LAN Topologies

LAN topologies are dependant upon the LAN technology in use, such as Ethernet or Token Ring. The LAN technology used in turn depends upon user applications, such as file sharing, Web hosting, or mainframe data centers.

There are four LAN topologies:

  • Star (Hub-and-Spoke)

  • Bus

  • Ring

  • Tree

Each of these is discussed in the following sections.

NOTE

LAN hosts are any device attached to a LAN; for example, workstations, servers, printers, routers, and so on.

Star (Hub-and-Spoke)

In a star, or hub-and-spoke, topology, the LAN hosts are connected by point-to-point links to a central hub. Several different cable types can be used for these point-to-point links, such as shielded twisted-pair (STP), unshielded twisted-pair (UTP), and fiber optic. Wireless transmission media also can be used for these point-to-point links.

The advantage of a hub-and-spoke topology is that no cable segment is a single point of failure. If one of these cables fail or develops a problem only that LAN host is affected, all other hosts remain operational.

The disadvantage of a hub-and-spoke topology is the central hub itself. This central hub is a single point-of-failure in that if it fails, every attached host is impacted; that is, out of service.

Figure 5-1 illustrates a hub-and-spoke topology with a file server, printer, and two workstations. As is shown here, if a cable to one of the LAN hosts fails, the rest of the devices are unaffected, unless they need to access the "disconnected" host. If the central hub fails, however, all attached hosts are affected and the LAN is considered down, or out-of-service.

Figure 5-1. Star (Hub-and-Spoke) Topology

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Hub-and-spoke topologies are often found in small office/home office (SOHO) environments where there are a small number of LAN hosts.

Bus

The bus topology is a simple design using a single length of cable with attached LAN hosts sharing the cable segment. Every host on this segment sees transmissions from every other host on the cable segment. This operation categorizes bus topologies as broadcast medium. The LAN hosts are endpoints to the cable segment and are known as bus network termination points.

This bus cable segment is a single point of failure. If the cable is broken, no LAN host will have connectivity and the LAN is out-of-service.

Ethernet (IEEE 802.3) best represents the bus topology, which is illustrated in Figure 5-2.

Figure 5-2. Bus Topology

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Bus topologies are similar to hub-and-spoke topologies and are often found in SOHO environments.

Ring

Ring topologies are the most redundant of network topologies. All hosts in a ring topology are considered repeaters and are enclosed in a loop. Unlike the star topology, there are no endpoints in a ring topology. The repeater in a ring topology is the LAN host's network interface card (NIC).

The NIC in a LAN host repeats any signal that is on the network, whether it is destined for that particular host or not. If for any reason, a NIC fails to perform this repeater function, the entire network could come down. The NIC controller recognizes and handles the problem of a defective repeater by pulling itself off the ring, allowing the ring to stabilize and continue operation.

Token Ring (IEEE 802.5) best represents a ring topology, albeit physically cabled in a star topology, as illustrated in following Figure 5-3 and 5-4.

Figure 5-3. Ring Topology (Logical)

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Figure 5-4. Ring Topology (Physical)

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Fiber Data Distributed Interface (FDDI) is another example of a ring topology implementation. Like Token Ring, FDDI rings are physically cabled in a star topology. FDDI hosts are configured either as a single-attached station (SAS) or as a dual-attached station (DAS). Single-attached stations are connected to one of the two FDDI rings, whereas dual-attached stations are connected to both rings by an A and B port on the FDDI stations and concentrator.

The significant advantage of ring topologies is the inherent redundancy; if a cable or NIC fails on one side of a LAN host in a ring, data traffic will traverse the other side of the failure.

Ring topologies are found in environments where high-availability and redundancy are critical components of a LAN segment.

Tree

The tree topology is a logical extension of the bus topology and is best described as multiple interconnected bus networks. Tree topologies enable a network to expand dynamically with only one active data path between any two-network endpoints (hosts). The tree topology is illustrated in Figure 5-5.

Figure 5-5. Tree Topology

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A tree network is one that does not employ loops in its infrastructure. An example of a tree network is one using the Spanning Tree Algorithm, often found with Ethernet (IEEE 802.3) networks. The Spanning Tree Algorithm prevents loops and ensures only one active path exists between any two LAN hosts.

Tree topologies are often found in larger enterprise or campus environments.



Network Sales and Services Handbook
Network Sales and Services Handbook (Cisco Press Networking Technology)
ISBN: 1587050900
EAN: 2147483647
Year: 2005
Pages: 269

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