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OSI - 1 www.comnets.uni-bremen.de

ISO OSI Reference Model ISO OSI Referenzmodell

Vorlesung Kommunikationsnetze

Kapitel 2 [Wa0001]

OSI = Open System Interconnection

ISO = International Standards Organization

RM = Reference Model

Dr.-Ing Umar Toseef


ISO/OSI Motivation and History

OSI – Open System Interconnection

OSI - 3 INTRO - 3

mailbag is unloaded from truck

empty mailbag

letter sorting machine

deliver and open letter

read letter

Translate from English to German

Presentation Layer

ideas for a description

Session Layer

write letter

Physical Layer

mailbag is transferred to plane

Data Link Layer

letter is put in mailbag

Transport Layer

put letter in envelope, address it and take it

to the post office

Network Layer

letter sorting

machine

Physical Layer

mailbag is unloaded from plane

Data Link Layer

empty mailbag

Anwendungsschicht

German engineer understands

patent

Präsentationsschicht

Sitzungsschicht

Physikalische Schicht

Sicherungsschicht

Transportschicht

Netzschicht

Physikalische Schicht

mailbag is transferred to truck

Sicherungsschicht

letter is put in mailbag

Network Layer

letter sorting machine

plane truck

Functionality of the OSI Layers

(Aufgaben der OSI-Schichten)

OSI: Open System Interconnection

ISO-Standard

Application Layer

American inventor has a patent


James Bond meets the 7 layer OSI model James Bond meets Number One on the 7th floor of the spy headquarters

building. Number One gives Bond a secret message that must get through to the US Embassy across town.

Bond proceeds to the 6th floor where the message is translated into an intermediary language, encrypted and compressed.

Bond takes the elevator to the 5th floor where security checks the message to be sure it is all there and puts some checkpoints in the message so his counterpart at the US end can be sure he’s got the whole message.

On the 4th floor the message is analyzed to see if it can be combined with some other small messages that need to go to the US end. Also if the message was very large it might be split into several small packages so other spies can take it and have it reassembled on the other end.

The 3rd floor personnel check the address on the message and determine who the address is and advising Bond of the fastest route to the Embassy.

On the 2nd floor the message is put into a special courier pouch (packet). It contains the message, the sender and destination ID. It also warns the recipient if other pieces are still coming.

Bond proceeds to the 1st floor where Q has prepared the Aston Martin for the trip to the Embassy. Bond departs for the US Embassy with the secret packet in hand.

On the other end the process is reversed. Bond proceeds from floor to floor

where the message is decoded. The US Ambassador is very grateful the message got through safely. "Bond, please tell Number One I’ll be glad to meet him for dinner tonight."

Application Layer

Presentation Layer

Session Layer

Transport Layer

Network Layer

Data Link Layer

Physical Layer

OSI - 6 INTRO - 6

Functionality of OSI Layers

7 Application Layer (Anwendungsschicht)

6 Presentation Layer (Präsentationsschicht, Darstellungsschicht)

5 Session Layer (Sitzungsschicht)

4 Transport Layer (Transportschicht)

3 Network Layer (Vermittlungsschicht, Netzschicht)

2 Data Link Layer (Übermittlungsschicht, Sicherungsschicht)

1 Physical Layer (Bitübertragungsschicht, physikalische Schicht)


OSI Layers 7 - 4

The Application layer represents the level at which applications access network services. This layer represents the services that directly support applications such as software for file transfers, database access, and electronic mail.

The Presentation layer translates data from the Application layer into an intermediary format. This layer also manages security issues by providing services such as data encryption, and compresses data so that fewer bits need to be transferred on the network.

The Session layer allows two applications on different computers to establish, use, and end a session. This layer establishes dialog control between the two computers in a session, regulating which side transmits, plus when and how long it transmits.

The Transport layer handles error recognition and recovery. It also repackages long messages when necessary into small packets for transmission and, at the receiving end, rebuilds packets into the original message. The receiving Transport layer also sends receipt acknowledgments.

http://www.lewistech.com/rlewis/Resources/JamesBondOSI3.aspx


OSI Layers 3 - 1

The Network layer addresses messages and translates logical addresses and names into physical addresses. It also determines the route from the source to the destination computer and manages traffic problems, such as switching, routing, and controlling the congestion of data packets.

The Data Link layer packages raw bits from the Physical layer into frames (logical, structured packets for data). This layer is responsible for transferring frames from one computer to another, without errors. After sending a frame, it waits for an acknowledgment from the receiving computer.

The Physical layer transmits bits from one computer to another and regulates the transmission of a stream of bits over a physical medium. This layer defines how the cable is attached to the network adapter and what transmission technique is used to send data over the cable.


www.wikipedia.org

OSI - 12 INTRO - 12

Protocol (Protokoll) Protocol (Protokoll) a formal description of (i) message formats (PDU: Protocol Data

Units, Protokoll-Dateneinheiten) (ii) and the rules which two or more machines must follow to exchange those messages

e.g.: TCP: Transport Control Protocol, IP: Internet Protocol Protocols usually exist in two forms:

1. In a textual form for humans to understand. E.g. the majority of Internet protocols are distributed as RFCs

(Request for Comments), which can (and should) be read to understand the protocols' design and operation.

2. As formal descriptions or programming code for computers to understand.

Both forms should ultimately specify the precise interpretation of every bit of every message exchanged across a network.

OSI - 13 INTRO - 13

Protocol Description

Protocols can be described by extended finite state machines (erweiterte endliche Automaten)

with state changes depending on variables, e.g., time dependent. Every state (Zustand) can be described by a set of variables.

by a formal specification language, e.g., SDL (Specification and Description Language)

Protocol description contains Syntax: defines syntactical units, e.g., sequence of characters Semantics (Semantik): describes the meaning and usage of

syntactically correct constructs Pragmatics (Pragmatik): information and knowledge about the

effects of actions Timing requirements (Zeitvorgaben): e.g., timeouts,

are a main part of protocols to guarantee the synchronization of the communicating protocol state machines, e.g., after transmission errors

OSI - 14

Protocol Description

www.comnets.uni-bremen.de

Finite State Machine

Specification and Description Language


Type of Networks (Netztypen)

Circuit switching (Leitungsvermittlung)

a connection is set up between sender and receiver

Packet switching (Paketvermittlung)

packets are routed through the network Connection-less mode (verbindungslos)

Connection-oriented mode (verbindungsorientiert)

virtual connection/circuit:

a route is reserved in the network

(cell switching: see ATM)

Examples: ISDN? Internet? GSM? GPRS?


Circuit Switching Example (Leitungsvermittelt)

Telephone Connections


Circuit Switching (CS) (Leitungsvermittlung) A circuit can be ...

a physical line (physikalische Leitung) a time slot in a frame in a TDM (Time Division Multiplex)

system (Zeitschlitz in einem Rahmen) a carrier frequency in an FDM (Frequency Division Multiplex)

system a wavelength in a WDM (Wave Division Multiplex) system a code in a CDMA (Code Division Multiplex) system

A connection - during its existence - uses one circuit or a selection of circuits in parallel (multi-channel switching)

Connection set-up find path through network and through switches towards

destination establish path

Communication send fixed data rate into the connection

Release connection after use


Packet Switching (PS) (Paketvermittlung)

Send packets of data instead of a fixed bit or byte rate

idle time between packets can be used by other communication relations

variable data rate is possible

connectionless (CL) or connection oriented (CO) modes

Circuit Switching

Packet Switching

Connection oriented

Connection less

Telephone ISDN GSM

X.25 ATM

Internet/IP GPRS

Examples: Telephone

X.25

ISDN

ATM

Internet/IP

GSM

GPRS


Connection Oriented Packet Switching

Connection establishment before data transmission routing performed only for connection establishment

data transmitted along established path

data packets carry only connection identifier

for packet switching: “virtual connection” or “virtual circuit”

connection state (next hop address) can be stored in network elements along the path

destination address given during connection set-up

"meta signaling" or default signaling connections needed

(e.g. connection set-up and release)


Connection Oriented Packet Switching

Virtual circuit

Packet Packet

Packet

Packet


Connectionless Packet Switching

data packets transmitted without connection on Network Layer

routing performed along with forwarding for each packet but: route cache

destination address carried in each packet

no network layer signaling needed

no information stored in network nodes along the path network nodes are less complex

cheap high speed packet forwarding

higher layer connections can survive network path outages


Connectionless Packet Switching

Packet 2

Packet 1

Packet 1

Packet 2

Packet 2


Communication of (N+1)-Entities by (N)-layer

• The (N)-layer provides its services to the (N+1)-layer, by using the provided (N)-functions and, if necessary, services of the (N-1)-layer.

• Each (N)-Entity is able to provide and use services for one or more (N+1)-Entities. This is called the service provider - service user - model.

Layer (N + 1)

(N + 1)-

entities

Layer (N)


(N+1)-

layer

System A System B

service user service user

service access points (SAP)

(N)- layer

Service-provider

Service model (SM)


Recursive application of the service user – service provider - model

User of service (N)

User of service (N)

(N+1) – protocol

Open system A Open system B

User of service (N-1)


(N) – protocol

Service of layer (N-1)

(N-1) – protocol User of service (N-2)


Provider of service (N-1)

Provider of service (N)

Service of layer (N)


Possibilities of address allocation

K

C

M L K D E D

allocation table

(N)-layer

A

B A

M L K Bc Bb Ba

C D E hierarchical one-to-one

One-to-One: direct mapping

Hierarchical: (N)-address consists of two parts

Allocation Table: flexible mapping possible


Illustration of data units and their structure

Assumption: (N)-SDUs are neither segmented nor blocked.

Relative position of control- and user data in a PDU is not fixed.

An (N)-PDU can be mapped one-to-one to an (N-1)-SDU.

(N-1)-SDU

PDU = Protocol-Data-Unit SDU = Service-Data-Unit PCI = Protocol-Control-Information

(N-1)-PCI

(N)-PDU

(N-1)-PDU

(N)-layer

(N-1)-layer

Structure of PDU

Head

Control-Data

Service -Data

Body


Relation between (N)-SDU, (N)-PDU and (N-1)-SDU

(N)- PCI

(N)-SDU

(N)-PDU

a) Neither segmenting nor blocking b) Segmenting/Reassembling

(N)- PCI

(N)- SDU

(N)-PDU (N)-PDU

(N)- PCI

(N)-SDU (N)- PCI

(N)-SDU

(N)-PDU

c) Blocking/Deblocking

SDU/PDU = service/protocol data unit

PCI = protocol control information

(N)-PDU (N)-PDU

(N-1)-SDU

d) Concatenation/Separation

(N-1)- PCI

(N-1)-PDU


Possible relations between (N)- and (N-1)-connections

(N)-layer

(N)-Connection Endpoint (CE)

(N-1)-CE

One-to-one Multiplex Splitting The splitting of connections can be necessary - to increase reliability where more than one (N-1)-C is available - to reach the demanded quality of service by using multiple (N-1)-C - to realise cost advantages by using several cheap (N-1)-C, each having less than the demanded quality of service (QoS).

Multiplex and Splitting assume special functions, which are not necessary in one-to-one connections.


Relation between T- and N-addresses

Functions in the T-layer Addresses: In the case of an S-entity demanding the installation of T-connection from the T-layer to another S-entity indicated by its T-address, the T-layer determines the network N-address, that is fixed by the T-entity, which supports the corresponding S-entity. Due to the fact that T-entities provide their services end-to-end, no further T-entity is involved as relay in this service. Therefore the T-layer maps T-addresses onto N-addresses, in order to fix the T-entity.

Session-entity

Transport- entity

Transport- entity

Session-entity End transport-

addresses

End network- addresses

Network layer


Allocation of several T- to one N-address

Session- entity A

Session- entity B

Network-entity

Transport addresses

Transport-entity

Network-address

Each T-entity can work for several S-entities at the same time.

One T-entity is able to connect several T-addresses (which name different S-entities) with the same N-address:


Multiplexing: several T-Connections over one N-Connection

Connection-Multiplexing and -Splitting:

In order to optimize the costs of network services, a T-connection is not always mapped 1:1 onto an N-connection, but also by using Multiplexing and Splitting. The connection installation makes the following functions necessary: - Selection of one N-connection, that fulfils the requirements of the S-Entity concerning costs and quality of service. - Decision about the use of Multiplexing or Splitting - Fixing the optimal size of T-PDU - Allocation of T- to N- addresses - Differentiation between T-connection and between the same pair of T-SAPs

SAP SAP

Network connection

Transport connections

Network services

Transport Layer

Session Layer


Network services

Transport connections

Network connection

Transport Layer

Session Layer

... ...

SAP

SAP

Splitting: one T-Connection over several N-Connections


Structure of Protocol Data Units (PDU)

Data

A-PCI A-SDU

Anwendungsschicht-PDU

Darstellungsschicht-PDU

Sicherungsschicht-PDU

Fra

me

Hea

der

Netz-PDU (Paket)

Net

work

H

eader

Transport- PDU

Tra

nsp

ort

H

eader

Sitzungsschicht-PDU

Ses

sio

n

Hea

der

Pre

senta

tion

H

eader

App

lica

tion

H

eader

Fra

me

end


7

6

5

4

3

2

1

End system Repeater Bridge Router End system

Network Relays (Gateways)


TCP/IP Protocol Stack

F

T

P

T

E

L

N

E

T

R

L

O

G

I

N

S

M

T

P

D

N

S . . .

H

T

T

P

R

T

P

S

F

T

P

TCP

IP

LAN/PPP/…

UDP


Reference Model Example: E-Mail

End System

Network Node

Network Node

End System

Transport Layer

(Inter)network Layer

Internet

Subnetwork Layer

Application Layer

OSI

Transport Layer

Network Layer

Link Layer + PHY

Application, Presentation Session

PPP

TCP

IP

SMTP

ATM PPP

IP Routing

Ethernet

TCP

IP

SMTP

E-Mail Client

(Outlook)

Mail Server (sendmail)

Eth. ATM

IP Routing


External

Network Uu

Iur

Iub

RNC

Internet

IP

Packet

……

…

RNC

RNC

Core

Network

UE

UE

B NodeB

NodeB

CN UTRAN UE Iu

Example: Iub Interface UMTS


Application

Layer

TPAL

Layer

TCP

Layer

IP

Layer

FP PDU

Layer

AAL2

Layer

ATM

Layer

Application

Layer

TPAL

Layer

TCP

Layer

IP

Layer

FP PDU

Layer

AAL2

Layer

ATM

Layer

RNC NodeB ATM Link

File Download Time

TCP Delay

TCP Segment Delay

FP PDU Delay

AAL2 Delay

12000 bytes 12000 bytes

1460 bytes

341 bytes

53 bytes

48 bytes

1500 bytes

1460 bytes

1500 bytes

341 bytes

48 bytes

53 bytes

ATM Cell Delay

Example UMTS

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