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Yüksek hızlı sayısal abone hatlarının (HDSL) incelenmesi

High bit rate digital subscriber lines (HDSL)

  1. Tez No: 46322
  2. Yazar: M.MUSTAFA GÜNDÜZ
  3. Danışmanlar: PROF.DR. GÜNSEL DURUSOY
  4. Tez Türü: Yüksek Lisans
  5. Konular: Elektrik ve Elektronik Mühendisliği, Electrical and Electronics Engineering
  6. Anahtar Kelimeler: Belirtilmemiş.
  7. Yıl: 1995
  8. Dil: Türkçe
  9. Üniversite: İstanbul Teknik Üniversitesi
  10. Enstitü: Fen Bilimleri Enstitüsü
  11. Ana Bilim Dalı: Belirtilmemiş.
  12. Bilim Dalı: Belirtilmemiş.
  13. Sayfa Sayısı: 73

Özet

ÖZET Son yıllarda teknolojinin hızlı gelişmesi ve özellikle bilgisayarların tıouzlayarak ekonomik hale gelmesi ile veri haberleşmesine olan talepte artmıştır. Mevcut telefon hatlarından veri iletilmesi uzun yıllardan beri gündemdedir ve uygulanmaktadır. Ancak talep edilen sayısal iletim hızlarının artması mevcut, abone hatları üzerinde yeni çalışmaları gerektirmiştir. Bu tezde halen üzerinde çalışmaların devam ettiği yüksek hızlı sayısal abone hatları (High Bit Rate Digital Subsriber Lines, HDSL) incelenmiştir. İncelemede HDSL 'in önemi ve niçin gerekli olduğu vurgulanmıştır. HDSL 'in mevcut bakır hatlar üzerinden herhangi bir ön koşul yada kuvvetlendirici kullanılmadan ulaştığı servis mesafesi ve bunu etkileyen faktörler üzerinde durulmuştur. Bu amaçla 1. Bölümde, konuya giriş yapılarak konunun önemi anlatılmış, 2. Bölümde HDSL" in gelişimi verilmiş ve sayısal abone hattının sağlaması gereken temel prensipler üzerinde durulmuştur. 3. Bölümde, mevcut bakır hatlar incelenerek kablo yapıları hakkında bilgi verilmiştir. Yine bu bölümde servis mesafesini etkileyen diafoni, darbe gürültüsü v.b. faktörler ayrıntılı olarak incelenmiştir. Bölüm 4 'de HDSL teknolojisi üzerinde durulmuş, bazı uygulama örnekleri verilmiştir. Bölüm 5' de Tayvan'da bu yönde yapılan geniş çaptaki bir araştırma ele alınmış ve bulunan pratik sonuçlar irdelenmiştir. 6. Bölümde ise Türkiye'de durum değerlendirilmesi yapılmış, bazı sonuçlar ve öneriler getirilmiştir.

Özet (Çeviri)

SUMMARY HIGH BIT RATE DIGITAL SUBSCRIBER LINES (HDSL) In recent years there has been rapid growth in demand for digital transmission links between subscribers, for example for interconnection of PBXs,. computer networks and data services. HDSL (High Bit Rate Digital Subscriber Lines) thus offers a bridge to the future, allowing the present copper cable network to be used for modern modern data services, whilst simultaneously cutting operating costs, and facilating to introduction of fibre-based services at later date. Over the last ten years there has been considerable investment in digital transmission systems to enable this vision of a digital network capable of supporting a wide range of services data, voice, video etc. to be imlemented. However weak link in this digital network has always been the access network which connect customers to the local exchange via local lines. A tpical connection of a custumer consist of a cascade of cable sections of different diameters of cable sections of different diameters and lenghts, bundled together with up to a hundered other such pairs, resulting in a low bandwidth, interference prone medium. This medium is sufficient for 4 kHz bandwidth analogue telephony, but poses a challange for digital transmission. Of course for meeting this greater bandwidth means laying fiber optic cable or special coaxial cables, microwave links using repeaters on the existing copper cables. This way is not only expensive but also takes time and extra personel for maintenance the equipment. Installing HDSL equipment using the existing copper lines is easy, cheap and very quick. For example laying a fibre-optic cable can take from 3 to 6 months, using HDSL and the existing copper-based equipment the custumer could have this connetion within one week. The connections between customers' premises and the - vi -local exchange are known as the access network. It consist of thousands of copper-quad cables radiating out from each local exchange. A local line from the local line local exchange to the subscriber's telephone consist of one or more cable sections which are spliced or interconnected together, with large cable bundles at the exchanga end, reducing to smaller cables or individual pairs at the customer's premises. The individual sections consist of just one of many cable types, with wire diameters ranging from 0.32 mm to 0.9 mm, with polyethylene, poiyvinylchloride or paper insulation and quad or twisted - pair structure, all packed in layers or bundles together with hundreds of other cables. These copper cables were originally installed to provide a 4 kHz transmission bandwidth for analogue telephony signals. They exhibit high frequency attenuation, phase distortion.interference, longitudinal imbalance, all of which pose problems for high bit rate digital transmission. Typical cable transmission characteristics are shown in Figure 1, in which the rapidly increasing attenuation at higher frequencies can be clearly seen. In addition to this frequency-dependent attenuation, the performance is limited by external interference. This interference is caused either by electromagnetic coupling from external sources or by signal“leakage”between adjacent wire-pairs in a cable. These are often classified as impulsive noise and crosstalk respectively. Impulsive noise is due to interference from sources such as dial pulses, ringing currents, electromechanical switches such as signalling relays, power-transmission and switching gear or electrostatic discharges. It is unpredictable but can generally be considered as expo nentially decaying pulses with peak amplitudes in the range 5 mV to 20 mV and durations of between 30 micro seconds and İ50 micro seconds, with a random rate of occurence. Crosstalk is used to describe the unwanted coupling of neighbouring transmission signals into the system and can be divided into“near-end”and“far-end”,“inter-system”and“ intra-system”crosstalk. This is illustrated in Figure 2 Ihtra-system (or self) crosstalk refers to crosstalk from an adjacent system using the same transmission method, and inter-system refers to crosstalk from a different system. Near-end crosstalk (NEXT) refers to signals coupled into a receiver from a transmitter at the same end of the cable, and far-end crosstalk (FEXT) refers to signals coupled into a receiver from a transmitter at the opposite end of the cable. - vii -Figure i. Typical cable transmission characteristics Tntra-system, near-end crosstalk, is often considered as the dominant form of crosstalk for HDSL systems. Although the crosstalk between any given adjacent pairs varies considerably, studies have shown that for large numbers of interferers (as would be the case at an exchange with many HDSL systems coming in on one cable bundle) the crosstalk can be characterized by a simplified model with a slope of 15 dB per decade at the frequencies of interest. - vi ii -Cable Transmitterfr/ \ Receiver C * } ^Next - i- --*.; Fext Receiver A r^ Receiver Figure 2. NEXT and FEXT generation in muitipair cable An additional degradation encountered in duplex systems, in which one wire-pair is used for the simultaneous transmission in both directions, is caused by echoes. It has been common practice in analogue telephony to use what is known as a“hybrid”splitter to permit simultaneous two-way operation on a single line, but there is always a small amount of leakage between the transmit and receive directions. Echoes can be due either to nonideal hybrid performance or to reflections from discontinuities on the telephone line, and result in part of the signal transmitted in one direction being super imposed on the signal received from the other direction. This is illustrated in Figure 3. From the previous discussion, it can local line environment is not ideal for transmission. The famous the capacity of a channel noise ratio (SNR) and the theory, the capacity of a noise impairments can be increased by power. However * in practice there be seen that the high speed data Hartley-Shannon law states that is a function of the signal to bandwidth. This means that in channel with given bandwidth and increasing the signal are two objections to this procedure. One is that increasing the signal power increases the disturbance to other systems. The second and main limitation is that of“self NEXT”: increasing the signal power increase the self NEXT, thus keeping the SNR constant. From the Shannon equation,. the theoretically possible maximum channel capacity for typical local lines is approximately 3 B,lbit/s. Comparing this with baseband binary transmission, where a SNR of about 15 dB is IX -required for acceptable performance self NEXT is achieved at 200 kHz rate of roughly 400 kbit/s. this margin against giving a maximum bit It can be shown that, despite the imperfections of the transmission medium, it is theoretically possible to transmit bit rates of up to 3 Mbit/s over typical local lines. In order to overcome the transmission impairments described above, several techniques had been developed for ISDN Basic Rate Access ; and these extended to HDSL. This has been recent enormous improvements in VLSI signal processing. have been improved and made possible by the for high-speed digital Transmitted Signal Ichoes from branches and gauge changer Receiver 4-Echo* Signal Figure 3. Echoes in a transmission system As can be seen in Figure 3, part of the transmitted signal in a duplex system appears at the receiver due to -*-imperfections the hybrid or the transmission line. An echo canceller subtracts a filtered version of the transmitted signal from the received signal, thus cancelling a great part of the echo. Using digital signal processing techniques, it is possible to achieve very large reductions in the echo (up to 60 dB). Signal coding techniques can be used to increase the bandwidth efficiency of the digital transmission. An example of a baseband coding technique is 2B1Q. This is a multi-level code which transmits two binary bits as one quaternary symbol, or quat. The incoming data stream is grouped into pairs of bits, and each bit-pair is then translated into one of four levels. This was the first line code used for HDSL systems and allows the equivalent of 800 kbit/s binary data to be transmitted within a bandwidth of 200 kHz. More complicated coding schemes result in longer transmission delays, which for digitally coded speech can be unacceptable. Further advanced digital signal processing techniques are used to compensate for the imperfections in the channel. A training sequence can be used at start up to identify the imperfections in the channel, and an adaptive pre-equalization can be used to cancel out the time- invariant imperfections. Decision feedback equalizers and noise prediction techniques can be used during operation to compensate for the short or long-term change in channel characteristics and interference. Figure 4 shows typical transmission distances that can be attained by the HDSL system of Ascom Ericsson Transmission Ltd. using such a transceiver. xi -Figure 4. Typical transmission distances 1*1 xii -

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