ECE2305: Circuit and Packet Switching Basics
Communication and Networking
Circuit 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
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Dedicated link between each
user
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Lots of wires
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Lots of network ports
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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:
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Dedicated circuit between sender and receiver
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Very low delay (essentially just propagation delay)
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Efficient for analog transmission of voice signals
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Can be inefficient for digital transmissions since channel capacity is
dedicated for the duration of connection
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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:
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More users than actual circuits available in network
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May be unable to connect users in periods of high use because all
circuits are busy
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Usually acceptable (although inconvenient) for voice traffic
Non-blocking network:
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Enough circuits available to permit all users to connect (in pairs)
simultaneously
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Usually expected for data traffic
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May require buffering
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ECE2305: Circuit and Packet Switching Basics
Space Division Switching
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Originally developed for analog links
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Also applicable to digital links
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Signal paths are physically separate from one
another
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Path is dedicated solely to transfer signals
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Basic building block of switch is a metallic
crosspoint or semiconductor gate
Images from: http://www.forensicgenealogy.info/contest 28 results.html and
http://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.
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ECE2305: Circuit and Packet Switching Basics
Public Circuit Switched Network
Note that trunks might be synchronous TDM lines, e.g., DS-1 or SONET
The main idea here is that, from the point of view of the users, there is a
dedicated circuit between them.
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ECE2305: Circuit and Packet Switching Basics
Modern “Circuit” Switching: Time-Division Switching
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Most analog signals are now digitized before transmission through a
network
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Low cost of digital hardware
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Telephone operators replaced by smart digital switches that
automatically establish and release dedicated “circuits”
Synchronous TDM multiplexing usually used:
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Multiplex low rate data streams into dedicated timeslots in a high rate
data stream
Guaranteed data rate through circuit
Low delay
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, synchronous
TDM, circuit switching)
I Dynamic allocation of on-demand channel capacity (statistical TDM,
packet switching)
1968:
I Almost all voice/data networks were circuit switched
I 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 switching
If computing is cheap: use packet switching
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ECE2305: Circuit and Packet Switching Basics
Packet Switching
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Station breaks long message into packets
Packets sent one at a time to the network
Network dynamically allocates capacity and delivers packets to
receiver without establishing a dedicated link
Two common approaches:
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Virtual circuit packet switching
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 they
have been completely received.
Assume:
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40 byte/octet data
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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 call
setup phase)
I All packets follow the same route
I Packets will arrive in order
I No per-packet routing decisions need
to be made (low per-packet overhead)
I Can be affected by network problems
I Network can provide sequencing and
error control
I Typically more efficient for long
messages (low per-packet overhead
outweighs fixed call setup overhead)
Note this is not a dedicated circuit (no
reserved 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 to
network problems)
I More flexible
I Typically more efficient for short
messages (no fixed call setup overhead
but 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:
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Call setup phase results in more fixed overhead
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Less per-packet overhead during data transmission since no routing
decisions have to be made for each packet
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Network can provide sequencing and error control
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Susceptible to single point of failure
Datagram:
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No call setup phase results in very small fixed overhead
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More per-packet overhead during data transmission since routing
decisions have to be made for each packet
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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
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Circuit switching (originally developed for analog voice
communication)
Packet switching (1969 ARPANET)
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Performance depends on several factors
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Virtual circuit
Datagram
Propagation delays
Length of message that will be transmitted
Application (continuous data or intermittent?)
Size of packets
Switching/routing delays
Bottom line:
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Tradeoff between fixed overhead and per-packet overhead
Datagram packet switching preferred in most modern applications
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