OSI

Layered Network Model

Networking models describe networking should work and 2 models are
- OSI Model
- TCP/IP Model
Model defines a networking framework for implementing protocols in different layers.
Control moves between layers, from one to next
starting at top most layer to bottom layer
over physical channel to next station on network, and then back as per hierarchy.

Layering Concepts and Benefits

Interoperability – interoperability between devices from different manufacturers.
Compatibility – devices/systems /networks should be compatible
Better Flexibility – flexibility in options and choices for configuration and installation.
Increased Life Expectancy – Devices from different technology generations can co-exist thus the older units do not get discarded immediately newer technologies are adopted.
Scalability – Experience shows that a layered design scales better than the horizontal approach.
Modularity Plug-ins and add-ons are easily added from use of a layered approach.
Standardization and Certification –easy compliance ot standards and according certification for use by end users.
Portability – Layered networking protocols are much easier to port from one system to another.

OSI Basics

The ISO or International Standards Organization, has defined a 7-layer model for standardizing networking processes.
7 Layers are
- Physical Layer
- Data Link Layer
- Network Layer
- Transport Layer
- Session Layer
- Presentation Layer
- Application Layer

Physical Layer

lowest layer of the OSI mode
about transmission and reception of the raw bit stream over physical medium like wire, air
describes electrical/optical, mechanical, and functional interfaces to physical medium
carries signals for all higher layers.
It provides
- Data encoding – digital signal to characteristics of physical medium
- Transmission technique
- transmits digital bits as electrical or optical signals on the physical medium

Data Link Layer

It gives error-free data frames transfer from one node to another over the physical layer
Provides top layers an error-free transmission over the link.
It provides
- Link establishment and termination
- Frame traffic control
- Frame sequencing
- Frame acknowledgment
- Frame delimiting
- Frame error checking
- Media access management

Network Layer

Transport Layer

ensures messages are delivered
- error-free
- in sequence
- no losses or duplications
relieves higher layer from concern of data transfer
In a reliable network layer with virtual circuit capability, a minimal transport layer is required
unreliable one needs datagrams
include extensive error detection and recovery
It provides
- Message segmentation
- Message acknowledgment
- Message traffic control
- Session multiplexing
strict message size limits are imposed

Session Layer

It allows session establishment between processes running on different nodes
It provides
- Session establishment, maintenance and termination
- Session support

Presentation Layer

It formats data to be presented to the application layer
translate data from application layer format into a common format at the source
It provides
- Character code translation
- Data conversion
- Data compression
- Data encryption

Application Layer

It serves as window for users and application processes to access network services.
It has functions
Resource sharing and device redirection
Remote file access
Remote printer access
Inter-process communication
Network management
Directory services
Electronic messaging (such as mail)
Network virtual terminals

TCP/IP

Also called, Internet protocol suite
It was in use before OSI model was established
Facilitates end-to-end connectivity
Also specify data should be
- Formatted
- Addressed
- Transmitted
- routed
- received at the destination.
It is maintained by the (IETF) or Internet Engineering Task Force.
has 4 abstraction layers, every layer has own protocols.
From highest to lowest, the layers are
Application layer (process-to-process)- Has all higher level protocols (like FTP, HTTP) for specific data communications services on a process-to-process level
Transport layer (host-to-host)- deals with opening and maintaining connections between Internet hosts. It does flow-control, error-correction, and connection protocols exist, such as TCP.
Internet layer (internetworking)- connects local networks, and establish internetworking. Defines addressing and routing structures. Has Internet Protocol or IP, which defines IP addresses.
Link layer- Also called Ethernet. Has communication technologies for a local network. Defines hosts communication on local network without routers.