Communication Networks I

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Communication Networks I

• OPEN SYSTEMS INTERCONNECTION (OSI) REFERENCE
  MODEL
• Prof. U. Killat
• Hamburg University of
• Technology

Department of Communication Networks Prof. Dr.
U. Killat
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OSI Reference Model

• Introduction
• Concepts of the OSI model
• Implementation aspects
• Discussion of the OSI model

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Introduction

• Incompatibilities are a problem in Interworking.
• Standardization is an approach to cure this evil.
  
• However, the complexity of the problem asks for
  structuring and modularization of the problem.
• International Standards Organization (ISO) since
  1977 has worked on the OSI model.
• They have developed a methodology and detailed
  implementation descriptions.

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Introduction

• Typical system decomposition in a communication
  network is to look at individual boxes
• which house some type of equipment and
• which can be described by their input/output
  behaviour at their interfaces.
• This is the preferred view of the ITU and can be
  referred to as vertical system decomposition.

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Introduction

• The OSI approach is a different one.
• Its background is software structuring rather
  than thinking in hardware components.
• The technique developed is referred to as
  layering - a concept that is also known from
  operating systems.

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Introduction
Hierarchy of layers in an operating system
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Introduction
OSI model
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Concepts of the OSI model

• The notions of service and protocol are
  important.
• Layer N on one machine talks to layer N on
  another machine to achieve a certain goal, i.e.
  the N service that is offered to layer (N1).
• The rules and conventions used in this
  conversation are collectively known as the layer
  N protocol.

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Concepts of the OSI model

• The entities executing a protocol on different
  machines are called peers.
• The entities in layer N implement a service used
  by layer (N 1). In this case layer N is called
  the service provider and layer (N 1) is called
  the service user.
• Obviously the N service is not a result of the
  execution of the N protocol alone. But layer N
  makes use of the service(s) offered by layer
  (N-1).

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Concepts of the OSI model

• The service is described by a set of service
  primitives that a layer provides to the layer
  above it.
• The upper layer has no idea how this service is
  being performed. By analogy, if I ask my
  secretary to organize a meeting I do not need to
  know whether the attendees are informed about the
  meeting by phone, mail, e-mail or fax.

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Concepts of the OSI model

• A service is like an object in an object-oriented
  language. It defines operations that can be
  performed on an object but does not specify how
  these operations are implemented.
• A protocol relates to the implementation of the
  service and as such is not visible to the user of
  the service.

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Concepts of the OSI model

• As a consequence one protocol might be replaced
  by another one which meets the same service
  specification.
• Lesson learned from the OSI model was
• Define a protocol only after having specified
  the service it should support.

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Concepts of the OSI model

• Services are available at service access points
  (SAPs).
• An SAP is identified by its address.
• In the telephone system an SAP is the wall socket
  and the SAP address is the telephone number.
• The address to reach the SAP of the IP layer is
  the IP address, the address to reach the SAP of
  the TCP layer is the port address.

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Concepts of the OSI model

• To exchange information between two layers there
  has to be an agreed upon set of rules about the
  interface involving some interface control
  information (ICI).
• Note! The information transfer between layers
  (inside a host) is considered as absolutely
  reliable, whereas the communication between peers
  in general is not.

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Concepts of the OSI model
N Request
N Confirmation
N Indication
N Response
SAP
SAP
N Layer
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Concepts of the OSI model

• The service primitives supporting a simple
  connection-oriented service

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Concepts of the OSI model

• TANENBAUMs example of inviting Aunt Millie for
  tea

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Concepts of the OSI model

• Invitation for tea seen from an OSI perspective

1
5
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Layer N1
Computer 1 You
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Layer N
3
1
2
4
5
6
7
8
9
10
3
5
Layer N1
Computer 2 Aunt Millie
2
6
8
Layer N
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Concepts of the OSI model

• The user data parameters associated with an N
  service primitive are known as N service data
  unit (N SDU).
• As this represents the information sent by the
  (N 1) entity to its peer, i.e. the (N
  1) protocol data unit, (N 1) PDU, we have

N SDU (N1)PDU N PDU N PCI (N1)PDU
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Concepts of the OSI model

• Layer interactions at SAPs

PCI stands for Protocol Control Information
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Concepts of the OSI model

• Layering results in a generic header structure
  

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Concepts of the OSI model

• SAPs are identified by an address.
• To address an application process (AP) in the OSI
  environment we have to consider the concatenation
  of SAP addresses
• AP address P SAP S SAP T SAP N
  SAP
• where the N SAP represents the network-wide
  address of the system in which the AP is
  resident.
• All other SAP addresses are created within the
  host.
• The mapping between the symbolic name of an
  application process and the AP address is done by
  the system directory.

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Concepts of the OSI model

• If an application process runs multiple
  transactions in parallel, so-called connection
  endpoint identifiers can be assigned to different
  transactions.
• These identifiers are, however, not part of the
  SAP address structure.

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Implementation aspectsof the OSI model

• In general communication protocols are not run by
  the host itself but by a local processor on the
  communication subsystem board.
• At the communication subsystem a shared memory is
  used to organize the communication
• between the host,
• the local processor and
• the network interface unit.
• This shared memory mainly contains data
  representing service primitives and their
  parameter.

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Implementation aspectsof the OSI model

• Hardware of the communication subsystem

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Implementation aspectsof the OSI model
Interlayer communication (a) transport ECB
structure, (b) relationship between ECB and UDP
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Implementation aspectsof the OSI model

• A task representing a protocol entity
  communicates with other tasks (and in particular
  the timer task) through a set of FIFO queues or
  mailboxes.
• Inter-task communication is managed by the local
  (real-time) operating system.
• The operating system will call a task, if it is
  idle and if a message arrives at one of its input
  queues.

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Implementation aspectsof the OSI model

• To pass service primitives between layers (tasks)
  the initiating task invokes the inter-task
  communication primitive of the operating system
  with the address pointer of the event control
  block (ECB) as a parameter.
• The operating system will then insert the pointer
  at the tail of the appropriate queue.
• When a task is scheduled to run, it will examine
  each of its queues for ECBs waiting for
  processing.
• The user data associated with a primitive is held
  in a separate buffer the user data buffer (UDP).

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Implementation aspectsof the OSI model

• As each layer adds its own PCI to the existing
  contents, it increments the UDP length by the
  corresponding amount.
• The UDP contains the accumulated PCIs for each of
  the higher layers. Thus the UDP at the physical
  layer will have the maximum length.
• Since the ECBs and UDBs must be accessible by
  each layer, they are each declared as global data
  structures.

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Implementationaspects

• program Comniunication_Subsystem
• global Intertask queues (mailboxes)
• Event Control Blocks
• User Data Buffers
• task Transport_Layer
• local typ Events (TCONreq, TCONresp, NCONconf,
  CR, CC, - - -)
• States (CLOSED, WFNC, WFCC, OPEN,
  WFTRESP, - - -)
• var EventStateTable array Events, States
  of 0..N
• PresentState States
• EventType Events
• ECB ECB Buffer
• UDB UDB Buffer
• procedure Initialize Initialize EventStateTable
  contents and state variables
• procedure TCONind
• procedure TCONconf
• procedure TDISmd
• function P0 boolean
• function P1 boolean

Outline program structure of a protocol layer
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Discussion of the OSI model

• When looking to the model, an obvious question
  is
• Why 7 layers?
• There are no first principles to arrive at 7
  layers.
• 7 is rather the result of a set of concerns
  outlined below
• 1. A layer should be created where a different
  level of abstraction is needed.
• 2. Each layer should perform a well-defined
  function.
• 3. The function of each layer should be chosen
  with an eye towards defining internationally
  standardized protocols.
• 4. The layer boundaries should be chosen to
  minimize the information flow across the
  interfaces.
• 5. The number of layers should be large enough
  that distinct functions need not be thrown
  together in the same layer out of necessity, and
  small enough that the architecture does not
  become unwieldy.

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Discussion of the OSI model

• The layers chosen were not very stable. For
  example, local area networks introduced the MAC
  layer considered as part of layer 2.
• Aspects like presentation where not very stable
  in residing in a certain layer and other aspects
  like authentication and encryption were forgotten
  altogether.
• Today the importance of the OSI mode lies in
  methodological achievements (protocol development
  after service definition).
• In practice TCP/IP protocols and their direct
  interfacing to a couple of applications have won
  the race.

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Discussion of the OSI model
OSI
TCP/IP
Application
Application
7
Presentation
6
Not present in this model
Session
5
Transport
Transport
4
Network
Internet
3
Data Link
Host to Network
2
Physical
1