1. Wireless NETWORKS NET 434 Topic No 6 Wireless LANS
Lte Capacity Workstream RMEA | Ericsson Internal | Uen, Rev DRAFT | 18-May | Page ‹#›

2. Wireless Lans
A local area network (LAN) is a computer network that interconnects computers within a smaller geographic area such as a home, school, computer laboratory, or office building, using network media.
A wireless local area network (WLAN) is a wireless computer network that links two or more devices using a wireless medium within a limited area such as a home, school, computer laboratory, or office building.
It is sometimes called wireless Ethernet.
Some countries, including the United States, the public uses the term WiFi (short for wireless fidelity) as a synonym for wireless LAN.

3. IEEE Architecture
The IEEE defines two kids of service sets:
The Basic Service Set
Ad-hoc BSS
Stationary or Mobile wireless stations
An optional central base station or Access Point (AP)
BSS without the AP is a standalone networks and cant communicate with other BSS (Ad-hoc Architecture/ Mode)
In this mode/Architecture a collection of computers are associated so that they can directly send frames to each other.

4. IEEE802.11 Architecture Infrastructure BSS Extended Service Set
A BSS with an AP is called the Infrastructure Mode.
In infrastructure mode, each client is associated with an AP (Access Point) that is in turn connected to the other network.
Example Internet Connection using WIFI
The client sends and receives its packets via the AP.
Extended Service Set
Several access points may be connected together typically by a wired network called a distribution system, to form an extended network.
The distribution system connects the APS in the BSSs and communication between a station in a BSs and outside the BSS occurs via Aps.
Mobile stations (Normal Stations in BSs) and stationary stations (AP stations that are part of a wired LAN)
The stations within reach of each other can communicate without the use of the AP. This idea is similar to a cellular network. If we consider each BSS as a cell and each AP to be a base station.
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5. Summary architecture

6. Wireless lan configurations

7. Station Types
IEEE defines three types of stations based on their mobility in a wireless LAN.
No transition (Mobile/Stationary only within a BSS)
BSS-transition (Mobile between two BSS but confined within ESS)
ESS-transition mobility (Mobile between two ESS, No guarantee of continuous service)

8. Protocol Architecture/Stack
IEEE has defined the specifications for a wireless LAN, called IEEE , which covers the physical and data-link layers.
The physical layer corresponds fairly well to the OSI physical layer
The data link layer in all the 802 protocols is split into two or more sub-layers.
The upper sub-layer of the data-link layer (LLC)-Logical Link Control.
Data link control functions include framing and flow and error control.
The lower sub-layer, multiple access control (MAC)
In , the MAC (Medium Access Control) sub-layer determines how the channel is allocated, that is, who gets to transmit next.
When nodes or stations are connected and use a common link, called a multipoint or broadcast link, we need a multiple-access protocol to coordinate access to the link.
Many protocols have been devised to handle access to a shared link.
All of these protocols belong to a sub layer in the data-link layer called media access control (MAC).

9. Protocol architecture

10. Physical Layer
Each of the transmission techniques makes it possible to send a MAC frame over the air from one station to another. They differ, however, in the technology used and speeds achievable.
All of the techniques use short-range radios to transmit signals in either the 2.4-GHz or the 5-GHz ISM frequency bands except the IEEE Infrared.
ISM Frequency bands used for Wireless LANs are unlicensed bands available to any transmitter willing to meet some restrictions, such as radiated power of at most 1 W (though 50 mW is more typical for wireless LAN radios).
Garage door openers, cordless phones, microwave ovens, and countless other devices, operate in the ISM band and compete with laptops for the same spectrum.
The 2.4-GHz band tends to be more crowded than the 5-GHz band. Less interference in 5-GHz band than 2.4 GHz
5 GHz can be better for some applications even though it has shorter range due to the higher frequency.
All of the transmission methods also define multiple rates.
Different rates can be used depending on the current conditions.
If the wireless signal is weak, a low rate can be used.
If the signal is clear, the highest rate can be used. This adjustment is called rate adaptation.
The band is divided into 79 sub-bands of 1 MHz (and some guard bands).
A pseudorandom number generator selects the hopping sequence.
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11. Physical Layer IEEE 802.11 FHSS IEEE 802.11 DSSS
Frequency-hopping spread spectrum (FHSS) method,
Band –4.835 GHz ISM band.
Modulation technique---two-level FSK or four-level FSK
Data rate of 1 or 2 Mbps
IEEE DSSS
Direct-sequence spread spectrum (DSSS) method,
Modulation technique----PSK (BPSK or QPSK)

12. Physical layer IEEE 802.11 Infrared IEEE 802.11b DSSS
IEEE infrared uses infrared light in the range of 800 to 950 nm.
This does not operate in the ISM band
Modulation technique is called pulse position modulation (PPM).
Data Rates---1 0r 2Mbps
IEEE b DSSS
High-rate direct-sequence spread spectrum (HRDSSS)
2.400–4.835 GHz ISM band.
Data Rates of 5.5 and 11Mbps

13. Physical layer IEEE 802.11a OFDM IEEE 802.11g IEEE 802.11n
OFDM using QAM
5.725–5.850 GHz ISM band.
Data rate –up to 54Mbps
IEEE g
This new specification defines forward error correction and OFDM.
2.400–4.835 GHz ISM band.
The modulation technique achieves a 22- or 54-Mbps data rate.
IEEE n
The goal is to increase the throughput of wireless LANs.
Data Rates of up to 600 Mbps data rate.
The standard uses 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, we are in a better position to eliminate noise.
The first is called a and uses a different frequency band, 5 GHz.
The second stuck with 2.4 GHz and compatibility. It is called g. Both give rates up to 54 Mbps.
In OFDM (Orthogonal Frequency Division Multiplexing), the channel bandwidth is divided into many subcarriers that independently send data (e.g., with QAM).
Usually, one high-rate stream of digital information is split into many low-rate streams that are transmitted on the subcarriers in parallel.
This division is valuable because degradations of the channel are easier to cope with at the subcarrier level; some subcarriers may be very degraded and excluded in favor of subcarriers that are received well.
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14. Physical Layer-summary

15. Medium Access Control
The IEEE MAC layer covers three functional areas
Reliable data delivery
Access control
Security
Like any wireless network, in a wireless LAN physical and MAC layer are unreliable due to noise interference and other propagation effects can result in a loss of significant number of frames.
A number of MAC frames may not be successfully received.
Reliability at higher layer such as the TCP
Frame exchange protocol at the MAC Layer
When a station receives a data frame from another station, it returns an acknowledgment (ACK) frame to the source station.
If the source does not receive an ACK within a short period of time, either because its data frame was damaged or because the returning ACK was damaged, the source retransmits the frame.

16. MEDIUM access Control
To further enhance reliability, a four-frame exchange may be used.
A source first issues a Request to Send (RTS) frame to the destination.
The RTS alerts all stations that are within reception range of the source that an exchange is under way; these stations refrain from transmission in order to avoid a collision between two frames transmitted at the same time.
The destination then responds with a Clear to Send (CTS).
The CTS alerts all stations that are within reception range of the destination that an exchange is under way
After receiving the CTS, the source transmits the data frame, and the destination responds with an ACK.

17. MEDIUM access Control-Access Modes
The working group considered two types of proposals for a MAC algorithm
DCF
PCF
Distributed Coordination Function
The DCF sub-layer makes use of a simple CSMA/CA (carrier sense multiple access) algorithm.
A station with a frame to transmit senses the medium. If the medium is idle, it waits to see if the medium remains idle for a time equal to IFS. If so, the station may transmit immediately.
If the medium is busy (either because the station initially finds the medium busy or because the medium becomes busy during the IFS idle time), the station defers transmission and continues to monitor the medium until the current transmission is over.

18. MEDIUM access Control
Once the current transmission is over, the station delays another IFS. If the medium remains idle for this period, then the station backs off a random amount of time and again senses the medium. If the medium is still idle, the station may transmit.
During the back off time, if the medium becomes busy, the back off timer is halted and resumes when the medium becomes idle.
If the transmission is unsuccessful, which is determined by the absence of an acknowledgement, then it is assumed that a collision has occurred.

19. MEDIUM access Control Three types of IFS
SIFS (short IFS): The shortest IFS, used for all immediate response actions. Highest Priority.
Receiver responds with an ACK frame after waiting only for an SIFS gap.
After getting a (RTS) frame, The station to which this frame is addressed should immediately respond with a CTS frame.
PIFS (point coordination function IFS): A middle length IFS, used by the centralized controller in the PCF scheme when issuing polls.(Point Coordinator Function. Used to take precedence over normal contention traffic.
DIFS (distributed coordination function IFS): The longest IFS. Used for ordinary traffic

20. DCF Frame Exchange

21. DCF Frame exchange

22. DCF-Frame Exchange Network Allocation Vector
How do other stations defer sending their data if one station acquires access?
Station sends an RTS frame, which includes the duration of time that it needs to occupy the channel.
The stations that are affected by this transmission create a timer called a network allocation vector (NAV) that shows how much time must pass before these stations are allowed to check the channel for idleness.
Each time a station accesses the system and sends an RTS frame, other stations start their NAV.
Collision During Handshaking
What happens if there is a collision during the time when RTS or CTS control frames are in transition, often called the handshaking period?
Two or more stations may try to send RTS frames at the same time. These control frames may collide.
The sender assumes there has been a collision if it has not received a CTS frame from the receiver. The back-off strategy is employed, and the sender tries again.

23. DCF Frame Exchange Hidden-Station Problem
The solution to the hidden station problem is the use of the handshake frames (RTS and CTS). Figure shows that the RTS message from B reaches A, but not C.
B and C are within the range of A, the CTS message, which contains the duration of data transmission from B to A, reaches C.
Station C knows that some hidden station is using the channel and refrains from transmitting until that duration is over.

24. Exposed station problem

25. Point Coordination Function
The point coordination function (PCF) is an optional access method that can be implemented in an infrastructure network (not in an ad hoc network).
It is implemented on top of the DCF and is used mostly for time-sensitive transmission.
PCF has a centralized, polling access method. The AP performs polling for stations that are capable of being polled.
Polling works with topologies in which one device is designated as a primary station and the other devices are secondary stations.
All data exchanges must be made through the primary device even when the ultimate destination is a secondary device.
The primary device controls the link; the secondary devices follow its instructions.
It is up to the primary device to determine which device is allowed to use the channel at a given time.
The primary device, therefore, is always the initiator of a session.
This method uses poll and select functions to prevent collisions. However, the drawback is if the primary station fails, the system goes down.

26. Point Coordination Function
The stations are polled one after another, sending any data they have to the AP.
To give priority to PCF over DCF, another inter-frame space, PIFS, has been defined. PIFS (PCF IFS) is shorter than DIFS.

27. OFDM Basics

28. Spread Spectrum basics
Behrouz forouzan Section 6.2