S-72.1130 Telecommunication Systems

1
S-72.1130 TelecommunicationSystems

• Wireless Local Area Networks

2
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

3
S-72.1130 TelecommunicationSystems

• LAN Basics

4
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

5
Typical Wired LAN

• Transmission Medium
• Network Interface Card (NIC)
• Unique MAC physical address

Serial format
Ethernet Processor
ROM
NIC implements MAC protocol physical port.
Parallel interface to PC
Reference A. Leon-Garcia, I. Widjaja,
Communication Networks , Instructor's Slide Set
6
IEEE 802-series of LAN Standards

• 802 standards free to download from
  http//standards.ieee.org/getieee802

WiMAX
Demand priority A round-robin (see token
rings-later) arbitration method to provide LAN
access based on message priority level
DQDB Distributed queue dual buss, see PSTN
lecture
7
Example How Ring Networks Work

• A node functions as a repeater
• only destination copies frame to it, all other
  nodes have to discarded the frame
• Unidirectional link
• Signal propagatesencoded by line codes
• Example 802.5
• Problem in reliability ifa station fails

A
A
B
C
B
C
B transmits frame addressed to A
C ignores frame
A
A
B
C
B
C
A copies frame
C absorbs returning frame
8
Token Ring

• A ring consists of a single or dual (FDDI) cable
  in the shape of a loop. Ring reservation
  supervised by rotating token.
• Each station is only connected to each of its two
  nearest neighbors. Data in the form of packets
  passes around the ring from one station to
  another in uni-directional way.
• Advantages
• (1) Access method supports heavy load without
  degradation of performance because the medium is
  not shared.
• (2) Several packets can simultaneous circulate
  between different pairs of stations.
• Disadvantages
• (1) Complex management - especially for several
  rings
• (2) Re-initialization of the ring whenever a
  failure occurs

9
Example Bus Network

• In a bus network, one nodes 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
• (2) Easy to add or remove stations
• Disadvantages
• (1) Poor efficiency with high network load
• (2) Relatively insecure, due to the shared
  medium

C
D
A
B
D
term
term
- Line coded, serial data - twisted pair or
coaxial cable
term terminator impedance
10
S-72.1130 TelecommunicationSystems

• IEEE 802 LAN Standard

11
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)
IEEE 802.11 Wireless
IEEE 802.3 Carrier Sense
IEEE 802.4 Token Bus
IEEE 802.5 Token Ring
b Wi-Fi
MAC
a
b
g
Ethernet
Physical Layers - options twisted pair,
coaxial, optical, radio paths (not for all MACs
above!)
OSI Layer 1 (physical)
Bus (802.3)
Star (802.3u)
Ring (802.5)
12
IEEE 802 Layers

• Logical Link Control (LLC) Sublayer
• Utilizes services of HDLC (High-level Data Link
  Control)
• Therefore, LLC SAPs separate upper layer data
  exchanges gt NIC applies different buffer
  segments for each SAP (port)
• LLC provides means to exchange frames between
  LANs using different MACs
• 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

IEEE 802.2 Logical Link Control (LLC)
LLC
b Wi-Fi
IEEE 802.11 Wireless
IEEE 802.3 Carrier Sense
IEEE 802.4 Token Bus
IEEE 802.5 Token Ring
MAC
Ethernet
a
b
g
Physical layers
PHY

• Physical 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)
13
S-72.1130 TelecommunicationSystems

• IEEE 802 LAN Standard Logical Link Layer (LLC)

14
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
LLCs Protocol Data Unit (PDU) (SAP Service
Access Point)
15
LLC Protocol Data Unit (PDU)
1 to 2 bytes
1 byte
1 byte
Information (network layer packet)
Destination SAP Address
Control
Source SAP Address
Source SAP Address
Destination SAP Address
C/R
I/G
7 bits
1
7 bits
1
I/G Individual or group address C/R Command
or response frame
Examples of SAP Addresses 06 IP packet E0
Novell IPX FE OSI packet AA Sub Network
Access protocol (SNAP)
IP Packet
IP Packet
LLC header
IP Packet
LLC header
MAC header
FCS
Packet encapsulation into a MAC frame
FCS Frame Check Sequence
16
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
• Unnumbered frame mode of HDLC
• 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

17
A TCP/IP Packet in 802.11Encapsulation
TCP makes logical connection to deliver the
packet
Control header
LLC constructs PDU by adding a control header
SAP (service access point)
MAC lines up packets using by using a MAC protocol
MAC frame withnew control fields
PHY layer transmits packet using a modulation
method (DSSS, OFDM, IR, FHSS)
Traffic to the target BSS / ESS
BDU protocol data unit
18
Encapsulation
Reference W. Stallings Data and Computer
Communications, 7th ed
19
SAP Addressing
IEE802.11 (CSMA/CA)...
ATM...
IEE802.11 (CDMA)...
Reference W. Stallings Data and Computer
Communications, 7th ed
20
S-72.1130 TelecommunicationSystems

• IEEE 802 LAN Standard Media Access Control
  (MAC) Layer

21
Media Access Control Ways to Share a Medium

• Medium sharing required for multiple users to
  access the channel
• Communications by
• unicasting
• multicasting
• broadcasting

Medium sharing techniques
Static channelization
Dynamic medium access control

• FDMA,TDMA, CDMA
• Uses partition medium
• Dedicated allocation to users
• Examples
• Satellite transmission
• Cellular Telephone

Scheduling
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)

22
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)

23
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
  of FDMA (uplink/dowlink)

24
Example 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

25
Throughput Performance of CSMA/CD
r (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
26
S-72.1130 TelecommunicationSystems

• IEEE 802.11 Wireless Local Area Networks (WLANs)

27
Why WLANs?

• Mobility
• Increases working efficiency and productivity
• Roaming support extended on-line times -gt
  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

28
WLAN Technology Challenges

• High date rates
• IEEE 802.11b supports rates up to 11 MBps (in
  practice 6 Mb/s), and 802.11g reaches up to 54
  Mb/s, need to have the bandwidth
• Interference
• Working in ISM band means sharing the frequency
  bands with microwave oven, 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
• Roaming, especially with GSM and UMTS
• Inter-operability between different vendors
• Only few basic functionality are interoperable,
  other vendors features cant be used in a mixed
  network

29
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
30
802.11 WLAN Architecture Overview

• LLC provides addressing and data link control
  common to all 802 LANs
• 802.11 MAC provides
• Access to wireless medium
• CSMA/CA (DCF)
• Contention-free access (PCF)
• Joining the network (NAV, addressing)
• Services
• Station service Authentication, privacy, MSDU
  delivery
• Distributed system Association, participates
  to data distribution
• Three physical layers (PHY)
• FHSS Frequency Hopping Spread
  Spectrum (SS)
• DSSS Direct Sequence SS
• IR Infrared transmission

IEEE 802.2 Logical Link Control (LLC)
LLC
b Wi-Fi
IEEE 802.11 Wireless
IEEE 802.3 Carrier Sense
IEEE 802.4 Token Bus
IEEE 802.5 Token Ring
MAC
Ethernet
a
b
g
Physical layers DSSS, FHSS, IR ...
PHY
CSMA/CA Carrier Sense Multiple Accesswith
Collision Avoidance LLC Logical Link Control
Layer MAC Medium Access Control Layer SS Spread
Spectrum FHSS Frequency hopping SS DSSS Direct
sequence SS IR Infrared light NAV Network
Allocation Vector SAP Service Access Point DCF
Distributed Coordination Function PCF Point
Coordination Function
MSDU MAC service data unit with an access
point in ESS or BSS
31
S-72.1130 TelecommunicationSystems

• IEEE 802.11 Wireless Local Area Networks
  (WLANs) Service Sets

32
IEEE 802.11 Architecture

• 802.11 networks can work in
• Basic service set (BSS)
• Extended service set (ESS)
• BSS can also be used in ad-hocnetworking

Network
LLC
802.xx
MAC
PHY
FHSS
DSSS
IR
Propagation boundary
Internet
Distribution system
Station B
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
Station A
BSS 1
BSS 2
Basic (independent) service set (BSS)
Extended service set (ESS)
Access Point
Portal gateway access to other networks/Internet
33
Basic and Extended Service Sets

• Basic Service Set (BSS) tens of meters
• Operate in Basic Service Area (BSA) that is much
  like the are 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 as access
  to Internet or other ESS

34
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 stationss preference
• ESS looks like a single BSS to LLC layer

Propagation boundary
Internet
II
III
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
Distribution system
Station B
IIIb
Station A
I
BSS 1
BSS 2
Basic (independent) service set (BSS)
Extended service set (ESS)
Access Point
Portal gateway access to other networks/Internet
35
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 dont support roaming with GSM!

- Address to destination mapping - seamless
integration of multiple BSS
ESS 1
ESS 2
36
S-72.1130 TelecommunicationSystems

• IEEE 802.11 Wireless Local Area Networks
  (WLANs) Media Access Protocol

37
Hidden Terminal Problem
(a)
Data Frame
A transmits data frame
C senses medium, station A is hidden from C
RTS Request to Send CTS Clear to Send

• New MAC CSMA with Collision Avoidance

Reference A. Leon-Garcia, I. Widjaja,
Communication Networks , Instructor's Slide Set
38
CSMA with Collision Avoidance
(c)
B
C remains quiet
Data Frame
A sends
RTS Request to Send CTS Clear to Send
Reference A. Leon-Garcia, I. Widjaja,
Communication Networks , Instructor's Slide Set
39
IEEE 802.11 Coordination Functions
Reference W. Stallings Data and Computer
Communications, 7th ed
40
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

41
IEEE 802.11 MAC (DWFMAC) Timing in Basic Access
duration depends on MAC load type
duration depends on network condition
MAC frame Control, management , data
headers(size depends on frame load and type)
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
Reference W. Stallings Data and Computer
Communications, 7th ed
42
Collisions, Losses Errors

• Collision Avoidance
• When station senses channel busy, it waits until
  channel becomes idle for DIFS period then
  begins random backoff time (in units of idle
  slots)
• Station transmits frame when backoff timer
  expires
• If collision occurs, recompute backoff over
  interval
• Receiving stations of error-free frames send ACK
• Sending station interprets non-arrival of ACK as
  loss
• Executes backoff and then retransmits
• Receiving stations use sequence numbers to
  identify duplicate frames

43
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
44
Carrier Sensing in 802.11 MAC

• Physical Carrier Sensing
• Analyze all detected frames
• Monitor relative signal strength from other
  sources
• 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

Reference A. Leon-Garcia, I. Widjaja,
Communication Networks , Instructor's Slide Set
45
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
NAV
Other
Wait for Reattempt Time
Defer Access
Reference A. Leon-Garcia, I. Widjaja,
Communication Networks , Instructor's Slide Set
46
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
Data
RTS
Source
SIFS
SIFS
SIFS
CTS
Ack
Destination
DIFS
NAV (RTS)
NAV (CTS)
Other
NAV (Data)
RTS Request to Send CTS Clear to Send
Defer access
Reference A. Leon-Garcia, I. Widjaja,
Communication Networks , Instructor's Slide Set
47
PCF Frame Transfer
Fixed super-frame interval
TBTT
Contention-free (CF) repetition interval
SIFS
SIFS
SIFS
SIFS
SIFS
Contention period (DCF)
CF End
B
PIFS
Reset NAV
NAV
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_Max_duration
D1, D2 frame sent by point coordinator U1, U2
frame sent by polled station TBTT target
beacon transmission time B beacon frame
48
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
49
MAC Frame Types

• 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
50
MAC Frame

• NOTE This frame structure is common for all data
  send by a 802.11 station

control info (WEP, data type as management,
control, data ...)
frame orderinginfo for RX
next frame duration
frame specific,variable length
-Basic service identification-source/destination
address-transmitting station-receiving station
frame check sequence (CRC)
BSSID a six-byte address typical for a
particular access point (network
administrator sets) CRC Cyclic Redundancy
Check WEP Wired Equivalent Privacy
51
S-72.1130 TelecommunicationSystems

• IEEE 802.11 Wireless Local Area Networks
  (WLANs) Physical Level

52
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
83.5 MHz
200 MHz
255 MHz
902 928 2400 2484
5150 5350 5470
5725 f/MHz
200 mW indoors only
EIRP power in Finland
1 W
100 mW
EIRP Effective Isotropically Radiated Power -
radiated power measured immediately after antenna
Equipment technical requirements for radio
frequency usage defined in ETS 300 328
53
Physical Levelof 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

PPDUBaseband Data Frame Unit, BPSK Binary Phase
Shift Keying, QPSK Quadrature PSK DSSS Direct
Sequence Spread Spectrum, PNPseudo Noise
54
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)

55
Example PHY of 802.11a

• 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 multi-carrier signals to single time
  domain symbol

56
References and Supplementary Material

• - A. Leon-Garcia, I. Widjaja Communication
  Networks (2th ed.)
• - W. Stallings Data and Computer
  Communications, 7th ed
• - Kurose, Ross Computer Networking (2th ed.)
• - Jim Geier Wireless LANs, SAMS publishing
• - 802 Standards, IEEE
• Supplementary Material
• 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