISM) bands:
26 MHz
902
EIRP power
in Finland
928
83.5 MHz
2400
2484
100 mW
200 MHz
5150
5350
255 MHz
5470
200 mW
indoors only
5725 f/MHz
1W
EIRP: Effective Isotropically Radiated Power - radiated power measured immediately after antenna
Equipment technical requirements for radio frequency usage defined in ETS 300 328
85
Physical Level
of 802.11: DSSS
DSSS-transmitter





802.11 supports 1 and 2 Mbps data transport, uses BPSK and QPSK modulation
(802.11b,a,g apply higher rates)
802.11 applies 11 chips Barker code for spreading - 10.4 dB processing gain
Defines 14 overlapping channels, each having 22 MHz channel bandwidth, from
2.401 to 2.483 GHz
Power limits 1000mW in US, 100mW in EU, 200mW in Japan
Immune to narrow-band interference, cheaper hardware
PPDU:Baseband Data Frame Unit, BPSK: Binary Phase Shift Keying, QPSK: Quadrature PSK
DSSS: Direct Sequence Spread Spectrum, PN:Pseudo Noise
86
Physical Level of 802.11: FHSS






Supports 1 and 2 Mbps data transport and applies two level - GFSK
modulation* (Gaussian Frequency Shift Keying)
79 channels from 2.402 to 2.480 GHz ( in U.S. and most of EU
countries) with 1 MHz channel space
78 hopping sequences with minimum 6 MHz hopping space, each
sequence uses every 79 frequency elements once
Minimum hopping rate
2.5 hops/second
Tolerance to multi-path,
narrow band interference,
security
Low speed, small range
due to FCC TX power
regulation (10mW)
* f  f c  f , f nom  160 kHz
87
26 MHz
902
Example: PHY of 802.11a




928
83.5 MHz
2400
2484
200 MHz
5150
5350
255 MHz
5470
5725 f/MHz
Operates at 5 GHz band
Supports multi-rate 6 Mbps, 9 Mbps,… up to 54 Mbps
Uses Orthogonal Frequency Division Multiplexing (OFDM) with 52
subcarriers, 4 us symbols (0.8 us guard interval)
Applies inverse discrete Fourier transform (IFFT) to combine multicarrier signals to single time domain symbol
88
802.3 Ethernet PHY
 10 Mb DIX Ethernet uses baseband transmission, that is,
the adapter sends a digital signal directly into the broadcast
channel.
 The interface card does not shift the signal into another
frequency band, as do ADSL and cable modem systems. DIX
Ethernet (10 Mb/s) uses Manchester encoding (next slide)
 With Manchester encoding each bit contains a transition; a 1
has a transition from up to down, whereas a zero has a
transition from down to up.
 The reason for Manchester encoding is that the clocks in
the sending and receiving adapters are not perfectly
synchronized. By including a transition in the middle of each
bit, the receiving host can synchronize its clock to that of
the sending host.
Manchester encoding
 used in 10BaseT (DIX Ethernet)
 each bit has a transition
 allows clocks in sending and receiving nodes to
synchronize to each other

no need for a centralized, global clock among nodes!
Ref: Kurose, Ross: Computer Networking
5: DataLink Layer
5-90
References and Supplementary Material
- A. Leon-Garcia, I. Widjaja: Communication Networks (2nd
ed., 1st ed. also quite ok :)
- W. Stallings: Data and Computer Communications
- Kurose, Ross: Computer Networking
- Jim Geier: Wireless LANs, SAMS publishing
- 802 Standards, IEEE
See especially:
 HDLC: A. Leon-Garcia, I. Widjaja: Communication
Networks, 2th ed.: pp. 333-340
 WLANs: W. Stallings: Data and Computer
Communications, 7th ed, pp. 544-568
91