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Understanding Long-Distance Digital Telephone Technologies

Explore the digital technologies used to connect computers across vast distances via public right-of-ways, such as copper wires, fiber optics, and satellite channels. Learn about the process of converting analog signals to digital for efficient transmission and the equipment required for different types of connections.

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Understanding Long-Distance Digital Telephone Technologies

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  1. Ch 12 Long-Distance and Local Loop Digital Technologies

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

  3. Digital Telephone • Analog signals degrade as they pass over copper wires • Amplifier along a path distorts the signal slightly and introduce noise • Digitalization: convert an analog signal to digital form • Analog-to-digital (AD) converter

  4. Digital Telephone • 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

  5. Illustration of Digitized Signal • Pick neatest digital value for each sample • coded digitally as 000-010-100-100-111-001-001-001-001-001…

  6. Illustration of Digitized Signal • Nyquist’s Sampling Theorem • If a continuous signal is sampled at a rate greater than twice the highest significant frequency, the original signal can be reconstructed from the samples • Human voice up to 4k Hz • Telephone standard known as Pulse Code Modulation (PCM) • Sample rate 8k Hz • Sample value in range 0 to 255

  7. Long-Distance Transmission Technologies • 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

  8. Equipment for Leased Connections • Analog circuit • Modem required at each end • Digital Circuit • DSU / CSU required at each end

  9. Digital Circuit and DSU/CSUs • Needed because telephone industry digital encoding differs from computer industry digital encoding

  10. Digital Circuit and DSU/CSUs • Channel Service Unit (CSU) portion • Handles line termination and diagnostics • Line connection test • Loop-back capacity when installing and testing • Prohibits excessive consecutive 1’s (non-zero voltage for 1’s) to prevent excessive current over the copper • Balanced encoding scheme: success 1 bits alternating between +3V and -3V • Data Service Unit (DSU) portion • Translates data between two encodings

  11. Telephone Standards • Cost of digital circuit depends on • Distance • Capacity

  12. Common Data Rates • Most common in North America • T1 circuit • T3 circuit (28 times T1) • Also available • Fractional T1 (e.g., 56 Kbps circuit) since T1 is too expensive to private individual • The phone company uses TDM to subdivide a T1 circuit

  13. Inverse Multiplexing • Combines two or more circuits to produce intermediate capacity circuit • A DSU/CSU is required at the ends of each T1 circuit • How to efficiently use the bandwidth? • Proxy • Load balancing

  14. Highest Capacity Digital Circuits • Also available from phone company as the major trunk connections across the country • Use optical fiber • Electrical standards called Synchronous Transport Signal (STS) • Optical standards called Optical Carrier (OC)

  15. Highest Capacity Digital Circuits

  16. Local Subscriber Loop • Telephone terminology • Refers to connection between residences / businesses and the phone company central office (CO) • Crosses public right-of-way • Originally analog

  17. Local Subscriber Loop

  18. Digital Local Loop Technologies • Integrated Services Digital Network (ISDN) • Provides digitalized voice and data over conventional TP local loop wires • Provides simultaneous voice / data services • Basic Rate Interface (BRI) offers three separate (2B+D) digital channels • B channel: 64k bps for voice/data • D channel: 16k bps for controlling purpose • ISDN now is an expensive alternative that offers little throughput

  19. Digital Local Loop Technologies • Asymmetric Digital Subscriber Line (ADSL) • Runs over conventional POTS wiring • Provides simultaneous voice / data services • Higher capacity, but asymmetric • Downstream can reach 6.14 Mbps • Upstream can reach 640 Kbps • Optimized for typical users who receive much more information than they send

  20. Illustration of ADSL Wiring

  21. Illustration of ADSL Wiring

  22. Illustration of ADSL Billing

  23. Other DSL Technologies • Symmetric Digital Subscriber Line (SDSL) • Uses a different encoding scheme than ADSL • Can operate over local loops for which ADSL is inappropriate • Business policy • High-Rate Digital Subscriber Line (HDSL) • Provides 1.544M bps in two directions • Short distance limitation • Requires two independent TPs, but can tolerate failure gracefully (i.e., operate at one-half rate)

  24. Cable Modem Technology Source: IEC On-Line Education

  25. Cable Modem Technology • Wiring (e.g., TPs) places an upper bound on how fast data can be transferred • Deliver over existing CATV coaxial cable • Cable system has unused bandwidth • Group of subscribers in neighborhood share the downstream bandwidth (up to 36 Mbps) by FDM

  26. Cable Modem Technology • Upstream communication • Dual path approach in dial-up modem without changing the existing cable system • Modification to the basic infrastructure (e.g., HFC) • VOD services beyond computer network connections

  27. Hybrid Fiber Coax (HFC) Neighborhood area Trunk FDM TDM

  28. Hybrid Fiber Coax (HFC) • Allow 2-way communication across a cable system • Optical fiber • Highest bandwidth • Replaces trunk from CO to neighborhood concentration points • Coaxial cable • Less bandwidth • Extends from neighborhood concentration point to individual subscribers (feeder circuit) • Replaces all amplifier / modem with 2-way devices

  29. Hybrid Fiber Coax (HFC) • Combination of FDM & TDM • 5-50 MHz : upstream • 50-450 MHz : analog TV(6M/ch) • 450-750 MHz : downstream • The group shares one carrier frequency with TDM

  30. Fiber To The Curb (FTTC) (人行道的)路緣 Source: Bell Labs Technique Journal

  31. Fiber To The Curb (FTTC) • Differs from HFC • Run OF closer to the end subscribers • Uses two media in each feeder circuit to provide an additional service

  32. Broadband Access Technologies Source: TEN-Telecom Workshop

  33. Summary • 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

  34. Summary (continued) • High capacity circuits available • Popular capacities known as OC-3, OC-12 • Local loop refers to connection between central office and subscriber • Local loop technologies include • DSL (especially ADSL) • Cable modems

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