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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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Circuit Switching
idle connection
A
B
time-slot unused by its connection
A
C
periodic frame
A
B
A
frame
C
A
B
frame
Data are transmitted inside fixed, periodic frames; each circuit
(connection) is allocated a fixed subset of the time-slots in
each frame; connectionID and routing information is provided
implicitely by the time-slotID in which a datum is transmitted.
The transmission capacity of a link is partitioned into a fixed
number of circuits, each of them having a fixed rate;
unused capacity in one circuit cannot be used by other circuits.
Advantage: simple.
Disadvantage: wasteful in transmission capacity, especially when
actual rate of connections varies widely with time.
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Multiplexing - Demultiplexing
at fixed aggregate capacity (circuit-switching style)
1
n
C1
C1
1
CM = C1+...+Cn
Cn
n
Cn
Examples:
circuit switching: frames & time-slots
wide (bit-parallel) buses inside switch elements
Minimal buffering requirements:
one time-slot-worth of data per mux'ed/demux'ed link
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Partitioned versus Shared Link Capacity
C1
C2
f1
f2
f3
Partitioned
Shared
f1
f2
f3
C1
f4
f5
C2
f1
f2
f3
f4
f5
f4
f5
CM
CM
Resource Partitioning leads to Underutilization:
In a link carrying multiplexed traffic of fixed aggregate
capacity type, the flows in one partition may lack capacity,
while other partitions may have excess capacity.
This is the disadvantage of circuit switching.
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Packet Switching
A
B
C
A
A
C
A
B
Non-periodic multiplexing of packets, on a demand basis; each
packet carries its own source and destination (connection) ID,
and can be stored and forwarded at any later time.
The transmission capacity of a link is shared among all flows
(connections) that pass through it, on a demand basis; any
capacity that is not used by one flow can be used by another.
Advantage: no waste of transmission capacity.
Challenges:
dynamic control (per packet), rather than static (at conn. set-up);
unpredictability of traffic, leading to contention for resources.
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Packet Switching: Statistical Multiplexing
C1 + ... + Cn = CM > Cm
C1
CM
Cn
fixed
aggregate
capacity
multiplexing
not fully utilized
all the time
partitioned
capacity
(circuit switching)
Buffer
buffer
required
to absorb
transients
when
λ(t) > Cm
Cm
shared
capacity
(packet
switching)
statistical multiplexing
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in1
out1
in2
out2
in1:
in2:
out1:
Store-and-Forward
d
d
time
versus
Cut-Through
in1:
Cut-through reduces delay.
in2:
Hiccup-less cut-through requires:
hiccup-less incoming packets
out1:
d'
time
controlled rate difference
between input and output
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