Wireless LAN A competing method to wired LAN. Course: Wireline Communication Instructor: Prof. Werner Henkel Student: Chin Yung Lu

Transcription

1 Wireless LAN A competing method to wired LAN Course: Wireline Communication Instructor: Prof. Werner Henkel Student: Chin Yung Lu

2 Outline of the presentation Introduction Background Problem Environment Traffic and Services Network Structure System Architecture Interference Throughput calculations IEEE Vs HIPERLAN/2

3 Introduction The increased demand for mobility and flexibility leads to the development from wired LANs to wireless LANs (WLANs) Users have as high demands in the performance of wireless LAN as they do in the wired LANs Two competing standards operating in 5GHz band: HIgh PErformance Radio Local Area Network-type 2 (HIPERLAN/2) defined by ETSI IEEE a defined by IEEE

4 Background Both standards offer decoded transmission rates up to 54 Mbps in the 5 GHz band IEEE a has been developed from earlier standard in the 2.4 GHz band The spectrum in the 5 GHz band was allocated to meet the increased demand on throughput for WLANs It is wider than the allocated spectrum in 2.4 GHz WLANs can be used in different environments: Home: interconnecting electronic equipments Public: support mobile access networks, infrastructure replacement Disadvantages: complexity of the radio environment, dynamic nature, security problem power consumption since the stations must be battery powered

5 Data rates versus mobility in different communication systems

6 The problem here What do these standards offer? HIPERLAN/2 or IEEE a? Or, should we jump into wireless now?

7 Environment Environment Examples Common factors Corporate Offices, hospitals, universities, and hotels They are rather large, have one owner, and have a lot of internal walls Public (urban) Home Exhibition halls, airports, and town squares Homes and small offices Large open areas where many operators could coexist Places with small areas and a lot of neighbours

8 Environment Wave propagation The common part: The corporate environmental part: The specific part: L fs = 2 4 π d λ (db), L w = W K ( K + 2 b ) K + 1 (db), L o = α d (db), d (m) - separating distance λ (m) - wavelength W - wall or floor attenuation, b - constant K - number of walls or floors d (m) - distance between the antennas α(db/m) - constant that reflects αthe amount of objects the path loss between two isotropic antennas in free space equal for all types of environments path loss due to walls and floors path loss due to objects

9 Environment Wave propagation Path loss mentioned in the previous slide is simplified and static A deviation is added for compensation which is denoted as fading: Consists of a slow and fast part Fast fading arises from the delay spread Slow fading represents people and objects in motion modelled as a lognormal first order Auto Regression (AR) process with a standard deviation equal to σ (db). L f = 2 ρ L f(t - t) + 1 ρ N(0, σ ) (db) Lf(t - t) is the previous sample, ρ is the correlation coefficient The total path loss (L) between two antennas in the corporate environment is now calculated as L = L fs + L w (#walls) + L w (#floors)+l f (db), and in the public environment L = L fs + L o + L f (db).

10 Traffic and Services High MultiMedia (HMM), e.g., web browsing, file transfer and video surveillance. High Interactive MultiMedia (HIMM), e.g., video conference, web hosting and client server. Very High MultiMedia (VHMM), e.g., streaming video and spooling video. Service HMM HIMM VHMM Corporate DL kbit/s Corporat e UL kbit/s Public DL kbi t/s Public UL kbi t/s Table 2. Net user bit rates for different services. Traffic model data come in as bursts

11 Network Structure 2 different types of network structures Infrastructure network AP cell = A number of Mobile Terminals (MTs) and an Access Point (AP) controlled by a single Coordination Function (CF) a logical function that determines when a station, i.e., a MT or an AP, is allowed to access the wireless medium. is connected to a Backbone Network AP cell MT MT AP Backbone Network MT AP MT AP cell Infrastructure network.

12 Network Structure Ad hoc network is typically created in a spontaneous manner is limited in time no connection to the Backbone Network, therefore no AP any Mobile Terminal (MT) can establish a direct communication with any other station, through a centralized access point MT MT MT Ad hoc network.

13 System Architecture Both HIPERLAN/2 and IEEE a are based on a layer architecture as follows: HIPERLAN/2 Backbone Network CL CL Layer IEEE802.11a Backbone Network RLC MAC Error control DLC Layer MAC MAC Layer PHY PHY Layer PHY PHY Layer Layer architecture for HIPERLAN/2 and a.

14 System Architecture MAC in IEEE a Medium Access Control (MAC) layer in IEEE a Takes care of all incoming packets from the BN Handles up to 4095 bytes w/o segmentation Based on CSMA, 1. A terminal with data to transmit first listens to the medium 2. If the medium is idle the station does not transmit immediately but continues to monitor the medium. 3. If the medium remains idle for a period known as the inter-frame space (IFS) then the station starts to transmit. 4. If the medium is in use, the transmission is deferred by one IFS period + a random back-off period. 5. If the medium is idle after the back-off period then the transmission starts. 6. If the medium comes into use before the back-off period finishes then the process is repeated from 4 with a longer back-off period. Collision Avoidance (CA) is implemented also takes care of Error Control (EC), authentication, key management, association, disassociation, and encryption seed distribution

15 System Architecture MAC in IEEE a (Frame types and formats) 3 types of frames used Transmission frame for association and authentication management control frame for handshaking and acknowledgement Request To Send (RTS) Clear To Send (CTS) Acknowledgement (ACK) data frame for transmitting data A MAC header that consists of frame control, duration, address, and sequence control information. MAC header 30 bytes Data frame Data bytes CRC 4 bytes The length of the MAC header depends on the frame type MAC header 16 bytes RTS frame CRC 4 bytes A data field of variable length MAC header 10 bytes CRC 4 bytes A 32-bits CRC used for error detection CTS/ACK frame MAC layer frame formats in IEEE a.

16 System Architecture - MAC function Medium access DIFS Distributed coordination function Inter Frame Space ACK Acknowledgement message sent back SIFS Short Inter Frame Space virtual carrier sensing method Medium access with handshaking RTS is sent before transmission A CTS is responded This is to avoid the hidden terminal problem

17 System Architecture - Convergence Layer(CL) and Data Link Control layer (DLC) in HIPERLAN/2 Convergence Layer (CL) in HIPERLAN/2 receives and transmits packets from and to higher layers Converts incoming traffic to packets of fixed length (48bytes) + 12 bits overhead Consists of Error Control (EC) sub-layer, Radio Link Control (RLC) sublayer, and a MAC layer EC handles 3 different modes of transmission strategies: Acknowledgement mode based on Selective Repeat Automatic Repeat request Repetition mode all transmissions are repeated a number of times w/o ACK Unacknowledged mode RLC is in charge of 3 control functions: Association function Radio Resource Control (RRC) function DLC user connection control function

18 System Architecture - MAC frame structure in HIPERLAN/2 the medium access is based on TDMA/TDD, using a fixed MAC frame of 2ms BCH = broadcast control, FCH = frame control, ACH = access feedback control, RCH = Random Channel A Long Channel (LCH) contains control or user data A Short Channel (SCH) contains only control data. Info contains control information. MAC frame MAC frame 2 ms MAC frame The Sequence Number (SN) is provided for the Error Control (EC) function. Broadcast phase Downlink phase Uplink phase RCH phase Burst Burst The CL bits are overhead added by the convergence layer. The CRC is used for error detection. BCH 15 bytes Frame type 4 bits FCH Info 52 bits SCH 9 bytes ACH 9 bytes CRC 2 bytes SCH SCH LCH LCH Frame type 2 bits SN 10 bits CL 12 bits LCH 54 bytes Data 48 bytes CRC 3 bytes Figure 12. MAC frame structure in HIPERLAN/2

19 System Architecture - MAC function in HIPERLAN/2 Uses a centralized control mechanism. AP is responsible for assigning radio resources within the MAC frame MT uses the RCH for sending a Resource Request (RR) to the AP If collision occurs, AP informs MT in a following ACH A back-off procedure takes place in RCH If RR is successful, AP schedules the MT transmission A RR contains the number of pending LCHs and the MT and the AP assigning resources The FCH sent by the AP contains an exact description of the resource allocation within the MAC frame BCH is used by AP to broadcast cell information

20 System Architecture - Physical layer (PHY) Breaks down the packets from the MAC layer into Orthogonal Frequency Division Multiplex (OFDM) symbols and transmits them. Both standards PHY work similarly Packet from MAC/DLC layer Scrambling Coding and Puncturing Interleaving Mapping OFDM To transmitter Physical layer functions. Scrambling prevents long runs of 1s and 0s in the end of modulation process Coding uses a convolutional encoder, by adding bits, to detect and correct errors Puncturing systematically removes some of the encoded bits thus increases code rate Interleaving to prevent error bursts Mapping the encoded and interleaved data is mapped to data symbols according to schemes depending on the requested transmission mode OFDM chosen due to its excellent performance on highly dispersive channels

21 System Architecture - Physical layer (PHY) - OFDM The channel spacing is 20 MHz, which allows high bit rates per channel but still has a reasonable number of channels in the allocated spectrum (e.g. 19 channels in Europe). 52 sub-carriers are used per channel, where 48 sub-carriers carry actual data 4 sub-carriers are pilots which facilitate phase tracking for coherent demodulation. The duration of the guard interval is equal to 800 ns, which is sufficient to enable good performance on channels with delay spread of up to 250 ns. An optional shorter guard interval of 400 ns may be used in small indoor environments. This modulation is chosen because of its high ability to combat Inter Symbol Interference (ISI) As a result, there is no need for an equalizer in the receiver A drawback is a required back-off in the transmitter power amplifier, which influences the power efficiency. About OFDM: The available bandwidth is divided into several sub-channels where each sub-channel is modulated and transmitted in parallel The sub carrier frequencies are chosen so that the different signals carried by these frequencies are orthogonal in time.

22 System Architecture - Physical layer frame format (IEEE a) A preamble used for Automatic Gain Control (AGC) convergence, antenna selection, timing/frequency acquisition, and channel estimation. The RATE field states the bit rate at which the rest of the frame is transmitted with. The length field states the number of bytes in this frame sent from the MAC layer. The tail bits enable decoding of the signal field immediately after reception, which is necessary since the decoding of the rest of the frame depends on the RATE and LENGTH fields. The service field is used to synchronize the descrambler. The second tail is used for decoding the rest of the frame. Pad bits are used to map the transmission into an integer number of OFDM symbols. RATE 4 bits Reserved 1 bit LENGTH 12 bits Parity 1 bit Tail 6 bits Preamble 16µs Signal 4µs Service 2 bytes MAC header 30 bytes Data Variable number of bytes CRC 4 bytes Tail 6 bits Pad bits From MAC layer Figure 14. IEEE802.11a physical layer frame format.

23 System Architecture - Physical layer frame format (HIPERLAN/2) non-specific fields are added to the LCH pr SCH frames from the DLC layer The physical layer adds 1 preamble to a burst of SCHs and LCHs in the downlink and uplink 1 preamble to the broadcast phase 1 preamble to the RCH phase Preamble Broadcast phase Downlink phase Preamble Burst Uplink phase Preamble Burst Preamble RCH phase HIPERLAN/2 physical layer frame format

24 System Architecture - Spectrum allocation and rules Frequency band (MHz) Number of channels RF power limit Comment mw mean EIRP Indoor use only W mean EIRP (200 mw for the highest channel) Indoor/outdoo r Spectrum allocation and rules for HIPERLAN in Europe. For HIPERLAN/2, different parts of the 5-GHz band have different operational conditions in order to co-exist with other systems The IEEE a standard is adapted to the US where spectrum allocation and rules differ from the European. For both standards a channel spacing of 20 MHz is used, this allows high bit rates with a reasonable number of channels.

25 System Architecture - Quality of Service (QoS) Throughput: the highest possible transmission rate is always used which results in decoded transmission rates up to 54 Mbps. HIPERLAN/2 uses a centralized medium access control allocates transmission time according to specific session It is not the same case for IEEE a Correctness: The EC function in both standards use CRC and ARQ-scheme in the acknowledged mode to decrease the PER on higher layers. Delays: If a session suffers from a low transmission rate and/or a high PER, the session will consequently suffer from delays.

26 System Architecture - Radio Resource Management (RRM) Dynamic Frequency Selection (DFS) Required in HIPERLAN/2 but not yet supported in IEEE a Implemented to ensure that each AP cell is transmitting in the lease interfered frequency band. For HIPERLAN/2 to operate in the allocated 5GHz spectrum bands in Europe Necessary to allow co-existence with other systems Implementation is vendor specific A straightforward and easy method is shown here. 1. Start the DFS with regular intervals, e.g., every second. 2. If a certain criteria is fulfilled, for example if more than half of the sessions in an AP cell experience a bad channel (low C/I), go to step 3 otherwise quit. 3. Stop all transmissions in this AP cell and let all active MTs and the AP start measuring different frequency bands until all available bands are measured. 4. Repeat 3, e.g., every other second during 10 seconds and calculate average values. 5. Choose the least interfered channel.

27 System Architecture - Radio Resource Management (RRM) Power Control (PC) Used in HIPERLAN/2 but not supported in IEEE a Reduce the amount of interference Simplify the AP receiver Save power in the MT Uplink: The transmission power of MTs is individually adjusted so that the AP receives equal signal power from each MT. Received power expected at the AP is specified in, where the three lowest values are mandatory. An MT shall use power levels, accuracy and regulations specified in. Downlink: The AP shall not increase the transmitter power more than 9 db during any 5-minute interval. The AP shall use power levels, accuracy and regulations specified in. The PC shall adjust the AP transmission power to a level sufficient to achieve reliable communication between the AP and the most distant MT, with the above restrictions.

28 System Architecture - Radio Resource Management (RRM) Link Adaptation (LA) Both standards use this function PHY provides several transmission modes with different coding rates and modulations schemes LA uses the best possible transmission mode under present conditions Can be based on (C/I) values at the receiver or depending on PER Modulation and coding rate Mode No. Decoded transmission rate (Mbps) C/I PER<1%(dB) C/I PER<5%(dB) Minimum required C (dbm) BPSK ½ BPSK ¾ QPSK ½ QPSK ¾ QAM 9/ QAM ¾ QAM ¾ 7 54 >30 > Transmission modes and requirements for HIPERLAN/2.

29 System Architecture - Radio Resource Management (RRM) Modulation and coding rate Mode No. Decoded transmission rate (Mbps) C/I PER<1%(dB) C/I PER<5%(dB) Minimum required C (dbm) BPSK ½ BPSK ¾ QPSK ½ QPSK ¾ QAM 1/ QAM ¾ QAM 2/ QAM 3/ >32.1 > Transmission modes and requirements for IEEE a (700 bytes)

30 System Architecture - Radio Resource Management (RRM) Handover In HIPERLAN/2 it is MT initiated Sector handover is only supported if the AP uses sector antennas. An MT associated with an AP selects the most appropriate sector. Radio handover is only supported if the AP uses multiple transceivers. Network handover is initiated when an associated MT moves between different AP cells. In IEEE a Support for handover will be found in layers above MAC.

31 Interference HIPERLAN/2 uses Dynamic Frequency Selection (DFS) to select a least-disturbed channel Power Control to reduce the average transmission power LA to select a mode with the highest transmission rate IEEE a uses LA together with the carrier sense function Since no DFS is implemented and a manual frequency plan is used, severe interference might occur.

32 Throughput calculation - HIPERLAN/2 The total duration of a MAC frame in HIPERLAN/2 is 2ms. Time for overhead must be subtracted from the 2 ms MAC frame to get the available time (Tavl) for user data carried in LCH packets. Incoming data of 48 bytes is portioned into 54 bytes LCH packets at the DLC layer. The overhead in the MAC frame is listed below. BCH with preamble: 36 µs (15 bytes + a preamble of 16 µs) # sessions _ in _ uplink+ # sessions _ in _ downlink FCH: 36 µs 3 ACH: 12 µs (9 bytes) RCH with preamble: 28 µs (9 bytes + a preamble of 16 µs) # sessions _ in _ downlink Time _ for _ a _ SCH Control signals in downlink: ( ) ( ) Control signals in uplink: (# sessions _ in _ uplink) ( Time _ for _ two _ SCHs) Preambles in downlink: (# MTs _ in _ downlink) 8 µs Preambles in uplink: (# MT _ in _ uplink) 12 µs Guard time in uplink: (# MTs _ in _ uplink) 2 µs Radio turn around time: 2.6 µs

33 Throughput calculation - HIPERLAN/2 PHY-Mode Mode number Decoded transmission rate (Mbps) Time/LCH (µs) BPSK ½ BPSK ¾ QPSK ½ QPSK ¾ QAM 9/ QAM ¾ QAM ¾ Time for a LCH/SCH depending on PHY-mode Throughput is defined as: # ms ( LCHs _ in _ a _ MAC _ frame) Throughput for HIPERLAN/2, 5MTS Mbps Decoded data rate Throughput 10 If we have 5 MTs Mode

34 Throughput calculation - IEEE a A data packet consists of a frame body with variable length. The maximum length is 4095 bytes A transmitted frame consists of: Always Preamble 16 µs Signal field 4 µs A service field of 16 bits Mode dependent A tail of 6 bits Mode dependent One of the following depending on frame type MAC header + Data + CRC = 734 bytes (data frame) Mode dependent MAC header + CRC = 14 bytes (ACK/CTS frame) 44 µs MAC header + CRC = 20 bytes (RTS frame) 52 µs Sometimes Pad bits (to map into an integer number of OFDM symbols) Mode dependent Throughput = Data / Total time

35 Throughput calculation - IEEE a Frame type Mode Number Decoded Transmission Rate (Mbps) Transmission time (µs) ACK RTS CTS IEEE a Throughput Data Mbps Decoded data rate Throughput Data Data Data Data Data Data Mode Data

36 IEEE Versus HIPERLAN/2

37 Conclusions IEEE a, when compared to HIPERLAN/2 needs a better priority mechanism in order to support real time services The use of power control in HIPERLAN/2 reduced the average mobile terminal transmission power significantly in comparison to IEEE a The choice between these two standards is dependent on the network purpose Wireless is still not as good as wireline communication, in terms of quality, stability, costs, security issues etc. History of telecommunication shows that gradually wireless communication will catch up with the fixed-line communication. E.g. telephone So it takes sometime before we all go wireless. Meanwhile, stay with your LAN cables! The End

38 The following slides are only for those people who want to know more about the Physical Layer of HIPERLAN/2!!!!

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