Metropolitan Area Networks
- Designed to extend over an entire city
- It may be a single network e.g. Cable TV Network
Or
- Interconnection of a No. of LANs into a larger network
Example: A company can use a MAN to connect the LANs in all of its offices throughout a city
Internetworks
- When two or more networks are connected they become an internetwork or internet
- Individual networks are joined together by the use of Internetworking Devices like Routers, Gateways etc.
- internet: Combination of Networks
- Internet: Specific World wide Network
The OSI MODEL
- International Standards Organization (ISO) 1947
- Multinational body dedicated to worldwide agreement on International Standards
- An ISO Standard that covers all aspects of Network Communication is Open System Interconnection Model (OSI)
- Open System: A model that allows two different systems to communicate regardless of their underlying network
- Vendor –Specific Models close off communication
Purpose of the OSI MODEL
- Open Communication between different systems without requiring changes to the underlying hardware and software
OSI Model is not a Protocol. It is a model for understanding and designing a network architecture that is flexible, robust and interoperable
Definition of the OSI MODEL
A layered framework for the design of network systems that allows communication across all types of computer systems reg ardless of their underlying architecture
Layers of the OSI Model
- Physical (Bits)
- Data Link (Frames)
- Network (Packets)
- Transport (Segment)
- Session (Dialog units)
- Presentation (Raw Data)
- Application (Text, Numbers)
Please Do not Touch Steve’s Pet Alligator
What happen when a message travels from device A to Device B?
- As the message travels from A to B , it may pas through many intermediate “Nodes”
- These nodes usually involve only the first three layers of the OSI Model
- In developing the OSI model, designers identified which networking functions had related uses and collected those functions into discrete groups that became the layers
- Each layer defines a family of functions distinct from other layers
- By defining and localizing functionality in this fashion , the designer o s created an architecture that is both comprehensive and flexible
- The OSI model allows complete transparency b/w otherwise incompatible systems
Peer-to-Peer Processes
- Within a single machine, each layer provides services to the layer above it and all upon the services from the layer below it.
- For example Layer 3
- Between machines, layer x on one machine communicates with layer x on the other machine.
- The communication is governed by Protocols
- The processes on each m/c that communicate at a given layer are called Peer –to peer processes
Headers and trailers
- Control data added to a data parcel
- Sender appends header and passes it to the lower layer
- Receiver removes header and passes it to upper layer
- Headers are added at layer 6,5,4,3,2. Trailer is added at layer 2
Passing of data and network information down through the layers of sending machine AND Back up through the layers of the receiving machine is made possible by an
INTERFACE
- Each interface defines what information and services a layer must provide for the layer above it
- Interface provides MODULARITY
- Each layer works as a separate module
- Any modification or replacements can be made without changes in surrounding layers
- Organization of Layers
- Network Support Layers
- Deals with the Physical aspect of moving data from one device to anothe
- Layers 1, 2, 3
- User Support Layers
- Allows interoperability among unrelated software systems
- Layers 5, 6, 7
Organization of Layers
Layer 4
- Ensures end-to-end reliable transmission
- Upper OSI Layers always implemented in Software
- Lower Layers are a combination of software and hardware
- Physical layer is mostly Hardware
The OSI Model
Summary
- Categories of Networks (MANs)
- Internetworks
- The OSI Model
Reading Sections
- Section 2.5, 3.1, 3.2 “Data Communications and Networking” 4th Edition by Behrouz A. Forouzan
