What are Network Topologies and their access Methods

Network topology refers to the arrangement of computers or devices in a network.

Physical topology refers to the placement of network components including device location & cable installation. Logical topology refers to how data is sent or received in a network.

A graphical representation of a physical network is best illustrated through topology. Basic topologies include Bus, Star, Ring, Mesh and Hybrid.

Bus Topology

In bus topology, each node (computer) is connected through a single cable (known as backbone or trunk) used as a common transmission medium for communication. Signal from the source computer travels to all computers connected to the cable until the destination computer accepts the data; if not, the machines ignore the data

In bus topology, the nodes are interconnected using co-axial cables through the T – Connector that splits the connection between nodes. Terminators are used at both ends to absorb the signal.

Advantages include ease of installation and low cost; however, since all the computers depend on a single cable, a single break or loose connection can cause the entire network to be down and troubleshooting can be difficult.

Bus-topology

Though bus topology is the simplest form for connecting multiple computers,issues may arise when two computers have a need to transmit at the same time. To handle such collisions, CSMA/CD protocol is used in Bus (Ethernet) implementations.

IEEE Standards related to bus topology are 10BASE2 (Thinnet) and 10BASE5(Thicknet).

10BASE2 is a variant of Ethernet that uses Co-axial cable; with maximum segment length of 200 meters through practical limit is 185 meters. Maximum number of nodes within a segment is limited to 30. 10BASE5 is similar to 10BASEZ with the exception of distance up to 500 meters and maximum number of nodes up to 100. With the introduction of Ethernet over Twister pair, both 10BASEZ & 10BASES are obsolete with very rare exceptions.

Devices called repeaters are used to accommodate more number of computers on a segment. Repeaters amplify and retransmit weak signals to cover longer distances. However, the IEEE design guideline covers the use of repeaters and maximum segments through the 5-4-3 rule; this means that in a collision domain there can be a maximum of 5 segments connected through 4 repeaters, with 3 segments containing active senders. Note that rule is applicable only for 10BASE2 and 10BASE5 shared Ethernet networks (Not for switched Ethernet).

Star Topology

Star topology is the most common and widely used topology today. Each computer is connected to a centralized device called the hub or switch using dedicated cable such as the Twisted-Pair. All signals need to pass through the centralized device Star topology is considered to be the easiest topology to design and implement as adding additional nodes is simple and easy to troubleshoot in case of single cable failure. However, entire network is affected if the hub or switch goes down.

Ring Topology

In a ring topology, each node connects to exactly two other nodes, forming a single continuous pathway for signals through each node Data travels from node to node, with each node along the way handling every packet. Ring topology provides only one, pathway between any two nodes, ring networks may be disrupted by the failure of a single link. A node failure or cable break can isolate every node attached to the ring. Ring topology uses physical star topology and logical ring for communication.

Devices used in ring topology include the Token Ring Network Interface card. Twister pair or fiber optic cables connected to a centralized device called the MSAU (Multistation Access Unit).

IEEE 802.5: IEEE 802.5 standard was derived from IBM token ring networks, this was originally developed by IBM. This has a logical ring topology and token based Media Access Control (MAC).

Mesh Topology

In a mesh topology, all nodes are connected to each other node. A full mesh requires complete connectivity between every node to ensure the data can be delivered one way or the other. This means that there is a high level of redundancy. Since this topology requires connectivity between every node, it is also the most expensive.

Hybrid Topology

Hybrid topology is a combination of two or more topologies mentioned above. For example, two physical star topology based network may be interconnected through a single bus topology.

Twisted Pair

Twisted-Pair cables are widely used in Local Area networks and telephone networks. In a twisted pair cable, two conductors of a single circuit are twisted together for cancelling out Electromagnetic interference (EMI) from external sources. Types include the UTP (Unshielded Twisted Pair) and STP (shielded Twisted Pair). UTP cables are found in Ethernet networks and telephone systems. RJ-45 (Registered Jack) connectors are used to connect the twisted pair cables to end-points on computer networks. RJ-11 is a connecter used on telephone networks.

IEEE Standards related to star topology are 10BASET, 100BASE-TX, 1000BASET &10GBASET. Maximum distance supported by twisted pair is 100 meters.

Twisted-pair Ethernet cables can be wired “straight-through” or “Crossover”.

To connect a network interface card to a switch, hub or router, straight-through or patch cables are used.

To connect similar devices (network interface card on computer to another network interface on another computer, hub to hub or switch to switch), crossover cables are used.

Fiber Optic

A fiber optic cable is a cable containing one or more optical fibers. Fiber Optic cables are ideal for transmitting data over very long distances at great speeds as light is used for the medium for transmission. Fiber optic cables are not susceptible to any EMI, Near-end Crosstalk (NEXT), or Far-end Crosstalk (FEXT).

Note that you require special network interface cards & network switch that support the fiber optic interface which is usually expensive and common only in large enterprise networks or locations that are susceptible to EMI such as factories that use heavy machineries.

Fiber Optic cables consist of a high quality glass or plastic strands and a plastic jacket made of Teflon or PVC that protects the cable.

Two types of Fiber-Optic cable exist: Single-Mode Fiber (SMF) used for longer distances and Multi-Mode Fiber (MMF) used for shorter distances. Signals are transmitted as light signals from source to destination. Either LED or Laser is used. In multi-mode fiber, light signals are transmitted in numerous dispersed path (single-mode fiber use single light source) and making it un-suitable for long distance transmissions.

In some cases, plenum rated cables are used that have a special jacket to protect against fire and emit less smoke than normal cables. However, this is rare and often seen only in industrial or manufacturing sites.

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