SME
Computer Networks · Problem type
Bottleneck link
Use this for “A 4 MB file crosses a 10 Mb/s link, then a 2 Mb/s link. How long does the transfer take?”
This is an example. Type your own values to start a new problem.
Rb (Mb/s)
=
min(
R₁ (Mb/s)
,
R₂ (Mb/s)
)
t (s)
=
8
×
F (MB)
Rb (Mb/s)
Formulas
Rb
=
min(
R₁
,
R₂
)
2 = min(10, 2)
t
=
8
×
F
Rb
16
=
8
×
4
2
Assumptions
•
The file streams through both links at once, so the slower link sets the rate for the whole path.
•
No other traffic shares the links, and propagation and queueing delays are small next to the transfer.
•
File sizes and rates use decimal prefixes: 1 MB is 10⁶ bytes and 1 Mb/s is 10⁶ bits a second.
Step-by-step
WE KNOW
F = 4 MB, R₁ = 10 Mb/s, R₂ = 2 Mb/s
FIND
bottleneck rate (Rb), transfer time (t)
Step 1 · Find bottleneck rate (Rb)
Use Rb = min(R₁, R₂)
Bits
can’
t
leave
the
path
faster
than
they
cross
its
slowest
link.
Rb = min(10, 2)
Rb = 2 Mb/s
Step 2 · Find transfer time (t)
Use t = 8 × F ÷ Rb
Eight bits a byte make the file megabits; over Mb/s that gives seconds.
t
=
8
×
4
2
t
=
32
2
t
=
16
s
Check
Put the numbers back in. Both sides should match.
2 = min(10, 2) ✓
16
=
8
×
4
2
✓
Topic check
See what you know: 5 questions on this topic
New questions each time. Pass to mark it complete.
Related lessons
Network performance
Main lesson
Bandwidth-delay product
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Queueing delay
Use this for “Packets reach a router at 800 a second and it serves 1000 a second. Find the average delay and number queued.”