Digital Data Services

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Digital Data Services
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Digital Services

• Two types of Digital Services
• Dedicated
• Switched

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Local loop

• Local Loop
• The physical connection (typically, twisted pair
  of copper wire) connecting the end user
  (subscriber) to the central office.
• Central Office
• Typically where the local loop is terminated for
  connection to the public switched telephone
  network (PSTN).

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Local loop

• Customer Premises
• Typically the building owned by the end user who
  uses services provided from the central office to
  enter the public switched telephone network
  (PSTN).
• Access Device
• Physical equipment that terminates the local
  loop, such as a CSU/DSU for T1.

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Local loop
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Local loop - Analog vs Digital

• The information content of an analog signal is
  conveyed by continuously varying some
  characteristic such as amplitude, frequency or
  phase of a voltage or other characteristic of the
  signal.
• Computers, motors, lights and other electrical
  devices generate electrical noise (unwanted
  electrical signals), which produce undesirable
  variations on the information content of an
  analog signal, thus making it error prone.
• Both voice and data (from modems - sent as
  sounds) are commonly transmitted as analog
  information.

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Local loop - Analog vs Digital

• A digital signal is conveyed through discrete
  changes in the state of the signal such as the
  presence or absence of voltage (1 or 0) or
  discrete changes in voltage (3, 0, -3) thereby
  making the information content of a digital
  signal less susceptible to electrical noise.
• Since a digital signal consists of 1s and 0s, it
  is easy to identify the signal sent (1 or 0) and
  re-create the signal and transmit it further.
• This is the primary reason digital communications
  improve the quality, reliability and speed of
  communications. Any noise or distortion
  introduced along the circuit is eliminated when
  the signal is regenerated.

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Digital Services

• Dedicated service is a circuit between fixed
  endpoints. The circuit is owned by an end user or
  leased from a common carrier.
• The service is available 24 hours a day, 7 days a
  week, 52 weeks a year, for exclusive use by that
  user.
• This service may also be referred to as a leased
  line or nailed-up circuit.

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Digital Services

• Switched service is a circuit for which the
  endpoints may vary with each usage.
• The circuit is provided by a common carrier,
  which is routed through a switched network,
  providing circuit switching between public end
  users.
• Dial-Up internet access is switched

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Digital Services

• There are two types of switched technology
• Packet switched.
• Circuit switched
• In a switched circuit, a call is established only
  for as long as needed and then the session is
  disconnected.

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Digital Services

• Dedicated Service
• Point-to-point service
• Customer has exclusive use 24x7
• Connection is always available
• Customer must pay for mileage

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Digital Services

• Switched Service
• Physical path may vary
• Shared network
• Customer connects on demand
• Customer pays for usage
• Service provider can oversubscribe capacity

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T1 Technology

• T1 circuit identifies a multiplexed 24 channel,
  1.544 Mbps digital data circuit (E1 in most
  countries)
• Provides communications between two facilities or
  from a local service provider. T1 refers to the
  transport of a DS-1 formatted signal onto a
  copper, fiber or wireless medium

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T1 Technology

• Used to deploy voice, data or video-conferencing
  services.
• Part of a digital hierarchy that starts with 24
  DS0s at 64 kbps.
• By combining multiple DS0s, a high-speed
  interface can be provided to support a
  synchronous interface to a LAN router or voice
  PBX. For distances longer than one mile,
  repeaters are required

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T1 Technology
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xDSL Technology

• xDSL is the generic name used to represent a wide
  variety of digital subscriber line technologies
  including HDSL, ADSL and IDSL.
• xDSL Services can be used on the same pair as a
  normal voice line or on separate dedicated pairs.

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xDSL Technology
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xDSL Technology
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xDSL Technology
Footnotes 1. Designations are
not acronyms, but CCITT recommendation numbers
2. 192 Kbps divides into
two B channels (64 kbps), one D channel
(16 kbps) and link
administration. 3.
"Duplex" means data of the same rate both
upstream and downstream
at the same time.
4. Requires two twisted-pair lines
5. Requires three twisted-pair
lines 6. A new system
called SDSL, for Single Line DSL, operates at 1.5
or 2.0 Mbps
duplex over one line
7. Also called BDSL, VADSL, or, at times, ADSL.
VDSL is ANSI and
ETSI designation. 8.
"Down" means downstream, from the network to the
subscriber. "Up"
means upstream. 9.
Future VDSL systems may have upstream rates
equal to downstream,
but on much shorter lines.