Data Communications 1
What is Open Systems Interconnection?
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The OSI reference model
lecturer:
Gorry Fairhurst
International standards describing how computers communicate
(to replace proprietary solutions)
Define functions of layers
(to define the services required and provided by protocols)
web site:
http://www.erg.abdn.ac.uk/users/gorry/eg3567
Provide framework for interaction between dissimilar systems
(interworking)
After OSI
The OSI Reference Model
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A small number of
"standard" protocols
(IP, CLNP, X.25, etc)
Well-understood
interfaces
Better protocols
More suppliers
May interchange
products
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Applications Programs
L7 Applications Layer
L6 Presentation Layer
L5 Session Layer
L4 Transport Layer
Communications between
networks simplified using
wide range of routers
Shared cost
of internets
L3 Network Layer
L2 Link Layer
L1 Physical Layer
Cabling
ES
IS
ES
Network Functions v End User Functions
Subnetwork
OSI Layers 1-3
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End System B
End System A
Application Layer
Middleware
Presentation Layer
Rôle
3Network Layer
Network routing,
call establishment and clearing,
addressing, individual call management.
Presentation Layer
End User
Function
Session Layer
Transport Layer
Transport Layer
Network Layer
Network Layer
Data Link Layer
Layer
Application Layer
Session Layer
Network
Services
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Network Functions
Data Link Layer
Physical Layer
2(Data) Link Layer Data framing, data transparency,
error control, management,
link establishment and clearing.
1Physical Layer
Physical Layer
Physical Medium
Network Service
Mechanical interface,
electrical interface,
signalling and interface control.
End System Software
OSI Layers 4-7
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End User A
End User B
Application Layer
Application Layer
Higher Layers
Presentation Layer
Presentation Layer
Session Layer
Session Layer
Transport Layer
Transport Layer
"Pipe"
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Layer
Rôle
7Application Layer
Application services:
Reliability, library functions.
6Presentation Layer
Data-oriented communication tasks:
Transfer syntax , data transformation.
5Session Layer
Application-oriented communication :
Dialogue and synchronisation control.
4Transport Layer
End-to-end communication:
Connection management, segmentation,
resequencing, error control, flow control.
"Pipe"
Network
Network
Services
End System Software
Protocol Layer
OSI Layers 4-7
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G Fairhurst
Layer
Rôle
7Application Layer
Application services:
Reliability, library functions.
N-Service Access Point
N- Protocol
Layer
6Presentation Layer
Data-oriented communication tasks:
Transfer syntax , data transformation.
5Session Layer
Application-oriented communication :
Dialogue and synchronisation control.
Protocol
Entity
N-1 Service Access Point
4Transport Layer
End-to-end communication:
Connection management, segmentation,
resequencing, error control, flow control.
Each entity (instance of a protocol) sends and receives data
via the Service Access Points SAPs using service primitives
OSI Layer Services
OSI Layer Services
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G Fairhurst
OSI introduced the notion of a “protocol layer” providing a “service”
Layer
A
layer providesUser
a service
to theService
layers above.
Each
User B
provided
N+1
request data)confirm
indication
is providing using the response
layer below
The service (moving
Layer
N
Protocol entity
Service
used
Peer
protocol entity
Layer
N-1
Layer
N+1
Layer
N
User A
request
confirm
Protocol entity
Service
provided
Service
used
User B
response
indication
Peer
protocol entity
Layer
N-1
Protocol Data Units (PDUs)
The packets / frames sent by a layer are called “PDU”s
Protocol Data Units (PDUs)
PDU Encapsulation
PDU Encapsulation / OSI RM
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N+1 Protocol
Layer
7
PDU
Protocol Control Information
6 7
5 6 7
N- Protocol
Layer
4 5 6 7
PCI +
PCI
Layer N+1
Protocol Data Unit
becomes a Layer N
Service Data Unit
SDU
SDU
3 4 5 6 7
Layer N PDU
2 3 4 5 6 7
1 2 3 4 5 6 7
1
3
5 6 7
N-1 Protocol
Layer
PCI +
SDU
Service Primitives
Service Primitives
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Client (ES)
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N+1-Data-Request
Server (ES)
N+1-Data-Indication
Request
Indication
Response
N-Layer
Confirm
N-Layer
Request
Indication
(Unconfirmed Service)
Segmentation / Fragmentation
OSI Interfaces
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G Fairhurst
From Layer N+1
System A
N- Protocol
Layer
System B
Peer Protocol
Layer (N+1)
Layer (N+1)
Service Interface
Service Interface
SDU
Header
SAP
PCI +
SDU
PCI + SDU
SAP
PCI SDU
Layer (N)
Layer (N)
Peer Protocol
Two calls to
N-1 Protocol Layer
OSI Tutorial Question
The rôle of OSI
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Advantages of OSI:
Equipment X
Vendor independent communication
Fewer "standard" protocols
Client
Server
Standard transportation networks
1) Assume the equipment X is a 100 Mbps Ethernet hub.
Problems with OSI:
ISO defined its own OSI protocol stack
Throughput lower than
with application-oriented protocols
A reference framework for describing protocols
(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.