Mobility

This page will cover various topics related to:
 * idle mode mobility
 * handover
 * roaming
 * call routing

Mobility
Mobility is a process that allows transferring the mobile service as the mobile station moves from one cell to another. Mobility in GSM networks has the purpose of monitoring subscribers and their location (in terms of the base station they are registered to), to enable them to receive voice calls, SMSs or data services.

Mobility parameters
Mobility has various parameters which will be listed in the sections below.

MCC (Mobile Country Code) and MNC (Mobile Network Code)
These parameters define the mobile operator's identity.

LAC (Location Area Code)
LAC is a unique number assigned to each location area, which is area served by several cells. A powered on mobile station will perform a location update whenever it crosses the boundaries of a location area. The location area update can be done via the VLR, if the old and new location area are served by the same MSC/VLR. Otherwise, the subscriber is moved to a different VLR if the new location area is served by a different MSC/VLR. This case involves a subscriber updated in the HLR with the new serving MSC/VLR as well.

CI (Cell ID)
CI is a unique identifier for each base station in the network.

NCC (Network Color Code)
NCC is a 3-bit network identity code, unique for each network in a particular area, assigned by regulators.

BCC (Base station Color Code)
BCC is a unique 3-bit cell identity, assigned in a color map.

C0 ARFCN
C0 ARFCN is the beacon radio channel for a particular base station, usually assigned in a 7-ARFCN color map.

Neighbor List
The Neighbor List is the C0 ARFCNs of neighboring base stations.

Cell Reselection Hysteresis
Cell Reselection Hysteresis refers to a cell's signal strength in order for it to trigger the MS to switch to it.

NCCs Permitted
NCCs Permitted is a list of NCCs that a MS should monitor; others will be ignored, so make sure your own NCC is within the list.

Idle Mode Mobility
Idle mode mobility enables the MS to be reached by the network at any time. The MS doesn't have an assigned dedicated channel and essentially it controls the idle mode.

Although inactive, the MS is turned on and is registered to the network. The device can start and receive calls and data packets from the network.

During idle mode, the MS will perform:
 * network selection
 * cell selection
 * cell reselection
 * paging
 * location updating

YateBTS Configuration for Mobility
All base stations will have the same NCC, MCC and MNC, since they belong to the same network.

All base stations have different CIs because they are different units.

All base stations have different LACs because the MS needs to register each time it moves to a different unit.

The neighbor list contains IP addresses of neighboring YateBTS units.

Neighbor list information is exchanged using a peering protocol based on SIP.

Neighboring base stations have different BCCs, as the specifications require, and are usually assigned in a color map.

Neighboring base stations also have different ARFCNs because they will not work otherwise, and are usually assigned from a color map.

Cell Reselection Hysteresis leads to choosing either the quality of service or the MS activity. If too much is applied, the MS is forced to use weak signal. Also, if too little is applied, transaction rates will increase and the standby battery life will decreases.

Handover
Handover is the process of transferring a proceeding voice call or data session from one cell to another.

Mobility is essential for performing handover. While conventional mobile networks perform handover within their BSCs, a YateBTS powered network will do handover peer-to-peer via SIP, since it doesn't have BSCs.

Handover is conditioned by the following factors:


 * RSSI: the Received Signal Strength Indicator (RSSI) of the current serving cell must be weak enough
 * RSSI level: the neighboring cell must have an RSSI significantly higher than than of the serving cell
 * previous handover rejections: there mustn't have been any recent handover rejections related to the neighboring cell since this is always a congestion indicator

If all these conditions have been fulfilled, the serving cell will initiate handover.

Neighbor List
The neighbor list is similar as in the case of idle mode mobility.

Minimum Received Power
The minimum received power is the power threshold given in dBm; if the serving cell exceeds a particular RSSI at the MS, handover is not performed.

RSSI Difference
RSSI Difference is expressed in dBm; indicates that if the RSSI of the strongest neighboring cell doesn't exceed a particular value, handover is not performed.

Handover Hold-off
Handover hold-off implies that if a handover procedure has been rejected by the neighboring cell within the handover hold-off period, handover is not performed.

Roaming
Roaming refers to providing a customer with voice and data services while abroad, through a visited network. It is a technical component provided through two protocols: SS7-MAP and SS7-CAP. Roaming also has a commercial component: the roaming agreement.

Common transactions:
 * Location Update – announces the home network HLR what is the current serving VLR and transfers the subscriber data to the VLR
 * Authentication – exchange of authentication tuples from the HLR/AuC
 * Inbound Call Routing – directs the inbound call to the current serving MSC
 * Outbound Call Routing – similar to a non-roaming outbound call, with a caller ID from the VLR

YateBTS and Roaming
Integration into a legacy PLMN uses the same SS7-MAP/CAP protocols. The HLR/VLR architecture is significantly scalable.