Network Devices,Frame Relay and X.25

Hardware/Networking Devices: Networking hardware may also be known as network equipment computer networking devices.

Network Interface Card (NIC): NIC provides a physical connection between the networking cable and the computer’s internal bus. NICs come in three basic varieties 8 bit, 16 bit and 32 bit. The larger number of bits that can be transferred to NIC, the faster the NIC can transfer data to network cable.

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Repeater: Repeaters are used to connect together two Ethernet segments of any media type. In larger designs, signal quality begins to deteriorate as segments exceed their maximum length. We also know that signal transmission is always attached with energy loss. So, a periodic refreshing of the signals is required.

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Hubs: Hubs are actually multi part repeaters. A hub takes any incoming signal and repeats it out all ports.

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Bridges: When the size of the LAN is difficult to manage, it is necessary to breakup the network. The function of the bridge is to connect separate networks together. Bridges do not forward bad or misaligned packets.

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Switch: Switches are an expansion of the concept of bridging. Cut through switches examine the packet destination address, only before forwarding it onto its destination segment, while a store and forward switch accepts and analyzes the entire packet before forwarding it to its destination. It takes more time to examine the entire packet, but it allows catching certain packet errors and keeping them from propagating through the network.

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Routers: Router forwards packets from one LAN (or WAN) network to another. It is also used at the edges of the networks to connect to the Internet.

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Gateway: Gateway acts like an entrance between two different networks. Gateway in organisations is the computer that routes the traffic from a work station to the outside network that is serving web pages. ISP (Internet Service Provider) is the gateway for Internet service at homes.

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ARP:Address Resolution Protocol (ARP) is a protocol for mapping an Internet Protocol address (IP address) to a physical machine address that is recognized in the local network. For example, in IP Version 4, the most common level of IP in use today, an address is 32 bits long. In an Ethernet local area network, however, addresses for attached devices are 48 bits long. (The physical machine address is also known as a Media Access Control or MAC address.) A table, usually called the ARP cache, is used to maintain a correlation between each MAC address and its corresponding IP address. ARP provides the protocol rules for making this correlation and providing address conversion in both directions.

There are four types of arp messages that may be sent by the arp protocol. These are identified by four values in the “operation” field of an arp message. The types of message are:
1)ARP request
2)ARP reply
3)RARP request
4)RARP reply

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Frame Relay:

Frame Relay is a standardized wide area network technology that operates at the physical and logical link layers of OSI model. Frame relay originally designed for transport across Integrated Services Digital Network (ISDN) infrastructure, it may be used today in the context of many other network interfaces.
Frame relay is an example of a packet switched technology. Packet switched network enables end stations to dynamically share the network medium and the available bandwidth.
Frame Relay is often described as a streamlined version of X.25, it is because frame relay typically operates over WAN facilities that offer more reliable connection services. Frame relay is strictly a layer 2 protocol suite, where as X.25 provides services at layer 3.
Some important characteristics of frame relay are,
  • It allows bursty data.
  • It allows the frame size 9000 bytes, which can accumulate all LANs.
  • It is less expensive than other traditional WANs.
  • It has error detection only at data link layer, there is no any flow control and error control.
  • There is also a retransmission policy if frame is damaged.
  • 56 kbps, 64 kbps, 128 kbps, 256 kbps, 512 kbps and 1.5 Mbps.

For most services, the network provides a permanent virtual circuit (PVC), which means that the customer sees a continuous, dedicated connection without having to pay for a full-time leased line, while the service provider figures out the route each frame travels to its destination and can charge based on usage. Switched virtual circuits (SVC), by contrast, are temporary connections that are destroyed after a specific data transfer is completed.In order for a frame relay WAN to transmit data, data terminal equipment (DTE) and data circuit-terminating equipment (DCE) are required. DTEs are typically located on the customer’s premises and can encompass terminals, routers, bridges and personal computers. DCEs are managed by the carriers and provide switching and associated services.

Frame Relay Virtual Circuits:

Frame Relay provides connection-oriented data link layer communications. This means that a defined communication exists between each pair of devices and that these connections are associated with a connection identifier (ID). This service is implemented by using a FR virtual circuit, which is a logical connection created between two DTE devices across a Frame Relay packet-switched network (PSN).Virtual circuits provide a bidirectional communication path from one DTE device to another and are uniquely identified by a data-link connection identifier (DLCI). A virtual circuit can pass through any number of intermediate DCE devices (switches) located within the Frame Relay PSN.

Frame Relay virtual circuits fall into two categories: switched virtual circuits (SVCs) and permanent virtual circuits (PVCs).

Switched Virtual Circuits (SVCs)

Switched virtual circuits (SVCs) are temporary connections used in situations requiring only sporadic data transfer between DTE devices across the Frame Relay network. A communication session across an SVC consists of the following four operational states:

Call setup—The virtual circuit between two Frame Relay DTE devices is established.

Data transfer—Data is transmitted between the DTE devices over the virtual circuit.

Idle—The connection between DTE devices is still active, but no data is transferred. If an SVC remains in an idle state for a defined period of time, the call can be terminated.

Call termination—The virtual circuit between DTE devices is terminated.

Permanent Virtual Circuits (PVCs)

Permanent virtual circuits (PVCs) are permanently established connections that are used for frequent and consistent data transfers between DTE devices across the Frame Relay network. Communication across a PVC does not require the call setup and termination states that are used with SVCs. PVCs always operate in one of the following two operational states:

Data transfer—Data is transmitted between the DTE devices over the virtual circuit.

Idle—The connection between DTE devices is active, but no data is transferred. Unlike SVCs, PVCs will not be terminated under any circumstances when in an idle state.

DTE devices can begin transferring data whenever they are ready because the circuit is permanently established.

X.25:

X.25 Packet Switched networks allow remote devices to communicate with each other over private digital links without the expense of individual leased lines. Packet Switching is a technique whereby the network routes individual packets of HDLC data between different destinations based on addressing within each packet. An X.25 network consists of a network of interconnected nodes to which user equipment can connect. The user end of the network is known as Data Terminal Equipment (DTE) and the carrier’s equipment is Data Circuit-terminating Equipment (DCE) . X.25 routes packets across the network from DTE to DTE.

X.25 Packet Switching

The X.25 standard corresponds in functionality to the first three layers of the Open Systems Interconnection (OSI) reference model for networking. Specifically, X.25 defines the following:

  • The physical layer interface for connecting data terminal equipment (DTE), such as computers and terminals at the customer premises, with the data communications equipment (DCE), such as X.25 packet switches at the X.25 carrier’s facilities. The physical layer interface of X.25 is called X.21bis and was derived from the RS-232 interface for serial transmission.
  • The data-link layer protocol called Link Access Procedure, Balanced (LAPB), which defines encapsulation (framing) and error-correction methods. LAPB also enables the DTE or the DCE to initiate or terminate a communication session or initiate data transfer. LAPB is derived from the High-level Data Link Control (HDLC) protocol.
  • The network layer protocol called the Packet Layer Protocol (PLP), which defines how to address and deliver X.25 packets between end nodes and switches on an X.25 network using permanent virtual circuits (PVCs) or switched virtual circuits (SVCs). This layer is responsible for call setup and termination and for managing transfer of packets.



IP Addressing

IP address is short for Internet Protocol (IP) address. An IP address an identifier for a computer or device on a TCP/IP network. Networks using the TCP/IP protocol route messages based on the IP address of the destination. Contrast with IP, which specifies the format of packets also called datagrams, and the addressing scheme.
An IP is a 32-bit number comprised of a host number and a network prefix, both of which are used to uniquely identify each node within a network.To make these addresses more readable, they are broken up into 4 bytes, or octets, where any 2 bytes are separated by a period. This is commonly referred to as dotted decimal notation.The first part of an Internet address identifies the network on which the host resides, while the second part identifies the particular host on the given network. This creates the two-level addressing hierarchy.All hosts on a given network share the same network prefix but must have a unique host number. Similarly, any two hosts on different networks must have different network prefixes but may have the same host number. Subnet masks are 32 bits long and are typically represented in dotted-decimal (such as 255.255.255.0) or the number of networking bits (such as /24).
*Class A addresses 127.0.0.0 to 127.255.255.255 cannot be used and is reserved for loopback and diagnostic functions.
The host’s formula will tell you how many hosts will be allowed on a network that has a certain subnet mask. The host’s formula is 2n – 2. The “n” in the host’s formula represents the number of 0s in the subnet mask, if the subnet mask were converted to binary.

Network Masks

A network mask helps you know which portion of the address identifies the network and which portion of the address identifies the node. Class A, B, and C networks have default masks, also known as natural masks, as shown here:

Class A: 255.0.0.0
Class B: 255.255.0.0
Class C: 255.255.255.0

An IP address on a Class A network that has not been subnetted would have an address/mask pair similar to: 8.20.15.1 255.0.0.0. In order to see how the mask helps you identify the network and node parts of the address, convert the address and mask to binary numbers.

8.20.15.1 = 00001000.00010100.00001111.00000001
255.0.0.0 = 11111111.00000000.00000000.00000000

Once you have the address and the mask represented in binary, then identification of the network and host ID is easier. Any address bits which have corresponding mask bits set to 1 represent the network ID. Any address bits that have corresponding mask bits set to 0 represent the node ID.

8.20.15.1 = 00001000.00010100.00001111.00000001
255.0.0.0 = 11111111.00000000.00000000.00000000
            -----------------------------------
             net id |      host id             

netid =  00001000 = 8
hostid = 00010100.00001111.00000001 = 20.15.1

A subnet mask is what tells the computer what part of the IP address is the network and what part is for the host computers on that network. 

Subnetting

Subnetting is a process of breaking large network in small networks known as subnets. Subnetting happens when we extend default boundary of subnet mask. Basically we borrow host bits to create networks. Let’s take a example

Being a network administrator you are asked to create two networks, each will host 30 systems.Single class C IP range can fulfill this requirement, still you have to purchase 2 class C IP range, one for each network. Single class C range provides 256 total addresses and we need only 30 addresses, this will waste 226 addresses. These unused addresses would make additional route advertisements slowing down the network.With subnetting you only need to purchase single range of class C. You can configure router to take first 26 bits instead of default 24 bits as network bits. In this case we would extend default boundary of subnet mask and borrow 2 host bits to create networks. By taking two bits from the host range and counting them as network bits, we can create two new subnets, and assign hosts them. As long as the two new network bits match in the address, they belong to the same network. You can change either of the two bits, and you would be in a new subnet.

Advantage of Subnetting

  • Subnetting breaks large network in smaller networks and smaller networks are easier to manage.
  • Subnetting reduces network traffic by removing collision and broadcast traffic, that overall improve performance.
  • Subnetting allows you to apply network security polices at the interconnection between subnets.
  • Subnetting allows you to save money by reducing requirement for IP range.

CIDR [ Classless Inter Domain Routing]:CIDR is a slash notation of subnet mask. CIDR tells us number of on bits in a network address.

  • Class A has default subnet mask 255.0.0.0. that means first octet of the subnet mask has all on bits. In slash notation it would be written as /8, means address has 8 bits on.
  • Class B has default subnet mask 255.255.0.0. that means first two octets of the subnet mask have all on bits. In slash notation it would be written as /16, means address has 16 bits on.
  • Class C has default subnet mask 255.255.255.0. that means first three octets of the subnet mask have all on bits. In slash notation it would be written as /24, means address has 24 bits on.

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Introduction of Computer Networks

Data Communication:When we communicate,when we share information.It can be local or remote.Local communication occur face to face while remote communication take place over distance.Data communication are the exchange of data between two devices via the some form of transmission medium such as wire cable.

Characteristics Of Data Communication

The Data communication must have major three fundamental characteristics:

  • Delivery
  • Accuracy
  • Time Line

1) Where Delivery means system must delivered data to correct destination. Data must be received by the intended device.

2) Accuracy mean data delivered in accurately. Means that data should not be altered during transmission.

3) Time line means data should be delivered in time. When data in form of video audio is transfer as they produced at same time to other location is called real time transition.

Types Of Data Communication

There ore two types of data communication

  • Serial communication
  • Parallel communication

Serial   communication

In telecommunication and computer science, serial communication is the process of sending data one bit at one time, sequentially  on a single wire, over a communication channel or computer bus. Serial is  a common communication protocol that is used by many devices. Serial communication has become the standard for intercomputer communication. Serial communication is used for all long-haul communication and most computer networks its save the costs of cable. Serial communication is a popular means of transmitting data between a computer and a peripheral device such as a programmable instrument or even another computer. its also easy to established and no extra devices are used because  most of  computers have one or more serial ports.Examples isR-232,Universal Serial Bus,R-423,PCI Express.

Parallel communication

Parallel communication is fast method of communication. in Parallel transmission transmit the data across a parallel wire. These Parallel wires are flat  constituting multiple, smaller cables. Each cable can carry a single bit of information . A parallel cable can carry group of data at the same time. In telecommunication and computer science, parallel communication is a method of sending several data signals over a communication link at one time.  Examples is Industry Standard Architecture(ISA),Parallel ATA,IEEE 1284,Conventional PCI.

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  • For synchronous data transfer, both sender and receiver access the data according to the same clock. Therefore, a special line for the clock signal is required. A master(or one of the senders) should provide clock signal to all the receivers in synchronous data transfer mode. Synchronous data transfer supports very high data transfer rate.
  • For asynchronous data transfer, there is no common clock signal between the senders and receivers. Therefore, the sender and the receiver first need to agree on a data transfer speed. This speed usually does not change after data transfer starts. The data transfer rate is slow in asynchronous data transfer..

Data Flow Communication between two devices can be simplex, half-duplex, or full-duplex:

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In simplex mode, the communication is unidirectional, as on a one-way street. Only one of the two devices on a link can transmit; the other can only receive. Keyboards and traditional monitors are examples of simplex devices. The keyboard can only introduce input; the monitor can only accept output. The simplex mode can use the entire capacity of the channel to send data in one direction.

In half-duplex mode, each station can both transmit and receive, but not at the same time. : When one device is sending, the other can only receive, and vice versa . The half-duplex mode is like a one-lane road with traffic allowed in both directions. When cars are traveling in one direction, cars going the other way must wait. Walkie-talkies and CB (citizens band) radios are both half-duplex systems. The half-duplex mode is used in cases where there is no need for communication in both directions at the same time; the entire capacity of the channel can be utilized for each direction.

In full-duplex mode,data transmission means that data can be transmitted in both directions on a signal carrier at the same time. For example, on a local area network with a technology that has full-duplex transmission, one workstation can be sending data on the line while another workstation is receiving data. Full-duplex transmission necessarily implies a bidirectional line (one that can move data in both directions).

Network:A network is a set of devices (often referred to as nodes) connected by communication links. A node can be a computer, printer, or any other device capable of sending and/or receiving data generated by other nodes on the network.

Type of Connection: A network is two or more devices connected through links. A link is a communications pathway that transfers data from one device to another. For visualization purposes, it is simplest to imagine any link as a line drawn between two points. For communication to occur, two devices must be connected in some way to the same link at the same time.

There are two possible types of connections:

a)Point-to-Point

b)Multipoint.

a)Point-to-Point: A point-to-point connection provides a dedicated link between two devices. The entire capacity of the link is reserved for transmission between those two devices. Most point-to-point connections use an actual length of wire or cable to connect the two ends, but other options, such as microwave or satellite links. When you change television channels by infrared remote control, you are establishing a point-to-point connection between the remote control and the television’s control system.

b)Multipoint: A multipoint (also called multidrop) connection is one in which more than two specific devices share a single link. In a multipoint environment, the capacity of the channel is shared, either spatially or temporally. If several devices can use the link simultaneously, it is a spatially shared connection. If users must take turns, it is a timeshared connection.

Network Topology:Network topology is the arrangement of the various elements of a computer or biological network. Essentially it is the topological structure of a network, and may be depicted physically or logically. Physical topology refers to the placement of the network’s various components, inducing device location and cable installation, while logical topology shows how data flows within a network, regardless of its physical design.

Devices on the network are referred to as ‘nodes.’ The most common nodes are computers and peripheral devices. Network topology is illustrated by showing these nodes and their connections using cables.

Factors to be taken into consideration while choosing a Network topology:

1)  Scale of your project (in terms of number of components to be connected).
2)  Amount of traffic expected on the network.
3)  Budget allotted for the network i.e. amount of money you are willing to invest.
4)  Required response time

Types of Network Topology:

1)Bus Topology

2)Ring Topology

3)Star Topology

4)Mesh Topology

5)Tree Topology

1)Bus Topology: In networking a bus is the central cable — the main wire — that connects all devices on a local-area network (LAN). It is also called the backbone. This is often used to describe the main network connections composing the Internet.  Bus networks are relatively inexpensive and easy to install for small networks. Ethernet systems use a bus topology.A signal from the source is broadcasted and it travels to all workstations connected to bus cable. Although the message is broadcasted but only the intended recipient, whose MAC address or IP address matches, accepts it. If the MAC /IP address of machine doesn’t match with the intended address, machine discards the signal. A terminator is added at ends of the central cable, to prevent bouncing of signals. A barrel connector can be used to extend it.

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Advantages of Bus Topology

  1. It is cost effective.
  2. Cable required is least compared to other network topology.
  3. Used in small networks.
  4. It is easy to understand.
  5. Easy to expand joining two cables together.

Disadvantages of Bus Topology

  1. Cables fails then whole network fails.
  2. If network traffic is heavy or nodes are more the performance of the network decreases.
  3. Cable has a limited length.
  4. It is slower than the ring topology.

2)Ring Topology:All the nodes are connected to each-other in such a way that they make a closed loop. Each workstation is connected to two other components on either side, and it communicates with these two adjacent neighbors. Data travels around the network, in one direction. Sending and receiving of data takes place by the help of TOKEN.

Token Passing: Token contains a piece of information which along with data is sent by the source computer. This token then passes to next node, which checks if the signal is intended to it. If yes, it receives it and passes the empty to into the network, otherwise passes token along with the data to next node. This process continues until the signal reaches its intended destination.
The nodes with token are the ones only allowed to send data. Other nodes have to wait for an empty token to reach them. This network is usually found in offices, schools and small buildings.

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Advantages of Ring Topology

  1. Transmitting network is not affected by high traffic or by adding more nodes, as only the nodes having tokens can transmit data.
  2. Cheap to install and expand

Disadvantages of Ring Topology

  1. Troubleshooting is difficult in ring topology.
  2. Adding or deleting the computers disturbs the network activity.
  3. Failure of one computer disturbs the whole network.

3)Star Topology: In a star network devices are connected to a central computer, called a hub. Nodes communicate across the network by passing data through the hub.

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Advantages of Star Topology

1)  As compared to Bus topology it gives far much better performance, signals don’t necessarily get transmitted to all the workstations. A sent signal reaches the intended destination after passing through no more than 3-4 devices and 2-3 links. Performance of the network is dependent on the capacity of central hub.
2)  Easy to connect new nodes or devices. In star topology new nodes can be added easily without affecting rest of the network. Similarly components can also be removed easily.
3)  Centralized management. It helps in monitoring the network.
4)  Failure of one node or link doesn’t affect the rest of network. At the same time its easy to detect the failure and troubleshoot it.

Disadvantages of Star Topology

1)  Too much dependency on central device has its own drawbacks. If it fails whole network goes down.
2)  The use of hub, a router or a switch as central device increases the overall cost of the network.
3)   Performance and as well number of nodes which can be added in such topology is depended on capacity of central device.

4)Mesh Topology:In a mesh network, devices are connected with many redundant interconnections between network nodes. In a true mesh topology every node has a connection to every other node in the network.

There are two types of mesh topologies:

Advantages of Mesh Topology

  1. Each connection can carry its own data load.
  2. It is robust.
  3. Fault is diagnosed easily.
  4. Provides security and privacy.

Disadvantages of Mesh Topology

  1. Installation and configuration is difficult.
  2. Cabling cost is more.
  3. Bulk wiring is required.

5)Tree Topology:Tree Topology integrates the characteristics of Star and Bus Topology. Earlier we saw how in Physical Star network Topology, computers (nodes) are connected by each other through central hub. And we also saw in Bus Topology, work station devices are connected by the common cable called Bus. After understanding these two network configurations, we can understand tree topology better. In Tree Topology, the number of Star networks are connected using Bus. This main cable seems like a main stem of a tree, and other star networks as the branches. It is also called Expanded Star Topology

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Advantages of Tree Topology

  1. Extension of bus and star topologies.
  2. Expansion of nodes is possible and easy.
  3. Easily managed and maintained.
  4. Error detection is easily done.

Disadvantages of Tree Topology

  1. Heavily cabled.
  2. Costly.
  3. If more nodes are added maintenance is difficult.
  4. Central hub fails, network fails.

6)Hybrid Topology:A hybrid topology is a type of network topology that uses two or more other network topologies, including bus topology, mesh topology, ring topology, star topology, and tree topology.

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Hybrid network topology has many advantages. Hybrid topologies are flexible, reliable, have increased fault tolerance. The new nodes can be easily added to the hybrid network, the network faults can be easily diagnosed and corrected without affecting the work of the rest of network. But at the same time hybrid topologies are expensive and difficult for managing.

Types of Network:

1)LAN:A LAN connects network devices over a relatively short distance. A networked office building, school, or home usually contains a single LAN, though sometimes one building will contain a few small LANs (perhaps one per room), and occasionally a LAN will span a group of nearby buildings. In TCP/IP networking, a LAN is often but not always implemented as a single IP subnet.A LAN typically relies mostly on wired connections for increased speed and security, but wireless connections can also be part of a LAN. High speed and relatively low cost are the defining characteristics of LANs.the maximum span of 10 km.

2)WAN:A wide area network, or WAN, occupies a very large area, such as an entire country or the entire world. A WAN can contain multiple smaller networks, such as LANs or MANs. The Internet is the best-known example of a public WAN.

3)MAN:A metropolitan area network (MAN) is a hybrid between a LAN and a WAN. Like a WAN, it connects two or more LANs in the same geographic area. A MAN, for example, might connect two different buildings or offices in the same city. However, whereas WANs typically provide low- to medium-speed access, MAN provide high-speed connections, such as T1 (1.544Mbps) and optical services.
The optical services provided include SONET (the Synchronous Optical Network standard) and SDH (the Synchronous Digital Hierarchy standard). With these optical services, carriers can provide high-speed services, including ATM and Gigabit Ethernet. These two optical services provide speeds ranging into the hundreds or thousands of megabits per second (Mbps). Devices used to provide connections for MANs include high-end routers, ATM switches, and optical switches.

4)PAN:

A Personal Area Network (PAN) is a computer network used for communication among computer devices, including telephones and personal digital assistants, in proximity to an individual’s body. The devices may or may not belong to the person in question. The reach of a PAN is typically a few meters. PANs can be used for communication among the personal devices themselves (intrapersonal communication), or for connecting to a higher level network and the Internet .

5)Campus Area Network – This is a network which is larger than a LAN, but smaller than an MAN. This is typical in areas such as a university, large school or small business. It is typically spread over a collection of buildings which are reasonably local to each other. It may have an internal Ethernet as well as capability of connecting to the internet.

6)Storage Area Network – This network connects servers directly to devices which store amounts of data without relying on a LAN or WAN network to do so. This can involve another type of connection known as Fibre Channel, a system similar to Ethernet which handles high-performance disk storage for applications on a number of professional networks.




OSI Layer

OSI (Open Systems Interconnection) is reference model for how applications can communicate over a network. A reference model is a conceptual framework for understanding relationships. The purpose of the OSI reference model is to guide vendors and developers so the digital communication products and software programs they create will interoperate, and to facilitate clear comparisons among communications tools. Most vendors involved in telecommunications make an attempt to describe their products and services in relation to the OSI model. And although useful for guiding discussion and evaluation, OSI is rarely actually implemented, as few network products or standard tools keep all related functions together in well-defined layers as related to the model. The TCP/IP protocols, which define the Internet, do not map cleanly to the OSI model.
                                                                                 
                                                                                     OSI layers
The main concept of OSI is that the process of communication between two endpoints in a telecommunication network can be divided into seven distinct groups of related functions, or layers. Each communicating user or program is at a computer that can provide those seven layers of function. So in a given message between users, there will be a flow of data down through the layers in the source computer, across the network and then up through the layers in the receiving computer. The seven layers of function are provided by a combination of applications, operating systems, network card device drivers and networking hardware that enable a system to put a signal on a network cable or out over Wi-Fi or other wireless protocol).
The seven Open Systems Interconnection layers are:

Layer 1: The Physical Layer :

  1. It is the lowest layer of the OSI Model.
  2. It activates, maintains and deactivates the physical connection.
  3. It is responsible for transmission and reception of the unstructured raw data over network.
  4. Voltages and data rates needed for transmission is defined in the physical layer.
  5. It converts the digital/analog bits into electrical signal or optical signals.
  6. Data encoding is also done in this layer.

Layer 2: Data Link Layer :

  1. Data link layer synchronizes the information which is to be transmitted over the physical layer.
  2. The main function of this layer is to make sure data transfer is error free from one node to another, over the physical layer.
  3. Transmitting and receiving data frames sequentially is managed by this layer.
  4. This layer sends and expects acknowledgements for frames received and sent respectively. Resending of non-acknowledgement received frames is also handled by this layer.
  5. This layer establishes a logical layer between two nodes and also manages the Frame traffic control over the network. It signals the transmitting node to stop, when the frame buffers are full.

Layer 3: The Network Layer :

  1. It routes the signal through different channels from one node to other.
  2. It acts as a network controller. It manages the Subnet traffic.
  3. It decides by which route data should take.
  4. It divides the outgoing messages into packets and assembles the incoming packets into messages for higher levels.

Layer 4: Transport Layer :

  1. It decides if data transmission should be on parallel path or single path.
  2. Functions such as Multiplexing, Segmenting or Splitting on the data are done by this layer
  3. It receives messages from the Session layer above it, convert the message into smaller units and passes it on to the Network layer.
  4. Transport layer can be very complex, depending upon the network requirements.
Transport layer breaks the message (data) into small units so that they are handled more efficiently by the network layer.

Layer 5: The Session Layer :

  1. Session layer manages and synchronize the conversation between two different applications.
  2. Transfer of data from source to destination session layer streams of data are marked and are re-synchronized properly, so that the ends of the messages are not cut prematurely and data loss is avoided.

Layer 6: The Presentation Layer :

  1. Presentation layer takes care that the data is sent in such a way that the receiver will understand the information (data) and will be able to use the data.
  2. While receiving the data, presentation layer transforms the data to be ready for the application layer.
  3. Languages(syntax) can be different of the two communicating systems. Under this condition presentation layer plays a role of translator.
  4. It performs Data compression, Data encryption, Data conversion etc.

Layer 7: Application Layer :

  1. It is the topmost layer.
  2. Transferring of files disturbing the results to the user is also done in this layer. Mail services, directory services, network resource etc are services provided by application layer.
  3. This layer mainly holds application programs to act upon the received and to be sent data.

Merits of OSI reference model:

  1. OSI model distinguishes well between the services, interfaces and protocols.
  2. Protocols of OSI model are very well hidden.
  3. Protocols can be replaced by new protocols as technology changes.
  4. Supports connection oriented services as well as connectionless service.

Demerits of OSI reference model:

  1. Model was devised before the invention of protocols.
  2. Fitting of protocols is tedious task.
  3. It is just used as a reference model.