1xBTS 1xBTS

Sessions & UATI

Before a device can carry data on an HRPD sector it needs a session identity. In HRPD the device is the AT (Access Terminal) and the network radio/session side is the AN (Access Network). After the AT acquires the sector, it asks the AN for a session, and the AN assigns it a UATI.

Sessions and connections

A session is the AT/AN relationship keyed by the UATI: the negotiated state and identity that persists while the device is attached to the sector. A connection is a separate, shorter-lived thing — an active traffic channel opened on top of an existing session when there is data to move. A device keeps its session across periods with no connection, and opens and closes connections as needed. Opening a connection is covered in Connections & Data Path.

The UATI exchange

After acquisition, the AT uses the reverse Access Channel to ask for a session identity. The main exchange is:

  1. The AT sends UATIRequest on the Access Channel.
  2. The AN returns ACAck and UATIAssignment on the forward Control Channel.
  3. The AT sends UATIComplete, echoing the assignment sequence.
  4. The AN now has a UATI-keyed HRPD session for that AT.

What a UATI is

The UATI is not a phone number or IMSI. It is the HRPD session address used on the air interface. 1xBTS allocates it from the configured HRPD subnet and uses it to correlate access messages, traffic events, packet-core sessions, and web UI session state. That single identity is what ties a device’s activity together across the access channel, the traffic channel, the packet core, and the dashboard.

See also