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Chapter 9. High-Speed Digital Access
1. DSL Technology
2. Cable Modem
3. SONET
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ADSL
• Asymmetrical Digital Subscriber Line
• ADSL is an asymmetric communication technology designed for residential users; it is not suitable for businesses
• The existing local loop can handle bandwidths up to 1.1 MHz
• ADSL is an adaptive technology. The system uses a data rate based on the condition of the local loop line.
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DMT (Discrete Multitone Technique)
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DMT: Bandwidth Division
• There is no set way that the bandwidth is divided
• Upstream– 24 channels * 4 Kbaud/sec * 15 bits/baud = 1.44 Mbps
• Downstream
– 224 channels * 4 Kbaud/sec * 15 bits/baud = 13.4 Mbps
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DSL: Actual Bit Rate
• Because of the high signal/noise ratio
• Upstream– 64 Kbps to 1 Mbps
• Downstream– 500 Kbps to 8 Mbps
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Customer Site: ADSL Modem
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Telco Site: DSLAM
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Other DSL Technologies: xDSL
• SDSL (Symmectric DSL)– For businesses that send and receive data in large volues in both di
rections
• HDSL (High-bit-rate DSL)– Designed as an alternative to the T-1 line
• VDSL (Very-high-bit-rate DSL)– Alternative to ADSL
– Uses coaxial, fiber-optic, twisted-pair cable for short distances (300-1800m)
– Uses DMT with a bit rate of 50-55 Mbps (downstream) and 1.5-2.5 Mbps (upstream)
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Traditional Cable TV Network
• Started to distribute video signals to locations with poor or no reception in the late 1940s: Unidirectional
• It was called Community Antenna TV
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HFC (Hybrid Fiber Coaxial) Network
• RCH (Regional Cable Head) feeds the distribution hubs
• Fiber node splits the signal to each coaxial cable
• Communication in an HFC network can be bidirectional
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Coaxial Cable Bands
• Video band: 6 MHz per TV channel
• Downstream data band: 64-QAM with TCM– Theoretical rate is 30 Mbps (6 Mbaud/sec * 5 bits/baud)
• Upstream data band: QPSK– Theoretical rate is 12 Mbps (6 Mbaud/sec * 2 bits/baud)
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CM (Cable Modem)
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CMTS (Cable Modem Transmission System)
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SONET/SDH
• Digital transmission standards for fiber-optic cable
• Independently developed in USA & Europe
– SONET(Synchronous Optical Network) by ANSI
– SDH(Synchronous Digital Hierarchy) by ITU-T
• Multiplexed transport mechanism
• Carrier for ATM and B-ISDN
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Interesting Features of SONET/SDH
• SONET/SDH is a synchronous network
– A single clock is used across the entire network
• It contains the standards for fiber-optic equipments
• Very flexible to carry other transmission systems (DS-0, DS-1, etc)
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SONET/SDH Rates
• SONET defines a signal hierarchy– STS (Synchronous Transport Signals)
• A hierarchy of signaling levels
– OC (Optical Carriers)
• Physical links defined for optical signals
– STS(OC)-1 (51Mbps) STS(OC)-192 (10Gbps)
• SDH specifies a similar hierarchy– STM (Synchronous Transport Module)
– STM-1 (155Mbps) STM-64 (10Gbps)
• OC-1, OC-3, OC-12, OC-48, OC-192
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SONET
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Frame Format: STS-1
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SONET Frames: STS-1
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SONET Rates
STS OC Rate (Mbps) SPE (Mbps) User (Mbps)
STS-1STS-1 OC-1 51.84 50.12 49.536
STS-3STS-3 OC-3 155.52 150.336 148.608
STS-9STS-9 OC-9 466.56 451.008 445.824
STS-12STS-12 OC-12 622.08 601.344 594.432
STS-18STS-18 OC-18 933.12 902.016 891.648
STS-24STS-24 OC-24 1244.16 1202.688 1188.864
STS-36STS-36 OC-36 1866.23 1804.032 1783.296
STS-48STS-48 OC-48 2488.32 2405.376 2377.728
STS-192STS-192 OC-192 9953.28 9621.604 9510.912
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Virtual Tributaries
• Partial payload that is inserted into an STS-1 frame
• Each component of subdivided SPE
• Provides backward compatibility
• Four types of VTs
– VT1.5 : For DS-1(T-1: 1.544Mbps)
– VT2: For CEPT-1(E-1: 2.048Mbps)
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VT Types
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STS Multiplexing
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STS-n
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