Communications Circuits

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Chapter 9

• Communications Circuits

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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

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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

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ISDN BRI and PRI
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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

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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

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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.

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T-Carrier Systems

• High-speed digital transmission system
• Time-division multiplexing
• T- identifies the carrier type
• DS identifies the signal

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T-Carrier System
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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

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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

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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 carriers office at each
  end of connection
• Carrier handles circuit in between end offices

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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

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ADSL Bandwidth Allocation
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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

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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

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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

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Trunk Cable with Multiple Coax
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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

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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

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Optical Cable

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

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Atlantic Ocean Cables
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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

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Microwave Tower
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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

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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

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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

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Comparison of Media
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Circuit Ownership

• Private circuit
• Maintained by company other than a common carrier
• Typical application
• within a companys campus or buildings
• Very inexpensive once installed

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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

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Circuit Ownership

• Switched circuits (dial-up)
• Normal telephone circuit (half duplex)

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Multiplexing and Concentrating

• Time-Division Multiplexing (TDM)
• A circuits 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

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FDM and TDM Time x Bandwidth
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TDM
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STDM
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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

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Network Access Technology Tradeoffs
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Case Study Dow Cornings 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

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Case Study Dow Cornings 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

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Case Study Dow Cornings Data Communications

• Broadband coax
• Used in Midland plant for in-plant television now
• Overseas
• Using frame relay
• 128 kbps to 512 kbps