Notes for 10/11/00

PART VI

Long-Distance and Digit Connection Technologies

- Connections used for large geographic distances are usually leased by telephone companies
- They provide a point to point connection
- They provide connections through
    - Copper wire
    - Microwave
    - Optical fiber
    - Satellite channel
-Customer chooses between
    - Analog: modem is needed
    - Digital: DSU/CSU devices needed at each end
        DSU: Data Service Unit.
                  translates digital data from the computer to the digital circuit of the Phone Company and back. Computer and Phone Company may use different       representations for digital data.
 
       CSU: Channel Service Unit
                - Diagnostics
                - Balances the number of 0’s and 1’s by using special encoding. This prevents excessive use of power.
 
Digital Circuit Technology
   - Developed by telephone companies
   - Designed for voice and data
   - Voice is converted to digital format and back
   - The telephone standard for digitalized voice is called PCM – Pulse Code Modulation
   - PCM:
        - Sample signal at a fixed rate. 3000-600 Hz voice
        - Send a number for each sample
        - Choose the closest digital value

 

Standard for Digital Circuits
     - Specified by telephone industry
     - Different around the World
     - Standards are known by a 2-character name
 
Name
Bit rate
Voice channels
T1
1.544 Mbps
24
T2
6.312 Mbps
96
T3
44.736 Mbps
672

      T3 circuit is 28 times faster than T1.

      T1, T3 are the most common.
      Also available:
           Fractional T1 (example, 56 Kbps circuit)

    -  It is also possible to combine two or more circuits to produce an intermediate capacity.

    - The hardware used is called "Inverse Multiplexer"
        -  The Inverse Multiplexer alternates data between circuits. It can be every other bit or every other byte.

 
 

High Capacity Digital Circuits

     - Also available from phone companies
     - Uses optical fiber
     - Signal standard is called STS (Synchronous Transport Signal)
     - Optical standard is called OC (Optical carrier)

 
Standard
Optical
Bit rate
Voice
STS
OC-1
51.84 Mbps
810
 
OC-3
155.52 Mbps
2430

              OC-3 is the most popular
 
 

Digital Services for Home, Small Companies

      - Local loop: Standard phone connection between home/office and central office
      - Analog Service: Standard phone line also called POTS – Plain Old Telephone System
      - Digital service:
                    - ISDN: Integrated Services Digital Network
                                  -Voice and data
                                  - Longer distances
                                  - Old
                     - DSL: Digital Subscriber Line
                                 - New technology
                                 - Higher speed