WLAN 1

IEEE Overview Adopted in 1997 Defines: MAC sublayer MAC management protocols and services Physical (PHY) layers – IR – FHSS – DSSS Lecture 4: and more2

3 Wifi and WLAN Wi-Fi (Wireless Fidelity) is a generic term that refers to the IEEE communications standard for Wireless Local Area Networks (WLANs). Wi-Fi Network connect computers to each other, to the internet and to the wired network. Maximum radiated power 1 W in US and 100 mW in EU countries (India commonly follows EU regulations but exact figures are not known to me right now)

particulars b (WiFi) – Frequency: Ghz DSSS – Modulation: DBPSK (1Mbps) / DQPSK (11 Mbps) – Orthogonal channels: 3 There are others, but they interfere. (!) – Rates: 1, 2, 5.5, 11 Mbps a: Faster, 5Ghz OFDM. Up to 54Mbps g: Faster, 2.4Ghz, up to 54Mbps

IEEE b Appeared in late 1999 Operates at 2.4GHz radio spectrum 11 Mbps (theoretical speed) - within 30 m Range 4-6 Mbps (actual speed) feet range Most popular, Least Expensive Interference from mobile phones and Bluetooth devices which can reduce the transmission speed.

IEEE a Introduced in 2001 (less popular) Operates at 5 GHz 54 Mbps (theoretical speed) Mbps (Actual speed) feet range More expensive Not compatible with b

7 IEEE g Introduced in 2003 Combine the feature of both standards (a and b) feet range 54 Mbps Speed 2.4 GHz radio frequencies Compatible with IEEE b

Physical Layer There are three sublayers in physical layer: Direct Sequence Spread Spectrum (DSSS) Frequency Hoping Spread Spectrum (FHSS) Diffused Infrared (DFIR) - Wide angle

Lecture 4: and more DSSS Direct sequence signaling technique divides the 2.4 GHz band into MHz channels. Adjacent channels overlap one another partially, with three of the 11 being completely non-overlapping. Data is sent across one of these 22 MHz channels without hopping to other channels.

details Three types of frames (see the WLAN tutorial manual for details): – data frame – control frame – management frame Fragmentation – can fragment large packets (this is separate from IP fragmentation). Preamble (check this carefully) – 72 1Mbps, 48 2Mbps – Note the relatively high per-packet overhead. Control frames – RTS/CTS/ACK/etc. Management frames – Association request, beacons, authentication, etc.

Overview, Architecture STA AP ESS BSS Existing Wired LAN Infrastructure Network Ad Hoc Network BSS: Basic Service Set ESS: Extended Service Set

Distribution System Portal 802.x LAN Access Point LAN BSS LAN BSS 1 Access Point Architecture of infrastructure network Station (STA) – terminal with access mechanisms to the wireless medium and radio contact to the access point Basic Service Set (BSS) – group of stations using the same radio frequency Access Point – station integrated into the wireless LAN and the distribution system Portal – bridge to other (wired) networks Distribution System – interconnection network to form one logical network (EES: Extended Service Set) based on several BSS STA 1 STA 2 STA 3 ESS

Architecture of an ad- hoc network Direct communication within a limited range – Station (STA): terminal with access mechanisms to the wireless medium – Basic Service Set (BSS): group of stations using the same radio frequency LAN BSS LAN BSS 1 STA 1 STA 4 STA 5 STA 2 STA 3

modes Infrastructure mode – All packets go through a base station – Cards associate with a BSS (basic service set) – Multiple BSSs can be linked into an Extended Service Set (ESS) Handoff to new BSS in ESS is pretty quick – Wandering around CMU Moving to new ESS is slower, may require re-addressing – Wandering from CMU to Pitt Ad Hoc mode – Cards (network interface card or NIC) communicate directly – Perform some, but not all, of the AP functions

Services: Station services ( activity within a single cell) : – authentication, – de-authentication, – privacy, – delivery of data Distribution services (manage cell membership and interact with stations outside the cell): – association – disassociation – Re-association – distribution – Integration A station maintain two variables: authentication state (≥ 1) i.e., can authenticate with more than one AP-s association state (≤ 1) i.e., can associate with one or zero APs

IEEE standard mobile terminal access point server fixed terminal application TCP PHY MAC IP MAC PHY application TCP PHY MAC IP MAC PHY LLC infrastructure network LLC

Layers and functions PLCP Physical Layer Convergence Protocol – clear channel assessment signal (carrier sense) PMD Physical Medium Dependent – modulation, coding PHY Management – channel selection, MIB Station Management – coordination of all management functions PMD PLCP MAC LLC MAC Management PHY Management MAC – access mechanisms, fragmentation, encryption MAC Management – synchronization, roaming, MIB, power management PHY DLC Station Management MIB: management information base

MAC Management Functions The MAC Management is responsible for the following: – maintaining time synchronization between stations transmitting beacons – channel scanning – forming, joining or leaving a BSS or IBSS – power management – association and re-association The last four functions above are carried out in response to requests from the Station Management Entity (SME)

19 MAC Management Structure

20 Time Synchronization and Beacons

Time Synchronization All stations maintain a Time Synchronization Function (TSF) The TSF is a timer, counting in microseconds with a modulus (i.e. length) of 2 64 (2 10 ) 6 = Stations within a BSS or IBSS synchronize their TSF by transmitting or receiving Beacons. Each beacon contains the value of the TSF of the transmitting station All TSF values are adjusted to take account of propagation and processing delays

Beacon: Beacons are management frames and contain all network information Beacons are transmitted periodically to announce the presence of a BSS (or IBSS) In an infrastructure network, only the AP transmits the beacon In an IBSS beacon generation is distributed amongst the participating STA-s The Beacon frame consists of MAC header (to be discussed later), frame body and FCS

Beacon (contd): Important parameters in a beacon are: – Time interval: beacons are transmitted periodically. This field provides the period of beacon transmission – Time stamp: Timestamp is the time at which an event is recorded by the computer- and not necessarily the time when the event occurred. The beacon timestamp enables the receiving STA to update its local clock and synchronise with the transmitting STA – Service set identifier (SSID): the SSID identifies a specific ESS. An AP transmits the SSID in the beacon frame and an STA is usually required to configure itself with the same SSID. Noted afterwards that it poses security problems and it is possible to disable this field – Supported rate: a beacon may inform that only 1 and 5.5 Mbps rates are supported, or 11 Mbps is not supported etc (for example) – Parameter sets: includes information about modulation, hopping parameters (for frequency hopped system) etc

Beacons: – Capability set: the requirements that the STA-s in the BSS should adhere to; for example, if they must use wired equivalent privacy (WEP) – Traffic indication map (TIM): is sent by an AP to indicate if an STA in a power saving mode would have data to receive (from the AP) An probe –response frames are quite close to beacon frames except that they do not contain the TIM field A beacon interval is a configurable parameter and expressed in time unit (TU). A time unit is 1024 micro-sec (note the difference from a mili-sec). The beacon interval is commonly set to 100 TU. In an IBSS, an initiating STA sends a beacon; if an STA does not hear a beacon within a random time interval it may send its own beacon frame to instantiate an IBSS

Beacon Frame

26 Synchronization in a BSS In a BSS, the AP is the only station to transmit beacons. Beacons indicate – that it is an Access Point – the TSF value of the AP – the beacon period (i.e. time between beacons) All other stations shall update their TSF to the value in the beacons sent by the AP

27 Beacon Transmission in a BSS Beacons are supposed to be transmitted at regular intervals However, If a beacon transmission is delayed due to the medium being busy, subsequent beacons shall be transmitted according to the original schedule

Authentication Process IEEE defines two different MAC layer authentications: - Open System Authentication - Shared Key Authentication These authentication mechanisms are defined with respect to infrastructure network only. Pre-authentication: Pre-authentication is typically done by a STA while it is already associated with an AP. If the authentication is left until re- association time, this may impact the speed with which a STA can reassociate between APs, limiting BSS-transition mobility performance. Authentication will be discussed afterwards

Synchronization in an IBSS The station that instantiates the IBSS sets the beacon period All other stations shall wait until they have received a beacon before sending any beacons A station shall update its TSF to the value in the received beacon if the received value is greater than the station’s TSF Given the time the beacon was received, and the beacon period, stations can calculate Target Beacon Transmission Time (TBTT) for all following beacons

Management Operations Scanning Association/Reassociation Time synchronization (using Time synchronization function (TSF)) Power management

Scanning & Joining Goal: find networks in the area Passive scanning – No require transmission  saves power – Move to each channel, and listen for Beacon frames Active scanning – Requires transmission  saves time – Move to each channel, and send Probe Request frames to solicit Probe Responses from a network Joining a BSS: listen to the beacon and get the following done: – Synchronize in TSF and frequency (using beacon time stamp): Adopt PHY parameters : authenticate

I wish to discuss authentication separately A glimpse of the authentication process Data protection 802.1X authentication 802.1X key management RADIUS-based key distribution Security capabilities discovery Authentication Server Access Point Station

Association in AP 1: Association request 2: Association response 3: Data traffic Client

Reassociation in New AP 1: Reassociation request 3: Reassociation response 5: Send buffered frames Old AP 2: verify previous association Client 6: Data traffic

Time Synchronization in Timing synchronization function (TSF) – AP controls timing in infrastructure networks – All stations maintain a local timer – TSF keeps timer from all stations in sync Periodic Beacons convey timing – Beacons are sent at well known intervals – Timestamp from Beacons used to calibrate local clocks – Local TSF timer mitigates loss of Beacons

Power Management in A station is in one of the three states – Transmitter on – Receiver on – Both transmitter and receiver off (dozing) AP buffers packets for dozing stations AP announces which stations have frames buffered in its Beacon frames Dozing stations wake up to listen to the beacons If there is data buffered for it, it sends a poll frame to get the buffered data Lecture 4: and more36

Lecture 4: and more37