Multiplexing and Data Rate Essays

Multiplexing and Data Rate Essays Multiplexing and Data Rate Essay Multiplexing and Data Rate Essay Multiplexing CHAPTER 4 Outline * Frequency Division Multiplexing(FDM) * Synchronous Time Division Multiplexing * Statistical Time Division Multiplexing * Asymmetric Digital Subscriber Line(ADSL) Multiplexing * Set of techniques that allows the simultaneous transmission of multiple signals across a single link * allows several transmission sources to share a larger transmission capacity Link = physical path Channel = portion of a link that carries a transmission between a given pairs of lines 2 CATEGORY OF MULTIPLEXING WDM FDM TDM ADSL Frequency Division Multiplexing * FDM – numerous signals are combined for ransmission on a single communications line or channel. Each signal is assigned a different frequency (subchannel) within the main channel. * Useful bandwidth of medium exceeds required bandwidth of channel * e. g. broadcast radio and cable television * Channel allocated even if no data Frequency Division Multiplexing Diagram * Each signal is modulat ed to a different carrier frequency * Carrier frequencies separated by guard bands (unused bandwidth) – to prevent interference so signals do not overlap. 3 FDM System FDM is an analog multiplexing technique that combines signals. FDM process FDM Demultiplexing Example 1 Assume that a voice channel occupies a bandwidth of 4 KHz. We need to combine three voice channels into a link with a bandwidth of 12 KHz, from 20 to 32 KHz. Show the configuration using the frequency domain without the use of guard bands. Solution Shift (modulate) each of the three voice channels to a different bandwidth, as shown in next figure Example 2 Five channels, each with a 100-KHz bandwidth, are to be multiplexed together. What is the minimum bandwidth of the link if there is a need for a guard band of 10 KHz between the channels to prevent interference? Solution For five channels, we need at least four guard bands. This means that the required bandwidth is at least 5 x 100 + 4 x 10 = 540 KHz, as shown in next Figure. Example 3 Four data channels (digital), each transmitting at 1 Mbps, use a satellite channel of 1 MHz. Design an appropriate configuration using FDM Solution The satellite channel is analog. We divide it into four channels, each channel having a 250-KHz bandwidth. Each digital channel of 1 Mbps is modulated such that each 4 bits are modulated to 1 Hz. One solution is 16- QAM modulation. Figure 6. 8 shows one possible configuration. Analog Carrier Systems ATT (USA) * Hierarchy of FDM schemes * Group 1. 12 voice channels (4kHz each) = 48kHz 2. Range 60kHz to 108kHz * Supergroup 1. 60 channel 2. FDM of 5 group signals on carriers between 420kHz and 612 kHz * Mastergroup 1. 10 supergroups Analog Hierarchy To maximize the efficiency infrastructure, multiplexed signals from lower bandwidth lines onto higher-bandwidth signals FDM of Th ree Voiceband Signals FDM Applications 1. Common used radio broadcasting AM and FM * AM (530 – 1700KHz) – shared with all radio stations * FM uses a wider band (88 – 108MHz) – each station needs more bandwidth, 200KHz 2. Television Broadcasting * Each TV channel has own bandwidth of 6 Mhz 3. 1st Generation of Cellular telephones * Voice signal 3KHz (300 – 3300Hz) channels * Bt = 10 x Bm , therefore each channel has 30KHz channels * each user has been allocated two 30KHz channel, therefore 60KHz. Example 4 The Advanced Mobile Phone System (AMPS) uses two bands. The first band, 824 to 849 MHz, is used for sending; and 869 to 894 MHz is used for receiving. Each user has a bandwidth of 30 KHz in each direction. The 3 5 Four 1-Kbps connections are multiplexed together. A unit is 1 bit. Find : (1) the duration of 1 bit before multiplexing, (2) the transmission rate of the link, (3) the duration of a time slot, and (4) the duration of a frame? Solution We can answer the questions as follows: 1. The duration of 1 bit is 1/1 Kbps, or 0. 001 s (1 ms). 2. The rate of the link is 4 Kbps. 3. The duration of each time slot 1/4 ms or 250 ms. 4. The duration of a frame 1 ms. Interleaving switches are synchronized and rotate at the same speed but opposite direction process of sending a unit data onto the path when the connection on the multiplexing and de 6 Four channels are multiplexed using TDM. If each channel sends 100 bytes/s and we multiplex 1 byte per channel, show the frame traveling on the link, the size of the frame, the duration of a frame, the frame rate, and the bit rate for the link. Solution The multiplexer is shown in Figure 6 Example 7 A multiplexer combines four 100 8 We have four sources, each creating 250 characters per second. If the interleaved unit is a character and 1 synchronizing bit is added to each frame, find (1) the data rate of each source, (2) the duration of each character in each source, (3) the frame rate, (4) the duration of each frame, (5) the number of bits in each frame, and (6) the data rate of the link. Solution Answer as follows: 1. The data rate of each source is 250 x 8 = 2000 bps = 2 Kbps. 2. The duration of a character is 1/250 s = 4 ms. 3. The link needs to send 250 frames per second. 4. The duration of each frame is 1/250 = 4 ms. 5. Each frame is 4 x 8 + 1 = 33 bits. 6. The data rate of the link is 250 x 33 = 8250 bps. Example 9 Two channels, one with a bit rate of 100 Kbps and another with a bit rate of 200 Kbps, are to be multiplexed. How this can be achieved? What is the frame rate? What is the frame duration? What is the bit rate of the link? Solution We can allocate one slot to the first channel and two slots to the second channel. Each frame carries 3 bits. The frame rate is 100,000 frames per second because it carries 1 bit from the first channel. The frame duration is 1/100,000 = 10 ms. The bit rate is 100,000 frames/s x 3 bits/frame = 300 Kbps. Pulse Stuffing * Problem Synchronizing various data sources * Each source has separate clock, variation among clock cause loss synchronization * Data rates from different sources not related by simple rational number * Solution Pulse Stuffing as effective remedy * Outgoing data rate (excluding framing bits) higher than sum of incoming rates * Stuff extra dummy bits or pulses into each ncoming signal until it matches local clock * Stuffed pulses inserted at fixed locations in frame and removed at demultiplexer TDM of Analog and Digital Sources Digital Carrier Systems * Long distance carrier system * Designed to transmit voice signal over high capacity transmission lonks usuch as optical fiber, coax and microwave * ATT developed hierarchy of TDM structures of various capacities * USA/Canada/Japan use one system * ITU-T use a similar (but different) system * US system based on DS-1 format which Multiplexes 24 channels * Each frame has 8 bits per channel plus one raming bit, 24 x 8 +1 =193 bits per frame Digital Carrier Systems (2) * For voice each channel contains one word of digitized data (using PCM at 8000 samples per sec) * Data rate 8000193 = 1. 544Mbps * Five out of six frames have 8 bit PCM samples * Sixth frame is 7 bit PCM word plus signaling bit * Signaling bits form stream for each channel containing control and routing info * Same format for digital data * 23 channels of data * 7 bits per frame plus indicator bit for data or systems control * 24th channel is sync DS Hierarchy TDM Carrier Standard North American and International TDM Carrier Standards Statistical TDM * In Synchronous TDM many slots are wasted * Statistical TDM allocates time slots dynamically based on demand * Multiplexer scans input lines and collects data until frame full * Data rate on line lower than aggregate rates of input lines * Improve efficiency is to allow multiple data resources to be packed in one single frame Statistical TDM Frame Formats Performance * Output data rate less than aggregate input data rates cause by average amount of input capacity of multiplexed line * Difficulty: May cause problems during peak periods when the input exceed capacity * Solution Buffer inputs to hold temporary excess input * Keep buffer size to minimum to reduce delay Cable Modem Outline * To support data transfer to and from a cable modem, two channels from cable TV provider dedicated to data transfer * One in each direction * Each channel shared by number of subscribers * Scheme needed to allocate capacity * Statistical TDM Cable Modem Operation Downstream * Cable sc heduler delivers data in small packets * If more than one subscriber active, each gets fraction of downstream capacity May get 500kbps to 1. 5Mbps * Also used to allocate upstream time slots to subscribers Upstream * User requests timeslots on shared upstream channel Dedicated slots for this * Headend scheduler sends back assignment of future time slots to subscriber Cable Modem Scheme Asymmetrical Digital Subscriber Line ADSL Link between subscriber and network * Local loop Uses currently installed twisted pair cable * Can carry broader spectrum * 1 MHz or more ADSL Design Asymmetric -Greater capacity downstream than upstream -Expected for video on demand and related services – high speed access -Users require higher capacity for downstream than upstream Frequency division multiplexing (FDM) element of ADSL strategy Reserve lowest 25kHz for voice – Plain old telephone service (POTS) – Voice carried only 0 – 4KHz band – Additional bandwidth for prevent crosstalk between voice and data channel Use echo cancellation or FDM to give two bands – smaller upstream band and larger downstream band Use FDM within upstream and downstream bands  œ Single bit stream multiple parallel bit streams – each portion carried separate frequency band Range up to 5. 5km -Depending of diameter cables and quality ADSL Channel Configuration Discrete Multitone * DMT * Use multiple carrier signals at different requencies * Sending some bits on each channel * Available transmission band =4kHz subchannels * Send test signal and use subchannels with better signal to noise ratio * ASL/DMT design employ 256 downstream subchannels at 4kHz (60kbps) * Possible to transmit at rate15. 36MHz * Impairments bring this down to 1. 5Mbps to 9Mbps DTM Bits Per Channel Allocation Each channel can carry data rate from 0-60 Kbps Shows increasing attenuation and decreasing signalto- noise ratio at higher frequencies DMT Transmitter xDSL * High data rate DSL * Single line DSL * Very high data rate DSL

The Blackstone Commentaries and Womens Right

The Blackstone Commentaries and Women's Right In the 19th century, American and British womens rights- or lack of them- depended heavily on the commentaries of William Blackstone which defined a married woman and man as one person under the law. Heres what William Blackstone wrote in 1765: By marriage, the husband and wife are one person in law: that is, the very being or legal existence of the woman is suspended during the marriage, or at least is incorporated and consolidated into that of the husband; under whose wing, protection, and cover, she performs every thing; and is therefore called in our law-French a feme-covert, foemina viro co-operta; is said to be covert-baron, or under the protection and influence of her husband, her baron, or lord; and her condition during her marriage is called her coverture. Upon this principle, of a union of person in husband and wife, depend almost all the legal rights, duties, and disabilities, that either of them acquire by the marriage. I speak not at present of the rights of property, but of such as are merely personal. For this reason, a man cannot grant anything to his wife, or enter into covenant with her: for the grant would be to suppose her separate existence; and to covenant with her, would be only to covenant with hims elf: and therefore it is also generally true, that all compacts made between husband and wife, when single, are voided by the intermarriage. A woman indeed may be attorney for her husband; for that implies no separation from, but is rather a representation of, her lord. And a husband may also bequeath any thing to his wife by will; for that cannot take effect till the coverture is determined by his death. The husband is bound to provide his wife with necessaries by law, as much as himself; and, if she contracts debts for them, he is obliged to pay them; but for anything besides necessaries he is not chargeable. Also if a wife elopes, and lives with another man, the husband is not chargeable even for necessaries; at least if the person who furnishes them is sufficiently apprized of her elopement. If the wife be indebted before marriage, the husband is bound afterwards to pay the debt; for he has adopted her and her circumstances together. If the wife be injured in her person or her p roperty, she can bring no action for redress without her husbands concurrence, and in his name, as well as her own: neither can she be sued without making the husband a defendant. There is indeed one case where the wife shall sue and be sued as a feme sole, viz. where the husband has abjured the realm, or is banished, for then he is dead in law; and the husband being thus disabled to sue for or defend the wife, it would be most unreasonable if she had no remedy, or could make no defence at all. In criminal prosecutions, it is true, the wife may be indicted and punished separately; for the union is only a civil union. But in trials of any sort they are not allowed to be evidence for, or against, each other: partly because it is impossible their testimony should be indifferent, but principally because of the union of person; and therefore, if they were admitted to be witness for each other, they would contradict one maxim of law, nemo in propria causa testis esse debet; and if against each other, they would contradict another maxim, nemo tenetur seipsum accusare. But, where the offence is directly against the person of the wife, this rule has been usually dispensed with; and therefore, by statute 3 Hen. VII, c. 2, in case a woman be forcibly taken away, and married, she may be a witness against such her husband, in order to convict him of felony. For in this case she can with no propriety be reckoned his wife; because a main ingredient, her consent, was wanting to the contract: and also there is another maxim of law, that no man shall take advantage of his own wrong; which the ravisher here would do, if, by forcibly marrying a woman, he could prevent her from being a witness, who is perhaps the only witness to that very fact. In the civil law the husband and the wife are considered as two distinct persons, and may have separate estates, contracts, debts, and injuries; and therefore in our ecclesiastical courts, a woman may sue and be sued without her husband. But though our law in general considers man and wife as one person, yet there are some instances in which she is separately considered; as inferior to him, and acting by his compulsion. And therefore any deeds executed, and acts done, by her, during her coverture, are void; except it be a fine, or the like manner of record, in which case she must be solely and secretly examined, to learn if her act be voluntary. She cannot by will devise lands to her husband, unless under special circumstances; for at the time of making it she is supposed to be under his coercion. And in some felonies, and other inferior crimes, committed by her through constraint of her husband, the law excuses her: but this extends not to treason or murder. The husband also, by the old law, might give his wife moderate correction. For, as he is to answer for her misbehaviour, the law thought it reasonable to intrust him with this power of restraining her, by domestic chastisement, in the same moderation that a man is allowed to correct his apprentices or children; for whom the master or parent is also liable in some cases to answer. But this power of correction was confined within reasonable bounds, and the husband was prohibited from using any violence to his wife, aliter quam ad virum, ex causa regiminis et castigationis uxoris suae, licite et rationabiliter pertinet. The civil law gave the husband the same, or a larger, authority over his wife: allowing him, for some misdemeanors, flagellis et fustibus acriter verberare uxorem; for others, only modicam castigationem adhibere. But with us, in the politer reign of Charles the second, this power of correction began to be doubted; and a wife may now have security of the peace against he r husband; or, in return, a husband against his wife. Yet the lower rank of people, who were always fond of the old common law, still claim and exert their ancient privilege: and the courts of law will still permit a husband to restrain a wife of her liberty, in the case of any gross misbehaviour. These are the chief legal effects of marriage during the coverture; upon which we may observe, that even the disabilities which the wife lies under are for the most part intended for her protection and benefit: so great a favourite is the female sex of the laws of England. Source ï » ¿William Blackstone. Commentaries on the Laws of England. Vol, 1 (1765), pages 442-445.