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Digital Data Services

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... is the primary reason digital communications improve the quality, reliability ... wide variety of digital subscriber line technologies including HDSL, ADSL ... – PowerPoint PPT presentation

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Title: Digital Data Services


1
Digital Data Services
2
Digital Services
  • Two types of Digital Services
  • Dedicated
  • Switched

3
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).

4
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.

5
Local loop
6
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.

7
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.

8
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.

9
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

10
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.

11
Digital Services
  • Dedicated Service
  • Point-to-point service
  • Customer has exclusive use 24x7
  • Connection is always available
  • Customer must pay for mileage

12
Digital Services
  • Switched Service
  • Physical path may vary
  • Shared network
  • Customer connects on demand
  • Customer pays for usage
  • Service provider can oversubscribe capacity

13
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

14
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

15
T1 Technology
16
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.

17
xDSL Technology
18
xDSL Technology
19
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.
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