Data communication and computer networks Introduction

Data communication and computer networks Introduction What is computer network? A computer network is a group of computer systems and other computing hardware devices that are linked together through communication channels to facilitate communication and resource-sharing among a wide range of users. Networks are commonly categorized based on their characteristics. Reasons for setting up networks Networks are used to: Facilitate communication via email, video conferencing, instant messaging, etc. Enable multiple users to share a single hardware device like a printer or scanner Enable file sharing across the network Allow for the sharing of software or operating programs on remote systems Make information easier to access and maintain among network users Different types of Networks There are many types of networks, including: Local Area Networks (LAN) Personal Area Networks (PAN) Home Area Networks (HAN) Wide Area Networks (WAN) Campus Networks Metropolitan Area Networks (MAN) Enterprise Private Networks Internetworks Backbone Networks (BBN) Global Area Networks (GAN) The Internet Differences between WANs, LANs & MANs Coverage Area LANs cover a small area, typically a room or building MANs cover a larger area, typically a city or county WANs have no limit on size or area covered Difference in ownership LANs are private - not owned MANs can be private or public WANS can be private or public Difference in Transmission Rates (Speed) LANs typically have high data rates compared to WANs MANs have higher rates than LANs WANs have low data rates Network topologies Network topology refers to the physical or logical layout of a network. It defines the way different nodes are placed and interconnected with each other; alternately, network topology may describe how the data is transferred between these nodes. There are two types of network topologies: physical and logical. Physical topology emphasizes the physical layout of the connected devices and nodes, while the Logical topology focuses the pattern of data transfer between network nodes. Physical and network topologies can be categorized into five basic models: Bus Topology: All the devices/nodes are connected sequentially to the same backbone or transmission line. This is a simple, low-cost topology, but its single point of failure presents a risk. Star Topology: All the nodes in the network are connected to a central device like a hub or switch via cables. Failure of individual nodes or cables does not necessarily create downtime in the network but the failure of a central device can. This topology is the most preferred and popular model. Ring Topology: All network devices are connected sequentially to a backbone as in bus topology except that the backbone ends at the starting node, forming a ring. Ring topology shares many of bus topology's disadvantages so its use is limited to networks that demand high throughput. Tree Topology: A root node is connected to two or more sub-level nodes, which themselves are connected hierarchically to sub-level nodes. Physically, the tree topology is similar to bus and star topologies; the network backbone may have a bus topology, while the low-level nodes connect using star topology. Mesh Topology: The topology in each node is directly connected to some or all the other nodes present in the network. This redundancy makes the network highly fault tolerant but the escalated costs may limit this topology to highly critical networks. Ring Topology As the name implies, in a ring topology all the devices on a network are connected in the form of a ring or circle. This topic describes a ring topology. a ring type of topology has no beginning or end that needs to be terminated. In 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. In one implementation, a “token” travels around the ring, stopping at each device. If a device wants to transmit data, it adds that data and the destination address to the token. The token then continues around the ring until it finds the destination device, which takes the data out of the token. The advantage of using this type of method is that there are no collisions of data packets. What is a token 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. Advantages of Ring Topology 1)   This type of network topology is very organized. Each node gets to send the data when it receives an   empty token. This helps to reduces chances of collision. Also in ring topology all the traffic flows in only one direction at very high speed. 2)   Even when the load on the network increases, its performance is better than that of Bus topology. 3)   There is no need for network server to control the connectivity between workstations. 4)   Additional components do not affect the performance of network. 5)   Each computer has equal access to resources. Disadvantages of Ring Topology 1)   Each packet of data must pass through all the computers between source and destination. This makes it slower than Star topology. 2)   If one workstation or port goes down, the entire network gets affected. 3)   Network is highly dependent on the wire which connects different components. 4)   Network cards are expensive as compared to Ethernet cards and hubs. What is 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. Ethernet protocol is commonly used in this type of topology. The diagram below will make it clear. Tree Topology . Advantages of Tree Topology 1. It is an extension of Star and bus Topologies, so in networks where these topologies can't be implemented individually for reasons related to scalability, tree topology is the best alternative. 2. Expansion of Network is possible and easy. 3. Here, we divide the whole network into segments (star networks), which can be easily managed and maintained. 4. Error detection and correction is easy. 5. Each segment is provided with dedicated point-to-point wiring to the central hub. 6. If one segment is damaged, other segments are not affected. Disadvantages of Tree Topology 1. Because of its basic structure, tree topology, relies heavily on the main bus cable, if it breaks whole network is crippled. 2. As more and more nodes and segments are added, the maintenance becomes difficult. 3. Scalability of the network depends on the type of cable used. Hybrid Topology Hybrid, as the name suggests, is mixture of two different things. Similarly in this type of topology we integrate two or more different topologies to form a resultant topology which has good points (as well as weaknesses) of all the constituent basic topologies rather than having characteristics of one specific topology. This combination of topologies is done according to the requirements of the organization. For example, if there exists a ring topology in one office department while a bus topology in another department, connecting these two will result in Hybrid topology. Remember connecting two similar topologies cannot be termed as Hybrid topology. Star-Ring and Star-Bus networks are most common examples of hybrid network. Hybrid Network Topology Image Advantages of Hybrid Network Topology 1)  Reliable : Unlike other networks, fault detection and troubleshooting is easy in this type of topology. The part in which fault is detected can be isolated from the rest of network and required corrective measures can be taken, WITHOUT affecting the functioning of rest of the network. 2) Scalable: Its easy to increase the size of network by adding new components, without disturbing existing architecture. 3) Flexible: Hybrid Network can be designed according to the requirements of the organization and by optimizing the available resources. Special care can be given to nodes where traffic is high as well as where chances of fault are high. 4) Effective: Hybrid topology is the combination of two or more topologies, so we can design it in such a way that strengths of constituent topologies are maximized while there weaknesses are neutralized. For example we saw Ring Topology has good data reliability (achieved by use of tokens) and Star topology has high tolerance capability (as each node is not directly connected to other but through central device), so these two can be used effectively in hybrid star-ring topology. Disadvantages of Hybrid Topology 1)  Complexity of Design: One of the biggest drawbacks of hybrid topology is its design. Its not easy to design this type of architecture and its a tough job for designers. Configuration and installation process needs to be very efficient. 2)  Costly Hub: The hubs used to connect two distinct networks, are very expensive. These hubs are different from usual hubs as they need to be intelligent enough to work with different architectures and should be function even if a part of network is down. 3)  Costly Infrastructure: As hybrid architectures are usually larger in scale, they require a lot of cables, cooling systems, sophisticate network devices, etc. Bus topology Bus Topology is the simplest of network topologies. In this type of topology, all the nodes (computers as well as servers) are connected to the single cable (called bus), by the help of interface connectors. This central cable is the backbone of the network and is known as Bus (thus the name). Every workstation communicates with the other device through this Bus. 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. Advantages Bus Topology 1)  It is easy to set-up and extend bus network. 2)  Cable length required for this topology is the least compared to other networks. 3)  Bus topology costs very less. 4) Linear Bus network is mostly used in small networks. Good for LAN. Disadvantages Bus Topology 1)  There is a limit on central cable length and number of nodes that can be connected. 2)  Dependency on central cable in this topology has its disadvantages. If the main cable (i.e. bus) encounters   some problem, whole network breaks down. 3)  Proper termination is required to dump signals. Use of terminators is must. 4)  It is difficult to detect and troubleshoot fault at individual station. 5)  Maintenance costs can get higher with time. 6)  Efficiency of Bus network reduces, as the number of devices connected to it increases. 7)  It is not suitable for networks with heavy traffic. 8)  Security is very low because all the computers receive the sent signal from the source. Star topology In Star topology, all the components of network are connected to the central device called “hub” which may be a hub, a router or a switch. Unlike Bus topology (discussed earlier), where nodes were connected to central cable, here all the workstations are connected to central device with a point-to-point connection. So it can be said that every computer is indirectly connected to every other node by the help of “hub”. All the data on the star topology passes through the central device before reaching the intended destination. Hub acts as a junction to connect different nodes present in Star Network, and at the same time it manages and controls whole of the network. Depending on which central device is used, “hub” can act as repeater or signal booster. Central device can also communicate with other hubs of different network. Unshielded Twisted Pair (UTP) Ethernet cable is used to connect workstations to central node. 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. What is Mesh Topology? In a mesh network topology, each of the network node, computer and other devices, are interconnected with one another. Every node not only sends its own signals but also relays data from other nodes. In fact a true mesh topology is the one where every node is connected to every other node in the network. This type of topology is very expensive as there are many redundant connections, thus it is not mostly used in computer networks. It is commonly used in wireless networks. Flooding or routing technique is used in mesh topology. Types of mesh topologies Full mesh and partial mesh Full Mesh Topology Mesh Topology Diagram In this, like a true mesh, each component is connected to every other component. Even after considering the redundancy factor and cost of this network, its main advantage is that the network traffic can be redirected to other nodes if one of the nodes goes down. Full mesh topology is used only for backbone networks. 2) Partial Mesh Topology:- This is far more practical as compared to full mesh topology. Here, some of the systems are connected in similar fashion as in mesh topology while rests of the systems are only connected to 1 or 2 devices. It can be said that in partial mesh, the workstations are ‘indirectly’ connected to other devices. This one is less costly and also reduces redundancy. Advantages of Mesh topology 1) Data can be transmitted from different devices simultaneously. This topology can withstand high traffic. 2) Even if one of the components fails there is always an alternative present. So data transfer doesn’t get affected. 3) Expansion and modification in topology can be done without disrupting other nodes. Disadvantages of Mesh topology 1) There are high chances of redundancy in many of the network connections. 2) Overall cost of this network is way too high as compared to 3) Set-up and maintenance of this topology is very difficult. Even administration of the network is tough. Chapter two Communication Architectures Definition - What does Data Communications (DC) mean? Data communications (DC) is the process of using computing and communication technologies to transfer data from one place to another, and vice versa. It enables the movement of electronic or digital data between two or more nodes, regardless of geographical location, technological medium or data contents OR Data communication is the transmission of electronic data over some media. The media may be cables, microwaves. Data communications incorporates several techniques and technologies with the primary objective of enabling any form of electronic communication. These technologies include telecommunications, computer networking and radio/satellite communication. Data communication usually requires existence of a transportation or communication medium between the nodes wanting to communicate with each other, such as copper wire, fiber optic cables or wireless signals. For example, a common example of data communications is a computer connected to the Internet via a Wi-Fi connection, which uses a wireless medium to send and receive data from one or more remote servers. Some devices/technologies used in data communications are known as data communication equipment (DCE) and data terminal equipment ( DTE). DCE is used at the sending node, and DTE is used at the receiving node. Elements of Data Communication Four basic elements are needed for any communication system. 1.         Sender.   The computer or device that is used for sending data is called sender, source or transmitter. In modern digital communication system, the source is usually a computer. 2.         Medium.  The means through which data is sent from one location to another is called transmission medium. If the receiver and transmitter are within a building, a wire connects them. If they are located at different locations, they may be connected by telephone lines, fiber optics or microwaves. Receiver.   The device or computer that receives the data is called receiver. The receiver can be a computer, printer or a fax machine. 4.         Protocols.   There are rules under which data transmission takes place between sender and receiver. The data communication s/w are used to transfer data from one computer to another. The s/w follows same communication protocols can communicate and exchange data. What does Communication Media mean? Communication media refers to the means of delivering and receiving data or information. In telecommunication, these means are transmission and storage tools or channels for data storage and transmission. The term is also commonly used in place of mass media or news media. Different media are employed for transmitting data from one computer terminal to the central computer or to other computer systems inside some kind of network. Forms of communication media Analog Digital Analog Signal. The transfer of data in the form of electrical signals or continuous waves is called analog signal or analog data transmission. An analog signal is measured in volts and its frequency is in hertz (Hz). Advantages of Analog Signaling Allows multiple transmissions across the cable. Suffers less from attenuation. Disadvantages of Analog Signaling Suffers from EMI. Can only be transmitted in one direction without sophisticated equipment. Digital Signal The transfer of data in the form of digit is called digital signal or digital data transmission. Digital signals consist of binary digits 0 & 1. Electrical pulses are used to represent binary digits. Data transmission between computers is in the form of digital signals. Advantages of Digital Signaling Equipment is cheaper and simpler than analog equipment. Signals can be transmitted on a cable bidirectional. Digital signals suffer less from EMI. Disadvantages Digital Signaling Only one signal can be sent at a time. Digital signals suffer from attenuation. Techniques of Data Communication There are two possible techniques of sending data from the sender to receiver, i.e.:- (1) Parallel transmission. (2) Serial transmission. Parallel Transmission. In parallel transmission each bit of character / data has a separate channel and all bits of a character are transmitted simultaneously. Here the transmission is parallel character by character. Serial Transmission. In serial transmission, the data is sent as one bit at a time having a signal channel for all the bit. What does Media mean? Media refers to the tools used to store and deliver information or data. The term media can refer to advertising, digital media, electronic media, hypermedia, mass media and multimedia, among many others. Digital media is an electronic medium where data is stored in digital form. It refers to the technical aspect of the storage and transmission of information, or to the end product consumed by a user, such as digital video, digital art or virtual reality. Transformation of an analog signal to digital information through an analog-to-digital converter is called sampling. The majority of digital media is based on translating analog signals to digital data and vice versa. Digital media can be processed in a number of ways using standard computer hardware and software, where the performance is critical in high-performance digital hardware such as an application-specific integrated circuit (ASIC). The processing process involves editing, content creation and filtering. Electronic media use electronics or electro-mechanical energy to enable end users to access content. This is different from static media, which is often accessed by end users in printed form. Broadcasting is the distribution of audio and video content to a dispersed audience via radio, television, or other methods. Receiving parties may include the general public or a relatively large subset thereof. Electronic broadcasting includes telephone broadcasting, radio broadcasting, television broadcasting, cable radio, satellite television and webcasting. Communication channel The communication channel is used to link various computing devices so that they may interact with each other. The most commonly used data communication channels Wire pairs Coaxial cable Microwave transmission Communication satellites Fiber optics Contemporary communication media facilitate communication and data exchange among a large number of individuals across long distances via email, teleconferencing, Internet forums, etc. Traditional mass media channels such as TV, radio and magazines, on the other hand, promote one-to-many communication. Problems associated with communication The task of a communication is very complex. To give an understanding of the complexity, below is a small sample of the type of problems which must be solved by the communications software:- How to connect two users together - communications channel. How to represent signals on a communications channel. How to detect and correct errors on channel to ensure error free transmission. How to allow users to gain access to the communications channel. How to route data to the correct user across a network. How to ensure the receiver interprets the data correctly - the receiving machine may differ from the sending machine. How to allow the user to run applications over the channel. Transmission Modes There are two transmission modes:- Asynchronous Transmission Synchronous Transmission Fundamental difference between the two modes is :- Asynchronous Transmission With asynchronous transmission signal timing is not required; signals are sent in an agreed pattern of bits and if both ends are agreed on the pattern then communication can take place. Bits are grouped together and consist of both data and control bits. If the signal is not synchronised the receiver will not be able to distinguish when the next group of bits will arrive. To overcome this the data is preceded by a start bit, usually binary 0, the byte is then sent and a stop bit or bits are added to the end. Each byte to be sent now incorporates extra control data. In addition to the control data small gaps are inserted between each chunk to distinguish each group. In asynchronous transmission each bit remains timed in the usual way. Therefore, at bit level the transmission is still synchronous (timed). However, the asynchronous transmission is applied at byte level, once the receiver realises that there is a chunk of incoming data timing synchronization takes place for the chunk of data. Asynchronous transmission is relatively slow due to the increased number of bits and gaps. It is a cheap and effective form of serial transmission and is particularly suited for low speed connections such as keyboard and mouse. Synchronous Transmission Synchronous transmission sends data as one long bit stream or block of data. There are no gaps in transmission; each bit is sent one after the other. The receiver counts the bits and reconstructs bytes. It is essential that timing is maintained as there are no start and stop bits and no gaps. Accuracy is dependent on the receiver keeping an accurate count of the bits as they come in. Synchronous transmission is faster than asynchronous because fewer bits have to be transmitted; ie: only data bits and no extra control bits. For this reason it is the choice for network communications links See all 3 photos Modes of Data Communication. The manner in which data is transmitted from one location to another location is called data transmission mode. There are three ways or modes for transmitting data from one location to another. These are: (1) Simplex. (2) Half duplex. (3) Full duplex. Half Duplex. In half duplex mode, data can be transmitted in both directions but only in one direction at a time. During any transmission, one is the transmitter and the other is receiver. So each time for sending or receiving data, direction of data communication is reversed, this slow down data transmission rate. In half duplex modes, transmission of data can be confirmed. Half Duplex Mode Wireless communication is an example of half duplex. Advantages of Half Duplex Costs less than full duplex. Enables for two way communications. Disadvantages of Half Duplex Costs more than simplex. Only one device can transmit at a time. Simplex Mode In simplex mode, data is transmitted in only one direction. A terminal can only send data and cannot receive it or it can only receive data but cannot send it. Simplex mode is usually used for a remote device that is meant only to receive data. It is not possible to confirm successful transmission of data in simplex mode. This mode is not widely used. Speaker, radio and television broadcasting are examples of simplex transmission, on which the signal is send from the transmission to your TV antenna. There is no return signal. Simplex Mode Advantages of Simplex Cheapest communication method. Disadvantage of Simplex Only allows for communication in one direction. Full Duplex. Full-duplex 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). Advantages of full duplex Speed Speed is a big advantage of a full duplex network infrastructure. When a device is set at half duplex, it can receive and transmit data, but not at the same time. With a full duplex network environment, that data can be sent and received at the same time. That can result in faster throughput speeds, fewer network bottlenecks and a marked increase in network performance Homogeneous Environment It can be much easier for network administrators to work with a single homogeneous environment. Troubleshooting is easier when every piece of network equipment, from the servers, switches and hubs in the data room to the terminals and computers on the floor, are set to full duplex. When a problem does occur, network personnel can run reports to identify problem clients and make the changes necessary to bring them back online. Compatibility Not all network equipment supports a full duplex setting. Many older routers, switches, hubs and network clients do not support full duplex, so network administrators need to check all their equipment carefully before moving forward with a full duplex network environment. Once the network is moved to full duplex, any devices that do not support full duplex will no longer be able to connect. The cost of replacing that old equipment can be quite high, and that expense should play a role in any decision to upgrade the network. Stability You may find that you have network clients that are capable of full duplexing, even if the underlying network infrastructure is not. If this is the case, switching that network client to a full duplex mode could cause instability across the entire local or wide area network. It can sometimes be difficult for network administrators to diagnose these types of issues, since they can be sporadic, occurring only when the problem terminal is sending or receiving data. Communication Architectures The result of breaking the overall communication problem down into a set of small, well defined tasks. is typically referred to as a communications architecture or structure. There are a number of such architectures, and two of the most widely used are :- OPEN SYSTEMS INTERCONNECTION MODEL ( OSI) Introduction The Open Systems Interconnection (OSI) model is a reference tool for understanding data communications between any two networked systems. It divides the communications processes into seven layers. Each layer both performs specific functions to support the layers above it and offers services to the layers below it. The three lowest layers focus on passing traffic through the network to an end system. The top four layers come into play in the end system to complete the process. This white paper will provide you with an understanding of each of the seven layers, including their functions and their relationships to each other. This will provide you with an overview of the network process, which can then act as a framework for understanding the details of computer networking. Since the discussion of networking often includes talk of “extra layers”, this paper will address these unofficial layers as well. Finally, this paper will draw comparisons between the theoretical OSI model and the functional TCP/IP model. Although TCP/IP has been used for network communications before the adoption of the OSI model, it supports the same functions and features in a differently layered arrangement. An Overview of the OSI Model A networking model offers a generic means to separate computer networking functions into multiple layers. Each of these layers relies on the layers below it to provide supporting capabilities and performs support to the layers above it. Such a model of layered functionality is also called a “protocol stack” or “protocol suite”. Protocols, or rules, can do their work in either hardware or software or, as with most protocol stacks, in a combination of the two. The nature of these stacks is that the lower layers do their work in hardware or firmware (software that runs on specific hardware chips) while the higher layers work in software. The Open System Interconnection model is a seven-layer structure that specifies the requirements for communications between two computers. The ISO (International Organization for Standardization) standard 7498-1 defined this model. This model allows all network elements to operate together, no matter who created the Protocols and what computer vendor supports them. The main benefits of the OSI model include the following: • Helps users understand the big picture of networking • Helps users understand how hardware and software elements function together • Makes troubleshooting easier by separating networks into manageable pieces • Defines terms that networking professionals can use to compare basic functional relationships on different networks • Helps users understand new technologies as they are developed • Aids in interpreting vendor explanations of product functionality Layer 1 – The Physical Layer The physical layer of the OSI model defines connector and interface specifications, as well as the medium (cable) requirements. Electrical, mechanical, functional, and procedural specifications are provided for sending a bit stream on a computer network. Components of the physical layer include: • Cabling system components • Adapters that connect media to physical interfaces • Connector design and pin assignments • Hub, repeater, and patch panel specifications • Wireless system components • Parallel SCSI (Small Computer System Interface) • Network Interface Card (NIC) In a LAN environment, Category 5e UTP (Unshielded Twisted Pair) cable is generally used for the physical layer for individual device connections. Fiber optic cabling is often used for the physical layer in a vertical or riser backbone link. The IEEE, EIA/TIA, ANSI, and other similar standards bodies developed standards for this layer. Note: The Physical Layer of the OSI model is only part of a LAN (Local Area Network). Layer 2 – The Data Link Layer Layer 2 of the OSI model provides the following functions: • Allows a device to access the network to send and receive messages • Offers a physical address so a device’s data can be sent on the network • Works with a device’s networking software when sending and receiving messages • Provides error-detection capability Common networking components that function at layer 2 include: • Network interface cards • Ethernet and Token Ring switches • Bridges NICs have a layer 2 or MAC address. A switch uses this address to filter and forward traffic, helping relieve congestion and collisions on a network segment. Bridges and switches function in a similar fashion; however, bridging is normally a software program on a CPU, while switches use Application-Specific Integrated Circuits (ASICs) to perform the task in dedicated hardware, which is much faster. Layer 3 – The Network Layer Layer 3, the network layer of the OSI model, provides an end-to-end logical addressing system so that a packet of data can be routed across several layer 2 networks (Ethernet, Token Ring, Frame Relay, etc.). Note that network layer addresses can also be referred to as logical addresses. Initially, software manufacturers, such as Novell, developed proprietary layer 3 addressing. However, the networking industry has evolved to the point that it requires a common layer 3 addressing system. The Internet Protocol (IP) addresses make networks easier to both set up and connect with one another. The Internet uses IP addressing to provide connectivity to millions of networks around the world. To make it easier to manage the network and control the flow of packets, many organizations separate their network layer addressing into smaller parts known as subnets. Routers use the network or subnet portion of the IP addressing to route traffic between different networks. Each router must be configured specifically for the networks or subnets that will be connected to its interfaces. Routers communicate with one another using routing protocols, such as Routing Information Protocol (RIP) and Open version of Shortest Path First (OSPF), to learn of other networks that are present and to calculate the best way to reach each network based on a variety of criteria (such as the path with the fewest routers). Routers and other networked systems make these routing decisions at the network layer. When passing packets between different networks, it may become necessary to adjust their outbound size to one that is compatible with the layer 2 protocol that is being used. The network layer accomplishes this via a process known as fragmentation. A router’s network layer is usually responsible for doing the fragmentation. All reassembly of fragmented packets happens at the network layer of the final destination system. Two of the additional functions of the network layer are diagnostics and the reporting of logical variations in normal network operation. While the network layer diagnostics may be initiated by any networked system, the system discovering the variation reports it to the original sender of the packet that is found to be outside normal network operation. The variation reporting exception is content validation calculations. If the calculation done by the receiving system does not match the value sent by the originating system, the receiver discards the related packet with no report to the sender. Retransmission is left to a higher layer’s protocol. Some basic security functionality can also be set up by filtering traffic using layer 3 addressing on routers or other similar devices. Layer 4 – The Transport Layer Layer 4, the transport layer of the OSI model, offers end-to-end communication between end devices through a network. Depending on the application, the transport layer either offers reliable, connection-oriented or connectionless, best-effort communications. Some of the functions offered by the transport layer include: • Application identification • Client-side entity identification • Confirmation that the entire message arrived intact • Segmentation of data for network transport • Control of data flow to prevent memory overruns • Establishment and maintenance of both ends of virtual circuits • Transmission-error detection • Realignment of segmented data in the correct order on the receiving side • Multiplexing or sharing of multiple sessions over a single physical link The most common transport layer protocols are the connection-oriented TCP Transmission Control Protocol (TCP) and the connectionless UDP User Datagram Protocol (UDP). Layer 5 – The Session Layer Layer 5, the session layer, provides various services, including tracking the number of bytes that each end of the session has acknowledged receiving from the other end of the session. This session layer allows applications functioning on devices to establish, manage, and terminate a dialog through a network. Session layer functionality includes: • Virtual connection between application entities • Synchronization of data flow • Creation of dialog units • Connection parameter negotiations • Partitioning of services into functional groups • Acknowledgements of data received during a session • Retransmission of data if it is not received by a device Layer 6 – The Presentation Layer Layer 6, the presentation layer, is responsible for how an application formats the data to be sent out onto the network. The presentation layer basically allows an application to read (or understand) the message. Examples of presentation layer functionality include: • Encryption and decryption of a message for security • Compression and expansion of a message so that it travels efficiently • Graphics formatting • Content translation • System-specific translation Layer 7 – The Application Layer Layer 7, the application layer, provides an interface for the end user operating a device connected to a network. This layer is what the user sees, in terms of loading an application (such as Web browser or e-mail); that is, this application layer is the data the user views while using these applications. Examples of application layer functionality include: • Support for file transfers • Ability to print on a network • Electronic mail • Electronic messaging • Browsing the World Wide Web TCP/IP MODEL OVERVIEW The OSI model describes computer networking in seven layers. While there have been implementations of networking protocol that use those seven layers, most networks today use TCP/IP. But, networking professionals continue to describe networking functions in relation to the OSI layer that performs those tasks. The TCP/IP model uses four layers to perform the functions of the seven-layer OSI model. The network access layer is functionally equal to a combination of OSI physical and data link layers (1 and 2). The Internet layer performs the same functions as the OSI network layer (3). Things get a bit more complicated at the host-to-host layer of the TCP/IP model. If the host-to-host protocol is TCP, the matching functionality is found in the OSI transport and session layers (4 and 5). Using UDP equates to the functions of only the transport layer of the OSI model. The TCP/IP process layer, when used with TCP, provides the functions of the OSI model’s presentation and application layers (6 and 7). When the TCP/IP transport layer protocol is UDP, the process layer’s functions are equivalent to OSI session, presentation, and application layers (5, 6, and 7). SWITCHING Switched network is made up of a series of interconnected nodes called switches. There are 3 types of switching methods Circuit switching Packet switching message switching Circuit switching Circuit Switching is generally used in the public networks. It came into existence for handling voice traffic in addition to digital data. How ever digital data handling by the use of circuit switching methods are proved to be inefficient. Packet switching In Packet Switching, messages are broken up into packets and each of which includes a header with source, destination and intermediate node address information. Individual Packets in packet switching technique take different routes to reach their respective destination. Independent routing of packets is done in this case for following reasons: Bandwidth is reduced by the splitting of data onto different routes for a busy circuit. For a certain link in the network, the link goes down during transmission the remaining packet can be sent through another route. Advantages switching methods Private Secure Not subject to congestion Disadvantages of circuit switching Inefficient use of bandwidth, pay for time call is connected regardless of amount of data Packet switching Shared use of high cost components Efficient use of bandwidth, Only pay for data in transit Disadvantages of circuit switching But not secure or private, subject to congestion  DELAYS ASSOCIATED WITH NETWORKS Propagation Delay Time taken for a signal to travel from the transmitter to the receiver Speed of light is the fastest a signal will propagate 3 X 108 m/sec through space 2 X 108 m/sec through copper Transmission Delay (Time) Time taken to put the bits on the transmission media Transmission speed of 2Mbps means 2 X 106 bits can be transmitted in 1 second Processing Delay Time taken to execute protocols check for errors Send Acks etc. Queuing Delay Time spent waiting in buffer for transmission Only in packet switched networks Increases as load on network increases Round Trip Delay Round trip delay is defined as the time between the first bit of the message being put onto the Transmission medium, and the last bit the acknowledgement being received back by the Transmitter. It is the sum of the all the delays detailed above. The round trip delay is a critical factor in the performance of packet switched protocols and networks. Indeed, it has been stated that a good algorithm for estimating the round trip delay is at the heart of a good packet switch protocol. PROPERTIES OF SIGNALS Bandwidth Capacity for a given system to transfer data over a connection. Bandwidth is a measurement of the width of a range of frequencies and is measured in hertz (Hz). In data networks bandwidth is normally specified as bits per second (BPS) Shannon-Hartley Theorem states that Dmax = Blog2(1 + S/N) where Dmax is the maximum bit rate B is the bandwidth in Hz and S/N is the signal to noise ratio All transmission mediums are degraded by ‘noise’. If the average power of the signal is given by S, and the average power of the noise is given by N, then the signal to noise ratio is given by S/N. The greater the value of S/N then the greater is the theoretical transmission rate of that medium. Signal distortion Attenuation Attenuation is gradual loss of signal strength over either wired or wireless network connections or decrease in the amplitude of the transmitted signal. Solution Attenuation increases with distance, repeaters must be used to restore signal to transmitted level. Attenuation increases with frequency, repeaters must also take this into consideration. Propagation delay The time required for a signal to travel from one point to another, generally from a transmitter through a medium to a receiver. Propagation delay is dependent on the nature of the electromagnetic signal, as not all signals travel at the same speed through a medium. Propagation delay also is influenced by the distance between the two points, the density of the medium, and the presence of passive devices such as loading coils that might increase the impedance of the medium Propagation delay varies with frequency. So various frequencies of a signal propagate at different rates. Clearly they will incur different amounts of delay. As bit rate increases, so does the probability of frequencies from one signal interfering with the next. The longer the transmission media then the more is the ‘spread’ of the component frequencies of the original transmitted square wave. Noise Noise is unwanted sound in signal in transmission Types of noise that affect media Thermal noise. All electronic components generate ‘noise’ internally; the level of this noise is related to the temperature of the electronic components, thus the term ‘thermal noise’ Atmospheric noise. This is electrical interference induced into the electronics as a result of external electromagnetic radiation. This includes interference from nearby electrical equipment, such as computers, mains switches and CRTs, and also interference from radio waves. These sources of interference are also known as RFI, or Radio Frequency Interference, and EMC, or Electro-Magnetic Coupling. RFI/EMC can be reduced by ‘good wiring practice’. This means ensuring that there are quality connections between cables, that good earth connections are made , and that protective shields are wrapped around transmission cables. Ringing. If cables are incorrectly terminated, then some of the energy in the transmitted signal is reflected back down the cable from which it came. This results in an effect that is similar to an optical interference pattern, with a characteristic ‘ringing’ distortion of the received signal. It is a major cause of distortion in high speed networks that are not correctly terminated, or have used the wrong (typically cheaper) cables.