IEEE 802 Architecture

IEEE 802 Architecture

• It offers standardized protocol architecture for LANs

• It covers physical, medium access control, and logical link control layers

• Protocols in OSI layers 3 and 4 are network architecture-independent

• The Higher-layer protocols are applicable to LANs, MANs, and WANs

• The IEEE 802 architecture is widely adopted for LAN standards

• It is referred to as the IEEE 802 reference model

• The lowest layer of the IEEE 802 reference model is the OSI model's physical layer

• Functions of the lowest layer include signal encoding/decoding, preamble handling, and bit transmission/reception

• The physical also specifies the transmission medium and topology

• This is considered to be below the lowest layer of the OSI model

• For some IEEE 802 standards, the physical layer is further subdivided into sublayers

• The Physical Layer Convergence Procedure (PLCP) maps 802.11 MAC layer protocol data units into a framing format

• This framing format is for user data and management information transfer between stations, using the associated PMD sublayer

• The Physical Medium Dependent Sublayer (PMD) defines the the user data transmission/receiving characteristics for a wireless medium between stations

• The functions above the physical layer provide service to LAN users

• On transmission, data is assembled into a frame with address and error detection fields

• On reception, frame is disassembled and address recognition and error detection occur

• Medium access to the LAN transmission is governed

• An interface is provided to higher layers, and flow & error control is performed

• Layer 2 separation is done because shared-access medium management logic is absent in traditional layer 2 data link control

• The same LLC can accommodate several MAC options

• Higher-level data reaches LLC, which adds control information as a header

• This creates an LLC protocol data unit (PDU)

• This control information facilitates the operation of the LLC protocol

• The entire LLC PDU goes to the MAC layer, which puts control information at the front and back

• This produces a MAC frame

• The control information inside the frame is required for the MAC protocol's operation

IEEE 802.11 Architecture: Basic Service Set

• IEEE defined specifications for wireless LANs, as IEEE 802.11, covering physical and data link layers
• IEEE 802.11 defines the basic service set (BSS) as the building block of a wireless LAN
• A BSS includes stationary or mobile wireless stations and an optional central base station known as the access point (AP)
• A BSS without an AP is a stand-alone network incapable of transmitting data to other BSSs
• A BSS without an AP is an ad hoc architecture where stations can form a network and locate each other to be part of a BSS without needing an AP
• A BSS with an AP is an infrastructure network

IEEE 802.11 Architecture: Extended Service Set

• An extended service set (ESS) consists of two or more BSSs with APs

• The BSSs are connected through a distribution system, commonly a wired LAN

• The distribution system facilitates connection between the APs in the BSSs

• The IEEE 802.11 specification doesn't specify the distribution system, such as an Ethernet example

• ESS uses mobile and stationary stations

• Mobile stations exist inside a BSS, while stationary stations are AP stations that are part of a wired LAN

• Stations within reach of one another can communicate without the use of an AP when BSSs are connected

• Two stations in two different BSSs typically communicate via two APs

• Each BSS is like a cell, and each AP is like a base station

• It also includes a mobile station that can simultaneously belong to more than one BSS

IEEE 802.11 Architecture: Station Considerations

• IEEE 802.11 defines three station types by means of their mobility in a wireless LAN

• They are no-transition, BSS-transition, and ESS-transition mobility

• A station with no-transition mobility is stationary or only moves inside a BSS

• A station with BSS-transition mobility can move between BSSs, but it is confined within an ESS

• A station with ESS-transition mobility is able to move between ESSs, however, continuous communication during the move is not guaranteed per IEEE 802.11

• All stations must maintain an association with the AP

• With association, an initial association is established with an AP for frame transmission/reception, so the device's ID and address must be known

• With reassociation, one transfers an association from one AP to another, allowing a mobile station to move from one BSS to another

• With disassociation, either a station or AP provides a notification that the association is terminated

• A station should notify others before leaving or shutting down, but MAC management does protect against stations disappearing without notice

• IEEE 802.11 defines two MAC sublayers: the distributed coordination function (DCF) and point coordination function (PCF)

IEEE 802.11 Architecture: Distributed Coordination Function

• DCF uses CSMAICA as the access method
• Wireless LANs cannot implement CSMA/CD for three reasons -For collision detection, a station has to simultaneously send data and receive collision signals, increasing bandwidth requirements -Collision detection may not occur because of the hidden station problem -The distance between stations means signal fading might prevent one station from hearing a collision at the far end

CSMA/CA Explained

• Source stations sense the medium by checking the carrier frequency energy level prior to frame transmission

• After sensing the medium the channel invokes with a persistence strategy with back-off until it is idle

• Wait for the distributed interframe space (DIFS) after an idle station is found before the station sends the request to send (RTS)

• After receiving the RTS the short interframe space (SIFS) begins

• After SIFS the destination station sends a control frame, clear to send, to the original source station

• This indicates that the destination station is ready to receive data

• The source station sends its data once this waiting period is completed.

• The destination station after waiting SIFS sends an acknowledgement to the source station

• Acknowledgement is necessary because the station cannot check for a successful arrival of data

• The lack of collision in CSMA/CD is an indication that the data has arrived

The Binary Exponential Backoff Procedure

• Wait either 0 or 1 slot times after the first collision
• On the second collision, pick either 0, 1, 2, or 3 at random and wait that many slot times
• After the kth collision, stations choose 0 to (2^k)-1 slots if k < 10
• If k>=10, stations randomly choose 0~1023 slots to defer transmission

Exchange of Data & Control Messages

• When a station sends an RTS frame, it needs to communicate the time it occupies the channel
• Affected stations establish a timer called a network allocation vector (NAV)
• The NAV indicates how much longer all stations should be allowed to check the channel for idleness
• When an RTS frame is sent other stations initiate NAV
• Before sensing the channel each station checks NAV to see if it has expired.

Collision During Handshaking

• Control frames may transition during handshaking
• Two or more stations send RTS frames together causing them to collide
• Absent of a collision detection mechanism means that the sender will only know a collision happened if no CTS frame is received
• When the aforementioned situation happens a back-off strategy is employed and the sender tries again

Interframe Space Values

• SIFS (Short IFS) which is the shortest IFS use for immediate response actions
• PIFS (Point coordination function IFS) which is a middle length IFS used by the controller in the PCF scheme
• DIFS (Distributed coordination function IFS) which is the longest IFS and is used as a minimum delay for asynchronous frames contending for access

Point Coordination Function (PCF)

• This is an optional access method for infrastructure networks, not for ad hoc networks

• It runs atop of the DCF protocol

• It is utilized mostly for time-sensitive applications.

• With PCF comes a centralized, contention-free polling access method

• The AP performs polling for stations that are capable of being polled

• Any data that stations have is then sent to the AP

• In order to give preference to PCF protocols; PIFS and SIFS protocols came into being

• SIFS is identical to that of DCF whereas PCF ( PIFS ) is shorter than DIFS.

• This means AP will have priority over a local server if it needs to perform PCF at the same time

• To prevent DCF using stations to be locked out from accessing the medium because of PCF a repetition internal has been designed that encompasses both PCF and DCF protocols.

• A Beacon frame ( special control frame ) begins each repetition interval

• Upon stations hearing the beacon they will initiate NAV for the duration of the specific contention-free period of the repetition interval

Fragmentation

• Wireless environments are noisy and a corrupt frame needs retransmission.
• Fragmentation is recommended to create smaller frames.
• The recommendation to fragment is important because smaller frames are simpler to re-transmit.

Frame Structure

• Frame control field (FC) is 2 bytes long

• Frame Control specifies frame types and control information

• Data, management, and control are the three major categories of frames defined under IEEE 802.11

• Management frames are used to make connections between access points and stations

• Control frames do the work of channel accessing and acknowledgement

Frame Control Field Types

• Table Explanations
  • Version - Current version is 0
  • Type specifies - manegement(00)/control(01)/data(10)
  • Subtype - specific to each type
  • To/From DS - Defined later
  • More Flag - setting to 1 means more framents
  • Retry - setting means to retry retransmission
  • Power Managment - setting means that it is in power managment mode
  • More data specifies stations has to send.
  • Wire encryption takes place
  • Rsvd - is reserved
• Important details
  • In many types of frames this field defines NAV, that is different in control frame
  • Addressing is performed with 4 x 6 byte field which is reliant on to/from flag, discussed now
  • Sequence control specifies flow number
  • Frame body may have a max number of 2312bytes, also heavily reliant of type and subtype
  • Framing checking via CRC-32, 4 byte FCS

General Addressing

• IEEE 802.11 uses 4 scenarios dependent on the flag of the aforementioned FC field
• The I/0 in these flags allows for 4 cases

Problems Preventing CSMA/CD Usage

• Hidden station problem
  • Assume B sends data to A. Station A transmits normally
  • C then sends data in station B outside transmission area
  • Collision happens that requires CTS in the framework to be prevented
• Exposed Station Protocol
  • A is sending code to B
  • C then is prohibited from talking to D that wastes bandwidth
• Exposed/Hidden usage may not be alleviated from the protocol, causing issue

Physical Layer Considerations

• Bands other than infrared operate on ISM, which in the US specifies 902-928MHz, 2.4-2.4835GHz and 5.725-5.850GHz

Protocol Considerations

• IEEE 802.11 FHSS
  • FHSS means frequency hopping spectrum
  • ISM operates on 2.4 GHz that is divided into 79 bands of 1mhz with guard bands
  • PNRG selects the hopping sequence
  • Binary FSK is normally selected to get a bandwidth of 1-2mbps
• IEEE 802.11 DSSS
  • DSSS - operates direct sequence spread spectrum
  • 2.4 GHz ISM bands
  • 1Mbauld/s operates on Binary PSK

Other Considerations

• 802.11 Infrared

  • light between 800-9500 nms
  • Encoding using Pulse position modulation
  • 1 bit at once for 1 mhz
  • light encodes bit setting on

• 802.11a OFDM

  • frequency division happens on 5Ghz,
  • ADSL is similar, reducing interference
  • PSK/QAM encoding

• 802.11b DSSS

  • Higher rate DSS
  • Backward Compatible with HR
  • data can go into 1,2,5.5, and 11mbps - which is 1+ mbh

• 802.11g

  • Forward correction on 2.4 G using OFDM
  • 22 -54 MB backwards comptible

Bluetooth

• short range technology for connecting such things as laptops and cameras Bluetooth LANs are Ad-Hoc network such that devices creates their own networks through piconets. Connection is possible through Bluetooth land but sizes are limited
• Some common Applications Peripheral devices for example Wireless mouse or keyboards, Communicate with computers Monitoring health sensors Home connections from one sensors Bluetooth was project started to 940-980s denmark. They are compliant IEEE 802.15 which functions as small area

Bluetooth: Network Structure

• Ad - hoc networks are two types -Piconet: small up to 8 users, masters at the top, others are in synchroncity. Can have one master and be in communication with either just the single or many -Scatternets: small up to 8 users, masters at the top, others are in synchroncity. Can have one master and be in communication with either just the single or many
  • Park state, as Bluetooth allows synchronization with up to 7, though up to 8 secondary devices may have access

Bluetooth: Usage & Communication

• Because activating the stations from the parked station involves active ones to be removed in the pool.

• Scatternets allows for connections of new data stream but will create a chain to the data that is currently running

• 2 gigaherts bandwidth can cause interface

• Bluetooth uses 7 internet models. Those models include the following:

  • Audio
  • Profiles
  • Data -L2Cap Layer -Baseband Layer -Radio Layer -COntrol Radio Layer - 10 mm low power devices Band = 2.4 ghz ISM with 79 channels per HZ FHSS which uses frequency hopping spectrum at 1600times per hertz every 625Us Modulation: transforms bits to signal using GPSK

Communication Details

• BaseBand: Mac layer, all access through media TDMA time slots access means devices is the means devices in 1 type of frequency. communication is only primary > secondary
• TDMA, bluetooth use to form TDD -TDMA secondary and receiver send + receive but not at same time using half duplex on single hop. single secondary communication if piconret are simply very simple that will occur. time slots 62us; the other use 0,2; secondary uses 135 Multiple secondary communication
  • the primary is still even at new odd packets. if the address points to that one device; the the secondary sends. Each of listening on even number but only one with a correct odd number

Access & Connections

• Access for the above is through pool based methods;
• Also pull/select method
• In addition, there comes 2 types of channels So - synchronous oriented - avoidance is the prime driver, it is created by reserving specific units. Is useed for real type audio at 64 kbps. ACL - more focused ensuring proper communication over a potential lower latency link, only comes through retransmission through 721 kbps.
• Frame Formation

Baseframe has 3 structure - one, three, five.

• Slot - 625 us
• Single: 1 MHZ with htz the frame.
• Multi: 3x Frame length
• 5x - uses one hop In detail we have - access code, header 54, ither three type to data; or frame, and CRC 3 bits for address and 4 bits for error The first the three - a/s hec The later, error checking bits. A sequence number bit There is a -Payload L2CAP - to get the large number Segmentation - Reas assembly using large bandwidth. The channel has ID a channel QOS- quality has service level. Bluetooth has 4 byes to get 655 large bytes using internet.