A real-life situation
A school has APs in every classroom, but also a storage room with no network cable, a bus garage across the car park, an old lab instrument with only an Ethernet port, and a sports field that needs coverage on match days. One topology can't serve them all. Wi-Fi has a different arrangement for each of these problems.
Infrastructure: BSS and ESS
In infrastructure mode every frame goes through an AP, even between two clients on the same AP. One AP radio with its clients is a BSS; several APs with the same SSID, joined by the wired distribution system, form an ESS that clients can roam across. This is the normal enterprise design, and the base the other topologies extend.
Ad hoc: IBSS
In an independent BSS two or more devices talk directly, with no AP. It needs no infrastructure, but it doesn't scale, has no central control and doesn't reach the wired network. Enterprises usually block it. Its modern relative, Wi-Fi Direct, connects a phone to a printer or a TV.
Extending coverage without a cable
- 1. Repeater: The laptop's frames cross the air twice, laptop to repeater and repeater to AP1.
- 2. Workgroup bridge: The WGB joins AP1 as a client and carries the wired devices' traffic over the air.
- Repeater: an AP with no cable that associates to a root AP and relays its clients' traffic. With one radio it repeats on the same channel, halving throughput; dual-radio repeaters can use a second channel for the link back. The repeater's cell should overlap the root AP's cell by about 50%.
- Workgroup bridge (WGB): a device (often an AP in WGB mode) that joins the WLAN as a client and gives wired-only devices behind it a connection, such as a lab instrument, a vending machine or the network in a vehicle. A Cisco WGB supports several wired clients; a universal WGB (uWGB) supports one, but also works with other vendors' APs.
Linking buildings: outdoor bridges
- 1. Bridged link: The two bridges act like one long cable: the LANs in both buildings are joined at Layer 2.
- Point-to-point: two bridges with directional antennas (Yagi, patch or dish) aimed at each other, linking two LANs. Distances of several kilometres are possible with a clear line of sight.
- Point-to-multipoint: one root bridge, often with an omnidirectional or sector antenna, links several remote buildings, each with a directional antenna pointing back at it.
A bridge carries no client traffic of its own; it just replaces a cable. Line of sight matters: the space around the straight line between the antennas (the Fresnel zone) should be largely clear of trees and buildings.
Mesh
- 1. Normal path: MAP1 sends the phone's traffic straight to the root AP over the wireless backhaul.
- 2. Self-healing: MAP3 has two possible paths. If MAP1 fails, it uses MAP2.
In a mesh, only the root AP (RAP) is cabled. Mesh APs (MAPs) connect to it, or to each other, over a wireless backhaul, usually on 5 GHz, while serving clients on their other radio. On Cisco controllers, AWPP (Adaptive Wireless Path Protocol) picks each MAP's best path to a RAP and changes it if a link fails. Mesh suits outdoor areas, campuses, ports and temporary sites where cabling is impossible or expensive. Each extra wireless hop adds delay and uses airtime.
Choosing a topology
| Need | Topology |
|---|---|
| Normal office or school coverage | Infrastructure ESS with cabled APs |
| One small area beyond the cable | A repeater (or better: run a cable) |
| A wired-only device where there is no cable | Workgroup bridge |
| Join two buildings' LANs | Point-to-point outdoor bridge |
| Join several buildings to one site | Point-to-multipoint bridge |
| Large outdoor area with few cables | Mesh |
| Two devices with no network at all | Ad hoc or Wi-Fi Direct |
Why it works this way
Cables are fast and reliable, but they can't go everywhere. Every topology beyond the cabled ESS trades some performance for reach: a wireless hop uses airtime that clients could have used, and adds a link that interference can break. That is why designers cable APs wherever they can and use wireless links only where they must.
Common mistakes
- Using repeaters to fix poor coverage everywhere, then wondering why the whole network is slow.
- Confusing a workgroup bridge (a client for wired devices) with an outdoor bridge (a link between LANs).
- Planning an outdoor bridge without line of sight; trees that grow in summer can break a link that worked in winter.
💡 Exam tip: the CCNA expects you to recognise BSS, ESS, IBSS, repeater, workgroup bridge, outdoor bridge and mesh from a description.
Key takeaways
- Infrastructure mode (BSS, ESS) sends everything through APs; ad hoc (IBSS) needs no AP.
- A repeater relays an AP's cell without a cable, at the cost of throughput.
- A workgroup bridge connects wired-only devices to a WLAN as a client.
- Outdoor bridges link buildings point-to-point or point-to-multipoint.
- Mesh: wired root APs and wireless mesh APs that find their own paths.
Check yourself
A factory machine has only an Ethernet port and sits far from any switch, but in range of an AP. What connects it?
Two buildings 2 km apart need their LANs joined without leasing a line. What fits best?
In a mesh network, which AP has a cable to the wired network?