October 11, 2000

Part VI

Long distance and Digital Connection Technologies

· Connections used for large geographic distances are usually leased by telephone companies

· They provide point to point connections

· They provide connections through

o Copper wire

o Microwave

o Optical fiber

o Satellite channel

· Customer chooses between

o Analog: modem is needed

o Digital: DSU/CSU devices needed at each end

DSU: Data Service Unit translates data from the computer to the digital circuit of the Phone Company and back. Computer and Phone Company may use different representation for digital data.

CSU: Channel Service Unit

• Diagnostic
• 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 (For e.g. 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.

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 control 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
• ADSL: Asymmetric Digital Subscriber Line