Wireless & Personal Communication Systems – CSE5807 Lecture: 07 1 Wireless Personal Communications Systems – CSE5807 Lecture: 07 Stephen Giles and Satha.
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1Wireless & Personal Communication Systems – CSE5807Lecture: 07
Wireless Personal Communications Systems – CSE5807
Lecture: 07
Stephen Giles and Satha K. Sathananthan
School of Computer Science and Software Engineering
Monash University
Australia
These slides contain figures from Stallings, and are based on a set developed by Tom Fronckowiak .
2Wireless & Personal Communication Systems – CSE5807Lecture: 07
Locating a WLAN • Service Set Identifier (SSID):
– Unique, case sensitive, alphanumeric value from 2-32 characters long.
– Used as a network name.
– Sent in beacons, probe requests, probe responses and other types of frames.
• Beacons:– To organize and synchronize wireless communications.
• From AP to station in infrastructure mode.
• From station to station in ad hoc mode. – Provide functions including
• Time synchronization
• FH or DS parameters
• SSID information
• Traffic Indication Map
• Supported rates
3Wireless & Personal Communication Systems – CSE5807Lecture: 07
Locating a WLAN • Scanning:
– Passive Scanning• Process of listening for beacons on each channel for a specific
period of time.
• Continuing process even after association.
– Active Scanning• Sending of probe request frame by a wireless station when it seeks
a network to join.
• The probe request frame contains either a particular network’s SSID or broadcast SSID.
4Wireless & Personal Communication Systems – CSE5807Lecture: 07
Authentication • Wireless client’s identity is verified by the network/access
point.
• Access Point => Accept/Deny
• Authentication Methods:– Open System Authentication
• Based on SSID only.
• Option of using WEP for only encrypting data.
– Shared Key Authentication
• Use WEP.
APLAN
Authentication Request Frame
Authentication Response Frame
5Wireless & Personal Communication Systems – CSE5807Lecture: 07
Association • Allowed to pass data through access point => “Associated”.
• Authentication => Association
• Wireless client can authenticate more than one access point at a time but can associate only one access point.
APLAN
Association Request Frame
Association Response Frame
6Wireless & Personal Communication Systems – CSE5807Lecture: 07
Roaming
• Wireless client determines based on the signal strength.
• IEEE802.11 does not define how should be performed.
– But some basic building blocks for this process.
• Active and passive scanning, reassociation process.
– New IEEE802.11f standard for roaming.
– Inter Access Point Protocol (IAPP).
APDisassociation Frame
Reassociation Frame
AP
7Wireless & Personal Communication Systems – CSE5807Lecture: 07
Load Balancing
• Multi-cell structure with co-located access points creating a common coverage area.
– Wireless clients automatically associate with the access point that is less loaded and provides the best signal quality.
A
B
BA
B
AB
8Wireless & Personal Communication Systems – CSE5807Lecture: 07
Adaptive Rate Selection (ARS)
• Speed adjustment with varying distance and interference.
– Switched between specified data rates.
• Important in planning:
– Network throughput
– Cell sizes
– Power outputs of access points and wireless clients
– Security
A
A
11 Mbps
2 Mbps
9Wireless & Personal Communication Systems – CSE5807Lecture: 07
Power Management
• Continuous Aware Mode:
– Uses full power and no sleep mode.
– Wireless client determines based on the signal strength.
• Power Save Polling (PSP):
– Wireless client powers down for a very short amount of time.
– In BSS, traffic indication map (TIM) is used to notify buffered traffic.
– In ad hoc, “Ad hoc traffic indication messages” are used to notify buffered traffic.
10Wireless & Personal Communication Systems – CSE5807Lecture: 07
IEEE802.11 Physical Layer
• Physical Layer Convergence Protocol (PLCP):– Responsible for carrier sensing assessment and forming packets for
different physical layers.
• Physical Medium Dependent (PMD) protocol:– Defines modulation and coding technique for signaling.
• Physical Layer Management:– Decides on channel tuning to different options for each physical layer.
Data Link Layer
Physical Layer
MAC
PLCP
MAC Management
PHY
LLC
PMD Management
11Wireless & Personal Communication Systems – CSE5807Lecture: 07
IEEE802.11 Standards
IEEE802.11 IEEE802.11b IEEE802.11a IEEE802.11g
Frequency 2.4GHz 2.4GHz 5GHz 2.4GHz
Max. Rate 2Mbps 11Mbps 54Mbps 54Mbps
Modulation FHSS
DSSS
DSSS OFDM OFDM
12Wireless & Personal Communication Systems – CSE5807Lecture: 07
IEEE802.11 Physical Layer: FHSS
SYNC (80) SFD (16) PLW (12) PSF (4) CRC (16) Whitened MPDU (<4096 Bytes)
PLCP (always 1Mbps) 1 or 2 Mbps)
Preamble Header
MPDU: MAC Protocol Data Unit
SYNC: Alternating 0 and 1
SFD: Start of Frame Delimiter – specific pattern of 16 bits (0000110010111101)
PLW: Packet Length Width
PSF: Packet Signaling Field
CRC: Cyclic Redundancy Check – to protect the PLCP bits
13Wireless & Personal Communication Systems – CSE5807Lecture: 07
IEEE802.11 Physical Layer: FHSS
SYNC (80) SFD (16) PLW (12) PSF (4) CRC (16) Whitened MPDU (<4096 Bytes)
PLCP (always 1Mbps) 1 or 2 Mbps)
Preamble Header
• FHSS PMD hops over 78 channels of 1 MHz in the center of the 2.44 GHz ISM bands.
• Modulation: Gaussian Frequency Shift Keying (GFSK)
– 1Mbps – Two levels of GFSK
– 2Mbps – Four levels of GFSK
• Three patterns of 26 hops => Selection by PHY Management layer.
– 0, 3, 6, 9,……75
– 1, 4, 7, 10, …..76
– 2, 5, 8, 11, …..77
• Minimum hop rate 2.5 hops per second.
• Maximum transmitted power is 100mW.
14Wireless & Personal Communication Systems – CSE5807Lecture: 07
IEEE802.11 Physical Layer: DSSS
MPDU: MAC Protocol Data Unit
SYNC: Alternating 0 and 1
SFD: Start of Frame Delimiter – specific pattern of 16 bits (1111001110100000)
Signal: Data rate
Service : Reserved for future use
Length: Length of MPDU in microsecond
FCS: PLCP header coding
SYNC (128) SFD (16) Signal (8) Service (8) Length (16) FCS (8) MPDU
PLCP (always 1Mbps) 1 or 2 Mbps)
Preamble Header
15Wireless & Personal Communication Systems – CSE5807Lecture: 07
IEEE802.11 Physical Layer: DSSS
• Barker code of length 11.
– Uses non-overlapping pulses at chip rate of 11Mcps occupying 26 MHz.
• Modulation:
– 1Mbps => DBPSK
– 2Mbps => DQPSK
• ISM band at 2.4 GHz divided into 11 overlapping channels spaced by 5 MHz.
• Maximum transmit power is 100mW.
SYNC (128) SFD (16) Signal (8) Service (8) Length (16) FCS (8) MPDU
PLCP (always 1Mbps) 1 or 2 Mbps)
Preamble Header
16Wireless & Personal Communication Systems – CSE5807Lecture: 07
IEEE802.11b Physical Layer• Defines a new coding, Complementary Code Keying (CCK)
to support data rates of 5.5 Mbps and 11Mbps.
– 1Mbps => Barker Code and DBPSK
– 2Mbps => Barker Code and DQPSK
– 5.5Mbps => CCK and DQPSK
– 11 Mbps => CCK and DQPSK
• Uses the same PLCP as the IEEE802.11 DSSS standard.
– Interoperates with IEEE802.11 networks.
17Wireless & Personal Communication Systems – CSE5807Lecture: 07
IEEE802.11a Physical Layer• Based on OFDM scheme.
• Operates at 5 GHz UNII bands.
– Eight non-overlapping channels of 20 MHz at the two lower bands of the 5 GHz UNII band.
– Each channel is divided into 52 subcarreirs, each approximately 300 kHz.
– Data is transmitted in parallel on each subcarrier.
• Forward Error Correction (FEC) codes are used to correct errors.
• Data rates: 6, 9, 12, 18, 24, 36, 48 and 54
• Modulation: BPSK, QPSK, 16-QAM and 64-QAM
18Wireless & Personal Communication Systems – CSE5807Lecture: 07
IEEE802.11g Physical Layer• Based on OFDM scheme.
• Operates at 2.4 GHz ISM bands.
– Backward compatibility with IEEE802.11b.
– Switch automatically to CCK/Other modulations.
• Data rates: 1, 2, 5.5, 6, 9, 11, 12, 18, 22, 24, 33, 36, 48 and 54 Mbps.
• Use optional CCK-OFDM.
19Wireless & Personal Communication Systems – CSE5807Lecture: 07
IEEE802.11a and HiperLAN-2
IEEE802.11a HiperLAN-2
Frequency 5 GHz 5 GHz
Max. trans. Rate 54 Mbps 54 Mbps
Medium Access Through sensing Centralized scheduling
QoS support PCF ATM/802.1p/RSVP
Wired backbone Ethernet Ethernet/ATM/ UMTS/PPP/IP
Connectivity Connectionless Connection-oriented
20Wireless & Personal Communication Systems – CSE5807Lecture: 07
Wireless LAN: Deployment• Requirements:
- Facility (Building plan).- Applications.- Users.- End user devices.- Battery longevity.- Coverage areas.- Security.
• Design:- System Architecture.- Identifying standards.- Selecting devices.
• Installation and Testing.
21Wireless & Personal Communication Systems – CSE5807Lecture: 07
Wireless LAN: Deployment
• RF interference (from other devices).
• Interoperability issues (eg.: IEEE 802.11 a & IEEE 802.b).
• Security holes.
• Application interfaces/requirements.
• Unclear requirements.
22Wireless & Personal Communication Systems – CSE5807Lecture: 07
Wireless LAN: Design
• Technical Considerations:• Adequate radio coverage throughout the service area.
• Adequate capacity to handle traffic load.
• Network performance.
• Main design steps:• Selection of AP locations.
• Assignment of radio frequencies to APs.
23Wireless & Personal Communication Systems – CSE5807Lecture: 07
Wireless LAN: Design• Radio propagation is mostly unpredictable. • Design is iterative process. • Steps involved:
1. Initial selection of AP locations.2. Test and redesign.
- Adjusting the AP locations based on signal strength measurements.
3. Creation of coverage map.4. Assignment of frequencies (or channels) to APs.5. Signal strength measurements and minimizing
co-channel coverage overlap.
24Wireless & Personal Communication Systems – CSE5807Lecture: 07
WLAN Design: Access Points
• Based on measurements.
• Layout and construction of buildings determine the coverage area of each AP.
• Must avoid coverage gaps.
• Space APs as far apart as possible to minimize:- the cost of equipment and installations.- the co-channel overlap.
25Wireless & Personal Communication Systems – CSE5807Lecture: 07
WLAN Design: Access Points
R
D
26Wireless & Personal Communication Systems – CSE5807Lecture: 07
WLAN Design: Access Points
R
D
27Wireless & Personal Communication Systems – CSE5807Lecture: 07
WLAN Design: Channel Allocations
• Once APs are located and their coverage areas are identified, radio channels are assigned to the APs.
• Radio channels are assigned in a way that provides the smallest possible co-channel overlap.
• In high-density areas:- Multiple radio channels.- Reducing the coverage areas of each APs.
• Coverage-oriented design: In low density areas, minimizing the number of APs.
• Capacity-oriented design: In high-density areas, assuring adequate capacity to serve all users.
28Wireless & Personal Communication Systems – CSE5807Lecture: 07
Wireless Metropolitan Area Networks
29Wireless & Personal Communication Systems – CSE5807Lecture: 07
Wireless Metropolitan Area Networks
• Defined in IEEE802.16 standard.
• Use licensed spectrum in 10 GHz – 66 GHz.
• Provide public network service to fee-paying customers.
• Use point-to-multipoint architecture with stationary rooftop or tower-mounted antennas.
• Provide efficient transport of heterogeneous traffic supporting quality of service (QoS).
• Are capable of broadband transmissions.
30Wireless & Personal Communication Systems – CSE5807Lecture: 07
IEEE 802.16 Protocol Architecture
31Wireless & Personal Communication Systems – CSE5807Lecture: 07
IEEE 802.16a• Support to mesh network topology.
• Line of sight is not required.
• Also operates at frequencies between 2 and 11 GHz.– Dynamic Frequency Selection (DFS) to avoid interference with WLAN.
• Further MAC and QoS support.
• Three radio technologies:– Single carrier modulation format.
– OFDM
– OFDMA
• Centralized and distributed MAC mechanism.
32Wireless & Personal Communication Systems – CSE5807Lecture: 07
Required Reading
• W. Stallings, “Wireless Communications and Networks” Prentice-Hall, 2000.
>> Chapter 13 & 14
Reference
• K. Pahlavan and K. Krishnamurthy “Principles of Wireless Networks”, Prentice-Hall, 2002.
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