Guest Author - Cathy Spearmon
What is a network? Simply put, a network is a group of two or more personal computers or devices linked together. There are many types of networks. However, the most common types of networks are local area networks (LANs) and wide area networks (WANs).
In a LAN network, computers are connected within a local area, such as an office or a home. In a WAN network, computers that are much farther apart are connected by way of telephone or communication lines, radio waves or any other means of communication.
Networks are categorized based on their three properties:
Protocols specify a common set of rules and signals which computers on the network use to communicate. Today, most networks use Ethernet. However, there are some networks that use IBM’s Token Ring protocol.
The topology of a network specifies the geometric arrangement of the network or a description of the layout of a specific region or area covered by a network. The complete physical structure of the cable (or data-transmission media) is called the physical topology. The way data flows through the network (or data-transmission media) is called the logical topology.
There are two types of connections that describe how multiple devices connect to a single cable or segment of data-transmission media. They are point-to-point and multi-point. Point-to-point connections provide a direct link between two devices; for example, a computer connected directly to a printer. Multi-point connections provide a link between three or more devices on a network.
All computer networks, regardless of their topologies, rely on point-to-point and multi-point connections.
The most common topologies are:
A bus topology uses one long cable (backbone) to which network devices are either directly attached or attached by using short drop cables. Because all computers share this bus, a computer checks for any information that might be coming down the backbone before sending its message. A message passes the other computers on the way to its destination. Each computer then checks the address of each message to see if it matches its own. Bus topology is a passive topology, which means that computers connected to a bus topology only listen for a signal being transmitted over the data-transmission media. They don´t move data from one computer to the next. Thus, if one computer goes down, it doesn´t affect the rest of the network.
In a bus network topology, the backbone must be terminated at both ends to prevent the signal from echoing back when it has reached the end of the bus. A terminator is a resistor that essentially absorbs signals when they reach the end of the cable. Thus, a terminator is said to have the effect of making the cable look infinite to a signal. Typically, with shielded cable installations, such as with coaxial cable, one of the terminators is connected to ground so as to dissipate static electricity that might build up in the shielding of the cable.
Ring topologies consist of several computers joined together to form a circle. Messages move from one computer to the next in one direction only. When a computer receives a message addressed to it, the message is copied and sent back with a modification to indicate that it was received. Ring topology is an active topology, which means that computers connected to a ring topology are responsible for moving data from themselves to the next computer. Thus, if one computer goes down, it affects the rest of the network.
Some networking schemes use a dual ring topology. In these configurations, data moves in opposite directions around the two rings. In most cases, one ring is the primary route over which data flows; the second ring serves as a backup in case of a break in the first ring. In the event of a primary ring break, the primary and secondary rings are automatically connected to form one larger ring. A single ring break won´t bring down a dual-ring network. Such dual ring configurations are called dual counter-rotating rings, because when un-joined, data flows in the opposite direction on the secondary ring as that on the primary ring.
The star topology can be used in either Ethernet or Token Ring networks. In Ethernet, the star topology is referred to as a star-bus. It uses a central device, called a hub, with drop cables extending in all directions. Each networked device is connected point-to-point to the hub. Hubs can be active or passive .The majority of hubs are active, meaning they regenerate and retransmit the signal. Some hubs, such as wiring panels, are passive, meaning they act only as connection points and don´t regenerate the signal. All messages in a star topology must go through the central hub before reaching their destination. Star topology is a passive topology. Computers connected to a star topology aren´t responsible for moving data from one computer to the next. Thus, if one computer goes down, it doesn´t affect the rest of the network. In Token Ring, the star topology is referred to as a star-ring. It connects workstations via patch cables. MSAUs are connected together to create the ring.
Just because a network connects to a central networking device does not mean it´s a star topology network. Take for instance 10BaseT Ethernet. Like all Ethernet networks, 10BaseT Ethernet uses a bus topology. However, with 10BaseT Ethernet, twisted-pair cables run from computers to hubs giving the appearance of a star topology. Inside the hub, however, each of the cables is connected forming the bus required for Ethernet. The star appearance is just an illusion.
A network’s architecture refers to one of two major types of networks
Within a peer-to-peer network configuration, there is no server. However, each client (computer) connects with one another in a workgroup where files, printers and Internet access are shared. This is most commonly used within home configurations where workgroups consist of about a dozen or less computers.
In a client/server network, an NT Domain Controller, which all computers login to, usually exists. This server provides a variety of services, which includes centralized routed Internet Access, e-mail, file & printer sharing, and security across the network.
Local Area Networks
Though, originally only afforded by medium to large-sized companies, the price of implementing a local area network has dropped tremendously, making a basic 4-PC 10BaseT home networking kit affordable at under $150.
This price reduction and convenience has contributed to the popularity of LANs. It’s becoming commonplace among households and office. Each computer on a LAN is capable of accessing shared files and devices anywhere within the LAN. By sharing expensive devices, a LAN is a cost-effective alternative to purchasing devices for each user.
Within an Ethernet LAN, you have a chose between 10BaseT (10 million bits per second or 10Mbps) or 100BaseT (100 million bits per second or 100Mbps), with the later being commonly referred to as Fast Ethernet. Most home networks and small businesses use 10BaseT because it is affordable and provides adequate performance. But for large businesses, 100BaseT may be required if a large number of computers are connect to the network or large amounts of data are being transmitted.
Ethernet networks have three different types of cabling: Unshielded Twisted Pair (UTP), coaxial and Shielded Twisted Pair (STP). UTP is the most popular and recommended because of its wide availability and low cost. STP cabling provides more shielding against electromagnetic interference (EMI) than UTP; however, it is more expensive. Coaxial cabling uses a much thicker and expensive cable. Also, it must run in a ring configuration from computer to computer. This is unlike twisted pair, where it is run from each computer to a port on a hub.
Another piece of equipment required for Ethernet is a hub. A hub is a device that allows all your computers to connect to the network through cabling. A hub acts as a go-between for devices connected to a network; it receives transmission signals from one device and sends them out to other connected devices. It has multiple ports to which network devices attach. It manages the receiving and transmitting of data to and from networked devices. Hubs come in various port configurations. The number of computers or devices that you wish to connect together within your network determines the configuration chosen. Each port on a hub supports a single 10BaseT or 100BaseT connection from the PC or the device. When using coaxial cabling, you will need to use a hub that has one coaxial port and minimal twisted pair ports.
Along with hubs, bridges and routers are devices used for linking together different LANs or LANs segments. Many companies have LANs at several offices. Routers allow the connection of these remote LANs across a Wide Area Network (WAN). Bridges are also used for this purpose. However, they are specifically used to connect two different types of networks.
There are maximum distances between workstations and hubs, hubs and hubs and workstations connected to a single LAN. Linking two LANs segments together with the use of a router or bridge exceeds these distances.
Bridges, which are simpler and less expensive as routers, acts as a traffic cop to make do and don’t decisions on which packets to send across the connected segments. Filtering is based on the destination address of the packet. If a packet’s destination is a PC on the same segment where it originated, it is not forwarded. If destined for a PC on another LAN, it’s connected to different bridge port and forwarded to that port.
Routers, similar to super-intelligent bridges, are more complex and expensive than bridges. They use information within each packet to route it from one LAN to another, communicate with each other and share information that allows them to determine the best route through a complex network of many LANs. It directs traffic to prevent head-on collisions. While bridges know the addresses of all the computers on each side of the network, routers know the addresses of all the computers, bridges and other routers that are on the network.
Switches are another type of device that is used to link several LANs, as well as route packets between them. A switch has multiple ports, each of which can support either a single station or entire Ethernet or Token Ring LAN. When different LANs are connected each port, packets between the LANs can be switched as needed.