chapter 2 data communications networks and protocols

Khoun Thavouth Data Communications: 

Khoun Thavouth Data Communications Chapter 2 Data Communications Networks and Protocols

Network: 

Network A network is a set of devices (often referred to as nodes) connected by communication links. 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.

Physical Structures: 

Physical Structures Type of Connection Point-to-Point Connection Point-to-Multipoint Connection Physical Topology Mesh Topology Star Topology Bus Topology Ring Topology

Type of Connection : 

Type 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. There are two possible types of connections: point-to-point and multipoint.

Point-to-Point Connection : 

Point-to-Point Connection 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.

Point-to-Point Connection : 

Point-to-Point Connection When you change television channels by infrared remote control, you are establishing a point-to-point connection between the remote control and the television's control system.

Multipoint Connection : 

Multipoint Connection A multipoint (also called multidrop ) connection is one in which more than two specific devices share a single link.

Multipoint Connection : 

Multipoint Connection 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 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

Mesh Topology : 

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

Mesh Topology : 

Mesh Topology To find the number of physical links in a fully connected mesh network with n nodes, we first consider that each node must be connected to every other node. We can use below formula: n(n-1)/2 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.

Advantages of Mesh Topology : 

Advantages of Mesh Topology A mesh offers several advantages over other network topologies. First, the use of 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. Second, a mesh topology is robust. If one link becomes unusable, it does not incapacitate the entire system.

Advantages of Mesh Topology : 

Advantages of Mesh Topology Third, there is the advantage of privacy or security. When every message travels along a dedicated line, only the intended recipient sees it. Physical boun-daries prevent other users from gaining access to messages. Finally, point-to-point links make fault identification and fault isolation easy. Traffic can be routed to avoid links with suspected problems. This facility enables the network manager to discover the precise location of the fault and aids in finding its cause and solution.

Disadvantages of Mesh Topology: 

Disadvantages of Mesh Topology The main disadvantages of a mesh are related to the amount of cabling and the number of I/O ports required. First, because every device must be connected to every other device, installation and reconnection are difficult. Second, the sheer bulk of the wiring can be greater than the available space (in walls, ceilings, or floors) can accommodate.

Disadvantages of Mesh Topology: 

Disadvantages of Mesh Topology Finally, the hardware required to connect each link (I/O ports and cable) can be prohibitively expensive. For these reasons a mesh topology is usually implemented in a limited fashion, for example, as a backbone connecting the main computers of a hybrid network that can include several other topologies.

Example of Mesh Topology: 

Example of Mesh Topology One practical example of a mesh topology is the connection of telephone regional offices in which each regional office needs to be connected to every other regional office.

Star Topology : 

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.

Star Topology : 

Star Topology

Advantages of Star Topology : 

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. This factor also makes it easy to install and reconfigure. Far less cabling needs to be housed, and additions, moves, and deletions involve only one connection: between that device and the hub.

Advantages of Star Topology : 

Advantages of Star Topology Other advantages include robustness. 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. As long as the hub is working, it can be used to monitor link problems and bypass defective links. Star topology has advantage in networking extension.

Disadvantages of Star Topology: 

Disadvantages of Star Topology 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. 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).

Example of Star Topology: 

Example of Star Topology The star topology is used in local-area networks (LANs). High-speed LANs often use a star topology with a central hub.

Bus Topology : 

Bus Topology A bus topology is multipoint. One long cable acts as a backbone to link all the devices in a network.

Bus Topology : 

Bus Topology Nodes are connected to the bus cable by drop lines and taps or sometime called T-Connector. The bus cable is end with a Terminator each end. A drop line is a connection running between the device and the main cable. A tap or T-Connector 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.

Bus Topology : 

Bus Topology 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 : 

Advantages of Bus Topology Advantages of a bus topology include ease of installation. Backbone cable can be laid along the most efficient path, then connected to the nodes by drop lines of various lengths. In this way, a bus uses less cabling than mesh or star topologies. In a star, for example, four network devices in the same room require four lengths of cable reaching all the way to the hub. In a bus, this redundancy is eliminated

Disadvantages of Bus Topology: 

Disadvantages of Bus Topology Disadvantages include difficult reconnection and fault isolation. Signal reflection at the taps can cause degradation in quality. This degradation can be controlled by limiting the number and spacing of devices connected to a given length of cable. Adding new devices may therefore require modification or replacement of the backbone.

Disadvantages of Bus Topology: 

Disadvantages of Bus Topology In addition, 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. Bus topology was the one of the first topologies used in the design of early local area networks. Ethernet LANs can use a bus topology, but they are less popular now because of its disadvantages.

Ring Topology : 

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.

Ring Topology : 

Ring Topology

Advantages of Ring Topology : 

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. In addition, fault isolation is simplified. Generally in a ring, a signal is circulating at all times. 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

Disadvantages of Ring Topology: 

Disadvantages of Ring Topology The only constraints are media and traffic consi-derations (maximum ring length and number of devices). However, unidirectional traffic can be a disadvan-tage. In a simple ring, a break in the ring (such as a disabled station) can disable the entire network. This weakness can be solved by using a dual ring or a switch capable of closing off the break.

Disadvantages of Ring Topology: 

Disadvantages of Ring Topology Ring topology was prevalent when IBM introduced its local area network Token Ring. Today, the need for higher-speed LANs has made this topology less popular.

Network Categories: 

Network Categories Today when we speak of networks, we are generally referring to three primary categories: Local Area Networks (LAN) Wide Area Networks (WAN) Metropolitan Area Network (MAN) The category into which a network falls is determined by its size.

Local Area Network: 

Local Area Network A local area network (LAN) is usually privately owned and links the devices in a single office, building, or campus.

Local Area Network: 

Local Area Network 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.

Local Area Network: 

Local Area Network

Local Area Network: 

Local Area Network 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. A common example of a LAN, found in many business environments, links a workgroup of task-related computers, for example, engineering workstations or accounting PCs.

Local Area Network: 

Local Area Network One of the computers may be given a large capacity disk drive and may become a server to clients. Software can be stored on this central server and used as needed by the whole group. In this example, the size of the LAN may be determined by licensing restrictions on the number of users per copy of software, or by restrictions on the number of users licensed to access the operating system.

Local Area Network: 

Local Area Network In addition to size, LANs are distinguished from other types of networks by their transmission media and topology. In general, a given LAN will use only one type of transmission medium. The most common LAN topologies are bus, ring, and star. Early LANs had data rates in the 4 to 16 megabits per second (Mbps) range. Today, however, speeds are normally 100 or 1000 Mbps.

Wide Area Network: 

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.

Wide Area Network: 

Wide Area Network

Wide Area Network: 

Wide Area Network

Wide Area Network: 

Wide Area Network We normally refer to the first as a switched WAN and to the second as a point-to-point WAN. The switched WAN connects the end systems, which usually comprise a router (internetworking connecting device) that connects to another LAN or WAN. The point-to-point WAN is normally a line leased from a telephone or cable TV provider that connects a home computer or a small LAN to an Internet Service Provider (lSP). This type of WAN is often used to provide Internet access.

Metropolitan Area Network: 

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.

Metropolitan Area Network: 

Metropolitan Area Network 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. Another example is the cable TV network that originally was designed for cable TV, but today can also be used for high-speed data connection to the Internet.

Interconnection of Networks: Internetwork: 

Interconnection of Networks: Internetwork Today, it is very rare to see a LAN, a MAN, or a WAN in isolation; they are connected to one another. When two or more networks are connected, they become an internetwork, or internet. As an example, assume that an organization has two offices, one on the east coast and the other on the west coast.

Interconnection of Networks: Internetwork: 

Interconnection of Networks: Internetwork The established office on the west coast has a bus topology LAN; the newly opened office on the east coast has a star topology LAN. The president of the company lives somewhere in the middle and needs to have control over the company from her home. To create a backbone WAN for connecting these three entities (two LANs and the president's computer), a switched WAN (operated by a service provider such as a telecom company) has been leased.

Interconnection of Networks: Internetwork: 

Interconnection of Networks: Internetwork To connect the LANs to this switched WAN, however, three point-to-point WANs are required. These point-to-point WANs can be a high-speed DSL line offered by a telephone company or a cable modern line offered by a cable TV provide.

Interconnection of Networks: Internetwork: 

Interconnection of Networks: Internetwork

Protocols and Standards: 

Protocols and Standards In this section, we define two widely used terms: protocols and standards. First, we define protocol, which is synonymous with rule. Then we discuss standards, which are agreed-upon rules.

Protocols: 

Protocols In computer networks, communication occurs between entities in different systems. An entity is anything capable of sending or receiving information. However, two entities cannot simply send bit streams to each other and expect to be understood. For communication to occur, the entities must agree on a protocol. A protocol is a set of rules that govern data communications. A protocol defines what is communicated, how it is communicated, and when it is communicated. The key elements of a protocol are syntax, semantics, and timing.

Protocols: 

Protocols Syntax : The term syntax refers to the structure or format of the data, meaning the order in which they are presented. For example, a simple protocol might expect the first 8 bits of data to be the address of the sender, the second 8 bits to be the address of the receiver, and the rest of the stream to be the message itself.

Protocols: 

Protocols Semantics : The word semantics refers to the meaning of each section of bits. How is a particular pattern to be interpreted, and what action is to be taken based on that interpretation? For example, does an address identify the route to be taken or the final destination of the message?

Protocols: 

Protocols Timing : The term timing refers to two characteristics: when data should be sent and how fast they can be sent. For example, if a sender produces data at 100 Mbps but the receiver can process data at only 1Mbps, the transmission will overload the receiver and some data will be lost.

Standards: 

Standards Standards are essential in creating and maintaining an open and competitive market for equipment manufacturers and in guaranteeing national and international interoperability of data and telecommunications technology and processes. Standards provide guidelines to manufacturers, vendors, government agencies, and other service providers to ensure the kind of interconnectivity necessary in today's marketplace and in international communications.

Standards: 

Standards For example: IEEE 802.11g Standard Connection: Wireless Frequency: 2.4 GHz Data Rate : 6, 9, 12, 18, 24, 36, 48, 54 Mbps Approximate indoor range: 38 meters Approximate outdoor range: 140 meters

Questions: 

Questions Describe point-to-point and multipoint connection. Describe Mesh topology. What are advantages and disadvantages of Mesh topology? Describe Star topology. What are advantages and disadvantages of Star topology? Describe Bus topology. What are advantages and disadvantages of Bus topology? Describe Ring topology. What are advantages and disadvantages of Ring topology?

Questions: 

Questions What is LAN, MAN, and WAN? What is internetwork? Give an example. Describe Protocol and its elements? Describe Standard and fine at least three standards.  The End