Dccn chapter 1

Chapter - 1

Overview of data communication and networking

  Data communication - Data communication refers to the exchange of data between a source and a receiver. Data communication is said to be local if communicating devices are in the same building or a similarly restricted geographical area.

The meanings of source and receiver are very simple. The device that transmits the data is known as source and the device that receives the transmitted data is known as receiver. Data communication aims at the transfer of data and maintenance of the data during the process but not the actual generation of the information at the source and receiver. 

Datum mean the facts information statistics or the like derived by calculation or experimentation. The facts and information so gathered are processed in accordance with defined systems of procedure. Data can exist in a variety of forms such as numbers, text, bits and bytes. The Figure is an illustration of a simple data communication system.

COMPONENTS OF A DATA COMMUNICATION  :

1. Message: It is the information or data to be communicated. It can consist of text, numbers, pictures, sound or video or any combination of these.

2. Sender: It is the device/computer that generates and sends that message.

3. Receiver: It is the device or computer that receives the message. The location of receiver computer is generally different from the sender computer. The distance between sender and receiver depends upon the types of network used in between.

4. Medium: It is the channel or physical path through which the message is carried from sender to the receiver. The medium can be wired like twisted pair wire, coaxial cable, fiber-optic cable or wireless like laser, radio waves, and microwaves.

5. Protocol: It is a set of rules that govern the communication between the devices. Both sender and receiver follow same protocols to communicate with each other.

1. Simplex:

In simplex mode, the communication is unidirectional, as on a one-way street. Only one of the two devices on a link can transmit; the other can only receive which can be represented in the following figure.

Keyboards and traditional monitors are examples of simplex devices. The keyboard can only introduce input; the monitor can only accept output. The simplex mode can use the entire capacity of the channel to send data in one direction.

2.Half Duplex

A half-duplex data communication system provides messages in both directions but only allows transfer in one direction at a time. Once a party begins sending a transmission, the receiver must wait until the signal stops before responding. If the two data transfers attempt to send at the same time, they both fail. For instance, if you talk on a CB radio, you press a button and speak. If the receiver attempts to press the button and speak at the same time, neither one of you hear either message. The system is not capable of sending both ways simultaneously.

3.Full Duplex

A full duplex is a communication that works both ways at the same time. Essentially, full duplex data communication is a set of two simplex channels, one works as a forward channel and the other as a reserve channel. The two channels link together at some point. An example of a full duplex communication system is a landline telephone. When talking on a telephone, both parties have the ability to speak at the same time. The data, carried both ways through the telephone line, runs simultaneously.

• Physical Structure

Types of connection - 

A network is two or more devices connected through links. A link is a communications pathway that transfers data from one device to another. For 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 multi point.

1. Point-to- point -

A point-to-point connection provides a dedicated link between two devices. The entire capacity of the link is reserved for transmission between those two devices. Most point-to-point connections use an actual length of wire or cable to connect the two ends, but other options, such as microwave or satellite links, are also possible which are shown in the following figure. 

2.Multipoint:

A multipoint (also called multidrop) connection is one in which more than two specific devices share a single link as shown in the following figure.

In a multi point 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 time shared connection.

 

Types of Topology:

The term 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 (usually called nodes) to one another.

There are four basic topologies possible: mesh, star, bus, and ring which are shown in the following figure.

 

1.Mesh Topology:

In a mesh topology, every device has a dedicated point-to-point link to every other device. The dedicated link carries traffic only between the two devices it connects. The number of physical links needed in a fully connected mesh network with n nodes are, n(n - 1). However, if each physical link allows communication in both directions (duplex mode), we can divide the number of links by 2. In other words, we can say that in a mesh topology, we need n(n -1) /2 duplex-mode links. To accommodate that many links, every device on the network must have n – 1 input/output (I/O) ports to be connected to the other n - 1 stations which are shown in the following figure:

Advantages of Mesh Topology:

1. The dedicated links guarantees that each connection can carry its own data load, thus eliminating the traffic problems that can occur when links must be shared by multiple devices.

2. A mesh topology is robust. If one link becomes unusable, it does not incapacitate the entire system.

3. Another advantage of Mesh topology is advantage of privacy or security. When every message travels along a dedicated line, only the intended recipient sees it. Physical boundaries prevent other users from gaining access to messages.

 4. point-to-point links make fault identification2 and fault isolation easy. Traffic can be routed to avoid links with suspected problems. This helps to discover the precise location of the fault and aids in finding its cause and solution.

Disadvantages of Mesh Topology: 

 1. Every device must be connected to every other device. So large amount of cabling and the number of I/O ports are required. So, the installation and reconnection are difficult.

 2. The sheer bulk of the wiring can be greater than the available space (in walls, ceilings, or floors) can accommodate.

 3. The hardware required to connect each link (I/O ports and cable) can be prohibitively expensive.

2.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. Unlike a mesh topology, a star topology does not allow direct traffic between devices. 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 as shown in the following Figure.

Advantages of Star Topology:

A star topology is less expensive than a mesh topology. In a star, each device needs only one link and one I/O port to connect it to any number of others.

 A star topology is robust. i.e If one link fails, only that link is affected. All other links remain active. This factor also lends itself to easy fault identification and fault isolation.

 Disadvantages Star Topology:

1.One big disadvantage of a star topology is the dependency of the whole topology on one single point, the hub. If the hub goes down, the whole system is dead.

 2.Although a star requires far less cable than a mesh, each node must be linked to a central hub. For this reason, often more cabling is required in a star than in some other topologies (such as ring or bus).

3.Bus Topology:

The preceding examples all describe point-to-point connections. A bus topology, on the other hand, is multipoint. One long cable acts as a backbone to link all the devices in a network which is shown in the following figure.

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. As a signal travels along the backbone, some of its energy is transformed into heat. Therefore, it becomes weaker and weaker as it travels farther and farther. For this reason there is a limit on the number of taps a bus can support and on the distance between those taps.

Advantages of Bus Topology: 

1. The main advantages of a bus topology is ease of installation. Backbone cable can be laid along the most efficient path, then connected to the nodes by drop lines of various lengths.

Disadvantages of Bus Topology:

 

1. The disadvantage of bus topology is difficult reconnection and fault isolation. A bus is usually designed to be optimally efficient at installation. It can therefore be difficult to add new devices. Signal reflection at the taps can cause degradation in quality.

 2. A fault or break in the bus cable stops all transmission, even between devices on the same side of the problem. The damaged area reflects signals back in the direction of origin, creating noise in both directions.

 

4. 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. A typical ring topology is as shown in the figure.

Advantages of Ring Topology:

 

A ring is relatively easy to install and reconfigure. Each device is linked to only its immediate neighbors (either physically or logically). To add or delete a device requires changing only two connections.

 A signal is circulating at all times (token) if one device does not receive a signal within specified period, it can issue an alarm. The alarm alerts the network operator to the problem and its location

 Disadvantages of Ring Topology:

 

The main disadvantage of ring topology is unidirectional traffic can be a disadvantage. In a simple ring, a break in the ring (such as a disabled station) can disable the entire network.

 

5. 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 the following figure.

If one link fails, only that link is affected. All other links remain active. This factor also lends itself to easy fault identification and fault isolation.

Types of Networks:

Network category is determined by its size, ownership, the distance it cover and its physical architecture. The types of networks are local-area networks and wide- area networks. The category into which a network falls is determined by its size. A LAN normally covers an area less than 2 miles, a WAN can be worldwide. Networks of a size in between are normally referred to as metropolitan area networks and span tens of miles.

 Personal area network :

A personal area network (PAN) is a computer network used for data transmission amongst devices such as computers, telephones, tablets and personal digital assistants. PANs can be used for communication amongst the personal devices themselves, or for connecting to a higher level network and the Internet (an uplink) where one master device takes up the role as gateway. A PAN may be carried over wired computer buses such as USB.

Local Area Network:

A Local Area Network (LAN) is usually privately owned and links the devices in a single office, building, or campus (see Figure 1.10). Depending on the needs of an organization and the type of technology used, a LAN can be as simple as two PCs and a printer in someone's home office; or it can extend throughout a company and include audio and video peripherals. Currently, LAN size is limited to a few kilometers.

LANs are designed to allow resources to be shared between personal computers or workstations. The resources to be shared can include hardware (e.g., a printer), software (e.g., an application program), or data.

Wide Area Network:

A Wide Area Network (WAN) provides long-distance transmission of data, image, audio, and video information over large geographic areas that may comprise a country, a continent, or even the whole world. A WAN can be as complex as the backbones that connect the Internet or as simple as a dial-up line that connects a home computer to the Internet.

 Metropolitan Area Network:

A Metropolitan Area Network (MAN) is a network with a size between a LAN and a WAN. It normally covers the area inside a town or a city. It is designed for customers who need a high-speed connectivity, normally to the Internet, and have endpoints spread over a city or part of city. A good example of a MAN is the part of the telephone company network that can provide a high-speed DSL line to the customer.

Different layers of OSI model

The OSI model is a layered framework for the design of network systems that allows communication between all types of computer systems. It consists of seven separate but related layers, each of which defines a part of the process of moving information across a network.

The different layers in OSI model are represented in the following figure.

1.Physical Layer:

The physical layer coordinates the functions required to carry a bit stream over a physical medium. It deals with the mechanical and electrical specifications of the interface and transmission medium. It also defines the procedures and functions that physical devices and interfaces have to perform for transmission to Occur. Figure shows the position of the physical layer with respect to the transmission medium and the data link layer.

 

The main functions of physical layer: 

1. Physical characteristics of interfaces and medium:

The physical layer defines the characteristics of the interface between the devices and the transmission medium. It also defines the type of transmission medium.

2. Representation of bits:

The physical layer data consists of a stream of bits (sequence of Os or 1s) with no interpretation. To be transmitted, bits must be encoded into signals--electrical or optical. The physical layer defines the type of encoding (how Os and I s are changed to signals).

3. Data rate:

The transmission rate-the number of bits sent each second-is also defined by the physical layer.

4. Synchronization of bits:

The sender and receiver not only must use the same bit rate but also must be synchronized at the bit level. In other words, the sender and the receiver clocks must be synchronized.

5. Line configuration:

The physical layer is concerned with the connection of devices to the media. In a point-to-point configuration, two devices are connected through a dedicated link. In a multipoint configuration, a link is shared among several devices.

6. Physical topology:

The physical topology defines how devices are connected to make a network. Devices can be connected by using a mesh topology (every device is connected to every other device), a star topology (devices are connected through a central device), a ring topology (each device is connected to the next, forming a ring), a bus topology (every device is on a common link), or a hybrid topology (this is a combination of two or more topologies).

7. Transmission mode: The physical layer also defines the direction of transmission between two devices: simplex, half-duplex, or full-duplex.

2. Data Link Layer:

The data link layer is responsible for moving frames from one hop (node) to the next. It makes the physical layer appear error-free to the upper layer (network layer). The following Figure shows the relationship of the data link layer to the network and physical layers.

The Data Link Layer Responsibilities:

1. Framing:

The data link layer divides the stream of bits received from the network layer into manageable data units called frames.

2. Physical addressing:

If frames are to be distributed to different systems on the network, the data link layer adds a header to the frame to define the sender and/or receiver of the frame. If the frame is intended for a system outside the sender's network, the receiver address is the address of the device that connects the network to the next one.

3. Flow control:

If the rate at which the data are absorbed by the receiver is less than the rate at which data are produced in the sender, the data link layer imposes a flow control mechanism to avoid overwhelming the receiver.

4. Error control:

The data link layer adds reliability to the physical layer by adding mechanisms to detect and retransmit damaged or lost frames. It also uses a mechanism to recognize duplicate frames. Error control is normally achieved through a trailer added to the end of the frame.

5. Access control:

When two or more devices are connected to the same link, data link layer protocols are necessary to determine which device has control over the link at any given time.

3.Network Layer:

The network layer is responsible for the delivery of individual packets from the source host to the destination host possibly across multiple networks (links). Whereas the data link layer oversees the delivery of the packet between two systems on the same network (links), the network layer ensures that each packet gets from its point of origin to its final destination. If two systems are connected to the same link, there is usually no need for a network layer. The following shows the relationship of the network layer to the data link and transport layers.

 

The Network Layer Responsibilities:

1. Logical addressing:

The physical addressing implemented by the data link layer handles the addressing problem locally. If a packet passes the network boundary, we need another addressing system to help distinguish the source and destination systems. The network layer adds a header to the packet coming from the upper layer that, among other things, includes the logical addresses of the sender and receiver.

2. Routing:

When independent networks or links are connected to create internetworks (network of networks) or a large network, the connecting devices (called routers or switches) route or switch the packets to their final destination. One of the functions of the network layer is to provide this mechanism.

4.Transport Layer:

The transport layer is responsible for process-to-process delivery of the entire message. A process is an application program running on a host. Whereas the network layer oversees source-to-destination delivery of individual packets, it does not recognize any relationship between those packets. It treats each one independently, as though each piece belonged to a separate message, whether or not it does. The transport layer, on the other hand, ensures that the whole message arrives intact and in order, overseeing both error control and flow control at the source-to-destination level.

The following Figure shows the relationship of the transport layer to the network and session layers.

 

The Transport Layer Responsibilities:

1. Service-point addressing:

Computers often run several programs at the same time. For this reason, source-to-destination delivery means delivery not only from one computer to the next but also from a specific process (running program) on one computer to a specific process (running program) on the other. The transport layer header must therefore include a type of address called a service-point address (or port address). The network layer gets each packet to the correct computer; the transport layer gets the entire message to the correct process on that computer.

2. Segmentation and reassembly:

A message is divided into transmittable segments, with each segment containing a sequence number. These numbers enable the transport layer to reassemble the message correctly upon arriving at the destination and to identify and replace packets that were lost in transmission.

3. Connection control:

The transport layer can be either connectionless or connection oriented. A connectionless transport layer treats each segment as an independent packet and delivers it to the transport layer at the destination machine. A connection oriented transport layer makes a connection with the transport layer at the destination machine first before delivering the packets. After all the data are transferred, the connection is terminated.

4. Flow control:

Like the data link layer, the transport layer is responsible for flow control. However, flow control at this layer is performed end to end rather than across a single link.

5. Error control:

Like the data link layer, the transport layer is responsible for error control. However, error control at this layer is performed process-to-process rather than across a single link: The sending transport layer makes sure that the entire message arrives at the receiving transport layer without error (damage, loss, or duplication). Error correction is usually achieved through retransmission

5.Session Layer:

The services provided by the first three layers (physical, data link, and network) are not sufficient for some processes. The session layer is the network dialog controller. It establishes, maintains, and synchronizes the interaction among communicating systems.

The session layer responsibilities:

1. Dialog control:

The session layer allows two systems to enter into a dialog. It allows the communication between two processes to take place in either half duplex (one way at a time) or full-duplex (two ways at a time) mode.

2. Synchronization:

The session layer allows a process to add checkpoints, or synchronization points, to a stream of data. For example, if a system is sending a file of 2000 pages, it is advisable to insert checkpoints after every 100 pages to ensure that each 100-page unit is received and acknowledged independently. In this case, if a crash happens during the transmission of page 523, the only pages that need to be resent after system recovery are pages 501 to 523. Pages previous to 501 need not be resent. The following Figure illustrates the relationship of the session layer to the transport and presentation layers.

6.Presentation Layer:

The presentation layer is concerned with the syntax and semantics of the information exchanged between two systems. The following Figure shows the relationship between the presentation layer and the application and session layers.

 

The Presentation Layer Responsibilities:

1. Translation:

The processes (running programs) in two systems are usually exchanging information in the form of character strings, numbers, and so on. The information must be changed to bit streams before being transmitted. Because different computers use different encoding systems, the presentation layer is responsible for interoperability between these different encoding methods. The presentation layer at the sender changes the information from its sender-dependent format into a common format. The presentation layer at the receiving machine changes the common format into its receiver-dependent format.

2. Encryption:

To carry sensitive information, a system must be able to ensure privacy. Encryption means that the sender transforms the original information to another form and sends the resulting message out over the network. Decryption reverses the original process to transform the message back to its original form.

3. Compression:

Data compression reduces the number of bits contained in the information. Data compression becomes particularly important in the transmission of multimedia such as text, Audio, and video.

7.Application Layer:

The application layer enables the user, whether human or software, to access the network. It provides user interfaces and support for services such as electronic mail, remote file access and transfer, shared database management, and other types of distributed information services. The following Figure shows the relationship of the application layer to the user and the presentation layer.

Where many application services available, the figure shows only three:

XAOO (message-handling services), X.500 (directory services), and file transfer, access, and management (FTAM). The user in this example employs XAOO to send an e-mail message.

The Application Layer Services:

1. Network virtual terminal:

A network virtual terminal is a software version of a physical terminal, and it allows a user to log on to a remote host. To do so, the application creates a software emulation of a terminal at the remote host. The user's computer talks to the software terminal which, in turn, talks to the host, and vice versa. The remote host believes it is communicating with one of its own terminals and allows the user to log on.

2. File transfer, access, and management:

This application allows a user to access files in a remote host (to make changes or read data), to retrieve files from a remote computer for use in the local computer, and to manage or control files in a remote computer locally

3. Mail services:

This application provides the basis for e-mail forwarding and storage. Directory services. This application provides distributed database sources and access for global information about various objects and services

Different Layers of TCP/IP

TCP/IP protocol suite is made of five layers: physical, data link, network, transport, and application.

The first four layers provide physical standards, network interfaces, inter networking, and transport functions that correspond to the first four layers of the OSI model. The three topmost layers in the OSI model, however, are represented in TCP/IP by a single layer called the application layer which is showing in the following figure.

The layers of the TCP/IP protocol suite contain relatively independent protocols that can be mixed and matched depending on the needs of the system. The term hierarchical means that each upper-level protocol is supported by one or more lower-level protocols.

At the transport layer, TCP/IP defines three protocols:

Transmission Control Protocol (TCP), User Datagram Protocol (UDP), and Stream Control Transmission Protocol (SCTP). At the network layer, the main protocol defined by TCP/IP is the Internetworking Protocol (IP). There are some other protocols that support data movement in this layer.

 1. Physical and Data Link Layers:

At the physical and data link layers, TCP/IP does not define any specific protocol. It supports all the standard and proprietary protocols. A network in a TCP/IP inter network can be a local-area network or a wide-area network.

2. Network Layer:

At the network layer (or, more accurately, the inter network layer), TCP/IP supports the Internet working Protocol. IP, in turn, uses four supporting protocols: ARP, RARP, ICMP, and IGMP.

a. Internet working Protocol (IP):

The Internet working Protocol (IP) is the transmission mechanism used by the TCP/IP protocols. It is an unreliable and connection less protocol-a best-effort delivery service. IP transports data in packets called data grams, each of which is transported separately. Data grams can travel along different routes and can arrive out of sequence or be duplicated. IP does not keep track of the routes and has no facility for reordering data grams once they arrive at their destination.
The limited functionality of IP should not be considered a weakness, however. IP provides bare-bones transmission functions that free the user to add only those facilities necessary for a given application and thereby allows for maximum efficiency.

b. Address Resolution Protocol:

The Address Resolution Protocol (ARP) is used to associate a logical address with a physical address. On a typical physical network, such as a LAN, each device on a link is identified by a physical or station address, usually imprinted on the network interface card (NIC). ARP is used to find the physical address of the node when its Internet address is known.

c. Internet Control Message Protocol:

The Internet Control Message Protocol (ICMP) is a mechanism used by hosts and gateways to send notification of data gram problems back to the sender. ICMP sends query and error reporting messages.

d. Internet Group Message Protocol:

The Internet Group Message Protocol (IGMP) is used to facilitate the simultaneous transmission of a message to a group of recipients.

3. Transport Layer:

Traditionally the transport layer was represented in TCP/IP by two protocols: TCP and UDP. IP is a host-to-host protocol, meaning that it can deliver a packet from one physical device to another. UDP and TCP are transport level protocols responsible for delivery of a message from a process (running program) to another process. A new transport layer protocol, SCTP, has been devised to meet the needs of some newer applications.

a. User Data gram Protocol:

The User Data gram Protocol (UDP) is the simpler of the two standard TCP/IP transport protocols. It is a process-to-process protocol that adds only port addresses, checksum error control, and length information to the data from the upper layer.

b. Transmission Control Protocol:

The Transmission Control Protocol (TCP) provides full transport-layer services to applications. TCP is a reliable stream transport protocol. The term stream, in this context, means connection-oriented: A connection must be established between both ends of a transmission before either can transmit data. 

At the sending end of each transmission, TCP divides a stream of data into smaller units called segments. Each segment includes a sequence number for reordering after receipt, together with an acknowledgment number for the segments received. Segments are carried across the internet inside of IP data grams. At the receiving end, TCP collects each data gram as it comes in and reorders the transmission based on sequence numbers.

c. Stream Control Transmission Protocol:

 The Stream Control Transmission Protocol (SCTP) provides support for newer applications such as voice over the Internet. It is a transport layer protocol that combines the best features of UDP and TCP.

 4. Application Layer:

The application layer in TCP/IP is equivalent to the combined session, presentation, and application layers in the OSI model. Many protocols are defined at this layer.

Peer-to-Peer Process:

Within a single machine, each layer calls upon the services of the layer just below it. Layer 3, for example, uses the services provided by layer 2 and provides services for layer 4. Between machines, layer x on one machine communicates with layer x on another machine. This communication is governed by an agreed-upon series of rules and conventions called protocols. The processes on each machine that communicate at a given layer are called peer-to-peer processes. Communication between machines is therefore a peer-to-peer process using the protocols appropriate to a given layer.

At the physical layer, communication is direct as shown in the following Figure. Device A sends a stream of bits to device B (through intermediate nodes). At the higher layers, however, communication must move down through the layers on device A, over to device B, and then back up through the layers. Each layer in the sending device adds its own information to the message it receives from the layer just above it and passes the whole package to the layer just below it.

 At layer I the entire package is converted to a form that can be transmitted to the receiving device. At the receiving machine, the message is unwrapped layer by layer, with each process receiving and removing the data meant for it. For example, layer 2 removes the data meant for it, and then passes the rest to layer 3. Layer 3 then removes the data meant for it and passes the rest to layer 4, and so on.

END OF CHAPTER 1 DCCN