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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.

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fundamentals of wireless lans 1 2

Fundamentals of Wireless LANs 1.2

Module 4:

Wireless Topologies

  • This chapter is just an overview of many topics that we will discuss in much more detail in later chapters.
devices on the wlan
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
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
Handheld Devices
  • There are three basic types of handheld devices.
    • key-based
    • pen-based
    • vehicle-mount
  • 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
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
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
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
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.

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

ap antennas
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
Bridge Antennas
  • 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 point-to-point transmission and omnidirectional configuration for point-to multi-point implementations.
wlan topologies

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
Cisco Three-layer Model

Includes APs, wireless bridges

  • Be familiar with this model (see online curriculum).
wireless topologies
Wireless Topologies
  • WLANs are access layer elements or products. WLAN products fit into two main categories:
    • Wireless in-building LANs
    • 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
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 lan1
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
Wireless repeater

50% overlap

Configuration Guide

Click Here

Root (Parent)


  • 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.

Repeater access points running Cisco IOS software cannot

associate to parent access points that that do not run

Cisco IOS software.

system redundancy and load balancing
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 unitstimeshare 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

system redundancy and load balancing1
System redundancy and load balancing
  • 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

hot standby
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 pointfails to respond, the standby access point comes online and takes the monitored access point's place in the network.
system redundancy and load balancing2
System redundancy and load balancing

Cisco Hot Standby Redundancy only

  • 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
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

  • 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.

  • 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.
  • 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.

IAPP: Please send buffered frames for…


* Packet - Source MAC of client…

  • 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.

  • 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 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

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.

APs are on different channels

channel setup1
Channel Setup

There aretwo 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
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 comparison1
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
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

Bridge Topologies

More on Bridges Later

root modes
Root modes
  • Cisco Aironet access points and bridgeshave 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.
point to point configuration
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 configuration1
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
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.



basic topologies
Basic Topologies

Peer-to-Peer (Ad Hoc) Topology (IBSS)

Basic Infrastructure Topology (BSS)

Extended Infrastructure Topology (ESS)

campus topologies
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
WLAN addition to AVVID
  • WLANs are part of Cisco’s Architecture for Voice, Video, and Integrated Data (AVVID).
vlan features
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
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
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.
  • 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.