Part VI

1. Part VI Long-Distance and Local Loop Digital Connection Technologies

2. Connect computers across Large geographic distance Public right-of-way Streets Buildings Railroads Motivation

3. General solution: lease transmission facilities from telephone company Point-to-point topology NOT part of conventional telephone system Copper, fiber, microwave, or satellite channels available Customer chooses analog or digital Long-Distance Transmission Technologies

4. Analog circuit Modem required at each end Digital Circuit DSU / CSU (Data Service Unit/Channel Service Unit) required at each end Equipment for Leased Connections

5. Developed by telephone companies Designed for use in voice system Analog audio from use’s telephone converted to digital format Digital format sent across network Digital format converted back to analog audio Digital Circuit Technology

6. Digital Circuit Technology

7. Pick nearest digital value for each sample Telephone standard known as Pulse Code Modulation (PCM) Illustration of Digitized Signal

8. Illustration of Digitized Signal PCM (Pulse Code Modulation) sampling and quantization Sampling is the periodic measurement of the signal every T seconds. These periodic measurements are called samples. Quantization is the approximation of the possible values of the samples by a finite set of (binary) values.

9. Illustration of Digitized Signal PCM (Pulse Code Modulation) Nyquist's sampling theorem A signal with maximum frequency fmax can be recovered exactly from samples that are measured more frequently than 2fmax every second.

10. Illustration of Digitized Signal PCM (Pulse Code Modulation) digitization of audio: (1) telephone voice (~4000 Hz) 8000 samples per second, every sample 8 bits=64kbps (DPCM: differential PCM, only encode the differences between samples) (2) compact discs (~20KHz) 41000 samples per second, encoded in 16 bits, two channels =1.3Mbps

11. Illustration of Digitized Signal Delta Modulation: use only 1 bit to mean a difference of +1 or -1

12. Illustration of Digitized Signal PCM (Pulse Code Modulation) T1 carrier 193x8000=1.544 Mbps

13. Illustration of Digitized Signal PCM (Pulse Code Modulation) Multiplexing T1 streams onto higher carriers

14. Perform two functions; usually a single “box” Needed because telephone industry digital encoding differs from computer industry digital encoding DSU portion Translates between two encodings CSU portion Terminates line Allows for maintenance DSU / CSU

15. Illustration of DSU / CSU • Cost of digital circuit depends on • Distance • Capacity DSU/CSU DSU/CSU Phone Company computer connection using computer standards digital circuit using telephone standards computer

16. Specified by the telephone industry in each country Differ around the world Are known by two-character standard name (T1, T2, etc.) Note: engineers refer to circuit capacity as “speed” Telephone Standards For Digital Circuits

17. Note: T2 not popular Example Circuit Capacities

18. Most common in North America T1 circuit T3 circuit (28 times T1) Also available Fractional T1 (e.g., 56 Kbps circuit) Common Digital Circuit Terminology

19. Combines two or more circuits Produces intermediate capacity circuit Special hardware required Needed at each end Called inverse multiplexor Inverse Multiplexing

20. Example of Inverse Multiplexing • Can alternate between circuits for • Every other bit • Every other byte inverse mux inverse mux phone company computer connection to computer pair of T1 circuits

21. Also available from phone company Use optical fiber Electrical standards called Synchronous Transport Signal (STS) Optical standards called Optical Carrier (OC) High-Capacity Digital Circuits

22. STS- is standard for electrical signals OC- is standard for optical signals Engineers usually use OC- terminology for everything OC-3 popular High-Capacity Circuits

23. The C Suffix In OC-3C (and others), the C stands for concatenated, which denotes a circuit with no inverse multiplexing. That is, an OC-3 circuit consists of three OC-1 circuits operating at 51.840 Mbps each, and is intended to be used as three circuits. An OC-3C (STS-3C) circuit, however, is a single circuit operating at 155.520 Mbps.

24. Synchronous Optical NETwork ( SONET ) In addition to the STS and OC standards, the phone companies have defined a broad set of standards for digital transmission. In North America, the standards use the term Synchronous Optical NETwork (SONET), while in Europe they are known as the Synchronous Digital Hierarchy (SDH).

25. Synchronous Optical NETwork ( SONET ) SONET frame on STS-1 (OC-1) circuit (Why 810 bytes?)

26. Synchronous Optical NETwork ( SONET ) Recall that 125 microseconds is a fundamental constant for digitalized voice because PCM requires taking an 8-bit sample every 125 microseconds . SONET uses the time to define frame size. At the STS-1 transmission rate of 51.840 Mbps, exactly 6480 bits are transferred in 125 microseconds, which means that a frame consists of 810 8-bit octets. The chief advantage of making the frame size depend on the bit rate of the circuit is that it makes synchronous multiplexing possible.

27. Telephone terminology Refers to connection between residence / business and central office Crosses public right-of-way Originally for analog POTS (Plain Old Telephone Service) Local Loop

28. Integrated Services Digital Network (ISDN) Handles voice and data (2B+1D) Extends to longer distance Expensive in North America Digital Subscriber Line (DSL) Newer technology Higher speed Several variants exist (xDSL) Digital Local Loop Technologies

29. Popular DSL variant Runs over conventional POTS wiring Higher capacity downstream Uses frequencies above POTS Asymmetric Digital Subscriber Line (ADSL)

30. Illustration of ADSL Wiring Standard twisted pair Connecting residence • Does not require any changes in local loop wiring • Does not preempt the local loop (can be used simultaneously) ADSL modem CO’s ADSL modem Digital connection To local network Digital connection To provider To telephone switch To analog phone residence Telephone central office

31. Illustration of ADSL Wiring To accommodate differences in local loop characteristics, ADSL is adaptive. When ADSL modems are powered on, they probe the line between them to find its characteristics, and then agree to communicate using techniques that are optimal for the line. ADSL uses a scheme known as Discrete Multi Tone modulation (DMT), which combines frequency division multiplexing and inverse multiplexing techniques. The standard (ANSI) ADSL system uses 256 frequency channels for the downstream data and 32 channels for the upstream. All channels have bandwidth of 4.3125KHz and the frequency difference between two successive channels is also 4.3125Khz.  (4.3125x256=1.104 MHz)

32. Illustration of ADSL Wiring FDM in ADSL 256 for downstream 32 for upstream

33. x DSL (High data rate DSL) (Very high data rate DSL)

34. x DSL

35. x DSL RADSL: Rate Adaptive DSL

36. x DSL SDSL: Symmetric DSL

37. Send / receive over CATV wiring Use FDM Group of subscribers in neighborhood share bandwidth Cable Modems

39. Wiring scheme for cable to allow digital access Optical fiber Highest bandwidth Extends from central office to neighborhood concentration points Coaxial cable Less bandwidth Extends from neighborhood concentration point to individual subscribers (e.g., residence) Hybrid Fiber Coax

40. 光線接入網採用光纖做為主要的傳輸媒體來取代傳統的雙絞線，。由於光纖上傳送的是光信號，因而需要在交換局將電信號進行電光轉換變成光信號後再在光纖上進行傳輸。在用戶端則要利用光網路單元（Optical Network Unit）再進行光電轉換恢復成電信號後送至用戶設備。 Fibers to the Rescue

41. 根據光纖向用戶延伸的距離，也就是ONU所設置的位置，光線接入網又有多種應用形式，其中最主要的三種形式是光纖到大樓（Fiber To The Building）、光纖到路邊（FTTCurb）、光纖到戶（FTTHome）。 Fibers to the Rescue

42. FTTC主要為住宅用戶提供服務。ONU放置在路邊，從ONU出來用同軸電纜傳送視像業務，雙絞線對傳送普通電話業務，每個ONU一般可為8～32個用戶服務，適合為獨門獨院的用戶提供各種寬頻業務，如VOD等。 Fibers to the Rescue

43. FTTB有分為兩種，一種是為公寓大樓用戶服務，實際上只是把FTTC中的ONU從路邊移至公寓大樓內；另一種是為辦公大樓服務的，ONU設置在大樓內的配線箱處，為大中型企事業單位及商業用戶服務，可提供高速數據、電子商務、影像圖文、遠程醫療、遠程教育等寬頻業務。FTTB與FTTC並沒有什麼根本不同，兩者的差異在於服務的對象不同，因而所提供的業務不同，ONU後面所採用的傳輸媒介也有所不同。FTTB有分為兩種，一種是為公寓大樓用戶服務，實際上只是把FTTC中的ONU從路邊移至公寓大樓內；另一種是為辦公大樓服務的，ONU設置在大樓內的配線箱處，為大中型企事業單位及商業用戶服務，可提供高速數據、電子商務、影像圖文、遠程醫療、遠程教育等寬頻業務。FTTB與FTTC並沒有什麼根本不同，兩者的差異在於服務的對象不同，因而所提供的業務不同，ONU後面所採用的傳輸媒介也有所不同。 Fibers to the Rescue

44. FTTH則是將ONU放置在住戶家中，有住戶專用，。為家庭提供各種綜合寬頻業務，如VOD、居家購物、多方線上遊戲等等。FTTH則是將ONU放置在住戶家中，有住戶專用，。為家庭提供各種綜合寬頻業務，如VOD、居家購物、多方線上遊戲等等。 Fibers to the Rescue

45. 光纖接入網，特別是FTTH光纖接入網，具有頻帶寬、容量大、信號質量好、可靠性高、可以提供多種業務，是實現B-ISDN的最佳方案，因而被認為是接入網的發展方向。但光纖接入網成本昂貴，FTTH平均用戶成本平均3000～5000美元，普通用戶難以承受。儘管FTTB、FTTC採用若干用戶共用ONU以分攤成本、降低平均成本的方式，但卻帶來供電困難等問題。此外，適合光纖接入網的網管程式還不完善，這些都限制了它的大規模應用。 Fibers to the Rescue

46. 近幾年來，隨著技術的進步，光電器件成本下降，FTTH與FTTC之間的成本差距正在逐步縮小。儘管FTTH初期投資高於FTTC，但由於FTTH無外部有源設備，因而可靠性高、供電容易且成本低、運營維護費低、規劃費用低等，綜合考慮系統壽命成本，FTTH與FTTC已不再存在巨大的成本差距，這將促進FTTC向FTTH的演化。 Fibers to the Rescue

47. In remote or rural areas, ADSL and cable are all unreachable. Even in areas where a local loop technology works for most subscribers, it may not work on all lines. Special Cases

48. Advantages: a satellite system has more bandwidth than a dialup connection, and can reach an arbitrary geographic location. Disadvantages: long delays, uplink is both expensive and large Broadcast Satellite Systems

49. To devise a low-cost local loop that uses satellites: Instead of treating satellites as point-to-point communication systems, a broadcast mechanism was devised. Instead of a large, expensive ground station, an alternative uplink transmission path was used. Broadcast Satellite Systems

50. Technologies exist that span long distances Leased analog lines (require modems) Leased digital circuits (require DSU / CSUs) Digital circuits Available from phone company Cost depends on distance and capacity Popular capacities called T1 and T3 Fractional T1 also available Summary