Fundamentals of Wireless LANs 1.2
Module 4:
Wireless Topologies
Overview
• This chapter is just an overview of many
topics that we will discuss in much more
detail in later chapters.
Components
Devices on the WLAN
• The most common devices used on WLANs are
workstations, which include both laptop and desktop
models.
– Many corporations are supplying their workforce with laptops,
instead of desktop models.
– While in the office, the laptop is typically connected to a docking
station with a large display monitor, full size keyboard, and a
mouse
• The use of only a laptop allows mobile workers to easily
keep track of work-related information and files w/o
having to transfer between two machines.
• Laptops can be kept at work (locked), taken home or
taken on the road.
– PDAs (personal digital assistants) have also become very
popular.
Increasing Mobility
• A result of user mobility is an increase in
productivity.
– Meetings and conferences are less challenging
– Access to resources is more available
• Printing, email, file shares and the Internet
• Desktops can easily be converted from wired to
wireless systems, by changing the NIC and
deploying access points but this is not as
practical.
– Wirelessly connected desktops are more practical if
Ethernet cable cannot be run to a specific area
(too costly, time consuming, historic buildings etc…)
Handheld Devices
•
There are three basic types of handheld
devices.
1. key-based
2. pen-based
3. vehicle-mount
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Key-based devices, are used for applications
requiring manual entry of character data.
Pen-based devices utilize a pen-like stylus and
typically do not have a keyboard or keypad.
Vehicle-mount mobile devices are intended for
use on a motorized pallet jack, forklift, or
mobile cart.
Devices and Operating Systems
Mobile Computing Operating Systems (OS)
• It is important to use only 802.11 compliant devices.
• The big advantages in doing this include interoperability,
speed, reliability, and real-time data communications.
• Other considerations and concerns include battery life
and durability.
Clients and adapters
• As with Ethernet, a driver is needed to
communicate with the OS on the computer.
• There are three types of drivers available
for wireless client adapters.
1.NDIS (Network Driver Interface Specification)
2. ODI Open Data-link Interface) 3.Packet
Wireless Adapter Drivers
• Network Driver Interface Specification (NDIS) – The primary
purpose of NDIS is to define a standard API for NICs.
– Current NDIS versions used by Windows are Microsoft proprietary
specifications. NDIS drivers are supported under 95/98, ME, NT, 2000,
and XP. The Novell NetWare Client32 is also supported.
• Open Data-link Interface (ODI) – ODI is analogous to NDIS, but is
NetWare specific. It is used in the Novell NetWare 3.x and 4.x
environments and will work with either NETX or VIMs.
– ODI drivers will also work under DOS.
• Packet – This interface is for use with DOS-based IP stacks. Some
of the more popular DOS-based IP stacks that work with Cisco
Aironet products include FTP Software and NetManage.
Access points and bridges
An access point (AP) is a WLAN
device that can act as the center
point of a stand-alone wireless
network or be used as the
connection point between wireless
and wired networks.
Workgroup bridge (WGB) product connects to
the Ethernet port of a device that does not
have a PCI or PCMCIA slot available. Will
connect up to eight wired machines to an
AP. It is ideal for connecting remote
workgroups to a wired LAN
Wireless Bridges are designed to
connect two or more networks that
are typically located in different
buildings.
AP
Antennas
• Cisco Aironet AP 2.4 GHz antennas are compatible with all Cisco RP-TNC
(Reverse Polarity –TNC (threaded Neill-Concelman)) equipped APs.
• The antennas are available with different gain and range capabilities, beam
widths, and form factors.
• Coupling the right antenna with the right AP allows for more efficient
coverage, as well as better reliability at higher data rates.
• Chapter 7 covers more about antennas
(Cisco Aironet Antennas and Accessories Reference Guide)
Bridge
Antennas
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Cisco Aironet bridge 2.4 GHz antennas provide transmission between two
or more buildings.
Antennas operate at Layer 1 of the OSI Model.
• These antennas are available in directional configurations for pointto-point transmission and omnidirectional configuration for point-to
multi-point implementations.
WLAN Topologies
Many of these features depend
upon the vendor and whether the
AP is a consumer wireless
product or business/enterprise
wireless product.
Cisco Three-layer Model
Includes
APs,
wireless
bridges
• Be familiar with this model (see online curriculum).
Wireless Topologies
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WLANs are access layer elements or products. WLAN products fit
into two main categories:
1. Wireless in-building LANs
2. Wireless building-to-building bridging
Wireless bridges allow two or more networks that are physically
separated to be connected on one LAN, without the time or
expense of dedicated cables or T1 lines.
Local Area Networks (LAN)
• The basic service set (BSS) is the area of RF coverage provided by
one access point. It is also referred to as a microcell.
• BSS can be extended by adding another AP.
• When more than one BSS is connected to a wired LAN, it is referred to
as an extended service set (ESS).
Local Area Networks (LAN)
• Adding an AP is also a way to add wireless devices
and extend the range of an existing wired system.
• If a single cell does not provide enough coverage, any
number of cells can be added to extend the range (ESS).
• It is recommended that adjacent BSS cells have a 10 to
15 percent overlap.
Wireless
repeater
Configuration Guide
Click Here
Root (Parent)
50%
overlap
Repeater
Repeater access points running Cisco IOS software cannot
associate to parent access points that that do not run
Cisco IOS software.
• A wireless repeater is simply an access
point that is not connected to the wired backbone.
• This setup requires a 50% overlap of the AP on the backbone and
the wireless repeater. (So they can reach each other.)
• The user can set up a chain of several repeater access points.
• However, the throughput for client devices at the end of the
repeater chain will be quite low.
• This is because each repeater must receive and then re-transmit
each frame on the same channel, similar to a wired repeater.
• For each repeater added to the chain, throughput is cut in half.
– It is recommended that not more than two hops be used.
• To set up repeaters, you must enable Aironet extensions on both
the parent (root) access point and the repeater access points.
System redundancy and load balancing
Multiple Vendors
Redundancy only
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In a LAN where it is essential to have communications, some customers will
require redundancy.
With the direct sequence spread spectrum (DSSS) products of a
different vendor, both AP units would be set to the same frequency and
data rate.
Since these units timeshare the frequency, only one unit can be
talking at a time.
If that one unit goes down for some reason, the remote clients will
hand off to the other active unit.
While this does provide redundancy, it does not provide any more
throughput than a single AP provides.
System redundancy and load balancing
Load Balancing and
Redundancy
• With the Cisco DS systems, the units are set to different
channels.
• Redundancy: If one unit goes down, remote clients will hand off to
the remaining unit and continue working.
• Load balancing: This can be configured based on the number of
users, the bit error rate, or signal strength.
– Distributes user connections across available access points
– Optimizes aggregate throughput
Hot Standby
• Hot Standby mode designates an access point as a
backup for another access point.
• The standby access point is placed near the access point it
monitors, configured exactly the same as the monitored
access point (except for the IP address)
• The standby access point associates with the monitored
access point as a client and sends IAPP queries to the
monitored access point through both the Ethernet and the
radio ports.
– IAPP = Inter-Access Point Protocol
• If the monitored access point fails to respond, the
standby access point comes online and takes the
monitored access point's place in the network.
System redundancy and load balancing
Cisco Hot Standby
Redundancy only
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Hot Standby is used when fault-tolerance and availability are critical
With Hot Standby, there is no load balancing (one AP is active, one is standby)
For business-critical deployments, a Cisco Aironet AP can be configured as a
redundant hot standby to another AP in the same coverage area.
The hot-standby AP continually monitors the primary AP on the same
channel, and assumes its role in the rare case of a failure of the primary AP.
The standby will be ready to take over, if the primary AP becomes
unavailable.
Hot Standby (cont.)
• The standby access point also must duplicate
several key settings on the monitored access
point. These settings are:
• SSID
• Default IP Subnet Mask
• Default Gateway
• Data rates
• Security settings
• Authentication types and authentication servers
– Check the monitored access point and record these
settings before you set up the standby access point.
Configuring Repeater and Standby Access Points and Workgroup Bridge Mode
Roaming
Not yet covered under
802.11.
• A WLAN designer must determine whether clients will
require seamless roaming from access point to access
point.
• Not yet standardized by IEEE 802.11 (working on it), most
vendors use IAPP (Inter-Access Point Protocol).
– Task Group F: A Standard IAPP
Roaming
• Initial Association:
– Probing (Probe Request, Probe Response)
• Note: 802.11 does not specify how the client determines
which AP to associate with , so it depends on vendor
implementation.
– Authentication (Authentication Request, Authentication
Response)
– Association (Association Request, Association Response)
• 802.11 does not allow associating with more than one AP.
Roaming
• Several factors need to be considered when designing a WLAN with
seamless roaming capabilities:
– Coverage must be sufficient for the entire path.
– A consistent IP address should be available throughout the entire
path. This means that all access points should be on the same VLAN.
– Until standardized by IEEE 802.11, access points will most likely need
to be from the same vendor.
Roaming
IAPP: Please
send buffered
frames for…
IAPP: Ok!
* Packet - Source
MAC of client…
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The client initiates the roaming (re-association)
process.
As the client is moving out of range of its associated
AP, the signal strength will start to drop off.
At the same time, the strength of another AP will
begin to increase.
The re-association process then occurs, including
authentication.
* AP(B) must update MAC
address tables on
infrastructure switches to
prevent the loss of data.
AP(B) sends an Ethernet
frame to AP(A) with the
source MAC address of the
client so all the switches
can update their CAM/MAC
tables.
Roaming
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•
The client NIC scans for a better
access point if the signal strength
falls below a threshold value.
The following options define signal
strength and wait thresholds that
trigger a new scan.
• When Adapter Has Been Associated for at Least—The number of
seconds the client adapter waits after connecting before searching for a
better access point. This threshold keeps the client adapter from jumping
from one access point to another too quickly after the initial connection.
• Signal Strength is Less Than—The signal strength threshold below
which the client adapter should search for a better access point. This
threshold keeps the client adapter from jumping from one access point to
another when both have strong signals.
• Example: When using the default values of 20 seconds and 50%, the
client adapter monitors the signal level 20 seconds after connecting and
every second thereafter. If the client detects that the signal strength is
below 50%, it scans for a better access point. After the access point
connects to a better access point, this scanning process repeats.
Scalability
APs are on different
channels
802.11b
• Scalability is the ability to locate more than one access point in the
same area.
• This will increase the available bandwidth of that area for all users
local to that access point.
• The current Cisco Aironet products are frequency agile.
• This means that they can look for and use the best channel.
– Least congested channel
• Three non-overlapping and non-interfering channels, up to a
theoretical 33 Mbps per cell.
• Users still only operate at a maximum theoretical value of 11 Mbps
Scalability
APs are on
different channels
• In the case of 802.11a, there are eight non-overlapping channels, each up
to a theoretical bandwidth of 54 Mbps.
• This means that a maximum of eight discrete systems can reside in the
same area, with no interference.
• Therefore, the highest aggregate total data rate for an 802.11a system is a
theoretical 432 Mbps, for a given cell area.
• Remember that any connected user will still only receive up to 54 Mbps.
Channel Setup
Channel Setup
There are two critical steps for a good WLAN deployment:
1. Determine placement of access points or bridges –
• This includes determining where they should be placed and deciding
how many are required for the desired coverage.
• Very few gaps in the coverage should be left.
• These gaps are essentially dead air and the client will lack
connectivity in these locations.
• As discussed before, bandwidth requirements have an impact on the
coverage areas.
2. Map out the channel assignments –
• There should be as little overlap as possible between channels that
use the same frequency.
Access point coverage and comparison
• As a client roams away from the access point, the transmission
signals between the two attenuate (weaken).
• Rather than decreasing reliability, the AP shifts to a slower data
rate, which gives more accurate data transfer.
• This is called data rate or multi-rate shifting.
• As a client moves away from an 802.11b access point, the data rate will
go from 11 Mbps, to 5.5Mbps, to 2 Mbps, and, finally, to 1 Mbps.
• Remember, the different data rates are because of different modulation
techniques – 11Mbps and 5.5Mbps/CCK, 2Mbps/QPSK, 1Mbps/BPSK
Access point coverage and comparison
• The Cisco Aironet 2.4 GHz radio delivers 100 mW of output and
offers a high degree of receiver sensitivity.
• The 5 GHz client radio has a 20 mW transmit power and the 5 GHz
access point has a 40 mW transmit power.
• It is possible to adjust the power level down, to create smaller
coverage cells.
• This would be done, for example, to prevent the coverage area of one
AP from extending too far into the coverage area of another AP.
Multirate implementation
• Provides for seamless roaming, but not at a constant speed.
• This example takes advantage of multi-rate technology, to step
down in bandwidth and gain greater coverage distances, with a
single access point.
• If 11 Mbps is required everywhere, the access points would need
to be relocated, so that only the 11-Mbps circles are touching each
other, with some overlap.
• This would require a greater number of APs, but consistent
bandwidth would be achieved.
Bridge Topologies
More on Bridges Later
Root modes
• Cisco Aironet access points and bridges have two
different root modes, in which to operate the following:
– Root = ON —
• The bridge or AP is a root.
• If it is a bridge, then it is called the master bridge.
– Root = OFF —
• The bridge or AP is not a root, non-root.
Root modes
Point-to-point configuration
VLAN 10
VLAN 10
• When using point-to-point wireless bridges, two LANs can be
located up to 40 km (25 miles) apart.
• The antennas must have line-of-site with each other.
• Obstacles such as buildings, trees, and hills will cause
communication problems.
• In this configuration, the Ethernet segments in both buildings act
as if they are a single segment.
• The bridge does not add to the Ethernet repeater count because this
segment is viewed by the network as a cable.
Point-to-point configuration
• Many corporations would like to have more bandwidth
between two locations, than the 11 Mbps provided by the
802.11b standard.
• Currently, with Cisco IOS, it is possible to use Fast
Etherchannel or multi-link trunking, to bond or
aggregate up to three bridges together.
• This gives the customer the potential for 33 Mbps .11b).
Point-to-multipoint configuration
Root
Non-root
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Non-root
For multipoint bridging, an omni directional antenna is typically used at
the main site.
Directional antennas are used at the remote sites.
In this configuration, again, all the LANs appear as a single segment.
Traffic from one remote site to another will be sent to the main site and then
forwarded to the other remote site.
Remote sites cannot communicate directly with one another.
Line of sight must be maintained between each remote site and the
main site.
Sample Topologies
Basic Topologies
Peer-to-Peer (Ad Hoc)
Topology (IBSS)
Basic Infrastructure
Topology (BSS)
Extended
Infrastructure
Topology (ESS)
Campus topologies
• One of the biggest benefits of a campus WLAN is the
ability for people to sit in common areas and work
together, and still easily get network access.
WLAN addition to AVVID
• WLANs are part of Cisco’s Architecture for
Voice, Video, and Integrated Data (AVVID).
VLAN, QoS, and Proxy Mobile IP
VLAN Features
• The Cisco Aironet APs only support the 802.1Q Trunking protocol
standard.
• Switches will not allow different VLANs to talk to one another.
• A router will be needed to allow different VLANs to communicate to
each other.
• The Cisco Aironet APs can be configured with 16 different VLANs
for system design flexibility.
• WLANs can now fit nicely into the larger network because VLANs have
been enabled on the APs.
• This allows WLAN users to roam from access point to access point
maintaining connectivity to the proper VLAN.
Quality of Service (QoS) feature
• Time critical data traffic such as voice and video benefit from
Quality of Service (QoS), which can be configured to give voice and
video higher priority.
• This allows for smooth voice communication, jitter free video, and
reliable delivery of E-Mail configured with a lower priority.
• Class of Service (CoS) uses the 802.1P standard to set the priority
field to network traffic.
Proxy
Mobile IP
• As the wireless station leaves one area and enters the
next, the new access point queries the station for its
home agent.
• The routers must support Home Agent/Forwarding Agent
(HA/FA) functionality. The HA/FA function allows the two
routers to create a tunnel between the two APs on
different Subnets.
• Data is then sent through the tunnel from one AP to the
other and the data is delivered by the AP to the end
station.
Summary
• This module discussed wireless topologies. First, the
components of WLANs were described, such as mobile
devices and client adapters, access points and bridges,
and antennas. A WLAN was defined as an extension to
a wired network environment.
• The two major topologies covered were WLANs and
wireless bridges. Some variations, which include the
wireless repeater and the workgroup bridge, were also
discussed. Additional features such as fault tolerance,
load balancing, and roaming were also mentioned.
• Channel setup was also discussed along with the
importance of planning channel usage, with as little
overlap as possible between channels that use the same
frequency.
• Finally, some examples of different topologies were
covered, concluding with an overview of Cisco AVVID.