Download Data communication and Internetworking and more Lecture notes Data Communication Systems and Computer Networks in PDF only on Docsity!
Module- Chapter-1 : “ Data communication , Data networking and the Internet” Syllabus : Chapter-1: Data communication, Data networking and the Internet A Communication model, Data communications, Networks, The internet. Chapter-2: Protocol Architecture Need for protocol architecture, TCP/IP protocol architecture, OSI model, TCP/IP Vs OSI model. 1 Introduction: Data communications : “ It deals with the transmission of signals in a reliable and efficient manner”. Or “Data communications are the exchange of data between two devices via some form of transmission medium such as a wire cable”. For data communications to occur, the communicating devices must be part of a communication system made up of a combination of hardware (physical equipment) and software (programs). Components of communication system Five components of a communication-system
- Message
- Sender
- Receiver
- Transmission-Medium
- Protocol (i) Message Message is the information (or data) to be communicated. Message may consist of number/text
picture or audio/video 2) Sender Sender is the device that sends the data-message Sender can be computer and mobile phone 3) Receiver Receiver is the device that receives the message. Receiver can be computer and mobile phone 4) Transmission Medium Transmission-medium is physical-path by which a message travels from sender to receiver. Transmission-medium can be wired or wireless. Examples of wired medium: twisted-pair wire (used in landline telephone), coaxial cable (used in cable TV network), fiber-optic cable Examples of wireless medium: radio waves, microwaves, infrared waves (ex: operating TV using remote control) 5) Protocol A protocol is a set of rules that govern data-communications. In other words, a protocol represents an agreement between the communicating-devices. Without a protocol, 2 devices may be connected but not communicating. Characteristics of Data communications: The effectiveness of a data communications system depends on four fundamental Characteristics: delivery, accuracy, timeliness, and jitter. i. Delivery: The system must deliver data to the correct destination. Data must be received by the intended device or user and only by that device or user. ii. Accuracy: The system must deliver the data accurately. Data that have been altered in transmission and left uncorrected are unusable. iii. Timeliness: The system must deliver data in a timely manner. Data delivered late are useless. In the case of video and audio, timely delivery means delivering data as they are produced, in the same order that they are produced, and without significant delay.
Transmitter Transmission system Receiver Destination Source : This device generates the data to be transmitted; Examples : Telephones and personal computers. Transmitter : The data generated by a source system are not transmitted directly in the form in which they were generated. Transmitter transforms and encodes the information in such a way as to produce electromagnetic signals that can be transmitted across some sort of transmission system. Example : A modem takes a digital bit stream from an attached device such as a personal computer and transforms that bit stream into an analog signal that can be handled by the telephone network. Transmission system : This can be a single transmission line or a complex network connecting source and destination. Receiver : The receiver accepts the signal from the transmission system and converts it into a form that can be handled by the destination device. Example : A modem will accept an analog signal coming from a network or transmission line and convert it into a digital bit stream. Destination : Takes the incoming data from the receiver. Communication tasks : The communication task that must be performed in a data communications system are listed below: (a) Transmission system utilization: Need to make efficient use of transmission facilities that are typically shared among a number of communicating devices. Various techniques Techniques like multiplexing are used to allow multiple users to share total capacity of a transmission medium Congestion control techniques may be required to assure that the system is not overwhelmed by traffic (b) Interface : To communicate, a device must interface with the transmission system.
All the forms of communication uses electromagnetic signals propagated over a transmission medium. (c) Signal generation : Electromagnetic signals travel over transmission medium. Once an interface is established, signal generation is required for communication. The properties of the signal, such as form and intensity, must be such that the signal is (1) Capable of being propagated through the transmission system, and (2) Interpretable as data at the receiver. (d) Synchronization: There must be some form of synchronization between transmitter and receiver. Synchronization means receiver must be able to determine when a signal begins to arrive and when it ends. It must also know the duration of each signal element. (e) Exchange management If data are to be exchanged in both directions over a period of time, the two parties must cooperate. Example: For two parties to engage in a telephone conversation, one party must dial the number of the other, causing signals to be generated that result in the ringing of the called phone. The called party completes a connection by lifting the receiver. For data processing devices, more will be needed than simply establishing a connection; certain conventions must be decided on. These conventions may include whether both devices may transmit simultaneously or must take turns, the amount of data to be sent at one time, the format of the data, and what to do if certain contingencies such as an error arise. (f) Error Detection and Correction: In all communications systems, there is a potential for error; transmitted signals are distorted to some extent before reaching their destination. Error detection and correction are required in circumstances where errors cannot be tolerated. This is usually the case with data processing systems. Example: In transferring a file from one computer to another, it is simply not acceptable for the contents of the file to be accidentally altered. (g) Flow - control : It is required to assure that the source does not overwhelm the destination by sending data faster than they can be processed and absorbed.
The personal computer is connected to some transmission medium, such as a local network or a telephone line, by an I/O device (transmitter), such as a local network transceiver or a modem. The input data are transferred to the transmitter as a sequence of voltage shifts [ g ( t )] representing bits on some communications bus or cable. The transmitter is connected directly to the medium and converts the incoming stream [ g ( t )] into a signal [ s ( t )] suitable for transmission. The transmitted signal s ( t ) presented to the medium is subject to a number of impairments before it reaches the receiver. Thus, the received signal r ( t ) may differ from s ( t ). The receiver will attempt to estimate the original s ( t ), based on r ( t ) and its knowledge of the medium, producing a sequence of bits These bits are sent to the output personal computer, where they are briefly buffered in memory as a block of bits. In many cases, the destination system will attempt to determine if an error has occurred and, if so, cooperate with the source system to eventually obtain a complete, error-free block of data. These data are then presented to the user via an output device, such as a printer or screen. The message as viewed by the user will usually be an exact copy of the original message ( m ). Figure 2 (a) : Simplified Data Communications Consider a telephone conversation. The message as viewed by the user will usually be an exact copy of the original message ( m ). In this case the input to the telephone is a message ( m ) in the form of sound waves. The sound waves are converted by the telephone into electrical signals of the same frequency. These signals are transmitted without modification over the telephone line. Hence the input signal g ( t ) and the transmitted signal s ( t ) are identical. The signals ( t ) will suffer some distortion over the medium, so that r ( t ) will not be identical to s ( t ). The signal r ( t ) is converted back into a sound wave with no attempt at correction or improvement of signal quality. Thus, is not an exact replica of m. However, the received sound message is generally comprehensible to the listener.
3.2 The Transmission of Information The basic building block of any communications facility is the transmission line. The manager is not concerned about technical detail like how information is encoded and transmitted across but the manager looks on whether the particular facility provides the required capacity, with acceptable reliability, at minimum cost. One of the basic choices facing a business user is the transmission medium. The two major approaches to increase the efficiency of transmission medium are multiplexing and compression. Multiplexing refers to the ability of a number of devices to share a transmission facility. Compression , as the name indicates, involves squeezing the data down so that a lower-capacity, cheaper transmission facility can be used to meet a given demand. 3.3 Transmission and Transmission Media Information can be communicated by converting it into an electromagnetic signal and transmitting that signal over some medium, such as a twisted-pair telephone line. The most commonly used transmission media are twisted-pair lines, coaxial cable, optical fiber cable, and terrestrial and satellite microwave. The data rates that can be achieved and the rate at which errors can occur depend on the nature of the signal and the type of medium. 3.4 Communication Techniques 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. The successful transmission of information involves a high degree of cooperation between the various components. 3.5 Transmission Efficiency A major cost in any computer/communications facility is transmission cost. It is important to maximize the amount of information that can be carried over a given resource or, alternatively, to minimize the transmission capacity needed to satisfy a given information communications requirement. Two ways of achieving this objective are multiplexing and compression.
4. Data Representation Information today comes in different forms such as text, numbers, images, audio and video. (a) Text : Text is represented as a bit-pattern. (Bit-pattern sequence of bits: 0s or 1s). Different sets of bit-patterns are used to represent symbols (or characters). Each set is called a code. The process of representing symbols is called encoding.
Entire-capacity of channel is used to send the data in one direction. (b) Half duplex : Both the stations can transmit as well as receive but not at the same time. (For ex: The half-duplex mode is like a one-lane road with 2 directional traffic). When one station is sending, the other can only receive and vice-versa. For example : Walkie-talkies Entire-capacity of a channel is used by one of the 2 stations that are transmitting the data. (c) Full duplex : Both stations can transmit and receive at the same time. (For ex: The full-duplex is like a 2-way street with traffic flowing in both directions at the same time). For Example : Mobile phones (When 2 people are communicating by a telephone line, both can listen and talk at the same time) Entire-capacity of a channel is shared by both the stations that are transmitting the data.
6. NETWORKS 6.1 Introduction “A network is defined as a set of devices interconnected by communication-links”. Or “A network is a set of devices called as nodes connected by communication links”. This interconnection among computers facilitates information sharing among them. Computers may connect to each other by either wired or wireless media (Often, devices are referred to as nodes). A node can be any device capable of sending/receiving data in the network. For example: 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. The best-known computer network is the Internet.
6.1 Network Criteria A network must meet following 3 criteria’s: (i) Performance Performance can be measured using i) Transit-time or ii) Response-time. Transit Time is defined as time taken to travel a message from one device to another. Response Time is defined as the time elapsed between enquiry and response. The network-performance depends on following factors: Number of users Type of transmission-medium Efficiency of software Often, performance is evaluated by 2 networking-metrics: i) throughput and ii) delay. Good performance can be obtained by achieving higher throughput and smaller delay times (ii) Reliability Reliability is measured by frequency of network-failure time taken to recover from a network-failure network's robustness in a disaster More the failures are, less is the network's reliability. (iii) Security Security refers to the protection of data from the unauthorized access or damage. It also involves implementing policies for recovery from data-losses. 6.2 Physical Structures: Type of Connection : Two types of connections 6.2.1 Point-to-Point Only two devices are connected by a dedicated-link Entire-capacity of the link is reserved for transmission between those two devices. Example: Point-to-Point connection b/w remote-control & TV for changing the channels. Figure : Point-to-point 6.2.2 Multi-point (Multi-drop) Three or more devices share a single link The capacity of the channel is shared, either spatially or temporally. If link is used simultaneously by many devices, then it is spatially shared connection.
Disadvantages:
- Difficult to detect and troubleshoot fault.
- Signal reflection at the taps can cause degradation in quality.
- A fault/break in the cable stops all transmission.
- There is a limit on i) Cable length ii) Number of nodes that can be connected.
- Security is very low because all the devices receive the data sent from the source. RING Topology: Each device is connected to the next, forming a ring There are only two neighbours for each device. Data travels around the network in one direction till the destination is reached. Sending and receiving of data takes place by the help of token. Each device has a repeater. A repeater receives a signal on transmission-medium & regenerates & passes the signal to next device. Advantages:
- Easy installation and reconfiguration. To add/delete a device, requires changing only 2 connections. 2)Fault isolation is simplified. If one device does not receive a signal within a specified period, it can issue an alarm. The alarm alerts the network-operator to the problem and its location.
- Congestion reduced: Because all the traffic flows in only one direction. Disadvantages:
- Unidirectional traffic.
- A fault in the ring/device stops all transmission. The above 2 drawbacks can be overcome by using dual ring.
- There is a limit on i) Cable length &
ii) Number of nodes that can be connected.
- Slower: Each data must pass through all the devices between source and destination. STAR Topology All the devices are connected to a central controller called a hub There exists a dedicated point-to-point link between a device & a hub. The devices are not directly linked to one another. Thus, there is no direct traffic between devices. The hub acts as a junction: If device-1 wants to send data to device-2, the device-1 sends the data to the hub, then the hub relays the data to the device-2. Advantages:
- Less expensive: Each device needs only one link & one I/O port to connect it to any devices.
- Easy installation & reconfiguration: Nodes can be added/removed w/o affecting the network.
- Robustness: If one link fails, it does not affect the entire system.
- Easy to detect and troubleshoot fault.
- Centralized management: The hub manages and controls the whole network. Disadvantages:
- Single point of failure: If the hub goes down, the whole network is dead.
- Cable length required is the more compared to bus/ring topologies.
- Number of nodes in network depends on capacity of hub. MESH Topology All the devices are connected to each other There exists a dedicated point-to-point link between all devices. There are n(n-1) physical channels to link n devices. Every device not only sends its own data but also relays data from other nodes. For ‘n’ nodes, there are n(n-1) physical-links there are n(n-1)/2 duplex-mode links Every device must have (n–1) I/O ports to be connected to the other (n-1) devices.
6.4 Network Types : Two popular types of networks:
- WAN (Wide Area Network)
- LAN (Local Area Network) 6.4.1 LAN LAN is used to connect computers in a single office, building or campus LAN is usually privately owned network. A LAN can be simple or complex.
- Simple: LAN may contain 2 PCs and a printer.
- Complex: LAN can extend throughout a company. Each host in a LAN has an address that uniquely defines the host in the LAN. A packet sent by a host to another host carries both source host’s and destination host’s addresses. LANs use a smart connecting switch. The switch is able to recognize the destination address of the packet & guide the packet to its destination. The switch reduces the traffic in the LAN & allows more than one pair to communicate with each other at the same time. **Advantages:
- Resource Sharing** Computer resources like printers and hard disks can be shared by all devices on the network. 2) Expansion Nowadays, LANs are connected to WANs to create communication at a wider level.
6.4.2 Wide Area Networks : WAN is used to connect computers anywhere in the world. WAN can cover larger geographical area. It can cover cities, countries and even continents. WAN interconnects connecting devices such as switches, routers, or modems. Normally, WAN is created & run by communication companies (Ex: BSNL, Airtel) leased by an organization that uses it. A WAN can be of 2 types: 1) Point-to-point WAN A point-to-point WAN is a network that connects 2 communicating devices through a transmission media 2) Switched WAN A switched WAN is a network with more than two ends. The switched WAN can be the backbones that connect the Internet. A switched WAN is a combination of several point-to-point WANs that are connected by switches 6.4.3 Internetwork
- A network of networks is called an internet. (Internet --> inter-network)
- For example (Assume that an organization has two offices, i) First office is on the east coast & ii) Second office is on the west coast. Each office has a LAN that allows all employees in the office to communicate with each other.
In a circuit-switching network, a dedicated communications path is established between two stations through the nodes of the network. That path is a connected sequence of physical links between nodes. i. On each link, a logical channel is dedicated to the connection. ii. Data generated by the source station are transmitted along the dedicated path as rapidly as possible. iii. At each node, incoming data are routed or switched to the appropriate outgoing channel without delay but may lead to congestion.
(ii) Packet Switching
Data is not sent along a dedicated path through the network. i. Data are sent out in a sequence of small chunks, called packets. ii. Each packet is passed through the network from node to node along some path leading from source to destination. iii. At each node, the entire packet is received, stored briefly, and then transmitted to the next node. iv. Packet-switching networks were designed with a data rate to the end user of about 64 kbps
(iii) Frame Relay
There is a considerable amount of overhead built into packet-switching schemes to compensate for errors(error detection). The overhead includes additional bits added to each packet to introduce redundancy and additional processing at the end stations and the intermediate switching nodes to detect and recover from errors. With modern high-speed telecommunications systems, this overhead is unnecessary. i. Frame relay was developed to take advantage of these high data rates and low error rates. ii. Frame relay uses variable-length packets , called frames , iii. Frame relay networks are designed to operate efficiently at user data rates of up to 2 Mbps. iv. The key to achieving these high data rates is to strip out most of the overhead involved with error control.
(iv)ATM(Asynchronous transfer mode)
It is also referred to as cell relay. i. It is a culmination of developments in circuit switching and packet switching. ii. ATM can be viewed as an evolution from frame relay and ATM uses fixed-length packets , called cells. iii. ATM provides little overhead for error control. iv. The result is that ATM is designed to work in the range of 10s and 100s of Mbps, and in the Gbps range. v. By using small, fixed-size cells, ATM is so efficient that it can offer a constant-data-rate channel even though it is using a packet-switching technique. vi. ATM extends circuit switching to allow multiple channels with the data rate on each channel dynamically set on demand. Differences between LANs and WANs:
- 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. For WANs, this is less often the case, or at least a significant fraction of the network assets is not owned. This has two implications. a) First, care must be taken in the choice of LAN, because there may be a substantial capital investment (compared to dial-up or leased charges for WANs) for both purchase and maintenance. b) Second, the network management responsibility for a LAN falls solely on the user.
- The internal data rates of LANs are typically much greater than those of WANs. 7. INTERNET 7.1 Origins of the Internet The Internet evolved from the ARPANET, which was 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. 7.2 Key Elements Figure 4(a) illustrates the key elements that comprise the Internet. The purpose of the Internet is to interconnect end systems , called hosts ; these include PCs, workstations, servers, mainframes, and so on.
