WHAT IS REFERENCE MODELS & WHAT IS NETWORKING DEVICES

WHAT IS REFERENCE MODELS.?

One of the most difficult software to be developed is the networking software. Reference models were designed to standardize the layer of functions and activities. The following section describes one of the most important reference models used in Computer Networks.

OSI (Open System Interconnection) Model The OSI model is an abstract description for layered communications and computer network protocol design open system means that it can communicate with any other system that follows the specified standards, formats and semantics. Protocols give the rules that specify how the different parties may communicate. In its most basic form, it divides network architecture into seven layers which from top to bottom are the Application, Presentation, Session, Transport, Network, Data-Link, and Physical Layers. It is therefore often referred to as the OSI Seven Layer Model (Figure).

A layer is a collection of conceptually similar functions that provide services to the layer above it and receives service  from   the layer below it. On each layer an instance provides services to the instances at the layer above and requests service from the  layer below. For example, a layer that provides error-free communications across a network provides the path needed by  applications above it, while it calls the next lower layer to send and receive packets that make up the contents of the path. Any  two instances at one layer are connected by a horizontal protocol connection on that layer.

The following are the layers of OSI model:

                                                                                                            
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 In transmission side data flows from layer 7 to layer 1, then to cabling or suitable medium. When data reaches the reception side, it flows from layer 1 to layer 7.

Application Layer: This layer is the layer for user interaction. We must have application software for dealing with the data. Presentation Layer: It converts the data into suitable format. It does tasks like compression, decompression, encryption and decryption.

Session Layer: This layer manages connections between different application layers.

Transport Layer: This layer converts data into segments and re-assembles the data stream. TCP and UDP are the protocols used in this layer. In this layer, data is converted into so called segments.

Network Layer: This layer translates logical address into physical address. This layer also fixes the route for data path. Router works in this layer. In this layer data is called a packet.

Data-Link Layer: This layer provides physical identification of a device using Media Access Control Address. It adds source and destination address to packets and convert them into frames. This is the layer that provides error free transmission. Physical Layer: This layer provides the functional requirements for activating a physical link. In this layer, data is carried from one device to another. Now, we can better understand the OSI layer with an example. Consider that you have to send a word document to a different network or through internet. The following are the process that will take place:

 1. In the APPLICATION LAYER, the user can edit the file by using application software like a word processor.

 2. In the PRESENTATION LAYER, user can compress the word file by using WINRAR or WINZIP and convert the data into different format for example.zip or .rar. You can also convert the word document into different formats.

3. In the SESSION LAYER, the particular file has to be integrated with browser for attaching it to email or likewise clients.

4. In the TRANSPORT LAYER, data is converted to segments. Source IP and destination IP are added to each packet. Frame checks and parity bits are also added in this layer.

5. In the NETWORK LAYER, the data is handed over to a router. The router calculates the best path for data transmission

6. In the DATA-LINK LAYER, transmission errors are handled and also flow of data is regulated so that receivers are not swamped by fast senders.

7. In the PHYSICAL LAYER, frames are transmitted as bits through media such as Optical fibre.

 WHAT IS NETWORKING DEVICES.?

For creating a network, you need certain basic devices. This section details some of these basic devices which are used to form a network. Network Interface cards, Hubs, bridges, repeaters, and routers are the devices that let you connect one or more computers to other computers, networked devices, or to other networks. Each has two or more connectors called ports (this term presently is used in the context of hardware) into which you plug in the cables to make the connection. Let us discuss each of these communication devices in detail.

Network Interface Cards

The network interface card (NIC) provides the physical connection between the network and the computer. Most NICs are internal, with the card fitting into an expansion slot inside the computer. Network interface cards are a major factor in determining the speed and performance of a network. It is a good idea to use the fastest network card available for fast transfer of data. The most common network interface connection today is Ethernet cards. Ethernet cards Ethernet cards that contain connections for twisted pair cables have a RJ-45 connection. The Ethernet card is sometimes also called as network adapter card. Each for the Ethernet card is identified by a unique number called the Media Access Control (MAC) address. Your mobile phone on which you use internet, generally has a MAC address.

                                                                                                                  
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Modem

Modem is an acronym for modulator demodulator. The meaning of the word modulator is to change and the meaning of the word demodulator is to restore to an original condition. A modem is a communication device that converts (i.e., modulates)

binary signal into analog signals for transmission over telephone lines and converts (i.e., demodulates) these analog signals back into binary form at the receiving end. Figure  shows the data transmission through modem.

                                                                                                             

You can use a modem to send data and files to other computer users using standard telephone lines. You can transfer data, exchange electronic files, and even carry on a typed conversation in real time. Modems are of two types: Internal and External. Internal modems are hardware cards and External modems, are kept outside of your computer, connected either by a USB or Serial Port. Internal modems are good for general usage, as they take up less desk space, and do not require a power supply, and for most purposes, internal modems work fine. External modems tend to be slightly more expensive than internal modems. Many experts consider them superior because they contain lights which indicate how the modem is functioning. In addition, they can easily be moved from one computer to another. However, they do use up one COM port.

                                                                                                   
​Repeaters

When a signal travels a network cable (or any other medium of transmission), they lose strength, degrade and become distorted in a process that is called attenuation A repeater is a device that electrically amplifies the signal it receives and re-broadcasts it (Figure 1.17). They are used when the total length of your network cable exceeds the standards set for the type of cable being used. A good example of the use of repeaters would be in a local area network using a star topology with unshielded twisted-pair cabling. If a cable is long enough, the attenuation will finally make a signal unrecognizable by the receiver.

                                                                                                       

Bridge

Like a repeater, a bridge can join several LANs. However, a bridge can also divide a network to isolate traffic problems. For example, if the volume of traffic from one or two computers or a single department is flooding the network with data and slowing down entire operation, a bridge can isolate those computers or that department. A bridge (Figure 1.18) is used to connect two segment i.e., segment 1 (LAN 1) and segment 2 (LAN 2). Each segment can have several computer attached to it.

                                                                                          

Hub

A hub sends any data packet coming from one port to all other ports. It is up to the receiving computer to decide if the packet is for it. Typically used to connect segments of a local area network (LAN), a hub contains multiple ports. You can imagine packets going through a hub as messages going into a mailing list. The mail is sent out to everyone and it is up to the receiving party to decide if it is of interest. The biggest problem with hubs is their simplicity. Since every packet is sent out to every computer on the network, there is a lot of wasted transmission. This means that the network can easily become bogged down.

                                                                                 

Hubs are typically used on small networks where the amount of data going across the network is never very high. A hub is typically the least expensive, least intelligent, and least complicated of the hub, router and switches. Every computer connected to the hub ―sees‖ everything that every other computer on the hub sees.

Switches

A switch does essentially what a hub does but more efficiently. By paying attention to the traffic that comes across it, it can ―learn‖ where particular addresses are. For example, if it sees traffic from machine A coming in on port 2, it now knows that machine A is connected to that port and that traffic to machine A needs to only be sent to that port and not any of the others. The net result of using a switch over a hub is that most of the network traffic only goes where it needs to rather than to every port. On busy networks this can make the network significantly faster.

                                                                           
​A switch (or Switching Hub) is a device that can segment a larger local area network to reduce the traffic load. One should implement a switch when you have a network with 20 or more users that have bogged down the network by excess traffic. It splits the network into two or more segments with devices that normally talk with each other. Conceptually – switching takes data from one interface and delivers it to another interface.

Router

A router translates information from one network to another; it is similar to an intelligent bridge. Router selects the best path to route a message, based on the destination address and origin. The router can direct traffic to prevent head-on collisions, and is smart enough to know when to direct traffic along shortcuts. Routers can even ―listen‖ to the entire network to determine which sections are busiest—they can then redirect data around those sections until they are removed. If you have a LAN that you want to connect to the internet, you will need to purchase a router. In this case, the router serves as the translator between the information on your LAN and the internet. It also determines the best route to send the data over the internet. Routers maintain a map of the physical networks on a Internet (network) and forward data received from one physical network to other physical networks.

                                                                                       

 Gateway

If you are connected to the internet, you have to enter through a Gateway. A Gateway connects your smaller network to the internet. A gateway passes information from one network to another network as your information travels across the internet. Gateway Interconnects networks at higher layers than bridges or routers. A gateway usually supports address mapping from one network to another, and may also provide transformation of data between the environments to support end to end application connectivity. Gateway typically limits the interconnectivity of two networks to a subset of the application protocols supported on either one. Routers exemplify special cases of gateways. Gateway, also called protocol converters, can operate at any layer of the networking model. The job of a gateway is much more complex than that of a router or a switch. Typically, a gateway must convert one protocol into another. The main function of a gateway is to convert protocols among communications networks. A router by itself transfers, accepts and relays packets only across networks using similar protocol. A gateway, on the other hand, accepts data formatted for one protocol and convert it to data formatted for another protocol before forwarding it. A gateway can be implemented in hardware, software or both, but they are usually implemented by the software installed within a router. A gateway must understand the protocols used by each network linked into the router. Gateways are slower than bridges, switches and (non- gateway) routers. The computers of Internet users and the computers that serve pages to users are host nodes, while the nodes that connect the network in between are gateways. For example, the computers that control traffic between company networks or the computers used by internet service providers (ISPs) to connect users to the internet are gateway nodes.

Check Your Progress 2

1) Identify the characteristics of bus, ring and star topologies and write the best characteristic that you think while designing a topology.

The basic characteristic of topology is its organization. This leads to properties that may be used to differentiate them. Some of these characteristics are: Network Scalability, Cost, Length of Cable etc.

2) Differentiate amongst LAN, MAN and WAN in terms of their coverage area.

  LAN – about 2 Kms it is high speed network; MAN up to 50 Kms - may use devices like Modem; WAN – covers large distances like states countries

3) Identify the examples of networks around you and classify them in terms of LAN, MAN and WAN.

LAN – Network in a University, Small office, Internet Cafes

MAN – Cable TV network WAN – Internet, VPN etc.

4) Differentiate between a bridge and a router.

A bridge connects several LANs. A bridge can also divide a network to isolate traffic or problems. Bridge Interconnects LAN segments at the network interface layer level and forwards frames between them.

A router translates information from one network to anther; it is more intelligent than a bridge. Routers select the best path   to route a message, based on the destination address and origin. In contrast to bridges few of the large routers may include   programs for their operations.

5) Explain the function of hub and where should we use this communication device?

A hub is the simplest of connection devices. Any data packet coming from one port is sent to all other ports. It is then up to the receiving computer to decide if the packet is for it. A hub is typically the least expensive, least intelligent, and least amount of date going across the network is network is never very high.

6) What is the function of gateway?

Gateway Interconnects networks at higher layers than bridges or routers. A gateway usually supports address mapping from one network to another, and may also provide transformation of the data between the environments to support end-to-end application connectivity. The main functionality of a gateway is to convert protocols among communications networks. A gateway on the other hand can accept a packet formatted for one protocol and convert it to packet formatted for another protocol before forwarding it. Gateways work on all seven OSI layers. 

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