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Introduction to Network Layer
Lesson 09NETS2150/2850
http://www.ug.cs.usyd.edu.au/~nets2150/
School of Information Technologies
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Lesson Outline Switching is an effective way of sharing network resources Circuit switching Packet Switching
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Position of network layer
McGraw-Hill ©The McGraw-Hill Companies, Inc., 2004
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Network Layer in an Internetwork
McGraw-Hill ©The McGraw-Hill Companies, Inc., 2004
Link 1
Link 2
Link 3
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Network layer functions
transport packet from sending to receiving hosts
network layer protocols in every host, router
three important functions: path determination: route
taken by packets from source to dest. Routing algorithms
forwarding: move packets from router’s input to appropriate router output
call setup: some network architectures require router call setup along path before data flows
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
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Switching in Wide Area Switched Networks
McGraw-Hill ©The McGraw-Hill Companies, Inc., 2004
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Switching Networks Long distance transmission is
typically done over a network of switched nodes not through dedicated mesh lines
Nodes not concerned with content of data
Data routed by being switched from node to node
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Switching Nodes Switching nodes may connect to
other nodes only, or to end systems and other nodes
Some redundant connections are desirable for reliability
Two different switching technologies: Circuit switching Packet switching
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An Example Network
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Circuit Switching Developed for voice traffic Provides dedicated communication path
between two stations Connected sequence of links Resources reserved for exclusive use Done at the physical layer Transparent connection Three phases in communication
Establish Transfer Disconnect
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Circuit Switching (2) Connection setup takes time Once connected, transfer is
transparent Developed for voice traffic (phone)
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Public Circuit Switched Network
Twisted-pair
(subscriber line)
Subscribers, subs. Line, exchanges, and trunks
Multiple voice frequency circuits
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Circuit Establishment
Local call
Long-distance call
Line c + one channel on the trunk to the ex
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Circuit switching Disadvantages Circuit switching designed for voice Resources dedicated to a particular call Much of the time a data connection is
idle, unused capacity is wasted Data rate is fixed
Both ends must operate at the same rate
Solution: Packet Switching
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Packet Switching Principles Developed for bursty data traffic Data transmitted in small packets
Typically 1000 octets Longer messages split into series of packets Each packet contains a portion of user data plus
some control info (header) Control info
Routing (addressing) info Packets are received, stored briefly (buffered)
and passed on to the next node Store and forward Not concerned with the content of the data
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The use of packets
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A packet’s trip from Src to Dest
Source
Destination
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Packet Switching…
Source and Dest.
layer 2 addresses
Source and Dest.
IP addresses
Routing table
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Advantages
Line efficiency Single node to node link can be shared by many
packets over time Packets queued and transmitted
Data rate conversion Each end system connects to the local node at
its own speed Nodes buffer data if required to equalize rates
Packets are accepted even when network is busy (as opposed to call dropping)
Delivery may slow down Priorities can be used
Based on the priority, some packets can experience less delay
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Packet Switching Technique End system breaks a long message
into packets Packets sent one at a time to the
network Packets handled in two modes:
Datagram used in today’s Internet Virtual circuit used in ATM, frame-
relay, X.25
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Datagram Mode: the Internet model
No call setup at network layer Each packet treated independently
No reference to packets handled before from the same message
no network-level concept of “connection” Packets can take any practical route Packets may arrive out of order Packets may go missing: Best-effort
service! Up to receiver to re-order packets and
recover from missing packets
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Datagram Mode Illustration
•Packets for same destination may not follow the same route•May arrive out of sequence•Exit node or the destination does the re-ordering
Exit node
Pkt re-ordered
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Virtual Circuit Mode Preplanned route established before any packets sent Call request and clear packets to establish and drop
circuit (handshake) Each packet contains a virtual circuit identifier instead
of destination address Every router on source-dest path maintains “state” for
each passing virtual circuit (VC) No routing decisions required for each packet transport-layer connection only involved two end
systems link, router resources (bandwidth, buffers) may be
allocated to VC to get circuit-like performance.
Non-dedicated path
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Virtual circuits: signaling protocols used to setup, maintain teardown VC used in ATM, frame-relay, X.25 not used in today’s Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1. Initiate call 2. incoming call
3. Accept call4. Call connected5. Data flow begins 6. Receive data
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VirtualCircuitDiagram
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Packet Size Packet size and transmission time Breaking a message into smaller
packets Transmission time drops Too many smaller packets is not good
either! Processing and queuing delays increase
when there are more packets to handle, for a single message
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Packet Size & Transmission time…
Total tx time: 43*3=129 octet-times
92 octet-times
77 octet-times 84 octet-times!
More and smaller packets meanmore of the headers, increasing theoctet-time
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Virtual Circuits vs Datagram Virtual circuits
Network can provide sequencing and error control Packets are forwarded more quickly
No routing decisions to make Less reliable
Loss of a node looses all circuits through that node Datagram
No call setup phase Better if few packets
More flexible Routing can be used to avoid congested parts of the
network
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Datagram or VC network: why?
Internet data exchange among
computers “elastic” service, no
strict timing req. “smart” end systems
(computers) can adapt, perform
control, error recovery simple inside network,
complexity at “edge” many link types
different characteristics uniform service difficult
ATM evolved from telephony human conversation:
strict timing, reliability requirements
need for guaranteed service
“dumb” end systems telephones complexity inside
network
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Circuit v Packet Switching Performance comparison involves:
Propagation delay Transmission time Node processing delay
Constant factorsVariable factor
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Event Timing Sequence
Node delay
Single block
Packetiseddata
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Network layer functions
transport packet from sending to receiving hosts
network layer protocols in every host, router
three important functions: path determination: route
taken by packets from source to dest. Routing algorithms
forwarding: move packets from router’s input to appropriate router output
call setup: some network architectures require router call setup along path before data flows
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
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Required Reading Circuit Switching Packet Switching
Virtual circuit Datagram
Read Stallings 10.1,10.2, and 10.6
Next: Routing