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CS144An Introduction to Computer Networks
Packet SwitchingWhat is packet switching?
Nick McKeown Stephen IbanezProfessor of Electrical Engineering and Computer Science, Stanford University
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Outline1. What is Circuit Switching?2. What is Packet Switching?3. Why does the Internet use Packet Switching?
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Circuit Switching
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DedicatedWire
DedicatedWire
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Circuit Switching
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DedicatedWire
DedicatedWire
Circuit Switch
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Circuit Switching
Source“Caller”
Central Office(C.O.)
Destination“Callee”
Central Office(C.O.)
TrunkExchange
Each phone call allocated 64kb/s. A 10Gb/s trunk line can carry over
150,000 calls.
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Circuit Switching- Each call has its own private, guaranteed, isolated
data rate from end-to-end.
- A call has three phases:1. Establish circuit from end-to-end (“dialing”)2. Communicate3. Close circuit (“tear down”)
- Originally, a circuit was an end-to-end physical wire.
- Nowadays, a circuit is like a virtual private wire.
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Problems1. Inefficient. Computer communication tends to be very
bursty. e.g. typing over an ssh connection, or viewing a sequence of web pages. If each communication has a dedicated circuit, it will be used very inefficiently.
2. Diverse Rates. Computers communicate at many different rates. e.g. a web server streaming video at 6Mb/s, or me typing at 1 character per second. A fixed rate circuit will not be much use.
3. State Management. Circuit switches maintain per-communication state, which must be managed.
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Outline1. What is Circuit Switching?2. What is Packet Switching?3. Why does the Internet use Packet Switching?
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Packet Switching
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Address Next-hop
B S2
C S3
D S3
AB
Packet Switch
S1
S2
S4
S3
C D
Data HeaderData B
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Packet Switching
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Packet Switch
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Packet switches have buffers
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Data H
Data H
Buffers hold packets:- When two or more packets arrive at the same time- During periods of congestion
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Packet SwitchingA
R1
R2
R4
R3
B
Source Destination
-Packets are routed individually, by looking up address in router’s local table.-All packets share the full capacity of a link.- The routers maintain no per-communication state.
H
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Outline1. What is Circuit Switching?2. What is Packet Switching?3. Why does the Internet use Packet Switching?
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Efficient use of expensive links- Links were assumed to be expensive and scarce. - Packet switching allows many, bursty flows to share the
same link efficiently.- “Circuit switching is rarely used for data networks, ...
because of very inefficient use of the links” – Bertsekas/Gallager
Resilience to failure of links & routers- “For high reliability, ... [the Internet] was to be a datagram
subnet, so if some lines and [routers] were destroyed, messages could be ... rerouted” - Tanenbaum
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Summary1. What is Circuit Switching?2. What is Packet Switching?3. Why does the Internet use Packet Switching?
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CS144An Introduction to Computer Networks
Packet SwitchingEnd to End Delay
Nick McKeown Stephen IbanezProfessor of Electrical Engineering and Computer Science, Stanford University
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Outline1. Useful definitions2. End-to-end delay3. Queueing delay
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Propagation Delay, tl: The time it takes a single bit to travel over a link at propagation speed c.
l
Example: A bit takes 5ms to travel 1,000km in an optical fiber with propagation speed 2 x 108 m/s.
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Packetization Delay, tp: The time from when the first to the last bit of a packet is transmitted.
p
Example 1: A 64byte packet takes 5.12μs to be transmitted onto a 100Mb/s link.Example 2: A 1kbit packet takes 1.024s to be transmitted onto a 1kb/s link.
Note: Packetization delay is sometimes called “serialization” delay because it’s the time it takes to serialize a packet onto the link.
r bits/s
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Outline1. Useful definitions2. End-to-end delay3. Queueing delay
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End-to-end delay
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l1, r1 l2, r2
l3, r3 l4, r4
Example: How long will it take a packet of length p to travel from A to B, from when the 1st bit is sent, until the last bit arrives? Assume the switches store-and-forward packets along the path.
A
B
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l1, r1 l2, r2 l3, r3 l4, r4A B
S1 S2 S3
A
B
S1
S2
S3
p/r2
l2/c
p/r3
l3/c
p/r4
l4/ctime
l1/c
p/r1 time
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l1, r1 l2, r2 l3, r3 l4, r4A B
S1 S2 S3
Other packetsData H
Q2(t)
p/r1A
B
S1
S2
S3
p/r2
l1/c
l2/c
p/r3
p/r4
l3/c
l4/c
Q2(t)
time
*Queueing = UK spelling, adopted by Kleinrock at UCLA in 1960s. Queueing and queuing (US spelling) are both widely used.
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Insert the “ping” video here.
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Packet delay variation
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CDF (%)
RTT (ms)
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Packet delay variation
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CDF (%)
RTT (ms)
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Summary
End to end delay is made up of three main components:- Propagation delay along the links (fixed)- Packetization delay to place packets onto links
(fixed)- Queueing delay in the packet buffers of the
routers (variable)
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CS144An Introduction to Computer Networks
Packet SwitchingPlayback Buffers
Nick McKeown Stephen IbanezProfessor of Electrical Engineering and Computer Science, Stanford University
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Real-time applications (e.g. YouTube and Skype)
have to cope with variable queueing delay
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Playback buffers
30From: youtube.com
Playing here
Bufferedto here
Playback buffer
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Playback buffers1Mb/s
CumulativeBytes
Time
1Mb/s
Sent b
y Serve
r
Variable delay
Received by Laptop
1Mb/s
Playback rate
Buffer
Server
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Playback buffers1Mb/s
Playback point
Videodecoder
Screen
Playback buffer
Server
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If the buffer is too small1Mb/s
CumulativeBytes
Time
1Mb/s
Sent b
y Serve
r
Variable delay
Received by Laptop
Server
1Mb/sBuffer
Make the buffer bigger!
“Rebuffering”
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Playback buffer- With packet switching, end-to-end delay is variable.- We use a playback buffer to absorb the variation.
- We could just make the playback buffer very big, but then the video would be delayed at the start.- Therefore, applications estimate the delay, set the
playback buffer, and resize the buffer if the delay changes.
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l1, r1 l2, r2 l3, r3 l4, r4A B
S1 S2 S3
Other packetsData H
Q2(t)
p/r1A
B
S1
S2
S3
p/r2
l1/c
l2/c
p/r3
p/r4
l3/c
l4/c
Q2(t)
time
Queueing adds variable and unpredictable delay
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SummaryReal-time applications use playback buffers to absorb the variation in queueing delay.
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