Fixed-Assignment For Voice Oriented Networks Ppt Background

Presentation on theme: "Wireless & Personal Communication Systems – CSE5807 Lecture: 03 1 Wireless Personal Communications Systems – CSE5807 Lecture: 03 Stephen Giles and Satha."— Presentation transcript:

1 Wireless & Personal Communication Systems – CSE5807 Lecture: 03 1 Wireless Personal Communications Systems – CSE5807 Lecture: 03 Stephen Giles and Satha K. Sathananthan School of Computer Science and Software Engineering Monash University Australia Modified by Peter Granville August 2006 These slides contain figures from Stallings, and are based on a set developed by Tom Fronckowiak.

2 Wireless & Personal Communication Systems – CSE5807 Lecture: 03 2 Access Technologies Base station Downlink / Forward ChannelUplink / Reverse Channel Responsible for interacting with the medium (wireless channel) to coordinate the successful operation of multiple terminals. Separation between uplink and downlink: - Frequency Division Duplexing (FDD): Uplink and downlink channels use different carrier frequencies. - Time Division Duplexing (TDD) Uplink and downlink channels use same frequency band but different time slots.

3 Wireless & Personal Communication Systems – CSE5807 Lecture: 03 3 Access Technologies Fixed-Assignment Access: Voice-oriented Networks –Frequency Division Multiple Access (FDMA) –Time Division Multiple Access (TDMA) –Code Division Multiple Access (CDMA) Random or Dynamic Access: Data-oriented Networks –Aloha Techniques –Carrier Sense Multiple Access Techniques (CSMA)

4 Wireless & Personal Communication Systems – CSE5807 Lecture: 03 4 Access Technologies ReservationCollision Resolution e.g.: Pure Aloha Fixed Allocation e.g.: FDMA & TDMA Dynamic Allocation Use of round-robin ordering e.g.: Token ring, token bus Use of collision resolution with implicit reservation e.g.: CSMA/CD

5 Wireless & Personal Communication Systems – CSE5807 Lecture: 03 5 Frequency Division Multiple Access (FDMA) Based on Frequency Division Multiplexing (FDM). All users transmit signal simultaneously, and they are separated from one another from by their frequency of operation. Time Frequency F1F1 F2F2 F3F3 F4F4 F5F5 Used in most of the First Generation (1G) cellular networks. –AMPS (Advanced Mobile Phone Service) system allocates 30 kHz of bandwidth for each forward and reverse channel.

6 Wireless & Personal Communication Systems – CSE5807 Lecture: 03 6 Frequency Division Multiple Access (FDMA) Adjacent channel interference –Use of guard bands.. –Power control. TDD and FDD: Time Frequency F1F1 F2F2 F3F3 F1F1 F2F2 F3F3 Uplink Downlink Time Frequency F1F1 F2F2 F3F3 F4F4 Uplink Downlink FDMA/FDD FDMA/TDD

7 Wireless & Personal Communication Systems – CSE5807 Lecture: 03 7 Time Division Multiple Access (TDMA) Based on Time Division Multiplexing (TDM). A number of users share the same frequency band by taking assigned turns in using the channel. –Users are assigned different time slots. –Synchronous – time slots pre-assigned to sources and fixed. Frequency Time T1T1 T2T2 T3T3 T4T4 T5T5

8 Wireless & Personal Communication Systems – CSE5807 Lecture: 03 8 Time Division Multiple Access (TDMA) Digital format (more flexible). Used in most of the Second Generation (2G) cellular networks. –GSM (Global System for Mobile Communications) allocates 8 time slots within a 200 kHz carrier bandwidth. TDD and FDD: Time Frequency F1F1 F1F1 Uplink Downlink TDMA/FDD T2T2 T3T3 T1T1 Downlink Uplink Time Frequency F1F1 TDMA/TDD

9 Wireless & Personal Communication Systems – CSE5807 Lecture: 03 9 Code Division Multiple Access (CDMA) Based on Spread Spectrum technology. A number of users use the same band at the same time, and the users are differentiated by a code. Accommodate different bandwidth requirements and switching methods without any need for coordination. Each user contributes to interference => Power control C3C3 C2C2 C1C1 Frequency Time Code

10 Wireless & Personal Communication Systems – CSE5807 Lecture: 03 10 Code Division Multiple Access (CDMA) More capacity and better quality. Used in 2G and 3G cellular networks. –2G: IS-95 –3G: IMT-2000 Uses TDD and FDD Uplink Downlink Frequency Time CDMA/FDD CDMA/TDD Time Code UplinkDownlink Frequency

11 Wireless & Personal Communication Systems – CSE5807 Lecture: 03 11 Random Access Methods Allocating a single channel among competing users. Belongs to a sublayer of the data link layer, called the “Medium Access Control (MAC)” sublayer. - Each node has a queue of frames to be transmitted. - Nodes normally send new frames immediately, hoping for no interference from other nodes. - How frames are transmitted when collision occur? => Pure Aloha => Slotted Aloha => Carrier Sense Multiple Access (CSMA) => CSMA/CD => CSMA/AD

12 Wireless & Personal Communication Systems – CSE5807 Lecture: 03 12 Pure Aloha Wireless access to a mainframe: “Alohanet”, invented and demonstrated in 1970 in Hawaii. mainframe :::: Stations/terminals wireless (common channel)     When stations are ready for transmission, they will transmit immediately hoping that other stations will not transmit at the same time. If two or more stations transmit at the same time, a transmission collision occurs and the receivers will not be able to read the transmissions correctly.

13 Wireless & Personal Communication Systems – CSE5807 Lecture: 03 13 Pure Aloha A sender will generally realize its transmission is unsuccessful, by not receiving an acknowledgement some time after the completion of its transmission. The sender then retransmits the same data frame again. Why use such an unreliable method? Simple – a complicated mechanism will be more difficult and expensive to implement Chances of transmission collision is small - if small number of terminals. Chances of collision increase with load, maximum utilization of channel is18%.

14 Wireless & Personal Communication Systems – CSE5807 Lecture: 03 14 Slotted Aloha Time on the channel is organized into uniform slots whose size equals the frame transmission time. Central clock needed to synchronize all stations Stations when they are ready, can only transmit at the beginning of a timeslot. Thus, frames that do overlap will do so totally. Maximum utilization of channel increases to 37% Slotted Aloha with TDMA: - Also called Reservation-Aloha (R-Aloha) - Used in GSM for initial contact between MS and BS.

15 Wireless & Personal Communication Systems – CSE5807 Lecture: 03 15 Carrier Sense Multiple Access (CSMA) If the signal propagation time is small compared to frame transmission time, a station should listen Carrier Sense - Listen to determine if another transmission is in progress before transmit. Reduces collisions - will only occur when two stations begin to transmit (almost) simultaneously Increased utilization of channel. non-persistent CSMA: When terminals sense a busy channel as they are ready, they will backoff for a while and return back to sense the channel some time later.

16 Wireless & Personal Communication Systems – CSE5807 Lecture: 03 16 CSMA – What should a station do if medium busy ? non-persistent CSMA: 1.If medium is idle, transmit; else go to 2 2.If medium is busy, wait an amount of time (the retransmission delay) drawn from a probability distribution; go to 1. –Some capacity wasted Random delay reduces the probability of collisions 1-persistent CSMA: 1.If medium is idle, transmit; else go to 2 2.If medium is busy, continue to listen until channel is sensed idle; then transmit immediately (with p = 1) –If two stations are waiting to transmit, a collision will occur.

17 Wireless & Personal Communication Systems – CSE5807 Lecture: 03 17 CSMA – What should a station do if medium busy ? p-persistent CSMA: 1.If medium is idle, transmit with probability p, and delay one time unit with probability (1 – p). The time unit is equal to maximum propagation delay. 2.If medium is busy, continue to listen until the channel is idle, to go 1. 3.If transmission is delayed 1 time unit, after waiting, go to 1.

18 Wireless & Personal Communication Systems – CSE5807 Lecture: 03 18 Carrier Sense Multiple Access/Collision Detection (CSMA/CD) 1.If the medium is idle, transmit; else go to 2. 2.If the medium is busy, continue to listen until channel is idle, then transmit immediately 3.If a collision is detected during transmission, transmit a brief jamming signal to assure that all stations know that there has been a collision then cease transmission 4.After transmitting the jamming signal, wait a random amount of time, the backoff, then go to 1.

19 Wireless & Personal Communication Systems – CSE5807 Lecture: 03 19 CSMA/CD Operation

20 Wireless & Personal Communication Systems – CSE5807 Lecture: 03 20 CSMA/CD with Binary Exponential Backoff Number of collisions Possible slots for experienced in the past (c) retransmission 1 to 10[n+1 to n+2 c ] 11 to 16[n+1 to n+2 10 ] >16=ABORT= collision nn+1n+2n+3… time For 1 to 10 retransmission attempts, mean value of random delay doubled. Mean value remains same for 11 to 16 Ethernet Protocol = 1-p CSMA/CD + BEB

21 Wireless & Personal Communication Systems – CSE5807 Lecture: 03 21 CSMA/CD and Wireless LANs Collisions consume bandwidth - want to avoid. Collision detection requires a station to be able to send data and receive collision signals at the same time, requires more complex electronics in wireless interface card. Collisions may not be detected - a station may be hidden from in a wireless environment due to natural obstacles such as mountains or artificial obstacles such as buildings The distance between stations in a wireless environment can be great – signal fading could prevent a station hearing a collision

22 Wireless & Personal Communication Systems – CSE5807 Lecture: 03 22 Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA)     CSMA with Collision Avoidance. An idle channel must be detected before a transmission can take place. A receiver must reply with a short ACK to inform the sender that the transmission is successful so that unnecessary retransmission will not occur.

23 Wireless & Personal Communication Systems – CSE5807 Lecture: 03 23 Spread Spectrum Occupies a much wider bandwidth. Two types: –Frequency Hopping Spread Spectrum (FHSS) –Direct Sequence Spread Spectrum (DSSS) More robust to interference and interception. –First used in military communication applications and later adopted for commercial use. –Superior performance over traditional radio signaling over multipath channel. CDMA system offers more flexibility, superior performance and more capacity over FDMA/TDMA based systems. –Adopted for 3G Cellular systems. Can be overlaid with other systems operating in the same band. –WLAN operating in unlicensed band.

24 Wireless & Personal Communication Systems – CSE5807 Lecture: 03 24 Spread Spectrum Frequency Power Narrowband Signal – High peak power (Traditional Signal) Spread Spectrum Signal – Low peak power

25 Wireless & Personal Communication Systems – CSE5807 Lecture: 03 25 Spread Spectrum What can be gained from apparent waste of spectrum? –Immunity from various kinds of noise and multipath distortion. –Can be used for hiding and encrypting signals. –Several users can independently use the same higher bandwidth with very little interference.

26 Wireless & Personal Communication Systems – CSE5807 Lecture: 03 26 Frequency Hoping Spread Spectrum (FHSS) Signal is broadcast over seemingly random series of radio frequencies. –A number of channels allocated for the FH signal. –Width of each channel corresponds to bandwidth of input signal. Signal hops from frequency to frequency at fixed intervals. –Transmitter operates in one channel at a time. –Bits are transmitted using some encoding scheme. –At each successive interval, a new carrier frequency is selected.

27 Wireless & Personal Communication Systems – CSE5807 Lecture: 03 27 Frequency Hoping Spread Spectrum (FHSS) Large number of frequencies used => Spread Spectrum. Channel sequence dictated by spreading code. Receiver, hopping between frequencies in synchronization with transmitter, picks up message. Advantages: –Eavesdroppers hear only unintelligible blips. –Attempts to jam signal on one frequency succeed only at knocking out a few bits. –Results in a system that is quite resistant to jamming. Jammer must jam all frequencies. With fixed power, this reduces the jamming power in any one frequency band.

28 Wireless & Personal Communication Systems – CSE5807 Lecture: 03 28 Direct Sequence Spread Spectrum (DSSS) Each bit in original signal is represented by multiple bits in the transmitted signal. Spreading code spreads signal across a wider frequency band. –Spread is in direct proportion to number of bits used. One technique combines digital information stream with the spreading code bit stream using exclusive-OR.

29 Wireless & Personal Communication Systems – CSE5807 Lecture: 03 29 Direct Sequence Spread Spectrum (DSSS)

30 Wireless & Personal Communication Systems – CSE5807 Lecture: 03 30 Direct Sequence Spread Spectrum (DSSS)

31 Wireless & Personal Communication Systems – CSE5807 Lecture: 03 31 Code-Division Multiple Access (CDMA)

32 Wireless & Personal Communication Systems – CSE5807 Lecture: 03 32 Code-Division Multiple Access (CDMA) Break each bit into k chips. –Chips are a user-specific fixed pattern Chip data rate of new channel = kD where D = rate of original data signal. If k=6 and code is a sequence of 1s and -1s –For a ‘1’ bit, A sends code as chip pattern –For a ‘0’ bit, A sends complement of code Receiver knows sender’s code and performs decoding. = received chip pattern = sender’s code

33 Wireless & Personal Communication Systems – CSE5807 Lecture: 03 33 CDMA Example User A’s code = –To send a 1 bit use chip pattern –To send a 0 bit use chip pattern User B’s code = –To send a 1 bit = Base Station receives chip pattern and applies A’s code –(A’s code) x (received chip pattern) User A ‘1’ bit: 6 ==> 1 User A ‘0’ bit: -6 ==> 0 User B receives chip pattern and applies its code, result: 0 ==> unwanted signal; ignored

34 Wireless & Personal Communication Systems – CSE5807 Lecture: 03 34 Required Reading W. Stallings, “ Wireless Communications and Networks ” Prentice-Hall, 2000. >> Chapter 2.5 and Chapter 7 Optional Reference K. Pahlavan and K. Krishnamurthy “Principles of Wireless Networks”, Prentice-Hall, 2002.

35 Wireless & Personal Communication Systems – CSE5807 Lecture: 03 35 The following slides are for interest ONLY

36 Wireless & Personal Communication Systems – CSE5807 Lecture: 03 36 Spreading Sequences Spreading Sequence Categories: –PN sequences: M-sequences – Constructed by using shift registers. Gold sequences – Constructed by using two m-sequences. –Orthogonal codes: Walsh codes Variable-Length Orthogonal codes For FHSS systems –PN sequences most common For DSSS systems not employing CDMA –PN sequences most common For DSSS CDMA systems –PN sequences –Orthogonal codes

37 Wireless & Personal Communication Systems – CSE5807 Lecture: 03 37 PN Sequences PN generator produces periodic sequence that appears to be random. PN Sequences: –Generated by an algorithm using initial seed. –Sequence isn’t statistically random but will pass many test of randomness. –Sequences referred to as pseudorandom numbers or pseudonoise sequences. –Unless algorithm and seed are known, the sequence is impractical to predict.

38 Wireless & Personal Communication Systems – CSE5807 Lecture: 03 38 Orthogonal Codes Orthogonal codes –All pairwise cross correlations are zero. –Fixed- and variable-length codes used in CDMA systems. –For CDMA application, each mobile user uses one sequence in the set as a spreading code. Provides zero cross correlation among all users. Types: –Welsh codes –Variable-Length Orthogonal codes

39 Wireless & Personal Communication Systems – CSE5807 Lecture: 03 39 Typical Multiple Spreading Approach Spread data rate by an orthogonal code (channelization code). –Provides mutual orthogonality among all users in the same cell. Further spread result by a PN sequence (scrambling code). –Provides mutual randomness (low cross correlation) between users in different cells.

40 Wireless & Personal Communication Systems – CSE5807 Lecture: 03 40 Orthogonal Frequency Division Multiplexing (OFDM) Transmission of Data in Parallel on Multiple Subcarriers (or Multiple Frequency Bands). Orthogonal Subcarriers.

41 Wireless & Personal Communication Systems – CSE5807 Lecture: 03 41 Orthogonal Frequency Division Multiplexing (OFDM) Advantages: Resistance to Multipath fading Spectral Efficiency Easy Implementation using FFT chips

42 Wireless & Personal Communication Systems – CSE5807 Lecture: 03 42 Orthogonal Frequency Division Multiplexing (OFDM) Adopted in several wireless communication system standards. –Digital Audio Broadcasting (DAB) –Digital Video Broadcasting (DVB) –Wireless LAN IEEE802.11a and IEEE802.11g (Wi-Fi) HiperLAN-1 and HiperLAN-2 –Fixed Broadband Wireless Access Networks IEEE802.16 (Wi-Max)

43 Wireless & Personal Communication Systems – CSE5807 Lecture: 03 43 OFDM and Multi-Access Technology Multicarrier-CDMA (MC-CDMA): –OFDM and CDMA –Multiple users are separated by CDMA codes. –It is being considered for 4G Cellular Networks. Orthogonal Frequency Division Multiplexing Access (OFDMA). –Multiple users are assigned group of subcarriers. –Used in IEEE802.16 networks.

44 Wireless & Personal Communication Systems – CSE5807 Lecture: 03 44 Ultra Wide Band (UWB) Transmission Transmission of narrow pulses (order of few tenths of a nanosecond) with low power. –Very wide band (several GHz). –Can coexist with existing radio systems (in licensed band too). Robust to interference and multipath fading. Suitable for short-range communication. –Is being considered for Wireless Personal Area Networks (WPAN). –May replace Bluetooth?

45 Wireless & Personal Communication Systems – CSE5807 Lecture: 03 45 Non-Preemptive Multiple Access (NPMA) Used in HiperLAN-1. Listen before talk (similar to CSMA/CA). Uses the characteristics of Slotted-Aloha and TDMA. If a terminal sense a medium free for a certain duration, then it immediately transmits. If the channel is busy, the terminal goes through three phases when the channel become available. - Prioritization phase - Contention phase - Transmission phase

Какая-то бессмыслица. Вначале был зарегистрирован нормальный ввод замка, в тот момент, когда она выходила из помещения Третьего узла, однако время следующей команды отпирания показалось Сьюзан странным. Две эти команды разделяло меньше одной минуты, но она была уверена, что разговаривала с коммандером больше минуты.

Сьюзан просмотрела все команды. То, что она увидела, привело ее в ужас.

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