Introduction to network layer
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Introduction to Network Layer. Lesson 09 NETS2150/2850 http://www.ug.cs.usyd.edu.au/~nets2150/. School of Information Technologies. Lesson Outline. Switching is an effective way of sharing network resources Circuit switching Packet Switching. Position of network layer. McGraw-Hill.

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Introduction to network layer

Introduction to Network Layer

Lesson 09

NETS2150/2850

http://www.ug.cs.usyd.edu.au/~nets2150/

School of Information Technologies


Lesson outline

Lesson Outline

  • Switching is an effective way

    of sharing network resources

  • Circuit switching

  • Packet Switching


Introduction to network layer

Position of network layer

McGraw-Hill

  • The McGraw-Hill Companies, Inc., 2004


Introduction to network layer

Network Layer in an Internetwork

Link 1

Link 2

Link 3

McGraw-Hill

  • The McGraw-Hill Companies, Inc., 2004


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

network

data link

physical

network

data link

physical

network

data link

physical

network

data link

physical

network

data link

physical

network

data link

physical

network

data link

physical

network

data link

physical

application

transport

network

data link

physical

application

transport

network

data link

physical

Network layer functions


Introduction to network layer

Switching in Wide Area Switched Networks

McGraw-Hill

  • The McGraw-Hill Companies, Inc., 2004


Switching networks

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


Switching nodes

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


An example network

An Example Network


Circuit switching

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


Circuit switching 2

Circuit Switching (2)

  • Connection setup takes time

  • Once connected, transfer is transparent

  • Developed for voice traffic (phone)


Public circuit switched network

Public Circuit Switched Network

(subscriber line)

Twisted-pair

Multiple voice frequency circuits

Subscribers, subs. Line, exchanges, and trunks


Circuit establishment

Local call

Long-distance call

Circuit Establishment

Line c + one channel on the trunk to the ex


Circuit switching disadvantages

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


Packet switching principles

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


The use of packets

The use of packets


A packet s trip from src to dest

A packet’s trip from Src to Dest

Destination

Source


Packet switching

Packet Switching…

Routing table

Source and Dest.

IP addresses

Source and Dest.

layer 2 addresses


Advantages

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


Packet switching technique

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


Datagram mode the internet model

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


Datagram mode illustration

Datagram Mode Illustration

Pkt re-ordered

Exit node

  • Packets for same destination may not follow the same

  • route

  • May arrive out of sequence

  • Exit node or the destination does the re-ordering


Virtual circuit mode

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


Virtual circuits signaling protocols

used to setup, maintain teardown VC

used in ATM, frame-relay, X.25

not used in today’s Internet

application

transport

network

data link

physical

application

transport

network

data link

physical

Virtual circuits: signaling protocols

6. Receive data

5. Data flow begins

4. Call connected

3. Accept call

1. Initiate call

2. incoming call


Virtual circuit diagram

VirtualCircuitDiagram


Packet size

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


Packet size transmission time

Packet Size & Transmission time…

More and smaller packets mean

more of the headers, increasing the

octet-time

77 octet-times

84 octet-times!

92 octet-times

Total tx time: 43*3=129 octet-times


Virtual circuits vs datagram

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


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

Datagram or VC network: why?


Circuit v packet switching

Circuit v Packet Switching

  • Performance comparison involves:

    • Propagation delay

    • Transmission time

    • Node processing delay

Constant factors

Variable factor


Event timing sequence

Event Timing Sequence

Node delay

Single block

Packetised

data


Network layer functions1

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

network

data link

physical

network

data link

physical

network

data link

physical

network

data link

physical

network

data link

physical

network

data link

physical

network

data link

physical

network

data link

physical

application

transport

network

data link

physical

application

transport

network

data link

physical

Network layer functions


Required reading

Required Reading

  • Circuit Switching

  • Packet Switching

    • Virtual circuit

    • Datagram

  • Read Stallings 10.1,10.2, and 10.6

  • Next: Routing


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