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Chapter Outline
Introduction (slides and 7.A) Layered Architecture (slides and 7.B &
7.D)
Routing (slides and 7.D) Reliable Transmission & Flow Control
(slides and read 7.E)
Congestion Control (slides and read 7.F)
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This Lecture
What is a network? Sharing the infrastructure
Circuit switching
Packet switching Best Effort Service
Analogy: the mail system Internets Best Effort Service
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Networks Why they are interesting?
Overcome geographic limits Access remote data Separate clients and server
Goal: Universal Communication (any to any) Design the cloud
Network
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Connectivity
LinkDSL, T1, T3, ...Characterized by
Capacity or bit-rate (1.5 Mb/s, 100Mb/s, )
Propagation delay (10us, 10ms, 100ms, ..) Transfer time on a link = #bit/bit-rate + propagation delay
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Connectivity
A mesh requires N2 links too costly
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This Lecture
What is a network? Sharing the infrastructure
Circuit switching
Packet switching Best Effort Service
Analogy: the mail system Internets Best Effort Service
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Two ways to share
Circuit switching (isochronous)Packet switching (asynchronous)
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Circuit Switching
DATA
Caller Callee
BostonSwitch LASwitch
propagation
delay
between
caller and
and Boston
switch
processing delay at switch
Its the methodused by thetelephone network
A call has threephases:
1. Establish circuit fromend-to-end (dialing),
2. Communicate,3. Close circuit (tear
down). If circuit not
available: busysignal
(1)
(2)
(3)
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Switch
Circuit Switching:Multiplexing/Demultiplexing
Time divided into frames and frames divided into slots Relative slot position inside a frame determines which
conversation the data belongs to E.g., slot 0 belongs to the red conversation Need synchronization between sender and receiver
Frames
0 1 2 3 4 5 0 1 2 3 4 5Slots =
One way for sharing a circuit is TDM:
Lecture notes use the word frame for slot
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Circuit Switching
Assume link capacity is C bits/sec Each communication requires R bits/sec #slots = C/R Maximum number of concurrent communications is C/R
What happens if we have more than C/R communications? What happens if the a communication sends less/more
than R bits/sec?
Design is unsuitable for computer networks wheretransfers have variable rate (bursty)
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Internet Traffic Is Bursty
Daily traffic at an MIT-CSAIL router
Max In:12.2Mb/s Avg. In: 2.5Mb/s
Max Out: 12.8Mb/s Avg. Out: 3.4 Mb/s
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Packet Switching Used in the Internet
Data is sent in Packets(header contains controlinfo, e.g., source anddestination addresses)
Per-packet routing At each node the entire
packet is received,stored, and thenforwarded (store-and-forward networks)
No capacity is allocated
Header Data
Packet 1
Packet 2
Packet 3
Packet 1Packet 2
Packet 3
Packet 1
Packet 2
Packet 3
processing
delay of
Packet 1at Node 2
propagationdelay
between
Host 1 &
Node 2transmission
time of
Packet 1
at Host 1
Host 1 Host 2
Node 1 Node 2
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Router
Packet Switching:Multiplexing/Demultiplexing
Multiplex using a queue Routers need memory/buffer
Demultiplex using information in packet headerHeader has destination Router has a routing table that contains information about
which link to use to reach a destination
Queue
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Queues introduce Variable Delay
Delay = Queuing delay + propagation delay + transmissiondelay + processing delay
Losses When packets arrive to a full queue/buffer they are
dropped
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Packet switching also show reordering
Host A
Host BHost E
Host D
Host C
Node 1Node 2
Node 3
Node 4
Node 5
Node 6 Node 7
Packets in a flow may not follow the same path (depends
on routing as we will see later)
packets may bereordered
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This Lecture
What is a network? Sharing the Infrastructure
Circuit switching
Packet switching Best Effort Service
Analogy: the mail system Internets Best Effort Service
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The mail system
Dina Nick
MIT Stanford
Admin Admin
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Characteristics of the mail system Each envelope is individually routed No time guarantee for delivery No guarantee of delivery in sequence
No guarantee of delivery at all! Things get lost How can we acknowledge delivery? Retransmission
How to determine when to retransmit? Timeout?
If message is re-sent too soon duplicates
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The mail system
Dina Nick
MIT Stanford
Admin Admin
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The Internet
Dina Nick
Nms.csail.mit.edu Leland.Stanford.edu
O.S. O.S.HeaderData HeaderData
Packet
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Characteristics of the Internet Each packet is individually routed No time guarantee for delivery No guarantee of delivery in sequence
No guarantee of delivery at all! Things get lost Acknowledgements Retransmission
How to determine when to retransmit? Timeout?
If packet is re-transmitted too soonduplicate
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Best Effort
No Guarantees: Variable Delay (jitter) Variable rate
Packet loss Duplicates Reordering
(notes also state maximum packet length)
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Differences Between Circuit & Packet Switching
Circuit-switching Packet-Switching
Guaranteed capacity No guarantees (best effort)
Capacity is wasted if data isbursty
More efficient
Before sending dataestablishes a path
Send data immediately
All data in a single flowfollow one path Different packets mightfollow different paths
No reordering; constantdelay; no pkt drops
Packets may be reordered,delayed, or dropped
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This Lecture
We learned how to share the networkinfrastructure between manyconnections/flows
We also learned about the implications ofthe sharing scheme (circuit or packetswitching) on the service that the
traffic receives