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Unformatted text preview: T
= Td
S + L/B = (Np + N – 1)(P/B)
Circuit Switching vs. Virtual Circuit Packet Switching
TV = EndtoEnd Delay, Virtual Circuit Packet Switching
TV = S + Td
TC = TV
L/B = (Np + N – 1)(P/B)
Datagram vs. Virtual Circuit Packet Switching
Td = T V – S
3.2 From Problem 3.1, we have
Td= (Np + N – 1)(P/B) + N × D
For maximum efficiency, we assume that Np = L/(P – H) is an integer. Also, it is
assumed that D = 0. Thus
Td = (L/(P – H) + N – 1)(P/B)
To minimize as a function of P, take the derivative:
0
= dTd/(dP)
0
= (1/B)(L/(P – H) + N – 1)  (P/B)L/(P – H)2
0
= L(P – H) + (N – 1) (P – H)2  LP
0
= –LH + (N – 1)(P – H)2
(P  H)2 = LH/(N – 1)
LH
P=H+
N−1
3.3 Each telephone makes 0.5 calls/hour at 6 minutes each. Thus a telephone occupies
a circuit for 3 minutes per hour. Twenty telephones can share a circuit (although
this 100% utilization implies long queuing delays). Since 10% of the calls are long
distance, it takes 200 telephones to occupy a long distance (4 kHz) channel full time.
The interoffice trunk has 106 /(4 × 103 ) = 250 channels. With 200 telephones per
channel, an end office can support 200 × 250 = 50,000 telephones.
3.4 The argument ignores the overhead of the initial circuit setup and the circuit
teardown.
3.5 Yes. A large noise burst could create an undetected error in the packet. If such an
error occurs and alters a destination address field or virtual circuit identifier field,
the packet would be misdelivered.
3.6 The number of hops is one less than the number of nodes visited.
a. The fixed number of hops is 2. 10 ...
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This note was uploaded on 02/20/2012 for the course ENC 3250 taught by Professor Dr.timothycerner during the Fall '11 term at UNF.
 Fall '11
 Dr.TimothyCerner

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