This Chapter Discusses How To Design A Wireless Network, And ...
This chapter discusses how to design a
wireless network, and includes the following
sections:
■
Making the Business Case
■
Wireless Technology Overview
■
Wireless Security
■
Wireless Management
■
Wireless Design Considerations
C H A P T E R
5
Wireless LAN Design
This chapter discusses wireless LAN (WLAN) technology and describes how it improves
mobility. After introducing WLANs as strategic assets to corporate networks, we discuss
WLAN standards and components. The security and management of WLANs are explored,
followed by design considerations for WLANs.
NOTE
Appendix B, “Network Fundamentals,” includes material that we assume you under-
stand before reading the rest of the book. Thus, we encourage you to review any of the material
in Appendix B that you are not familiar with before reading the rest of this chapter.
Making the Business Case
The popularity of WLANs is undeniable. The following three main driving forces play in favor
of WLANs:
■
Flexibility
■
Increased productivity
■
Cost savings compared to wired deployment
WLANs let users access servers, printers, and other network resources regardless of their
location, within the wireless reach. This flexibility means that, for example, a user’s laptop stays
connected working from a colleague’s cubicle, from a small meeting room, or from the
cafeteria. Recognizing the benefits brought about by WLAN flexibility, businesses are now
deploying WLANs in record numbers.
According to a 2003 NOP World research study,1 WLAN users stayed connected to their
corporate network 3.64 hours per day longer than their wired peers, thus increasing their
productivity by 27 percent. Through the flexibility of WLANs, not only does the productivity
go up, but the response times are also significantly improved.
132 Chapter 5: Wireless LAN Design
The benefits of wireless mobility don’t stop at laptops and personal digital assistants (PDAs). IP
telephony and videoconferencing are also supported over WLANs, integrating quality of service
(QoS) to ensure that the interactive traffic has priority over the less-time-sensitive data transfers.
Another significant benefit of WLANs is their low-cost deployment in locations where the costs
of running LAN wire would be prohibitive. The total cost of ownership (TCO) of a WLAN is very
low compared to the benefits they bring to an organization, providing that a WLAN is secured and
managed properly.
Companies that are not deploying WLANs quickly enough find that employees take the matter in
their own hands and install their own WLANs, potentially creating significant breaches in the
corporate network security infrastructure. Therefore, wireless security is an important topic to
discuss in conjunction with wireless design.
WLANs, seen just a few years ago as a novelty, are now seen as critical to corporate productivity.
Wireless Technology Overview
In its most simplistic form, a WLAN is an LAN that uses radio frequency (RF) to communicate
instead of using a wire. As shown in Figure 5-1, wireless clients connect to wireless access points
(WAPs).
Figure 5-1
Wired and Wireless Networks
Layer 2
Layer 2
or
or
Layer 3
Layer 3
Devices
Devices
Hub
Wireless Access Points
Wired Clients
Wireless Clients
Wireless Technology Overview 133
Because WLANs use RF, the throughput (speed) is inversely proportional to the distance between
the transmitter and the receiver.2 Therefore, everything being equal (notwithstanding interfer-
ences), the closer a wireless client is to a transmitter, the greater is the throughput, as shown in
Figure 5-2.
Figure 5-2
Throughput (Coverage) Is Related to the Distance from the RF Transmitter
250–350 feet 2 Mbps DSS
150–250 feet 5.5 Mbps DSS
150 feet 11 Mbps D
–
S
0
S
0
1
DSS = Digital Spread Spectrum
However, wireless communication brings a trade-off between flexibility and mobility versus
battery life and usable bandwidth.
Wireless Standards
WLAN standards that are currently supported by major vendors were developed by the working
group 11 of the Institute of Electrical and Electronics Engineers (IEEE) 802 committee. The most
common standards are shown in Table 5-1.
134 Chapter 5: Wireless LAN Design
Table 5-1
Wireless Standards
Maximum
Throughput
Frequency
Standard
(Mbps)
(GHz)
Compatibility
Ratified
802.11b
11
2.4
—
1999
802.11a
54
5
—
1999; Product
availability 2001
802.11g
54
2.4
Backward-
2003
compatible with
802.11b
The 802.11a standard operates in the unlicensed 5-GHz band, which makes the transmission
vulnerable to interference from microwave ovens and cordless phones. The strength of 802.11b
and 802.11g signals, which operate in the 2.4-GHz band, is affected negatively by water, metal,
and thick walls.
The 802.11b and 802.11g standards divide the 2.4 GHz into 14 overlapping individual channels.
Channels 1, 6, and 11 do not overlap and therefore can be used to set up multiple networks. The
802.11a standard is an amendment to the original standard. The advantage of using 802.11a is that
it suffers less from interference, but its use is restricted to almost line of sight, thus requiring the
installation of more access points than 802.11b to cover the same area.
The medium access method of the 802.11 standards, called the Distribution Coordination Method,
is similar to the carrier sense multiple access collision detect (CSMA/CD) mechanism of Ethernet.
The following types of frames are transferred over the airwaves:
■
Data frame—Network traffic.
■
Control frame—Frame controlling access to the medium, similar to a modem’s analog
connection control mechanism, with its Request To Send (RTS), Clear To Send (CTS), and
acknowledgment (ACK) signals.
■
Manager frame—Frames similar to data frames, pertaining to the control of the current
wireless transmission.
Wireless Technology Overview 135
Other Wireless Standards
Other wireless standards include the following:
HomeRF—In 1998, a consortium was formed to promote the idea of HomeRF to be used with
products in the home market. The participants were, among others, Siemens, Motorola, and
Compaq.
Bluetooth—This is a specification for short-range radio links between mobile computers, mobile
phones, digital cameras, and other portable devices, such as headsets. Bluetooth could be
considered a standard for personal area networks.
Wireless Components
The main components of wireless networks are as follows:
■
Wireless access points
■
Wireless client devices
Wireless Access Points
WAPs provide connectivity between wireless client devices and the wired network, as shown
earlier in Figure 5-1.
Integrated Access Point
The WAP does not need to be a stand-alone device. Cisco offers integrated access point
functionality3 for some small- to medium-business (SMB) routers, as shown in Figure 5-3. By
installing a high-speed wireless interface card (HWIC) in Cisco 1800, 2800, or 3800 routers,
customers can run concurrent routing, switching, and security services and include IEEE 802.11
wireless LAN functionality in a single platform.
136 Chapter 5: Wireless LAN Design
Figure 5-3
Integrating Routing and Wireless Functionality
Access Point
(Router with
Access Point
Card)
Wireless Client Devices
A wireless client device is equipped with a wireless interface card (WIC), which the device uses
to communicate over RF with WAPs. Wireless clients can be the following items, among other
things:
■
User workstations and laptops
■
PDAs
■
Wireless IP phones
User Workstations and Laptops: Ad-Hoc Network
In addition to connecting to a WLAN access point, two wireless end stations can form an
exclusive, point-to-point, wireless network without the intervention of an access point. This type
of independent network is known as an ad-hoc network.
PDAs
Wireless PDAs—PDAs that connect directly on the corporate network—play a significant role in
an organization where time is extremely sensitive. An example of where 802.11b-compatible
Wireless Security 137
devices (wireless PDAs) are put to benefit is triage nurses who are faster at inputting their
assessment and sharing their findings on the spot rather than walking back to the nurses’ station
to do so.
Wireless IP Phones
Absolute campus mobility is probably best demonstrated by Cisco wireless IP phones.4 These
802.11b phones have built-in security, QoS, and management features. Wireless IP phones
leverage existing IP telephony deployments, as shown in Figure 5-4.
Figure 5-4
Deploying Wireless IP Phones
Call Processing
WAP
IP Phones
Workstation
Workstation
Wireless IP Phone
Wireless Security
Although security was originally included with 802.11 standards, it soon became obvious that it
wasn’t enough. Wireless security—or the lack of it—has been a major contributor to IT managers’
reluctance to adapt wireless LANs.
138 Chapter 5: Wireless LAN Design
Recently, wireless security has improved dramatically, providing IT managers with an acceptable
level of comfort to proceed with the installation of WLANs. IEEE 802.11i, released in June 2004,
addresses current security concerns.
In addition to the 802.11 suite of standards, the 802.1x standard can be used for wireless security.
More precisely, 802.1x addresses port-based access control.
Wireless Security Issues
A main issue with wireless communication is unauthorized access to network traffic or, more
precisely, the watching, displaying, and logging of network traffic, also known as sniffing.
Contrary to a wired network, where a hacker would need to be physically located at the corporate
premises to gain access through a network drop, with a wireless network, the intruder can access
the network from a location outside the corporate building. WLANs use radio frequencies, and
their signals propagate through ceilings and walls. Therefore, wireless eavesdropping, known as
war driving or walk-by hacking, and rogue WAPs—unauthorized WAPs that allow a hacker access
to a network—are two significant security issues with wireless networks.
Moreover, wireless equipment tends to ship with open access. Not only is traffic propagated in
clear text, but WAPs also voluntarily broadcast their identity, known as Service Set Identifiers
(SSIDs).
Wireless Threat Mitigation
Thanks to the wireless open-access default mode, we can join a wireless network from our favorite
coffee shop or hotel room; however, this unrestricted access is not advisable for corporate
networks. Wireless network security can be classified into the following three categories:
■
Basic wireless security
■
Enhanced wireless security
■
Wireless intrusion detection
Basic Wireless Security
Basic wireless security is provided by the following built-in functions:
■
SSIDs
■
Wired Equivalent Privacy (WEP)
■
Media Access Control (MAC) address verification
Wireless Security 139
SSIDs
An SSID is a code that identifies membership with a WAP. All wireless devices that want to
communicate on a network must have their SSID set to the same value as the WAP SSID to
establish connectivity with the WAP.
By default, a WAP broadcasts its SSID every few seconds. This broadcast can be stopped so that
a drive-by hacker can’t automatically discover the SSID and hence the WAP. However, because
the SSID is included in the beacon of every wireless frame, it is easy for a hacker equipped with
sniffing equipment to discover the value and fraudulently join the network.
Beacon Frame
The WAP periodically advertises SSID and other network information using a special 802.11
management frame known as a beacon.
Being able to join a wireless network by the mere fact of knowing the SSID is referred to as open
authentication.
Wired Equivalent Privacy
WEP can be used to alleviate the problem of SSID broadcasts by encrypting the traffic between
the wireless clients and WAPs. Joining a wireless network using WEP is referred to as shared-key
authentication, where the WAP sends a challenge to the wireless client who must return it
encrypted. If the WAP can decipher the client’s response, the WAP has the proof that the client
possesses valid keys and therefore has the right to join the wireless network. WEP comes in two
encryption strengths: 64-bit and 128-bit.
NOTE
Even if a user manages to proceed with open authentication—for example, he guesses
the SSID—if WEP is activated, he could not communicate with the WAP until he obtains the keys.
However, WEP is not considered secure: A hacker sniffing first the challenge and then the
encrypted response could reverse-engineer the process and deduce the keys used by the client
and WAP.
MAC Address Verification
To further wireless security, a network administrator could use MAC address filtering, in which
the WAP is configured with the MAC addresses of the wireless clients that are to be permitted
access.
140 Chapter 5: Wireless LAN Design
Unfortunately, this method is also not secure because frames could be sniffed to discover a valid
MAC address, which the hacker could then spoof.
Enhanced Wireless Security
Stronger security standards, shown in Table 5-2, were created to replace the weaknesses in WEP.
Table 5-2
Wireless Security Standards
802.11 Original
Security Component
Standards
Security Enhancement
Open authentication or
802.1x
Authentication
shared-key
WEP
Wireless Fidelity (Wi-Fi)
Protected Access (WPA),
Encryption
then 802.11i
802.1x
IEEE 802.1x is a port-based network access control standard. It provides per-user, per-session,
mutual strong authentication, not only for wireless networks but also for wired networks, if
need be.
Depending on the authentication method used, 802.1x can also provide encryption. Based on the
IEEE Extensible Authorization Protocol (EAP), 802.1x allows WAPs and clients to share and
exchange WEP encryption keys automatically. The access point acts as a proxy, doing the heavier
computational load of encryption. The 802.1x standard also supports a centralized key manage-
ment for WLANs.
Wi-Fi Protected Access
WPA was introduced as an intermediate solution to WEP encryption and data integrity insecurities
while the IEEE 802.11i standard was being ratified.
When WPA is implemented, access to the WAP is provided only to clients that have the right
passphrase. Although WPA is more secure than WEP, if the preshared key is stored on the wireless
client and the client is stolen, a hacker could get access to the wireless network.
WPA supports both authentication and encryption. Authentication done through preshared keys is
known as WPA Personal; when done through 802.1x, it is known as WPA Enterprise.
Wireless Management 141
WPA offers Temporal Key Integrity Protocol (TKIP) as an encryption algorithm and a new
integrity algorithm known as Michael. WPA is a subset of the 802.11i specification.
802.11i
In June 2004, the IEEE ratified the draft for the 802.11i standard, also known as WPA2. The
802.11i standard formally replaces WEP and other security features of the original IEEE 802.11
standard.
WPA2 is the product certification attributed to wireless equipment that is compatible with the
802.11i standard. WPA2 certification provides support for the additional mandatory 802.11i
security features that are not included in WPA. WPA2, like WPA, supports both Enterprise and
Personal modes for authentication.
In addition to stricter encryption requirements, WPA2 also adds enhancements to support fast
roaming of wireless clients by allowing a client to preauthenticate with the access point toward
which it is moving, while maintaining a connection to the access point that it is moving away from.
Wireless Intrusion Detection
Many products provide rogue access point detection. However, some third-party products
integrate better than others with Cisco Aironet WAPs and the CiscoWorks Wireless LAN Solution
Engine (WLSE), discussed in the next section. One such third-party product is from AirDefense.5
This product provides wireless intrusion detection that uses the access points to scan the airwaves
and report wireless activity.
Wireless Management
Wireless LANs require the same level of security, dependability, and management that wired
networks do.
Network management tasks related to WLANs are as follows:
■
RF management services
■
Interference detection
■
Assisted site survey
■
RF scanning and monitoring
142 Chapter 5: Wireless LAN Design
Cisco Integrated Wireless Network6 is an evolution of the Cisco Structured Wireless-Aware
Network (SWAN), which has been available from Cisco since 2003. The main components of
Structured Wireless-Aware Networks7 are as follows:
■
Cisco Aironet WAP
■
Management and security servers, specifically CiscoWorks WLSEs
■
Wireless clients
■
SWAN-aware Cisco Catalyst switches and Cisco routers
Cisco Integrated Wireless Network addresses wireless security, deployment, management, and
control issues. It seeks to provide the same level of security, scalability, reliability, ease of
deployment, and management for wireless LANs as is expected from wired LANs.
Cisco Integrated Wireless Network requires wireless clients to send RF management (RM) data to
a Cisco Aironet WAP, Cisco IOS router, or Cisco Catalyst switch running Wireless Domain
Services (WDS), as shown in Figure 5-5.
Figure 5-5
Campus Infrastructure and Cisco Integrated Wireless Network
Management Module
Cisco
ACS
Network
Mangement
Core
Cisco Works
Wireless LAN
Solution Engine
RM Aggregation
Cisco Catalyst Switch
Running Cisco IOS
Building
Software with WDS
Distribution
RM
Building
RM
Rogue Access Point
Wireless Design Considerations 143
The WDS devices aggregate all the RM data. All access points and clients register with WDS using
802.1x. The WDS devices forward the authentication information to a CiscoWorks WLSE.
One of the many benefits of WDS is Fast Secure Roaming, which assists a wireless client when
migrating from one WAP to another. Another significant benefit of Cisco Integrated Wireless
Network is the alert generated should a rogue WAP or rogue wireless client connect to the
network, because all connecting devices are reported to the WDS device for further authentication.
Although they should be concerned with wireless security, organizations shouldn’t forget the
basics of physical security, as demonstrated by the following story. In 2005, a Middle Eastern bank
was broken into. The thief didn’t take anything, but rather left something—a WAP in the wiring
closet connecting to the bank’s LAN. The hacker was already inside the bank network and
therefore effortlessly proceeded to transfer money until his stratagem was discovered.
Wireless Design Considerations
The following sections discuss some items that should be considered when designing and
provisioning a wireless network.
Site Survey
Site surveys, originally introduced to make the most of scarce resources, are sometimes seen as
unnecessary in this age of inexpensive WAPs, where wireless saturation seems so economical.
Maybe the days of serious physical surveying, where one would look under the ceiling tiles, are
long gone, but you should still perform surveying to determine the optimal locations for WAPs to
minimize channel interference while maximizing the range.
Whether you are performing an in-depth site survey or a rudimentary one, you should ask the
following questions:
■
Which wireless system is best suited for the application?
■
Does a line-of-sight requirement exist between antennas?
■
Where should the WAP be located so that it is as close as possible to clients?
■
What potential sources of interference are in this building? Example sources are cordless
phones, microwave ovens, natural interference, or other access points using the same channel.
■
Should any federal, provincial, or local regulations and legislation be considered in this
deployment?
144 Chapter 5: Wireless LAN Design
Site Surveys Have Their Purpose
Some WAPs have an autoconfiguration option with which, after listening on the network, they can
autoconfigure themselves for the least-used wireless channel. This is not always desirable, though.
For example, if a WAP is installed on the sixth floor of a multi-WAP, multistory building, it might
select a channel that it perceived to be available. If that channel is already used by a WAP on the
first floor, a client on the third floor could have difficulty staying connected because the channels
overlap there.
Overlapping channels in a wireless network perform similarly to an overcrowded wired network
plagued by continuous collisions. Undoubtedly, performance will suffer and clients might not be
able to establish consistent connectivity to the wireless network.
This problem could be more easily solved with rudimentary planning and by using
nonoverlapping channels. Channels 1, 6, and 11 do not overlap, as mentioned in the “Wireless
Standards” section, earlier in this chapter.
WLAN Roaming
WLANs are relatively inexpensive to deploy compared to wired networks, and because, as shown
earlier in Figure 5-2, throughput is directly related to the proximity of WAPs, network managers
often install WAPs to provide overlapping signals, as shown in Figure 5-6. Using this overlapping
design, coverage (radius) area is traded for improved throughput.
Figure 5-6
Overlapping Signals Eliminate Dead Spots
Access Point 1
Access Point 2
Wireless Design Considerations 145
Note that these overlapping signals must be in nonoverlapping channels. This scenario, however,
requires WLAN roaming. WLAN roaming plans consider that as a user moves away from a WAP
and is therefore losing signal strength, his connection should seamlessly jump to a WAP that
provides a stronger signal.
Point-to-Point Bridging
It is not always feasible to run a network cable between two buildings to join their respective LANs
into a single Layer 3 broadcast domain. If the two buildings are a reasonable distance apart and
preferably in direct line of sight with each other, wireless bridges can be configured, as shown in
Figure 5-7. It takes two WAPs to create one logical two-port bridge. In this mode, WAPs are
operating in a dedicated point-to-point bridge mode and therefore are no longer operating as
wireless access points for clients.
Figure 5-7
Point-to-Point Bridging
Building 1
Building 2
Design Considerations for Wireless IP Phones
Because wireless IP phones have different coverage and wireless characteristics than common
wireless clients, a system administrator should conduct another site survey.
Another consideration for wireless IP phones is roaming. The roaming described in the “WLAN
Roaming” section, earlier in this chapter, is Layer 2 roaming. With Layer 2 roaming, devices keep
their IP address and therefore the changing to another switch would not be noticeable by users.
Layer 3 roaming would mean that a device would have to change its IP address; this would mean
an interruption in the user’s connection. If the connection was to a wireless IP phone, the call
would be disconnected; this scenario would likely be unacceptable to users. When wireless IP
146 Chapter 5: Wireless LAN Design
phones are used, the network needs to be equipped with a Cisco Catalyst 6500 Series Wireless
LAN Services Module (WLSM). WLSM, an integral component of SWAN, provides aggregation
of access point radio management information, thus enabling Layer 2 and Layer 3 roaming and
client mobility management.
Layer 2 roaming refers to an IP phone switching WAP within its subnet of origin. Layer 3 roaming
refers to an IP phone switching connectivity from a WAP in its subnet to a WAP located in another
subnet. Prior to WLSM, Layer 3 roaming was an issue because the phone would end up in a subnet
to which its IP address and default gateway wouldn’t belong.
Summary
In this chapter, you learned about wireless technology and implementation. The following topics
were covered:
■
The prevalence and rapid growth of wireless networks
■
Industry standards pertaining to wireless LANs
■
Equipment comprising wireless LANs
■
Wireless security and threat mitigation such as the following:
— WEP
— WPA and 802.11i
— 802.1x
— Wireless intrusion detection
■
Wireless management using Cisco Integrated Wireless Networks
■
Design considerations for wireless networks such as site surveys and roaming capabilities
Endnotes
1“Cisco Business Ready Campus Solutions” http:// www.cisco.com/application/pdf/en/us/ guest/
netsol/ns431/c654/cdccont_0900aecd800d8124.pdf.
2“Wireless LANs At-A-Glance,” Cisco 2004, http://www.cisco.com/application/pdf/en/us/guest/
netsol/ns24/c643/cdccont_0900aecd800dc92e.pdf.
Summary 147
3“Cisco HWIC-AP WLAN Module for Cisco 1800 (Modular), 2800 and 3800.” http://
www.cisco.com/en/US/products/ps5949/products_data_sheet0900aecd8028cc7b.html.
4“Cisco Wireless IP Phone 7920,” http://www.cisco.com/en/US/partner/products/hw/phones/
ps379/ps5056/index.html.
5http://www.airdefense.net/cisco.
6“Cisco Integrated Wireless Network,” http://www.cisco.com/en/US/netsol/ns340/ns394/ns348/
ns337/networking_solutions_package.html.
7“Cisco Structured Wireless-Aware Network (SWAN) Multimedia Presentation.” http://
www.cisco.com/en/US/netsol/ns340/ns394/ns348/ns337/
networking_solutions_presentation0900aecd8022d512.shtml.
