Long Distance and Local Loop Digital Technologies

1
Long Distance and Local Loop Digital Technologies

• Chaminade University
• Department of Computer Science and Computer
  Information Systems

2
Goals

• In this chapter you will learn about networks
  that overcome the size and distance limitations
  of LANs.
• Point-to-point digital communication circuits
• High speed digital access

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Digital Telephony
4
Digital Circuits
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Digital Circuits

• DSU Data Service Unit
• CSU Channel Service Unit
• NIU Network Interface Unit
• The purpose of the DSU/CSU is to interface a
  computer to a digital circuit provided by a
  telephone company.
• DSU features
• Handles data
• Manages the translation between digital formats

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Digital Circuits

• CSU Handles diagnostics (line disconnected,
  loopback test etc..)
• The CSU also prohibits excessive consecutive 1
  bits.
• The interface standard used on the computer side
  can be RS-232 (if less than 56kpbs) or RS449.

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DS Terminology
8
Intermediate Capacity Circuits
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Highest Capacity Circuits
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SONET
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Digital Subscriber Line (DSL)

• Researchers observed that a scheme like ADSL
  might be possible because many local loops
  accommodate signals at frequencies higher than
  those used by the telephone system.
• DSL There are several variants. They are
  collectively referred as xDSL.
• Downstream data flowing out to the user
• Upstream data flowing from the user.
• There are two main variants
• Symmetric
• Asymmetric

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ADSL
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ADSL

• Maximum downstream is 6.144Mbps.
• Maximum upstream is 640kbps.
• The effective upstream is 576k (because a
  mandatory network control channel that requires
  64kpbs).
• ADSL does not require any changes in the local
  loop wiring.
• It uses the same twisted pair used for the analog
  telephone service.

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ADSL

• ADSL is adaptive when ADSL modems are powered
  on, they probe the line between them to find its
  characteristics.
• They agree to communicate using techniques that
  are optimal for the line.

15
Other DSL Technologies

• SDSL Symmetric Digital Subscriber provides
  symmetric bit rates in both directions.
• Businesses that provide information to others
  tend to have the opposite symmetry that their
  users.
• Few phone companies offer ADSL with the direction
  of asymmetry reversed.
• Businesses that export information may prefer
  SDSL.

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Cable Modem Technology

• Uses the coaxial cable. Therefore, it has high
  capacity and is immune to electromagnetic
  interference
• The cable system uses broadband signaling (FDM)
  to deliver multiple television channels
  simultaneously.
• Unused channels can be used to send data.

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Cable Modem Technology

• A pair of modems is required, one that resides at
  a subscribers site, and another at the CATV
  center.
• Both modems in a pair must be tuned to the same
  carrier frequency, which is multiplexed onto the
  cable along with television signals.

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Cable Modem Technology

• Since there are millions of subscribers in a
  large metropolitan area, it is impossible to
  allocate a separate carrier frequency to each.
• One frequency is used for a set of subscribers
  (e.g. neighborhood).
• Each subscriber is assigned an address.
• The system resembles a shared LAN.