Quality Control - Ericsson EF738 Service Manual

If the mobile telephone makes a call, a call request will be sent to the area base sta-
tion, which will select a voice channel and direct the mobile to the selected channel.
As in the previous case, the call will be transmitted over the selected voice channel,
until a zone boundary is passed.
The mobile telephone uses the control channel for different purposes, namely to
report to the system when it passes from one service area into another. One of the
purposes is for page response, which is an answer to a cell access from a mobile sta-
tion , that is a call from the mobile phone or an order confirmation. When messages
are sent to all mobile phones being tuned to a control channel, this is called over-
head message. The system parameters are transmitted during this message and con-
tain for instance the following information about the cellular system:
• AID (Area Identity) indicates system A or B, system identity number, MSC serv-
ice area and location area.
• DCC (Digital Colour Code) identifies which group of cells the message belongs
to.

Quality Control

Each time a connection is established, the exchange automatically measures the
field intensity of the mobile telephone. During conversation, the signal-to-noise
ratio is continuously measured by a monitoring signal, SAT (Supervisory Audio
Tone), sent out by the base station and returned by the mobile telephone. If the base
station finds that the signal strength is below a certain value, it alerts the exchange.
The exchange now requests neighbouring base stations to transmit their monitoring
signals and measure the signal strength. The exchange then decides which base sta-
tion provides the best conditions, and orders the mobile telephone to switch to a
channel of this base station instead. The mobile sends a signalling tone (ST) as
response or synchronization to the base station, for instance during call setup and
during handoff.
System Introduction
9