Network Topologies

 

There are three primary types of network topologies, which refer to the physical and logical layout of the network cabling. They are Bus, Star, and Ring. Bus and Star are the most widely used for Ethernet networks and Ring is used for Token Ring networks.

 

 

Low Level Standards

Prior to discussing network topologies, it is necessary to define low level standards. These are guidelines that describe how data (frames) are transmitted across the physical and data link layers of a network. They are developed by the Institute of Electrical and Electronic Engineers. The 802.x standard describes guidelines for Ethernet and Token Ring networks. Standards such as 10 BASE T and 10 BASE 2 describe a specific cable type and other limitations for Ethernet, such as Category 5 unshielded twisted pair for 100 BASE T, or Fast Ethernet.

 

 

Bus topology

Bus topology refers to a single cable that connects all the workstations, servers, printers and other devices on the network. The cable runs from device to device by using tee connectors that plug into the network adapter cards. Each end device has a terminator on one end of the tee and a cable going out to the next device on the other end, while all devices in the middle have one cable coming in and one going out. The terminators on each end device simply stop the network signal from reflecting back into the cable and colliding with other transmissions. The most common type of network cable used for a bus topology is RG-58 thin net. The network speed is limited to 10 megabits per second, making it a suitable media for only 10 BASE 2 Ethernet. There are also network size limitations. You may have a maximum of twenty network devices on a segment, and the segment cannot exceed 185 meters in total length. By using a device called a repeater that boosts the signal, you can have up to five segments on a network. However, only three of these segments can have devices attached to them. The other two segments are used to link the three populated segments, giving you a maximum number of sixty devices with a total network length of 925 meters. This topology works equally well for either peer to peer or client server.

Advantages

• Less expensive than a star topology due to less footage of cabling and no network hubs

• Good for smaller networks not requiring higher speeds

 

Disadvantages

• Limited in size and speed

• One bad connector can take down entire network

• Difficult to troubleshoot

 

 

Star Topology

In a star topology, each network device has a home run of cabling back to a network hub, giving each device a separate connection to the network. If there is a problem with a cable, it will generally not affect the rest of the network. The most common cable media in use for star topologies is unshielded twisted pair copper cabling. Category 3 is still found frequently in older installations. It is capable of 10 megabits per second data transfer rate, making it suitable for only 10 BASE T Ethernet. Most new installations use Category 5 cabling. It is capable of data transfer rates of 100 megabits per second, enabling it to employ 100 BASE T Ethernet, also known as Fast Ethernet. More importantly, the brand new 1000 BASE T Ethernet standard will be able to run over most existing Category 5. Finally, fiber optic cable can be used to transmit either 10 BASE T or 100 BASE T Ethernet frames.

Two variations of the star topology used by most large Ethernet networks today are the star bus and star tree topologies. Essentially, the star bus topology has multiple data closets interconnected by bus trunk lines of thin net, while the star tree topology links multiple data closets with twisted pair or fiber optic. These types of network topologies allow a network to cover a much larger physical area.

There are size limitations to star topologies utilizing Ethernet. The maximum number of network devices is 1,024 and the maximum number of data closets is four. When using Category 3 or 5 twisted pair cabling, individual cables cannot exceed 100 meters. In regard to total network length, the maximum when linking data closets with twisted pair is 500 meters between the furthest two devices. If multi-mode fiber optic is used to link closets, then the distance between closets can be up to 2,000 meters.

 

Advantages

• More suited for larger networks

• Easy to expand network

• Easy to troubleshoot because problem usually isolates itself

• Cabling types can be mixed

 

Disadvantages

• Hubs become a single point of network failure, not the cabling

• Cabling more expensive due to home run needed for every device

 

 

Ring Topology

Ring topologies are used on token ring networks. Each device processes and retransmits the signal, so it is capable of supporting many devices in a somewhat slow but very orderly fashion. A token, or small data packet, is continuously passed around the network. When a device needs to transmit, it reserves the token for the next trip around, then attaches its data packet to it. The receiving device sends back the packet with an acknowledgment of receipt, then the sending device puts the token back out on the network. Most token ring networks have the physical cabling of a star topology and the logical function of a ring through use of multi access units (MAU). In a ring topology, the network signal is passed through each network card of each device and passed on to the next device. All devices have a cable home runned back to the MAU. The MAU makes a logical ring connection between the devices internally. When each device signs on or off, it sends an electrical signal which trips mechanical switches inside the MAU to either connect the device to the ring or drop it off the ring. The most common type of cabling used for token ring networks is twisted pair, although there are nine different types that can be used. With IBM Type 1 Shielded cable, you can have up to 33 network segments with 260 devices on each. Transmission rates are at either 4 or 16 megabits per second.

 

Advantages

• Very orderly network where every device has access to the token and the opportunity to transmit

• Performs better than a star topology under heavy network load

• Can create much larger network using Token Ring

 

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 are much more expensive than Ethernet cards and hubs

• Much slower than an Ethernet network under normal load