Data Communication, Data Networking and the Internet PDF

UNIT-I Data communication, Data networking and the Internet A Communication Model The fundamental purpose of a communication system is the exchange of data between two parties as shown in the figure below. The key elements of this model are:  Source - generates data to be transmi...

UNIT-I Data communication, Data networking and the Internet A Communication Model The fundamental purpose of a communication system is the exchange of data between two parties as shown in the figure below. The key elements of this model are:  Source - generates data to be transmitted  Transmitter - converts data into transmittable signals  Transmission System - carries data from source to destination  Receiver - converts received signal into data  Destination - takes incoming data This simple narrative conceals a wealth of technical complexity. Table below lists a selection of the key tasks that must be performed in a data communication system.  Communication Tasks  Transmission system utilization - need to make efficient use of transmission facilities typically shared among a number of communicating devices.  A device must interface with the transmission system.  Once an interface is established, signal generation is required for communication.  There must be synchronization between transmitter and receiver, to determine when a signal begins to arrive and when it ends. There is a variety of requirements for communication between two parties that might be collected under the term exchange management.  Error detection and correction are required in circumstances where errors cannot be tolerated.  Flow control is required to assure that the source does not overwhelm the destination by sending data faster than they can be processed and absorbed.  Addressing and routing, so a source system can indicate the identity of the intended destination, and can choose a specific route through this network.  Recovery allows an interrupted transaction to resume activity at the point of interruption or to condition prior to the beginning of the exchange.  Message formatting has to do with an agreement between two parties as to the form of the data to be exchanged or transmitted.  Frequently need to provide some measure of security in a data communication system.  Network management capabilities are needed to configure the system, monitor its status, react to failures and overloads, and plan intelligently for future growth. Data Communication deals with the most fundamental aspects of the communication function, focusing on the transmission of signals in a reliable and efficient manner.  Figure below provides a new perspective on the communication model discussed earlier. Assume a PC user wants to send an email message ‘m’ to another user. The process is modeled as follows:  User keys in message ‘m’ comprising bits ‘g’ buffered in source PC memory.  Input data is transferred to I/O device (transmitter) as sequence of bits g(t) using voltage shifts.  Transmitter converts these into a signal s(t) suitable for transmission media being used.  Transmitted signal s(t) presented to the medium is subject to a number of impairments, so received signal r(t) may differ from s(t).  Receiver decodes signal recovering g’(t) as estimate of original g(t).  Decoded bits is buffered in destination PC memory as bits g’ being the received message m’. The Transmission of Information The basic building block of any communication facility is the transmission line. One of the basic choices facing a business user is the transmission medium. For use within the business premises, this choice is generally completely up to the business. For long-distance communication, the choice is generally but not always made by the long- distance carrier. In either case, changes in technology are rapidly changing the mix of media used. The ever-increasing capacity of fiber optic channels is making channel capacity a virtually free resource. However, switching is now becoming the bottleneck. The growing use of wireless transmission is a result of the trend toward universal personal telecommunication and universal access to communication. Despite the growth in the capacity and the drop in cost of transmission facilities, transmission services remain the most costly component of a communication budget for most businesses. Thus, the manager needs to be aware of techniques that increase the efficiency of the use of these facilities, such as multiplexing and compression.. Multiplexing refers to the ability of a number of devices to share a transmission facility. Compression involves squeezing the data down so that a lower-capacity, cheaper transmission facility can be used to meet a given demand. The transmission of information across a transmission medium involves more than simply inserting a signal on the medium. The technique used to encode the information into an electromagnetic signal must be determined. There are various ways in which the encoding can be done, and the choice affects performance and reliability. Types of Connections 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 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: point-to-point and multipoint. 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. 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. Physical Topology Physical topology refers to the way in which a network is laid out physically. Two or more devices connect to a link; two or more links form a topology. The topology of a network is the geometric representation of the relationship of all the links and linking devices to one another. There are four basic topologies possible: Point to point: Mesh and Star Multipoint: Bus and Ring Mesh Topology In a mesh topology, every device has a dedicated point-to-point link to every other device. The term dedicated means that the link carries traffic only between the two devices it connects. In a fully connected mesh network, we need n(n -1) /2 duplex-mode physical links, where ‘n’ is number of nodes of the network. Star Topology In a star topology, each device has a dedicated point-to-point link only to a central controller, usually called a hub. The devices are not directly linked to one another. The controller acts as an exchange: If one device wants to send data to another, it sends the data to the controller, which then relays the data to the other connected device. Bus Topology A bus topology is multipoint. One long cable acts as a backbone to link all the devices in a network. Nodes are connected to the bus cable by drop lines and taps. A drop line is a connection running between the device and the main cable. A tap is a connector that either splices into the main cable or punctures the sheathing of a cable to create a contact with the metallic core. Ring Topology In a ring topology, each device has a dedicated point-to-point connection with only the two devices on either side of it. A signal is passed along the ring in one direction, from device to device, until it reaches its destination. Each device in the ring incorporates a repeater. When a device receives a signal intended for another device, its repeater regenerates the bits and passes them along. Hybrid Topology A network can be hybrid. For example, we can have a main star topology with each branch connecting several stations in a bus topology as shown in Figure below: a star backbone with three bus networks. NETWORKS The number of computers in use worldwide is in the hundreds of millions, with pressure from users of these systems for ways to communicate among all these machines being irresistible. Advances in technology have led to greatly increased capacity and the concept of integration, allowing equipment and networks to deal simultaneously with voice, data, image, and even video. Have two broad categories of networks: Local Area Networks (LAN) and Wide Area Networks (WAN). Local Area Networks (LAN) A LAN is a communication network that interconnects a variety of devices and provides a means for information exchange among those devices.  The scope of the LAN is small, typically a single building or a cluster of buildings.  The LAN is owned by the same organization that owns the attached devices.  The internal data rates of LANs are typically much greater than other networks. Metropolitan Area Network (MAN)  Collection of LANs with the same geographical area, for instance a city.  A network of computers located at different sites within a large physical area, such as a city.  MAN often acts as a high speed network (although not as fast as LAN) to allow sharing of regional resources. Wide Area Networks (WAN)  Wide area networks generally cover a large geographical area, require the crossing of public right-of-ways, and rely at least in part on circuits provided by a common carrier.  It consists of a number of interconnected switching nodes. A transmission from any one device is routed through these internal nodes to the specified destination device.  WANs have been implemented using one of two technologies: Circuit switching and Packet switching. More recently, frame relay and ATM networks have assumed major roles. WANs have been implemented using the following technologies:  Circuit switching  Packet switching  Frame relays  Asynchronous Transfer Mode (ATM) Circuit switching In a circuit-switching network, a dedicated communication path is established between two stations through the nodes of the network. Path should be reserved in advance. That path is a connected sequence of physical links between nodes, with a logical channel dedicated to the connection. Data generated by the source station are transmitted along the dedicated path as rapidly as possible. The most common example of circuit switching is the telephone network. Packet switching A packet-switching network uses a quite different approach, without need to dedicate transmission capacity along a path through the network. Rather, data is sent in a sequence of small chunks, called packets. Each packet is passed through the network from node to node along some path leading from source to destination. At each node, the entire packet is received, stored briefly, and then transmitted to the next node. Packet-switching networks are commonly used for terminal-to-computer and computer- to-computer communication. Frame relay Packet switching was developed at a time when digital long distance transmission facilities exhibited a relatively high error rate compared to today’s facilities. As a result, there is a considerable amount of overhead built into packet-switching schemes to compensate for errors. With modern high-speed telecommunication systems, the rate of errors has been dramatically lowered and any remaining errors can easily be caught in the end systems by logic that operates above the level of the packet-switching logic. Frame relay was developed to take advantage of high data rates and low error rates on modern WAN links. The key to achieving these high data rates is to strip out most of the overhead involved with error control. Asynchronous transfer mode (ATM) Asynchronous transfer mode (ATM), is a culmination of developments in circuit switching and packet switching. ATM can be viewed as an evolution from frame relay. ATM uses fixed-length packets, called cells. As with frame relay, ATM provides little overhead for error control, depending on the inherent reliability of the transmission system and on higher layers of logic in the end systems to catch and correct errors. By using a fixed packet length, the processing overhead is reduced even further for ATM compared to frame relay. The result is that ATM is designed to work in the range of 10s and 100s of Mbps, and in the Gbps range. ATM allows the definition of multiple virtual channels with data rates that are dynamically defined at the time the virtual channel is created. THE INTERNET The Internet evolved from the ARPANET, developed in 1969 by the Advanced Research Projects Agency (ARPA) of the U.S. Department of Defense. It was the first operational packet-switching network. ARPANET began operations in four locations. Today the number of hosts is in the hundreds of millions and the number of users in the billions. ARPA started to develop methods and protocols for internetworking-communicating across arbitrary, multiple, packet-switched networks, eventually leading to the TCP (Transmission Control Protocol) and IP (Internet Protocol) protocols, which, in turn, formed the basis for the TCP/IP protocol suite. Key Elements The purpose of Internet, is to interconnect end systems, called hosts- these include PCs, workstations, servers, mainframes. Hosts are connected to a network, such as a LAN or a WAN. Networks are in turn connected by routers (Each router is attached to two or more networks). The Internet operates as follows:  A host may send data to another host anywhere on the Internet.  The source host breaks the data to be sent into a sequence of packets, called IP datagrams or IP packets.  Each packet includes a unique numeric address of the destination host. This address is referred to as an IP address, because the address is carried in an IP packet.  Based on this destination address, each packet travels through a series of routers and networks from source to destination.  Each router, as it receives a packet, makes a routing decision and forwards the packet along its way to the destination. Internet Architecture The Internet today is made up of thousands of overlapping networks. A key element of the Internet is the set of hosts attached to it. Hosts are sometimes grouped together in a LAN. Individual hosts and LANs are connected to an Internet service provider (ISP) through a point of presence (POP). The connection is made in a series of steps starting with the customer premises equipment (CPE). ISPs can be classified as regional or backbone, with peering links between. Internet Terminology Central Office (CO) The place where telephone companies terminate customer lines and locate switching equipment to interconnect those lines with other networks. Customer Premises Equipment (CPE) Equipment placed at the customer’s end of the telephone line and usually owned by the telephone company. Internet Service Provider (ISP) A company that provides other companies or individuals with access to, or presence on, the Internet. Network Access Point (NAP) A physical facility that provides the infrastructure to move data between connected networks. One of several major Internet interconnection points that serve to tie all the ISPs together. Network Service Provider (NSP) A company that provides backbone services to an Internet service provider (ISP). An ISP connects at a point called an Internet exchange (IX) to a regional ISP that in turn connects to an NSP backbone Point of Presence (POP) A site that has a collection of telecommunication equipment. An ISP POP is the edge of the ISP’s network; connections from users are accepted and authenticated here EXAMPLE CONFIGURATION Figure below illustrates some of the typical communication and network elements in use today. In the upper-left-hand portion of the figure, there is an individual residential user connected to an Internet service provider (ISP) through some sort of subscriber connection. The Internet consists of a number of interconnected routers that span the globe. The routers forward packets of data from source to destination through the Internet. The lower portion shows a LAN implemented using a single Ethernet switch. This is a common configuration at a small business or other small organization. **** End of Chapter 1 ****