|Mobile Phone Technology at a Microscopic
| Information System Engineering, Year 2|
Article 2 by Rajkumar Periannan,
Supervised by Dr T.Field
Paired with Fadi Joseph Fahham
|Control Channel Functions||Subscriber Initialisation|
|Subscriber Initialisation||Call Progression|
|Call Progression||Call Origination and Termination|
|Mobile To Land||Traffic Channel Access|
|Land to Mobile||Maintaining the Link|
|Fixed Channel Assignment Strategies||Forward Channel Concept|
|Dynamic Channel Assignment Strategies||Queuing Handover Request|
|Flexible Channel Assignment Strategies|
Mobile phones....Is it a small electronic device? It looks like a television remote control doesn't it?
The way it works is simple isn't it? Yes! Yes! No!!!!! This article explains why the answer to the last question
is no by using detailed explanation on how a cellular system processes calls. The cellular network used to explain this
has dual mode-digital and analogue(D-AMPS in North America). The call processing events for both analogue and digital cellular
system covered here are :-
Each carrier (non-wireline, wireline) has 21 channels allocated for control purposes. One of these channels is assigned to each cell site. A control channel is on the air 24 hours a day, 365 days a year. The control channel actually combines three functions into one channel:
The information that is continuously transmitted is on the Forward Control Channel(FOCC). Continuos transmission is needed here because the mobile phone monitors the level of the received FOCC to determine when they approach the edge of the cell and needs to be passed to an adjacent cell base station. There are three types of messages that are multiplexed in this channel. They are :-
|Overhead Messages||These are messages that are intended for all mobiles. Information in this message includes the cellular system's SID, combined paging and access channel information, and subscriber registration information.|
|Mobile Control Messages||These messages contain information for a specific mobile. This includes paging data, voice channel assignment, power level, or other commands that cause a mobile to respond.|
|Control Filler Messages||These are messages that are transmitted when there are no other messages to be transmitted on the FOCC. This maintains the mobile's synchronisation to the Overhead Message Train. This message can also be used to tell the mobiles what power level to use when accessing the system on the Reverse Control Channel.|
Before a call can be originated or received, the mobile unit must go through an initialisation
Upon application of power, the mobile does a series of internal diagnostics. Memory is checked and sanity tests are done on the processor(s). A tone is injected into the audio circuitry. If the circuits are good, the tone is heard from the speaker. All indicator lights are activated and "8's" are shown in the display. If any portion of the diagnostic routine fails, the mobile powers down.
Control Channel Access
Next, the mobile scans the 21 control channels looking for an active channel. If it is programmed for the A side it scans from channel 333 down 21 channels. If it is assigned to the B side, it scans from channel 334 up 21 channels. The mobile looks for the strongest control channel and starts decoding the transmitted information. This includes system SID, combined paging and access channel information, and how many channel to scan for paging/access channels. Now the mobile scans the 21 channels a second time. This time it is looking for the strongest paging channel. (It got the number of channels to scan during the first scan.) The mobile now locks onto the strongest paging channel and goes into the "idle task". Although the mobile scanned the 21 control channels a second time looking for a "paging" channel, it actually returned to the same control channel. Control, paging, and access functions are multiplexed onto the same channel.
In the idle task, the mobile is constantly monitoring the overhead message train. It is looking for control information which will most likely be a page of its directory number. It is also scanning its keyboard looking for data entry. This would be when the user makes a call.
This section of Call Processing will deal with various call types. Each of the call types covered
assumes that the mobile phone is in the idle task and in its home system. The first one discussed
is the Mobile to Land call.
Before this section is covered there are two signals that the reader needs to know.
They are :-
The diagram on the left shows the SAT assignments to cell clusters.
When a landside party calls a cellular mobile the PSTN determines that the call must be routed to
the MSC. A CDR is opened and the MSC performs validation and translation of the dialled
A page message is sent from the MSC to the system's cell sites. This page message is transmitted on all FOCC in all cell sites. Systems can also be configured so that just the last known paging area the mobile was in will transmit the page message.
When the mobile sees the page on the FOCC it rescans the control channels looking for an access channel and locks on to the strongest one. The mobile checks the busy/idle bits (just like the mobile to land Call scenario) and transmits a page responds message on the RECC.
The cell site receives the page response message and selects an idle voice channel. The selected voice channel keys its transmitter and begins transmitting SAT. The cell site transmits on the FOCC an IVCDM to the mobile that contains the voice channel assignment, SAT, and VMAC information.
When the mobile receives the IVCDM on the FOCC, it tunes to the assigned voice channel, checks and transponds SAT. The MSC updates the CDR with the IVCDM information.
Next, the cell site transmits an alert order on the FOVC to the mobile. This message tells the mobile to start ringing. When the mobile starts ringing, it transmits ST continuously until the mobile is answered. When the mobile goes off hook, ST is shut off. This loss of ST at the cell indicates that the call has been answered. The MSC now connects the landside party with the mobile. Both forward and reverse audio paths are unmuted, conversation takes place, and the CDR is updated.
Landside to mobile call termination was previously discussed under the Call Termination section.
This section will discuss the process the mobile goes through when placing a digital call. There
are many areas in the processing of a digital call that is similar to an analogue call. This section will
concentrate on the differences between the two. A knowledge of analogue call processing is
The call progression description will be centred around a dual mode mobile. This is a mobile (or portable) that is capable of operation in either the analogue or digital mode. An assumption is made that the mobile is operating in the digital mode.
The dual mode mobile is programmed almost exactly like the analogue phone. The major exception
is the Station Class Mark (SCM). This is an entry into the mobile's memory that tells the switch
the unit has digital capability. When the phone makes a call origination it tells the switch what
type of phone it is.
These tests are similar to the analogue phone power up tests.
After the power up tests, the mobile scans the 21 control channels. It will scan the dedicated 21
control channels, either the A side channels (313-333) or the B side channels (334-354)
depending on how the phone is programmed. It will look on the strongest control channel for the
Protocol Compatibility Indicator (PCI) bit. This bit tells the mobile whether or not the system has
digital capability. If this bit is set, the mobile proceeds with decoding the overhead message train
and going into the idle task just as in the analogue system. If the PCI bit is not set the phone will
scan the secondary control channels for the strongest signal.. These are a second set of control
channels that be from 696-716 for system A or 717-737 for system B. These secondary control
channels are for digital use only. The phone then attempts to decode a possible overhead message
train. If an overhead message train(OMT) cannot be found (this could be an analogue voice channel!), the phone retunes to
the primary control channels and operates in the analogue mode.
Call progression in these areas are similar to analogue operation. A major difference is when the
mobile is instructed to tune to a digital traffic channel. In an analogue system the mobiles is
assigned a voice channel.
If digital traffic channels are available (digital voice/data channels), the base station informs the mobile of the channel and time slot assignment. The mobile is also informed of the Digital Verification Colour Code (DVCC) to expect on the traffic channel. The DVCC is an 8 bit word that is transponded on each time slot. It performs the same function as SAT in an analogue call. The DVCC word has an additional 4 bits added for error correction to make it a Coded Digital Verification Colour Code (CDVCC).
When the mobile receives the digital traffic channel designation on the Forward Control Channel,
it attempts to synchronise to the Forward Digital Traffic Channel. When synchronised, the
mobile transmits in its allocated time slot at its assigned power level and will transpond CDVCC.
The signal from the mobile to the base station takes a finite amount of time to travel to the cell site. A special Time Alignment procedure is used to ensure that transmission from other mobiles on the same frequency (but different time slots) do not collide. When the mobile first joins the traffic channel the distance to the base station is not known. The mobile needs to know how much to delay its transmissions in its time slot to avoid collisions, or glare.
Before the mobile starts transmitting actual data, it first transmits Shortened Bursts. This allows the base station to determine time alignment information. The base station sends the mobile timing information until it starts receiving valid data. The mobile then will begin transmitting in its normal mode.
Once the mobile is on the Digital Traffic Channel it will be operating in its assigned time slot and
with the proper time alignment. As the mobile moves through the cell's coverage area messages
are sent to adjust the mobile's power level and time alignment to maintain call quality.
During a call there are conditions that can cause the call quality to deteriorate. This results in an increasing Bit Error Rate (BER). Some causes can be:
Doppler shift causes a change in receive frequency when there is relative motion between the base
station and the mobile. These frequency (phase) changes in an analogue system are relatively minor.
However, in a digital system, the Doppler shift can cause problems. A car travelling 60 mph to or
from a cell site causes a frequency shift of 80 Hz. The digital modulation scheme interprets this
as a phase change and errors can occur.
Fading occurs when the signal from a transmitter is attenuated by physical objects such as a
building or a hill. The receiver has an Automatic Gain Control (AGC) circuit to compensate for
Multipath occurs when two or more signals arrive at a receiver at different time because of
reflections. There is a time delay between the two signals because each signal travelled a different
distance. If the two signals are 180 degrees out of phase, complete cancellation occurs. This is
also a concern in analogue systems.
As we have seen, there are many factors at work to degrade the digital signal which causes the
BER to increase. There are many techniques use to combat these effects such as space diversity
to minimise the effects of multipath and AGC to help control fading. But the combined effects of
all modes of interference can be to much for the radio to handle. There is one more tool in the
arsenal to fight signal degradation.....an equaliser.
In dual mode radios the equaliser compensates for the effects of distortion. SYNC - Synchronisation is a 14 symbol field. Each time slot has its own unique synchronisation sequence. The receiver takes SYNC(known bit sequence) and compares it to the received signal. The equaliser can then dynamically adjust its filters to reverse the effects of distortion.
At the end of the conversation one of the parties will go on hook. If the call is on an analogue
channel the process will be as discussed in the analogue call processing section. If the call is on a
digital traffic channel, a Release Message will be sent either by the base station or by the mobile,
depending on which side of the call goes on-hook first. The receiving end will acknowledge the
order, the mobile turns off its transmitter and retunes to the primary control channels. The mobile
then starts the initialisation procedures.
In the text above, the channels used and the purpose they serve in both analogue and digital cellular
system during call processing have been discussed
This section identifies and explains strategies that could be used to assign the needed channels.
The three main assignment strategies are :-
The common underlying theme in all fixed assignment strategies is the permanent assignment of a set of channels
to each cell. The same set of radio frequencies is reused by another cell at some distance away.
A call attempt at a cell site can only be served by the unoccupied channels of the predetermined set of channels at that cell
site(see diagram 2); otherwise, the call is blocked. Here, when handover occurs, the MSC informs the new BS and receive a
confirmation or rejection message from the new BS regarding the handover.
Fixed channel assignment strategy. A-G denote different sorts of channels permanently assigned to cells.
A very high number of users would have their call blocked(blocked traffic). The number of users serviced at any one time is
Borrowing strategies. channel a4 is borrowed and now locked to cells marked 'N'. Cells marked 'X' were already prohibited from using a4.
If all permanent channels of a cell are busy, a channel can be borrowed from a neighbouring cell, provided that this channel
does not interfere with the existing calls.
When a channel is borrowed, additional cells are prohibited from using it. The MSC supervises the borrowing procedure, following an algorithm that favours channels of cells with the most unoccupied channels to be borrowed. The algorithm 'locks' the borrowed channels towards the cells that are one or two cell unit away from the borrower cells.
The MSC keeps record of free, serving and borrowed(therefore locked) channels and informs all the BSs about locked channels.
In this proposed strategy, permanent channels of a cell are divided into two groups
Minimizes the number of calls on the relatively more 'borrowable' channels in order to reduce the locking effect of borrowed channels in additional cells. This effectively minimises the number of blocked traffic.
In this strategy, cells have no channels themselves but refer all call attempts to the MSC, which manages all channel assignment in its region.
Each time a call attempt arrives, the BS asks the MSC for the channel with the minimum cost to be assigned. The cost function depends on the future
blocking probability, usage frequency of the candidate channel, the reuse distance of the channel, and so on. The MSC decides, on a call-by-call basis, which channel
to assign to which call attempt by searching for the available channel for which the cost function is minimum.
The MSC needs to have information regarding channel occupancy distribution under current traffic conditions and other network-directed criteria, as well as
Flexible channel assignment strategies combine aspects of both the fixed and dynamic strategies because it
has two types of channels mentioned below :-
|Permanent Channel||each cell is assigned a set of these channels and would normally suffice under light traffic loads.|
|Flexible Channel||held by the MSC and assigns these to cells whose permanent channels have become inadequate under increasing traffic loads.|
|Schedueled manner||Assumption is made that future changes in traffic distribution can be accurately pin-pointed in time and space. The change in assignment of flexible channels is then made at the predetermined peaks of traffic change|
|Predictive manner||The traffic intensity or equivalently the blocking probability is constantly measured at every cell site so that the reallocation of the flexible channels can be carried out by the MSC at any point in time|
This section explains why forced termination of a call occurs and then followed by a discussion of the methods that could be used to minimise this problem.
A break during conversation occurs when handover takes place. This is not noticed by the user. But once the break is thought to become noticeable to the user the call is forced to terminate.
Call blocking occurs when there are too many users accessing the system simultaneously and the system is unable to allocate free channels to the users.
If the system has a channel assignment strategy in which the BS handles the handover request in exactly the same manner as it handles originating calls, the probability of forced termination of an on-going call due to unsuccessful handover would equal the probability of blocking an originating call. From the user point of view, termination of an on-going call is more undesirable than blocking new calls. Therefore handover request should be given a higher request than originating call request. Below, two possible handover prioritisation scheme is discussed. They are :-
This scheme improves the probability of successful handovers by simply reserving a number of channels exclusively for handovers. The remaining channels can be equally shared between handovers and originating calls.
There are two problems with this scheme. They are :-
(i) Total carried traffic is reduced since less channels are granted to originating calls(originating calls and not on-going calls add up to carried traffic).
Allow queuing of originating calls
(ii) Risk of inefficient spectrum utilisation when fixed channel assignment strategies used.
Use flexible or dynamic channel assignment. Instead of the cells keeping guard channels in their possession, the MSC should keep a collection of channels dedicated for handover requests, or it can have a number of flexible channels with associated probabilities of being allocated for handover requests.
Queueing Handover Requests
Handover and receiver thresholds. Linear motion from BS 1 and BS 2 is assumed; handover must occur in [t0,t1]
The handover threshold is set at the point where the power received from a neighbouring cell site has started to exceed the power received from the current BS for certain amount and/or for a certain time.
The receiver threshold is the point at which the received power from the BS is at the minimum acceptable level.
The overlapping circle area shown in diagram 4 is the handover area. The mobile phone must be assigned a new BS while in the handover area. Call termination occurs when the power level from the current BS falls below the receiver threshold prior to the mobile phone being assigned a channel by the target BS. Therefore the handover request can be queued within the interval in which the mobile phone is in the handover area.
In this section the Measurement Based Prioritisation Scheme(MBPS) is discussed. This is then compared with the typical First In First Out(FIFO) scheme.
Measurement Based Prioritisation Scheme(MBPS)
MBPS is designed as a handover protection method which a cellular communication network can utilise along with any channel allocation strategy. In MBPS, the handover area is divided into power level ratios. This power ratio decreases from the handover threshold to the receiver threshold(decreases when the mobile phone is moved away from the current BS). This is shown in diagram 4.
This scheme uses a multipriority non-pre-emptive queue.
The properties of this queue are :-
|High priority||Assigned to the requests
||Low priority||Assigned to request
|The flow chart on the left explains the behaviour a system using this scheme. Queue holds the handover requests|
|Graph 1. Pobability of Forced Termination vs. offered load for 20% handover traffic||Graph 2. Probability of Forced Termination vs. offered load for 50% handover traffic|
|Graph 3. Pobability of Call Blocking vs. offered load for 20% handover traffic||Graph 4. Probability of Call Blocking vs. offered load for 50% handover traffic|
The call processing event have been discussed in detail for both digital and analogue cellular systems. Various channel assignment strategies have been explained and it was found that an improvement in the channel assignment performance leads to an increase in complexity of the MSC. In order to minimise forced termination of calls, the MBPS should be used instead of the common FIFO queuing scheme. This is because it gives lower probability of call termination and blocked call compared to the FIFO scheme.GLOSSARY
|AGC||Automatic Gain Control|
|CDR||Call Detail Record|
|ESN||Electronic Serial Number|
|FOCC||Forward Control Channel|
|FORC||Forward Voice Channel|
|IRCDM||Initial Voice Channel Designation Usage|
|MIN||Mobile Identification Number|
|MSC||Mobile Switching Centre|
|NAM||Number Assignment Mobile|
|REVC||Reverse Voice Channel|
|REVCC||Reverse Control Channel|
|SAT||Supervisory Audio Tone|
|SID||System Identification Number|
|VMAC||Voice Channel Mobile Attenuation Code|
|1.||Title:||Mobile Communication Systems|
|Author(s):||Parson J.D., Gardiner J.G.|
|Source:||2nd Edition, Blackie.|
|Chapter 8, pg 253.Gives a reasonable explanation about workings of cellular networks.|
|2.||Title:||Comparisons of Channel Assignment Strategies in Cellular Mobile Telephone Systems|
|Author(s):||Lee M., T.P. Yum|
|Source:||IEEE Trans. on vehicular Tech., vol 38,Nov. 1989|
|Detailed explanation of the topic.|
|3.||Title:||A strategy for Flexible Channel Assignment in Mobile Communication Systems|
|Author(s):||Tajima J., Imamura K.|
|Source:||IEEE Trans on Vehicular Tech., vol 37, May 1988|
|Detailed explanation about the topic.|
|4.||Title:||Increasing Channel Occupancy in Large-Scale Mobile Radio Systems:Dyanamic Channel Assignment|
|Author(s):||Tajima J., Imamura K.|
|Source:||IEEE Trans on Vehicular Tech., vol 37, May 1988|
|Detailed explanation about the topic.|
|5.||Title:||A Hybrid Channel Assignment Scheme in Large-Scale Cellular Structured Mobile Communication Systems|
|Author(s):||Kanwa T.J., Georganas N.D.|
|Source:||IEEE Trans on Vehicular Tech., vol 26, Apr 1988|
|Detailed explanation about the topic.|
|6.||Title:||An effective Prioritisation Scheme for Handovers in Cellular Networks|
|Author(s):||Tekinay S., Jabbari B.|
|Source:||IEEE Communication Magazine|
|Detailed explanation about the topic. An excellent material|