SpectraLink Cordless Telephone NetLink Wireless Telephones Best Practices White Paper Wireless Telephone User Manual

Deploying NetLink Wireless Telephones  
Best Practices  
White Paper  
Version 1.0  
May 2004  
 
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One of the most critical issues in deploying NetLink Wireless  
2.1  
Coverage  
Telephones is ensuring sufficient wireless coverage. Often enterprise  
Wi-Fi networks are designed only for data applications and may not  
provide adequate coverage for wireless telephone users. Quite often  
these networks are designed to cover only areas where data terminals  
will be used, and do not include coverage in other areas such as  
stairwells, bathrooms, building entrances, or lobby areas where NetLink  
handsets may be used.  
The overall quality of coverage is also more important with telephony  
applications. Coverage that is suitable for data applications may not be  
adequate for Wi-Fi telephony. Most data communication protocols  
provide a mechanism for retransmission of lost or corrupted packets.  
Delays caused by retransmissions are not harmful, or even discernable,  
for most data applications. However, the real-time nature of a full-duplex  
telephone conversation requires that voice packets need to be received  
correctly within tens of milliseconds of their transmission. There is little  
time for retransmission; lost or corrupted packets must be discarded. In  
areas of poor coverage, data application performance may be  
acceptable due to retransmission protocols, but real-time voice quality  
may not be acceptable.  
Another factor to consider in determining the coverage area is the device  
usage. Wireless telephone devices are used differently than wireless  
data terminals. Telephone users tend to walk as they talk, while data  
users are most often stationary. NetLink Wireless Telephones are  
usually held next to the user’s head, introducing additional radio signal  
attenuation. Data terminals are usually set on a surface or held out at  
arms length so the user’s body has little affect. Because of these  
factors, a wireless telephone may have less range than a data terminal  
and the wireless LAN layout should account for a general reduction in  
radio signal propagation.  
To provide comprehensive coverage for Wi-Fi telephony applications,  
APs need to be positioned with sufficient overlapping coverage to  
ensure that there are no coverage gaps, or dead spots, between them.  
As NetLink Wireless Telephones move out of range of a particular AP,  
they seek out another AP to hand-off to, or re-associate with, in order to  
maintain their network connection. A properly designed Wi-Fi network  
will provide seamless hand-offs between APs, ensuring excellent voice  
quality throughout the facility.  
2.1.1 Overlapping  
Coverage  
The wireless LAN layout must factor in the transmission settings that will  
be configured within the APs. The transmission of voice requires  
relatively low data rates and a small amount of bandwidth compared to  
other applications. The 802.11 standard includes data rate reduction  
specifications so that as a user moves away from the access point, the  
radio adapts and uses a less complex and slower mechanism to send  
the data. The result is increased range (coverage) when operating at  
reduced transmission rates. Access points should generally not be  
configured to limit the transmission to only the higher rates if wireless  
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voice is an application on the wireless LAN because the coverage area  
of the AP will be greatly reduced. If a site requires configuring the APs  
to only negotiate at the higher rates, the layout of the wireless LAN must  
account for the reduced coverage and additional APs will be required to  
ensure seamless overlapping coverage at the higher rates.  
Wireless bridges are used to connect Ethernet LANs or extend the range  
of existing wireless LANs. Such devices generally create bottlenecks for  
network capacity and add delay to the overall network, which is not  
tolerable in the boundaries of QoS requirements. SpectraLink does not  
support a configuration that includes wireless bridges and does not  
recommend using wireless bridges within any wireless voice network.  
2.1.2 Wireless Bridges  
Adjacent APs need to use different radio channels to prevent  
interference between them. The 802.11b standard utilized by NetLink  
Wireless Telephones provides three non-interfering channels: channels  
1, 6, and 11 for North America. Access points within range of each other  
should always be set to non-interfering channels to maximize the  
capacity and performance of a wireless LAN, as shown in the diagram  
below:  
2.2  
Channel and  
Power  
Considerations  
Non-interfering, Overlapping 802.11b Channel Coverage  
The transmission power of APs can also be increased or decreased to  
provide more or less AP coverage area. Generally, the transmit power  
setting should be the same for all APs in a facility. This minimizes the  
chance of higher-power APs interfering with nearby lower-power APs,  
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and provides consistent coverage.  
Performing a site survey can minimize the possibility of dead spots. The  
AP equipment provider can usually perform a site survey. While many  
tools exist that allow customers to perform their own assessment,  
SpectraLink recommends that every site employ a professional site  
survey to ensure optimum coverage and minimize interference.  
2.2.1 Site Surveys  
Site surveys are a necessity for large or complex facilities. An extensive  
site survey will ensure that the minimum number of APs are deployed,  
but at the risk of having to significantly reconfigure the network if the  
coverage area is remodeled. Wi-Fi infrastructure providers are making  
significant developments to reduce the time, cost, and complexity of  
determining AP locations. Because the cost of APs has significantly  
dropped over the last few years, in some cases it is more cost effective  
to install more APs with overlapping coverage rather than try to  
maximize coverage for each AP with a rigorous site survey.  
To verify coverage with an existing Wi-Fi network, NetLink Wireless  
Telephones offer a site survey mode that can be used to test the AP’s  
signal strength in the wireless LAN coverage area. This mode detects  
the four strongest AP signals and displays the signal strength and the  
AP channel assignments. This mode can be used to detect areas with  
poor coverage or interfering channels. With the NetLink e340 and i640  
Wireless Telephones, the entire coverage area should be checked to  
ensure that at least one access point’s reading is stronger than –70 dBm  
in all areas. Also, if the site survey mode indicates 2 APs using the  
same channel, then at least one other AP must be indicated at 10 dBm  
stronger than those APs to avoid channel conflicts. After a site survey is  
completed, coverage issues can be resolved by adding and/or relocating  
APs and overlap issues may be resolved by reassigning channels or by  
relocating some access points. Another complete site survey should be  
performed after any adjustments are made to ensure that the changes  
are satisfactory and have not impacted other areas.  
All APs on the wireless network used by the NetLink Wireless  
Telephones must be set to the same supported and basic data rates. If  
this is not adhered to, the NetLink Wireless Telephones may not  
associate to the closest AP if a more distant one supports a higher data  
rate.  
2.2.2 Access Point  
Data Rates and  
Power Output  
In addition, all APs must be set to operate at the same power output.  
SpectraLink highly recommends a power output setting of 100 mW. If  
this cannot be accommodated, SpectraLink recommends a 50 mW  
setting and requires a minimum of 30 mW. With lower power output  
settings, special attention must be made to AP placement to ensure  
there are no frequency re-use issues. These problems may not be  
evident when using the handset’s site survey tool as it is assumes 100  
mW transmission power from the APs.  
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The network capacity requirements also factor into the number of APs  
required, although in most cases the coverage area is the primary factor.  
Data traffic is very bursty and sporadic, but data applications can tolerate  
network congestion with reduced throughput and slower response times.  
On the other hand, voice traffic cannot tolerate unpredictable delays, but  
at least the bandwidth requirements are constant and consistent for  
every phone call. Also, telephone traffic can be predicted using  
probabilistic usage models, allowing a network to be designed with high  
confidence in meeting anticipated voice capacity requirements. Beyond  
the normal IP telephony design guidelines, there are several additional  
considerations that need to be addressed for Wi-Fi telephony with  
NetLink Wireless Telephones.  
2.3  
Capacity  
There are several factors that determine the AP bandwidth utilization of  
a telephone call. The first is the VoIP protocol used and its  
2.3.1 Access Point  
Bandwidth  
characteristics. The type of codec utilized combined with the packet rate  
will determine the size of the voice packets, along with any additional  
overhead information required for the protocol. The payload information  
makes up a little more than half of a typical voice packet, with 802.11  
and IP protocol overhead filling the rest. The 802.11 protocols include  
timing gaps for collision avoidance, which means bandwidth utilization is  
more accurately quantified as a percentage rather than actual data  
throughput. The percentage of bandwidth used increases for lower data  
rates, but it is not a linear function because of the bandwidth consumed  
by the timing gaps and overhead. For example, a call using standard 64  
kb/s voice encoding (G.711) utilizes about 4.5% of the AP bandwidth at  
11 Mb/s, and about 12% at 2 Mb/s. In this example, four simultaneous  
calls on an AP would consume about 18% of the available bandwidth at  
11 Mb/s or about 48% at 2 Mb/s.  
Considerations  
The following table lists the theoretical percentage of available  
bandwidth used per telephone call for each 802.11b data rate:  
1 Mb/s  
15.7%  
2 Mb/s  
10.0%  
5.5 Mb/s  
6.4%  
11 Mb/s  
5.4%  
NetLink Telephony Gateway (24  
kb/s), 20 ms sample rate  
G.711 (64 kb/s), 30 ms sample rate  
G.729 (8 kb/s), 30 ms sample rate  
20.5%  
9.3%  
11.7%  
6.1%  
6.1%  
4.1%  
4.5%  
3.5%  
Theoretical Call Bandwidth Utilization of 802.11b Access Points  
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The maximum number of simultaneous telephone calls an AP can  
support is determined by dividing the total available bandwidth by the  
percentage of bandwidth used for each individual call. Approximately 20-  
40% of the AP bandwidth is reserved for channel negotiation and  
association algorithms, so 60-80% of the total available bandwidth  
should be used for calculating the maximum call capacity per AP. Lower  
overall bandwidth is available when there are a greater number of  
devices associated with an AP. For example if all calls on an AP are  
using a theoretical 4.5% of the bandwidth at 11 Mb/s, the actual number  
of calls expected at that rate would be about 13 (60% of bandwidth  
available / 4.5% theoretical bandwidth utilized per call). The actual  
number of calls expected at 2 Mb/s using the NetLink Telephony  
Gateway and a 20 ms sample rate is about 7 (70% of bandwidth  
available / 10% theoretical bandwidth utilized per call).  
Even with all of the known variables, there are many other vendor-  
specific characteristics associated with individual access points that  
make it difficult to quantify the concurrent calls per AP without thoroughly  
testing specific configurations. As a general rule based on lab tests and  
experience, wireless LAN designs for NetLink Wireless Telephones  
should consider no more than 12 simultaneous calls at 11 Mb/s or no  
more than 7 calls at 2 Mb/s using either G.711 or NetLink Telephony  
Gateways. Using the G.729 codec will yield roughly 50% more calls at  
these mentioned data rates, but the general performance of NetLink  
Wireless Telephones using this codec on various APs has not been well  
tested.  
To allow for bandwidth to be available for data traffic, SpectraLink  
provides the ability to limit the number of calls per access point within the  
NetLink Telephony Gateway and SVP Server. The “Calls per Access  
Point” setting will limit the number of active NetLink Wireless Telephone  
calls on each access point. Wireless Telephones are free to associate  
with other APs within range that have not reached the set maximum  
number of calls. SpectraLink recommends this setting be equal to or  
below the maximum number of calls discussed in the previous  
paragraph.  
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The push-to-talk (PTT) mode of the NetLink i640 Wireless Telephone  
2.3.2 Push-to-Talk  
Multicasting  
uses SpectraLink’s proprietary SpectraLink Radio Protocol (SRP)  
ADPCM encoding. If a PTT broadcast is active (i.e. a user presses the  
PTT button), the feature will use the bandwidth as indicated in the table  
above for the single transmitting i640 Wireless Telephone and one half of  
the bandwidth for all of the receiving i640 Wireless Telephones. The data  
rate used for PTT depends on the AP’s settings for multicast traffic. This  
bandwidth used is independent of the number of handsets receiving the  
PTT call. Because the PTT mode uses IP multicasting, all APs on the  
subnet will transmit a PTT broadcast unless the network is running  
Internet Group Management Protocol (IGMP), in which case the  
broadcast will only go to those APs that are associated with NetLink i640  
Wireless Telephones with the PTT feature enabled.  
Considerations  
Because the data rate and the packet rate are constant, Wi-Fi telephony  
calls may be modeled in a manner very similar to circuit-switched calls.  
Telephone users (whether wired or wireless) generally tend to make calls  
at random times and of random durations. Because of this, mathematical  
models can be applied to calculate the probability of calls being blocked  
based on the number of call resources available.  
2.3.3 Telephone  
Usage  
Telephone usage is measured in units of Erlangs. One Erlang is  
equivalent to the traffic generated by a single telephone call that lasts for  
one hour. A typical office telephone user will generate 0.10 to 0.15  
Erlangs of usage, which equates to six to nine minutes on the telephone  
during a one-hour period. Heavy telephone users may generate 0.20 to  
0.30 Erlangs, or 12 to 18 minutes of phone usage in an hour. Note that  
traffic analysis is based on the aggregate traffic for all users, so users with  
higher or lower usage are averaged out.  
The traffic engineering decision is a tradeoff between additional call  
resources and an increased probability of call blocking. Typical systems  
are designed to a blocking level (or grade of service) of 0.5% to 2% at the  
busiest times. Traffic model equations use the aggregate traffic load,  
number of users, and number of call resources to determine the blocking  
probability. The blocking probability can also be used along with the  
aggregate traffic load to determine the number of call resources required.  
Traffic model equations and calculators are available at www.erlang.com.  
Consider a system with APs that can support six active telephone calls. If  
a blocking probability of 1% or less is desired, each AP can support about  
13 moderate wireless telephones users. If the AP coverage can support  
12 simultaneous calls per AP, each AP can support about 39 moderate  
users.  
The following table shows maximum users per AP, based on the AP’s  
ability to handle simultaneous calls:  
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User Calling Intensity  
Light  
Moderate  
Heavy  
Erlangs per User  
0.10  
0.15  
0.20  
Max Active Calls per AP  
Users Supported per AP (1% Blocking Probability)  
1
2
1
1
2
1
2
2
3
4
3
3
4
8
6
4
5
13  
19  
25  
31  
37  
44  
51  
58  
9
7
6
13  
17  
21  
25  
30  
34  
39  
10  
13  
16  
19  
22  
26  
29  
7
8
9
10  
11  
12  
Users Supported per Access Point  
Areas where more Wireless Telephone usage is expected, such as  
cafeterias and auditoriums, can be provided with additional capacity to  
support more users by installing addition APs with smaller coverage  
areas. But for most enterprise applications, the number of calls  
supported within the coverage area of an AP should be sufficient.  
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3.0 Network Infrastructure Considerations  
The NetLink Wireless Telephone infrastructure components should  
3.1  
Physical  
Connections  
connect to the facility’s local area network (LAN) using Ethernet  
switches, as opposed to Ethernet hubs, to provide adequate bandwidth  
and limit traffic collisions.  
Ethernet switches should be configured to negotiate the connection  
requirements automatically. NetLink Telephony Gateways require  
10Base-T, half-duplex transmission and the NetLink SVP Server utilizes  
10/100Base-T, half or full-duplex transmissions and can be set to  
automatically negotiate or be configured to a specific transmission  
configuration.  
Network wiring is an important component of any Ethernet based system  
and is subject to local and state building code specifications. Category  
5, 4-pair 10/100Base-T Ethernet cabling should be used for NetLink  
Wireless Telephone infrastructure equipment.  
NetLink Wireless Telephones operate as LAN client devices and  
therefore require IP addresses to work with the network. IP addresses  
can be assigned statically through the configuration menus on the  
handsets, or dynamically using standard DHCP protocol. For dynamic  
IP addressing, a DHCP server must be available.  
3.2  
Assigning IP  
Addresses  
NetLink Telephony Gateways and NetLink SVP Servers also require IP  
addresses and support either static or DHCP address assignment.  
When utilizing multiple NetLink SVP Servers with an IP telephony  
server, the master NetLink SVP server must be assigned a static IP  
address. When operating with an IP telephony server, the NetLink  
SVP Server also requires a range of IP addresses that covers the total  
number of Wireless Telephones supported by that NetLink SVP server.  
When a NetLink Wireless Telephone registers with the telephony server,  
one of the IP address within this range is used to communicate between  
the NetLink SVP Server and the telephony server. This IP address is  
used by the IP telephony server as an alias for the NetLink Wireless  
Telephone, but will not be equivalent to the handset’s IP address that  
was either statically assigned or obtained from the DHCP server. The  
range of alias IP addresses must not be used within any DHCP range or  
cover the IP address used by any other device. In the case where  
multiple NetLink SVP Servers are used for added capacity, an exclusive  
range of IP addresses equivalent to the number of total users each  
NetLink SVP Server can support is required per NetLink SVP Server.  
All NetLink components can be field-upgraded with new software to add  
features or capabilities and bug fixes. NetLink Wireless Telephones  
utilize a TFTP client to automatically download new code when  
available. NetLink Telephony Gateways have an integrated TFTP  
server to support Wireless Telephone and OAI Gateway software  
upgrades. For installations that do not use NetLink Telephony  
3.3  
Software  
Updates Using  
TFTP  
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Gateways, a separate TFTP server must be provided. Also, the NetLink  
SVP Server requires a separate TFTP server for software updates. The  
NetLink Telephony Gateway cannot be used as a TFTP server for the  
NetLink SVP Server code. NetLink Telephony Gateways receive  
software updates only through an FTP server.  
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4.0 Quality of Service  
Quality of Service (QoS) is a means of guaranteeing a level of service  
that will result in a network connection of adequate quality. Typically this  
results in providing different levels of service for different applications,  
depending on their requirements. When data and voice are competing  
for bandwidth it is necessary to have a prioritization method that provides  
a controlled preference to voice packets. The initial 802.11 standards  
did not provide a practical QoS mechanism, so SpectraLink developed  
SpectraLink Voice Priority to allow real-time voice applications to coexist  
with data applications on a Wi-Fi network without compromising voice  
quality.  
4.1  
SpectraLink  
Voice Priority  
(SVP)  
Voice quality is ensured on a shared network with SVP, a QoS  
mechanism for quality of service that is fully compatible with Wi-Fi  
networks. Adopted by the leading wireless LAN vendors, SVP  
guarantees audio quality in a shared voice and data network. Access  
points generally use random backoff intervals and require all types of  
traffic to contend for bandwidth with equal rights. Treating all traffic  
equally can cause significant delays to voice traffic. Modifying the AP  
behavior to recognize and prioritize voice packets increases the  
probability of better performance while continuing to treat asynchronous  
data packets normally. The two operations that comprise SVP in the  
AP, minimizing random backoff and priority queuing, require a packet  
filtering mechanism. Packet filtering requires recognizing the packet’s  
protocol identifier, which for SpectraLink packets is registered protocol ID  
119 for the SpectraLink Radio Protocol (SRP). The NetLink SVP Server  
also performs packet delivery timing in the link to the Wireless  
Telephones that is critical for ensuring seamless handoffs among APs  
and for enhanced battery management processes.  
SVP-enabled APs are required for all NetLink Wireless Telephone  
installations, even if the wireless LAN is being used only for voice. SVP  
is required to ensure the timing and delivery of SpectraLink voice  
packets. Without a method of prioritization for voice packets, the lack of  
a controlled delivery method will result in poor audio quality, even with  
only voice devices on the network.  
4.1.1 SVP-enabled  
Access Points  
Information regarding APs that are compliant with SVP, or otherwise  
support a compatible voice QoS mechanism, can be found on  
SpectraLink’s website in the NetLink product section at  
specific AP models are also available on the website are should be  
closely followed to ensure the proper implementation of SVP.  
To trigger SVP in the APs from the wired side of the network, a NetLink  
Telephony Gateway and/or NetLink SVP Server is required. NetLink  
Telephony Gateways can provide SVP support for small installations  
with four or fewer Gateways. If NetLink Telephony Gateways are used  
for SVP, the NetLink Wireless Telephones are limited to a maximum  
4.1.2 SVP  
Infrastructure  
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data transmission rate of 2 Mb/s.  
A NetLink SVP Server is required for applications using an IP telephony  
server or using more than four NetLink Telephony Gateways. A NetLink  
SVP Server can also be used with four or fewer NetLink Telephony  
Gateways to allow a maximum data transmission rate of 11 Mb/s.  
A single NetLink SVP Server supports 120 simultaneous calls when  
used with NetLink Telephony Gateways, or 80 simultaneous calls with  
an IP telephony server. Multiple NetLink SVP Servers can be used to  
increase capacity to support up to 850 total calls and 8,000 Wireless  
Telephones for IP telephony server interfaces. When used with NetLink  
Telephony Gateways, the total number of users is limited to 640 total  
users (40 NetLink Telephony Gateways). Refer to the NetLink SVP  
Server Installation, Setup, and Maintenance for more information about  
the maximum number of simultaneous calls and Wireless Telephones  
supported by multiple NetLink SVP Servers.  
For installations with multiple NetLink SVP Servers, call resources are  
automatically allocated between the APs and the NetLink Wireless  
Telephones by those devices’ MAC addresses. Allocation is done by  
dividing the MAC address by the number of NetLink SVP Servers and  
assigning the device based on the remainder. For example, if three  
NetLink SVP Servers are used, the first NetLink SVP Server is assigned  
to all APs and NetLink handsets with MAC addresses that are even  
multiples of three. The second NetLink SVP Server is assigned to MAC  
addresses with a remainder of one when divided by three, and the third  
is assigned to the MAC addresses with a remainder of two. In most  
instances, because of the large number of Wireless Telephones and  
APs expected in such an application, the distribution of call processing  
will be relatively even across all NetLink SVP Servers.  
If a NetLink SVP Server other than the SVP Server assigned as the  
‘master’ fails and can be no longer detected, the call processing will be  
automatically redistributed among the remaining servers. Some active  
calls may be lost during this process, but the process does not require  
any manual re-configuration. To minimize downtime related to a failed  
master NetLink SVP Server or a single server, a spare NetLink SVP  
Server can reside on the network and in the case of a failure, the  
network administrator can assign the IP address of the failed unit to the  
replacement SVP Server  
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5.0 Security  
Security provisions are critical for any enterprise Wi-Fi network.  
5.1  
Security  
Wireless technology does not provide any physical barrier to the  
network, since radio waves penetrate walls and can be monitored and  
accessed from outside a facility. The extent of security measures  
utilized are typically proportional to the value of the information  
accessible on the network. The security risk for Wi-Fi telephony is not  
limited to the typical wired telephony concerns of eavesdropping on  
telephone calls or making unauthorized toll calls, but is equivalent to the  
security risk of the data network that connects to the APs. Several  
different security solutions can be implemented with NetLink Wireless  
Telephones. Determining the proper level of security should be based  
on identified risks, corporate policy, and an understanding of the pros  
and cons of the available security methods.  
Concerns  
NetLink Wireless Telephones support Wired Equivalent Privacy (WEP)  
encryption as defined by the 802.11 standard. The handsets can use  
either 40-bit or 128-bit key lengths. WEP is intended to provide the same  
level of security over a wireless LAN as on a wired Ethernet LAN.  
Although security flaws have been identified, WEP still provides strong  
encryption that requires an experienced and dedicated hacker to break.  
5.1.1 Wired Equivalent  
Privacy (WEP)  
802.1x based authentication protocols such as EAP-TLS or Cisco’s  
LEAP were developed to provide a higher level of security for wireless  
networks. These advanced methods require a back-end authentication  
server to authenticate users and generate new keys. This authentication  
and re-keying process can take up to several seconds and is required  
each time a user hands-off from one AP to the next in the same subnet.  
While this is taking place, the client device is not authenticated to an AP  
and there is an interruption in the data stream and therefore in the voice  
conversation. This interruption caused by the authentication process is  
unacceptable for voice communication in most enterprise applications.  
5.1.2 Cisco Fast  
Secure Roaming  
(FSR)  
To address the voice quality issues with most security mechanisms,  
SpectraLink and Cisco have worked together to deliver a Fast Secure  
Roaming (FSR) mechanism. FSR allows the authentication process to  
be done in a way that minimizes the number of messages required  
between the NetLink Wireless Telephones and the Cisco wireless LAN  
infrastructure. It is designed to be compatible with wireless standards  
and allow backward compatibility with devices utilizing previous security  
mechanisms, such as Cisco’s LEAP.  
Implementation of FSR for Cisco Aironet APs utilizes several standard  
and proprietary security components, including Cisco Client Key  
Management (CCKM), LEAP authentication, Michael message integrity  
check (MIC), and Temporal Key Integrity Protocol (TKIP). FSR not only  
addresses the roaming issue, but also provides strong security  
measures for authentication, privacy, and data integrity.  
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Recognizing the need for stronger security standards, the IEEE is  
5.1.3 Emerging  
Security  
developing the 802.11i standard, which is expected to be ratified in late  
2004. The 802.11i standard includes stronger encryption, key  
management, and authentication mechanisms. An interim solution  
endorsed by the Wi-Fi Alliance is Wireless Protected Access (WPA),  
which is a subset of the 802.11i standard.  
Standards  
SpectraLink is committed to industry standards and will implement the  
802.11i security standard once it is ratified. Depending on the required  
components of this standard, an enhanced security method that is  
conducive to mobile voice requirements, like the Cisco FSR mechanism,  
may be required to provide the best voice quality.  
Virtual LANs (VLANs) can be used to segregate traffic into different  
security classes. By using separate VLANs, data traffic can utilize the  
most robust, but process intensive, security methods.  
5.2  
Utilizing VLANs  
The 802.1Q standard establishes a method for inserting VLAN  
membership information into Ethernet frames via header information  
tags. NetLink infrastructure equipment and SpectraLink Voice Priority  
are not compatible with 802.1Q tags. The Ethernet switch must remove  
802.1Q tags prior to forwarding packets destined for NetLink Telephony  
Gateways or a NetLink SVP Server. In other words, the Ethernet switch  
ports must not be configured as trunked ports.  
Access points can be configured to filter certain MAC addresses, which  
can be used as a method of securing the wireless LAN. This process  
generally works, but does cause some performance issues on some  
APs.  
5.3  
MAC Filtering  
and  
Authentication  
A more robust method of using MAC addresses to secure the network  
utilizes authentication back to a RADIUS server. In general, the delays  
caused by this authentication are not acceptable for voice traffic. Having  
the RADIUS server on the local network will help reduce delays, but the  
response time of the server may still be an issue. Adding any network  
delays will compound the issue. Network administrators should evaluate  
whether such delays are not great enough to affect the voice quality of  
NetLink Wireless Telephones.  
The traffic filtering capabilities of firewalls, Ethernet switches, and  
wireless switches can be used as security methods by allowing only  
certain types of traffic to pass onto specific areas of the LAN. To  
properly provide access control, it is necessary to understand the kind of  
IP traffic utilized by the NetLink Wireless Telephones.  
5.4  
Firewalls and  
Traffic Filtering  
When using NetLink Telephony Gateways to interface to a traditional  
PBX, the NetLink Wireless Telephones utilize the SpectraLink Radio  
Protocol (ID 119). This protocol in on a peer level with TCP and UDP  
and does not uses ports unique to TCP and UDP.  
For an IP telephony server interface, the ports that are used depend on  
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the IP telephony protocol used on the telephony switch interface. The  
telephony switch vendor should be able to supply the port numbers used  
by the protocol.  
The NetLink Wireless Telephones, NetLink Telephony Gateways, and  
NetLink SVP Server use TCP and UDP and other common IP protocols  
from time to time. These include DHCP, DNS, WINS, TFTP, FTP,  
Telnet, ARP, and ICMP. SpectraLink uses proprietary UDP channels  
between the infrastructure components that use UDP ports 5454 - 5458.  
The push-to-talk (PTT) mode of the NetLink i640 Wireless Telephone  
uses the multicast IP address 224.0.1.116, which is also used by the  
NetLink Wireless Telephones and infrastructure components to locate  
and maintain each other.  
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Virtual Private Networks are secured private network connections.  
5.5  
Virtual Private  
Networks  
(VPNs)  
VPNs typically employ some combination of encryption, digital  
certificates, strong user authentication and access control to provide  
security to the traffic they carry. They usually provide connectivity to  
many devices behind a VPN concentrator. The network can be broken  
into two portions, protected and unprotected:  
1. The area behind the VPN server is referred to as the “protected”  
portion of the network. Sensitive, private network equipment  
such as file servers, email servers and databases would reside  
in this portion.  
2. The area in front of the VPN server is referred to as the  
“unprotected” or demilitarized zone (DMZ), where the wireless  
APs and less sensitive network equipment may reside.  
Utilizing VPNs can be an extremely effective method of securing a  
wireless network. Many customers have been implementing VPNs to  
maintain the integrity of their wireless LANs by requiring wireless users  
who need access to the protected portion of the network to connect  
through a firewall.  
Voice devices, such as the NetLink Wireless Telephone do not require  
access to the protected portion of the network. Placing the NetLink  
Wireless Telephones, NetLink SVP Server(s), and NetLink Telephony  
Gateways in the demilitarized zone, and requiring data users to utilize  
the VPN ensures that the network is protected against hackers seeking  
to access sensitive information within the network core.  
NetLink Wireless  
Telephones  
NetLink  
VPN Concentrator  
Telephony  
Gateway  
Devices that  
require access to  
the network core  
utilize a secure  
VPN connection  
(dashed line).  
Protected Network Core  
Unprotected DMZ  
Deploying NetLink Wireless Telephones with a VPN  
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6.0 NetLink Wireless Telephones and Subnets  
Subnets are used to create a boundary between network segments.  
Although these boundaries are logical, they become somewhat of a  
physical boundary for mobile network devices moving throughout the  
enterprise. When a device with an established IP data stream (such as  
with an active phone call) attempts to roam across a subnet boundary, it  
needs to obtain a valid IP address within the new subnet. During this  
process the data stream cannot be re-established automatically and the  
connection (voice call) is dropped. In the case of the NetLink Wireless  
Telephones, the handsets should be power-cycled to obtain a new  
DHCP address. The handsets can automatically recover in the new  
subnet from a lost network connection with the original subnet, but the  
40-second failure and recovery time generally warrants cycling the  
power.  
Some APs, Ethernet switches, and third-party devices have  
implemented methods to facilitate device mobility. While these methods  
are transparent to the client device, they often cause enough delay and  
latency to manifest poor voice quality. In addition, many of these  
methods do not work well under loaded conditions, such as might be  
experienced with a large number of highly mobile wireless voice users.  
NetLink Wireless Telephones must reside within the same subnet as the  
source of the SpectraLink Voice Priority (SVP) control. SVP can be  
controlled from a NetLink Telephony Gateway, a NetLink SVP Server, or  
a combination of the two. Because the NetLink SVP Server can only  
operate in a single PBX interface mode, Wireless Telephones cannot  
operate with a NetLink Telephony Gateway and in a native IP interface  
to an IP telephony server on the same NetLink SVP server. All SVP  
Servers on the same subnet must operate in the same PBX interface  
mode (either native IP or through NetLink Telephony Gateways).  
There are additional subnet requirements for NetLink Wireless  
Telephones based on the infrastructure components that are used.  
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NetLink Wireless Telephones, NetLink Telephony Gateways, NetLink  
SVP Server(s), and the wireless APs must reside on the same subnet.  
One reason for this requirement is that the NetLink Wireless Telephones  
use IP multicast messages to initialize the Wireless Telephone  
registration on the NetLink Telephony Gateways. Most routers deployed  
in multi-subnet Ethernet environments are configured to filter out  
multicast and broadcast messages. If a NetLink Wireless Telephone is  
powered up on a different subnet than the NetLink Telephony Gateway  
to which it is registered, the multicast message will never reach the  
NetLink Telephony Gateway.  
6.1  
6.2  
Subnets and  
NetLink  
Telephony  
Gateway  
Interfaces  
Although not recommended, NetLink Wireless Telephones can be  
deployed across multiple subnets when used with an IP telephony  
server interface. This can help facilitate subnet roaming when the  
subnets are geographically separated by defined boundaries.  
Subnets and IP  
Telephony  
Server  
Interfaces  
Each subnet must have its own NetLink SVP Server. This is necessary  
because Ethernet packets containing voice as their payload have short  
interesting lifetimes, making the timely delivery of voice packets  
essential. Routers can introduce latency and delay between the NetLink  
SVP Server and the APs, which manifests as poor voice quality.  
Ethernet connectivity between the NetLink SVP Server and the IP  
telephony server should never exceed 100 ms of network delay and 10  
ms of network jitter regardless of the physical properties of the link. The  
ability to cross a subnet boundary exists in this scenario, but the NetLink  
handsets will need to be power-cycled to obtain new IP address within  
the new subnet. In addition, other configuration considerations need to  
be addressed. Because users will not want to re-administer the  
Wireless Telephones to get them to work on another subnet, the  
ESSIDs should be broadcast using the “Learn Always” mode, the WEP  
key should be the same or turned off, and DHCP should be used.  
7.0 Conclusion  
Voice telephony over a wireless LAN represents the convergence of  
voice and data technology in the wireless environment. There are some  
specific network design criteria that must be applied to effectively  
implement a wireless telephony solution that is suitable for the  
demanding requirements of both voice users and data network  
administrators. With a little background study, both network and  
telephony professionals will be able to easily and confidently design and  
deploy a SpectraLink Wi-Fi telephony solution.  
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