LAN topologies
topology is a physical structure or layout of the systems connected in a network.
types of network topologies:
- star topology
- ring topology
- bus topology
- tree topology
Star Topology (Hub-and-Spoke topology) :
All stations are attached by cable to a central point, usually a wiring hub or other device
operating in a similar function.
Several different cable types can be used for this point-to-point link, such as shielded
twisted-pair (STP), unshielded twisted-pair (UTP), and fiber-optic cabling. Wireless media
can also be used for communications links.
The advantage of the star topology is that no cable segment is a single point of failure
impacting the entire network. This allows for better management of the LAN. If one of the
cables develops a problem, only that LAN-attached station is affected; all other stations
remain operational.
The disadvantage of a star (hub-and-spoke) topology is the central hub device. This central
hub is a single point-of-failure in that if it fails, every attached station is out of service.
These central hubs, or concentrators, have changed over the years. Today, it is common to
deploy hubs with built-in redundancy. Such redundancy is designed to isolate a faulty or
failed component, such as the backplane or power supply.
Ring Topology:
All stations in a ring topology are considered repeaters and are enclosed in a loop. Unlike
the star (hub-and-spoke) topology, a ring topology has no end points. The repeater in this
case is a function of the LAN-attached station’s network interface card (NIC).
Because each NIC in a LAN-attached station is a repeater, each LAN station will repeat any
signal that is on the network, regardless of whether it is destined for that particular station.
If a LAN-attached station’s NIC fails to perform this repeater function, the entire network
could come down. The NIC controller is capable of recognizing and handling the defective
repeater and can pull itself off the ring, allowing the ring to stabilize and continue operating.
Token Ring (IEEE 802.5) best represents a ring topology. Although the physical cabling is
considered to be a star topology, Token Ring is a ring in logical topology, as demonstrated
by the following figures. Although physical topology is a physical layer attribute, the media
access method used at the data link layer determines the logical topology. Token Ring
defines a logical ring and contention, as Ethernet defines a logical bus. Even when attached
to a hub, when one Ethernet device transmits, everyone hears the transmission, just as
though on a bus.
Bus Topology:
Sometimes referred to as linear-bus topology, Bus is a simple design that utilizes a single
length of cable, also known as the medium, with directly attached LAN stations. All
stations share this cable segment. Every station on this segment sees transmissions from
every other station on the cable segment; this is known as a broadcast medium. The LAN
attachment stations are definite endpoints to the cable segment and are known as bus
network termination points.
This single cable segment lends itself to being a single point of failure. If the cable is
broken, no LAN station will have connectivity or the ability to transmit and receive.
Ethernet (IEEE 802.3) best represents this topology. Ethernet has the ability to utilize many
different cable schemes. Further discussion of Ethernet and these cable schemes will be
found in greater detail in Chapter 3. Figure 2-7 is an example of a bus topology.
Tree Topology:
The tree topology is a logical extension of the bus topology and could be described as
multiple interconnected bus networks. The physical (cable) plant is known as a branching
tree with all stations attached to it. The tree begins at the root, the pinnacle point, and
expands to the network endpoints. This topology allows a network to expand dynamically
with only one active data path between any two network endpoints.
A tree topology network is one that does not employ loops in its topology. An example of
a tree topology network is a bridged or switched network running the spanning tree
algorithm, usually found with Ethernet (IEEE 802.3) networks. The spanning tree
algorithm disables loops in what would otherwise be a looped topology. Spanning tree
expands through the network and ensures that only one active path exists between any two
LAN-attached stations. Figure 2-8 is an example of a tree topology.
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