Topologies | Choosing the Correct Cabling

The network's topology refers to the physical layout of the elements of the telecommunications cabling system structure (e.g., main equipment room, cross-connections, telecommunications rooms and enclosures, switches, and hubs) that make up the network. Choosing the right topology is important because the topology affects the type of networking equipment, cabling, growth path, and network management.

Today's networking architectures fall into one of three categories:

• Hierarchical star
• Bus
• Ring

Topologies are tricky because some networking architectures appear to be one type of technology but are in reality another. Token Ring is a good example of this because Token Ring uses hubs (multistation access units or MAUs). All stations are connected to a central hub, so physically it is a hierarchical star topology; logically, though, it is a ring topology. Often two topology types will be used together to expand a network.

Physical vs. Virtual Topology

Topology refers to the physical layout of the wiring and key connection points of a network, and it also refers to the network's method of transmitting data and to the logical, orvirtual, layout of its connection points. Before the advent of structured wiring, physical and logical topology were often the same. For example, a network that had a ring topology actually had the wiring running from point to point in a ring. This can be confusing these days. The implementation of structured wiring standardized a hierarchical star configuration as the physical topology for modern networks, and network electronics take care of the logical topologies.

Hierarchical Star Topology

When implementing a hierarchical star topology, all computers are connected to a single, centrally located point. This central point is usually a hub of servers and switches located in the main equipment room and interconnected through the main cross-connection. All cabling used in a hierarchical star topology is run from the equipment outlets back to this central location. Typically in commercial buildings, there is a horizontal cross-connection with a workgroup switch located in a telecommunications room (TR) that allows backbone cabling to interconnect with horizontal cabling. Usually, the utilization of switch ports is low. In other words, only 70–80 percent of the ports in a switch are connected. Essentially, you are paying for more ports than you are using.

A lower-cost method involves placing horizontal cross-connections and workgroup switches in telecommunications enclosures. This is standardized in ANSI/TIA-568-C and is commonly referred to as fiber-to-the-telecommunications enclosure (FTTE). These house mini-patch panels and switches and are located in enclosures, installed overhead or on wall space, very close to clusters of equipment outlets. One benefit of this implementation of the hierarchical star is that the utilization of switch ports is typically 90 percent or greater. Another benefit is that TRs can be smaller, reducing power and HVAC requirements since TRs do not house the equipment and patch panels. Studies performed by TIA's Fiber Optic LAN Section have shown that the reduction of TR space and utility requirements, combined with lower costs in switches and ports, can lead to savings of 25 percent or more compared to a traditional implementation of hierarchical star where switches are located in TRs. One practical disadvantage of FTTE that has been raised by some users is the need to service equipment out in the work space environment (for example, above office cubes) as opposed to in a TR.

Another alternative implementation of a hierarchical star topology per ANSI/TIA-568-C is called centralized cabling. Centralized cabling is a hierarchical star topology that extends from the main cross-connection in an equipment room directly to an equipment outlet by allowing a cable to be pulled through a telecommunications room (or enclosure) without passing through a switch. The cable can be a continuous sheath of cable from the equipment room, or two separate cables may be spliced or interconnected in the TR. In either case, there is no need to use a workgroup switch in a TR to interconnect a backbone cable to a horizontal cable since all electronics are centralized in the main equipment room. This subset of the hierarchical star topology is commonly referred to as fiber-to-the-desk (FTTD) since it employs fiber to support the greater than 100 meter distances from the main equipment room/cross-connection to the equipment/telecommunications outlet. However, media converters or optical NICs are required to convert from optical to electrical near the equipment outlets. The centralized cabling topology can produce savings by reducing the size of TRs and need for HVAC since no active equipment is used in the TR. TIA's Fiber Optic LAN Section (www.fols.org) has studied this topology in detail and provides good information on its benefits.

 Hierarchical star topology with a central hub to support FTTD

Cabling @ Work: High School Decides to Use Optical Fiber Cabling

A new school recently designed a new communications network. They decided to use an optical fiber network. Why? Let's take a look.

Why ABC High Uses Optical Fiber Cabling

ABC High chose to use optical fiber cabling for two primary reasons:

Link lengths By taking advantage of the long data runs supported by multimode fiber, network planners were able to consolidate electronics into a single, centrally located wiring closet. This approach drastically reduced the amount of electronics needed for the installation and lowered the cost of (and simplified) network maintenance.

Network longevity School administrators realized that additional "headroom" would become essential in future programs and/or applications. This cabling system provides a quick and easy way to upgrade the system in years to come.

Installation Details

The new high school has 32 classrooms and accommodates approximately 650 students in grades 9–12. School administrators chose a centralized cabling network topology with a switch serving as the backbone.

Within the building, a single, centrally located wiring closet forms the heart of the network cabling plant, with OM3 50/125 micron (per ISO 11801 Ed. 2) multimode fiber links radiating to each of the classrooms and other areas throughout the building. There are at least seven network drops in each classroom.

The use of small form-factor connectors is valuable as it allows large numbers of fibers to be terminated in a small panel space.

From the perspective of cabling, the hierarchical star topology is now almost universal. It is also the easiest of the three networking architectures to cable. The ANSI/TIA-568-C and ISO/IEC 11801 Ed. 2 standards assume that the network architecture uses a hierarchical star topology as its physical configuration. If a single node on the star fails or the cable to that node fails, then only that single node fails. However, if the hub fails, then the entire star fails. Regardless, identifying and troubleshooting the failed component is much easier than with other configurations because every node can be isolated and checked from the central distribution point. In this topology, the network speed capability of the backbone is typically designed to be 10 times that of the horizontal system. For example, if the equipment outlets and desktops are provisioned to run 100Mbps in the horizontal, then these links are usually connected to a workgroup switch that has a 1Gbps uplink connected to the building riser backbone cable. If the horizontal links were designed to operate at 1Gbps, then the backbone should be capable of operating at 10Gbps if it is expected that the aggregate load of the horizontal connections on the switch will be close to 10Gbps. Although you may not need to implement the hardware to support these speeds on day one, it is smart to design the cabling media with a 10:1 ratio to support future bandwidth growth.

From this point on in the chapter, we will assume you understand that the physical layout of a modern network is a hierarchical star topology and that when we discuss bus and ring topologies we're referring to the logical layout of the network.

Killing an Entire Hierarchical Star Topology

Although a single node failure cannot usually take down an entire star topology, sometimes it can. In some circumstances, a node fails and causes interference for the entire star. In other cases, shorts in a single cable can send disruptive electrical signals back to the hub and cause the entire star to cease functioning. Of course, failure of the hub will also affect all nodes in a star topology. (Just as you need diversity and redundancy in cabling/network paths, you need diversity and redundancy in your switches as well.)

Bus Topology

The bus topology is the simplest network topology. Also known as a linear bus, in this topology all computers are connected to a contiguous cable or a cable joined together to make it contiguous. 

 Bus topology

Ethernet is a common example of a bus topology. Each computer determines when the network is not busy and transmits data as needed. Computers in a bus topology listen only for transmissions from other computers; they do not repeat or forward the transmission on to other computers.

The signal in a bus topology travels to both ends of the cable. To keep the signal from bouncing back and forth along the cable, both ends of the cable in a bus topology must be terminated. A component called a terminator, essentially nothing more than a resistor, is placed on both ends of the copper (coax) cable. The terminator absorbs the signal and keeps it from ringing, which is also known as overshoot or resonance; this is referred to as maximum impedance. If either terminator is removed or if the cable is cut anywhere along its length, all computers on the bus will fail to communicate.

Coaxial cabling was most commonly used in true bus-topology networks such as thin/thick Ethernet. However, UTP-based cabling Ethernet applications like 10Base-T, 100Base-T, and above still function as if they were a bus topology even though they are wired as a hierarchical star topology.

Ring Topology

A ring topology requires that all computers be connected in a contiguous circle. The ring has no ends or hub. Each computer in the ring receives signals (data) from its neighbor, repeats the signal, and passes it along to the next node in the ring. Because the signal has to pass through each computer on the ring, a single node or cable failure can take the entire ring down.

Ring topology

A true ring topology is a pain in the neck to install cable for because the circular nature of the ring makes it difficult to expand a ring over a large physical area. Token Ring is a ring topology. Even though Token Ring stations may be connected to a central MAU (and thus appear to be a star topology), the data on the Token Ring travels from one node to another. It passes through the MAU each time.