Cisco WLAN - Chapter. 04 : wireless topologies

Ch. 4 – Wireless Topologies
Cisco Fundamentals of Wireless LANs version 1.1
Rick Graziani
Cabrillo College

Rick Graziani graziani@cabrillo.edu 2
Overview
• This chapter is just an overview of many topics that we will
discuss in much more detail in later chapters.

Devices and Operating Systems
• 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.
Mobile Computing Operating Systems (OS)

Clients and adapters
• As with Ethernet, a driver is needed to communicate with the OS on
the computer.
• NDIS Driver Version - The version of the NDIS 3 device driver that is
installed on the computer. (Network Driver Interface Specification)

Access points and bridges (more later)
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.
Wireless Bridges are designed to
connect two or more networks that
are typically located in different
buildings.
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
Not currently covered under 802.11

• Cisco Aironet AP 2.4 GHz antennas are compatible with all Cisco
RP-TNC 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 efficient
coverage in any facility, as well as better reliability at higher data rates.
• A detailed coverage of antennas will be provided later in the
course.
AP
Antennas

• Cisco Aironet bridge 2.4 GHz antennas provide transmission between two or
more buildings.
• Antennas operate at Layer 1 of the OSI Model.
• Remember that the physical layer defines the electrical, mechanical,
procedural, and functional specifications for activating, maintaining, and
deactivating the physical link between end systems.
• Characteristics such as voltage levels, timing of voltage changes, physical data
rates, maximum transmission distances, physical connectors, and other,
similar, attributes are defined by physical layer specifications.
Bridge
Antennas

WLAN Topologies
Many of these features depend upon the vendor and
whether the AP is a consumer wireless product or
business/enterprise wireless product.
Not all of these features are available on all APs or by
all vendors.

Cisco Three-layer Model
• Be familiar with this model (see online curriculum).
Includes
APs,
wireless
bridges

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.
• It is recommended that adjacent BSS cells have a 10 to 15 percent
overlap.

Minimal Overlap Coverage Option
• By arranging the access points so that the overlap in a coverage area is
minimized, a large area can be covered with minimal cost.
• The total bandwidth available to each wireless client device depends on the
amount of data each mobile station needs to transfer and the number of
stations located in each cell.
• Seamless roaming is supported as a client device moves in and out of
range of each access point, thereby maintaining a constant connection to
the wired LAN.
• Each device in the radio network must be configured with the same SSID
to provide roaming capability. (Roaming will discussed later.)
SSID = Student
Channel 1
SSID = Student
Channel 6

Wireless repeater
• 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.
Not covered under 802.11
50%
overlap

System redundancy and load balancing
• 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.
Multiple Vendors
Redundancy only

• 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
Load Balancing and
Redundancy

Multiple Overlapping Networks Coverage
Option
• Multiple networks can operate in the same vicinity.
• The architecture provides multiple channels that can exist in the same
area with virtually no interference to each other.
• In this mode, each system must be configured with different SSIDs
and different channels, which may (depending on configurations)
prevent clients from roaming to access points of a different wireless
network.
SSID = Student
Channel 1
SSID = Staff
Channel 6
SSID = Public
Channel 11

• Another option, when fault-tolerance and availability are critical, is a hot-
standby AP.
• In this case, there is no load balancing.
• 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.
Cisco Hot Standby
Redundancy only

Roaming
• 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
Not yet covered under
802.11.

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.
– Until standardized by IEEE 802.11, access points will most likely
need to be from the same vendor.

Roaming
• 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.
IAPP: Please
send buffered
frames for…
IAPP: Ok!
* AP(B) must update MAC
address tables on
infrastructure switches to
prevent to 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 SAT/MAC
tables.
* Packet - Source
MAC of client…

Roaming
• 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
• 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.
• 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
APs are on
different channels

Scalability
• 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.
APs are on
different channels

Scalability
• Specifies the channel number and frequency that the client adapter
uses for communications. The channels conform to the IEEE 802.11
Standard for your regulatory domain.
• In infrastructure mode, this option is set automatically and cannot be
changed. The client adapter listens to the entire spectrum, selects the
best access point, and then uses the same channel as that access
point.
• In ad hoc mode, the channel of the client adapter must match the
channel used by the other clients in the wireless network. If the client
adapter does not find any other ad hoc client adapters, this option
specifies the channel on which the client adapter broadcasts beacons.

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.
• This happens without losing the connection, and without any interaction
from the user.

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 pico-cells, or
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.

• Sets the transmit power level of the radio. Select a value for Transmit
Power that is no greater than the maximum allowed by the regulatory
body in your country (FCC in the United States, ETSI in Europe, and
MKK in Japan). Reducing the transmit power conserves battery power,
but it reduces the range of the radio. The default power level is the
maximum power allowed by the regulatory agency in your country.
• Note: If World Mode is enabled, the transmit power is limited to the
maximum level allowed by the regulatory agency of the country where
the adapter is used.

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.

Channel usage and interference
• Remember that the 802.11 standard uses the unlicensed spectrum
and, therefore, anyone can use these frequencies.

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

Root modes
on on
off
off
off
off

Point-to-point configuration
• 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.

Point-to-multipoint configuration
• 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.
root
Non-root Non-root

Distance limitations
• The 802.11 standard sets a time limit for the acknowledgement of packets.
• Remember that 802.11 also defines a Local Area Network, which means a
typical wireless range of up to 305 m (1000 ft), not several kilometers or
miles.
• The bridge products have a parameter that increases this timing, whereas
the workgroup bridge and AP does not.
• The timing is increased, by violating the 802.11 standard.
• This allows the Cisco devices to operate at greater distances.
• Any wireless bridge that supports distances over one mile must violate
802.11.
• This means that radios of other 802.11 vendors may not work with the
Cisco bridges when the distances are greater than 1.6 km (1 mile).

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.
• 802.11e is supplementary to the MAC layer to provide QoS support for
LAN applications.

• Cisco’s Proxy Mobile IP is designed for use in even the most
complex network environments.
• As the wireless station leaves one area and enters the next, the new
access point queries the station for its home agent.
• After it has been located, packet forwarding is established
automatically between the new and old access points to ensure the
user can transparently exchange data.
Proxy mobile IP

Cisco WLAN - Chapter. 04 : wireless topologies

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