Wireless LAN

10.1 Introduction

• Wireless LAN is a local area network without wires.

• 10.2 Advantages• Schools, manufacturing plants install wireless

LAN for many reasons. They are• 1. mobility: it offers wire free access to

information within the operating range of WLAN

• 2.Low implementation costs:it is easy to set up, relocate, change and manage.

• 3. Installation speed and simplicity: installation is fast and easy• 4. Network expansion: network reach where wires cannot

reach.• 5. Reduced cost of ownership: long term cost benefit• 6.higher user to install base ratio: for 100 channels ,2500

subscribers.• 7.reliability: failure in wired network is downtime due to cable

fault. It is not in wlan.• 8. scalability: configutations are easily changeable and range

from small subscribers to thousnd subscirbers.• 9. usage of ISM band: wlan available in unregulated ism band.

In india 2.4 ghz is free for WLAN.• WLAN is commercially known as wifi i.e wireless fidelity

10.2.1 Wireless LAN evolution

• 802.11 is the first generation systems.• Second generation Standards 802.11a and

802.11g offer high bandwidth.• It offers horizontal roaming. i.e. a user move from

one AP to another AP. seamlessly.• Third generation offers vertical roaming .i.e.

seamless roaming between different networks. • Security is extended. • Standards are 802.11f and 802.11i

10.2.2. Wireless LAN applicaitons

• WLAN suits for dynamic environments.1.Office /campus environment a person move with his laptop to the meeting room and

continue working.2. Factory shop floor:In these floor layout change very freuqently. objects

within the building are constantly moving.3. homes:Networking of different home devices like phones,

computers and appliances.

4. Workgroup environment:It helps to small workgroups or teams to work

together within a building or in a neighborhood. E.g. rescue members after a disaster or accident site or civil construction.

5. Heritage buildings:To set up virtual reality show6. Public places:Airports , railway stations access net.7. war/defense sites:In war sites and war games.

10.3 IEEE 802.11 standards• IEEE 802 committee was set up in February 1980 for LAn.• WLAN is different at the physical and media access layer.• WiFi or IEEE 802.11 became popular WLAN protocol.• It is published in 1997 with a goal to support 2.4 ghz RF and

infrared transmission.• Other standards carry the alphabets 802.11a,ieee 802.11b

etc.• Some standards are given in table 10.1• IEEE 802.11- 2mbps in 2.4 GHz ism band• IEEE 802.15.1 – WPAN based on Bluetooth specification, in

2.4 GHz ism band.• IEEE 802.1x – port based network to provide authentication

to devices attached to a lan port

• To avoid confusion, IEEE consolidated IEEE std 802.11-2007 standard entitled “ IEEE standard for information technology - telecommunications and information exchange between systems - local and metropolitan area networks – specific requirements part 11: wireless lan medium access control and physical layer specifications”.

• It is shown in table 10.2 see book.• Next standards are released in 2011 like IEEE

802.11k, IEEE 802.11y are merged into IEEE std 802.11-2011

10.4 Wireless LAN architecture• 10.4.1 types of wireless LAN1. 802.11 :In June 1997 IEEE finalized this std. it operates at 2.4 GHz

at data rate 1 Mbps and 2 Mbps. It is evolved into various specifications like 802.11b,802.11a,802.11g. Today it offers at rate of 54 Mbps.

2. HyperLAN:It is started in Europe as a specification en 300 652 in

1996 by ETSI broadband radio access network group. HyperLAN/1 version works at 5 GHz band and offers 24 Mbps.

3. HomeRF: In 1998, it provides shared wireless access protocol(swap).

It offers interoperability between pc and consumer electronic devices within the home. Swap uses fhssm and offers 1 mbps and 2 mbps at 2.4 GHz band.

4. Bluetooth:It is promoted by IBM, Ericsson, Intel, Lucent, 3com,

Microsoft, Nokia, Motorola And Toshiba.It is WPAN at 2.4 GHz at rate 1 mbps. It uses fhssm with

low power and smaller range.5. MANET:It is a working group within IETF to develop standard for

mobile ad hoc networks.

10.4.2 Ad hoc versus Infrastructure Mode• Wireless networks are 2 types1. Infrastructure – in this mode, the MS are connected to

base station or Access Point (AP). Fig 10.1 shows wireless LAN in this mode. 802.11 LAN is based on cellular infrastructure where system is divided into small clusters ore cells. Each cell is called as BSS or Basic Service Set

2. ad hoc mode – in this mode, there is no access point. A number of MS form a cluster to communicate with each other. It is shown in fig 10.2. Here BSS is completely independent. so Independent BSS(IBSS). Multiple BSS from Extended Service SET(ESS). ESS is connected to the backbone LAN or the distribution system.

10.4.3 802.11 Architecture

• In 802.11 BSS is controlled by one Base station. It is called as Access Point(AP).

• Many AP are connected through backbone. This backbone is called as Distributed System(DS). It is either Ethernet or wireless itself as in fig 10.3

• The whole interconnected wireless LAN is single 802 network is seen to the upper layers of OSI model is called as Extended Service Set(ESS).

• 802.11 standard defines the concept of a “portal”.• A portal is a device that interconnects between and

802.11 and another 802 LAN.

• Cell Design in Wireless LAN• For proper functioning of wireless LAN, neighboring

cells(BSS) are on different frequencies.• Wireless LAN cards can not interfere with each other.• In DSSS, It uses 13 different channels. In fig 10.3• In FHSS, it uses 79 channels. These frequencies are non

overlapping.• In fig 10.4 each cell is using non overlapping

frequencies cell A and cell D use same frequency.• Two frequencies use by two cells will not have any

effect as long as distance “X” is great enough to ensure effective radio isolation from each other.

• Radio isolation or separation means that a device in cell A cannot detect the signal transmitted by any device in cell D.

IEEE 802.11 Layers Description

• The standard defines a single MAC, which interacts with 3 PHY fig 10.5 as follows.

1.FHSS2. DSSS3.InfraRed.

Physical Layer(Layer 1) Architecture• The architecture of the physical layer fig 10.5 comprises two

sub layers for each station.1.Plcp(physical Layer Convergence Procedure): PLCP sub layer is responsible for carrier sense(CS) part of the

carrier sense multiple access / collision avoidance (CSMA / CA) protocol.

It prepares MAC protocol Data unit(MPDU) fir transmission.It delivers the incoming frames from the wireless medium to

MAC layer.PLCP appends fields to MPDU that contains information

needed by the physical layer transmitter and receiver.This frame is called PLCP protocol Data Unit(PPDU).

2. PMD (Physical Medium Dependent): It provides actual transmission and reception of

physical layer entities between stations through the wireless media.

It provides the modulation / demodulation of the transmission.

FHSS (Frequency Hopping Spread Spectrum) Physical Layer

• This layer carries the clocking information to synchronize the receiver clock with the clock of the transmitted packet.

• Fig 10.6 shows FHSS PPDU packet.• The fields in the FHSS PLCP is1.SYNC: it is made up of alternate zeros and ones. This

bit pattern is to synchronize the clock of the receiver.2. Start Frame Delimiter: it indicates the beginning of

the frame and the content of this field is fixed and is always 0000110010111101.

3. PSDU Length Word (PLW): This field specifies the PSDU in octets.

4. PLCP signaling (PSF): This field contains information about the data rate of the fields. The PLCP preamble is transmitted at 1 mbps. This field contains the speed of the link. For e.g., 0000 means 1 mbps and 0111 signifies 4.5 mbps bandwidth.

5. Header Error Check: this field contains the CRC (Cyclic Redundancy Check) according to CCITT CRC-16 algorithm.

• FHSS PMD • It is responsible for converting the binary bit

sequence into analog signal and transmit the PPDU frame into the air. For this it uses FHSS.

• It transmits PPDU by hopping from channel to channel according to a particular pseudo - random hopping sequence.

• Once the hopping sequence is set in the access point, stations automatically synchronize to the correct hopping sequence.

Direct Sequence Spread Spectrum (DSSS) Physical Layer.

• DSSS PLCP is responsible for synchronizing and receiving the data bits correctly.

• Fig 10.7 shows the DSSS PPDU packet.• This field in the DSSS PLCP are as follows:1.sync: This field is made up of alternate zeroes and

ones. This bit pattern is to synchronize the clock of the receiver with the received frame.

2. Start Frame Delimiter: This field indicates the beginning of the frame and the content of this field is fixed and is always 111001110100000.

3. Signal: This field defines the type of modulation the receiver must use to demodulate the signal.

When the value of the field is multiplied by 100 kbps we get the bandwidth of the transmission.

4. Service: This field is not used and is always 0.5. Length: This field contains an unsigned 16 bit integer

indicating the length of the frame. it uses microseconds. The receiver use this to synchronize with clock to determine the end of frame.

6. Frame Check Sequence: This is a 16 bit checksum based on CCITT CRC -16 algorithm.

• DSSS PMD: It translates the binary digital sequence into analog radio signals and transmits the PPDU frame into the air.

• It is same as DSSS CDMA.

The MAC Layer (Layer 2) Architecture

• It defines two different access methods.1. The Distributed Coordination Function2. The Point Coordination Function1.The Basic Access Method: CSMA/CAIt is called the Distributed Coordination Function

by IEEE standard.It is Carrier Sense Multiple Access With Collision

Avoidance Mechanism (CSMA/CA).

• In a wired Environment(Ethernet) every station connected to the wire sense the signal in the wire.• This is not true in wireless media.• In wireless LAN, it uses CSMA/CA protocol and not possible to detect a collision of data packets in the

air.• Consider the scenario with 3 mobile nodes as in fig 10.9(a).• The transmission of A reaches B, but not C.• The transmission of C reaches B, but not A.• Radio signal of B reaches both A and C.•

• Consider another version as in fig 10.9 b.• The radio transmission signal of A reaches C and

B.• The radio signal of c reaches both A and D.• Collision detection mechanisms are a good idea

on wired LAN, but cannot good for wireless LAN for two reasons.

• Implementing a collision detection mechansim requires implementing full duplex transmission.

• In a wireless environment, we cannot assume that all stations will be able to receive radio signals from each other and does not mean that medium is free.

• The mechanism behind CSMA/ CA1. When a wireless station wants to

communicate, it first listens to its spectrum to check if it can sense radio waves from any other wireless station.

2. If the medium is free for a specified time then the is allowed to transmit. This time interval is called distributed inter frame space (DIFS).

3. If the current device senses a carrier signal of another wireless device on the same frequency as it wants to transmit on, it backs off and initiates a random timeout.

4. After the timeout has expires, the wireless station again listens to the radio spectrum and if it senses another wireless station transmitting, it continues to initiate random timeouts until it does not detect or sense another wireless station transmitting on the same frequency.

5. When it does not sense another wireless station transmitting, the current wireless station starts transmitting its own carrier signal to communicate with other wireless station, and once synchronized transmits the data.

6. The receiving station checks the CRC of the received packet and sends an acknowledgement packet(ACK).

The reduced opportunity for data transmission leads to less effective access to wireless bandwidth.

It reduces the speed of the operation.In worst case, the wireless connection is lost.

Virtual Carrier Sense• The two stations colliding because they cannot sense each

other’s presence.• To reduce the probability of colliding, the standard defines a

virtual carrier sense mechanism. i.e.• A station wanting to transmit a packet first transmits a short

control packet called RTS(Request To Send), which includes the source, destination and the duration of the following transaction(the data packet and the respective ACK).

• The destination station after receiving this request packet responds with a response control packet called CTS(Clear To Send), which includes the same duration transformation.

• All stations receiving either RTS/CTS, set their virtual sense indicator called Network Allocation Vector or NAV, for the given duration.

• This mechanism reduces the probability of a collision on the receiver side by a station that is hidden from the transmitter to the short duration of the RTS transmission.

• Because the station senses the CTS and reserves the medium as busy until the end of the transaction.

• The diagrams fig 10.10 show a transaction between stations A and B, and the NAV setting of their neighbors.

Fragmentation and Reassembly

• There are several reasons why it is preferable to use smaller packets in a wireless LAN environment.

1. Due to higher bit error rate, a packet is getting corrupted.

2. In case of packet corruption, the smaller the packet, the less overhead.

3. In fhss, the smaller the packet, smaller the chance that the transmission will be postponed.

• Any change in the protocol for wireless LAN cause a major change in the protocol of the higher layers.

• So, IEEE solves the problem by adding a simple fragmentation / reassembly mechanism at the MAC layer.

• The mechanism is simple send and wait algorithm, where the transmitting station in slow speed until one of the following conditions happens:

1. Receives an ACK for the said fragment2. Decides that the fragment was transmitted too many

times and drops the whole frame.This standard allows the station to transmit to a different

address between retransmissions of a given fragment.fig 10.11 shows frame fragmentation.

Inter Frame Spaces

• The standard defines four types of spacing intervals.

• These are called Inter Frame Spaces (IFs).• Ifs is used to defer a station’s access to the

medium and provide various level of priorities.1.SIFS (Short Inter Frame Space), is the shortest

inter frame space with highest priority.RTS,CTS use SIFS intervals. SIFS value is a fixed

value as per PHY.

2. PIFS (Point Coordination IFS), is used by the access point (or point coordinatior), to gain access to the medium before any other station. This value of PIFS is SIFS + a slot tiem i.e 78 microseconds.

3. DIFS(DISTRIBUTED IFS), is the inter frame space used for a station willing to start a new transmission, which is calculated as PIFS plus one slot time i.e., 128 microseconds.

4.EIFS(Extended IFS), is a longer IFS used by a station that has received a packet that it could not understand. This is needed to prevent the station from colliding with a future packet belonging to the current dialog.

Maintaining synchronized stations

• Stations need to maintain synchronization.• On an infrastructure BSS, synchronization is

achieved by all the stations updating their clocks according to the AP’s clock.

• The AP periodically transmits frames called beacon frames.

• These frames contain the value of the AP’s clock at the moment of transmission.

• This is the time when physical transmission actually happens, and not when the packet was put in the queue for transmission.

• The receiving stations check the value of their clocks the moment the signal is received, and correct it to be synchronized with AP’s clock.

Power saving

• Power saving enables station to got into sleep mode without losing information.

• The AP maintains a continually updated record of all stations in power saving mode.

• AP buffers the packets the packets addressed to these stations until

• The stations request these by sending polling request or

• The stations change their operation mode.