taxonomy of switched wide area networks - eecs.yorku.ca · telecommunication. systems....
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Circuit vs. Packet Switching
Taxonomy of Switched Wide Area Networks
intelligent coredumb nodes
Virtual Circuit(e.g. ATM)
Datagram Networks(e.g. the Internet)
connectionless connection-oriented
WANTelecommunication
Systems
Circuit-SwitchedNetworks
(e.g. telephone networks)
Packet-SwitchedNetworks
connection-orientedservice (TCP)
connectionlessservice (UDP)
dumb coreintelligent nodes
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Circuit vs. Packet Switching (cont.)Network Core – mesh of routers/switches that interconnect end systems
• two fundamental approaches to building a WAN core:
(1) circuit switching (example: telephone networks)• a sequence of links (communication path) between
two communicating nodes is determined ahead ofthe actual communication
• on each physical link, a channel is dedicated to the connection
• data is sent as a stream of bits through the network
(2) packet switching (example: the Internet)• data is sent through network in short blocks – packets• network links are dynamically shared by many packets;
each packet uses full link bandwidth
LAN 1
LAN 2WAN Core Network
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Circuit vs. Packet Switching (cont.)
Circuit-SwitchedNetworks
(e.g. telephone networks)
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Circuit vs. Packet Switching (cont.)
Packet Switching:Datagram Networks(e.g. the Internet)
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Communication via – involves three phases:Circuit Switching
Circuit Switching
(1) circuit establishment• before any data is transmitted, an end-to-end circuit
must be established, i.e. network resources on path/links between end-devices must be reserved
(2) data transfer• data transmission and signaling may each be digital
or analog
(3) circuit disconnect• after some period of data transfer, the connection
is terminated, by action of one of two stations, anddedicated resources are released
Vancouver
Toronto
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Circuit Switching (cont.)
Multiplexing inCircuit-SwitchedNetworks
– each link can be shared among (up to) n “circuits” ⇒each circuit gets a fraction 1/n of the link’s bandwidth• multiplexing = set of techniques that allows simultaneous
transmission of multiple signals across a single data link
• frequency division multiplexing (FDM) = each circuitcontinuously gets a fraction of the link’s bandwidth
• time division multiplexing (TDM) = each circuit gets allof the bandwidth periodically during brief intervals of time
frequency
frequency
FDM
time
time
TDM
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Circuit Switching (cont.)
Advantages of Circuit Switching
• inefficient use of capacity – channel capacity isdedicated for the duration of a connection, even ifno data is being transferred(example: silent periods in a phone call)
• circuit establishment delay – circuit establishmentintroduces ‘initial delay’
• network complexity – end-to-end circuit establishmentand end-to-end bandwidth allocation is complicatedand requires complex signaling software to coordinateoperation of switches
Disadvantages of Circuit Switching
• guaranteed Quality of Service – data are transmittedat fixed (guaranteed) rate; delay at nodes is negligible
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Packet Switching
Communication viaPacket Switching
(1) message segmentation• longer message is broken up into series of packets• packets contain user’s data + control data• control data (header) contains information that network
requires to route the packet
(2) data transfer• intermediate nodes perform following operations:
(a) receive entire packet(b) determine next node and link on route(c) queue packet to go out on that link
• when link is available, packet is transmitted to nextnode
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Further Details ofPacket Switching
– each packet is treated independently withno reference to packets that have gonebefore!
• each packet contains the full (IP) addressits destination as well as its source
• each packet switch has a forwarding (routing)table that maps destination addresses to an output link
• when packet arrives at a packet switch, theswitch examines packet’s destination addressand chooses the next node on packet’s pathbased on current traffic, line failure, etc.
• packets with the same destination addressdo not necessarily follow the same route⇒ packets may arrive out of sequence atthe destination !
• if packets arrive out of order, resequencing must be performed at the destination
Packet Switching (cont.) 9
Packet Switching (cont.)Main Principle ofPacket Switching
• statistical multiplexing () on-demand rather thanpre-allocated sharing of resources – link capacity is sharedon packet-to-packet basis only among those users whohave packets that need to be transmitted over the link(1) router buffers packets and arranges them in a queue(2) as the transmission line becomes available, packets
are transmitted one by one …
• store-and-forward () switch must receive entire packetbefore it can begin to transmit the first bit of the packet ontothe outbound link
A
B statistically multiplexed packets: packets are interleaved
based on the statistics of the senders
queue of packets waiting for output
Bandwidth division into “pieces”Dedicated allocationResource reservation
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Packet Switching (cont.)
Example [ circuit switching vs. packet switching ]
• N=35 users share a 1 Mbps link• each user generates 100kbps when “active”• each user is active 10% of time
How many users can be supported with circuitand how many with packet switching?
Circuit SwitchingWith circuit switching, 100kbps must be reserved for each user at all times. Hence, theoutput link can support 1Mbps/100kbps = 10 simultaneous users.
Packet Switching• 10 or fewer simultaneously active users ⇒ aggregate rate ≤ 1 Mbps ⇒ users’
packets flow through output link without delay, as in case of circuit switching
• more than 10 simultaneously active users ⇒ aggregate rate exceeds output capacity
With 35 users, probability of 10 or less simultaneously active users = 0.9996.Thus, packet switching can support all 35 users with virtually no delay!
N users 1 Mbps link
100 kbps
100 kbps
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Advantages of Packet Switching
• transmission delay – each time a packet passes througha packet-switching node, it incurs a delay not present incircuit switching = the time it takes to absorb the packetinto an internal buffer
• variable delay – each node introduces additional variabledelay due to processing and queueing
• overhead – to route packets through a packet-switchingnetwork, overhead information including the address ofdestination and/or sequence information must be added toeach packet
Disadvantages of Packet Switching
• greater line efficiency – node-to-node link dynamicallyshared by many packets / connections
• data rate conversion – two stations of different datarates can exchange packets, because each connects toits node at its proper data rate ⇒ nodes act as buffers
• no blocked calls – packets are accepted even underheavy traffic, but delivery delay increases
Packet Switching (cont.) 12