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Data
Communications
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Switching
- A
switched network consists of a series of interlinked nodes
called switches.
- Switches
are hardware and/or software devices capable of creating temporary
connections between two or more devices linked to the switch
but not to each other.
- Circuit
switching -- creates a direct physical connection between
two devices such as phones or computers.
- A
circuit switch is a device with n inputs and m
outputs that creates a temporary connection between an
input link and an output link.
- Designed
for voice communications and less well suited to data
and other nonvoice communications.
- A
circuit-switched link creates the equivalent of a
single cable between two devices and thereby assumes
a single data rate for both devices.
- Inflexible,
retains the established path.
- Sees
all transmissions as equal.
- An
example is the Public Switched Telephone Network (PSTN).
The switching centers are organized into five classes:
regional offices (class 1), sectional offices (class 2),
primary offices (class 3), toll offices (class 4), and
end offices (class 5).
- Space-division
switches
- Paths
in the circuit are separated from each other spatially
- Crossbar
switch connects n inputs to m outputs
in a grid, using electronic microswitches at each
crosspoint. Major limitation is the number of crosspoints
required.
- Multistage
switches combine crossbar switches in several
stages
- Provide
several options for connecting each pair of linked
devices.
- The
reduction in the number of crosspoints results
in blocking during periods of heavy traffic.
- In
a single stage switch, blocking does not occur.
- Advantage
-- Instantaneous
- Disadvantage
-- the number of crosspoints required to make space-divsion
switching acceptable in terms of blocking.
- Time-Division
switches
- Uses
time-division multiplexing to achieve switching.
- Advantage
-- Needs no crosspoints
- Disadvantage
-- processing each connection creates delays.
- Time-slot
interchange (TSI) changes the ordering of
the slots based on the desired connections.
- TDM
Bus -- the input and output lines are connected
to a high-speed bus through input and output gates.
- Multistage
switches combine space-division and time-division
technology to optimize both the physical (the
number of crosspoints) and the temporal (the amount
of delay).
- Packet
Switching
- Data
are transmitted in discrete units of potentially variable
length blocks called packets.
- Datagram
approach
- Each
packet is treated independently from all others. Packets
are referred to as datagrams.
- Responsibility
of the transport layer to reorder the datagrams before
passing them on to the destination port.
- The
link joining each pair of nodes can contain multiple
channels. Multiplexing can be done with FDM or TDM.
- Virtual
circuit approach
- The
relationship between all packets belonging to a message
or session is preserved.
- A
single route is chosen between sender and receiver
at the beginning of the session.
- In
the switched virtual circuit (SVC) format a
virtual circuit is created whenever it is needed
and exists only for the duration of the specific exchange.
The network could pick an alternate route in response
to failure or congestion.
- Permanent
virtual circuits provide the same virtual circuit
between two users on a continuous basis.
- Circuit-switched
vs Virtual-Circuit connection
- A
circuit-switched connection creates a path between two
points, whereas a virtual-circuit creates a route between
two points.
- The
links that make a path are dedicated, whereas links that
make a route can be shared by other connections.
- Message
switching stores and forwards messages from secondary
storage devices. Unpopular for direct communication.
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