Cisco 7920 IP User Manual

Cisco − Wireless IPT Design Guide for the Cisco 7920 IP Phone

Wireless IPT Design Guide for the Cisco 7920 IP Phone................................................................................1

Introduction.............................................................................................................................................1

Prerequisites............................................................................................................................................1

Requirements....................................................................................................................................1

Components Used.............................................................................................................................2

Conventions......................................................................................................................................2

Cisco 7920 IP Phone Overview..............................................................................................................2

WLAN Overview....................................................................................................................................2

RF Overview  Site Surveys and Design Recommendations................................................................2

VoIP QoS................................................................................................................................................3

Security for the Cisco 7920 and WLAN Networks................................................................................5

Network Sizing.......................................................................................................................................6

Number of 802.11b Devices per AP................................................................................................6

Number of 802.11b IP Phones per AP.............................................................................................6

Numbers of 802.11b Phones per Layer 2 Subnet or VLAN............................................................8

Understanding Layer 2 and Layer 3 Roaming........................................................................................9

Understanding Roaming Terminology.............................................................................................9

Layer 2 Roaming............................................................................................................................10

Layer 3 Roaming............................................................................................................................12

VLANs..................................................................................................................................................13

WLAN QoS for VoIP...........................................................................................................................14

Interconnecting WLANs to Cisco Campus Infrastructure....................................................................18

Connecting APs to the Catalyst 3550 SMI or EMI........................................................................19

Connecting APs to the Catalyst 2950 EI........................................................................................20

Connecting APs to the Catalyst 2950 SI........................................................................................20

Using Cisco Emergency Responder for E911 calls with the Cisco 7920.............................................20

Caveats and Limitations........................................................................................................................21

Call Admission Control..................................................................................................................22

Designing Around the Lack of Layer 3 Roaming..........................................................................22

Other Caveats and Limitations.......................................................................................................22

Related Information..............................................................................................................................23

Wireless IPT Design Guide for the Cisco 7920 IP Phone

Introduction Prerequisites

Requirements Components Used Conventions

Cisco 7920 IP Phone Overview WLAN Overview RF Overview  Site Surveys and Design Recommendations VoIP QoS Security for the Cisco 7920 and WLAN Networks Network Sizing

Number of 802.11b Devices per AP Number of 802.11b IP Phones per AP Numbers of 802.11b Phones per Layer 2 Subnet or VLAN

Understanding Layer 2 and Layer 3 Roaming

Understanding Roaming Terminology Layer 2 Roaming Layer 3 Roaming

VLANs WLAN QoS for VoIP Interconnecting WLANs to Cisco Campus Infrastructure

Connecting APs to the Catalyst 3550 SMI or EMI Connecting APs to the Catalyst 2950 EI Connecting APs to the Catalyst 2950 SI

Using Cisco Emergency Responder for E911 calls with the Cisco 7920 Caveats and Limitations

Call Admission Control Designing Around the Lack of Layer 3 Roaming Other Caveats and Limitations

Related Information

Introduction

This document provides design guidelines and deployment recommendations for customers adding the Cisco 7920 IP Phone to an existing Architecture for Voice, Video and Integrated Data (AVVID) network. The assumption is made that you have either an existing AVVID network for wired IP phones, or that you have previously read the AVVID design guides. It is not assumed that you have previous Wireless LAN (WLAN) experience. This document makes reference to several existing design guides for 802.11, IP Telephony, and campus network design.

Prerequisites

Requirements

Readers of this document should be knowledgeable of the following:

Cisco − Wireless IPT Design Guide for the Cisco 7920 IP Phone

AVVID design guides and administration.•

Components Used

The information in this document is based on the software and hardware versions below:

The Cisco 7920 IP Phone.•

The information in this document was created from the devices in a specific lab environment. All of the devices used in this document started with a cleared (default) configuration. If your network is live, make sure that you understand the potential impact of any command.

Conventions

For more information on document conventions, see the Cisco Technical Tips Conventions.

Cisco 7920 IP Phone Overview

The Cisco 7920 IP Phone extends the AVVID family of IP phones from 10/100 Ethernet to 802.11b WLANs. The Cisco 7920 provides a multi−line appearance with functionality similar to existing Cisco 79xx IP Phones. In addition, the Cisco 7920 provides enhanced WLAN security and Quality of Service (QoS) for operation in

802.11b networks. Future enhancements to the Cisco 7920 will add support for XML−based data access and services, as well as Universal Serial Bus (USB) connectivity for additional data services.

WLAN Overview

802.11b WLAN networks allow multiple devices to transmit data, voice, and video at data rates up to 11Mb. While WLAN networks allow similar types of traffic to be transmitted over them, it does have certain characteristics that differ from wired networks:

WLANs operate as a shared medium, which means that communication on the WLAN is half−duplex

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and that all devices within a single WLAN share the 11Mb. WLAN bandwidth varies based on the distance that a WLAN client is from a WLAN Access Point

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(AP). The farther the distance, the lower the data rates that traffic is supported. Since all WLAN traffic is seen by all other WLAN devices (within range), additional security

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considerations need to be taken to ensure that traffic is not captured or manipulated by intruders.

RF Overview  Site Surveys and Design Recommendations

Before getting started with any WLAN deployment, the first activity that every network administrator should undertake is a complete site survey of the environment where the WLAN devices are deployed. The site survey should be performed to understand the number of APs required to provide Radio Frequency (RF) coverage. It should take into consideration which types of antennas provide the best coverage, as well as where sources of RF interference exist.

An important factor to keep in mind is that rarely do two physical environments have the same RF characteristics. Because of this, the sections of this document that are RF specific are generalized. Network administrators may need to adjust those parameters to their specific RF environment and requirements.

While you may have already performed RF site surveys for an initial WLAN deployment, it is important to

Cisco − Wireless IPT Design Guide for the Cisco 7920 IP Phone

remember that the Cisco 7920 has somewhat different roaming characteristics than Cisco Aironet Network Interface Cards (NICs). This includes thresholds on the Cisco 7920 that determine when the phone roams or how much the RF environment needs to change in order to determine when a roaming event should occur. Because of this, it is recommended that a secondary site survey be performed for the Cisco 7920s. This gives you the opportunity to tune the APs to ensure that the Cisco 7920s have enough RF coverage and bandwidth to provide proper voice quality.

The design recommendation is that the Received Signal Strength Indication (RSSI) level in the RF network is at least 20 throughout the network. This provides the Cisco 7920 with the minimum signal coverage to be able to not only initiate a new call, but also roam properly between APs.

RSSI 15 30 45 60

The design recommendation is that the QoS Basis Service Set (QBSS) level on the APs is kept lower than 40. If the Cisco 7920 attempts to initiate a call and the QBSS element in the beacon is more than 40, the call will fail with a network busy tone to the user.

The power levels of the APs vary from network to network and need to be adjusted to meet site−specific requirements. On the APs, the coverage area increases as the power levels increase. But this does not necessarily mean that all the APs should be set to 50 or 100mW. This could potentially create an RF environment where the Cisco 7920s roam too frequently because of RF coverage overlap. Frequent roams could potentially cause interruptions in the overall voice quality. On the Cisco 7920, another tradeoff to consider is that the higher the power−level, the shorter the battery life.

−85

−70

−50

−35

dBm

VoIP QoS

While Voice over IP (VoIP) does convert voice signals into IP data packets and converge them with data traffic, the requirements of voice and data are very different when transmitted over an IP network. Data traffic can be classified with the following characteristics:

Bursty  Traffic can be sent in large or small bursts, depending on the application.• Bandwidth Greedy  Transmission Control Protocol (TCP) applications attempt to use as much

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bandwidth as the network allows. Packet Loss Insensitive  The retransmission capabilities of TCP allow data to continue to work

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correctly even with packet loss on the network. Delay Insensitive  Most TCP applications can handle some packet delay without effecting overall

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performance.

Cisco − Wireless IPT Design Guide for the Cisco 7920 IP Phone

Voice traffic can be classified with the following characteristics:

Smooth  VoIP packets are sent at consistent intervals with uniform packet sizes.• Benign  VoIP packets only attempt to use the bandwidth necessary to send from end to end. VoIP

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does not use any windowing to determine data rates. Packet Loss Sensitive  VoIP traffic is extremely sensitive to packet loss. Excessive loss degrades

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overall voice quality. Delay Sensitive  While VoIP can tolerate some amount of delay, excessive delay or excessive delay

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variation (jitter) degrade overall voice quality. User Datagram Protocol (UDP) Best Effort  VoIP sends Real Time Protocol (RTP) packets using

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UDP. UDP does not have a mechanism to retransmit lost packets.

The network guidelines defined by Cisco AVVID for proper VoIP operation are as follows:

Delay  Not to exceed 150 ms (one way).• Delay Variation (Jitter)  Not to exceed 30ms.• Packet Loss  Not to exceed 1 percent.•

Note: While isolated testing may show that VoIP calls could operate in a network outside of these guidelines,

deploying a VoIP network under those conditions cannot be predictably engineered. Support from the Cisco

Cisco − Wireless IPT Design Guide for the Cisco 7920 IP Phone

Technical Assistance Center (TAC) (if problems arise) is limited for this situation.

Security for the Cisco 7920 and WLAN Networks

The Cisco 7920 is supported in the architecture of the Cisco Wireless Security Suite.

The architecture is discussed in detail in documents for Wireless LAN Security Solution.

The architecture fits into the overall Cisco SAFE security architecture. For more information, refer to Cisco SAFE: Wireless LAN Security in Depth.

Note: The Cisco 7920 does not support Cisco Temporal Key Integrity Protocol (TKIP) or Cisco Message Integrity Check (MIC) in the initial software release. Future versions of the Cisco 7920 software will add support for TKIP and MIC as well as Wi−Fi Protected Access (WPA).

The Cisco 7920 supports both static Wired Equivalency Protocol (WEP), 802.1, and Extensible Authentication Protocol (EAP) − Cisco Light Extensible Authentication Protocol (LEAP) for authentication and data encryption. When either encryption model is used, both the signaling (Signaling Connection Control Part (SCCP)) and media (RTP) are encrypted between the Cisco 7920 and AP.

Static WEP requires that a 40 or 128 bit key be manually entered on all of the Cisco 7920s as well as the APs. It performs AP−based authentication based on the device (such as the Cisco 7920) having a matching key.

LEAP allows devices (such as the Cisco 7920 and the AP) to be mutually authenticated (Cisco 7920 > AP, AP > Cisco 7920) based on a username and password. Upon authentication, a dynamic key is used between the Cisco 7920 and the AP to encrypt traffic.

If LEAP is used, a LEAP−compliant RADIUS server such as the Cisco Secure ACS for Windows, is required to provide access to the user database. The ACS server can either store the username and password database locally, or it can access that information from an external Microsoft Windows directory.

Note: While it is a valid configuration option, it is not recommended that an external (off ACS) database be used to store the username and password credentials for Cisco 7920 phones. Because the ACS server must be queried whenever the Cisco 7920 roams between APs, the unpredictable delay to access this external database could cause excessive delay and poor voice quality.

The placement of the ACS server should be considered when deploying LEAP. This is because LEAP authentication is required every time a Cisco 7920 roams between APs, and RTP traffic (voice) does not flow until the LEAP authentication is completed. Reducing the amount of delay (such as router hops and WAN links) between APs and the ACS significantly improves the overall voice quality when Cisco 7920 users are roaming..

The three options for deploying the ACS functionality are as follows:

Centralized ACS server  All users access the ACS server in a central location within the network.• Remote ACS server  For remote offices that have slow speed WAN links or congested WAN links

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that might delay LEAP processing, a ACS server could be deployed locally in the office. Local and Fallback RADIUS server functionality in a Cisco AP  In Cisco IOS® Software

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Release 12.2(11)JA , the Cisco AP supports the ability to authenticate LEAP users without having to access an external ACS server. This functionality supports up to 50 users, and is supported for EAP−Cisco (LEAP) only. This functionality does not interact with a centralized or remote ACS server in terms of database synchronization. This functionality is designed to be used as the primary

Cisco − Wireless IPT Design Guide for the Cisco 7920 IP Phone

RADIUS functionality in a small office, but could also be used as a ACS server in case of WAN failure.

The following is a step−by−step example of configuring the fallback RADIUS server for LEAP authentication:

Configure the Network Access Server (NAS), by issuing the following commands:

radius−server local

nas 192.168.10.35 key Cisco nas 192.168.10.45 key Cisco

Configure the user database, by issuing the following commands:

radius−server local user BM−AP1200−one−SCM password Cisco user BM−AP1100−two−SCM password Cisco user testuser password Cisco

Configure the local RADIUS server in the APs RADIUS server list, by issuing the following

3. commands:

aaa group server radius rad_eap

server 192.168.10.45 auth−port 1812 acct−port 1813 radius−server host 192.168.10.45 auth−port 1812 acct−port 1813 key Cisco

Configure the RADIUS server timeouts, by issuing the following commands:

radius−server deadtime 10

Disable client holdoff, by issuing the following command:

no dot11 holdoff−time

Network Sizing

Outside of normal IP Telephony design guidance for sizing (such as a Public Switched Telephone Network (PSTN) gateway ports, transcoders, and WAN bandwidth), there are several 802.11b specific considerations to address.

Number of 802.11b Devices per AP

Between 15 to 25 802.11b devices per AP is recommended. Normal caveats apply in terms of traffic types, usages patterns, and physical space coverage.

Number of 802.11b IP Phones per AP

Before any discussion about network planning can take place, it helps to understand the basics of the overall network capacity. The chart below shows the theoretical maximum throughput of an 802.11b WLAN network. The highlighted sections show the data that is relevant for the packet sizes of VoIP traffic (G.711 and G.729).

Cisco − Wireless IPT Design Guide for the Cisco 7920 IP Phone

One of the key aspects when calculating network capacity for 802.11b networks is that it is a shared medium. Because of this, consideration must be given for radio contention among the various devices. This means that the back−off algorithms in 802.11b, that allow multiple devices to access the medium, affect the overall throughput.

For the VoIP calculations below, a VoIP call has the following characteristics:

The packets are made up of a 20 byte IP header, an 8 byte UDP header, a 12 byte RTP header, and the

1. RTP data. The RTP data is a 20ms voice sample. For G.729, this is 20 bytes. For G.711, this is 160 bytes.2. The total VoIP packet is 200 bytes of (IP+UDP+RTP) headers + RTP data. The 802.11 header (Layer

3. 2 MAC) is 24 bytes long, so the total packet is 224 bytes. RTP data is transmitted at 50 packets per second (pps) in each direction, or 100pps for a full−duplex

4. conversation.

Looking at the 11Mb column of the chart, we can make the following calculations for G.711:

256 byte packet size = 2,596,588 bits per second = 324,573 bytes per second (Theoretical packet rate)• 100 packets per second * 224 bytes per packet = 22,400 bytes per second (Bandwidth of a G.711

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VoIP call) 324,573 / 22,400 = 14.489 calls (Theoretical maximum VoIP capacity per 802.11b channel)•

14.489 * .6 = 8.69 calls (Theoretical maximum number of VoIP calls * 60 percent of the bandwidth)

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Note: Sixty percent of the bandwidth is used to calculate the number of VoIP calls, which allows for the following:

It allows bandwidth to be available for data traffic.♦ It provides bandwidth consideration for 802.11b management traffic and acknowledgements.♦

The design consideration for G.711 calls is not to exceed seven concurrent VoIP calls per AP. This number has been proven in lab testing to provide acceptable voice quality.

The design consideration for G.729 calls is not to exceed eight concurrent VoIP calls per AP. This number has been proven in lab testing to provide acceptable voice quality.

Note: The following information affects Voice Activity Detection (VAD) configuration:

Cisco − Wireless IPT Design Guide for the Cisco 7920 IP Phone

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Cisco 7920 IP User Manual

Table of Contents

Wireless IPT Design Guide for the Cisco 7920 IP Phone

Introduction

Prerequisites

Requirements

Components Used

Conventions

Cisco 7920 IP Phone Overview

WLAN Overview

VoIP QoS

Security for the Cisco 7920 and WLAN Networks

Network Sizing

Number of 802.11b Devices per AP

Number of 802.11b IP Phones per AP