1 Topologies: Bus, Star, Ring, Mesh, Hybrid
In computer networking, topology refers to the layout of connected devices. This article introduces the standard topologies of networking.
Topology in Network Design
Network topology is the arrangement of the various elements (links, nodes, etc.) of a computer network. Essentially, it is the topological structure of a network, and may be depicted physically or logically. Physical topology refers to the placement of the network's various components, including device location and cable installation, while logical topology shows how data flows within a network, regardless of its physical design. Distances between nodes, physical interconnections, transmission rates, and/or signal types may differ between two networks, yet their topologies may be identical.
Think of a topology as a network's virtual shape or structure. This shape does not necessarily correspond to the actual physical layout of the devices on the network. For example, the computers on a home LAN may be arranged in a circle in a family room, but it would be highly unlikely to find a ring topology there.
Bus topology is one which all of the devices on the network are connected with a single cable with terminators on each end. This single cable is often referred to as a backbone or trunk.
The typical Bus network uses coax as its cable. Coax is a cable similar to what you use for your cable TV. Coax is also referred to as 10Base2. The upside to using coax is that it is inexpensive, easy to install, and is not as susceptible to electromagnetic interference as twisted pair cable is.
The downside for a coax network is the speed is limited to 10 Mbps (Megabits per second) and that is an interruption occurs in the cable, all of the nodes (workstations) on the cable will lose connectivity. If a NIC fails or a cable is disconnected at any of the points in the network, it will not be terminated properly so all of the computers will lose connectivity to the network.
Bus networks (not to be confused with the system bus of a computer) use a common backbone to connect all devices. A device wanting to communicate with another device on the network sends a broadcast message onto the wire that all other devices see, but only the intended recipient actually accepts and processes the message.
Ethernet bus topologies are relatively easy to install and don't require much cabling compared to the alternatives. 10Base-2 ("ThinNet") and 10Base-5 ("ThickNet") both were popular Ethernet cabling options many years ago for bus topologies. However, bus networks work best with a limited number of devices. If more than a few dozen computers are added to a network bus, performance problems will likely result. In addition, if the backbone cable fails, the entire network effectively becomes unusable.
This diagram illustrates the bus network topology. A bus topology such as 10Base-2 or 10Base-5 Ethernet uses a single communication backbone for all devices.
Advantages of a Bus Topology
• Easy to connect a computer or peripheral to a linear bus.
• Requires less cable length than a star topology.
Disadvantages of a Bus Topology
• Entire network shuts down if there is a break in the main cable.
• Terminators are required at both ends of the backbone cable.
• Difficult to identify the problem if the entire network shuts down.
• Not meant to be used as a stand-alone solution in a large building.
The ring topology is one which the network is a loop where data is passed from one workstation to another. Commonly, you find the ring topology with token ring networks. Token ring networks are defined by IEEE 802.5 and were primarily developed by IBM. The token ring network is designed to transmit a token, or a special frame, designed to go from node to node around the ring. As the frame passes, if a workstation needs to transmit data, it modifies the frame, attaches its data and sends it on. If the data is intended for the next workstation on the network, it receives the data and the information stops at that workstation. If it is intended for somewhere else on the network, the data is retransmitted around the ring until it finds its intended location. Once the data finds its new home, a blank token is transmitted and another workstation can attach data and then that data travels around the ring.
There is a token holding timer to prevent a workstation from transmitting too much data. This protocol ensures all workstations on the network get an opportunity to send data. The original specification could only operate up to 16 Mbps though newer Fast Token Ring networks can transmit up to 1 Gbps (gigabit per second). Advantages for token ring networks include a 4k maximum frame size, longer distance capabilities than Ethernet, and each station is guaranteed access to a token at some point. Ethernet is a shared access medium meaning each workstation has equal access to the available bandwidth at any given time.
The recommended distance for Type 1 cabling on a token ring network is 300 meters, on Unshielded Twisted Pair (UTP) cabling, about 150 meters. More details will be discussed about token ring shortly.
In a ring network, every device has exactly two neighbors for communication purposes. All messages travel through a ring in the same direction (either "clockwise" or "counterclockwise"). A failure in any cable or device breaks the loop and can take down the entire network.
To implement a ring network, one typically uses FDDI, SONET, or Token Ring technology. Ring topologies are found in some office buildings or school campuses.
This diagram illustrates the ring network topology. A ring topology such as FDDI or SONET sends messages clockwise or counterclockwise through the shared link.
The most popular topology for business today - the star topology consists of all of the nodes on a network connected to a central switch or hub. A node is a device attached to the network - such as a computer.
Each node on the network has a cable back to the central switch. If one cable fails to a node, only that node (computer) is affected. You can combine several switches or hubs to create several stars, all connected together.
The Star topology is very inexpensive to maintain versus other topologies. 10BaseT is an example of Star topology. Think of the star topology as a big wheel. At the center of the wheel is a switch or hub and each spoke going out from the center goes to a node. Many home networks use the star topology. A star network features a central connection point called a "hub node" that may be a network hub, switch or router. Devices typically connect to the hub with Unshielded Twisted Pair (UTP) Ethernet.
Compared to the bus topology, a star network generally requires more cable, but a failure in any star network cable will only take down one computer's network access and not the entire LAN. (If the hub fails, however, the entire network also fails.)
This diagram illustrates the star network topology. A star topology typically uses a network hub or switch and is common in home networks.
Advantages of a Star Topology
• Easy to install and wire.
• No disruptions to the network when connecting or removing devices.
• Easy to detect faults and to remove parts.
Disadvantages of a Star Topology
• Requires more cable length than a linear topology.
• If the hub, switch, or concentrator fails, nodes attached are disabled.
• More expensive than linear bus topologies because of the cost of the hubs, etc.
A mesh topology is one which all of the nodes are directly connected with all of the other nodes. A mesh topology is the best choice when you require fault tolerance, however, it is very difficult to setup and maintain.
There are two types of mesh network: full mesh and partial mesh. A full mesh is one which every workstation is connected to the other ones in the network. In a partial mesh, the workstations have at least two NICs with connections to other nodes on the network. Mesh networks are commonly used in WANs.
Mesh topologies involve the concept of routes. Unlike each of the previous topologies, messages sent on a mesh network can take any of several possible paths from source to destination. (Recall that even in a ring, although two cable paths exist, messages can only travel in one direction.) Some WANs, most notably the Internet, employ mesh routing.
A mesh network in which every device connects to every other is called a full mesh. As shown in the illustration below, partial mesh networks also exist in which some devices connect only indirectly to others.
This diagram illustrates the mesh network topology. A mesh topology provides redundant communication paths between some or all devices (partial or full mesh).
It’s combination of any two or more network topologies. Instances can occur where two basic network topologies, when connected together, can still retain the basic network character, and therefore not be a hybrid network. For example, a tree network connected to a tree network is still a tree network. Therefore, a hybrid network accrues only when two basic networks are connected and the resulting network topology fails to meet one of the basic topology definitions. For example, two star networks connected together exhibit hybrid network topologies. A hybrid topology always accrues when two different basic network topologies are connected.