the operation of a BSS hinges on the AP, the BSS is bounded by usable. This is known as thebasic service area (BSA)or cell. the area where the AP’s signal is
the cell is shown as a simple shaded circular area that centers around the AP itself.
It advertises the existence of the BSS so that devices can find it and try to join. To do that, the AP
uses a unique BSS identifier (BSSID) that is based on the AP’s own radio MAC address.
Wireless devices must also have unique MAC addresses to send wireless frames at Layer 2 over
the air.
the AP advertises the wireless network with a Service Set Identifier (SSID), which is a text string containing a logical name.Think of the BSSID as a machine-readable name tag that uniquely
identifies the BSS ambassador (the AP), and the SSID as a nonunique, humanthat identifies the wireless service. -readable name tag
Membership with the BSS is called an association. request to the AP and the AP must either grant or deny the request. Once associated, a device A wireless device must send an association
becomes a client, or an 802.11 station (STA), of the BSS.
As long as a wireless client remains associated with a BSS, most communications to and from the client must pass through the AP,
By using the BSSID as a source or destination address, data frames can be relayed to or from the AP.
By sending data through the AP first, the BSS remains stable and under control.
Distribution System
The 802.11 standard refers to the upstream wired Ethernet as the distribution system (DS) for the wireless BSS,
the AP is in charge of mapping a virtual local-area network (VLAN) to an SSID.
The AP must be connected to the switch by a trunk link that carries the VLANs.
The AP uses the 802.1Q tag to map the VLAN numbers to the appropriate SSIDs.
The AP then appears as multiple logical APs—one per BSS—with a unique BSSID for each. With
Cisco APs, this is usually accomplished byfor each SSID. incrementing the last digit of the radio’s MAC address
Even though wireless clients can be distributed across many SSIDs, all of those clients must share the same AP’s hardware and must contend for airtime on the same channel.
Extended Service Set
When APs are placed at different geographic locations, they can all be interconnected by a switched infrastructure. The 802.11 standard calls this an extended service set (ESS),
Ideally,any SSIDs that are defined on one AP should be defined on all the APs in an ESS
each cell in Figure 26-8 has a unique BSSID, but both cells share one common SSID.
Passing from one AP to anotheris called roaming
Each AP offers its own BSS on its own channel, to prevent interference between the APs. As a client device roams from one AP to another, it must scan the available channels to find a new AP
(and BSS) to roam toward.
(and BSS) to roam toward.
The 802.11 standard allows two or more wireless clients to communicate directly with each other, with no other means of network connectivity. This is known as an ad hoc wireless network, or an
independent basic service set (IBSS),
One of the devices must take the lead and begin advertising a network name and the necessary
radio parameters, much like an AP would do.
Independent Basic Service Set
Other Wireless Topologies
Repeater
A wireless repeater takes the signal it receives and repeats or retransmits it in a new cell
area around the repeater.
Some repeaters can use two transmitters and receivers to keep the original and repeated signals isolated on different channels. One transmitter and receiver pair is dedicated to
signals in the AP’s cell, while the other pair is dedicated to signals in the repeater’s own cell.
Workgroup Bridge
WGB acts as an external wireless network adapter for a device that has none.
You might encounter two types of workgroup bridges:
You might encounter two types of workgroup bridges:
Universal workgroup bridge (uWGB): wireless network. A single wired device can be bridged to a
Workgroup bridge (WGB)wired devices to be bridged to a wireless network.:A Cisco-proprietary implementation that allows multiple
Outdoor Bridge
act as a bridge to form a single wireless link from one LAN to another over a long distance.
One AP configured in bridge mode is needed on each end of the wireless link. Special purpose antennas are normally used with the bridges to focus their signals in one direction
A point-to-multipoint bridged link allows a central site to be bridged to several other sites
Mesh Network
In a mesh topology, wireless traffic is bridged from AP to AP, in a daisy-chain fashion, using
another wireless channel.
Mesh APs can leverage dual radiosone a different range. Each mesh AP usually maintains a BSS on one channel, with which —one using a channel in one range of frequencies and
wireless clients can associate. Client traffic is then usually bridged from AP to AP over other channels as a backhaul network.
The mesh network runs its own dynamic routing protocol to work out the best path for backhaul traffic to take across the mesh APs.
RF Overview
Electromagnetic waves do not travel in a straight line. Instead, they travel by expanding in all directions away from the antenna.
frequencyin 1 secondof the wave, or the number of times the signal makes one complete up and down cycle
A second.hertz (Hz)is themost commonly used frequency unit and is nothing other than one cycle per
3 kHz to 300 GHz is commonly called radio frequency (RF).
Wireless Bands and Channels
Wireless Bands and Channels
Because a range of frequencies might be used for the same purpose, it is customary to refer to the range as a band of frequencies.
the range from 530 kHz to around 1710 kHz is used by AM radio stations; therefore, it is commonly called the AM band or the AM broadcast band.
One of the two main frequency ranges used for wireless LAN communication lies between 2.400
and 2.4835 GHz. This is usually called the 2.4entire range between 2.4 and 2.5 GHz -GHz band, even though it does not encompass the
The other wireless LAN range is usually called the 55.825 GHz. The 5-GHz band actually contains the following four separate and distinct bands:-GHz band because it lies between 5.150 and
5.150 to 5.250 GHz
5.250 to 5.350 GHz
5.470 to 5.725 GHz
5.725 to 5.825 GHz
Gap between 5.350 and 5.470 this gap exists and cannot be used for wireless LANs.
Do not worry about memorizing the band names or exact frequency ranges; just be aware of the two main bands at 2.4 and 5 GHz.
A frequency band contains a continuous range of frequencies.
bands are usually divided into a number of distinct channels. Each channel is known by a channel
number and is assigned to a specific frequency.
In the 5not encroach on or overlap the frequencies allocated for any other channel. In other words, the 5-GHz band, this is the case because each channel is allocated a frequency range that does -
GHz band consists of nonoverlapping channels.
APs and Wireless Standards
Wireless client devices and APs can be compatible with one or more amendments;
APs can usually operate on both bands simultaneously to support any clients that might be present on each band.
wireless clients typically associate with an AP on one band at a time, while scanning for potential
wireless clients typically associate with an AP on one band at a time, while scanning for potential APs on both bands. The band used to connect to an AP is chosen according to the operating
system, wireless adapter driver, and other internal configuration.
Cisco APs have dual radios (sets of transmitters and receivers) to support BSSs on one 2.4channel and other BSSs on one 5-GHz channel simultaneously. Some models also have two 5-GHz -GHz
radios that can be configured to operate BSSs on two different channels at the same time,
providing wireless coverage to higher densities of users that are located in the same vicinity.
RF signals propagate or reach further on the 2.4to penetrate indoor walls and objects easier at 2.4 GHz than 5 GHz. However, the 2.4-GHz band than on the 5-GHz band. They also tend -GHz band is
commonly more crowded with wireless devices. Remember that only three nonoverlapping channels are available, so the chances of other neighboring APs using the same channels is
greater. In contrast, the 5-GHz band has many more channels available to use, making channels
less crowded and experiencing less interference.
This chapter covers the following exam topics:
2.0 Network Access
2.6 Compare Cisco Wireless Architectures and AP modes
Autonomous AP Architecture
VLANs must be trunked from the distribution layer switch (where routing commonly takes place) to the access layer, where they are extended further over a trunk link to the AP.
Two wireless users that are associated to the same autonomous AP can reach each other through the AP without having to pass up into the wired network
configure SSIDs, VLANs, and many RF parameters like the channel and transmit power to be used.
An autonomous AP must also be configured with a management IP address so that you can remotely manage it.
management address is not normally part of any of the data VLANs, so a dedicated management
VLAN (i.e., VLAN 10) must be added to the trunk links to reach the AP.
Each AP must be configured and maintained individually unless you leverage a management platform such as Cisco Prime Infrastructure or Cisco DNA Center.
That might sound straightforward until you have to add a new VLAN and configure every switch
and AP in your network to carry and support it.
Cloud-based AP Architecture
Cisco Meraki is cloudnetworks built from Meraki products. For example, through the cloud networking service, you can -based and offers centralized management of wireless, switched, and security
configure and manage APs, monitor wireless performance and activity, generate reports, and so
on.
Cisco Meraki APs can be deployed automatically, once you register with the Meraki cloud. Each AP will contact the cloud when it powers up and will self-configure. From that point on, you can
manage the AP through the Meraki cloud dashboard.
From the cloud, you can push out code upgrades and configuration changes to the APs in the
enterprise
adds the intelligence needed to automatically instruct each AP on which channel and transmit power level to use.
Can also collect information from all of the APs about things such as RF interference, rogue or
unexpected wireless devices that were overheard, and wireless usage statistics.