Physical Topologies

Bus Topology

• A networking topology that connects networking components along a single cable or that uses a series of cable segments that are connected linearly.
• A network that uses a bus topology is referred to as a “bus network.” Bus networks were the original form of Ethernet networks, using the 10Base5 cabling standard. Bus topology is used for:
  • Small work-group local area networks (LANs) whose computers are connected using a thinnet cable
  • Trunk cables connecting hubs or switches of departmental LANs to form a larger LAN
  • Backboning, by joining switches and routers to form campus-wide networks

Advantages

• Easy to install
• Costs are usually low
• Easy to add systems to network
• Great for small networks

Disadvantages

• out of date technology.
• include difficult reconnection and fault isolation
• Can be difficult to troubleshoot.
• Unmanageable in a large network
• If cable breaks, whole network is down

Ring Topology

• In a ring topology, each device has a dedicated point-to-point connection with only the two devices on either side of it.
• A signal is passed along the ring in one direction, from device to device, until it reaches its destination.
• Each device in the ring incorporates a repeater. When a device receives a signal intended for another device, its repeater regenerates the bits and passes them along.
• A ring is relatively easy to install and reconfigure.
• Each device is linked to only its immediate neighbors (either physically or logically). To add or delete a device requires changing only two connections. The only constraint are media and traffic considerations (maximum ring length and number of devices).
• In addition, fault isolation is simplified. Generally in a ring, a signal is circulating at all times. If one device does not receive a signal within a specified period, it can issue an alarm.
• The alarm alerts the network operator to the problem and its location.
• However, unidirectional traffic can be a disadvantage.
• In a simple ring, a break in the ring (such as a disabled station) can disable the entire network.
• This weakness can be solved by using a dual ring or a switch capable of closing off the break.
• However, unidirectional traffic can be a disadvantage. In a simple ring, a break in the ring (such as a disabled station) can disable the entire network.
• This weakness can be solved by using a dual ring or a switch capable of closing off the break.

Advantages

• Very orderly network where every device has access to the token and the opportunity to transmit
• Performs better than a bus topology under heavy network load
• Does not require network server to manage the connectivity between the computers

Disadvantages

• One malfunctioning workstation or bad port in the MAU can create problems for the entire network
• Moves, adds and changes of devices can affect the network
• Network adapter cards and MAU’s a Multistation Access Unit are much more expensive than Ethernet cards and hubs
• Much slower than an Ethernet network under normal load

Mesh Topology

In a mesh topology, every device has a dedicated point-to-point link to every other device. The term dedicated
means that the link carries traffic only between the two devices it connects. To connect n nodes in Mesh
topology, we require n(n-1)/2 duplex mode links.

Advantages:

• The use of dedicated links guarantees that each connection can carry its own data load, thus eliminating
• the traffic problems that can occur when links must be shared by multiple devices.
• Robust, If one link becomes unusable, it does not incapacitate the entire system.
• Advantage of privacy or security.
• point-to-point links make fault identification and fault isolation easy , Traffic can be routed to avoid links with suspected problems.

Disadvantage:

• Required high amount of cabling and the number of I/O ports.
• the sheer bulk of the wiring can be greater than the available space (in walls, ceilings, or floors) can accommodate.
• the hardware required to connect each link (I/O ports and cable) can be prohibitively expensive.

One practical example of a mesh topology is the connection of telephone regional offices in which each regional office needs to be connected to every other regional office.

Star Topology

In a star topology, each device has a dedicated point-to-point link only to a central controller, usually called a hub. The devices are not directly linked to one another. Unlike a mesh topology, a star topology does not allow direct traffic between devices. The controller acts as an exchange: If one device wants to send data to another, it sends the data to the controller, which then relays the data to the other connected device .

Advantages:

• Less Expensive than Mesh topology.
• In a star topology, each device needs only one link and one I/O port to connect it to any number of other devices. This factor also makes it easy to install and reconfigure.
• Less Cabling, Addition and Deletion involves only one connection between the devices and the Hub or Switch.
• Easy for Fault identification and fault isolation. If one link fails, only that link is affected. All
• other links remain active.

Disadvantage:

• One big disadvantage of a star topology is the dependency of the whole topology on one single point, the hub. If the hub goes down, the whole system is dead

An extended star topology links individual stars together by connecting the hubs or switches. A hierarchical topology is similar to an extended star. However, instead of linking the hubs or switches together, the system is linked to a computer that controls the traffic on the topology.

Logical Topologies

The logical topology of a network determines how the hosts communicate across the medium. The two most
common types of logical topologies are broadcast and token passing.

The use of a broadcast topology indicates that each host sends its data to all other hosts on the network
medium. There is no order that the stations must follow to use the network. It is first come, first serve. Ethernet
works this way as will be explained later in the course.

The second logical topology is token passing. In this type of topology, an electronic token is passed sequentially
to each host. When a host receives the token, that host can send data on the network. If the host has no data to
send, it passes the token to the next host and the process repeats itself. Two examples of networks that use token
passing are Token Ring and Fiber Distributed Data Interface (FDDI). A variation of Token Ring and FDDI is
Arcnet. Arcnet is token passing on a bus topology.