Environmental Characteristics and Considerations

The campus environment, including the location of the network nodes, the distance between the nodes, and the transmission media used, influences the network topology. This section examines these considerations.

Network Geography Considerations

The location of Enterprise Campus nodes and the distances between them determine the network's geography.

Nodes, including end-user workstations and servers, can be located in one or multiple buildings. Based on the location of nodes and the distance between them, the network designer decides which technology should interconnect them based on the required maximum speed, distance, and so forth.

Consider the following structures with respect to the network geography:

■ Intrabuilding

■ Interbuilding

■ Distant remote building

These geographic structures, described in the following sections, serve as guides to help determine Enterprise Campus transmission media and the logical modularization of the Enterprise Campus network.

Intrabuilding Structure

An intrabuilding campus network structure provides connectivity for all end nodes located in the same building and gives them access to the network resources. The Building Access and Building Distribution layers are typically located in the same building.

User workstations are usually attached to the Building Access switches in the floor wiring closet with twisted-pair copper cables. Wireless LANs (WLAN) can also be used to provide intrabuilding connectivity, enabling users to establish and maintain a wireless network connection throughout—or between—buildings, without the limitations of wires or cables.

NOTE WLANs are covered in Chapter 9, "Wireless Network Design Considerations."

Access layer switches usually connect to the Building Distribution switches over optical fiber, providing better transmission performance and less sensitivity to environmental disturbances than copper. Depending on the connectivity requirements to resources in other parts of the campus, the Building Distribution switches may be connected to Campus Core switches.

Interbuilding Structure

As shown in Figure 4-5, an interbuilding network structure provides connectivity between the individual campus buildings' central switches (in the Building Distribution and/or Campus Core layers). These buildings are usually in close proximity, typically only a few hundred meters to a few kilometers apart.

Figure 4-5 Interbuilding Network Structure


Cam Core

Buildi Distr

Cam Core ng butio

Building Access n/

Building Access






Building Access

Building A

Building B

Because the nodes in all campus buildings usually share common devices such as servers, the demand for high-speed connectivity between the buildings is high. Within a campus, companies might deploy their own physical transmission media. To provide high throughput without excessive interference from environmental conditions, optical fiber is the medium of choice between the buildings.

Depending on the connectivity requirements to resources in other parts of the campus, the Building Distribution switches might be connected to Campus Core switches.

Distant Remote Building Structure

When connecting buildings at distances that exceed a few kilometers (but still within a metropolitan area), the most important factor to consider is the physical media. The speed and cost of the network infrastructure depend heavily on the media selection.

If the bandwidth requirements are higher than the physical connectivity options can support, the network designer must identify the organization's critical applications and then select the equipment that supports intelligent network services—such as QoS and filtering capabilities—that allow optimal use of the bandwidth.

Some companies might own their media, such as fiber, microwave, or copper lines. However, if the organization does not own physical transmission media to certain remote locations, the Enterprise Campus must connect through the Enterprise Edge using connectivity options from public service providers, such as traditional WAN links or Metro Ethernet.

The risk of downtime and the service level agreements available from the service providers must also be considered. For example, inexpensive but unreliable and slowly repaired fiber is not desirable for mission-critical applications.

NOTE Chapter 5, "Designing Remote Connectivity," includes further discussion of connecting remote locations.

Transmission Media Considerations

An Enterprise Campus can use various physical media to interconnect devices. The type of cable is an important consideration when deploying a new network or upgrading an existing one. Cabling infrastructure represents a long-term investment—it is usually installed to last for ten years or more. The cost of the medium (including installation costs) and the available budget must be considered in addition to the technical characteristics such as signal attenuation and electromagnetic interference.

A network designer must be aware of physical media characteristics, because they influence the maximum distance permitted between devices and the network's maximum transmission speed. Twisted-pair cables (copper), optical cables (fiber), and wireless (satellite, microwave, and Institute of Electrical and Electronics Engineers [IEEE] 802.11 LANs) are the most common physical transmission media used in modern networks.


Twisted-pair cables consist of four pairs of isolated wires that are wrapped together in plastic cable. With unshielded twisted-pair (UTP), no additional foil or wire is wrapped around the core wires. This makes these wires less expensive, but also less immune to external electromagnetic influences than shielded twisted-pair cables. Twisted-pair cabling is widely used to interconnect workstations, servers, or other devices from their network interface card (NIC) to the network connector at a wall outlet.

The characteristics of twisted-pair cable depend on the quality of the material from which they are made. As a result, twisted-pair cables are sorted into categories. Category 5 or greater is recommended for speeds of 100 megabits per second (Mbps) or higher. Category 6 is recommended for Gigabit Ethernet. Because of the possibility of signal attenuation in the wires, the maximum cable length is usually limited to 100 meters. One reason for this length limitation is collision detection. If one PC starts to transmit and another PC is more than 100 meters away, the second PC might not detect the signal on the wire and could therefore start to transmit at the same time, causing a collision on the wire.

One of the main considerations in network cabling design is electromagnetic interference. Due to high susceptibility to interference, twisted pair is not suitable for use in environments with electromagnetic influences. Similarly, twisted pair is not appropriate for environments that can be affected by the interference created by the cable itself.

NOTE Some security issues are also associated with electromagnetic interference. Hackers with access to the cabling infrastructure might eavesdrop on the traffic carried across UTP, because these cables emit electromagnetic signals that can be detected.

Distances longer than 100 meters may require Long-Reach Ethernet (LRE). LRE is Cisco-proprietary technology that runs on voice-grade copper wires; it allows higher distances than traditional Ethernet and is used as an access technology in WANs. Chapter 5 further describes LRE.

232 Chapter 4: Designing Basic Campus and Data Center Networks Optical Fiber

Typical requirements that lead to the selection of optical fiber cable as a transmission medium include distances longer than 100 meters and immunity to electromagnetic interference. Different types of optical cable exist; the two main types are multimode (MM) and single-mode (SM).

Multimode fiber is optical fiber that carries multiple light waves or modes concurrently, each at a slightly different reflection angle within the optical fiber core. Because modes tend to disperse over longer lengths (modal dispersion), MM fiber transmission is used for relatively short distances. Typically, LEDs are used with MM fiber. The typical diameter of an MM fiber is 50 or 62.5 micrometers.

Single-mode (also known as monomode) fiber is optical fiber that carries a single wave (or laser) of light. Lasers are typically used with SM fiber. The typical diameter of an SM fiber core is between 2 and 10 micrometers. Single-mode fiber limits dispersion and loss of light, and therefore allows for higher transmission speeds, but it is more expensive than multimode fiber.

Both MM and SM cables have lower loss of signal than copper cable. Therefore, optical cables allow longer distances between devices. Optical fiber cable has precise production and installation requirements; therefore, it costs more than twisted-pair cable.

Optical fiber requires a precise technique for cable coupling. Even a small deviation from the ideal position of optical connectors can result in either a loss of signal or a large number of frame losses. Careful attention during optical fiber installation is imperative because of the traffic's high sensitivity to coupling misalignment. In environments where the cable does not consist of a single fiber from point to point, coupling is required, and loss of signal can easily occur.


The inherent nature of wireless is that it does not require wires to carry information across geographic areas that are otherwise prohibitive to connect. WLANs can either replace a traditional wired network or extend its reach and capabilities. In-building WLAN equipment includes access points (AP) that perform functions similar to wired networking hubs, and PC client adapters. APs are distributed throughout a building to expand range and functionality for wireless clients. Wireless bridges and APs can also be used for interbuilding connectivity and outdoor wireless client access.

Wireless clients supporting IEEE 802.11g allow speeds of up to 54 Mbps in the 2.4-GHz band over a range of about 100 feet. The IEEE 802.11b standard supports speeds of up to 11 Mbps in the 2.4GHz band. The IEEE 802.11a standard supports speeds of up to 54 Mbps in the 5-GHz band.

NOTE Wireless issues are discussed further in Chapter 9.

Transmission Media Comparison

Table 4-2 presents various characteristics of the transmission media types.

Table 4-2 Transmission Media Type Characteristics


Copper Twisted Pair

MM Fiber

SM Fiber


Distance (range)

Up to 100 meters

Up to 2 kilometers (km) (Fast Ethernet)

Up to 550 m (Gigabit Ethernet)

Up to 300 m (10 Gigabit Ethernet)

Up to 10 km (Fast Ethernet)

Up to 5 km (Gigabit Ethernet)

Up to 80 km (10 Gigabit Ethernet)

Up to 500 m at 1 Mbps


Up to 10 Gigabits per second (Gbps)

Up to 10 Gbps

Up to 10 Gbps or higher

Up to 54 Mbps1




Moderate to expensive


Deployment area

Wiring closet

Internode or interbuilding

Internode or interbuilding

Internode or interbuilding

^Wireless is half-duplex, so effective bandwidth will be no more than half of this rate.

^Wireless is half-duplex, so effective bandwidth will be no more than half of this rate.

The parameters listed in Table 4-2 are as follows:

■ Distance: The maximum distance between network devices (such as workstations, servers, printers, and IP phones) and network nodes, and between network nodes. The distances supported with fiber vary, depending on whether it supports Fast Ethernet or Gigabit Ethernet, the type of fiber used, and the fiber interface used.

■ Bandwidth: The required bandwidth in a particular segment of the network, or the connection speed between the nodes inside or outside the building.

NOTE The wireless throughput is significantly less than its maximum data rate due to the half-duplex nature of radio frequency technology.

■ Price: Along with the price of the medium, the installation cost must be considered. For example, fiber installation costs are significantly higher than copper installation costs because of strict requirements for optical cable coupling.

■ Deployment area: Indicates whether wiring is for wiring closet only (where users access the network), for internode, or for interbuilding connections.

When deploying devices in an area with high electrical or magnetic interference—for example, in an industrial environment—you must pay special attention to media selection. In such environments, the disturbances might interfere with data transfer and therefore result in an increased number of frame errors. Electrical grounding can isolate some external disturbance, but the additional wiring increases costs. Fiber- optic installation is the only reasonable solution for such networks.

Cabling Example

Figure 4-6 illustrates a typical campus network structure. End devices such as workstations, IP phones, and printers are no more than 100 m away from the LAN switch. UTP wiring can easily handle the required distance and speed; it is also easy to set up, and the price-performance ratio is reasonable.

Figure 4-6 Campus Networks Use Many Different Types of Cables

Figure 4-6 Campus Networks Use Many Different Types of Cables

Fiber SM Gigabit Ethernet: <5 km

NOTE The distances shown in the figure are for a sample network; however, the maximum distance supported varies depending on the fiber interface used.

Optical fiber cables handle the higher speeds and distances that may be required among switch devices. MM optical cable is usually satisfactory inside the building. Depending on distance, organizations use MM or SM optical for interbuilding communication cable. If the distances are short (up to 500 m), MM fiber is a more reasonable solution for speeds up to 1 Gbps.

However, an organization can install SM fiber if its requirements are for longer distances, or if there are plans for future higher speeds (for example, 10 Gbps).

NOTE Selecting the less expensive type of fiber might satisfy a customer's current needs, but this fiber might not meet the needs of future upgrades or equipment replacement. Replacing cable can be very expensive. Planning with future requirements in mind might result in higher initial costs but lower costs in the long run.

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  • karolin
    Which three items are environmental characteristics that affect the campus network design?
    3 years ago
  • albertina
    What are the design considerations for transmission media?
    3 years ago

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