1 introduction to network layer lesson 09 nets2150/2850 nets2150/ school of information technologies

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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


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








application

transportnetworkdata linkphysical

application


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Switching in Wide Area Switched Networks


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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


application


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









application


application


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Required Reading Circuit Switching Packet Switching

Virtual circuit Datagram

Read Stallings 10.1,10.2, and 10.6

Next: Routing

1 introduction to network layer lesson 09 nets2150/2850 nets2150/ school of information technologies

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