Network Topologies

The topology of a network is the geometric representation of all links and nodes of a network—the structure, consisting of transmission links and processingnodes, that provides communications connectivity between nodes in a network. A link is the physical transmission path that transfers data from one device to another. A node is a network addressable device.

Graph theory describes certain characteristics of a network topology such as the average node degree for robustness (average number of links terminating at a node in a network), network diameter for size (the longest/shortest path between any two nodes in a network), number of paths for complexity (total number of paths between all node pairs), and cutsets for flow (minimum number of removed links to partition a network). However, the most dominant characteristic of a network topology is its shape.

The most general shape characteristics are symmetry and regular/irregular shape. There are five basic network topology regular shapes: mesh, star, tree, bus, and ring. The bus is a special case of a tree with only one trunk. The mesh has the highest node degree; the bus has the lowest node degree.

In a mesh topology, every node has a dedicated point-to-point link to every other node which requires n(n–1)/2 links to connect n nodes. This is the original way the telephone network started in major East Coast U.S. cities. Before long the sky was not visible on certain downtown intersections due to the amount of overhead wire! The mesh topology allows for robustness in presence of faults since the loss of links or nodes can be routed around due to the amount of connectivity. However, this comes at the cost of complex network management due to the number of links and expensive resource usage since each n node must have n–1 ports to connect in the mesh.

In a star topology, each node has a dedicated point-to-point link to a central hub. If one node wants to send data to another, it sends to the central hub, which then relays the data to the destination node. A star provides centralized control but also represents a performance bottleneck and single-point-of-failure.

A tree topology occurs when multiple star topologies are connected together such that not every node is directly connected to a central hub. Thus, a tree extends a star topology, allowing for community clustering around local hubs. The two fundamental trees upon which topologies are built are: the minimum spanning tree, which is the least-cost tree connecting all nodes in a graph; and the Steiner Tree (ST), which is the least-cost tree connecting a subset of member nodes in a graph. (The ST may contain non-member nodes also, which are called Steiner points). Cost is determined by placing weights on links and nodes based on predetermined metrics such as distance, supply/demand, economic cost, delay, or bandwidth.

In a bus topology, a shared medium connects all nodes in the network. This shared medium may be a single wire or radio frequency. The shared medium provides ease-of-installation and flexibility, since it initially consists of a single cable run alongside targeted computers or computers broadcasting on specific frequencies. However, the shared medium also creates two problems: collisions when two nodes broadcast simultaneously, and fault management, since any network problems affect allconnected computers. Isolating the problem requires physically separating the shared medium in a methodological manner.

The ring topology is a series of unidirectional, dedicated point-to-point links connecting in a physical ring. This topology provides inherent reliability since a signal from a source travels around the ring to the destination and back to the source as an acknowledgement. Least-cost rings may approach the cost of a least-cost tree but are generally more expensive and have more delay. Also a ring is not a flexible topology—adding and deleting links and nodes is disruptive.

Protocols are matched to these topologies to enable computer network usage. The bus topology requires a shared medium access protocol based on sensing transmission to avoid collisions with probabilistic retransmission (that is, retransmission after a probabilistically determined time). The ring topology requires a token-passing where a node needs to have a token in order to transmit. At high loads, the bus topology with a shared medium access protocol experiences collisions, and thus offers diminished performance beyond a particular usage threshold. The ring topology with a token-passing protocol has unnecessary overhead at low loads but its performance does not degrade at high loads.

In general, there are two alternatives for operation of a star: (1) the central hub broadcasts all traffic it receives (physically a star but logically a bus), or (2) the central hub selectively switches incoming traffic only to destination nodes. The performance of a star depends on the processing capability of the central hub as well as the capacity of the spoke links, and beyond a threshold connections may be blocked. The tree topology is used for multipoint or group communications and thus depends on the slowest link or node with lowest processing capability in the tree connecting the group.

Examples of networks matched to these topologies include local area network (LAN) standard ETHERNET, which is a bus topology using a shared media access protocol, and LAN standard TOKEN RING, which is a ring topology using a token-passing protocol. The star is the topology of the local loop, circuit-switched telephone network with the central hub being the local central office. The tree topology is the basis of emerging group communication applications that are not yet standardized.

William J. Yurcik

Bridging Devices; Internet; Network Design; Network Protocols; Office Automation Systems; Telecommunications.

Stallings, William. Data and Computer Communications, 6th ed. Upper Saddle River, NJ: Prentice Hall, 2000.

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