The OSI reference model

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23 Οκτ 2013 (πριν από 3 χρόνια και 11 μήνες)

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Data Communications 1
The OSI reference model
lecturer:
Gorry Fairhurst
web site:
http://www.erg.abdn.ac.uk/users/gorry/eg3567
G Fairhurst
W
hat is Open Systems Interconnectio
n?
International standards describing how computers communicate
(to replace proprietary solutions)
Define functions of layers
(to define the services required and provided by protocols)
Provide framework for interaction between dissimilar systems
(interworking)
G Fairhurst
A
fter OS
I
A small number of
"standard" protocols
(IP, CLNP, X.25, etc)
Communications between
networks simplified using
wide range of routers
Well-understood
interfaces
Better protocols
More suppliers
May interchange
products
Shared cost
of internets
G Fairhurst
Applications Programs
L7 Applications Layer
L6 Presentation Layer
L5 Session Layer
L4 Transport Layer
L3 Network Layer
L2 Link Layer
L1 Physical Layer
Cabling
T
he OSI Reference Mod
el
ES
ISES
G Fairhurst
End System A
End System B
Middleware
Network
Services
Application Layer
Presentation Layer
Session Layer
Transport Layer
Network Layer
Data Link Layer
Physical Layer
Application Layer
Presentation Layer
Session Layer
Transport Layer
Network Layer
Data Link Layer
Physical Layer
Physical Medium
End User
Function
Network Functions
Network Functions v End User Functions
G Fairhurst
Layer Rôle
3 Network Layer Network routing,
call establishment and clearing,
addressing, individual call management.
2 (Data) Link Layer Data framing, data transparency,
error control, management,
link establishment and clearing.
1 Physical Layer Mechanical interface,
electrical interface,
signalling and interface control.
O
SI Layers 1-
3
Subnetwork
G Fairhurst
Application Layer
Presentation Layer
Session Layer
Transport Layer
End User A
Application Layer
Presentation Layer
Session Layer
Transport Layer
End User B
Network
"Pipe""Pipe"
Network
Services
Higher Layers
Network Service
G Fairhurst
Layer Rôle
7 Application Layer Application services:
Reliability, library functions.
6 Presentation Layer Data-oriented communication tasks:
Transfer syntax , data transformation.
5 Session Layer Application-oriented communication :
Dialogue and synchronisation control.
4 Transport Layer End-to-end communication:
Connection management, segmentation,
resequencing, error control, flow control.
End System Software
O
SI Layers 4-
7
G Fairhurst
Layer Rôle
7 Application Layer Application services:
Reliability, library functions.
6 Presentation Layer Data-oriented communication tasks:
Transfer syntax , data transformation.
5 Session Layer Application-oriented communication :
Dialogue and synchronisation control.
4 Transport Layer End-to-end communication:
Connection management, segmentation,
resequencing, error control, flow control.
End System Software
O
SI Layers 4-
7
G Fairhurst
N- Protocol
Layer
N-Service Access Point
N-1 Service Access Point
Protocol
Entity
P
rotocol Laye
r
Each entity (instance of a protocol) sends and receives data
via the
Service Access Points SAPs using
service primitives
G Fairhurst
User A
User B
Layer
N+1
Layer
N-1
Layer
N
Protocol entity
request
confirmresponseindication
Service
provided
Service
used
Protocol Data Units (PDUs)
Peer
protocol entity
O
SI Layer Service
s
OSI introduced the notion of a “
protocol layer” providing a “
service”
Each
layer provides a
service to the
layers above.
The
service (moving data) is providing using the
layer below
The packets / frames sent by a layer are called “
PDU”s
G Fairhurst
User A
User B
Layer
N+1
Layer
N-1
Layer
N
Protocol entity
request
confirmresponseindication
Service
provided
Service
used
Protocol Data Units (PDUs)
Peer
protocol entity
O
SI Layer Service
s
G Fairhurst
P
DU Encapsulation / OSI R
M
7653
1
24
765324
76534
7654
765
76
7
7653
1
G Fairhurst
P
DU Encapsulatio
n
SDUPCI
+
SDUPCI
N- Protocol
Layer
Layer N PDU
Protocol Control Information
N+1 Protocol
Layer
N-1 Protocol
Layer
PDU
SDUPCI
+
Layer N+1
Protocol Data Unit
becomes a Layer N
Service Data Unit
G Fairhurst
Request
Request
Indication
Response
Confirm
Indication
S
ervice Primitive
s
Client (ES)
Server (ES)
G Fairhurst
S
ervice Primitive
s
N-Layer
N+1-Data-Request
N-Layer
N+1-Data-Indication
(Unconfirmed Service)
G Fairhurst
Layer (N+1)
Layer (N+1)
Layer (N)
Layer (N)
System ASystem B
Peer Protocol
Peer Protocol
Service Interface
Service Interface
SAPSAP
OSI Interfaces
G Fairhurst
N- Protocol
Layer
Header
SDUPCI
+
SDU
SDUPCI
SDUPCI
+
From Layer N+1
Two calls to
N-1 Protocol Layer
Segmentation / Fragmentatio
n
G Fairhurst
Advantages of OSI:
Vendor independent communication
Fewer "standard" protocols

Standard transportation networks

Problems with OSI:
ISO defined its own OSI protocol stack

Throughput lower than
with application-oriented protocols
T
he rle of OS
I
A reference framework for describing protocols
Client
Server
1) Assume the equipment X is a 100 Mbps Ethernet hub.
(a) Using the OSI reference model, sketch the protocol stack
(b) Sketch the Ethernet MAC frame header received by the server
2) Repeat above for equipment X being an Ethernet switch.
3) Repeat above for equipment X being a network layer router.
Equipment X
O
SI Tutorial Questio
n