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Chapter 9

Chapter 9 . Communications Circuits. Types of Circuits. Point-to-point Connects only two nodes Multipoint Connects several nodes 2-wire circuits Signal + return Half-duplex data, except can use FDM to get full duplex Phone line 4-wire circuits Full duplex data. Digital Circuits.

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Chapter 9

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  1. Chapter 9 Communications Circuits

  2. Types of Circuits • Point-to-point • Connects only two nodes • Multipoint • Connects several nodes • 2-wire circuits • Signal + return • Half-duplex data, except can use FDM to get full duplex • Phone line • 4-wire circuits • Full duplex data

  3. Digital Circuits • Signal distortions easier to correct than on analog lines • No A/D or D/A conversions

  4. ISDN • Basic Rate Interface (BRI) • Two B (bearer) channels + One D (delta) channel • B channel has 64 kbps capacity • D channel has 16 kbps capacity • B channels can be combined or separate, data or digitized voice • Primary Rate Interface (PRI) • 23 B (bearer) channels + One D (delta) channel • B channel has 64 kbps capacity • D channel has 64 kbps capacity • Total capacity is T1

  5. ISDN BRI and PRI

  6. ISDN Example • PRI can be set up with • 772 kbps (13 B channels) for video • 384 kbps (6 B) for 6 voice channels • 256 kbps (4 B) for data • Can reconfigure at later time

  7. Broadband ISDN (BISDN) • Services • Full duplex at 155.52 Mbps • Full duplex at 622.08 Mbps • Asymmetrical circuit with 2 simplex channels • One at 155.52 Mbps • One at 622.08 Mbps • Asynchronous Transfer Mode is used to transport ISDN • Being deployed slowly

  8. ISDN Benefits • Efficient multiplexed access to public network • Capability to support integrated voice and data • Signaling channel for network management • Internationally defined open system interface • Popular in Europe and Japan • Slow rollout in U.S.

  9. T-Carrier Systems • High-speed digital transmission system • Time-division multiplexing • T-# identifies the carrier type • DS# identifies the signal

  10. T-Carrier System

  11. T-1 • Uses two wire pair for full duplex • Carries 24 64 kbps channels plus 8 kbps signaling • One way • Channels multiplexed on T-1 line at transmitter and demultiplexed at receiver • May be cheaper to lease a T-1 line and use less than full capacity than to lease multiple low-capacity lines • Leased point-to-point

  12. Fractional T-1 • IXCs are offering capacities less than 1.544 Mbps at 128 kbps minimum plus 64 kbps intervals • CO runs full T-1 to customer, usually reserves all 24 channels for customer future expansion

  13. Switched Megabit Data Service (SMDS) • Connectionless • No dedicated line between locations • Based on Asynchronous Transfer Mode (ATM) technology • Speeds: • 1.544 Mbps • 44.736 Mbps • Not point-to-point like T-1 • Lease circuit to nearest carrier’s office at each end of connection • Carrier handles circuit in between end offices

  14. Digital Subscriber Line (DSL) • Asymmetric DSL (ADSL) • Developed in 1987 • Designed to deliver digitized video to homes over twisted pair • New application: Internet access • Full rate speed is 1.5 – 8 Mbps downstream, up to 1 Mbps upstream • Limit is 13,000 ft • Service available in Atlanta area

  15. ADSL Bandwidth Allocation

  16. DSL • Other DSLs • G.Lite • 1.5 Mbps downstream, 384 kbps upstream • Symmetric DSL (SDSL) • 784 kbps max each way • Mainly business customers • Very-high-rate DSL (VDSL) • Asymmetric • Downstream • 51 – 55 Mbps for 1000 feet or less • 13 Mbps for 4000 feet or less • Upstream • 1.6 – 2.3 Mbps

  17. Circuit Media – Copper Wire • Twisted pair • STP - shielded twisted pair • UTP - unshielded twisted pair • STP is better, but harder to terminate and more expensive • EIA standards for UTP • Category 1 (Cat 1) - basic, not for data • Cat 2 - up to 4 Mbps • Cat 3 - 10 to 16 Mbps, at least 3 twists per inch • Cat 4 - 16 Mbps • Cat 5 - 100 Mbps, 3 to 4 twists per inch • Current standard-use cable • Cat 5E – up to 1 Gbps • Look for Cat 6 and Cat 7 in the future

  18. Coaxial Cable • Bandwidth about 400 to 600 MHz • Used to carry 10,800 telco FDM voice calls • Repeaters every mile • Used by cable TV companies for video, data • Shielded • Improved immunity to electromagnetic interference • More difficult to work with than UTP • Bulky, more expensive

  19. Trunk Cable with Multiple Coax

  20. Optical Fiber • Advantages • High bandwidth • 135+ Mbps over 40 miles • 1.7+ Gbps over short distances • Much lower loss than copper • Immune to electrical interference • No crosstalk • Smaller and lighter than copper cables • Secure • Disadvantages • More expensive than copper • Harder to terminate

  21. Optical Fiber • Single mode • 5 micron core • Multimode • 62.5 micron core • More dispersion than single mode • Lower BW • Used in undersea cables • First was TAT-8 • 40,000 simultaneous calls

  22. Optical Cable • SONET • Synchronous optical network • Primarily used by phone companies • Replacing T-3 and T-4 • Data rates to Gbps

  23. Atlantic Ocean Cables

  24. Microwave Radio • Most common medium for long distance • 4 GHz to 28 GHz carrier frequencies • 30 MHz channels • Up to 6000 voice circuits • Line-of-sight repeater locations • 20 to 30 miles apart • Mostly analog, some digital • Transmission affected by rain • Sometimes used for private networks

  25. Microwave Tower

  26. Satellite • Usually placed in geosynchronous orbits (GEO) • 23,000 miles above the earth • Appears stationary • Positioned to cover certain areas • Uplink - transmission to satellite from base station • Downlink - transmission from satellite to a base station

  27. Satellite - Advantages and Disadvantages • Main advantage • Communication between widely separated locations without telco infrastructure • Disadvantages • Security • Anyone can receive transmissions • Must encrypt • Time delay • High orbit - delays on the order of 0.25 seconds minimum • Very annoying • Telcos switched back to terrestrial links where possible • Satellites still used for international calls • Weather can affect link

  28. Other Satellite Applications • Used for PCS and paging systems • Low Earth Orbit (LEO) • 435 miles up • 100 minute orbits • About 70 satellites required for full coverage • Medium Earth Orbit • 6200 miles up • 6 hour orbits • 9 to 12 satellites required for full coverage • Satellite TV • Direct Broadcast Satellite (DBS) also known as Direct Satellite Service (DSS) • Downlink only • Competes with cable TV

  29. Comparison of Media

  30. Circuit Ownership • Private circuit • Maintained by company other than a common carrier • Typical application • within a company’s campus or buildings • Very inexpensive once installed

  31. Circuit Ownership • Leased circuit • Circuits owned by common carrier but leased to a customer for their exclusive use • Reasons to lease a circuit • A private circuit cannot be installed • When leased circuit cost is less than dial up cost for time used • When 4-wire service is needed (full duplex) • For high-speed transmission • Cost • Based on speed and distance • Will involve LEC and/or IXC • Leased 4-wire circuit 10% more than leased 2-wire

  32. Circuit Ownership • Switched circuits (dial-up) • Normal telephone circuit (half duplex)

  33. Multiplexing and Concentrating • Time-Division Multiplexing (TDM) • A circuit’s capacity is divided into time slots • A TDM frame is made up of a bit, character, or message (depending on method) from each source • At receiver, frames disassembled and signals reconstructed • Ex.: A circuit with 9600 bps can contain four multiplexed 2400 bps channels • Statistical Time Division Multiplexing (STDM) • Time slots not fixed • STDM multiplexer samples the input sources and only sends data from active sources • Addresses accompany source characters

  34. FDM and TDM: Time x Bandwidth

  35. TDM

  36. STDM

  37. Multiplexing and Concentrating • Concentrator • Combines several low-speed circuits into one high-speed circuit • Inverse concentrator • Divides up data on a high-speed line and puts it on several low-speed lines

  38. Network Access Technology Tradeoffs

  39. Case Study: Dow Corning’s Data Communications • Large LAN at Midland headquarters • PCs and terminals communicate with servers and mainframe • Speed at least 16 Mbps • Response time less than 1 second • Facilities near Midland • Using high-speed digital lines • Excellent response times • Lines not heavily used

  40. Case Study: Dow Corning’s Data Communications • Outside Midland • Use 9.6 kbps connecting sales offices and warehouses on multipoint leased lines • Connections to large plants use digital circuits of 128 kbps or higher • Fiber optics • Used on campus • Carrying video and data • Several point-to-point T1s between facitilites • Multiplex voice, data, and video

  41. Case Study: Dow Corning’s Data Communications • Broadband coax • Used in Midland plant for in-plant television now • Overseas • Using frame relay • 128 kbps to 512 kbps

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