WLAN – IEEE 802.11

IEEE 802.11

802.11 standard only covers the physical layer PHY and medium access layer MAC.

PHY divided into two sublayers :-− Physical layer convergence protocol (PLCP)

− Physical medium dependent (PMD)

PLCP – it provides a carrier sense signal called clear channel assessment (CCA), and provides a common PHY service access point (SAP).

PMD – handles modulation and encoding /decoding of signals.

Cont..

MAC – support the association and re-association of a station to an access point and roaming between different access point ,control authentication,encryption, synchronization and power management.

PHY Layer

802.11 supports three versions of PHY layer − FHSS – Frequency hopping spread spectrum

− DSSS – Direct sequence spread spectrum

− Infra red

[Spread Spectrum-techniques involve spreading the bandwidth needed to transmit data.]

FHSS Frame Structure

Synchronization – 80 bit SYN (010101....), used to SYN potential receiver.

Start frame delimiter (SFD) – indicate the start of frame.

PLCP_PDU length word (PLW) – length of payload PLCP Signalling field (PSF) – indicate data rate of

payload 0000 – 1Mbits/sec, granularity 500 Kbits/sec. Header error check (HEC) – 16 bit checksum

DSSS Frame Structure

Self Study

MAC Layer

Three basic access mechanisms have been defined for IEEE 802.11:(based on CSMA/CA,RTS/CTS and polling) :-− First two method summarized as distributed

coordination function (DCF). offers ASYN service and not require any access point

to control.

− Third, point coordination function (PCF). offers both ASYN and time-bounded service but need

an access point to control medium access.

Medium Access and Inter-frame Spacing

Short inter-frame spacing (SIFS) – shortest waiting for medium access (highest priority) ex. Control msg.

PCF inter-frame spacing (PIFS) – used for time bounded services.

DCF inter-frame spacing (DCF) – longest waiting time and has the lowest priority for medium access.

[Contention – duration in which several nodes try to access the medium]

Access Mechanism

DFWMAC-DCF using access mechanism based on CSMA/CA.

Based on random access scheme with carrier sense and collision avoidance through random backoff.

Basic DFWMAC-DCF example

DFWMAC-DCF with RTS/CTS In this,after waiting for DIFS + backoff time , the sender can issue a request

to send(RTS).

RTS will have same priority , it includes the recevier of the data transmission and whole duration (data frame + ACK).

Other nodes receive this RTS will set its NAV(net allocation vector).

Receiver will answer RTS with CTS message after waiting for SIFS.

This CTS will also include duration of transmission

DFWMAC-PCF with polling

PCF- Point Coordination Function It requires an access point that controls

medium access and polls the single nodes.

MAC frames

Frame control – serve several purposes.

Duration/ID – indicating the period of time in which the medium is occupied, used to set the NAV.

Address 1 to 4 – four address fields contain standard IEEE 802 MAC addresses (48 bits each), meaning of each address is depend upon the DS bits.

Sequence control – seq. no used to filter duplicates.

Data – contain data transfer from sender to receiver.

Checksum (CRC) – error control

Protocol version – current protocol version.

Type – function of frame : management (00), control (01), or data (10) and value (11) is reserved.

Subtype – subtype for management frame: (0000) for association req. , (1000) beacon , RTS (1011), CTS (1100).

More fragments – 1 more frag. Present.

Retry – if retransmission set to 1.

Power Management – set to 1 then power saver mode.

More data – indicate more data is present between access point and station nodes.

Order – if set to 1 then strict order.

Interpretation of MAC addresses

Ad-hoc network – exchange of MAC frame between two wireless nodes without a distribution system.

Infrastructure network(from AP) – frame physically originates from an access point.

Infrastructure network (to AP) – station sends a packet to another station via the access point.

Infrastructure network (within DS) – packet transmitted between two access points.

BSSID – basic service set ID , DA – destination address , SA – source address,RA – receiver address , TA – transmitter address.

MAC Management

Perform following tasks:- SYN – finding a wireless LAN, SYN of internal clock,

generation of becon signals. Power Management – to control, transmitter activity

for power management (periodic sleep,buffering, without missing a frame).

Roaming – joining a network,changing of access points,scanning for access points.

Management information base(MIB) – parameters representing the current state, can be accessed via standardized protocol : simple network management protocol (SNMP).

SYN

To SYN clocks of all nodes , timing synchronization function (TSF).

Within a BSS, timing is conveyed by the periodic transmissions of a beacon frame.

Beacon contains a timestamp and other management information used for power management and roaming.

Transmission of a beacon frame is not always periodic as it deferred if the medium is busy.

In infra-based network , AP performs SYN by transmitting the periodic beacon signal.

SYN example

Beacon intervals are not shifted if one beacon is delayed.

Timestamp of beacon always reflects the real transmit time, not the scheduled time.

Power Management

Wireless devices are battery powered , i.e power saving mechanisms are crucial.

Basic idea of power management is to switch off the transceiver whenever it is not needed.

PM is simple for sending device, but PM for receiver can't known is advance.

Receiver “walk up” the transceiver periodically. Power saving includes two states for a station:-

Sleep and awake Buffering of data in sender.

Power Management (Cont.)

Sender communicate with power saving station then it has to buffer data.

If a station detects that it is a destination of a buffered packet it has to stay awake until the transmission takes place.

Walk up at right moment require the timing synchronization function(TSF).

Power management in infra-based network is simpler.

Beacon sent by the access point, a traffic indicating map(TIM) is transmitted.

Power Management example

DTIM – delivery traffic indication map

Power Management in Ad-hoc

Self Study

Roaming

Moving between AP is called roaming.

Steps are as follows :-

Check current link quality and scanning for another AP.

Scanning involves the active search for another BSS.

– Passive scanning means listening into the medium to find other n/w. i.e receiving beacon msg.

– Active scanning means also sending a probe

– and , beacon and prob responses used to join the new BSS Select the best AP and send association request.

New BSS response the association request

IEEE 802.11b & 802.11a

Self Study

HIPERLAN

HIPERLAN – High Performance Local Area Network

ETSI (European Telecommunication Standard Institute) standardized HIPERLAN

HIPERLAN is a WLAN allowing for node mobility and supporting ad-hoc and infrastructure-based topologies.

Bluetooth

In 1998 five companies (Ericsson,Intel,IBM,Nokia,Toshiba) founded the Bluetooth consortium with the goal of developing a single-chip,low-cost,radio-based wireless network technology.

Bluetooth Architecture

Networking Protocol Stack

Bluetooth Architecture (Networking)

It operate on 79 channels in the 2.4 GHz band with 1 MHz carrier spacing.

Each device perform frequency hopping with 1600 hops/s in a random fashion.

Piconet – it is a collection of Bluetooth device which are SYN to the same hopping sequence.

Piconet is a collection of devices with different roles.

Cont.

One device in piconet is master (M)

All other device connected to the master are act as a slaves(S).

Two additional devices : parket devices (P) and stand-by (SB)

P – known but they do not have connection.

SB – do not participated in the piconet.

M determine the hopping pattern in the piconet

Slaves have to SYN to this pattern.

Each piconet has one master and exctly 7 simultaneous slaves.

Formation of Piconet

As all devices use a same hopping sequence they must be SYN.

Step 1 Master sending its clock and device ID .

Step 2 hopping pattern is determined by the device ID , a 48-bit worldwide unique identifier.

Step 3 slaves adjust their internal clock according to the master and participated in the piconet.

All active devices are assigned a 3-bit active member address (AMA).

All parket devices use an 8-bit parket member address(PMA).

Formation of Scatternet

Groups of piconet called scatternet.

Many piconet with overlapping coverage can form scatternet.

If a device wants to participate in more then one piconet, then it has to SYN to the hopping seq. of the piconet it wants to take part in.

Bluetooth (Protocol Stack)

Protocol stack can be divided into two parts Core specification Profile specification

Core specification comprise following elements:- Radio Baseband Link Manager Protocol Logical Link Control and adaptation

protocol (L2CAP) Service discovery protocol

Bluetooth:Protocol Stack:Core:Radio

It specifies the air interface i.e frequencies, modulation and transmit power.

Bluetooth uses the license-free frequency band at 2.4 GHz.

A frequency hopping/time-division duplex scheme is used for transmission, with a fast hopping rate of 1600 hops/sec.

A time interval between two hops is 625 mico sec.

Each slot uses different frequency Bluetooth uses 79 hop carrier equally

spaced with 1 MHz.

Bluetooth:Protocol Stack:Core:Baseband

It manages frequncy hopping ,medium access and also defines the packet format.

It defines 1-slot, 3-slot and 5 slot for higher data rates.

No frequency hopping is performed within packets.

In above example the master or one out of seven slaves may transmit data in an alternative fashion.

Bluetooth:Protocol Stack:Core:Baseband (Cont..)

Components of a bluetooth packet at baseband layer, packet contains following three fields :-

Access code– used for timing SYN and piconet identification.

Packet header – used for addess, packet type , flow and error control and checksum.The three bit active member address (AMA) represent the active address of the slave.

Payload – up to 343 bytes payload can be transferred.

Bluetooth:Protocol Stack:Core:Physical Link

It offers two different types of links:- Synchronous connection-oriented link(SCO)

For this master reserve two consecutive slots (forward and return slots) at fixed intervals

Master can support upto three simultaneous SCO links to the same slave or to different slaves.

Asynchronous connectionless link(ACL) Data applications , point-to-multipoint

transfer scenarios Only one ACL links can exist between a master

and slave.

Bluetooth:Example data transmission

Master always uses the even number of frequency slots , odds slots are for the slaves

Every sixth slot is used for an SCO link

ACL link use single or multiple slots providing asymmetric bandwith

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