Call Handling in AMPS and ETACS
In the process of call handling the call is setup and this process involve various step like mobile registration receiving a call etc.
Mobile registration process
- When a mobile is powered on it runs a self-diagnostic.
- Then it starts to scan the forward control channels.
- It picks the cell with strongest signal. Mobile scans every 7 sec, or when signal strength falls below a certain threshold
- Once it picks the strongest signal, it decodes SIN to determine if it is at its home location.
- If too many errors it will pick next strongest signal
- When MS has successfully camped on to a FCC it transmits a message which contains info such as Electronic serial number (ESN), its phone number, its home system ID etc. The mobile sends this info in the RCC
- The base station sends this information to MSC, which consults with various databases to check the info out.
- If everything checks out the mobile is ready to either make a call or receive a call
Mobile receiving an incoming call
- Mobile continues to monitor control channel.
- If there is an incoming call for this mobile, a page is sent by MSC on all the base station’s FCCs. The page includes the mobiles MIN.
- If the intended mobile receives this page, it will send an ack on the RCC.
- When MSC receives the ack, the MSC directs the base station to assign FVC and RVC to the call..
- The base station also assigns to the subscriber unit a supervisory Audio tone (SAT), and voice mobile attenuation code (VMAC)
- After the RVC and FVC assignment the mobile moves to these new frequencies, and voice transmission can proceed.
- Also during a call, if any signaling is required it is done on the pair of voice channels
- The SAT is transmitted continuously on both the FVC and RFC.
- The VMAC instructs the mobile to transmit at a particular power level.
- Once a call is up on a voice channel, all signaling is done on the voice channel via a scheme known as "Blank and Burst".
- When the site needs to send an order to the mobile, such as hand off, power up, or power down, it mutes the SAT on the voice channel.
- This is filtered at the mobile so that the customer never hears it.
- When the SAT is muted, the phone mutes the audio path, thus the "blank", and the site sends a "burst" of data.
- The process takes a fraction of a second and is scarcely noticeable to the customer.
- Again, it's more noticeable on a Motorola system than on Ericsson or Lucent. You can sometimes hear the 'bzzt' of the data burst.
Mobile receiving an incoming call
- For blank and burst mode data transmission,
- FSK is used by both the base and mobile station in blank and burst mode to initiate handoff, to change the subscriber power , and provide other system data
Mobile initiating a call
- When a subscriber unit initiates a call, the mobile unit sends an originating message on RCC.
- The originating message includes
- Subscriber MIN, ESN, station class mark and the destination telephone number
- The station class mark or SCM tells the cell site and the switch what power level the mobile phone operates at.
- The cell site can turn down the power in your phone, lowering it to a level that will do the job while not interfering with the rest of the system.
- In years past the station class mark also told the switch not to assign older phones to a so called expanded channel, since those phones were not built with the new frequencies the FCC allowed.
- If received correctly at the base station it sends this info to MSC
- MSC checks out the information to determine if the subscriber is a registered user.
- Connects the subscriber to PSTN
- Assigns the call to FVC and RFC, with specific SAT and VMAC
- This info is sent to the subscriber units, who switches to the new frequency pair, adjusts it TX power level and returns the SAT back.
- During the call MSC will issue a number of blank and burst commands
What happens when all the voice channels are busy?
- The MSC will hold PSTN line open, and instructs the current base station to issue a directed retry to the subscriber on FCC.
- A directed retry forces the MS to switch to a different base station, from voice channel assignment
- Depending on propagation conditions, location of a subscriber unit and the current traffic in the base station the directed retry may or may not work.
- This lets the BS and MSC know that a call was terminated voluntarily by the user, as opposed to being dropped by the system