Communication and Networking Circuit and Packet Switching ...spinlab.wpi.edu/courses/ece2305_2014/switched_communication.pdf · ECE2305: Circuit and Packet Switching Basics Motivation

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)

D. Richard Brown III 1 / 20


Unswitched/Unmultiplexed Network

I Dedicated link between eachuser

I Lots of wires

I Lots of network ports

I Difficult to add more users



Switched Network

I No dedicated links between users

I Extra switching hardware needed

I Additional overhead to ensure data goes to the right place



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)



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



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.



Non-Blocking Space Division Switch



Three-Stage Space Division Switch

Blocking possible here.D. Richard Brown III 8 / 20


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.



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



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



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



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



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).



Virtual Circuit Packet Switching



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.



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



Typical Timing Diagrams



Comparison Table



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


Documents

Communication and Networking Circuit and Packet Switching ...spinlab.wpi.edu/courses/ece2305_2014/switched_communication.pdf · ECE2305: Circuit and Packet Switching Basics Motivation