important against duplicated frames due to lost ACKs Addresses

Wireless LAN- II Rajan Shankaran ITEC851Frame Format and Addressing Rajan Shankaran ITEC851802.11 – Frame format Types  control frames, management frames, data frames Sequence numbers  important against duplicated frames due to lost ACKs Addresses  receiver, transmitter (physical), BSS identifier, sender (logical) Miscellaneous  sending time, checksum, frame control, data Frame Control Duration/ ID Address 1 Address 2 Address 3 Sequence Control Address 4 Data CRC bytes 2 2 6 6 6 2 6 0-2312 4 Protocol version Type Subtype To DS More Frag Retry Power Mgmt More Data WEP 2 2 4 1 From DS 1 Order bits 1 1 1 1 1 1 Rajan Shankaran ITEC851Special Frames: ACK, RTS, CTS Acknowledgement Request To Send Clear To Send Frame Control Duration Receiver Address Transmitter Address CRC bytes 2 2 6 6 4 Frame Control Duration Receiver Address CRC bytes 2 2 6 4 Frame Control Duration Receiver Address CRC bytes 2 2 6 4 ACK RTS CTS Rajan Shankaran ITEC851 Duration computation in RTS and CTS??MAC address format scenario to DS from DS address 1 address 2 address 3 address 4 ad-hoc network 0 0 DA SA BSSID – infrastructure network, from AP 0 1 DA BSSID SA – infrastructure network, to AP 1 0 BSSID SA DA – infrastructure network, within DS 1 1 RA TA DA SA DS: Distribution System AP: Access Point DA: Destination Address SA: Source Address BSSID: Basic Service Set Identifier RA: Receiver Address TA: Transmitter Address Rajan Shankaran ITEC851 Filtering on address 1, ack- address 32Frame Fragmentation Rajan Shankaran ITEC851Fragmentation in 802.11  Fragmentation takes place when the length of a higher-level packet exceeds the fragmentation threshold configured by the network administrator.  All fragments have:  Same frame sequence number but have  Ascending fragment numbers  to aid in reassembly.  All of the fragments that comprise a frame are normally sent in a fragmentation burst, which is shown in Figure.  To indicate that it is a fragment, the MAC sets the More Fragments bit in the frame control field to 1: Frame control information: More Fragment (MF) bit. (set to 1)  All non final ACKs continue to extend the lock for the next data fragment and its ACK. Rajan Shankaran ITEC851Fragmentation in 802.11  Final data frame: MF bit set to 0  Final ACK: Sets the NAV to 0.  No limit is placed on the number of fragments, but the total frame length must be shorter than any constraint placed on the exchange by the PHY. Rajan Shankaran ITEC851802.11 – Handoff (Roaming) Handoff: is the transition for any given station from one access point to a geographically adjacent access point as the station moves around. Current Practice: No or bad connection? Then perform: Scanning  scan the environment, i.e., listen into the medium for beacon signals or send probes into the medium and wait for an answer (passive and active scanning) Reassociation Request  station sends a request to one or several AP(s) Reassociation Response  success: AP has answered, station can now participate  failure: continue scanning AP accepts Reassociation Request  signal the new station to the distribution system  the distribution system updates its data base (i.e., location information)  typically, the distribution system now informs the old AP so it can release resources Rajan Shankaran ITEC851802.11 Handoff Inter Access Point Protocol (IAPP)  In order to create a Distribution Service (and hence an ESS), APs must be interoperable and communicate using a common protocol  Currently, APs from different vendors do not communicate with each other in a standardized manner and hence may not interoperate  Problem with scanning: latency, power consumption  802.11 (f) Inter Access Point Protocol (IAPP): a Recommended Practice Rajan Shankaran ITEC851Requirements of the IAPP  The IAPP requires the following:  Access Points: TCP/UDP over IP  TCP/IP and UDP/IP packets over the Distribution System  The presence of a RADIUS server (optional)  No IAPP functionality is required in stations  Presence of IAPP – no effect on Data transfer  Stations see L2 network  DS be able to carry IP: Not necessary for user data. Rajan Shankaran ITEC851IAPP Functions The IAPP performs the following functions:  Support the mobility of stations  Creation and maintenance of an ESS  Assistance to Layer 2 devices  Enforcement of the rule of association RADIUS server : Security Rajan Shankaran ITEC851IAPP Context  Involves relevant data transfer between APs  Not mentioned in current standards  The new standard: 802.11r – Includes: QoS, Security  Speed(handoff) versus overhead (signaling) Rajan Shankaran ITEC851IAPP Procedures  There are two main IAPP Procedures which we classify according on the type of message that initiated the procedure:  Association  Reassociation  Both procedures ensure that a Station is not associated with more than one AP  Access Point Management Entity (APME) function Rajan Shankaran ITEC851The Need for Security  IAPP messages are sent over IP: Fraud occurrence  Insertion of bogus MOVE or ADD-Notify Possible.  State information deletion.  Possibility of an attacker capturing IAPP packets  Station confidentiality is compromised.  Hence, it is necessary to provide secure connections  Between APs for the transmission of IAPP messages  Recommended: IPSec (RFCs 2406, 2407) Rajan Shankaran ITEC851The use of RADIUS  RADIUS (Remote Authentication Dial In User Service) (RFC 2865) provides a centralized method for authenticating users  In addition, it can act as a directory for information, available only to authenticated users  The proposed use of RADIUS is to maintain, for each AP, the following information:  BSSID  Shared secret  IP address of the AP  Security methods supported by the AP  Radius Services: authentication, APID information, security parameters: Keys etc Rajan Shankaran ITEC851802.11 – Multicast Support Rajan Shankaran ITEC851Multicast in 802.11  The point of interest for this work arises from the fact that the RTS-CTS-ACK exchange and the Binary Exponential Backoff algorithm is defined only for unicast transmission.  The semantics for broadcast (transmission to all stations) and multicast (transmission to a group of stations) are completely different.  broadcast as a special case of multicast  Multicast transmission:  sender (the AP) senses the medium for a period of DIFS.  If the medium is found to be free for this period of time – transmit multicast frame.  No RTS-CTS mechanism in place.  Receiver status not checked” Busy/interfering transmissions on  No ack from destination.  No reliability with multicast transmission Rajan Shankaran ITEC851Leader based protocol  One popular scheme ; Leader based approach  Leader is elected for a multicast group,  Only the leader sends a CTS to the sender.  Other stations: remain silent or NCTS  If no NCTS was received the sender goes ahead and sends data.  Leader sends an ACK if data was successfully received.  Other stations: remain silent or NACK Rajan Shankaran ITEC851Leader Based Protocol The protocol abstractly works in terms of slots.  Slot 1: The Access Point sends multicast RTS.  Slot 2: The leader- CTS: if it is ready to receive data otherwise do nothing. Other stations in the group remain silent Else they send NCTS (not clear to send) – if not ready to receive.  Slot 3: If CTS was heard in slot 2, the Access Point starts multicast data operation. Else — execute the back-off scheme and start from slot 1.  Slot 4: After the Access Point has transmitted data, the leader sends an ACK or a NACK.  Other stations – Silent (success) or NACK (failure)  Slot 5: If a ACK was heard in slot 4, the transmission is cons
idered complete. Else, the access point retransmits the multicast RTS in slot 1.  Scheme makes an interesting use of collisions  Collision cases: station-leader, station- station  Either case, transmission unsuccessful, sender retries after timeout Rajan Shankaran ITEC851Leader Based Protocol – Issues  Leader selection  Old leader leaves  Intelligent selection mechanism  Location of leader close to sender when compared to other stations- Near Far Terminal Problem  The protocol doesn’t work when the RTS reaches certain stations and not others.  The protocol fails when a NCTS or a NACK is lost and the leader sends a CTS or an ACK. Rajan Shankaran ITEC851MAC – Power Management Rajan Shankaran ITEC851Beacon  Beacon Frame: A type of management frame, provides the “heartbeat” of a wireless LAN.  enabling stations to establish and maintain communications in an orderly fashion.  is approximately fifty bytes long, with about half of that being a common frame header and cyclic redundancy checking (CRC) field.  The beacon’s frame body resides between the header and the CRC field and constitutes the other half of the beacon frame.  Destination MAC addresses: Set to Broadcast  Each beacon frame carries the following information in the frame body:  Beacon interval  Timestamp.  Service Set Identifier (SSID)  Supported rates  Parameter Sets  Capability Information  Traffic Indication Map (TIM). Rajan Shankaran ITEC851Power Saving in 802.11 Networks  In general, the best way to save power for wireless communication devices would be to switch them off.  Unfortunately, one can not do this without losing the capability to communicate in both directions.  Therefore there are two problems to be addressed in power saving:  How does a station in power save mode receive packets from other stations?  How does a station send to another station in power save mode? Rajan Shankaran ITEC851Power Saving in 802.11 Networks  The basic idea of power saving includes two states for a station: sleep and awake, and buffering of data in senders.  All stations in PS mode to be synchronized : Waking times.  At Window start time: Sender announces buffered frames for the receiver. A station receives an announcement frame: Stays awake until the frame was delivered.  This is easy to be done in infrastructure networks: Presence of Access Point: Buffering and synchronization functions. Rajan Shankaran ITEC851Power Saving in 802.11 Networks  Power Saving in IEEE 802.11 therefore consists of a Timing Synchronization Function and the actual power saving mechanism.  Power saving:  Infrastructure Mode  Ad hoc Mode. Rajan Shankaran ITEC851Power Saving in Infrastructure Mode  The AP transmits together with the beacon a so-called Traffic Indication Map (TIM).  Traffic Indication Map (TIM)  list of unicast receivers transmitted by AP.  The mobiles afterwards poll the AP for the packets.  If broadcast/multicast frames are to be transmitted, they are announced by a Delivery TIM (DTIM) and sent immediately afterwards.  Delivery Traffic Indication Map (DTIM):  list of broadcast/multicast receivers transmitted by AP  Stations in power save mode have to wake up short before the end of the beacon interval and to stay awake until the beacon transmission is over. Rajan Shankaran ITEC851Power Saving in Ad Hoc Mode  Ad-hoc Packets for a station in doze state have to be buffered by the sender until the end of the beacon interval.  Announced using Ad-hoc TIMs (ATIMs),which are transmitted in a special interval (the ATIM window) directly after the beacon.  Ad-hoc Traffic Indication Map (ATIM)  announcement of receivers by stations buffering frames  more complicated – no central AP  collision of ATIMs possible (scalability?)  ATIMs are unicast frames which have to be acknowledged by the receiver.  After sending the acknowledgment, the receiver does not fall back into doze state (see Figure – Next slide).  Both ATIMs and the data packets have to be transmitted using the standard back-off algorithm. Rajan Shankaran ITEC851Best Practice Recommendation Physical WLAN Design: 802.11g Rajan Shankaran ITEC851Rajan Shankaran ITEC851 Best Practice Recommendations: Wireless Pick newest one, cost permitting  802.11n (in 2011) Placement of APs should be consideredRajan Shankaran ITEC851 Physical WLAN Design More challenging than designing a traditional LAN  Use a temporary AP and laptop to evaluate placement of APs  Locations are chosen to provide coverage as well as to minimize potential interference Begin design with a site survey, used to determine:  Feasibility of desired coverage  Measuring the signal strength from temporary APs  Potential sources of interference  Most common source: Number and type of walls  Locations of wired LAN and power sources  Estimate of number of APs requiredRajan Shankaran ITEC851 Physical WLAN Design Begin locating APs  Place an AP in one corner  Move around measuring the signal strength  Place another AP to the farthest point of coverage  AP may be moved around to find best possible spot  Also depends on environment and type of antenna  Repeat these steps several times until the corners are covered  Then begin the empty coverage areas in the middle Allow about 15% overlap in coverage between APs  To provide smooth and transparent roaming Set each AP to transmit on a different channelWLAN: IEEE 802.11b Data rate  1, 2, 5.5, 11 Mbit/s, depending on SNR  User data rate max. approx. 6 Mbit/s Transmission range  300m outdoor, 30m indoor  Max. data rate ~10m indoor Frequency  Free 2.4 GHz ISM-band Security  Limited, WEP insecure, SSID Availability  Many products, many vendors Connection set-up time  Connectionless/always on Quality of Service  Typ. Best effort, no guarantees (unless polling is used, limited support in products) Manageability  Limited (no automated key distribution, sym. Encryption) Special Advantages/Disadvantages  Advantage: many installed systems, lot of experience, available worldwide, free ISM-band, many vendors, integrated in laptops, simple system  Disadvantage: heavy interference on ISM-band, no service guarantees, slow relative speed only Rajan Shankaran ITEC851WLAN: IEEE 802.11a Data rate  6, 9, 12, 18, 24, 36, 48, 54 Mbit/s, depending on SNR  User throughput (1500 byte packets): 5.3 (6), 18 (24), 24 (36), 32 (54)  6, 12, 24 Mbit/s mandatory Transmission range  100m outdoor, 10m indoor  E.g., 54 Mbit/s up to 5 m, 48 up to 12 m, 36 up to 25 m, 24 up to 30m, 18 up to 40 m, 12 up to 60 m Frequency  Free 5.15-5.25, 5.25-5.35, 5.725-5.825 GHz ISM-band Security  Limited, WEP insecure, SSID Availability  Some products, some vendors Connection set-up time  Connectionless/always on Quality of Service  Typ. best effort, no guarantees (same as all 802.11 products) Manageability  Limited (no automated key distribution, sym. Encryption) Special Advantages/Disadvantages  Advantage: fits into 802.x standards, free ISM-band, available, simple system, uses less crowded 5 GHz band  Disadvantage: stronger shading due to higher frequency, no QoS Rajan Shankaran ITEC851WLAN: IEEE 802.11 – future developments (03/2005) 802.11c: Bridge Support  Definition of MAC procedures to support bridges as extension to 802.1D 802.11d: Regulatory Domain Update  Support of additional regulations related to channel selection, hopping sequences 802.11e: MAC Enhancements – QoS  Enhance the current 802.11 MAC to expand support for applications with Quality of Service requirements, and in the capabilities and efficiency of the protocol  Definition of a data flow (“connection”) with pa
rameters like rate, burst, period…  Additional energy saving mechanisms and more efficient retransmission 802.11f: Inter-Access Point Protocol  Establish an Inter-Access Point Protocol for data exchange via the distribution system  Currently unclear to which extend manufacturers will follow this suggestion 802.11g: Data Rates > 20 Mbit/s at 2.4 GHz; 54 Mbit/s, OFDM  Successful successor of 802.11b, performance loss during mixed operation with 11b 802.11h: Spectrum Managed 802.11a  Extension for operation of 802.11a in Europe by mechanisms like channel measurement for dynamic channel selection (DFS, Dynamic Frequency Selection) and power control (TPC, Transmit Power Control) Rajan Shankaran ITEC851WLAN: IEEE 802.11 – Newer Standards 802.11i: Enhanced Security Mechanisms  Enhance the current 802.11 MAC to provide improvements in security.  TKIP enhances the insecure WEP, but remains compatible to older WEP systems  AES provides a secure encryption method and is based on new hardware 802.11j: Extensions for operations in Japan  Changes of 802.11a for operation at 5GHz in Japan using only half the channel width at larger range 802.11k: Methods for channel measurements  Devices and access points should be able to estimate channel quality in order to be able to choose a better access point of channel 802.11m: Updates of the 802.11 standards 802.11n: Higher data rates above 100Mbit/s  Changes of PHY and MAC with the goal of 100Mbit/s at MAC SAP  MIMO antennas (Multiple Input Multiple Output), up to 600Mbit/s are currently feasible  However, still a large overhead due to protocol headers and inefficient mechanisms 802.11p: Inter car communications  Communication between cars/road side and cars/cars  Planned for relative speeds of min. 200km/h and ranges over 1000m  Usage of 5.850-5.925GHz band in North America Rajan Shankaran ITEC851WLAN: IEEE 802.11- Newer Standards 802.11r: Faster Handover between BSS  Secure, fast handover of a station from one AP to another within an ESS  Current mechanisms (even newer standards like 802.11i) plus incompatible devices from different vendors are massive problems for the use of, e.g., VoIP in WLANs  Handover should be feasible within 50ms in order to support multimedia applications efficiently 802.11s: Mesh Networking  Design of a self-configuring Wireless Distribution System (WDS) based on 802.11  Support of point-to-point and broadcast communication across several hops 802.11t: Performance evaluation of 802.11 networks  Standardization of performance measurement schemes 802.11u: Interworking with additional external networks 802.11v: Network management  Extensions of current management functions, channel measurements  Definition of a unified interface 802.11w: Securing of network control  Classical standards like 802.11, but also 802.11i protect only data frames, not the control frames. Thus, this standard should extend 802.11i in a way that, e.g., no control frames can be forged. Note: Not all “standards” will end in products, many ideas get stuck at working group level Info:,, Rajan Shankaran ITEC851ISM band interference Many sources of interference  Microwave ovens, microwave lightning  802.11, 802.11b, 802.11g, 802.15, Home RF  Even analog TV transmission, surveillance  Unlicensed metropolitan area networks  … Levels of interference  Physical layer: interference acts like noise  Spread spectrum tries to minimize this  FEC/interleaving tries to correct  MAC layer: algorithms not harmonized  E.g., Bluetooth might confuse 802.11 OLD © Fusion Lighting, Inc. NEW Rajan Shankaran ITEC851

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