2 Network Infrastructure and Connectivity

How network traffic moves

Network infrastructure connects end devices, carries traffic between network segments, and provides the physical environment for reliable communications. A useful way to follow traffic is from a device’s network interface through a wired or wireless link to a switch and, when needed, a router.

Ethernet and switching

Ethernet is the common wired LAN technology defined by IEEE 802.3. Ethernet frames contain source and destination MAC addresses, payload data, and error-checking information. Ethernet operates over copper, fiber, and other physical media at a range of speeds.

Modern switched Ethernet links normally operate full duplex, allowing simultaneous transmission and reception. Collisions are principally associated with older shared, half-duplex Ethernet.

A switch learns which MAC addresses are reachable through which ports by examining the source address of incoming frames. It forwards a known unicast frame toward the port associated with its destination MAC address. If the destination is unknown, or the frame is broadcast, the switch floods it to eligible ports in the same VLAN, except the port on which it arrived.

Switches reduce unnecessary traffic compared with a shared hub. However, a Layer 2 loop can cause broadcast storms, so redundant switched networks use loop-prevention mechanisms such as Spanning Tree Protocol (STP).

Switches commonly provide access ports for endpoint devices and trunk ports for links between network devices. A port’s negotiated speed and duplex must be compatible at both ends; mismatches can cause poor performance or connectivity problems.

VLANs and trunk links

A virtual LAN (VLAN) divides a switched network into separate logical Layer 2 broadcast domains. Devices in different VLANs are isolated at Layer 2 even if they connect to the same physical switch. An organization might, for example, place employee computers, voice phones, and guest devices in separate VLANs.

Communication between VLANs requires a Layer 3 device, such as a router or Layer 3 switch. Routing policies or access-control lists can control that communication. IEEE 802.1Q specifies VLAN-aware bridging.

• An access port typically carries traffic for one VLAN and connects to an endpoint.

• A trunk port carries traffic for multiple VLANs, commonly between switches or between a switch and a router or access point.

• 802.1Q tagging identifies the VLAN associated with frames on a trunk. Depending on configuration, a trunk may also carry one VLAN’s traffic untagged; this is often called the native VLAN.

Trunk settings, including the allowed VLANs and native VLAN, should agree at both ends. For example, a PC connected to an access port assigned to VLAN 20 sends ordinary untagged Ethernet frames. The switch associates them with VLAN 20 and adds the appropriate VLAN tag when forwarding them across a trunk. The receiving switch uses the tag to keep the traffic in VLAN 20.

Wi-Fi standards and radio conditions

Wi-Fi is based on the IEEE 802.11 family of wireless LAN standards. Familiar generation names correspond to particular 802.11 amendments; these names are convenient labels, not guarantees of a particular real-world speed. The consolidated IEEE 802.11 standard includes amendments through 2024.

• Wi-Fi 4 (802.11n): Operates in 2.4 GHz and 5 GHz and introduced MIMO capabilities.

• Wi-Fi 5 (802.11ac): Operates in 5 GHz.

• Wi-Fi 6 (802.11ax): Designed for more efficient operation, including in busy environments; operates in 2.4 GHz and 5 GHz.

• Wi-Fi 6E (802.11ax): Extends Wi-Fi 6 operation into the 6 GHz band where permitted.

• Wi-Fi 7 (802.11be): A newer generation that may use 2.4 GHz, 5 GHz, and 6 GHz, depending on equipment and regulatory availability.

Older standards include 802.11b and 802.11g in 2.4 GHz, and 802.11a in 5 GHz. The 2.4 GHz band generally offers broader reach but has fewer non-overlapping channel choices and is more prone to interference. The 5 GHz and 6 GHz bands offer more channel capacity, but usable range and building penetration can be more limited.

Actual performance depends on factors such as channel width, interference, signal strength, access-point placement, client capability, and the number of active users. A wireless access point (AP) bridges wireless clients to the wired network. The network name users select is the SSID, which can be mapped to a particular VLAN to keep traffic such as guest wireless separate from employee traffic.

A Wi-Fi generation name and formal Wi-Fi Alliance certification are related but distinct. Certification indicates that a product has passed the Alliance’s interoperability testing.

Copper cabling and terminations

Twisted-pair copper is widely used for endpoint connections and Power over Ethernet (PoE). Common categories include Cat 5e, Cat 6, and Cat 6A. Category describes cable performance, not an Ethernet speed by itself: the achievable link depends on the cable, connectors, installation quality, link length, and equipment.

As a common planning guideline, a twisted-pair Ethernet channel is limited to 100 m (328 ft). Cat 6A supports 10GBASE-T to 100 m when the complete channel meets the applicable requirements.

Copper cable may be unshielded (UTP) or shielded. Shielding can help in environments with significant electromagnetic interference, but it must be correctly installed and bonded. Sharp bends, excessive pulling force, and untwisting pairs too far at terminations can degrade performance. Solid-conductor cable is typically used for permanent building runs; stranded patch cords are more flexible.

The familiar Ethernet plug is commonly called RJ-45, although 8-position, 8-contact (8P8C) is the more precise description. T568A and T568B are wiring schemes for terminating twisted-pair cable; use a consistent scheme at both ends for a straight-through cable. Many modern Ethernet ports support auto-MDI-X, but correct, consistent termination remains important.

Fiber and other connectors

Fiber-optic cable carries light and is immune to electromagnetic interference. Multimode fiber is commonly used for shorter building or data-center links, while single-mode fiber is suited to longer distances and higher-capacity links, depending on optics and design.

Common fiber connector types include LC and SC; ST and multi-fiber MPO connectors may also be encountered. Connector type, fiber type, transceiver wavelength, and link budget must be compatible. Keep fiber end faces clean and avoid looking into an active fiber or transceiver.

Other connectors include RJ-11 for many telephone connections and coaxial connectors such as F-type for cable services. Connectors must match the medium and equipment; a physical fit alone does not guarantee compatibility.

Physical infrastructure and troubleshooting

Structured cabling organizes permanent cable runs, termination points, patch panels, and equipment connections. Work-area jacks connect to horizontal cabling, which terminates at a telecommunications room or intermediate distribution frame (IDF). A main distribution frame (MDF) commonly serves as a building’s central distribution point and may connect to IDFs over a backbone, often fiber.

Patch panels provide organized termination, and short patch cords connect panel ports to switch ports. Racks and cabinets hold switches, patch panels, and other equipment. Good practice includes clear labeling, cable management, adequate airflow and cooling, appropriate power capacity, and physical access control.

Uninterruptible power supplies (UPSs) can provide temporary backup power and protection from some power disturbances. When planning or troubleshooting, trace the full path to locate a fault: device, patch cord, wall jack, horizontal run, patch panel, switch port, and any uplink.