Voxtel W420, Руководство по ремонту

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5.7 Mobile Turn-On
1)Mobile Searches for Broadcast Channels (BCH)
2)Synchronizes Frequency and Timing
3)Decodes BCH sub-channels (BCCH)
4)Checks if Network Allowed by SIM
5)Location Update
6)Authentication
When a mobile first turns on, it searches all 124 channels in the downlink for signals. It will then order the
channels by received signal strengths and check to determine if the channel was a BCH (Broadcast CHannel).
Once the MS finds a BCH, it adjusts internal frequency and timing from the FCH and SCH, then checks to
determine if the BCH is from its PLMN (Public Land Mobile Network). This involves comparing the allowed
network and country codes stored on the SIM card with the information encoded on the BCCH. The mobile
repeats this cycle until a good broadcast channel is found. If the mobile recognizes that it's in a different cell
from the last time it was used, it needs to tell the network where it is. The network has to keep track of where
every mobile is so that it can route calls to the correct cell for any particular mobile. This process of telling the
network "here I am" is called a location update. The mobile sends a RACH, gets assigned to an SDCCH,
exchanges control information, then ends the call. The user will typically not be aware that this process is
taking place.
For Location Updates, this time showing which part of the network is involved in transactions.
The IMSI attach/detach process is a way of forcing all mobiles to inform the network when they have
camped and when they have turned off (or just before they turn off!). The SIM stores the last location Area
Code (LAC) when it is powered down and it compares this to the camping LAC on Power up and if they are
different it will perform an IMSI attach.
Mobile Originated Call
Mobile Sends RACH
Channel Assignment Posted on BCH (AGCH)
Mobile and Base Station communicate on SDCCH
Authentication
Mobile Assigned to Traffic Channel (TCH)
Speech Data sent and received
Once camped, the mobile is ready to send or receive calls.
When a user dials a number, and presses the send button on the mobile, call origination takes place. The
mobile transmits a short RACH burst on the uplink, using the same ARFCN as the BCH is using on the
downlink. The base station responds to the RACH by posting an AGCH (Access Grant CHannel) on the CCCH.
These are logical channels on the BCH physical channel. The mobile listens on the BCH for the AGCH, when it
receives it and decodes the instructions, it re-tunes to another ARFCN and/or timeslot and begins a two-way
dialogue with the base station on an SDCCH. One of the first things that the mobile will mreceive is the SACCH
associated with the SDCCH. Once it receives the SACCH, it will get timing advance and transmitter power
information from the base station. The base station will have calculated the correct timing advance from the
arrival time of the RACH. Once the mobile gets timing advance information, it can send normal length bursts.
The SDCCH is used to send messages back and forth, taking care of alerting (making the mobile ring) and
authentication (verifying that this mobile is allowed to use the network). After a short period of time (1 to 2
seconds), the mobile is commanded over the SDCCH to re-tune to a TCH. Once on the TCH, speech data is
transferred on the uplink and downlink.
Mobile Terminated Call
Mobile Sees Page
Mobile Sends RACH
Channel Assignment Posted on BCH (AGCH)
Mobile and Base Station communicate on SDCCH
Authentication
Mobile Assigned to Traffic Channel (TCH)
Speech Data sent and received
The process for base station originated calls is very similar. The base-station posts a PCH (paging CHannel)
on the CCCH part of the BCH. When the mobile receives the PCH, it responds by sending a RACH. The
remainder of the process is identical to the mobile originated case. If you can find a way to translate the GSM
bursts into audio tones (AM demodulate), it's interesting to hear the difference between the channel types as a
call is set up. A good way to do this is to use a GSM phone near an old TV set or a conventional wired phone.