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Data
Communications
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Multiplexing
- A
set of techniques that allows the simultaneous transmission
of multiple signals across a single data link.
- Definitions
- Path
refers to the physical link.
- Channel
refers to a portion of a path that carries a transmission
beween a given pair of devices.
- Multiplexers
(MUX) combine multiple transmission streams into a
single stream.
- Demultiplexers
(DEMUX) separates a stream into its component transmissions.
- Frequency
Division Multiplexing (FDM) is an analog technique
that can be applied when the bandwidth of a link is greater
than the combined bandwidths of the signals to be transmitted.
- Signals
generated by each sending device modulate different
carrier frequencies using either AM or FM modulation.
These modulated signals are then combined into a single
composite signal that can be transported by the link.
- Channels
must be separated by strips of unused bandwidth (guard
bands) to prevent signals from overlapping.
- The
demultiplexer uses a series of filters to decompose
the multiplexed signal into its constituent component
signals.
- Wave
division multiplexing (WDM) is conceptually the same
as FDM, except that the multiplexing and demultiplexing
involve light signals transmitted through fiber-optic
channels.
- Time
division multiplexing (TDM) is a digital process that
can be applied when the data rate capacity of the transmission
medium is greater than the data rate required by the sending
and receiving devices.
- In
synchronous time-division multiplexing the multiplexer
allocates exactly the same time slot to each device at
all times, whether or not a device has anything to transmit.
- Time
slots are grouped into frames. A frame consists
of one complete cycle of time slots, including one
or more slots dedicated to each sending device.
- By
assigning each time slot to a specific input line,
we end up with empty slots whenever not all the lines
are active.
- In
bit stuffing the multiplexer adds extra bits
to a device's source stream to force the speed relationships
among various devices into integer multiples of each
other.
- Asynchronous
time-division multiplexing, or statistical time-division
multiplexing allows a number of lower-speed input
lines to be mulitplexed to a single higher-speed line.
- The
total speed of the input lines can be greater than
the capacity of the path.
- If
we have n input lines, the frame contains no
more than m slots, with m less than
n.
- Given
the same link, asynchronous TDM can support more devices
than synchronous TDM.
- Rather
than being preassigned, each slot is available to
any of the attached input lines that has data to send.
- In
the absence of fixed positional relationships, each
time slot must carry an address telling the demultiplexer
how to direct the data.
- Adding
address bits to each time slot increases the overhead
of an asynchronous system and somewhat limits its
potential efficiency. It is efficient only when the
size of the time slots is kept relatively large.
- Can
accomodate traffic of varying data rates by varying
the length of the time slots.
- Inverse
multiplexing takes the data stream from one high-speed
line and breaks it into portions that can be sent across
several lower speed lines simultaneously, with no loss
in the collective data rate.
- The
telephone system
- Analog
services
- Analog
switched service uses two-wire twisted-pair cable
to connect the subscriber's handset to the network
via an exchange. This connection is called the local
loop. The network it joins is referred to as a
public switched telephone network (PSTN). The
signal is analog, and the bandwidth is usually between
0 and 4000 Hz. For analog lines, FDM is used.
- Analog
leased service offers a leased, or dedicated line
that is permanently connected to another customer.
- Conditioning
improves the quality of a line by lessening attenuation,
signal distortion, or delay distortion.
- Digital
services
- Less
sensitive than analog services to noise and other
forms of interference. It is also of lower cost.
- Switched/56
service allows data rates of up to 56Kbps. To
communicate through this service, both parties must
subscribe. A digital service unit (DSU) changes
the rate of the digital data created by the subscriber's
device to 56 Kbps and encodes it in a format used
by the service provider. Supports bandwidth on
demand, allowing subscribers to obtain higher
speeds by using more than one line.
- Digital
data service (DDS) is a digital leased line with
a maximum data rate of 64 Kbps.
- Digital
Signal (DS) Service is a hierarchy of digital
signals.
| Service |
Line |
Rate
(Mbps) |
Voice
Channels |
| DS-1 |
T-1 |
1.544 |
24 |
| DS-2 |
T-2 |
6.312 |
96 |
| DS-3 |
T-3 |
44.736 |
672 |
| DS-4 |
T-4 |
274.176 |
4032 |
- Fractional
T Lines allow several subscribers to share one
line by multiplexing their transmissions. Transmissions
are directed through a digital service unit/channel
service unit (DSU/CSU). This device divides the
capacity of the line into separate channels.
- Digital
Subscriber Line (DSL) uses the existing telecommunication
networks to accomplish high-speed delivery of data, voice,
video, and multimedia.
- Asymmetric
digital subscriber line (ADSL) provides higher bit
rates in the downstream direction than the upstream direction.
- Rate
adaptive asymmetrical digital subscriber line (RADSL)
is based on ADSL. It allows different data rates depending
on the type of communication: voice, data, or multimedia.
- High
bit rate digital subscriber line (HDSL) is an alternative
to a T-1 line. It uses 2B1Q encoding which is less susceptible
to attenuation.
- Symmetric
digital subscriber line (SDSL) is the same as HDSL
but uses one single twisted-pair cable.
- Very
high bit rate digital subscriber line (VDSL) is similar
to ADSL and uses coaxial, fiber-optic, or twisted-pair
cable for short distances.
- Fiber
to the curb (FTTC) uses copper twisted-pair cable coming
from individual premises that is multiplexed in the junction
boxes and converted to optical signals.
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