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Issues with Devices Roaming Between Sites

Figure 11-1 shows a phone device roaming between two internal sites as a user brings along his or her phone for business travel.

Figure 11-1

Figure 11-1 Roaming Devices Between Internal Sites

When users roam between sites, they might take their phones with them. This typically does not apply to Cisco IP Phones, but it's very common with softphones such as Cisco IP Communicator running on a laptop or Cisco Unified Wireless IP Phones.

Issues with Roaming Devices

When phones move between different CUCM sites, inaccurate phone settings may occur.

The configuration of an IP Phone includes personal settings and location-dependent settings that are all bound statically to the phone's MAC address and hence to the device itself. The physical device location has traditionally been assumed to be constant.

If a phone (or, more likely, a softphone such as IP Communicator) is moved between sites, the location-dependent settings become inaccurate. Some of these settings and their errors are as follows:

  • Region: Might cause wrong codec settings.
  • Location: Might cause wrong call admission control (CAC) and bandwidth settings.
  • Survivable Remote Site Telephony (SRST) reference: Might cause a malfunction of Cisco Unified SRST.
  • Automated Alternate Routing (AAR) group: Might cause a malfunction of the call redirection on no bandwidth.
  • Calling Search Space: Might cause usage of remote gateways instead of local ones.
  • Media Resource Groups and Media Resource Group Lists: Might cause allocation of wrong media resources such as conference bridges or transcoders.

For correct settings, CUCM needs to be aware of the physical location of all phones, including roaming devices.

Device Mobility Solves Issues of Roaming Devices

Device Mobility offers functionality that is designed to enhance the mobility of devices within an IP network. Table 11-1 summarizes the challenges and solutions of Device Mobility.

Table 11-1. Device Mobility Solves Issues of Roaming Devices

Issue Without Device Mobility

Device Mobility Feature to Solve the Issue

When the mobile user moves to a different location, Call Admission Control settings are not adjusted.

Location settings are dynamically assigned.

PSTN gateways to be used are fixed.

Dynamic phone CSS allows for site-independent local gateway access.

SRST reference is fixed.

SRST reference is dynamically assigned.

When the mobile user moves to a different region, codec settings are not adjusted.

Region settings are dynamically assigned.

AAR does not work for mobile users.

The AAR calling search space and the AAR group of the DN are dynamically assigned.

Media resources are assigned location-independently.

The media resource list is dynamically assigned.

AAR has Extension Mobility issues.

Extension Mobility also benefits from dynamic assignment.

Although devices such as IP Phones and IP Communicator still register with the same CUCM cluster with SCCP or SIP, they now will adapt some of their behavior based on the actual site where they are located. Those changes are triggered by the IP subnet in which the phone is located.

Basically, all location-dependent parameters can be dynamically reconfigured by Device Mobility. Thus, the phone keeps its user-specific configuration, such as directory number, speed dials, and call-forwarding settings. However, it adapts location-specific settings such as region, location, and SRST reference to the actual physical location. Device Mobility can also be configured so that dial plan-related settings such as the device Calling Search Space (CSS), AAR group, and AAR CSS are modified.

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