Cisco 7925G Administration Guide

Mid-call Roaming
A wireless IP phone user is actively engaged in a call and moves from one building to another. The
roaming event occurs when the phone moves into the range of a different AP, and then the phone
authenticates and associates with the new AP. The previous AP hands the call over to the new AP while
maintaining continuous audio connection without user intervention. As long as the APs are in the same
Layer 2 subnet, the wireless IP phone keeps the same IP address and the call continues. As a wireless
IP phone roams between APs, it must reauthenticate with each new AP. See Authentication Methods,
on page 39 for information about authentication.
If the user is roaming across Layer 2 boundaries, then Layer 3 mobility must be enabled to have seamless
roaming, and to preserve an existing call.
If the wireless IP phone user moves from an AP that covers IP Subnet A to an AP that covers IP Subnet
B without Layer 3 mobility enabled, the phone no longer has an IP address or gateway that is valid
within the new subnet and the call can disconnect.
Layer 3 Roaming
Layer 3 roaming is available with Cisco autonomous and unified deployments. With unified deployments,
Layer 3 roaming can be performed using intercontroller roaming or, if AP groups are used, to enable
Layer 3 roaming.
Layer 3 roaming with Cisco Unified Access Points is accomplished by controllers that use dynamic
interface tunneling and requires Layer 3 mobility to be enabled. Clients that roam across controllers
and VLANS can keep their IP address when using the same SSID.
Fast and Secure Roaming
Cisco Centralized Key Management (CCKM) enables authenticated client devices to roam securely
from one AP to another without any perceptible delay during reassociation. With the support of the
CCKM protocol, the wireless IP phone negotiates the handoff from one AP to another easily. During
the roaming process, the phone scans for the nearby APs, determines which AP can provide the best
service, and then reassociates with the new AP. When implementing stronger authentication methods,
such as WPA2 and EAP, the number of information exchanges increases and causes more delay during
roaming. To avoid delays, use CCKM.
CCKM, a centralized key management protocol, provides a cache of session credentials on the Cisco
Unified Wireless LAN Controller or a Wireless Domain Server (WDS). As the phone roams from one
AP to the next, CCKM compresses the number of message exchanges during roaming by providing a
master key stored on the WDS for the AP to use. The reassociation exchange is reduced to two messages,
thereby reducing the off-network or audio gap time.
For details about CCKM, see the “Cisco Fast Secure Roaming Application Note” at:
http://www.cisco.com/en/US/products/hw/wireless/ps4570/prod_technical_
reference09186a00801c5223.html
In dual-band WLANs, it is possible to roam between 2.4 GHz bands (802.11b/g)
and 5 GHz bands (802.11a). The phone moves out of range of one AP using
one band and into the range of another that has the same SSID but is using a
different band.
Note
Cisco Unified Wireless IP Phone 7925G, 7925G-EX, and 7926G Administration Guide
31
VoIP Wireless Network
WLANs and Roaming

Cisco 7925G Specifications

General

Keypad number of keys	12
Display diagonal	2 \
Display resolution	176 x 220 pixels
Call waiting	Yes
Lines quantity	6 lines
Voice codecs	G.711a, G.722, G.729a, iLBC
Security algorithms	128-bit WEP, SSL/TLS, WPA
Supported network protocols	DHCP, TFTP, DNS, SCCP, SRST
Input voltage	100 - 240 V
Dimensions (WxDxH)	52 x 20 x 127 mm
Minimum system requirements	Cisco Unified Communications Manager 4.1, 4.2, 4.3, 5.1, 6.0, 6.1, 7.0 + Cisco Unified Communications Manager Express 4.3 +
Input frequency	50/60 Hz
Storage temperature (T-T)	-30 - 60 °C
Operating temperature (T-T)	0 - 40 °C
Operating relative humidity (H-H)	10 - 95 %
Product type	IP Phone
Product color	Gray
Safety	UL 60950, CSA 22.2 No.60950, EN 60950, IEC 60950, AS/NZS 60950, IEC 60529 (IP 54)
Certification	47 CFR 15 B ICES-003 B EN 55022 B AS/NZS CISPR 22 B CISPR 22 B VCCI B EN 61000-3-2 EN 61000-3-3 KN 22 EN 55024 EN 50082-1 EN 61000-6-1 EN 300386 EN 60601-1-2 KN
Compliance industry standards	IEEE 802.11a, IEEE 802.11b, IEEE 802.11g, IEEE 802.1X, IEEE 802.11e

Weight and Dimensions

Weight	138 g