ECE2305: Circuit and Packet Switching Basics
Communication and NetworkingCircuit and Packet Switching Basics
D. Richard Brown III
(selected figures from Stallings Data and Computer Communications 10th edition)
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ECE2305: Circuit and Packet Switching Basics
Unswitched/Unmultiplexed Network
I Dedicated link between eachuser
I Lots of wires
I Lots of network ports
I Difficult to add more users
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ECE2305: Circuit and Packet Switching Basics
Switched Network
I No dedicated links between users
I Extra switching hardware needed
I Additional overhead to ensure data goes to the right place
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ECE2305: Circuit and Packet Switching Basics
Circuit Switching
Three phases:
1. Circuit establishment
2. Data transfer
3. Circuit disconnect
Once connected, the data transfer is transparent:
I Dedicated circuit between sender and receiver
I Very low delay (essentially just propagation delay)
I Efficient for analog transmission of voice signals
I Can be inefficient for digital transmissions since channel capacity isdedicated for the duration of connection
I Like structured multiplexing techniques, e.g., synchronous TDM,channel is reserved even if not used (until disconnect)
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ECE2305: Circuit and Packet Switching Basics
Blocking vs. Non-Blocking Circuit Switched Networks
Blocking network:
I More users than actual circuits available in network
I May be unable to connect users in periods of high use because allcircuits are busy
I Usually acceptable (although inconvenient) for voice traffic
Non-blocking network:
I Enough circuits available to permit all users to connect (in pairs)simultaneously
I Usually expected for data traffic
I May require buffering
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ECE2305: Circuit and Packet Switching Basics
Space Division Switching
I Originally developed for analog links
I Also applicable to digital links
I Signal paths are physically separate from oneanother
I Path is dedicated solely to transfer signals
I Basic building block of switch is a metalliccrosspoint or semiconductor gate
Images from: http://www.forensicgenealogy.info/contest 28 results.html andhttp://rhetoricaldevice.com/RingRingRing.html.
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ECE2305: Circuit and Packet Switching Basics
Non-Blocking Space Division Switch
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ECE2305: Circuit and Packet Switching Basics
Three-Stage Space Division Switch
Blocking possible here.D. Richard Brown III 8 / 20
ECE2305: Circuit and Packet Switching Basics
Public Circuit Switched Network
Note that trunks might be synchronous TDM lines, e.g., DS-1 or SONETThe main idea here is that, from the point of view of the users, there is adedicated circuit between them.
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ECE2305: Circuit and Packet Switching Basics
Modern “Circuit” Switching: Time-Division Switching
I Most analog signals are now digitized before transmission through anetwork
I Low cost of digital hardware
I Telephone operators replaced by smart digital switches thatautomatically establish and release dedicated “circuits”
I Synchronous TDM multiplexing usually used:I Multiplex low rate data streams into dedicated timeslots in a high rate
data streamI Guaranteed data rate through circuitI Low delayI Transparent to end users
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ECE2305: Circuit and Packet Switching Basics
Motivation for Packet Switching
Fundamental conflict/tradeoff in communication systems:
I Pre-allocation of dedicated channel capacity (FDM, synchronousTDM, circuit switching)
I Dynamic allocation of on-demand channel capacity (statistical TDM,packet switching)
1968:
I Almost all voice/data networks were circuit switchedI Real-time dynamic allocation of channel capacity was unrealistic given
current computer hardware
1969: ARPANET
I First demonstrations of packet switched computer network
If lines are cheap: use circuit switchingIf computing is cheap: use packet switching
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ECE2305: Circuit and Packet Switching Basics
Packet Switching
I Station breaks long message into packetsI Packets sent one at a time to the networkI Network dynamically allocates capacity and delivers packets to
receiver without establishing a dedicated linkI Two common approaches:
I Virtual circuit packet switchingI Datagram packet switching
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ECE2305: Circuit and Packet Switching Basics
Effect of Packet Size
Packets: X → a → b → Y
Packets forwarded only after theyhave been completely received.
Assume:
I 40 byte/octet data
I 3 byte header
Cases:
(a) 40 + 3 = 43 byte packets
(b) 20 + 3 = 23 byte packets
(c) 8 + 3 = 11 byte packets
(d) 4 + 3 = 7 byte packets
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ECE2305: Circuit and Packet Switching Basics
Virtual Circuit Packet Switching
I Route is pre-planned (requires a callsetup phase)
I All packets follow the same route
I Packets will arrive in order
I No per-packet routing decisions needto be made (low per-packet overhead)
I Can be affected by network problems
I Network can provide sequencing anderror control
I Typically more efficient for longmessages (low per-packet overheadoutweighs fixed call setup overhead)
Note this is not a dedicated circuit (noreserved capacity).
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ECE2305: Circuit and Packet Switching Basics
Virtual Circuit Packet Switching
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ECE2305: Circuit and Packet Switching Basics
Datagram Packet Switching
I No call setup phase
I Each packet is treated independently
I Packets may take different routes
I Packets may arrive out of order
I Usually more reliable (robust tonetwork problems)
I More flexible
I Typically more efficient for shortmessages (no fixed call setup overheadbut higher per-packet overhead)
As seen in lab 5, this is how IP works.
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ECE2305: Circuit and Packet Switching Basics
Virtual Circuits vs. Datagram Packet Switching
Virtual circuits:
I Call setup phase results in more fixed overhead
I Less per-packet overhead during data transmission since no routingdecisions have to be made for each packet
I Network can provide sequencing and error control
I Susceptible to single point of failure
Datagram:
I No call setup phase results in very small fixed overhead
I More per-packet overhead during data transmission since routingdecisions have to be made for each packet
I Typically more flexible and resilient to network problems
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ECE2305: Circuit and Packet Switching Basics
Typical Timing Diagrams
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ECE2305: Circuit and Packet Switching Basics
Comparison Table
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ECE2305: Circuit and Packet Switching Basics
Final Remarks
I Circuit switching (originally developed for analog voicecommunication)
I Packet switching (1969 ARPANET)I Virtual circuitI Datagram
I Performance depends on several factorsI Propagation delaysI Length of message that will be transmittedI Application (continuous data or intermittent?)I Size of packetsI Switching/routing delays
I Bottom line:I Tradeoff between fixed overhead and per-packet overheadI Datagram packet switching preferred in most modern applications
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