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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 time-slot unused by its connection
A B A C A B A C A B
periodic frameframe 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 C1 C1 1 CM = C1+...+Cn n n Cn 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
f1 Partitioned Shared C1 f2 f3 f1 f1 f2 C1 f2 f3 CM f3 CM f4 C2 f4 f4 f5 f5 C2 f5
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
AAAABBCC
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 Cm Buffer
Cn not fully utilized fixed all the time buffer aggregate required capacity partitioned to absorb shared capacity capacity multiplexing transients (circuit switching) when (packet λ(t) > Cm switching) statistical multiplexing
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in1 out1 in1: in2 out2 in2:
out1: Store-and-Forward dd time versus
Cut-Through in1: Cut-through reduces delay. in2: Hiccup-less cut-through requires: out1: hiccup-less incoming packets d' controlled rate difference time between input and output
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