Table 15.1 (continued)
ROAMING USERS LOGGING AWAY FROM OFFICE: HOME AND
WHILE TRAVELING
The price to pay is the more difficult determination
of location for emergency calls when on travel or
when connected with the home enterprise
network using VPN. See Chapter 16, “Emergency
and Preemption Communications services.”
Session mobility Users can move active sessions between terminals.
For example a conference call participant on a
mobile phone can move to a multimedia PC
without dropping out from the conference and
without losing any media content during the
switchover. Streaming media sessions can be
moved seamlessly from a mobile phone or PDA to
a living room TV set.Examples of SIP Application-Layer Mobility
The various types of mobility defined in Table 5.1 will be illustrated here with
several high-level examples.
The basic scenario for roaming users is shown in Figure 15.1, where there is
open Internet access, for example at professional meetings (IETF), conferences
(Voice on the NET, or VON), some hotels, and in some public places.
The first step (a) consists of acquiring an IP address, gateway address, and
DNS server address from the network using the Dynamic Host Configuration
Protocol (DHCP). The mobile host will send out DHCPDISCOVERmessages
and one or more DHCP servers will reply with a DHCPOFFER, followed by
DHCPACKto the mobile station (MS) [9]. The other DHCP messages are not
shown for brevity, since this topic is out of scope here. Once the mobile host
has network connectivity, the SIP UA can register with the outgoing SIP proxy.
There are several methods to configure the SIP UA with the outgoing SIP
proxy, one of them is specified in [10].256 Chapter 15