- Fact:
- Every (operational) complex software systems created by humans are designed using a modular (layer) approach
- Example:
- Operating Systems:
- UNIX Operating System is
layered:
- UNIX Operating System is
layered:
- Operating Systems:
- Communication Architecture:
- A system that allows
a large number of
computers
to
communicate with each other
over large distances
is a
very complex software system....
- The design of the present day communication systems are layered....
- A system that allows
a large number of
computers
to
communicate with each other
over large distances
is a
very complex software system....
- ISO:
- ISO = International Standard Organization
- Brief history:
- In the late 1970s, two projects began independently, with the same goal:
- to define a unifying standard for the architecture of networking (communication) systems.
- One project was administered by the
International
Organization for Standardization
(ISO),
The other was undertaken by the International Telegraph and Telephone Consultative Committee, or CCITT (the abbreviation is from the French version of the name).
-
These two
international standards bodies each developed a
document
that defined
similar networking models.
-
In 1983, these
two documents were
merged
to form a standard called:
- The Basic Reference Model for Open Systems Interconnection.
- In the late 1970s, two projects began independently, with the same goal:
- The 7 layers in the
ISO OSI reference model:
Description of the layers:
- Physical layer:
- Is the only
hardware "layer"
- The Physical
hardware "layer"
provides the ability to
transmit and receive
(electrical) signals.
- Is the only
hardware "layer"
- Datalink layer:
- The first
software layer
- The datalink layer
ensures
that the transmitted data
is received correctly:
Techniques used: (discussed later in course)
- Sequence numbers
- error detection/correction (e.g., checksum)
- Automatic re-transmission request
- The first
software layer
- Network layer:
- Determine
the next node
to forward a packet
to its
(final) destination
Techniques:
- Topology discovery
- Routing computation algorithms (e.g., Shortest Path (Dijkstra))
- Determine
the next node
to forward a packet
to its
(final) destination
- Transport layer:
- Ensure
that the data (packet) transmitted
between the source and the
destination node
is received correctly
Techniques used: (discussed later in course)
- Sequence numbers
- error detection/correction (e.g., checksum)
- Automatic re-transmission request
Note:
- You may have
heard
the term:
TCP/IP
- TCP = the Transport Control Protocol used in the Internet
- Ensure
that the data (packet) transmitted
between the source and the
destination node
is received correctly
- Session layer:
- manage
the connections and services
between
multiple end-points
(I.e.: group communication !!!)
- manage
the connections and services
between
multiple end-points
- Presentation layer:
- Translation of the message content so that it is suitable for consumption at a particular host
Examples:
- Language translation services:
- Application layer:
- The application layer are
user programs
that communicate with
each other
- These programs usually implement a request/reply exchange protocol
Example:
- Web service
- Email service
- The application layer are
user programs
that communicate with
each other
- Physical layer:
- Fact:
- The design of the Internet is very close to ISO OSI reference model
- Structre of the
Internet:
- The first 4 layers of
the Internet:
- Physical
- Datalink
- Network
- Transport
as identical to the the ISO OSI reference model
- Layers 5:
- Session layer
was implemented (later) around 1994
It's known as the IP Multicast service.
- Layer 6:
- Presentation layer
is not available.... (too fancy ???)
- Layer 7:
specifies the
network applications
- is nothing more than the network applications that user develop....
- The first 4 layers of
the Internet: