In networking terms, topology refers to the virtual design of network. It tells us how the computers will be connected to each other using cables.
To create an office LAN (Local Area Network), we can consider following one of the LAN topologies:
• Bus
• Ring
• Star
• Tree
Bus Topology: Simplest of all LAN topologies, which have single cable to connect multiple computers in a network. If this cable fails, the entire network crashes.
Ring Topology: All communication amongst computers travel through a ring in same direction. If any of the cable fails, the look breaks and thus make network unusable.
Star Topology: Require more cable than Bus topology. Failure in one cable only makes one cable inaccessible and not the entire LAN.
Tree Topology: Tree topology is nothing but multiple star topologies. Requires a lot more cables than star topology but is extra safe when one of the cables fails.
After considering the vulnerability and cost of network it is recommended that star topology is selected for the office LAN. This choice for the network is made based on requests that physical aspects of the network are easy to manage. The star topology uses a hub as a central point of connection for the computers or other devices on the Local Area Network (LAN). These devices that are connected to the network are connected to the hub with their own separate cable. The star topology allows the network to be expanded without difficulty. This depends on how many ports are on the hub that is being used. “If more ports are needed on the hub, two hubs can be linked together to provide more connectivity (Habraken, Joe, 2004)”.
Adding additional computers to the network will be easy. This is done by simply connecting the desired computer or computers to the port on the hub. This is an advantage of the star topology because it allows you to expand the network with out interfering with the other devices. This topology relies on a lot of cabling that can sometimes result in cost issues but if planned correctly it can fit within any reasonable budget.
Cables Specification
Cables are most important part of any network, if not wireless. It is always advisable to consider factors like cost, durability and speed before selecting cable for the network. Following are common types of cable which includes twisted-pair cable, coaxial cable, and fiber-optic cable.
Twisted-pair cables:
Twisted pair cables are most common types of cables which are used in telephone communications and also in almost all modern Ethernet networks. Twisted pair cables are made of pair of wires and are twisted to provide protection against crosstalk – When electric signals flow through the wire it makes a small circular magnetic field around the wire, and thus twisted pair cables are made in such a way to minimize this disturbance.
There are basically two typed of twisted-pair cables:
1.) Unshielded twisted pair (UTP)
2.) Shielded twisted pair. (STP)
Unshielded twisted-pair (UTP) cable are composed of pairs of wires, and has eight individual copper wires which are covered by insulating material to reduce ‘disturbance’. The twisted wires pairs in UTP reduce signal degradation caused by electromagnetic interferences. UTP cables have inexpensive, and has small diameter which makes it easy during installation. And as it’s easily available and can be used with major networking architecture, UTP put forth itself to be a strong contender when choosing cables for networks.
Characteristics of UTP
• Speed and throughput—10 to 1000 Mbps
• Average cost per node—Least expensive
• Media and connector size—Small
• Maximum cable length—100 m (short)
Shielded twisted-pair (STP) has four pairs of wires. Each pair is wrapped in metallic foil, and this four pair is again wrapped in metallic braid or fail. STP cables are designed to decrease electrical disturbance within the cable and/or external. STP cables prevents disturbance better than UTP cables but are more expensive and difficult to install. Because of its cost and difficulty in installation, STP cables are generally not preferred.
Characteristics of STP
• Speed and throughput—10 to 100 Mbps
• Average cost per node—Moderately expensive
• Media and connector size—Medium to large
• Maximum cable length—100 m (short)
Coaxial Cable:
Coaxial cables has hollow outer cylindrical conductor that surrounds a single inner wire made of two conducting elements. One of the elements (metal) is located in center of cable and is generally copper which again has a layer of flexible insulation. There is a woven copper braid or a metallic foil which acts as shield to reduce outside interference or a second wire if primary wire is defected. Coaxial cable can be used over longer distance than twisted-pair cables but are more expensive than UTP. As coaxial cables can transmit through longer distance, it decreases the number of repeaters to be installed within a broader network which helps bring down the cost. But because of the thickness of coaxial cables, it is difficult to install in some situations, and difficultly in installation means expensive to install
Characteristics of Coaxial Cable:
• Speed and throughput—10 to 100 Mbps
• Average cost per node—Inexpensive
• Media and connector size—Medium
• Maximum cable length—500 m (medium)
Below is a short table which can be referred to while choosing the type of cables in any network topology.
Media Type Maximum Segment Length Speed Cost Advantages Disadvantages
UTP 100 m 10 Mbps to 1000 Mbps Least expensive Easy to install; widely available and widely used Susceptible to interference; can cover only a limited distance
STP 100 m 10 Mbps to 100 Mbps More expensive than UTP Reduced crosstalk; more resistant to EMI than Thinnet or UTP Difficult to work with; can cover only a limited distance
Coaxial 500 m (Thicknet)
185 m (Thinnet) 10 Mbps to 100 Mbps Relatively inexpensive, but more costly than UTP Less susceptible to EMI interference than other types of copper media Difficult to work with (Thicknet); limited bandwidth; limited application (Thinnet); damage to cable can bring down entire network
Fiber-Optic 10 km and farther (single-mode)
2 km and farther (multimode) 100 Mbps to 100 Gbps (single mode)
100 Mbps to 9.92 Gbps (multimode) Expensive Cannot be tapped, so security is better; can be used over great distances; is not susceptible to EMI; has a higher data rate than coaxial and twisted-pair cable Difficult to terminate
Although, fiber optics and wireless network can be used for networking, we are not considering it in this paper as fiber optics is too expensive to implement it practically in office generally, while wireless network is too slow and vulnerable to implement either.
References:
http://compnetworking.about.com/od/networkdesign/a/topologies.htm
http://www.ciscopress.com/articles/article.asp?p=31276&seqNum=1