VIEWS: 6 PAGES: 39 POSTED ON: 10/21/2011
Name= 802.11v2.ppt Available in airports, hotels, McDonnells, Coffee Shops, train stations, Eircom plan to provide service from Payphones on streets. WLAN standardized by IEEE 802.11. Specifies only the physical and MAC layers, just like IEEE 802.3 Ethernet IEEE 802.5 Token Ring IEEE 802.4 Token Bus There are many versions of WLAN 802.11 original standard, 1 and 2 Mbps physical layer, CSMA/CA MAC adopted 1997 802.11a, enhanced to provide 54Mbps in 5GHz band 802.11b, provides 11Mbps in 2.4GHz, 1999 802.11g, provides 54Mbps in 2.4GHz band, 2004. 802.11? To provide 100Mbps Advantages of Wireless Operation No cabling or connector problems. No planning, (?) self configuring plug & play. Unobtrusive. Robust, reconfiguring. Flexible. Provides service in areas like airport lounges, coffee shops, train terminals etc Disadvantages: A lower QoS, lower bandwidth, (1-10 Mbps), higher error rates (10-4 as opposed to 10-10 for fibre), higher delays. Cost: WLAN adapters cost approx 10 times more Not fully standardized at present. Some proprietary solutions. Restrictions:Wireless products have to conform to national regulation to minimise interference i.e low power. Not as secure as wired networks. WLANs should operate uniformly throughout the world. Same frequencies. WLAN devices can be carried from one country to another. Should be low power with power-saving modes of operation. Should be licence-free, i.e legal in all countries. Should be simple to use. Wireless LAN Requirements •High Throughput. Efficient use of available spectrum •Capacity. May need to support hundreds of nodes in a small area. •Connection to a fixed backbone LAN •Service Area. A typical area of coverage is of diameter 100<x<300m. •Low Power. Mobile devices operate on batteries. Must conserve power •Robust & Secure. Wireless links are poor quality. Easily eavesdropped •Must be capable of operating in the presence of other networks •Licence-free operation. •Handover/Roaming. The MAC protocol must allow roaming over cells •Dynamic configuration to allow users to join and leave at random. Ad Hoc and Infrastructure based WLANs Many WLANs require an infrastructure. The infrastructure provides access to other networks, forwarding functions, and medium access control functions. Communication typically takes place only between the wireless nodes and an Access Point (AP), not directly between wireless nodes. The AP acts as a bridge to other networks, this way several interconnected wireless networks appear as one large logical wireless network, the interconnectivity provided by the bridge. Cellular ‘phone networks (e.g GSM) are infrastructure wireless networks. Also, satellite based ‘phones are infrastructure based. Ad hoc requires no infrastructure. Any node can communicate with any other within radio range. Ad hoc networks are more complex, since each node has to implement MAC. Example= Bluetooth. An ad-hoc network Ad-Hoc networks interconnected via an infrastructure. Communication is via the AP, not client-to-client. GSM is an infrastructure based wireless network, no mobile-to-mobile communication, except through MSC IEEE WLAN Objectives Specify a simple & Robust WLAN with time-bounded and asynchronous service. Specify physical and MAC layers only. Higher layers oblivious to type of physical network. MAC layer must be able to accommodate several different physical Layers, e.g infrared, spread-spectrum etc. Capable of world-wide operation, use unlicensed bands. WLANs are normally owned by a private organization, e.g Hotel Connection of 802.11 and 802.3 BSS: Basic Service Set DS: Distribution System AP:Access Point Portal ESS:Extended Service Set Note that IEEE does not specify form of DS. But it does specify its services. Bridging between 802.11 and 802.3 (Portal) Association:Establishes an association between a station and an AP within a BSS. The AP transfers the details of the Station to other APs when the station roams. This is similar to the registration of a mobile user in the HLR in GSM. Re-association: This is the transfer of a mobile station from one BSS to another. Disassociation: A notification from a station or from an AP that an existing association is terminated. i.e on shutdown or on exit from an ESS. Integration:enables the transfer of data between an 802.11 station and an 802.x (i.e Ethernet) wired LAN.Refers to a wired LAN that is physically connected to the DS and whose stations may be logically connected to the 802.11 LAN via the integration service Physical Layer There are 5 separate physical layer specifications in 802.11. 1. A frequency hopping spread spectrum in ISM 2.4 GHz band. Provides either 1Mbps or 2 Mbps depending on modulation used. 2. A direct sequence spread spectrum (DSSS), in ISM 2.4GHz band. Provides either 1Mbps or 2 Mbps. (2 Mbps is optional). 3. An Infrared specification. Either 1Mbps or 2Mbps. About 20m range 4. 802.11a. Uses a wider bandwidth, 5GHz, orthogonal frequency division multiplexing (OFDM).Provides a range of speeds, 6,9,12 18,24,36, 48, 54 Mbps. Not Spread Spectrum. 5. 802.11b. Uses the 2.4GHz ISM band and delivers either 5.5Mbps or 11Mbps. DSSS. Same bandwidth as 1. Different modulation (CCK) 6. 802.11.g with OFDM, data rate 54Mbps The Physical Layer Options With OFDM, the serial data stream is split into several parallel streams via a serial to parallel converter at the sending site. The reverse happens at the receiver site. The Physical Layer (Contd.) The FHSS specification permits the overlapping of several networks in the same area, each having a different hopping sequence. It defines 79 channels, each of width 1Mhz,(most countries except Japan, France Spain). The overall bandwidth of the ISM band is from 2.4-2.4835Mhz This band is unregulated, there is interference from garage doors, cordless phones, radio controlled toys, microwave ovens, Bluetooth. It uses Gaussian Frequency Shift Keying (GFSK) with either 2/4 states to deliver either 1or 2 Mbps. The DSSS, separates networks from each other with codes, not frequency. It offers a robust network in face of interference, and insensitivity to multipath effects. It uses Differential Binary Phase Shift Keying (DPSK) to yield 1Mbps and Quadrature Binary Phase Shift Keying (QPSK) to yield 2Mbps. The LLC Layer •Ethernet offers best effort transport. There is no error or flow control. •The LLC (IEEE protocol) hides the differences between the various kinds of 802.x networks by providing a common format and interface to the network layer. LLC is based on HDLC. It provides error and flow control. •LLC provides 1)unreliable datagram service 2)ack’ed datagram service 3)Reliable connection oriented service, the user chooses most appropriate service. •The frame header contains a source and destination access points addresses, (SSAP & DSAP), seq numbers and acks similar to HDLC. •When used over 802.3 Ethernet, the combination provides a reliable flow controlled transport service The MAC in Ethernet (CSMA/CD) The MAC is intended to reduce the amount of collisions. Collisions occur because of the non zero propagation delay of a stations transmission until it is observable by all other stations on the network. Before accessing the network, a station is required to ‘listen’ or sense the network to see whether it is idle or busy. If it is busy, the station defers, and tries again later. If it is idle, then the station transmits its frame, while at the same time monitoring the network for possible collisions which can occur during the vulnerable period within the collision window. If the vulnerable period expires without a collision, then the station has acquired the network, and will succeed in transmitting its frame. If a collision occurs, each station backs off for a random period of time after which they will retry. There are some differences between a fixed LAN and a wireless LAN. 1.Except during the collision window, in a wired LAN, all stations can ‘hear’ the transmissions of every other station, and therefore will know not to interfere. This is generally not the case in wireless LANs. Radio waves propagate omnidirectionally from the source and attenuate ~1/d2. Therefore it is possible that some stations may not hear the transmissions of other stations, i.e they may be out of range. Suppose A wishes to transmit to B, and suppose further that B is already receiving from another station C which is out of range of A. A can sense the medium and find it idle, and begin to transmit to B. It’s transmission will corrupt C’s transmission to B, i.e a collision. A cannot detect this collision. This is a case of a collision occurring at the destination. The conclusion is that CSMA with collision detection (CD) does not work in a wireless LAN. 2.Detecting a collision at the source is also difficult in a wireless LAN. It would require radio equipment which could receive its own transmission, i.e to be able to send and receive simultaneously. For cost reasons, in order to keep the equipment cheap, the standard does not require the ability to send and receive simultaneously. 3 For the above reasons, CSMA with collision avoidance is used in wireless LANs. (CSMA/CA). The Hidden Station Problem The MAC for 802.11 is different from Ethernet. Hidden station problem. Low power levels means that not all stations in a cell (BSS) are within radio range of each other. In (a), C is transmitting to B. If A senses the medium it will find it idle, and start transmitting to B. Collision. A’s transmission will corrupt C’s at B. C is hidden to A and vice versa. The exposed station problem (b), B does not transmit to C since it finds the medium busy (A is transmitting to D, not shown but out of range of C). It could since A’s transmission is out of range of C. B is exposed to A. Delay not collision. THE MAC LAYER There are 3 separate MAC Layer specifications. Each relies on a ‘Clear Channel Assessment’(CCA) from the physical layer to determine if the medium is idle. Each Physical layer specification is required to provide a CCA to the MAC. 1. A mandatory basic CSMA/CA. Supports broadcasting. 2. An optional method, an enhancement of the basic CSMA/CA to cope with the hidden station problem. 3. A contention free polling MAC for a time bounded service. 3. The methods 1 and 2 above are referred to as DCF, i.e Distributed Coordination Function. Method 3 is referred to as PCF, i.e Point Coordination Function. PCF requires a fixed infrastructure (polling performed by an AP). It is not available in ad-hoc networks. Inter Frame Spacing Idle network DCF 1. The MAC Basic CSMA/CA Protocol (DCF) If the medium is idle, and remains idle for interval DIFS, a node can access the medium at once. If the medium is busy, a node has to wait for DIFS. If still idle after DIFS, a node calculates a random backoff time interval within a contention window. (A slotted window, window size determined by propagation RTT, transmitter delay, and other physical characteristics). If, on expiry of the timer, the medium is idle, it can be seized at once. If, on expiry of the timer, the medium is busy, then the node stops it’s timer, waits for the channel to become idle again for DIFS, and starts the timer again. As soon as the timer expires the node seizes the channel. (The timer has not been initialized, but counts down from the stop-value. Deferred stations do not choose a randomized waiting time again, but count down). Longer waiting stations have advantage over newly contending stations, they only have to wait for the remainder of the backoff timer interval from the previous cycle. When the source transmits a frame, a timer is started. If an acknowledgement is not received in the lifetime of the timer, it is assumed there was a collision, and a retransmission is started. DIFS: Distributed Coordination Interframe Spacing PIFS: Point Coordination Interframe Spacing If the medium is busy, all access attempts are deferred. When the medium becomes idle, an interval of SIFS (Short Inter Frame Spacing) must elapse before the next transmission. This is the shortest delay, i.e the highest priority. Used for returning acks. MAC CSMA/CA Basic Protocol (Contd.) The MAC Basic protocol tries to adapt the size of the contention window to minimise the extent of collissions. Under heavy load, there can be significant collisions. The initial size of the contention window is 7 slots. Each time a collission occurs, its size is doubled to a max of 255. (Values are 7,15,31,63,127,255). Under light load conditions, a Small CW results in less delay in accessing the medium. For unicast transmission, an additional feature involving an ACK is provided to cope with poor quality radio links. After receiving a frame, the receiver returns an ACK, after waiting only SIFS. No other station can access the medium within SIFS to cause a collission. If no ACK is returned, sender retransmits. For a retransmission, sender has to compete with other users, no special priveliges. Max limit on number of retransmissions. Failure is reported to higher layers. It is more efficient to correct failed transmissions at this level than wait for recovery via upper layers e.g TCP. (See diagram 7.12 next slide.) 2. DCF with RTS/CTS The standard defines an optional additional mechanism using two Control packets, Request To Send (RTS) and Clear To Send (CTS). After waiting for DIFS plus a random backoff time the sender issues RTS. The RTS identifies the intended receiver, plus the duration necessary to transmit the whole data frame and its related ack. Every node that receives this RTS sets its Network Allocation Vector (NAV) to reflect the duration for which it must not attempt to access the medium. This is an internal timer set by those nodes which have sensed the RTS. When the receiver answers after SIFS with a CTS, this packet contains the duration required for data transmission also. All stations within reach of the CTS-issueing node (possibly a different set) set their NAV to allow the data transmission to take place. The sender sends the data after SIFS. An ACK is returned after SIFS if successful. NAVs are cancelled, cycle starts again. Collissions can only occur on sending RTS. The use of a returned Ack after SIFS in Unicasting. 2. Enhanced DCF with RTS/CTS A situation where a station can sense the presence of two other stations, but the other two stations cannot sense each other, there can be collisions. This is called the hidden station problem. Also there is the exposed station problem. To cope with this problem an enhanced MAC is specified. 3. The PCF MAC The PCF provides a contention-free medium access mechanism via polling. It’s use requires an Access Point (AP). Not in ad hoc networks. Works alongside DCF. Time is split into ‘superframes’. A superframe slot contains a contention-free period plus a contention period. During the contention- free period, nodes (which have signed up for polling via a special beacon frame) are polled. After the polling cycle is complete, a special end marker CFend is transmitted to indicate the start of the contention period. The length of the contention-free period depends on how many nodes have to be polled. Polling has priority over DCF. MAC Fragmentation To decrease the probability of corruption to transmitted frames, the MAC specifies a fragmentation mechanism. The frame size is adjusted by the MAC to reflect the link error rate. The sender sends RTS (after DIFS) to reserve the medium. It includes the duration parameter for first fragment and related ack. Nodes within reach set their NAVs. The CTS asserts the duration parameter also, nodes within reach set their NAVs. The first fragment is sent after SIFS. The first fragment also includes a duration parameter in respect of the next fragment. This serves to reserve the medium for 2nd fragment and related ACK. An ACK is returned with a duration parameter for 2nd Fragment. Stations within reach set their NAVs, the medium is reserved For 2nd fragment . The last fragment does not include a duration, so medium reservation is terminated. (See diagram in next slide 7.14) THE MAC FRAME Frame Control:Protocol version, frame-type (data,control,mgt), frag? 2 DS bits relating to the 4 addresses. Duration: Used with the RTS/CTS reservation scheme. Addresses: standard MAC addresses, source, source AP, dest AP and destination node. All 48 bits. Sequence Control: A sequence number to deal with duplicates. A four Bit field for fragmentation control, 12 bits for frame sequencing. Data: the payload of the frame from a higher layer. Max 2312 bytes. CRC: 4 octet checksum. Control Frames Synchronization Each 802.11 node maintains an internal clock. Synchronization of clocks is required for power management and for coordination of PCF, hopping sequence etc.Performed by AP in infrastructure based networks. Within a BSS, timing is provided by the quasi periodic transmission of a beacon frame, which contains a timestamp, power mgt data, and roaming data.Beacon frame transmission may be delayed due to busy medium, hence quasi periodic. For ad-hoc networks, synchronization is more difficult. Here each node transmits a beacon signal based on its own clock. There are fixed beacon transmission intervals. Standard Access rules apply to beacon transmission, so generally only one beacon signal survives. All stations base their clock on the received beacon signal. Power Management Wireless devices are normally battery powered.Power saving modes are required. Switch off transceiver when idle. A station has 2 states. Sleep and awake. When a sender wishes to communicate with a sleeping station it has to buffer the transmission until the station wakes up. Sleeping stations have to wake up periodically, and stay awake for a minimum time. During this time all senders announce the destinations of their bufferred data. When a station discovers that it is a target destination of some bufferred data, it must stay awake until it receives the data. Waking up a the right time requires the Timing Synchronization function (TSF). Power management in infrastructure networks is easier than in ad-hoc networks. The AP buffers all data for sleeping nodes. With every beacon frame sent by the AP, a Traffic Indication Map (TIM) is transmitted which contains a list of station for which there is buffered data (unicast) in the AP.For multicast/broadcast, station must stay awake. Roaming Typical networks within buildings require more than one AP. The range of an AP is (10-20)m. When a user is mobile the station moves from one AP to another. For uninterrupted service a handover has to take place from AP to AP. When the current link quality is too poor, the station starts scanning for another access point. In ad hoc networks, it can lead to the creation of a new BSS. Passive scanning involves listening for other networks. Active scanning involves sending a probe on each channel, and waiting for a response. Based on signal strength, the station selects an AP. It sends an association request to the selected AP. The AP responds with an association response message. The AP accepting an association request indicates the new station in its BSS to the DS. The DS updates its DataBase to reflect the current location of each wireless station. Also the DS innforms the old AP that the station is no longer in its BSS. A HLR for 802.11. Summary. WLAN flexible, lower quality than wire based. Ad hoc versus Infrastructure based WLAN concerned with Phy and MAC only. LLC mediates with upper layers. Unlicenced in 2.4 GHz band, available worldwide Large network via a DS. Phy based on either optical (infraRed) or radio. (FHSS or DSSS or OFD Roaming via AP IEEE 802.11b allows either 5.5 or 11 Mbps IEEE 802.11a allows up to 54 Mbps in 5GHz range Hidden Station and Exposed Station Problems 3 separate MACs. CCA always available from any of 5 Physical layers 1. Basic. No consideration of hidden Sation problem 2. Enhanced. Takes care of Hidden station problem via RTS/CTS 3. Contention free, via polling by AP. MAC does fragmentation, synchronization and power management References 1. Mobile Communications. Jochen Schiller. Addison Wesley. ISBN=0-201-39836-2 2. Wireless Communications and Networks.William Stallings. Prentice Hall. ISBN=0-13-040864-6 3. Computer Networks. Andrew S. Tanenbaum. 4th edition. Prentice Hall. ISBN=0-13-038488-7 4. IEEE 802.11 Wireless LAN Working Group Web Site.
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