Types of Wireless Networks
Wireless LANs are not the only type of wireless networks that exist.
Wireless Personal Area Network (WPAN)
WPAN uses low-powered transmitters to create a very short range network, usually 7 to 10 meters. Based on the 802.15 standard and includes technologies such as Bluetooth and ZigBee although ZigBee can have greater range. Unlicensed ISM frequencies are used including the 2.4 GHz band.
Wireless Local Area Network (WLAN)
Wireless service connecting multiple devices using IEEE 802.11 standard over medium-sized range, usually up to 100 meters. Uses unlicensed frequencies in 2.4 GHz and 5 GHz band.
Wireless Metropolitan Area Network (WMAN)
Wireless service over a large geographic area, such as all or portion of a city. One technology used is WiMAX, which is based on the 802.16 standard. Most commonly uses licensed frequencies.
Wireless Wide Area Network (WWAN)
Wireless data service for mobile phones offered over a very large geographic area, such as regional, national or even global by telecommunication carriers. Licensed frequencies are used.
Wireless LAN Topologies
Likelihood of interference increases as the number of wireless devices grows. Wireless devices use half duplex to avoid colliding with other transmissions. This means hosts can’t transmit and receive at the same time on a given frequency.
To transmit and receive at the same time, one frequency would have to be used for transmitting and another for receiving, much like full-duplex transmission in a wired network. 802.11 AC Wave 2 will somewhat ease that restriction through the use of downstream MU-MIMO.
Wireless devices compete for airtime and only one host should transmit at a time. 802.11 devices have to determine if the channel is clear and available before transmitting.
There is no inherent organization to a wireless medium or any inherent control over the number of devices that can transmit and receive frames. Any wireless device can try to communicate. A wireless network should at a minimum make sure that every device using a channel can support a common set of parameters, including data rates, 802.11 modulation types, channel width and so on. It should also be possible to control which devices (and users) that are allowed to use the wireless medium and the methods used to secure the wireless transmission.
Basic Service Set
A wireless service area consisting of a closed group of mobile devices that form around a fixed device. Devices that want to join have to advertise their capabilities and be granted permission to join. This is called a Basic Service Set (BSS). The AP operates in infrastructure mode, meaning that it offers services that are necessary to form the infrastructure of a wireless network.
The BSS is bounded by the area where the AP’s signal is usable and this is known as the Basic Service Area (BSA) or cell. The AP is a single point of contact for devices that want to use the BSS. The existence of the BSS is advertised by the AP through a unique BSS Identifier (BSSID), based on the AP’s own radio MAC address.
The AP advertises the wireless network with a Service Set Identifier (SSID) which is a text string with a logical name. The BSSID identifies the AP and the SSID the wireless service.
Membership with the BSS is called an association. Devices send association requests and the AP grants or denies them. If granted the device becomes a client, or 802.11 Station (STA), of the BSS.
Most of the traffic to and from the client has to pass through the AP and has the BSSID as the source or destination address of data frames.
The upstream wired Ethernet network is referred to as the Distribution System (DS) for the BSS in the 802.11 standard.
Think of an AP as a translational bridge, where frames are translated between two dissimilar media (wired and wireless) and then bridged at L2. The AP maps VLANs to SSIDs. For example clients associated to the “mywifi” SSID would appear to be connected to VLAN 100. Multiple VLANs can be mapped to multiple SSIDs. The AP must be connect to a trunk port on a switch (DS with 802.1q) to be able to do this.
The AP appears as multiple logical APs by providing multiple BSS with a unique BSSID for each. On Cisco APs this is normally accomplished by incrementing the last digit of the radio’s MAC for each SSID.
The multiple SSIDs cover the same geographic area due to sharing transmitter, receiver, antennas and channel.
Extended Service Set
Most often one AP can’t provide coverage for all clients. APs interconnected by a switch infrastructure are called an Extended Service Set (ESS).
The APs should cooperate and provide a consistent wireless service that is seamless from the client’s perspective. This means that the same set of SSIDs should be supported on the APs in the ESS. A client can distinguish the AP based on the BSSID even though the SSID is the same if it moves from AP to AP. Passing from one AP to another in the ESS is called roaming.
Independent Basic Service Set
Two clients communicating directly without an AP is called an ad-hoc network or Independent Basic Service Set (IBSS). This is for example used in wireless printing. One device needs to take the lead and advertise a network name and the necessary radio parameters. IBSS networks do not scale well beyond 8-10 devices.
Other Wireless Topologies
A repeater is used to provide wireless coverage for clients that are not within range of an AP. Normally APs connect back to the DS but not a repeater.It takes the signal it receives and retransmits it. If the repeater has a single radio the throughput can be halved because when the repeater repeats the AP’s signal, it’s received by the AP again. Some repeaters have two radios to keep the original and the repeated signal isolated. One radio is then dedicated to signals in the AP’s cell and the other to signals in the repeater’s own cell.
A Workgroup Bridge (WGB) can be used to provide access to the network for devices that only have wired Ethernet but there is no switch to connect to but it’s within range for wireless coverage. The device can then connect to the WGB through wired Ethernet and the WGB becomes a wireless client of a BSS, acting as an external wireless network adapter for a device that does not have one.
Universal Workgroup Bridge (uWGB) – A single wired device can be bridged to a wireless network.
Workgroup Bridge (WGB) – Cisco-proprietary implementation allowing several wired clients to be bridged to a wireless network.
An AP configured as a bridge to form a single wireless link from one LAN (switch) to another over a long distance. Commonly used for connectivity between buildings or cities.
For two LANs that need to be bridged, a point-to-point bridged link can be used, requiring one bridge at each location. Directional antennas are used to mazimize the link distance.
A point-to-multipoint bridged link allows several sites to be bridged to a central site. The central site uses an omnidirectional antenna and the other sites use a directional antenna, directed at the central site.
When it’s not practical to run Ethernet cabling to every AP, APs can be configured in mesh mode. In a mesh topology, traffic is bridged from AP to AP, in a daisy-chain fashion. Mesh networks are often used to provide wireless coverage over a large area, such as outdoor networks.
Leverages dual radios, one for 2.4 GHz and one for 5 GHz. A BSS is provided over the 2.4 GHz radio and the 5 GHz one is used to backhaul traffic. At the edge of the mesh, the backhaul traffic is bridged to the wired LAN infrastructure. A dynamic routing protocol runs in the mesh to figure out the best path for backhaul traffic to take across the mesh APs.