l2cn :network layers and physical layer

7/31/2019 L2CN :Network Layers and Physical Layer

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Lecture 2Network Layers and Physical Layer

Dr. Thaier Hayajneh

Department of Computer Engineering

The Hashemite University

408450 Computer Networks, Fall 2011/2012

Characteristics of Protocols

A rotocol is a set of mutuall a reed u on rules that

regiment the interactions between the communicatingentities.

e ey e ements o a protoco are:

Syntax defines the structure of informationcommunicated, including the data format, the coding, and

signal representations.

Semantics defines the meaning of the exchanged signals,includin control information for coordination and error

handling.

Timing defines the time at which data should be

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Characteristics of Protocols

,

WHAT is comunicated,

,

WHEN it is communicated.

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Standards

Defined by public organizations (de juro) or by widespread use (de facto)

Open standards

Avaliable to everyone (but not necessarily for free)

Everyone has the possibility to propose, criticize, and influence

tan ar s organ zat ons

ISO: International Organization for Standardization

IETF: Internet Engineering Task Force

IEEE: Institute of Electrical and Electronics En ineers

ANSI: American National Standards Institute

ETSI: European Telecommunications Standard Institute

ITU: International Telecommunication Union

ITU-T: International Telecommunication UnionTelecommunication Standards

Sector

EIA: Electronic Industries Association

...


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A La ered Model: Sendin a Letter

Modify one without affecting the other

Services and A lications

Service

What a network operator offers

to customers (subscribers)

Application

the service for

Examples

Telephone connection

Service: voice transmission

Application: conversation between two parties

Computer communication via modem

erv ce: same as a ove p us nternet connect v tyApplication: Web browsing, e-mail, file transfer, chat, etc

Network Architectures

complex to be handled as a monolithic unit.

An alternative a structured a roach.

Divide the communication task into manageable parts.

Need to describe the communication functions in terms of an

architecture.

The architecture defines the relationship and interactions

interfaces and protocols.

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La ered Architecture

v

.

Each layer performs a related subset of functions

re uired for communication and adds value to the

services provided by lower layers.

Layer Nrelies on services of layer N-1 to provide a service to

layer N+1

Service required from lower layer is independent of how that

Information and complexity hiding

Chan es in la er Ndo not affect other la ers

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La ered Model Interfaces

are called interfacesInterface protocols define the interaction between adjacent

layers in the same system

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Network Architecture

ys em es gn erpec ve

v v

Can the network be totally trusted?

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End-to-End Ar ument

functionalities in a complex system is what system

design is all about

End-to-End argument states that a function, such as

reliability or ordered delivery, should not be provided in

the lower levels of the system unless it can be

completely and correctly implemented at that level

unct ona re un ancy can e a owe per ormance

optimization is soughtExam le: error control on a ho -b -ho basis

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Statefull vs. Stateless

management

Establishment of a new connection causes a state change inthe switch

A direct side effect: fate sharing

ate o t e en -to-en connect on epen s on t e state o

intermediate nodes

Simple and more robust

Onl task re uired is to maintain end-to-end routin tables

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Statefull vs. Stateless

state complexity for more routing and processingrequirements at intermediate systems?

Connectionless networks have to perform address processing

on a packet basis

Not necessarily true, as successive pac ets usually refer

to the same address

ac ng rou ng compu a on grea y re uces process ngrequirements

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ISO/OSI Communication Model

v

the International Standard Organization (ISO) indefinining its Reference Model.

The model referred to as the Open System

Interconnection (OSI) defines a framework for the

specification of protocol standards for connecting

heterogenous computers.

e mo e prov es t e as s or two open systems t atconform to the reference model and the associated standards

to exchange information.

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Define the rules and conventions for various functions within

each layer.

Specify the general relations among these functions.

Determine the constraints on the types of functions and their

.

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Application Layer

Presentation Layer

H Data

Data

Session Layer

H Data

Network Layer

H Data

Data Link Layer

H Data

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Physical layer


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Presentation PresentationUpper Layers:End Nodes

Trans ort

Session

Trans ort

SessionOnly

Network Network Network

Physical

Data L n

Physical

Data L n

Physical

Data L nLower Layers:All Nodes

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Communication Medium

ISO/OSI Communication Model Architecture

v

Application layer,

Presentation layer,

Session layer,

Transport layer,

Network layer,

Data link layer, and

ys ca ayer.

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ISO/OSI Communication La ers

Process to process communication

All layers exist to support this layer

Applications include e-mail, teleconferencing, WWW, ftp,

distributed databases

Presentation Layer

Conversion of data to conform to a common format (e.g. little

, ,

representation)

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Bind two communicating entities into a relationship,

Session setup (authentication), failure recovery (brokensession), synchronization services

Transport Layer:

End-to-end data delivery

End-host to end-host communication

u t p ex ng o mu t p e ata streams rom g er ayers

(upward multiplexing)

Flow control between sender and receivers

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Data transmission and delivery between hosts

Controls access to the network

Provides routing of packets within the network

Manages contention and bottlenecks within the network

Data Link Layer:

Error free communication over a single link between adjacent

no esSpeed matching between senders and receivers

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Transmission of raw bits (0/1) over physical wires

Interfacing, data encoding, clock recovery, etc.

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different machines is only achieved at the physical

layer.

The interlayer interface defines a framework for

standards

No details of the implementation, no definition of the

interlayer interfaces.

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ISO/OSI Communication Model Interfaces

Peer-To-Peer

Layer N Layer Nrotoco

SAP SAPInterface Interface

Service Access Point

rotoco rotoco

Layer N-1 Layer N-1

Peer-To-Peer

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End System End System


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Interfaces

interlayer interfaces, since these are not visible fromoutside.

The interfaces is used to access services provided by a

lower layer to a higher layer.

The point at which service is provided is called Service

Access Point (SAP)

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Model Limitations

Focus entirely on listing of details, with no motivation

for techni ues ado ted.

Every reasonable suggestion becomes an option.

This results in an excessive number of options, for a protocol

architecture that is supposed to be an international standard.

The layering structure of the model is arbitrary

Appropriate placement of features in layers is not alwaysobvious.

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Model Limitations

v

communication mentality.

Techniques from communication fields have been sometimesinappropriate.

Use of interrupts, when implementation calls for use of a

.

Initial focus entirely on connection-oriented, rather than

connectionless service.

Connectionless service is currently being provided, but as

an afterthought.

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Model Limitations

The model does not allow ex licit b assin of ad acentlayers when not needed.

May lead to redundancy of functionalities.May result in inefficiency.

In general, layering has many conceptual advantages, .

Fanatical ahderence to layering as a religion may beproblematic.

Layer Nmay duplicate lower layer functionality,

Different layers may need the same information,-

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Internet Model

Communication Between La ers

- -

Betwen protocols at the same layer in different devices

Logical connection

Interfaces between layersBetween adjacent layers in the same device

down (sending) or up (receiving)

Communication takes place over well-defined interfaces

Communication Between La ers

Two types of communication between layersPeer-to- eer communicationsame la er different devices ,

Communication over interfaces between layersdifferent layers, same device

Interfaces Between La ers

v

can access it

Each la er has its own format for the Protocol DataUnit (PDU)

A layer in the sending device may add more rotocol

information to the data unit from the layer aboveHeaders and trailers

A layer in the receiving device may strip offprotocol

information


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Data Exchan e

A layer in the sender adds protocol information to the data

Headers and trailers

A layer in the receiver strips off protocol information

Function of La ers

Data Link Layer

Transport Layer

Ph sical La er

Transmission of bits between nodes

signals

Unguided medium (free space) for radio andoptical signals

Protocols

Mechanical and electrical interfaces

Connectors, cables, voltage levels

Transmission and reception of signals

digital modulation, line coding

Synchronous and asynchronous transmission

Standards, for example EIA RS-232, ITU-T SDH (ANSI SONET)

Data Link La erHo -to-Ho


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Data Link La er

Transmission offrames between nodes

Framing

Divides bit stream into larger data units, framesEthernet frame up to 12,144 bits (1518 bytes), including control information

Flow control

Prevent receiver from being overrunError control

Detect and (perhaps) retransmit damaged framesAccess control

c ev ce may sen on a s are nAddressing

Data Link Layer Frame Example

Header

address and control Data

Trailer

error control

8-bit

flag

8-bit

flag

Bit sequence for frame synchronization

AddressesSource and destination

ontroSequence number

Transmitted and expected

Link connect and disconnect

TrailerBit sequence for detecting bit errors

Data Link La er Standards

Ethernet

Family of protocols

Ethernet (10 Mb/s), Fast Ethernet (100 Mb/s), Gigabit

Ethernet 1 Gb/s

IEEE 802.11 Wireless LAN

IETF: Point-to-Point Protocol (PPP)

: u t -protoco a e w tc ng )

ISO: High-level Data Link Control (HDLC)

Link Access Procedure Balanced (LAP-B), ITU-T X.25

Normal Response Mode (NRM), SDLC

Network La erSource-to-Destination


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Network La er

Deliver of ackets from source to destination

possibly across multiple linksAddressing

Logical addresses

Unique within the network

RoutinCalculation of paths between pairs of nodes

IETF: Internet Protocol (IP)

Trans ort La erProcess-to-Process

Trans ort La er

Delivery between end users (processes)

Addressed by ports

Reliable (connections) or non-reliable (datagrams)Flow control

Reactive traffic control (prevent congestion)

rror an ng

Connection set-up and tear-down

Se mentation and reassembl

IETF: Transport Control Protocol (TCP), User Datagram Protocol (UDP)

ISO: Transport Protocol Class 0 4

Application Layer

Provides services to the end user


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Future Trends of Communication Networks

In recent ears, a lar e technolo ical ro ress occurred both in

the field of electronics and the field of opticsTechnological progress has opened the door to a new class of

applications: ea - me pp cat onsThese applications create the need for:

Greater flexibilit in the transfer mode

Transport of services other than pure data,

Transport of high bit rate services,

eve opmen o g er spee sys ems.

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l2cn :network layers and physical layer

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