ISO OSI Reference Model - Universität Bremen · OSI - 5 James Bond meets the 7 layer OSI model...
Transcript of ISO OSI Reference Model - Universität Bremen · OSI - 5 James Bond meets the 7 layer OSI model...
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
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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
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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
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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)
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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 - 8 www.comnets.uni-bremen.de
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.
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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.
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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
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Protocol Description
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Finite State Machine
Specification and Description Language
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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?
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Circuit Switching Example (Leitungsvermittelt)
Telephone Connections
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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
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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
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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)
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Connection Oriented Packet Switching
Virtual circuit
Packet Packet
Packet
Packet
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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
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Connectionless Packet Switching
Packet 2
Packet 1
Packet 1
Packet 2
Packet 2
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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)
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(N+1)-
layer
System A System B
service user service user
service access points (SAP)
(N)- layer
Service-provider
Service model (SM)
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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)
User of service (N-1)
(N) – protocol
Service of layer (N-1)
(N-1) – protocol User of service (N-2)
User of service (N-2)
Provider of service (N-1)
Provider of service (N)
Service of layer (N)
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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
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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
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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
OSI - 31 www.comnets.uni-bremen.de
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.
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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
OSI - 36 www.comnets.uni-bremen.de
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:
OSI - 37 www.comnets.uni-bremen.de
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
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Network services
Transport connections
Network connection
Transport Layer
Session Layer
... ...
SAP
SAP
Splitting: one T-Connection over several N-Connections
OSI - 39 www.comnets.uni-bremen.de
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
OSI - 40 www.comnets.uni-bremen.de
7
6
5
4
3
2
1
End system Repeater Bridge Router End system
Network Relays (Gateways)
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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
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Reference Model Example: E-Mail
End System
Network Node
Network Node
End System
Transport Layer
(Inter)net- work 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
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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
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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