- A network can be defined as a group of computers and other devices connected in some ways so as to be able to exchange data.
- Each of the devices on the network can be thought of as a node; each node has a unique address.
- Addresses are numeric quantities that are easy for computers to work with, but not for humans to remember.
- Example: 220.127.116.11
- Some networks also provide names that humans can more easily remember than numbers.
So what is a computer network? A network is a collection (two or more) of computers that are connected together for sharing data and resources. Networks are commonly categorized by their geographical size.
A local area network (a LAN) is a group of computers and associated devices that share a common communication line and are typically located within a small geographic area, such as in your home or in an office building, or even a single department in a company. A local area network may connect as few as two or three users, as in a home network, or as many as several hundred users, as in a business.
A metropolitan area network (a MAN) is a bit more diverse, incorporating computers and LANs that are distributed over a few miles of area, as you would find between corporate buildings or on a college campus.
A wide area network (a WAN) is a widely geographically dispersed computer network that is composed of multiple LANs and MANs and can be spread across a country or around the globe. The Internet (sometimes simply referred to as “the Net”) is a worldwide network of many computer networks (LANs, MANs, and WANs) that is accessible to hundreds of millions of people worldwide.
Domain Name System (DNS)
The numerical version of the IP address is usually represented by a name or series of names called the domain name—for instance, www.someplace.com or ftp. filearchive.edu, which is mapped into a static IP address using the Domain Name System (DNS).
The DNS is a hierarchical database used for translating the domain name to an IP address. When your computer needs to translate a domain name into a numerical IP address, it asks a domain name server to provide this information.
The Internet Address is also called an IP address or Host Address. It is a logical address assigned to the network interface card in your computer. An IP address (where IP means Internet Protocol) is how one computer can find another computer on a network. Each node must know its own address on the network and that of any other computer with which it will communicate. The IP address is a 32-bit binary number that identifies each packet of information sent across the network. The IP address is usually expressed as four decimal numbers, each representing eight bits, separated by periods. This is sometimes known as the “dot address” or as “dotted quad notation.” For instance, in the example network shown in Figure 2-2, address 192.168.27.16 is the IP address of one of the machines. This is internally stored and used as the integer 3,232,242,448 which is 11000000.10101000.00011011.00010000. This can be represented in decimal numbers as shown
Discussions of network communications often center on what is known as a protocol stack. A protocol is the set of rules that computers (or other network devices) in a network use when they communicate. In essence, a protocol is the language the network devices use to talk to each other. A protocol stack is an abstract model that divides the network up into layers, based on functions and communication protocols used in those functions. Each layer in the stack only talks to the layer above or below it, using the protocols defined in those layers. As information is passed down the stack, it is encapsulated. Encapsulation is basically a process of adding a protocol specific header to the information received from the layer above. As information is passed up the stack, the header specific to the current layer is stripped off and the data is sent to the layer above. By adhering to this protocol stack concept, software and hardware can be designed without worrying about the details of what’s going on in all the layers, just the neighboring layers. Things become reusable, transportable, device independent.
Types of Networks
There are two principle kinds of networks: Wide Area Networks (WANs) and Local Area Networks (LANs).
- Cover cities, countries, and continents.
- Based on packet switching technology
- Examples of WAN technology: Asynchronous Transfer Mode (ATM), Integrated Services Digital Network (ISDN)
- Cover buildings or a set of closely related buildings.
- Examples of LAN technology: Ethernet, Token Ring, and Fiber Distributed Data Interconnect (FDDI).
- Ethernet LANs: based on a bus topology and broadcast communication
- Token ring LANs: based on ring topology
- FDDI LANs: use optical fibbers and an improved token ring mechanism based on two rings flowing in opposite directions.
According to the protocols involved, networks interconnection is achieved using one or several of the following devices: →Bridge: a computer or device that links two similar LANs based on the same protocol.
Router: a communication computer that connects different types of networks using different protocols.
B-router or Bridge/Router: a single device that combines both the functions of bridge and router.
Gateway: a network device that connects two different systems, using direct and systematic translation between protocols.
What Is an Internetwork?
An internetwork is a collection of individual networks, connected by intermediate networking devices, that functions as a single large network. Internetworking refers to the industry, products, and procedures that meet the challenge of creating and administering internetworks.
Open System Interconnection Reference Model
The Open System Interconnection (OSI) reference model describes how information from a software application in one computer moves through a network medium to a software application in another computer. The OSI reference model is a conceptual model composed of seven layers, each specifying particular network functions. The model was developed by the International Organization for Standardization (ISO) in 1984, and it is now considered the primary architectural model for intercomputer communications. The OSI model divides the tasks involved with moving information between networked computers into seven smaller, more manageable task groups. A task or group of tasks is then assigned to each of the seven OSI layers. Each layer is reasonably self-contained so that the tasks assigned to each layer can be implemented independently. This enables
the solutions offered by one layer to be updated without adversely affecting the other layers. The following list details the seven layers of the Open System Interconnection (OSI) reference model:
• Layer 7—Application
• Layer 6—Presentation
• Layer 5—Session
• Layer 4—Transport
• Layer 3—Network
• Layer 2—Data link
• Layer 1—Physical
Characteristics of the OSI Layers
The seven layers of the OSI reference model can be divided into two categories: upper layers and lower layers.
The upper layers of the OSI model deal with application issues and generally are implemented only in software. The highest layer, the application layer, is closest to the end user. Both users and application layer processes interact with software applications that contain a communications component. The term upper layer is sometimes used to refer to any layer above another layer in the OSI model.
The lower layers of the OSI model handle data transport issues. The physical layer and the data link layer are implemented in hardware and software. The lowest layer, the physical layer, is closest to the physical network medium (the network cabling, for example) and is responsible for actually placing information on the medium.
OSI Model and Communication Between Systems
Information being transferred from a software application in one computer system to a software application in another must pass through the OSI layers. For example, if a software application in System A has information to transmit to a software application in System B, the application program in System A will pass its information to the application layer (Layer 7) of System A. The application layer then passes the information to the presentation layer (Layer 6), which relays the data to the session layer
(Layer 5), and so on down to the physical layer (Layer 1). At the physical layer, the information is placed on the physical network medium and is sent across the medium to System B. The physical layer of System B removes the information from the physical medium, and then its physical layer passes the information up to the data link layer (Layer 2), which passes it to the network layer (Layer 3), and so on, until it reaches the application layer (Layer 7) of System B. Finally, the application layer of System B passes the information to the recipient application program to complete the communication process.
Interaction Between OSI Model Layers
A given layer in the OSI model generally communicates with three other OSI layers: the layer directly above it, the layer directly below it, and its peer layer in other networked computer systems. The data link layer in System A, for example, communicates with the network layer of System A, the physical layer of System A, and the data link layer in System B
OSI Model Physical Layer
The physical layer defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating the physical link between communicating network systems.
Physical layer specifications define characteristics such as voltage levels, timing of voltage changes, physical data rates, maximum transmission distances, and physical connectors. Physical layer implementations can be categorized as either LAN or WAN specifications.
OSI Model Data Link Layer
The data link layer provides reliable transit of data across a physical network link. Different data link layer specifications define different network and protocol characteristics, including physical addressing, network topology, error notification, sequencing of frames, and flow control. Physical addressing (as opposed to network addressing) defines how devices are addressed at the data link layer.
Network topology consists of the data link layer specifications that often define how devices are to be physically connected, such as in a bus or a ring topology. Error notification alerts upper-layer protocols that a transmission error has occurred, and the sequencing of data frames reorders frames that are transmitted out of sequence. Finally, flow control moderates the transmission of data so that the receiving device is not overwhelmed with more traffic than it can handle at one time.
OSI Model Network Layer
The network layer defines the network address, which differs from the MAC address. Some network layer implementations, such as the Internet Protocol (IP), define network addresses in a way that route selection can be determined systematically by comparing the source network address with the destination network address and applying the subnet mask. Because this layer defines the logical network layout, routers can use this layer to determine how to forward packets. Because of this, much of the design and configuration work for internetworks happens at Layer 3, the network layer.
OSI Model Transport Layer
The transport layer accepts data from the session layer and segments the data for transport across the network. Generally, the transport layer is responsible for making sure that the data is delivered error-free and in the proper sequence. Flow control generally occurs at the transport layer. Flow control manages data transmission between devices so that the transmitting device does not send more data than the receiving device can process. Multiplexing enables data from several applications to be transmitted onto a single physical link. Virtual circuits are established, maintained, and terminated by the transport layer. Error checking involves creating various mechanisms for detecting transmission errors, while error recovery involves acting, such as requesting that data be retransmitted, to resolve any errors that occur.
The transport protocols used on the Internet are TCP and UDP.
OSI Model Session Layer
The session layer establishes, manages, and terminates communication sessions. Communication sessions consist of service requests and service responses that occur between applications located in different network devices. These requests and responses are coordinated by protocols implemented at the session layer. Some examples of session-layer implementations include Zone Information Protocol (ZIP), the AppleTalk protocol that coordinates the name binding process; and Session Control Protocol (SCP), the DECnet Phase IV session layer protocol.
OSI Model Presentation Layer
The presentation layer provides a variety of coding and conversion functions that are applied to application layer data. These functions ensure that information sent from the application layer of one system would be readable by the application layer of another system. Some examples of presentation layer coding and conversion schemes include common data representation formats, conversion of character representation formats, common data compression schemes, and common data encryption schemes. Common data representation formats, or the use of standard image, sound, and video formats, enable the interchange of application data between different types of computer systems. Conversion schemes are used to exchange information with systems by using different text and data representations, such as EBCDIC and ASCII. Standard data compression schemes enable data that is compressed at the source device to be properly decompressed at the destination. Standard data encryption schemes enable data encrypted at the source device to be properly deciphered at the destination.
Presentation layer implementations are not typically associated with a particular protocol stack. Some well-known standards for video include QuickTime and Motion Picture
Experts Group (MPEG). QuickTime is an Apple Computer specification for video and audio, and MPEG is a standard for video compression and coding. Among the well-known graphic image formats are Graphics Interchange Format (GIF), Joint Photographic Experts Group (JPEG), and Tagged Image File Format (TIFF). GIF is a standard for compressing and coding graphic images. JPEG is another compression and coding standard for graphic images, and TIFF is a standard coding format for graphic images.
OSI Model Application Layer
The application layer is the OSI layer closest to the end user, which means that both the OSI application layer and the user interact directly with the software application.
This layer interacts with software applications that implement a communicating component. Such application programs fall outside the scope of the OSI model. Application layer functions typically include identifying communication partners, determining resource availability, and synchronizing communication.
When identifying communication partners, the application layer determines the identity and availability of communication partners for an application with data to transmit. When determining resource availability, the application layer must decide whether sufficient network resources for the requested communication exist. In synchronizing communication, all communication between applications requires cooperation that is managed by the application layer. Some examples of application layer implementations include Telnet, File Transfer Protocol (FTP), and Simple Mail Transfer Protocol (SMTP).
What Is a LAN?
A LAN is a high-speed data network that covers a relatively small geographic area. It typically connects workstations, personal computers, printers, servers, and other devices. LANs offer computer users many advantages, including shared access to devices and applications, file exchange between connected users, and communication between users via electronic mail and other applications.
LAN Transmission Methods
LAN data transmissions fall into three classifications: unicast, multicast, and broadcast. In each type of transmission, a single packet is sent to one or more nodes. In a unicast transmission, a single packet is sent from the source to a destination on a network. First, the source node addresses the packet by using the address of the destination node. The package is then sent onto the network, and finally, the network passes the packet to its destination.
A multicast transmission consists of a single data packet that is copied and sent to a specific subset of nodes on the network. First, the source node addresses the packet by using a multicast address. The packet is then sent into the network, which makes copies of the packet and sends a copy to each node that is part of the multicast address.
A broadcast transmission consists of a single data packet that is copied and sent to all nodes on the network. In these types of transmissions, the source node addresses the packet by using the broadcast address. The packet is then sent on to the network, which makes copies of the packet and sends a copy to every node on the network.
LAN topologies define the manner in which network devices are organized. Four common LAN topologies exist: bus, ring, star, and tree. These topologies are logical architectures, but the actual devices need not be physically organized in these configurations. Logical bus and ring topologies, for example, are commonly organized physically as a star. A bus topology is a linear LAN architecture in which transmissions from network stations propagate the length of the medium and are received by all other stations.
A ring topology is a LAN architecture that consists of a series of devices connected to one another by unidirectional transmission links to form a single closed loop. Both Token Ring/IEEE 802.5 and FDDI networks implement a ring topology. Figure depicts a logical ring topology.
A star topology is a LAN architecture in which the endpoints on a network are connected to a common central hub, or switch, by dedicated links. Logical bus and ring topologies are often implemented physically in a star topology, which is illustrated in Figure. A tree topology is a LAN architecture that is identical to the bus topology, except that branches with multiple nodes are possible in this case. Figure 2-5 illustrates a logical tree topology.