The scenario for this work envisages the middle-sized company upgrading its local area network (LAN). Currently, the company operates Ethernet 10BaseT network with a bus topology. The goal is to upgrade the network to a star topology that would comply with the 100BaseT Ethernet standard. This paper provides an overview of the two standards, substantiating the upgrade decision, and outlines the major migration steps with regard to the network hardware and throughput.
Historically, the legacy bus topology had implied sharing the bandwidth of a single coaxial cable between all PCs on a LAN, restricting the possibility of simultaneous data transmission. As the twisted pair (10BaseT standard) replaced the coaxial cable (10Base2), every PC on the network segment became connected to a hub, or network concentrator, which provides a shared data transmission media. It is still a bus topology since hubs do not allow simultaneous transmission of multiple PCs. The only benefit of the 10BaseT over the 10Base2 is the increased network reliability, as the disconnection of any network cable does not fail the whole LAN segment (Dean, 2009, p.209).
The introduction of the network switches has enabled the high-speed simultaneous data transmission between multiple PCs. In this new star topology, the hubs are replaced with the LAN switches while the network cabling remains the same. The resulting performance increase amounts to 100/1000 Mbps speed dedicated to every network host instead of 10 Mbps shared between all network nodes in a bus topology. It became possible due to the media access control function incorporated into the switches. Apart from the physical addressing, the media access control (MAC) is an important tool for collision detection and handling. In a shared network segment with the bus topology, only one host can transmit at a time. If any two or more computers transmit simultaneously, a network collision occurs. It happens because the signal propagation over the physical line has its finite speed and the network node can start transmitting when the signal from another host has not arrived yet. Most commonly, the MAC mechanism handles the collisions by means of the CSMA/CD (Carrier Sense Multiple Access with Collision Detection) protocol. When collision occurs, all nodes stop transmitting and wait for some random time. This timeframe must exceed the so-called “round-trip time”, which is required for the signal propagation through the whole Ethernet bus segment (Spurgeon, 2009, p. 49). Then the network node makes sure that there are no other communications on the media and starts the transmission again. The collision handling is relevant only in the 10Base2/10BaseT networks. In the star topology, the switches know which MAC addresses are associated with every port and distribute network packets accordingly.
The whole upgrade could be effectively limited to the replacing hubs with switches if the appropriate cabling is already in place. Traditionally, LANs use the twisted pair cable, either shielded or unshielded (STP/UTP). The cable characteristics can limit the network performance, thus the specific cable type is required for the high-speed communication. The network speed of 1000 Mbps can be achieved using UTP Cat5E cable, each pair of which has certain number of twists per cable’s length (Andrews, 2009, p.488). The UTP/STP cable can connect network nodes only within the range of 100 meters.
Assuming that the proper cabling infrastructure exists, the upgrade can be conducted in a few hours with the minimal network downtime. All hubs must be removed from the racks in a closet and new switches should be mounted instead. The dedicated 100 Mbps network speed should be enough to provide the best desktop connectivity for end users. As the fastest throughput is required for the payroll department, all their PCs will be connected to a 1000 Mbps switch. In order to support the new speed, the network interface cards’ (NICs) upgrade may also be necessary.