Wireless Topology Modes & Service Sets (CCNA Notes)

IEEE 802.11 Overview

• Wireless LANs (WLANs) defined by IEEE 802.11 standard.

• Uses Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) to share a single communication channel among multiple devices.

  • Metaphor: "One-lane road"—only one vehicle (device) may occupy the road (channel) at any moment, so each must first sense the road, then proceed.

Wireless Topology Modes

• Two primary topologies plus one practical variant discussed:

  1. Ad Hoc Mode (a.k.a. Independent Basic Service Set – IBSS)

  2. Infrastructure Mode

  3. Tethering / Hotspot (quick-access variant of Infrastructure)

1 Ad Hoc Mode (IBSS)

Definition & Operation

• Devices connect directly, peer-to-peer (P2P) with no Access Point (AP).

• Suitable for temporary networks or locations without installed infrastructure.

Characteristics

• Independent Basic Service Set (IBSS)

  • Each device communicates on equal terms.

  • No centralized coordination.

• Scalability Limitations

  • Works fine for “a few” devices.

  • Performance and management issues arise as device count grows.

• Typical Use Cases

  • Quick file sharing.

  • Multiplayer gaming in a small room.

  • Disaster-recovery scenarios where APs are unavailable.

Key Terms & Identifiers

• BSSID = MAC address of the virtual IBSS (often the MAC of the device that created the network).

2 Infrastructure Mode

Definition & Core Idea

• All wireless clients communicate via a central Access Point (AP) that also acts as a bridge to a Distribution System (DS) (usually Ethernet).

• Dominant deployment in homes, offices, campuses, public hotspots.

Building Blocks

1. Basic Service Set (BSS)

2. Extended Service Set (ESS)

2.1 Basic Service Set (BSS)

• Single AP + its associated wireless clients.

• AP handles:

  • Frame transmission & reception.

  • Authentication / association.

  • Bridging to wired LAN.

• Basic Service Area (BSA)

  • Geographic coverage of one AP.

  • Similar to a cell tower’s cell in cellular networks.

  • Size determined by AP power, antenna gain, obstacles, etc.

• Identifier

  • BSSID = AP’s Layer-2 MAC address (unique per BSS).

2.2 Extended Service Set (ESS)

• Formed when 2 or more BSSs are interconnected through the same DS (e.g., Ethernet switch infrastructure).

• Goals

  • Extend coverage (larger area than single BSA).

  • Increase capacity (more concurrent clients).

  • Enable roaming: client can move from one BSA to another seamlessly within the same ESS.

• Identifier

  • SSID (Service Set Identifier) — user-visible network name broadcast by each AP in the ESS.

• Roaming Mechanics

  • Client monitors signal strength.

  • When threshold crossed, it re-associates to stronger AP sharing same SSID & security settings.

Infrastructure Mode Summary

• Pros: scalability, centralized management, integration with wired LAN, security policy enforcement.

• Cons: requires AP purchase/maintenance, single point of failure per BSS (unless redundant APs are used).

3 Tethering / Mobile Hotspot

Concept

• A smartphone or tablet uses its cellular data connection to create a mini-AP (hotspot).

• Acts as an Infrastructure-mode AP for nearby devices (laptops, tablets).

• Two radios involved:

  1. Cellular (3G/4G/5G) uplink to ISP.

  2. Wi-Fi downlink to local clients.

• Practical for travel, backup internet, or quick small-office connectivity.

CSMA/CA Refresher (Link-Layer Access Method)

• Each Wi-Fi device:

  1. Listens (Carrier Sense) to check if medium is idle.

  2. If idle, waits a random back-off period.

  3. Transmits frame.

  4. Uses Acknowledgments to confirm delivery; if none, assumes collision & retries.

• Collision Avoidance rather than collision detection (as in Ethernet) because simultaneous transmission corrupts the entire frame and radios cannot listen while transmitting.

Identifiers & Terminology Quick Sheet

• BSSID: \text{MAC}_{AP}\,\,(48\text{ bits}) — uniquely identifies a BSS.

• SSID: Human-readable name (up to 32 ASCII chars) — identifies an ESS.

• IBSS: Independent Basic Service Set (Ad Hoc).

• DS: Distribution System (usually Ethernet).

• BSA: Basic Service Area — RF coverage of one AP.

Practical / Ethical / Real-World Notes

• Security: Infrastructure mode supports WPA2/WPA3 enterprise policies; ad hoc is often unsecured.

• Channel Planning: Multiple APs in an ESS must be placed on non-overlapping channels to minimize co-channel interference.

• Power Control & Coverage Holes: Oversized BSAs can cause hidden-node problems; undersized can create coverage gaps.

• Roaming Contracts: In public Wi-Fi networks (airports, cafés), roaming across ESS may involve captive portals or paid hand-offs.

Example Scenarios

• Small Office/Home Office (SOHO): One AP, BSS, SSID "HomeWiFi"; laptops, phones connect; typical throughput >100\text{ Mbps}.

• University Campus: Dozens of APs per building, joined in one ESS "Campus-Secure"; seamless roaming so students stay connected while walking.

• Conference Sharing (Ad Hoc): Speakers share files quickly without relying on venue’s AP; <10 devices — IBSS preferred.

• Field-Work: Researchers in remote area create hotspot using satellite phone + tethering to provide internet to tablets.

Exam-Style Q&A (Self-Check)

1. Which topology enables two devices to communicate peer-to-peer without an AP?
   • Answer: Ad Hoc Mode (IBSS).

2. True or False: An ESS is created when two or more BSS need to be joined to support roaming clients.
   • Answer: True.

3. Define “Basic Service Area.”
   • Geographic RF coverage provided by a single AP within a BSS.

4. What identifier distinguishes one BSS from another at Layer 2?
   • BSSID (AP’s MAC address).

5. What technology allows a smartphone to act as a Wi-Fi AP for other devices?
   • Tethering / Mobile Hotspot.

IEEE 802.11 Overview

• Wireless LANs (WLANs) are defined by the IEEE 802.11 standard, which specifies the Physical (PHY) layer and Medium Access Control (MAC) sublayer of the Data Link Layer for wireless connectivity.

• The standard utilizes Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) through the Distributed Coordination Function (DCF) to manage shared access to a single communication channel among multiple devices. The primary goal is to minimize collisions in a half-duplex wireless medium.

• Metaphor: "One-lane road"—only one vehicle (device) may occupy the road (channel) at any moment. Before proceeding, each vehicle must first sense the road to ensure it is clear. Key 802.11 amendments like \text{802.11b} (2.4 GHz, 11 Mbps), \text{802.11a} (5 GHz, 54 Mbps), \text{802.11g} (2.4 GHz, 54 Mbps), \text{802.11n} (up to 600 Mbps, MIMO), \text{802.11ac} (Gigabit Wi-Fi), and \text{802.11ax} (Wi-Fi 6, OFDMA, higher efficiency) have evolved to offer increased speeds and wider frequency band usage.

Wireless Topology Modes

• Two primary topologies, plus one practical variant, are commonly discussed in \text{802.11}:

  1. Ad Hoc Mode (a.k.a. Independent Basic Service Set – IBSS)

  2. Infrastructure Mode

  3. Tethering / Hotspot (a quick-access variant of Infrastructure Mode)

1 Ad Hoc Mode (IBSS)

Definition & Operation

• Devices connect directly, peer-to-peer (P2P) without the need for a central Access Point (AP).

• This mode is particularly suitable for temporary networks or in locations without pre-installed network infrastructure.

Characteristics

• Independent Basic Service Set (IBSS)

  • Each device communicates on equal terms with other devices within the IBSS.

  • There is no centralized coordination; devices handle their own medium access and retransmissions.

• Scalability Limitations

  • Works acceptably for “a few” devices (typically less than 10-15).

  • Performance and management issues quickly arise as the number of devices grows due to increased collisions, lack of buffering, and absence of flow control or centralized quality of service (QoS).

  • Security is often limited to WEP or open configurations, lacking modern WPA/WPA2 enterprise features.

• Typical Use Cases

  • Quick file sharing between two laptops where a router is unavailable.

  • Multiplayer gaming in a small, localized setting.

  • Disaster-recovery scenarios or emergency communications where traditional APs are unavailable or damaged.

Key Terms & Identifiers

• BSSID = The MAC address of the virtual IBSS. In an Ad Hoc network, it is typically the MAC address of the device that initiated the network, serving as a unique identifier for that specific peer-to-peer group.

2 Infrastructure Mode

Definition & Core Idea

• All wireless clients communicate via a central Access Point (AP). The AP functions as a bridge between the wireless clients and a Distribution System (DS), which is typically a wired Ethernet Local Area Network (LAN).

• This is the dominant deployment model for Wi-Fi networks in homes, offices, campuses, and public hotspots due to its scalability, centralized management, and security features.

Building Blocks

1. Basic Service Set (BSS)

2. Extended Service Set (ESS)

2.1 Basic Service Set (BSS)

• A BSS consists of a single AP and all its associated wireless clients.

• The AP is responsible for several critical functions within the BSS:

  • Beaconing: Periodically broadcasting beacon frames containing network information (SSID, security, supported rates).

  • Frame transmission & reception: Managing all data traffic between wireless clients and the wired network.

  • Authentication / association: Authenticating and associating wireless clients to the BSS.

  • Bridging: Connecting the wireless medium to the wired LAN (DS).

  • Power management: Coordinating power-saving modes for client devices.

  • Quality of Service (QoS): Prioritizing traffic according to configured policies.

• Basic Service Area (BSA)

  • The BSA represents the geographic radio frequency (RF) coverage area provided by one AP.

  • It is analogous to a cell tower’s cell in cellular networks.

  • Its size and shape are determined by factors such as AP transmit power, antenna gain and type (omnidirectional vs. directional), environmental obstacles (walls, furniture), interference from other wireless devices, and the chosen Wi-Fi frequency band (2.4 GHz vs. 5 GHz).

• Identifier

  • BSSID = The AP’s Layer-2 MAC address is the unique identifier for a BSS. It is broadcast in beacon frames and distinguishes one BSS from another.

2.2 Extended Service Set (ESS)

• An ESS is formed when two or more BSSs are interconnected through the same Distribution System (DS), typically an Ethernet switch infrastructure.

• The primary goals of an ESS are:

  • Extend coverage: To provide a much larger contiguous coverage area than a single BSA can offer, enabling full building or campus-wide Wi-Fi access.

  • Increase capacity: By distributing clients across multiple APs, an ESS can support a higher total number of concurrent wireless clients.

  • Enable seamless roaming: Clients can move freely from one BSA (served by one AP) to another BSA (served by a different AP) within the same ESS without losing their network connection.

• Identifier

  • SSID (Service Set Identifier) — This is the user-visible network name (up to 32 ASCII characters) that is broadcast by each AP belonging to the ESS. All APs in the same ESS share the same SSID.

• Roaming Mechanics

  • Clients within an ESS continuously monitor the signal strength (RSSI) from nearby APs.

  • When the signal quality from the currently associated AP falls below a predefined threshold, or a stronger signal from another AP with the same SSID is detected, the client initiates a re-association process to the stronger AP. This Layer 2 mobility ensures continuous connectivity.

  • Roaming can be optimized through AP controllers and centralized management systems.

Infrastructure Mode Summary

• Pros: High scalability, centralized management (especially with controllers), seamless integration with existing wired LAN infrastructure, robust security policy enforcement (WPA2/WPA3 Enterprise, 802.1X, VLANs), and better performance due to central coordination and buffering.

• Cons: Requires hardware investment (APs, controllers) and ongoing maintenance, and a single AP failure can create a service disruption for its BSS (unless redundant APs or mesh capabilities are utilized).

3 Tethering / Mobile Hotspot

Concept

• Tethering describes the functionality where a smartphone or tablet leverages its cellular data connection (3G/4G/5G) to create a mini-AP or Wi-Fi hotspot.

• It effectively acts as a compact, personal Infrastructure-mode AP for nearby Wi-Fi-enabled devices like laptops, other tablets, or smart devices.

• Two distinct radio interfaces are involved in a tethering setup:

  1. The cellular radio provides the uplink connection to the Mobile Network Operator's (MNO) Internet Service Provider (ISP).

  2. The Wi-Fi radio provides the downlink connection to local clients, forming a localized Wi-Fi network.

• This solution is highly practical for use during travel, as a backup internet source, for providing quick internet access in small, temporary workgroups, or in areas without public Wi-Fi.

• Considerations include potential rapid battery drain on the host device and usage against cellular data caps.

CSMA/CA Refresher (Link-Layer Access Method)

• CSMA/CA is the protocol used by IEEE \text{802.11} devices to share the wireless medium. It avoids collisions by ensuring only one device transmits at a time. The process for each Wi-Fi device wishing to transmit is as follows:

  1. Carrier Sense: The device first listens (senses the carrier) to check if the wireless medium is idle for a specified duration (DIFS - Distributed Interframe Space).

  2. Random Back-off: If the medium is idle, the device doesn't transmit immediately. Instead, it waits for an additional random back-off period (determined by a contention window). This random delay helps to prevent multiple devices, sensing the channel idle simultaneously, from transmitting at the exact same moment.

  3. Transmission: If the medium remains idle throughout the back-off period, the device then transmits its frame.

  4. Acknowledgments (ACKs): After transmitting, the sender expects an Acknowledgment (ACK) frame from the receiver within a Short Interframe Space (SIFS). If an ACK is received, the transmission is successful. If no ACK is received (due to collision or corruption), the sender assumes a collision or error and schedules a retransmission, doubling its contention window for the next attempt (binary exponential back-off).

• Collision Avoidance rather than collision detection (as used in Ethernet) is crucial in wireless networks because:

  • Radios are half-duplex; they generally cannot listen for collisions while transmitting.

  • Collisions in wireless environments often corrupt the entire frame, making detection difficult and retransmission essential.

  • Hidden Node Problem: CSMA/CA can be augmented with RTS/CTS (Request to Send/Clear to Send) mechanisms. An RTS frame is sent by the sender, and the receiving AP responds with a CTS frame. Both frames include a Network Allocation Vector (NAV), a duration field that tells all other devices hearing either RTS or CTS to remain silent for the duration of the upcoming data exchange. This mitigates hidden node issues where a device might not hear another device's transmission but could still cause a collision at the AP.

Identifiers & Terminology Quick Sheet

• BSSID: ext{MAC}_{ ext{AP}} ext{ (48 bits)} — uniquely identifies a Basic Service Set (typically the MAC address of the AP's radio interface).

• SSID: Human-readable name (up to 32 ASCII characters) — identifies an Extended Service Set (ESS) and is the network name users see.

• IBSS: Independent Basic Service Set (Ad Hoc mode) — a peer-to-peer wireless network without an AP.

• DS: Distribution System (usually Ethernet) — the wired backbone connecting APs in an ESS.

• BSA: Basic Service Area — the geographic RF coverage area of a single Access Point.

• DIFS: Distributed Interframe Space — the minimum idle time a station must sense before starting its back-off timer.

• SIFS: Short Interframe Space — a shorter interframe space used for immediate responses like ACKs or CTS frames.

• NAV: Network Allocation Vector — a virtual carrier sense mechanism, indicating current channel reservation time based on RTS/CTS frames (or broadcast frames).

• RTS/CTS: Request to Send / Clear to Send — Optional handshake mechanism to mitigate hidden node problems and reserve the channel.

Practical / Ethical / Real-World Notes

• Security: Infrastructure mode significantly supports advanced security protocols such as WPA2 and WPA3, including both Pre-Shared Key (PSK) for home use and Enterprise versions (\text{802.1X} with RADIUS servers) for robust corporate authentication. Ad hoc mode, conversely, often uses less secure methods or no encryption at all, making it generally unsuitable for sensitive data.

• Channel Planning: For optimal performance and to minimize co-channel interference (CSI), multiple APs in an ESS, especially those within radio proximity, must be strategically placed on non-overlapping channels. In the 2.4 GHz band, channels \text{1, 6,} and \text{11} are typically used. The 5 GHz band offers more non-overlapping channels, making it preferable for high-density deployments.

• Power Control & Coverage Holes: Careful adjustment of AP transmit power and antenna selection is crucial. Oversized BSAs can lead to hidden-node problems (where two stations can't hear each other but both communicate with the same AP, leading to collisions). Undersized BSAs can create coverage gaps or