Computer Networks

NETWORK BASICS
NETWORK CLASSIFICATIONS
NETWORK CONFIGURATIONS
LOCAL AREA NETWORKS AND WIDE AREA NETWORKS
ETHERNET ADVANCES
NETWORK REMOTE ACCESS DEVICES
BIBLIOGRAPHY

Computers are an essential part of daily operations for most business in developed countries. Businesses rely on their computers to store and track information, communicate with customers and suppliers, design and manufacture products, and more. Businesses, therefore, typically have multiple computers in an office and throughout their company. These computers are connected through networks that allow information to be shared between computers.

A computer network, as defined in the Merriam-Webster dictionary, is “a system of computers, peripherals, terminals, and databases connected by communications lines.” In other words, networks are used to connect computers to other computers, as well as to other devices such as printers, scanners, and fax machines. Networks can be used to connect devices in the same building or they can be used to connect devices that are miles apart. By far, the most well-known network in use today is the Internet. Many individuals and businesses around the world connect to the Internet on a daily basis. Other examples of networks include library card catalogs, the displays of flight arrival and departure times used at airports, and credit card readers at retail stores.

Networking technology advances, namely the Internet, allow a business to be available 24 hours a day, seven days a week. This immediate nature of business places tremendous pressure on the underlying network infrastructure. In the twenty-first century, organizations must understand and carefully manage a very large volume and a wide variety of network devices to protect the health and performance of these real-time processes. Organizations must also make sure that configuration policies comply with both internal and external standards.

NETWORK BASICS

Basic networks have nodes connected together using hubs. As a network grows, this configuration begins to present challenges of scalability, latency, and network failure. Scalability affects the hub; hub networks have limited shared bandwidth, which makes it difficult to grow significantly and maintain performance. Hub networks keep the IT (information technology) department busy, requiring frequent redesign as organizations grow.

Latency, another issue of the hub network, is the time it takes a packet to get to its destination. Each node has to wait to transmit to avoid collisions, which happen when more than one device transmits data at the same time. Therefore, as more nodes are added to the hub, the latency effect increases.

Network failure is a huge threat for hubs. Just one device on a hub can cause problems for other devices attached to the hub due to incorrect speed settings or excessive broadcasts.

NETWORK CLASSIFICATIONS

Computer networks are classified in a variety of ways. Some of the more standard methods include the following:

• Networks are classified according to scale. These include the personal area network (PAN), local area network (LAN), campus area network (CAN), metropolitan area network (MAN), and the wide area network (WAN)).
• The network is classified according to connection method such as optical fiber, Ethernet, wireless LAN, HomePNA, or power line communication.
• The network is classified by functional relationship, such as active networking, client-server, and peer-to-peer.
• The network is classified according to network topology. These types of networks include the bus network, star network, ring network, mesh network, star-bus network, and tree or hierarchical topology network.

NETWORK CONFIGURATIONS

A solid network configuration strategy ensures the proper configuration of routers and switches. The IT department is charged with defining and carefully managing a specific set of device configurations designed to protect the enterprise. The implementation of this network configuration management strategy and solution can help to effectively manage compliance-related tasks and all configuration changes, such as moves, adds, and deletes, as well as automate the typical auditing activities.

Networks can be set up many different ways depending on the number of devices, the distances between those devices, the transmission speed requirements, and other factors. The topology is how computers, printers, and other devices are connected over a network. It describes the layout of wires, devices, and routing paths. The most popular configurations, or topologies, include the bus, token ring, star, and star bus topologies.

Bus. With a bus configuration, each node is connected sequentially along the network backbone. A node is any device connected to the network, such as a computer, printer, or scanner. Backbone is the term used to describe the main cables to which the network segments are connected. Resistors are placed at each end of the network to ensure that the signal is terminated when it reaches the end. When one node sends information to another node through the network, the information travels along the backbone until it reaches the desired receiving node.

Ethernet bus topologies are relatively easy to install and don't require much cabling compared to the alternatives. Bus networks work best with a limited number of devices. If more than a few dozen computers are added to

a network bus, performance problems will likely result. In addition, if the backbone cable fails, the entire network basically becomes unusable.

Token Ring. With a ring configuration, each node is connected sequentially along the network backbone. However, unlike the bus configuration, the end of the network connects to the first node, forming a circuit. Nodes on a token ring take turns sending and receiving information. In the token ring topology, a token travels along the backbone with the information being sent. The node with the token sends information to the next node along the backbone. The receiving node reads the information addressed to it and then passes the token and any additional information to

the next node. This continues until the token and data make it back to the first node in the network.

To implement a ring network, one typically uses FDDI, SONET, or Token Ring technology. Ring topologies are found in some office buildings or school campuses.

Star. With a star configuration, each node is connected to a central hub via network segments. When one node sends information to another node, the information passes through the hub. The hub does not filter or route the information in any way; it simply serves as a connector between network segments. Many home networks use the star topology.

Star bus. With a star bus configuration, the hubs of multiple star networks are connected together via the backbone. This is the most common network configuration in use.

Tree topology. In a tree topology, multiple star topologies are integrated onto a bus. Only hub devices connect directly to the tree bus. Each hub is considered the root of a tree of devices. This approach is considered a bus/star hybrid; it supports future expandability of the network much better than a bus or a star alone.

Mesh topology. Mesh topologies take messages on any of several possible paths from source to destination. The Internet, and some other WANS, employ mesh routing. A full mesh is a mesh network where every device connects to every other.

LOCAL AREA NETWORKS AND WIDE AREA NETWORKS

A LAN, as the name implies, is a network that connects devices that are local, or relatively close to each other. Nodes on a LAN are usually in the same building. In TCP/IP networking, a LAN is often but not always implemented as a single IP subnet. A WAN, on the other hand, is used to connect nodes that could be miles apart. LANs generally transmit data faster than WANs, and are usually more reliable. A WAN is a geographically-dispersed collection of LANs. A network device called a router connects LANs to a WAN. In IP networking, the router maintains both a LAN address and a WAN address.

A WAN is different from a LAN. Most WANs, like the Internet, are not owned by any one organization but exist under collective or distributed ownership and management. WANs tend to use technology like ATM, Frame Relay, and X.25 for connectivity over the longer distances. Fiber-optic cables are used for both LANs and WANs.

While LANs and WANs are the most popular network types, other networks include:

• The Wireless Local Area Network, which is a LAN based on WiFi wireless network technology.
• A Metropolitan Area Network is a network spanning a physical area larger than a LAN but smaller than a WAN, such as a city. A MAN is typically owned and
  
  
  operated by a single entity such as a government body or a large corporation.
• A Campus Area Network is a network spanning multiple LANs but smaller than a MAN, such as on a university or local business campus.
• A Storage Area Network connects servers to data storage devices through a technology like fiber channel.
• A System Area Network links high-performance computers with high-speed connections in a cluster configuration. This network is also known as Cluster Area Network.

Ethernet networking. Ethernet is a LAN protocol (i.e., a set of rules that governs communications) developed in the mid-1970s by Bob Metcalfe and David Boggs at Xerox Corporation's Palo Alto Research Center. Today, Ethernet is the most widely used network technology in the world. The original Ethernet used a bus topology and provided for transfer rates of up to 10 million bits per second (Mbps). This Ethernet specification was modified slightly and became the Institute of Electrical and Electronics Engineering (IEEE) 802.3 standard, which helped solidify Ethernet as a widely-recognized, open international standard.

Although networks using Ethernet protocol generally connect devices over short distances, technological advances now allow Ethernet to connect devices that are miles apart. Ethernet is widely accepted and largely installed because it is simple and efficient and because network interface cards (NIC) for Ethernet can be easily installed in personal computers, workstations, or high-end computers. Furthermore, it can run on a variety of media, including fiber optic, twisted-pair, cable, and wireless connections.

Repeaters. When Ethernet was first implemented, most people used a copper coaxial cable. However, the maximum length of this cable was 500 meters, which was not long enough for some networks. To address this problem, network engineers used repeaters to connect several Ethernet segments.

Actual network devices that serve as repeaters usually have some other name. Active hubs, for example, are repeaters and are also called multiport repeaters. Most often, they are referred to as hubs.

Bridges. Bridges provide a simple means for connecting LANs. A bridge is a device that connects physically separate LAN segments (such as different Ethernet cables) into one logical LAN segment. Bridges filter data traffic at a network boundary. Bridges, which operate at the data link layer of the OSI model, reduce the amount of traffic on a LAN by dividing it into two segments.

There are four categories of bridges: transparent, source routing, encapsulating, and translating. Transparent bridges are used for Ethernet, whereas source routing bridges are used for token ring networks. Encapsulating bridges connect two segments of the same media (such as token ring to token ring) over a medium. The receiving bridge removes the envelope, checks the destination, and sends the frame to the destination device. Translating bridges are used to connect different types of network media such as Ethernet and FDDI (fiber distributed data interface). FDDI is a set of protocols that uses a modified form of the token-passing method over fiber-optic cable.

An Ethernet bridge, for example, inspects each incoming Ethernet frame—including the source and destination MAC addresses, and sometimes the frame size—to make individual forwarding decisions.

Routers. LAN segments joined by a router are physically and logically separate networks. In contrast to a bridge, when multiple network segments are joined by a router they maintain their separate logical identities (network address space), but constitute an internetwork.

Routers specify the destination and route for each packet, and they can be used to direct packets and interconnect a variety of network architectures. A major difference between a bridge and a router is that the bridge distinguishes packets by source and destination address, whereas a router can also distinguish packets by protocol type. Routers provide for the interfaces to WANs such as frame relay and packet switching services. Some new bridge products have added router capabilities; hence, the practical distinction is becoming blurred, giving rise to the term “brouter.”

Routers can also be used to limit access to a network by the type of application (e.g., allowing electronic mail to pass, but not file transfer traffic). This capability provides a measure of security for the network, and is used extensively when creating firewalls. Firewalls are implemented to secure an organization's network when it is linked to the Internet.

Switches. Ethernet communicates across the network using the Carrier Sense Multiple Access with Collision Detection (CSMA/CD) process. A protocol using CSMA/CD monitors, or listens to, the media for network traffic, or information traveling through the network from one node to another. If a node does not sense any traffic, it will send frames or packets of information onto the media. A network frame is like a mailed letter. The letter is put in an envelope that has a return address and the

address of its destination. Data are like the letter and the frame is like the envelope. The data is placed in the frame and the frame has the addressing information and error-checking code. Each protocol has its distinctive frame. The device continues sending until it finishes or until a collision occurs.

A collision happens when more than one device transmits data at the same time. When a collision occurs, each device waits a random amount of time before trying to retransmit the data. By having each node wait a random amount of time, there is only a slim chance that the two devices will send out the data at the same time again. The collision detection and frame retransmission are part of the protocol.

One way to reduce the number of collisions is to add switches to the network. A switch, like a hub, connects nodes to each other. Switches speed things up by allowing different nodes of a network to communicate directly with one another in a smooth and efficient manner. However, while a hub requires each node to share the bandwidth (i.e., the amount of simultaneous data traffic the network can support), a switch allows each node to use the full bandwidth.

Switches that make a separate connection for each node in a company's internal network are called LAN switches. These types of switches develop a series of instant networks that contain only the two devices communicating with each other at that particular moment.

In a fully switched network, each node is connected to a dedicated segment of the network, which in turn is connected to a switch. Each switch supports multiple dedicated segments. When a node sends a signal, the switch picks it up and sends it through the appropriate segment to the receiving node. Ethernet protocol in a fully switched environment does not require collision detection because the switches can send and receive data simultaneously, thus eliminating the chance of collision.

Most companies do not use fully switched networks, as the cost of replacing each hub with a switch can be expensive. Instead, most use a mixed network configuration in which a combination of hubs and switches are used. For example, all of the computers in each department may be connected to their own departmental hub, and then all of the departmental hubs may be connected to a switch.

ETHERNET ADVANCES

The Ethernet has become a data transmission standard. Its dominance of Ethernet as a LAN technology for desktop PCs has made it difficult for other technologies to gain acceptance. The installed base of Ethernet networks is larger than any other alternative technology deployment in the upper spectrum of data rates. This is credited to the simplicity of Ethernet standards and the cost-effective equipment. Bandwidth requirements are growing with more interest and demand for IP applications and streaming video. Some experts believe that will replace voice as the dominating traffic type.

In May 1996 eleven network vendors (including Cisco Systems and Sun Microsystems) formed the Gigabit Ethernet Alliance. The goal of the alliance was to develop a standard for 1 Gigabit per second (Gbps) Ethernet transmissions. Soon thereafter, network vendors were successful in designing networks that achieved the 1 Gbps transmissions goal, and in 2002 the IEEE approved the fibre-only 10 Gbps Ethernet. Throughout 2004 great progress was made in the development of l0 Gbps Ethernet technology and its infrastructure. The increased speed of the 10 Gbps Ethernet in terms of data storage, system backup, teleconferencing, and surveillance systems will prove beneficial to blade servers, networked enterprise switches, video servers, and other applications. The higher density, reduced power, and improved cost-effectiveness appeal to all of the major system developers.

The IEEE has worked to continuously update a standard for the Ethernet. In developing the IEEE 802.3ba spec, two different standards were supported by the IEEE standards committee. One group wanted faster server-to-switch applications, and supported a 40 Gbps standard. Other members were more interested in developing a more robust network backbone and favored the higher 100 Gbps speed. This higher speed required more costly and power intensive equipment.

Ultimately, the IEEE voted, in December 2007, to standardize both the 40 Gbps and 100 Gbps speeds as part of the IEEE 802.3ba spec. The connection equipment for speed would have different physical specifications.

NETWORK REMOTE ACCESS DEVICES

Network remote access devices are used to connect remote (off-site) users to an organization's network. There are many options available. See Table 1 for some of the common line designations.

Modems. A modem is a device that converts data from digital to analog signals so it can travel over the public switched telephone network (PSTN) to its destination. Once the signal reaches its destination, the modem converts it back to digital. As the PSTN was designed to carry voice (analog signals), it is not the best option for carrying data. Digital data networks (DDNs) are replacing the PSTN. DDNs are used to transmit both data and digitized voice. Because of their slow data transmission speeds, modems are no longer used in most business environments.

ISDN. Integrated services digital network (ISDN) is a switched, high-speed data service. ISDN is an international telecommunications standard for transmitting voice, video, and data over digital lines running at 64 Kbps, and reaches 1.5 Mbps in North America and 2 Mbps in Europe. ISDN uses the same copper telephone lines as modems do, but at a rate approximately five times faster. Furthermore, it is extremely reliable.

T1. A T1 line carries data approximately 60 times faster than a modem on a normal telephone line. The higher speed and extreme reliability make this a popular choice for many medium-to large-sized businesses. T1 lines can handle hundreds of users simultaneously for general browsing. However, it cannot handle that many users simultaneously downloading large files, such as MP3 files or video files. For very large companies, T1 lines may not be sufficient.

Cable Modems. A cable modem is a device used to connect a computer to a coaxial cable, such as the kind used for cable television, in order to access online services. This device modulates and demodulates signals like a conventional modem. In addition, a cable modem functions like a router designed for installation on cable television networks. The most popular application for cable modems is high-speed Internet access, which provides much faster service than standard telephone-line modems, thus enabling users to access streaming audio, video, and other services.

Wireless Technology. Mobile telephones, laptop computers, and handheld computers are so affordable that they have become a part of everyday life for many people and businesses around the world. Advances in wireless technology have made it possible for people to access networks without having to physically connect to the network through cables. For example, it is not uncommon for business travelers to access networks on their wireless fidelity (Wi-Fi)-enabled laptop PCs or handheld computers while waiting at an airport.

Bluetooth is a wireless standard developed by a group of electronics manufacturers to allow any electronic device—such as computers, cell phones, keyboards, and headphones—to find and connect to other devices without any direct action from the user. The devices find one another and transmit data without any user input at all. Because Bluetooth technology is inexpensive and does not require the user to do anything special to make it work, it is gaining wide use around the world.

Wireless products are affordable and very reliable. With wireless connections, it is possible for people to move around while connected to a network. This is useful in environments such as hospitals, where health care professionals can access patient records from various locations around the campus. Many home and small-business users also use wireless networks to avoid the need to route twisted-pair wiring around their premises.

SEE ALSO Computer Security; The Internet

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