Patterson Chap 1 Q 16

Question

16. Calculate the latency (from first bit sent to last bit received) for
the following:
(a) 100-Mbps Ethernet with a single store-and-forward switch in
the path and a packet size of 12,000 bits. Assume that each
link introduces a propagation delay of 10 µs and that the
switch begins retransmitting immediately after it has finished
receiving the packet.
(b) Same as (a) but with three switches.
(c) Same as (a), but assume the switch implements “cutthrough” switching; it is able to begin retransmitting the
packet after the first 200 bits have been received.
 

Answer 1

1)transmission time=12000/100*10^6= 120 us

packer reaches at switch=120+10=130us

then from switch to destination=130

so total latency=260us

2)for 3 switches =130*4=520 us(as packet has to go over 4 links)

3)For “cut-through”, a switch needs to only decode the 200 bits before it begins to forward. This takes 2 us. This delay replaces the switch transmit delays in the previous answer for a total delay of one transmit delay + 3 cut through decoding delays + 4 propagation delays = 120+3*2+4*10=166 us

Comments

option c has only one switch so only one cut through should be counted.

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for option c will it be 120+10+2+10=142 microsecond. (considering only one switch)