FIT3165 / FIT4165 COMPUTER NETWORKS
WEEK 8 – WIRELESS NETWORKS
Faculty of Information Technology © 2022 Monash University
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Wireless Networks
8.1 WIRELESS LANS
8.2 DIFFERENT TYPES OF WIRELESS NETWORKS
Objectives
❑ We introduce wireless LANs, using IEEE project 802.11, the dominant standard.
❑ We also discuss WiMAX technology, which is the counterpart of last-mile wired networks such as DSL or cable.
Wireless LANs
• Wireless communication is one of the fastest-growing technologies.
• The demand for connecting devices without the use of cables is increasing everywhere.
• Wireless LANs can be found on college campuses, in office buildings, and in many public areas.
Architectural Comparison
❖ Isolated LANs
❖ Connection to Other Networks
❖ Moving between Environments
❑ Characteristics ❖ Attenuation
❖ Interference
❖ Multipath Propagation
Wired versus wireless
Isolated LANs
Connection of a wired LAN and a wireless LAN to other networks
Hidden Station Problem
The CSMA/CD algorithm does not work in wireless LANs for three reasons:
1.Wireless hosts do not have enough power to send and receive at the same time.
2.The hidden station problem prevents collision detection
3.The distance between stations can be great.
Hidden-Station Problem
The solution to the hidden station problem is the use of the handshake frames (RTS = Request to send, CTS = Clear to Send).
Wireless Networks – Characteristics and Access Control
● There are several characteristics of wireless LANs that either do not apply to wired LANs or the existence of which is negligible and can be ignored.
● We discuss some of these characteristics here to pave the way for discussing wireless LAN protocols.
● Wired LANs uses a dedicated medium (point-to-point communication). So the MAC protocol, such as
CSMA/CD, is not needed anymore.
● In the case of a wireless LAN, the medium (air) is still shared between the users. We need MAC protocols such as CSMA/CA or channelization protocols to control sharing the medium.
● Maybe the most important issue we need to discuss in a wireless LAN is access control—how a wireless host can get access to the shared medium (air).
● The CSMA/CD algorithm does not work in wireless LANs for three reasons:
○ Wireless hosts do not have enough power to send and receive at the same time.
○ The hidden station problem prevents collision detection
○ The distance between stations can be great.
IEEE 802.11 Project
● IEEE has defined the specifications for a wireless LAN, called IEEE 802.11, which covers the physical and data-link layers.
● In some countries, including the United States, the public uses the term WiFi (short for wireless fidelity) as a synonym for wireless LAN(WLAN).
● WiFi, however, is a wireless LAN that is certified by the WiFi Alliance, a global, nonprofit industry association of more than 300 member companies devoted to promoting the growth of wireless LANs.
IEEE 802.11 Project cont.
Architecture: The standard defines two kinds of services: the basic service set (BSS) and the extended service set (ESS).
Basic Service Set Extended Service Set Station Types
MAC Sublayer
Distributed Coordination Function (DCF) Point Coordination Function (PCF) Fragmentation
Frame Format
Frame Types
Addressing Mechanism Exposed Station Problem Physical Layer
❖ IEEE 802.11 FHSS
❖ IEEE 802.11 DSSS
❖ IEEE 802.11 Infrared
❖ IEEE 802.11a OFDM
❖ IEEE 802.11b DSSS
❖ IEEE 802.11g
❖ IEEE 802.11n
Architecture – BSS and ESS
Basic service sets (BSSs)
Extended service set (ESS)
Wireless Network Components & Architecture
▪ Basic Service Set (BSS)
– Smallest WLAN block
▪ Distribution System (DS) – Connects BSS blocks
▪ Access Points (AP)
– Functions as a bridge or relay
▪ Extended Service Set (ESS)
– Two or more BSS interconnected
MAC layers in IEEE 802.11 standard
MAC Sublayer: IEEE 802.11 defines two MAC sublayers: the distributed coordination function (DCF) and point coordination function (PCF). Figure shows the relationship between the two MAC sublayers, the LLC sublayer, and the physical layer. We discuss the physical layer implementations and MAC sublayer.
Wireless LAN has four frame types:
Shared-Wired LAN, the medium access process is achieved using the CSMA/CD collision detection mechanism. Wireless LAN, the medium access process is achieved using the CSMA/CA collision avoidance mechanism. The same task is done using the RTS, CTS, and ACK frames. We need all of these as well as DATA frames.
Carrier sense multiple access with collision avoidance (CSMA/CA)
Contention Window and Network Allocation Vector (NAV)
Contention window
DIFS = Distributed interframe space RTS = request to send (RTS).
SIFS = short interframe space (SIFS) CTS = Clear to send
NAV = network allocation vector No carrier Sensing RTS = Request to Send
CTS = Clear to Send
ACK = Acknowledgment
CSMA/CA and NAV
Example of repetition interval
PIFS =Point Coordination Function Inter-FrameSpace.
DIFS = Distributed interframe space RTS = request to send (RTS).
SIFS = short interframe space (SIFS) CTS = Clear to send
NAV = network allocation vector No carrier Sensing RTS = Request to Send
CTS = Clear to Send
ACK = Acknowledgment
Frame format
Control Frames
Addressing mechanisms – Case 1
Case 1: 00 In this case, To DS = 0 and From DS = 0.
This means that the frame is not going to a distribution system
(To DS = 0) and is not coming from a distribution system (From DS = 0). The frame is going from one station in a BSS to another without passing through the distribution system.
✔ Address 1 is always the address of the next device that the frame will visit.
✔ Address 2 is always the address of the previous device that the frame has left.
✔ Address 3 is the address of the final destination station if it is not defined by address 1 or the original source station if it is not defined by address 2.
✔ Address 4 is the original source when the distribution system is also wireless.
Addressing mechanisms – Case 2
❑ Case 2: 01 In this case, To DS = 0 and From DS = 1.
This means that the frame is coming from a distribution system (From DS = 1). The frame is coming from an AP and going to a station. Note that address 3 contains the original sender of the frame (in another BSS).
✔ Address 1 is always the address of the next device that the frame will visit.
✔ Address 2 is always the address of the previous device that the frame has left.
✔ Address 3 is the address of the final destination station if it is not defined by address 1 or the original source station if it is not defined by address 2.
✔ Address 4 is the original source when the distribution system is also wireless.
Addressing mechanisms – Case 3
❑ Case 3: 10 In this case, To DS = 1 and From DS = 0. This means that the frame is going to a distribution system (To DS = 1). The frame is going from a station to an AP. The ACK is sent to the original station. Note that address 3 contains the final destination of the frame in the distribution system.
✔ Address 1 is always the address of the next device that the frame will visit.
✔ Address 2 is always the address of the previous device that the frame has left.
✔ Address 3 is the address of the final destination station if it is not defined by address 1 or the original source station if it is not defined by address 2.
✔ Address 4 is the original source when the distribution system is also wireless.
Addressing mechanisms – Case 4
❑ Case 4: 11 In this case, To DS = 1 and From DS = 1. This is the case in which the distribution system is also wireless. The frame is going from one AP to another AP in a wireless distribution system. Here, we need four addresses to define the original sender, the final destination, and two intermediate APs.
✔ Address 1 is always the address of the next device that the frame will visit.
✔ Address 2 is always the address of the previous device that the frame has left.
✔ Address 3 is the address of the final destination station if it is not defined by address 1 or the original source station if it is not defined by address 2.
✔ Address 4 is the original source when the distribution system is also wireless.
Addressing mechanisms – Summary
Physical layer specifications
Physical Layer: We discuss six specifications, as shown in Table. All implementations, except the infrared, operate in the industrial, scientific, and medical (ISM) band, which defines three unlicensed bands in the three ranges 902–928 MHz, 2.400–4.835 GHz, and 5.725–5.850 GHz.
Physical layer of IEEE 802.11 FHSS
Physical layer of IEEE 802.11 DSSS
Physical layer of IEEE 802.11 infrared
IEEE 802.11 Family
• IEEE 802.11b DSSS: describes the high-rate direct-sequence spread spectrum (HRDSSS) method for signal generation in the 2.400–4.835 GHz ISM band. HR-DSSS is similar to DSSS except for the encoding method, DSSS, HR-DSSS defines four data rates: 1, 2, 5.5, and 11 Mbps.
• IEEE 802.11g: This new specification defines forward error correction and OFDM using the 2.400–4.835 GHz ISM band. The modulation technique achieves a 22- or 54-Mbps data rate. It is backward-compatible with 802.11b, but the modulation technique is OFDM.
• IEEE 802.11n: An upgrade to the 802.11 project is called 802.11n (the next generation of wireless LAN). The goal is to increase the throughput of 802.11 wireless LANs. This standard has higher bit rate of 600 Mpbs. The standard uses what is called MIMO (multiple-input multiple-output antenna) to overcome the noise problem in wireless LANs. The idea is that if we can send multiple output signals and receive multiple input signals.
• IEEE 802.11ac: IEEE 802.11ac physical layer is an extension of 802.11n, The theoretical 802.11ac maximum data rate is 6.93 Gb/s using 160-MHz bandwidth, eight spatial streams.
• IEEE 802.11ad: The IEEE 802.11ad standard is known for its 7 Gbps of throughput. The standard operates in 60 GHz bands, differing from 802.11n, which works in both the 2.4 and 5 GHz bands, and 802.11ac, which runs in the 5 GHz band.
• Worldwide Interoperability for Microwave Access (WiMAX) is an IEEE standard 802.16 (for fixed wireless) and 802.16e (for mobile wireless) that aims to provide the “last mile” broadband wireless access alternative to cable modem, telephone DSL service
• People want to have access to the Internet from home or office (fixed) where the wired access to the Internet is either not available or is expensive.
• People also need to access the Internet when they are using their cellular phones.
• WiMAX has been designed for these types of applications.
MobileWiMAX Fixed WiMAX
Wireless Impairments
❖ Atmospheric absorption – water vapour and oxygen contribute to attenuation
❖ Multipath – obstacles reflect signals so that multiple copies with varying delays
are received
Refraction – bending of radio waves as they propagate through the atmosphere
Sketch of three important Propagation: Reflection (R ) Scattering (S), Diffraction (D)
❖ Reflection – occurs when signal encounters a surface that is large relative to the wavelength of the signal
❖ Diffraction – occurs at the edge of an impenetrable body that is large compared to wavelength of radio wave
❖ Scattering – occurs when incoming signal hits an object whose size is nearly equal to or less than the wavelength of the signal
The Effects of Multipath Propagation
Multiple copies of a signal may arrive at different phases (i.e.. at different times due to different path length)
➢ If phases add destructively, the signal level relative to noise declines, making detection more difficult
Inter-Symbol Interference (ISI)
➢ One or more delayed copies of a pulse may arrive at the same time as the primary pulse for a subsequent bit
SNR is the ratio of the signal power to the noise power.
➢ If the signal power is decreased or the noise power is increased, SNR will decrease.
➢ In a wireless LAN, the signal power is less (using batteries). The noise power is higher in a wireless LAN because the noise is not controlled. The noise from any source can affect the signal exchanged between the sender and the receiver.
Types of Fading (Non-Examinable)
❖ Fast fading
❖ Slow fading
❖ Flat fading
❖ Selective fading
❖ Rayleigh fading – statistical model, random, r-dist
❖ Rician fading – random-stochastic model, radio propagation anomaly caused by partial cancellation of a radio signal by itself
● So far we have discussed
○ Wireless networks
○ Example wireless networks
○ Limitations and issues.
● Next week
○ Physical Layer Transmission – Digital Transmission
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