Cisco Systems 7920 IP manual Call Admission Control, Designing Around the Lack of Layer 3 Roaming

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Call Admission Control

Call Admission Control (CAC) provides the mechanism that prevents the presence of the next voice call to interrupt or provide poor QoS for all existing voice calls. Regarding Time−Division Multiplex (TDM), this is accomplished by a finite number of DS0 channels. In wired Ethernet AVVID networks, this is provided by an interaction between the Cisco CallManager regions and Cisco H.323 gatekeepers. However, these mechanisms only address CAC for the initial setup of IP Telephony calls. They do not address CAC when the underlying network is changing for the IP phone throughout the call. This is the case when a Cisco 7920 roams between two APs.

While the Cisco APs does send out QBSS information about the CU, and the Cisco 7920 can utilize this information to determine the best AP to associate with, this does not provide a 100 percent guarantee that calls retain proper QoS during a roam between APs. One example of when this degraded QoS could occur is an event that causes more than seven or eight active Cisco 7920s to roam into an area served by a single AP, thus exceeding the guidelines for calls per AP.

If the CU is above the CU threshold, the call is not setup and the phone displays a network busy message. If the CU of the candidate handoff AP is above the threshold, the Cisco 7920 remains associated to the existing AP for as long as possible. Once the current AP is lost (no probe response or beacons received), the Cisco 7920 handsoff to the candidate AP regardless of the CU. A beep is heard before this happens so the user can go back toward the current AP or stop and finish the conversation.

Another area of admission control that needs to be considered with the Cisco 7920 is when a user takes the Cisco 7920 from one location to another (such as campus to branch). When the Cisco 7920 re−registers with the Cisco CallManager, there is no mechanism that updates which Cisco CallManager region (or gatekeeper zone) the Cisco 7920 is associated with. This means that it is possible that calls to or from that Cisco 7920 would not properly be accounted for across a WAN link, and could cause poor voice quality to phones that are registered in that region.

Designing Around the Lack of Layer 3 Roaming

For customers that require roaming support for a large area (such as between floors of a buildings), large Layer 2 VLANs (spanning across access switches) can be created to eliminate the Cisco 7920s crossing an Layer 3 boundary when roaming. The following guidelines need to be considered for customers that are looking to deploy large Layer 2 VLANs:

∙Cisco ESE guidelines do not recommend that data VLANs span multiple access switches.

∙An Layer 2 VLAN should not cross a building boundary. If this is required, then an additional (overlay) Layer 2 core needs to be built to avoid creating instability and excessive traffic in the traditional Layer 3 core.

∙Both the voice VLAN and the native VLAN on the AP need to be trunked across the large Layer 2 VLAN. This is required to allow both voice traffic and IAPP traffic between the APs.

∙Having Layer 2 VLANs span multiple access switches creates the possibility of STP loops (if configured incorrectly) and overall network instability. Designs using this should model should be reviewed with your Cisco Systems Engineer before deployment.

∙For customers that may eventually be considering extending their Layer 3 networks down to the access switches, the model of building large Layer 2 VLANs is no longer valid.

Other Caveats and Limitations

The following are other things to be aware of regarding the Cisco 7920 IP Phone:

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

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Contents Table of Contents Requirements IntroductionPrerequisites Components Used Cisco 7920 IP Phone OverviewWlan Overview RF Overview Site Surveys and Design RecommendationsRssi VoIP QoS∙ Delay Variation Jitter Not to exceed 30ms Security for the Cisco 7920 and Wlan Networks Number of 802.11b IP Phones per AP Network SizingNumber of 802.11b Devices per AP Page Numbers of 802.11b Phones per Layer 2 Subnet or Vlan Understanding Roaming Terminology Understanding Layer 2 and Layer 3 RoamingLayer 2 Roaming Page Layer 3 Roaming VLANs Wlan QoS for VoIP Page Page Page Interconnecting WLANs to Cisco Campus Infrastructure Connecting APs to the Catalyst 3550 SMI or EMI Connecting APs to the Catalyst 2950 SI Connecting APs to the Catalyst 2950 EICaveats and Limitations Other Caveats and Limitations Call Admission ControlDesigning Around the Lack of Layer 3 Roaming Related Information

7920 IP specifications

The Cisco Systems 7920 IP phone represents a significant advancement in wireless communication technology, designed specifically for enterprises that require mobility and efficiency in their communication systems. This versatile device integrates seamlessly into a Cisco IP telephony network, allowing users to maintain connectivity while on the move, which is essential in fast-paced business environments.

One of the key features of the Cisco 7920 is its robust wireless capabilities. Operating on the IEEE 802.11b/g Wi-Fi standards, it provides a reliable connection to corporate networks, ensuring that users can access voice, data, and video services without being tethered to a desk. The phone supports dynamic call handling and features a responsive LCD display that enhances user interaction, making it easier to navigate menus and manage calls.

The Cisco 7920 also offers exceptional sound quality, thanks to its advanced audio technology. With Wideband audio support, users can experience clarity in conversations, which is particularly important in environments where clear communication is vital. The phone is equipped with noise-canceling capabilities that reduce background noise, allowing users to focus on their conversations without distractions.

Security is another critical aspect of the Cisco 7920. The device supports several security protocols, including WPA and WPA2, ensuring that all communications are encrypted and secure. This is crucial for organizations that handle sensitive information and need to comply with industry regulations.

Additionally, the Cisco 7920 features an extended battery life, enabling users to make calls throughout the day without frequent recharging. Its lightweight design and ergonomic build make it comfortable to use for extended periods.

In terms of integration, the Cisco 7920 IP phone is compatible with various Cisco Unified Communications Manager software versions, allowing for easy configuration and management within existing network infrastructures. Its support for multiple languages also makes it a suitable choice for international organizations, accommodating diverse user groups.

Overall, the Cisco Systems 7920 IP phone combines the necessary features, technologies, and characteristics to meet the demands of modern enterprise communication. Its blend of wireless capability, superior audio quality, security measures, and integration versatility makes it an excellent choice for organizations seeking efficient and reliable communication solutions.
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