Networking is an essential tool that is utilized in society today. Everywhere you look, networks are prevailing in our technology. Common devices that use networks are cell and landline telephones, pagers, computers, registers, security cameras, and cable/satellite television. Without this technology, our world would be a much different place. For this reason, I am going to spend some time discussing different aspects of networking in the next several posts.
The purpose of a network is to allow many users to access one common resource simultaneously. Currently, are you on the most popular network available, the Internet. Terms such as DSL, Cable Modems, Routers, and Wi-Fi are common in daily life, yet much more exists to interconnect computer resources.
Network Topology
A network topology is the physical and logical way of how the network operates. Four primary topologies exist within modern networking protocols, each with their own negatives and benefits.
Bus Topology
Bus topology networks are one of the oldest true network topology that still operates today. This simplistic method of interconnecting computers using a single cable. The terminology for this coaxial cable is a trunk, or backbone. Utilizing this method, it requires all computers, or hosts, on the network to view every piece of data sent over the network.
This form of network topology has a major benefit to others in only one factor, cost. Since it only requires once piece of network media, or cable, the implementation costs are fairly inexpensive. On the opposite spectrum, a bus topology requires that all hosts are functioning for the entire network. As a result, the network can fail from a single point of failure. In addition, due to the nature of each host required to listen to the network traffic, each host that is added to the network creates further performance degradation.
To make this work, a coaxial cable connects to each computer using a T-connector. Due to the “daisy chaining” of the computers on the network, it is required that the ends have a terminator. Without the terminator at both ends of the bus cable, the network will believe a section of the network is not functioning, and effectively not work. The following is a crude topological map of a bus network.
During standard bus topology communication, the host that is attempting to send the data will listen for a short period of time, then send data out on the network. For example, host A wishes to send data to host E. Host A will send the data, and hosts B, C, D, and E will all receive the data. Inside the data packet, it will specify the recipient, therefore, host B, C, and D will dismiss this data as non-relevant data and not store it. Host E will receive the data and store it locally for further use.
If two hosts were to listen, determine it is safe to send data, and send it simultaneously, an event known as network collision would occur. When this occurs, all hosts on the network disregard all data on the network, and wait a random period of time prior to sending data once again. By utilizing a random period of time generated by the network card, it almost always prevents a second network collision from happening.
Ring Topology
The next evolution of the network topologies is the ring topology. Much like the bus network, this form utilizes a single data cable throughout the entire network. The major difference is that the network media loops into itself rather than terminating. A token ring is the most popular version of a ring topology network
A continuation of sharing characteristics with the bus topology, the benefit exists in the low installation cost. Negatives are displayed by the single point of failure, and slowing of the network with each added host. Each computer is connected utilizing T-connectors until a complete loop has been completed.
The following is a rough topological map of a ring topology:
The procedure of sending data packets is different than that of the bus topology. Rather than the entire network listening at one point in time, it is received by one host at a time. With this method, it is possible to have several data packets on the network simultaneously since the communications occur only between the two hosts in tandem within the ring.
As an example, host B wishes to send data to host E. To initiate the communication, host B will wait to ensure there is no traffic on the network. Once it is clear, it will send the data packet to host C. Since the packet is addressed to host E, and not C, it will send the data packet along the ring to host D. Host D will recognize the address is not for it, and continue the ring by sending the data to host E. When arriving at host E, it will accept the data and store it locally. In this example, host A would not know that there had been a communication occurring on the network.
Star Topology
The star topology is a very common topology utilized in networks today, especially in home networking. This method utilizes a central hub or switch to interconnect computers together. Two forms of network media are prevalent with star topology, Cat5E and wireless media (Wi-Fi).
A major benefit to this topology is that it is extremely easy to troubleshoot in the event of network media, or host failure. In addition, unlike the previous two topologies if the host or network media does fail, it does cause a critical failure of the network. Only that host’s resources will be unavailable. A slight disadvantage of this method is a higher cost of implementation than the previous two since additional media and hardware is required.
The following is a basic diagram of the star topology:
Utilizing a basic setup with a hub, this network will operate similar to the bus topology. Briefly, hubs operate as a repeater only broadcasting data received to all ports except that of the sender. An Example utilizing a Hub is if host A is required to access a resource on computer C. As with all communications, host A will listen to ensure network traffic is not occurring. When clear, it will send a packet addressed to computer C. The hub will receive the packet, and redistribute it to hosts, B, C, D, and E. All of the computers will read the address of the packet and will discard the information except for host C, which will store the data locally.
If the star topology was being utilized by a switch, it would operate more efficiently. A switch directs packets by the address of the recipient. Utilizing the same example as with the hub, host A would listen for an available network, then send the packet destined for host C. The switch will receive the packet, read the address and forward it to host C. Hosts B, D, and E will not know about the existence of the data packet. Since switches can store data internally for a temporary period of time, theoretically, different hosts can send data simultaneously, and the switch will direct the data in order it was received to the appropriate host.
(More will be discussed about switches, hubs, routers, and bridges in a later document.)
Mesh Topology
A true mesh is the most redundant topology since computers are connected to each other directly. This becomes a major disadvantage since a vast number of network media would be required to connect each host to each other. As an example, in a network with 10 hosts, each host would require 9 runs of network media. Therefore, one 10 host network would require 99 lengths network media (9 cables per host X 10 hosts). Obviously, this is a major disadvantage that often overweighs the positive of complete redundancy.
Other disadvantages to these networks are they are difficult to manage and troubleshoot. A major example of a mesh network is the internet backbone that provides internet service providers with access to the internet.
In most cases, mesh networks are implemented as a partial mesh, with only two or three network media running from each computers. This still produces a high level of redundancy, while drastically reducing the costs utilizing only 20 – 30 media cables. To qualify as a true mesh, it requires at least three host computers.
The following is a representation of a full mesh network:
This topology utilizes a direct method of sending data. The host will recognize which network media the recipient resides upon and sends it. If that network media fails, it will send it along the next working piece of network media for that host to forward to the recipient.
Conclusion
Unless in a home environment, most networks are a hybrid of two or more of these types of network. Each has their own strengths and weaknesses. The major factor to decide which topology to utilize often comes down to reliability verses cost.
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