Post on 25-Jan-2021
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2.2 Circuit Switching, Time-Division Multiplexing (TDM), Time Switching,
Cut-through
• Circuit Switching versus Packet Switching
• Digital Telephony, Time-Division Multiplexing (TDM)
• Time Switching, Time-Slot Interchange (TSI)
• Switching and Computers: 1st and 2nd Generations
• Cut-through
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A BA CAA B CA
number of circuits, each of them having a fixed rate;unused capacity in one circuit cannot be used by other circuits.
actual rate of connections varies widely with time.
Data are transmitted inside fixed, periodic frames; each circuit
The transmission capacity of a link is partitioned into a fixed
Advantage: simple.
Disadvantage: wasteful in transmission capacity, especially when
B
frameperiodic frame frame
idle connection time-slot unused by its connection
(connection) is allocated a fixed subset of the time-slots ineach frame; connectionID and routing information is providedimplicitely by the time-slotID in which a datum is transmitted.
Circuit Switching
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Multiplexing - Demultiplexing
CM = C1+...+CnC1
Cn
1
n
1
n
C1
Cn
Examples:
Minimal buffering requirements:one time-slot-worth of data per mux'ed/demux'ed link
circuit switching: frames & time-slots
wide (bit-parallel) buses inside switch elements
at fixed aggregate capacity (circuit-switching style)
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This is the disadvantage of circuit switching.while other partitions may have excess capacity.capacity type, the flows in one partition may lack capacity,In a link carrying multiplexed traffic of fixed aggregate
f1f2f3
f5f4
f1f2f3
f5f4
Partitioned versus Shared Link Capacity
Resource Partitioning leads to Underutilization:
C1
C2
C1
C2
CM
f5f4f3f2f1
CM
Partitioned Shared
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Packet Switching
and can be stored and forwarded at any later time.packet carries its own source and destination (connection) ID,
dynamic control (per packet), rather than static (at conn. set-up);
Non-periodic multiplexing of packets, on a demand basis; each
B BC CA A A A
The transmission capacity of a link is shared among all flows
capacity that is not used by one flow can be used by another.(connections) that pass through it, on a demand basis; any
Challenges:
unpredictability of traffic, leading to contention for resources.
Advantage: no waste of transmission capacity.
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Packet Switching: Statistical Multiplexing
statistical multiplexing
λ(t) > Cm
C1
Cn
Buffer
bufferrequiredto absorbtransients
not fully utilizedall the timefixed
multiplexingcapacity
aggregate
CM
sharedcapacity
Cm
when (packetswitching)
partitionedcapacity
(circuit switching)
C1 + ... + Cn = CM > Cm
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d' time
in2:
out1:
in1:
timed d
in1
in2
out1
out2
Store-and-Forward
versus
Cut-Through
Hiccup-less cut-through requires:
Cut-through reduces delay.
controlled rate differencebetween input and output
in2:
out1:
in1:
hiccup-less incoming packets