A computer network connects 2 or many devices to share data and services. Multiple N/Ws connected together from an inter-network.
Inter-networking deliver challenges - inter operating between products of various manufacturers demands consistent standards. Network reference models were prepared to deal these disputes. A network reference model functions as a blueprint, detailing how communication between network devices should happen.
The 2 most acknowledged network reference models are:
• Open Systems Interconnection (OSI) model
• Department of Defense (DoD) model
Without the framework that network models furnish, all network hardware and software would undergo proprietary. Organizations would have got shut up into a individual vendor’s equipment, and worldwide networks suchlike the Internet would have been impractical, if not impossible.
Network models are formed into layers, on each layer standing for a specific networking function. These functions are operated by protocols, which are rules(formulas) that regulate end-to-end communication 'tween devices.
The TCP/IP suite is the most current protocol suite, and is the base of the Internet.
A network model is not a physical entity - there's none OSI device, It's logical. Manufacturers don't all of the time purely stick to a reference model’s design, and so not every communications protocol meets absolutely inside a individual layer. A few protocols can perform over multiple layers.
OSI Model:
The Open Systems Interconnection (OSI) model was developed by the International Organization for Standardization (ISO), and formalized in 1984. It's provided the first model regulating how data should be directed across a network.
The OSI model consists of 7 layers, from each one related to a particular network role:
7 Application
6 Presentation
5 Session
4 Transport
3 Network
2 Data-link
1 Physical
Take note that the bottom layer is Layer 1. Varied mnemonics get more at ease to remember the ordering of the OSI model’s layers:
7 Application - Advise
6 Presentation - Person's
5 Session - Sales
4 Transport - Trust
3 Network - Not
2 Data-link - Do
1 Physical - Please
International Organization for Standardization additional built up an entire protocol suite grounded on the OSI model; however, the OSI communications protocol suite was ne'er widely applied.
The OSI model itself is now slightly deprecated - current communications protocol suites, such as the TCP/IP suite, is awkward to match cleanly within the OSI model’s 7 layers. This is specially truthful of the upper three layers.
The bottommost (or lower) 4 layers are more clear specified, and terminology from those layers is even prevalently utilized. A lot protocols and devices are represented by which bottom layer they function at.
OSI Model - The Upper (top) Layers
The top three layers of the OSI model are frequently referred as upper layers:
• Layer 7 - Application layer
• Layer 6 - Presentation layer
• Layer 5 - Session layer
Protocols that function at these layers handle application-level roles, and are usually applied in software package.
The role of the upper layers of the OSI model may be tough to figure. Upper layer protocols don't e'er go perfectly within a layer, and oftentimes run over multiple layers.
Layer 7 - The Application Layer:
The Application layer (Layer-7) allows the interface 'tween the user applications programme and the network. A web browser and an e-mail client are good example of user applications programme.
The user applications programme itself doesn't dwell at the Application layer - the protocol does. The user interacts on the applications programme, which in turn interacts with the application protocol.
Cases of Application layer protocols includes:
• Telnet
• POP3 and SMTP, via an email clientHTTP, via a web browser
• HTTP, via a web browser
• FTP, via an FTP client
The Application layer supplies a kind of functions:
• Determines resource availability
• Synchronizes communication
• Discovers communication partners
The Application layer interacts with the Presentation layer at a lower place it. As it's the uppermost layer,there is no layer to upper it for interacts.
In addition, the Presentation layer can execute encryption and compression of data, as requisite. Still, these functions can also be executed at lower layers also. E.g., the Network layer can execute encryption, applying IPSec.
Layer 6- The Presentation Layer:
The Presentation layer (Layer-6) verifies the format and syntax of user data for the application layer. This assures that information from the transmitting application can be understood by the receiving applications programme.
Criteria have been built up for the format of information cases, specified text, images, audio, & video. Examples of Presentation layer formats includes:
• Text - RTF, ASCII, EBCDIC
• Images - GIF, JPEG, TIF,bmp
• Audio - MIDI, MP3, WAV,m4a
• video - MPEG, AVI, MOV,Mp4,3gp,mkv
If two devices don't support the equal format or syntax, the Presentation layer can render conversion or translation services to ease communication.
In addition, the Presentation layer can execute encryption and compression of data, as requisite. Still, these functions can also be executed at lower layers also. E.g., the Network layer can execute encryption, applying IPSec.
Layer 5 - The Session Layer:
The Session layer (Layer-5) is responsible for establishing, maintaining, and at long last terminating sessions 'tween devices. If a session is cracked, this layer can attempt to retrieve the session.
Sessions communication drops under one of 3 classes:
• Simplex - one way communication
• Half-Duplex - two-way communication, but not simultaneous (same time)
• Full-Duplex - simultaneous two-way communication
Several advanced protocol suites, such as transmission control protocol/internet protocol(tcp/ip) , don't carry out Session layer protocols. Connection direction is frequently held by lower layers, such as the Transport layer.
The lack of actual Session layer protocols can deliver challenges for high-availability and failover. Trust on lower-layer protocols for session management provides less flexibility than a strict adhesiveness to the OSI model.
OSI Model - The Lower
Layers:
The bottom 4 layers of the OSI model are very much adverted to as the lower layers:
• Layer 3 - Network layer
• Layer 2 - Data-Link layer
• Layer 1 - Physical layer
Protocols that functions at these layers insure the end-to-end transportation of data 'tween devices, and are enforced in both software and hardware.
Layer 4 - The Transport Layer:
The Transport layer (Layer-4) doesn't really transmit data, contempt its name. Rather, this layer is liable for the trustworthy transfer of data, by assuring that data reaches its destination error-free and in order.
Transport layer communication comes below two classes:
• Connectionless - needs no connection prior to data is sent.
• Connection-oriented - needs that a connection with particular agreed-upon parameters be accomplished prior to data is sent.
Connection-oriented protocols put up various crucial services:
• Segmentation and sequencing - data is segmented into smaller parts for transport. Each segment is allotted a chronological sequence number, so that the receiving host can reassemble the data on arrival.
• Connection establishment - connections are established, maintained, and finally dismissed between devices.
• Acknowledgments - receipt of data is affirmed through the use of acknowledgments. Otherwise, data is retransmitted, assuring delivery.
• Flow control (or windowing) - data transfer rate is managed to prevent congestion.
The TCP/IP protocol suite integrates 2 Transport layer protocols:
• User Datagram Protocol (UDP) - connectionless
• Transmission Control Protocol (TCP) - connection-oriented
Layer 3 - The Network Layer:
The Network layer (Layer-3) holds inter-network communication, and has 2 key duties:
• Logical addressing - grants a unique address that describes both the host, and the network that host connected.
• Routing - finds out the best path to a specific address network, and then routes data consequently.
2 of the basic Network layer protocols are:
• Novell’s Internetwork Packet Exchange (IPX).
• Internet Protocol (IP).
IPX is about entirely deprecated. IP version 4 (IPv4) and IP version 6 (IPv6) are covered in nauseating point in different guides.
Layer 2- The Data-Link Layer:
When the Network layer is occupied with transmitting data 'tween networks, the Data-Link layer (Layer-2) is liable for transmitting data within a network.
The Data-Link layer comprises of two sublayers:
• Media Access Control (MAC) sublayer
• Logical Link Control (LLC) sublayer
The MAC sublayer holds access to the physical medium, helping as mediator if multiple devices are contending for the similar physical link. Data-link layer technology have varied techniques of achieving this -
Ethernet applies Carrier Sense Multiple Access with Collision Detection (CSMA/CD), and Token Ring uses a token.
Ethernet is covered up in large detail in other guides.
The LLC sublayer functions as the intermediator 'tween the physical link and each higher layer protocols. It checks that protocols suchlike IP can operate irrespective of what type of physical technology is being utilized.
In addition, the LLC sublayer can execute flow-control and error-checking, though such roles are often supplied by Transport layer protocols, such as TCP.
The Data-link layer bundles the higher-layer data into frames, and then the data can be place onto the physical wire. This packaging method is named to as framing or encapsulation.
The encapsulation type will change depending upon the fundamental technology. Usual Data-link layer technology admit following:
• Ethernet - almost common LAN data-link technology
• Token Ring - most entirely deprecated
• 802.11 Wireless )
• Frame-Relay
• ATM (Asynchronous Transfer Mode)
• FDDI (Fiber Distributed Data Interface)
The data-link frame contains the source and destination physical (or hardware) address. Physical addresses uniquely describe a host inside a network, and are much hardcoded onto physical network interfaces. However, hardware addresses contain no mechanism for differentiating one network from another, and can only identify a host within a network.
The most familiar Physical address is the Ethernet MAC address.
Layer 1 - The Physical Layer:
The Physical layer (Layer-1) commands the signal and transmitting of raw bits onto the physical medium. The Physical layer is nearly related to the Data-link layer, as some technologies (such as Ethernet) contains both data-link and physical functions.
The Physical layer puts up specs for a kind of hardware:
• Connectors and transceivers
• Cables
• Hubs
• Wireless radios
• Network interface cards (NICs)
Layered Communication & Encapsulation:
As data is travelled from the user application down the virtual layers of the OSI model, all of the lower layers binds a header (and occasionally a trailer) containing protocol data specified to that layer. These headers are known as Protocol Data Units (PDUs), and the operation of adding these headers is known as encapsulation.
E.g., a Transport layer protocol specified as TCP will add a header carrying flow control and sequencing info. The Network layer header holds logical addressing info, and the Data-link header holds physical addressing and extra hardware precise info.
The PDU of each layer is described with a assorted terms:
Layer PDU Name
Application -
Presentation -
Session -
Transport - Segments
Network - Packets
Data-Link - Frames
Physical - Bits
All layers transmits with the related layer on the receiving device. E.g., With the sending device, source and destination hardware directing is ordered in a Data-link header. On the receiving device, that Data-link header is processed and stripped away before being sent out to the Network and other upper layers.
Network devices are usually identified by the OSI layer they function at; or, further specifically, what header or PDU the device works.
E.g., switches are mostly described as Layer-2 devices, as switches process data stored in the Data-Link header of a frame, like Ethernet MAC addresses. Likewise, routers are known as Layer-3 devices, as routers process logical addressing information @ the Network header of a packet, such as IPs.
OSI Reference Model Example:
A web browser processes as a good practicable example of the OSI model and the TCP/IP protocol suite:
• Τhe web browser assists as the user interface for accessing a internet site. The web browser itself doesn't run at the Application layer. Rather, the web browser appeals the Hyper Text Transfer Protocol (HTTP) to interface with the remote web server, that's why http:// precedes every web address.
• The Internet can provide information in a large kind of formats, a role of the Presentation layer. Usual formats on the Internet includes HTML, XML, PHP, GIF, and JPEG. Some encryption or compression mechanisms applied on a internet site are likewise counted a Presentation layer role.
• The Session layer is liable for building, maintaining, and ending the session 'tween devices, and deciding whether the communication is full-duplex or half-duplex. Even so, the TCP/IP stack doesn't admit session-layer protocols, and is dependent on lower-layer protocols to execute these roles.
• Hypertext transfer protocol utilizes the Transport layer protocol to secure the authentic delivery of data. TCP builds and holds a connection from the client to the net server, and bundles the higher-layer data into segments. A sequence amount is assigned to each segment so that data can be reassembled after arrival.
• The finest path to route the data 'tween the client and the web server is decided by IP, a Network layer protocol. IP is as well responsible for the assigned logical addresses on the client and server, and for encapsulate segments into packets.
• Data can't be sent direct to a logical address. Because packets go from network to network, IP addresses are translated to hardware addresses, which is a role of the Data-Link layer. The packets are encapsulated into frames to be placed upon the physical medium.
The data is ultimately transported onto the network medium at the Physical layer, in the kind of raw bits. Signaling and encoding mechanisms are settled at this layer.
IP and the DoD Model:
The Internet Protocol (IP) was primitively formulated by the Department of Defense (DoD), and was a foundation for a group of communications protocol that got familiar as the TCP/IP protocol suite.
TCP/IP consists 4 layers:
4.Application Layer
3.Host-to-Host Layer
2.Internet
1.Network Access
The following table exemplifies where primary protocols meet into the TCP/IP model:
Layer
|
Example Protocols
|
Application
|
FTP,
HTTP, SMTP
|
Host-to-Host
|
TCP, UDP
|
Internet
|
IP
|
Network
Access
|
Ethernet
|
In spite of the DoD model, the OSI model is still the base for most network terminology practically.
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